STUART ROJSTACZER, Editor 

STRONG GROUND MOTION AND GROUND FAILURE 

THOMAS L. HOLZER, Coordinator 

The Lorna Prieta, California, Earthquake of 

October 17, 1989-Hydrologic Disturbances 

- 

U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 1551-E 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1994 

CONTENTS 

By Stuart Rojstaczer 

The origin of the tsunami excited by the earthquakefaulting or slumping --- ---- ------------ ---------- - --------- 

By Kuo-Fong Ma, Kenji Satake, and Hiroo Kanamori 

Stream-channel adjustment in Fern Canyon near 

Watsonville, California, after the earthquake ---------- 11 

By Deborah R. Harden and Dennis Fox 

Sources and magnitudes of increased streamflow in 

the Santa Cruz Mountains for the 1990 water year after the earthquake---------------------------------------- 31 

By Robert R. Curry, Brett A. Emery, and Tom Gentry Kidwell 

I 

Hydrologic changes associated with the earthquake in 

the San Lorenzo and Pescadero drainage basinsÃ‘--- 51 

By Stuart Rojstaczer and Stephen Wolf 
THE LOMA PRIETA, CALIFORNIA, EARTHQUAKE OF OCTOBER 17,1989: 

STRONG GROUND MOTION AND GROUm FAILURE 

HYDROLOGIC DISTURBANCES 

INTRODUCTION 

By Stuart Rojstaczer, 

Duke University 

Seismic events have long been known to cause changes 

in the level of oceans, streams, lakes, and the water table. 

The great San Francisco earthquake of 1906 induced significant hydrologic changes (Lawson, 1908) that were 

qualitatively similar to those changes observed for the 

Loma Prieta earthquake. What is different is that the hydrologic data sets collected from the Loma Prieta event 

have enough detail to enable hypotheses on the causes for 

these changes to be tested. The papers in this chapter document changes in ocean level, stream morphology and 

flow, water table height, and ground-water flow rates in 

response to the earthquake. Although hydrologic disturbances may have occurred about 1 hour before the main 

shock (Roeloffs, 1993), the papers in this chapter deal 

strictly with postevent hydrologic changes. The hydrologic 

responses reported here reflect changes that are not the result of surface rupture. They appear to be the result of 

landslides, the static displacements induced by the earthquake, and changes in the permeability of the near surface. 

The Monterey Bay tsunami is examined by Ma and others. They use elastic half-space models of the earthquake 

in conjunction with a finite difference model of tsunami 

motion in Monterey Bay to examine the origin of the tsunami. The waveform of the tsunami cannot be explained 

solely by the static displacement of the ocean floor induced by the earthquake. To mimic the observed response, 

Ma and others include earthquake-induced slumping of 

0.01 km3 of sediment into the ocean from Moss Landing. 

The response of stream morphology to San Andreas fault 

motion is the subject of Harden and Fox. Stream channels 

in the Santa Cruz Mountains have been altered by prehistoric earthquakes in this region. Harden and Fox have 

made repeat surveys of a channel within Fern Canyon. 

This channel was chosen for detailed study because it 

shows evidence of geologically recent disruption by the 

San Andreas fault and because it has a relatively straight 

channel that enters the fault zone at a right angle. The surveys indicated that the channel elevation increased along a 

30-m segment upstream of the fault. The middle 17 m of 

this reach showed an average of 28 cm of aggradation; the 

aggradation appears to reflect a decrease in the gradient 

upstream of the fault. The postearthquake surveys of this 

channel suggest that earthquakes without significant surface 

rupture may induce changes in stream-channel morphology 

that, over geologic time, can produce the offset streams 

characteristic of the San Andreas fault. Tectonic deformation of stream channels occurs at a faster rate than the ability of the channel to readjust by aggradation or erosion. 

Changes in the magnitude of streamflow in response to 

the Loma Prieta earthquake are well documented and are 

the subject of three papers in this volume. Briggs examines 

changes in stream flow in Waddell Creek. The earthquake 

induced a transient increase in discharge in the Creek and 

associated springs that exponentially declined over a period 

of one to two months following the earthquake. Spring 

flow at higher elevations ceased at an earlier time than 

spring flow at lower elevations suggesting that the water 

table elevation declined over time. 

On a regional scale, Curry and others state that verifiable changes in stream discharge were noted as far away 

as 88 km from the epicenter. Increases in flow that persisted for several months following the earthquake were restricted to the Santa Cruz Mountains and San Francisco 

peninsula. The source of this water appears to be the 

ground-water system. Chemical analyses of about 100 surface water and ground-water samples demonstrated modest increases in ionic concentration in some, but not all, 

sampled waters. 

The ground-water and surface-water response of the San 

Lorenzo and Pescadero drainage basins is discussed by 

Rojstaczer and Wolf. Streamflow increased at most gaging 

stations within 15 minutes after the earthquake. Groundwater levels in the highlands parts of the basins were locally lowered by as much as 21 m within weeks to months 

after the earthquake. Streamflow reduction in these basins 

followed an exponential rate similar to that shown by 

Briggs. In the San Lorenzo basin, changes in stream chemistry were significant. Although cationfanion ratios of the 

major constituents remained relatively constant, overall 

ionic concentrations and the calcite saturation index of the 

streamwater increased. Solute concentrations declined significantly within several months after the earthquake. 
E2 HYDROLOGIC DISTURBANCES 

Although increases in stream and spring flow have been 

sometimes ascribed to fluid sources from the midcrust, the 

cause of streamflow increases examined in three of the papers described above appears to be a permeability increase 

of the near-surface aquifers and aquitards. Hence, the observed ground-water and surface-water response are not 

apparently coupled to the earthquake-generation process. 

The permeability increases seem to persist for a period 

longer than the time of increased streamflow. Rojstaczer 

and Wolf present a simple diffusipnal model that mimics 

the observed streamflow and ground-water changes. The 

cause for the permeability increases is not known. Curry 

and others speculate that the permeability increase is due 

to microbiologic processes. Rojstaczer and Wolf propose 

that the permeability increase is due to seismically induced fracturing and microfracturing. 

In the papers presented in this chapter, hydrologic 

changes associated with the Loma Prieta earthquake are 

spatially variable, but the overall character of the changes 

can be explained by relatively simple conceptual models. 

In a study of ground-water disturbances in South Carolina 

associated with the New Madrid earthquakes of 18 1 1- 18 12 

(Smith, 1819) the following was noted: Whatever may be 

the cause of this phenomenon, the effects are so inconvenient, and it is so generally believed that they are likely to 

be permanent, that the inhabitants of the town are beginning to build cisterns, in order to accumulate artificial reservoirs of water (p. 95). 

The hydrologic changes in response to the Loma Prieta 

earthquake are much more local in extent than those of the 

New Madrid, but the impact on communities in the region 

was significant. Unlike the hydrologic disturbances observed 

at the time of the New Madrid earthquakes, the causes for 

these changes appear to be well understood. They reflect the 

dynamic response of the ocean, streams, and ground water 

to earthquake-induced physical changes in the morphology 

and internal fabric of near-surface rocks and sediment. 

REFERENCES CITED 

Lawson, A.C., chairman, 1908, The California earthquake of April 18, 

1906; Report of the State Earthquake Investigation Commission: 

Carnegie Institution of Washington, Publication 87, 451 p. 

Roeloffs, E., 1993, A reported streamflow increase: U.S. Geological Survey Professional Paper 1550-C, p. 47-52. 

Smith, E.D., 1819, On the changes which have taken place in the wells of 

water situated in Columbia, South-Carolina, since the earthquakes of 

181 1-12: American Journal of Science, v, 1, p. 93-95. 
THE LOMA PRIETA, CALIFORNIA, EARTHQUAKE OF OCTOBER 17,1989: 

STRONG GROUND MOTION AND GROUND FAILUW 

HYDROLOGIC DISTURBANCES \ 1 

THE ORIGIN OF THE TSUNAMI EXCITED BY THE EARTHQUA=- 

FAULTING OR SLUMPING 

By Kuo-Fong Ma, California Institute of Technology, 

Kenji Satake, University of Michigan, 

and 

Hiroo Kanamori, California Institute of Technology 

CONTENTS 

Abskact ........................................................... hkoduction ....................................................... Dab ............................................................ --- Method .............................. ---- ---------- -- ----------- --- Fault model ........................... - ........................... Results ----------------------me-------- -- ----------- ----- -------- --- Conclusions ......................... ------- ------- -------- ----- --- References cited ------------------- - ------------- -- ------------ -- 

amplitude of the observed tsunami, the volume of sediments involved in the slumping is approximately 0.012 

h3. Thus the most likely Fause of the tsunami observed 

at Monterey is the combination of the vertical uplift of the 

sea floor due to the main faulting and a large-scale slumping near Moss Landing. 

ABSTRACT 

The first arrival of the tsunami recorded at Monterey, 

California, was about 10 min after the origin time of the 

earthquake. Using an elastic half space, we computed vertical ground displacements for many different fault models for the Loma Prieta earthquake and used them as the 

initial condition for computation of the tsunami in Monterey Bay. The synthetic tsunami computed for the uniform 

dislocation model determined from seismic data can explain the arrival time, polarity, and amplitude of the beginning of the tsunami. However, the period of the 

synthetic tsunami is too long compared with the observed. 

We tested other fault models with more localized slip distribution. None of the models could explain the observed 

period. The residua1 waveform, the observed minus the 

synthetic waveform, begins as a downward motion at 

about 18 min after the origin time of the earthquake and 

could be interpreted as due to a secondary source near 

Moss Landing. If the large-scale slumping near Moss 

Landing suggested by an eyewitness observation occurred 

about 9 min after the origin time of the earthquake, it 

could explain the residual waveform. To account for the 

INTRODUCTION 

The Loma Prieta earthquake (Mw=6.9) generated a 

tsunami in Monterey Bay, just south of the epicenter 

(fig. 1A). Such nedeld tsunamis are relatively rare in the 

United States; the 1906 San Francisco earthquake (Lawson, 1908), the 1927 Lompoc earthquake, the 1964 Alaskan earthquake, and the 1975 Kalapana earthquake are 

among the few examples. Since large coastal earthquakes, 

either onshore or offshore, can cause serious tsunami hazards, we investigated the tsunami excited by the Lorna Prieta earthquake in an attempt to understand the generation 

mechanism of such nearfield tsunamis. We will show that 

two elements contributed to tsunami excitation-the vertical deformation of the sea floor caused by faulting and 

the secondary submarine slumping presumably caused by 

shaking. 

DATA 

The tsunami was recorded (fig. 1B) on the tide gauge in 

Monterey Bay. Schwing and others (1990) described this 

instrument as a bubble gauge. We digitized and detrended 

the record (fig. 10 for one hour starting from the origin 

time of the earthquake. The first arrival of the tsunami is 

about 10 minutes after the origin time of the earthquake, 

and the peak-to-peak amplitude is about 40 cm. 
HYDROLOGIC DISTURE3ANCES 

METHOD 

Tsunami waveforms are computed either analytically for 

the case of uniform depth (see Takahashi, 1942; Kajiura, 

1963; Ward, 1982; Comer, 1984; Okal, 1988) or numerically 

for actual bathymetry (Hwang and others, 1972; Houston, 

1978; Aida, 1978; Satake, 1985). Since the bathymetry in 

Monterey Bay (fig. 2) is very complex-a canyon runs northeast to southwest-an assumption of uniform depth is not 

valid. We used a finite difference method to compute the 

tsunami in the bay using the actual bathymetryy which is 

known very accurately. 

As the initial condition for tsunami computation, we 

used the vertical ground displacement caused by faulting. 

For this computation, we used Okada's (1985) programy 

which computes ground deformation caused by faulting in 

a homogeneous half space. Since the source process time 

of the earthquake is less than 10 seconds and the water 

depth is much smaller than the scale length of the ground 

deformation, we assumed that the water surface is uplifted 

instantaneously exactly in the same way as the bottom deformation. The amplitude of the tsunami is of the order of 

10 cm and is much smaller than the water depth, about 

100 m. Also, the wavelength of the tsunami* about 10 km 

in the bay, is much longer than the water depth. Hence we 

can use the vertically integrated linear long-wave equation 

and continuity equation as basic equations of tsunami 

propagation. In a Cartesian coordinate system (x, y) these 

equations are given by 

v Monterey 

and 

where Qx and Qy are the flow rate obtained by integrating 

the velocity vertically from the bottom to the surface in the 

x and y directions respectively, g is the acceleration of gravity, 

D is the water depth, and H is the water height above the 

average surface. These equations are solved with a finite 

difference method. The bathymetry in Monterey Bay and the 

area for which the computation is made are shown in figure 

2. The grid size is 114 min, which is about 400 m and 500 m 

in the x and y directions, respectively, and the number of 

grid points is about 14,400. The time step of computation is 

2 s, which is chosen to satisfy the stability condition for the 

finite difference calculation. Since the bathymetry is known 

in detail, the tsunami can be computed very accurately. 

FAULT MODEL 

The fault model of the Lorna Prieta earthquake has been 

determined very accurately using seismicy geodetic, and 

Time (min) C 

Figure 1 .-A, Locations of earthquake epicenter (star) and fault (shaded strip) and tide gauge station (solid triangle). B, Tsunami record from tide gauge at 

Monterey (after Schwing and others, 19W). C, Detrended tsunami record for one hour starting from origin time of earthquake. 
THE ORIGIN OF THE TSUNAMI EXCITED BY THE EARTHQUAKE-FAULTING OR SLUMPING E5 

aftershock data. Kanamori and Satake (1990) inverted 

teleseismic body- and surface-wave data and obtained a 

mechanism with dip=70Â SW., rake=l38O, and strike=N. 

128' E. The seismic moment is 3x10~~ dyne-cm (Mw=6.9). 

The total length of the aftershock area is about 40 km, and 

the main shock is located near the center of the aftershock 

(U.S. Geological Survey Staff, 1990), which suggests bilateral faulting. Kanamori and Satake (1990) suggested a uniform fault model having a fault length, L, of 35 km. The 

coseismic slip on the fault is 238 cm, if the fault width, W, 

is assumed to be 12 km. Lisowski and others (1990) compared the observed geodetic data with several dislocation 

fault models; their preferred fault model has a fault length 

of 37 km and fault width of 13.3 km. The coseismic slip on 

the fault is 204 cm. The focal mechanism has dip=70Â SW., 

rake=1M0, and strike=N. 44O W. The total seismic moment 

determined from geodetic data is the same as that determined 

from seismic data by Kanamori and Satake (1990). 

RESULTS 

We first computed the vertical crustal deformation for the 

uniform seismic fault model (k35 km, W=12 km, and 

D=238 cm) determined by Kanamori and Satake (1990) and 

used it as the initial condition for tsunami computation. 

Figure 3A shows the location of the epicenter and the vertical 

crustal deformation. The displacement beneath the sea floor, a 

maximum of 25 cm, was responsible for tsunami generation. 

Figure 2.-Bathymetry in Monterey Bay and area over which tsunami computation was made. Contour lines indicate water depths 

in meters (contour intervals are variable). 

To see the contribution of the sea-floor displacement to 

the observed tsunami, we computed an inverse travel-time 

Figure 3.-A, Vertical crustal deformation with 10-cm contour intervals 

for uniform seismic fault model (L=35 km, W=12 km, and D=238 cm); 

earthquake epicenter indicated by asterisk. B, Inverse tsunami travel-time 

isochrons (contours indicate tsunami wavefronts at every 2 min); dashed 

box indicates area for inversion computation (see text for discussion). 
E6 HYDROLOGIC DISTURJ3ANCES 

diagram by placing a source at the tide-gauge station and 

propagating tsunamis backward into the bay. Figure 3B 

shows the inverse tsunami travel-times every 2 min. The 

isochron at 10 min is close to the south edge of the displacement field defined by the 0 cm contour line. This is 

consistent with the onset time of the tsunami at 10 min 

after the origin time of the earthquake. Figure 4 shows the 

snapshots of computed tsunamis at 5, 10, 15, 20, 25, and 

30 min after the origin time. 

Figure 5A compares the synthetic tsunami computed for 

this model with the observed. The synthetic tsunami can 

explain the arrival time, polarity, and amplitude of the beginning of the observed tsunami. However, the period of the 

synthetic tsunami is too long compared with the observed. 

The reason for the long period of the synthetic tsunami 

is that the sea floor deformation caused by faulting is very 

broad. If the slip on the fault is more localized than that in 

the model used in the above computation, the period of 

the synthetic tsunami could be decreased. To test this, we 

computed tsunamis for three localized sources and for the 

geodetic fault model obtained by Lisowski, Prescott, Savage, and Johnston (1990) for comparison. 

In the first case we localized the entire slip in the northwest half of the fault (fault length=l7.5 km). In the second 

case, the slip is localized in the southeast half (fault 

length=l7.5 km). In the third case, we localized the displacement in the bottom half of the fault plane (fault 

length=35 km, width=6 km). In all cases, the seismic moment is the same as for the uniform model. These cases 

represent the three extreme cases of localized sources. The 

fourth model is taken from Lisowslci, Prescott, Savage, 

and Johnston (1 990). Figures 5B-E compare the synthetics 

for these cases with the observed. The waveform of the 

synthetics is not very different from that for the uniform 

model. This result indicates that the displacement field 

caused by faulting is smoothed out in Monterey Bay, and 

it is not possible to explain the short period of the observed tsunami. 

The difference in the period suggests that a secondary 

source may be responsible for the tsunami observed at 

Figure 4.-Snapshots of the computed tsunami computed for the fault model at 5, 10. 15, 20, 25, and 30 

min. 
THE ORIGIN OF THE TSUNAMI EXCITED BY THE EARTHQUAKE-FAULTING OR SLUMPING 

Monterey. To explore this possibility, we computed the residual waveform; that is, the observed minus the synthetic 

waveforms. The residual waveform, shown in figure 5E 

begins as a downward motion at about 18 min after the 

origin time of the earthquake. Figure 3B shows that the 

isochron at 18 min is slightly north of Moss Landing. 

Schwing, Norton, and Pilskaln (1990) suggested the possibility of large-scale slumping near Moss Landing. Sea 

level fell by 1 m or more near Moss Landing soon after the 

earthquake. This sea level change is larger than the change 

expected solely from the direct effect of faulting. The inverse travel-time curve shown in figure 3B suggests that if 

this slumping occurred 9 min after the earthquake, the arrival time of the residual tsunami shown in figure 5F could 

be interpreted as due to the slumping at Moss Landing. 

To determine more details of the secondary source responsible for the tsunami, we divided the sea floor into 

four blocks (8x10 km2 each) as shown in figure 3B. Owing 

to the time delay of the secondary source, we shifted the 

residual waveform by 9 min and inverted the shified residual tsunami waveform to determine the displacement for 

each block. The inversion is formulated as 

where AJ{ti) is the tsunami amplitude at time ti due to a 

unit displacement at the jth block, xj is the displacement 

at the jth block, and bj{ti) is the observed tide gauge 

record at time ti. The displacement xj for each block is 

estimated with a linear least squares inversion of equation (2). 

Figure 6A shows the vertical displacement of the sea 

floor determined by the inversion. The displacement 

shows an isolated subsidence at the southeast block near 

Moss Landing, which is consistent with our assumption. 

The synthetic tsunamis computed for the displacement 

field shown in figure 6A and for a subsidence in the 

southeast block only are shown in figure 6B and 6C, respectively. Both can explain the period and the amplitude 

of the shifted residual tsunami. The southeast block near 

Moss Landing has a subsidence of about 15 cm over an 

area of 80 km2. Figure 6D compares the synthetic waveform computed for faulting and slumping combined with 

the observed. 

A slump may be most adequately modeled by a sudden 

subsidence followed by a gradual uplift. However, the details are unknown. If the later uplift was gradual, the tsunami source could be modeled using a single subsidence 

source. If this is the case, our result suggests that the volume of sediments involved in the slumping is approximately 0.013 km3. However, this estimate depends on the 

details of the slumping. Unfortunately, from the single observation we cannot determine further details. 
HYDROLOGIC DISTURBANCES 

CONCLUSIONS 

The uniform fault model determined from seismic data 

can explain the arrival time, polarity, and amplitude of the 

beginning of the observed tsunami, but the period of the 

synthetic tsunami is too long. We tested fault models with 

a wide range of nonuniform slip distribution, but none of 

them could explain the observed period satisfactorily. This 

suggests that a secondary source is required to explain the 

tsunami observed at Monterey. The residual waveform, the 

observed minus synthetic waveform computed for the seismic source, suggests that the most likely secondary source 

is a sediment slump near Moss Landing; evidence for such 

a slump has been reported by an eyewitness. 

Since the tsunami excited by the secondary source can be 

more extensive than that by the earthquake faulting itself, as 

is the case for the Loma Prieta earthquake, the possibility of 

tsunamis caused by secondary sources needs to be carefully 

evaluated in assessing the tsunami potential of nearshore 

earthquakes. 

Acknowledgments.-This work was supported by National Science Foundation grant EAR 89- 15987 and U.S. 

Geological Survey giant 14-08-00 1 -G1832. Contribution 

No. 4949, Division of Geological and Planetary Sciences, 

California Institute of Technology, Pasadena, California. 

REFERENCES CITED 

Aida, I,, 1978, Reliability of a tsunami source model derived from fault 

parameters: Journal of Physics of the Earth, v. 26, p. 57-73. 

Comer, R.P., 1984, The tsunami mode of a flat earth and its excitation by 

earthquake sources: Journal of the Royal Astronomical Society, 

V. 77, p.1-27. 

Houston, J.R., 1978, Interaction of tsunamis with the Hawaiian Islands 

calculated by a finite-element numerical model: Journal of Physics 

of the Ocean, p. 93-102. 

Hwang, L.-S., Butler, H.L., and Divoky, D.J.,1972, Tsunami model: Generation and open-sea characteristics: Bulletin of Seismological Society of America, v. 62, p. 1579-1596. 

Kajiura, K., The leading wave of a tsunami, 1963: Bulletin of Earthquake 

Research Institute, University of Tokyo, v. 41, p. 535-571. 

Kanamori, H., and Satake, K., 1990, Broadband study of the 1989 Loma 

Prieta earthquake: Geophysical Research Letter, v. 17, p. 1 1 79- 1 182. 

Lawson. A.C., Gilbert, G.K., Reid H.F., Branner, J.C. Leuschner, A.O., 

Davidson, G., Burkhalter, C., and Campbell, W.W., 1908, The California earthquake of April 18, 1906: report of the state earthquake 

investigation commission: Carnegie Institution, v. 2, p. 369-373. 

Lisowski, M., Prescott,W.H., Savage C.J., and Johnston, M.J., 1990. Geodetic estimate of coseismic slip during the 1989 Loma Prieta, California, earthquake: Geophysical Research Letters, v. 17, p. 1437- 1440. 

Okada, Y., 1985, Surface deformation due to shear and tensile faults in a 

half-space: Bulletin of the Seismological Society of America, v. 75, 

p. 1135-1154. 

Okal, E.A., 1988, Seismic parameters controlling far-field tsunami amplitudes: A review: Natural Hazards, v. 1, p. 67-96. 

Satake, K., 1985, The mechanism of the 1983 Japan Sea earthquake as 

inferred from long-period surface waves and tsunamis: Physics of 

the Earth and Planetary Interiors, v. 37, p. 249-260, 1985. 

Schwing, F.B., Norton, J.G., and Pilskaln, C.H., 1990, Earthquake and 

bay, response of Monterey Bay to the Loma Prieta earthquake: Eos, 

v. 71, p. 250-251. 
THE ORIGIN OF THE TSUNAMI EXCITED BY THE EARTHQUAKE-FAULTING OR SLUMPING E9 

Takahashi, R., 1942, On seismic sea waves caused by deformations of Ward, S.N., 1982, Earthquake mechanisms and tsunami generation: The 

the sea bottom: Bulletin of Earthquake Research Institute, Universi- Kuril Islands earthquake of 13 October, 1963: Bulletin of the Seisty of Tokyo, v. 20, p. 377-400. mological Society of America, v. 72, p. 759-777. 
THE LOMA PRIETA, CALIFORNIA, EARTHQUAKE OF OCTOBER 17,1989: 

STRONG GROUND MOTION AND GROUND FAILURE 

HYDROLOGIC DISTURBANCES 

STREAM-CHANNEL ADJUSTMENT IN FERN CANYON 

NEAR WATSONVILLE, CALIFORNIA, AFTER THE EARTHQUAKE 

CONTENTS 

By Deborah R. Harden and Dennis Fox, 

San Jose State University, San Jose, California 

Ell 

11 

14 

16 

18 

19 

20 

20 

ABSTRACT 

Fern Canyon, which contains a small stream crossing the 

San Andreas fault zone 8 krn northeast of Watsonville, has 

formed a broad floodplain upstream of two compressional 

ridges that deflect the present channel in a right-lateral sense 

and that have at least partly dammed it during late Holocene 

time. Between March 1990 and June 199 1, repeated surveys 

along the Fern Canyon channel revealed an increase in the 

channel elevation along 30 m of the channel upstream of the 

fault. The middle 17 m of this reach showed an average of 

28 cm of aggradation. We infer that the aggradation occurred 

during the March 1991 storms that produced the only significant postearthquake runoff during a period of less-thanaverage precipitation. Between March 1991 and June 1992, 

a period of less-than-normal runoff, about one-third of the 

total 1990-91 increase in bed elevation persisted. Aggradation, which resulted from the infilling of the streambed alluvium with fine-grained, organic-rich sediment, appears to 

reflect a decrease in the gradient upstream of the fault. Alluviation in the low-gradient reach upstream of the fault is 

one indirect hydrologic response to ongoing tectonic disruption in the fault zone. Results suggest that earthquakes without significant surface rupture may induce changes in stream 

channels that, over geologic time, can produce the offset 

streams characteristic of the San Andreas fault zone. Both 

the evidence of postearthquake aggradation and the anomalously flat longitudinal profiles of the streams where they 

cross the fault indicate that the stream channels are being 

deformed tectonically at a faster rate than their ability to 

readjust by aggradation or erosion. 

INTRODUCTION 

Quaternary slip rates and long-term earthquake frequency 

along segments of the San Andreas fault zone have been 

established in part by studies of Holocene alluvial deposits 

and surfaces (Wallace, 1990); however, little is known 

about the adjustments made by streams in response to lateral faulting. Fault-related alluvial deposits have been attributed to channel aggradation within the fault zone, 

ponding of drainage due to tectonic blockage, or capture 

and diversion. 

Stream channels of all sizes have been disrupted by 

movement along the traces of the San Andreas fault zone, 

including along segments which are inferred to have no 

large, ground-rupturing earthquakes. Sudden changes in 

channel geometry can clearly occur when earthquakes produce significant lateral surface offset of a stream crossing 

the fault trace. Even along traces with events that offset 

stream channels, channel deflection may occur during intervening periods, making it difficult to correctly identify 

earthquake-induced offset (Wesnousky and others, 1991). 

Along fault segments which experience only small or moderate earthquakes, channel adjustment may be an indirect, 

and perhaps long-term, process involving channel shifting, 

ponding and alluviation, or incision. Because these indirect 

adjustments can be accomplished only if significant runoff 

occurs, they may be hydrologically controlled rather than 

true tectonic adjustments. For example, a slight change in 

stream gradient following a moderate earthquake might 

result in channel changes only in cases where effective 

flows can modify the channel. The Loma Prieta earthquake 

provided an opportunity to investigate changes in channel 

Ell 
El2 HYDROLOGIC DISTURBANCES 

geometry following an earthquake that produced no major 

surface rupture. The results reported here document changes observed through June 1992, and we plan to continue 

measurements at the study site over the next several years. 

The area of study is in the Santa Cruz Mountains at the 

southern end of the rupture zone of the Lorna Prieta earthquake (U.S. Geological Survey Staff, 1990), approximately 10 km northeast of Watsonville (fig. 1). Several small 

tributaries of the Pajaro River with drainage areas of less 

than 8 km2 flow southwest across the trace of the San Andreas fault zone in the area, dropping from elevations of 

approximately 450 m to approximately 60 m at the Pajaro 

River floodplain (fig. 2). The active trace of the fault zone 

extends along the mountain slope approximately midway 

between the ridgecrest and the floodplain (Sarna-Wojcicki 

and others, 1975). The area is underlain by marine sedimentary rocks, predominantly sandstone and shale, of late 

Tertiary age (Allen, 1946; Brabb, 1989). The weak consolidation of many of the units is a likely contributing factor 

in the development of large complex slump-earthflows 

along the fault zone at the northern end of the study area. 

Streams in the study area show strong geomorphic evidence of disruption across the San Andreas fault zone 

(figs. 3,4). Most stream courses show marked right-lateral 

offset, and the drainage basins themselves are strongly 

skewed to the southeast, reflecting progressive right-lateral 

Figure 1.-Index map, modified from U.S. Geological Survey Staff 

(1990), showing location of study area. Earthquake rupture zone 

defined by occurrence of aftershocks. 

offset during drainage integration. Sag ponds and linear 

alluviated valleys parallel to the fault zone are common. 

Figure 2.-Map showing several streams crossing San Andreas fault in study 

area (from Watsonville East 1:24,000 quadrangle, contour interval 20 ft). 

Highlighted streams are shown in the topographic profiles in figure 4. 
Figure 3.Ã‘Stud area, showing vegetated channels of Mill Canyon and Fern Canyon in center. Note apparent leftlateral offset, caused by stream capture, of Mill Creek across San Andreas fault. Both streams are tributary to Coward 

Creek, which flows from right center to lower center, View to east across the Santa Clara Valley. 

Figure 4.-Longitudinal profiles of small streams crossing San Andreas fault zone in portion of Watsonville East quadrangle, Horizontal distance along each stream is plotted relative to the point where 

each stream crosses the fault zone, Profiles of Mill Canyon, Fern Canyon, and other tributaries to Coward Creek and Mattos Gulch do not intersect the profiles of those streams because of this projection 

technique (profiles modified from U.S. Geological Survey Staff and Cummings, 1972). Vertical exaggeration x3.8. 
~ 

El4 HYDROLOGIC DISTURBANCES 

Longitudinal profiles of these streams show marked flattening where the streams cross the fault. In addition, many 

profiles show evidence of recent capture in the form of 

steep, irregular reaches, such as the downstream reaches of 

Fern Canyon, Mattos Gulch, and Captured Creek (fig 4). 

Natural exposures of deposits within alluviated valleys 

show evidence of episodic deposition and (or) ponding in 

the form of buried organic-rich horizons and alternating 

layers of clay and gravelly alluvium. Organic deposits 

cored by A.M. Sarna-Wojcicki from between 4.4 and 5.1 

m depth in a sag pond in Mattos Gulch in 1973 yielded 

four radiocarbon ages between 3,200 and 3,860 years B.P. 

(Kelley and others, 1978), suggesting a long history of 

drainage disruption in the area. 

During February 1990, we conducted our initial field 

surveys of Fern Canyon (fig. 5), which contains a small 

tributary to Coward Creek (figs. 2, 3), to examine the topography near the fault zone in detail and to detect possible changes in channel geometry after the earthquake. 

Theodolite and electronic distance meter surveys enabled 

construction of topographic maps at 1:500 scale with a 

0.5-m contour interval (fig. 6). Reoccupation of marked 

points during repetitive surveys indicates that horizontal 

coordinates of points are precise to within at least 0.15 m 

and elevations within 0.05 m. Points and profiles were 

plotted using Surfer and Grapher, produced by Golden 

Software, Inc., Both programs can be used on a DOS personal computer. Hand-contoured topographic maps proved 

' 

more representative of the topography than computer-gencrated maps (fig. 6). Longitudinal profiles of the channel 

of Fern Canyon and the floodplain surface were also generated from the survey data (fig. 7) and were used to examine alluviation of the channel. 

FERN CANYON 

Fern Canyon was selected for detailed study because it 

shows clear evidence of recent disruption by the San Andreas fault and because it has a relatively straight channel 

that enters the fault zone at right angles. The fault zone 

near Fern Canyon is marked by classic geomorphic features diagnostic of strike-slip faults, including two linear 

compressional ridges flanked by a linear valley on the 

northeast that contains small sag ponds (figs. 5, 6, 8). The 

juxtaposition of dark clay-rich gravel (east side) against 

sandy alluvium (west side) across the fault trace was revealed in preliminary trenching by D. Schwartz of the 

U.S. Geological Survey. 

The modem channel of Fern Canyon is diverted to the 

north around the compressional ridges. The gradient flattens markedly above the fault (fig. 7), and a steeply incised canyon cuts through the northern pressure ridge and 

flows into Coward Creek. A 0.8-m-high nick point marks 

the apparent fault trace (fig. 7). Riparian vegetation indicates that the modem lower channel, downstream of the 

Figure 5.Ã‘Compressiona ridges and sag pond deposits near Fern Canyon. Fern Canyon channel bends northwest in center of photograph. Denser vegetation upstream from compressional ridges reflects ponding of water 

upstream of fault. View west-northwest. 
- . -.- Fern Canyon channel 

Meters 

Figure 6.-Topographic map of Fern Canyon, hand contoured using surveyed points and field notes, and geologic map of surficial units. Contour interval is 0.5 m, and elevations are 

relative to reference station, assigned elevation 167.64 m. End points and projection line of profiles shown in figure 7 are shown by letters and solid line. Large dot shows location of site 

in figure 1 1. 
El6 HYDROLOGIC DISTURBANCES 

northward bend (fig. 5), has been established for at least a 

few hundred years. Within the canyon, stumps of redwood 

trees at least 250 years old at the time of cutting are surrounded by stump-sprouted trees at least 80 years old. Because redwoods grow only in riparian zones in the study 

area, they establish a minimum age for the lower channel. 

An older course of the Fern Canyon channel is defined 

by the alluviated flat forming the main terrace in the area 

(figs. 6, 8). During formation of the flat, the stream flowed 

in a relatively direct path. We propose that at this time the 

upper and lower reaches of the Fern Canyon channel were 

directly opposite each other and that the current 60-m 

right-lateral offset represents more recent lateral displacement of the channel along the fault. 

Geomorphic evidence also suggests that, prior to assuming 

its current deflected course. Fern Canyon at least temporarily 

flowed through a steep abandoned channel between the compressional ridges (fig. 6), perhaps when the lower channel 

was blocked by the toe of a large landslide on the northern 

edge of lower Fern Canyon. The landslide deposits bury the 

northern end of the northern compressional ridge. At this 

time, we are uncertain as to the timing of the landsliding or 

its effect on the Fern Canyon channel. 

EFFECTS OF THE EARTHQUAKE 

ON FERN CANYON 

After the Loma Prieta earthquake, two zones of semilinear, subparallel cracks with 1-2 cm of southwest-side-up 

relief were observed at the base of the ridges (fig. 8). The 

cracks may be the result of differential settling during 

ground shaking, with the bedrock compressional ridges 

settling less than the alluvial and sag pond deposits. Hall 

and others (1991) noted two sets of fractures with up to 15 

cm of northeast-side-up offset along a ridgecrest approximately 7.5 krn southeast of Fern Canyon. 

Increased spring activity and the appearance of ponded 

water at the head of the abandoned Fern Creek channel 

were noted immediately after the earthquake (fig. 6). It is 

difficult to assess the hydrologic conditions in Fern Canyon because streamflow records are not available for the 

small streams in the study area. Conversations with local 

ranchers indicate that Fern Canyon maintains a perennial 

low discharge (estimated by us to be 1-2 ft3/s) because of 

discharge into the stream by springs. Along the sag pond 

near the fault trace, the water table is elevated, and standing water appears after even minor rainfall. 

B' 

I 

- - Active Channel Floodplain Surface 

I The active channel data Ints have been 

ro Ã§eto onto the flood &I profile ÃˆnÃ 

Serflco~ Ã§xog~ratio IO? 

point 

CUMULATIVE DISTANCE (m) 

Figure 7.-Longitudinal profiles of main alluvial surface and present Fern Canyon, with points projected onto 

common line. Note downstream flattening of the surface reflecting partial damming along the fault. 
STREAM-CHANNEL ADJUSTMENT IN FERN CANYON, CALIFORNIA, AFTER THE EARTHQUAKE El7 

Although no channel surveys were made prior to the 

earthquake, we postulate that the February 1990 survey of 

the channel provides a reasonable approximation of preearthquake conditions because no significant runoff occurred in the channel between November 1989 and March 

1990. The average annual precipitation at Watsonville, the 

climatological station closest to the area, is 533 rnm. No 

significant storms occurred between October 1989 and 

February 1990, and we infer that the channel surveyed in 

February 1990 had not been significantly modified since 

the earthquake. Between the initial survey and the June 

1991 survey, average precipitation at Watsonville was 

much less than normal, with the exception of two storms 

in March 1991 that produced 298 mm of rainfall. Channel 

modifications detected by the 1991 survey reflect modifications during those storms. During the June 1991-June 

1992 interval, rainfall was again somewhat less than average at Watsonville. Three storms in February and March 

1992, produced approximately as much rainfall as the two 

199 1 storms. Channel modifications made during 1990-92 

can therefore be assumed to be less than would have occurred during normal or wetter-than-average rainfall years. 

The June 1991 resurvey of the Fern Canyon channel was 

Figure 8.Ã‘Crack formed during the Lorna Prieta made in order to detect anv changes that " occurred during w 

earthquake along trace of the San Andreas fault zone 

near Fern Canyon. Cracks were accentuated by dig- the runoff events of March 1991, following the first signifgings of ground squirrels. Pencil for scale, view to icant storms to occur since the earthquake. The surveys 

southeast. detected no change in the channel planform within the detectable limits of the surveys (fig. 9). However, the longitudinal profile of the Fern Canyon channel showed significant 

February 1990 

* * * June 1991 

+ + + June 1992 

0 Meters 50 

Figure 9.-Plot showing surveyed points for 1990, 1991, and 1992. Position of Fern Canyon channel shows no appreciable shifting within limits of 

surveying precision. 
HYDROLOGIC DISTURBANCES 

elevation increase for 80 m upstream of the fault (fig. 10). 

The increase averaged 30 cm in the 17-m reach imrnediately upstream of the fault, with a maximum of 70 cm aggradation at some points. Field observations indicated that the 

increase in bed elevation was not caused by deposition of 

discrete flood deposits or sand and gravel bars, but rather 

resulted from the inundation of the streambed and surrounding low flats with fine-grained, organic-rich sediment. 

Between June 1991 and June 1992, approximately twothirds of the 1990-91 increase in bed elevation was removed (fig. 10). Field observations support survey results 

that indicate some aggradation persisted in the low-gradient reach immediately upstream of the fault trace. Here the 

inundation of the streambed with 15 cm or more of dark, 

fine-grained silt and clay between the 1991 and 1992 surveys caused the target pole to consistently sink into the 

bed, thus making the 1991-92 degradation to some extent 

an artifact of surveying problems. It will not be known 

whether any of the streambed elevation will persist long 

enough to provide a permanent record of post-Loma Prieta 

hydrologic adjustments in Fern Canyon until several more 

years of survey data are collected. 

HOLOCENE HISTORY OF FERN CANYON 

Natural streambank exposures reveal up to 2 m of gravelly alluvium underlying the Fern Canyon floodplain upstream of the fault trace. At one site investigated in detail, 

the deposits consist of alternating sandy gravel, with little 

or no silt and clay, and organic-rich clayey and silty gravel 

(fig. 11). Preliminary observations lead us to hypothesize 

that the gravels inundated by fine sediment represent times 

of partial damming of the creek, whereas sandy deposits 

may represent times of unimpeded flow. The sandy alluvial 

layers display cross bedding at some localities and may 

represent individual flood events. The gravelly clay layers, 

which contain little sand (fig. ll), are similar to those exposed on the northeast side of the fault trace in the excavated trench. The upper parts of the clay-rich layers, which are 

dark colored, are also enriched in organic carbon (fig. 1 I), 

suggesting that they may represent buried A horizons. 

The youngest recognized deposits along the upper 

streambanks are historical, as indicated by the presence of 

old bottle glass immediately upstream of the deposits 

shown in figure 1 1. The glass was found in deposits equivalent to the horizons immediately beneath the "disturbed" 

zone at the described site, where the upper 25-35 cm exposed show signs of human disturbance. These deposits 

overlie the first buried organic-rich horizon (depth 60- 

90 cm, fig. 1 l), which we interpret as the presettlement 

floodplain surface. Large samples of charcoal from the 

sandy alluvium between the disturbed horizon and the 

original floodplain surface (fig. 11) yielded two radiocarbon ages of modem and 430Â±9 yr B.P. The spread in the 

ages may partly reflect the presence of riparian redwoods 

along Fern Canyon; complete burning of trees several centimes old could yield radiocarbon ages spanning several 

centuries. The ages suggest that the upper deposits are 

probably no older than several hundred years old. 

Radiocarbon dating of small charcoal samples from sandy 

gravel below the upper buried soil and from a dark clay-rich 

- - February 1990 

- 

June 1991 

June 1992 cÃˆA 

\ Vertical Exaggeration: 20X 

Figure 10.-Longitudinal profiles of the Fern Canyon channel near San Andreas fault. Data were obtained from repeated field surveys. 

158 1 I 1 1 I 1 1 1 I I 1 1 1 1 1 

0 50 100 150 

DOWNSTREAM DISTANCE (m) 
STREAM-CHANNEL ADJUSTMENT IN FERN CANYON, CALIFORNIA, AFTER THE EARTHQUAKE El9 

layer at the base of the exposure yielded ages that are stratigraphically inverted (fig. 11). These results probably reflect 

the remobilization of detrital charcoal from upstream colluvial deposits and redepositioninFern Canyon alluvium. The 

lower sample was also contaminated with unknown sulfide 

material. Despite the lack of age resolution, it is clear that 

both clay-rich layers were deposited since late Holocene 

time. If these layers do indeed represent earthquake-induced 

partial ponding of Fern Canyon, then two late Holocene 

events prior to 1906 could be recorded in the alluvial record. 

DISCUSSION OF RESULTS 

Stream offsets can be accomplished by a variety of processes, including uplift of compressional ridges and largescale landsliding, as well as by simple lateral separation. It 

is clear that Fern Canyon, Mill Creek, and similar small 

streams crossing the San Andreas fault zone have responded to disruption by the fault with a variety of channel adjustments. Whether older alluvial deposits in Fern Canyon 

reflect periodic partial damming of the channel is uncertain at present because deposits are not well dated and correlated. In addition, the effects of variables such as major 

flood events, intense timber harvesting in the early 1900's, 

and currently active grazing have not been evaluated. 

The repeated surveys of the profiles of Fern Canyon and 

its floodplain demonstrate that gradient decreases upstream 

of the fault are the most obvious channel change effected 

by faulting. Alluviation of the Fern Canyon channel following the Loma Prieta earthquake has been concentrated in 

the low-gradient reach immediately upstream of the fault 

trace and has resulted partly from inundation of the streambed by fine-grained, organic-rich sediment. This recent aggradation may represent a hydrologic response to a gradient 

decrease created by the Lorna Prieta earthquake. However, 

several years of survey data must be collected to evaluate 

the pattern of alluviation during significant runoff events 

which do not follow moderate earthquakes. It is also unknown at this time whether sediment still stored in the 

reach upstream of the fault will remain there. Evidence of 

post-earthquake aggradation, combined with anomalously 

flat long profiles of the streams where they cross the San 

Andreas fault, indicates that the stream channels are being 

deformed tectonically at a faster rate than their ability to 

readjust by aggradation or erosion. Continued study of 

Percentage 

Radiocarbon Age Depth Soil Profile 

t 

Organic Carbon 

A Charcoal sample 

cm 

0 

220% modern Â 0.88% 

430 Â 90yr B.P. 

wsu-4252,4250 5 -. 

100 

2850 Â 2 15 yr B.P. 

WSU-4249 

Figure 11 .-Soil horizons and alluvial units exposed along floodplain edge tion, whereas sandy gravel indicates unimpeded streamflow. Organic carbon 

(fig. 6), showing relative positions and radiocarbon ages of charcoal Sam- content was determined by the Walkley-Black method (Singer and Janitzky, 

pies. Presence of buried soils is indicated by bulges in organic carbon. 1986), and radiocarbon ages were determined by Y. Welter, Washington 

Clay-rich gravel may be formed during times of partial ponding and alluvia- State University. 

Hori~on Moist color 0 123 

0 I I I 

C Historic: disturbed 

1090 Â 160 yr B.P. 

WSU-425 1 

200 200 - 

I 

-? 

r 

& 

---- - - 

sandy 

A 2C2 7.5 Y R 312 

- Ã 

-- -- - - - 

4Ab 

------ 

5C 1 ----- - - - 

A 

5C2 

? 4 - Preset tlemen t 

Ã‘ ground surface A 

I 

(.-OÃ 

I 

gravel 

sandy 

clayey 

gravel 

sandy 

sandy 

gravel 

, 7.5 YR 1/2 

5 YR 2.512 

7.5 YR 312 

7 YR I 

(2.5 YR 4/8 

flrBVel " - 

gravel 100 - 
HYDROLOGIC DISTURBANCES 

changes in bed elevation and sediment storage will yield 

valuable understanding of the mechanisms by which laterally offset streams become displaced. 

ACKNOWLEDGMENTS 

This project was supported by the U.S. Geological Survey National Earthquake Hazards Reduction Program and 

by the Department of Geology at San Jose State University. We thank Alex Gallego, Christine Ohlmann, Gregory 

Swanson, and John Wallace for assistance with surveys, 

data reduction, and laboratory work. Dr. Yvonne Welter, 

Washington State University, performed the radiocarbon 

analyses. Kathy Haller provided a constructive and critical 

review which improved the manuscript. Finally, we are 

grateful to Mr. Ed Kelly for access to the study area. 

REFERENCES CITED 

Allen, J.E., 1946, Geology of the San Juan Bautista quadrangle, California: California Division of Mines Bulletin 133, 112 p., map scale 

1 :62,500. 

Brabb, E.E., 1989, Geologic map of Santa Cruz County, California: U.S. 

Geological Survey Miscellaneous Investigations Map 1-1905, scale 

1 :62,500. 

Hall, N.T., Lettis, W.R., dewit, M.W., and Angell, M., 1991, Late Quaternary uplift along the Pajaro River, southern Santa Cruz Mountains, California: Final Technical Report, U.S. Geological Survey, 

National Earthquake Hazards Reduction Program, Contract No. 14- 

08-G1828,39 p. 

Kelley, L., Spiker, E., and Rubin, M., 1978, U.S. Geological Survey, 

Reston, Virginia, radiocarbon dates XIV: Radiocarbon, v. 20, no. 2, 

p. 283-312. 

Sarna-Wojcicki, A.M., Parnpeyan, E.H., and Hall, N.T., 1975, Recently 

active breaks, San Andreas Fault, Santa Cruz Mts. to Gabilan 

Range, central California: U.S. Geological Survey, Miscellaneous 

Field-Studies Map MF-650, scale 1 :24,000. 

Singer, M.J., and Janitzky, I?, 1986, Field and laboratory procedures used 

in a soil chronosequence study: U.S. Geological Survey Bulletin 

1648, p. 24-35. 

U.S. Geological Survey Staff, 1990, The Loma Prieta, California, earthquake: an anticipated event: Science, v. 247, p. 286-293. 

U.S. Geological Survey Staff and Cummings, J.C., 1972, Progress report 

on the USGS Quaternary studies in the San Francisco Bay Area: 

Friends of the Pleistocene Guidebook, p. 87-99. 

Wallace, R.E., ed., 1990, The San Andreas fault system, California: U.S. 

Geological Survey Professional Paper 1515, 283 p. 

Wesnousky, S.G., Prentice, C.S., and Sieh, K.E.,1991, An offset Holocene stream channel and the rate of slip along the northern reach of 

the San Jacinto fault zone, San Bernardino Valley, California: Geological Society of America Bulletin, v. 103, p. 700-709. 
THE LOMA PRIETA, CALIFORNIA, EARTHQUAKE OF OCTOBER 17,1989: 

STRONG GROUND MOTION AND GROUND FAILURE 

HYDROLOGIC DISTURBANCES 

EFFECTS OF THE EARTHQUAKE ON 

SURFACE WATERS IN WADDELL VALLEY 

By Robert 0. Briggs 

CONTENTS 

ABSTRACT 

Waddell Creek is a perennial stream near Santa Cruz, 

Calif., which discharges into the Pacific Ocean about 38 

km from the epicenter of the Lorna Prieta earthquake. The 

earthquake caused no significant changes or damage to 

structures or surface features in the area but was followed 

by transient increases in discharge of Waddell Creek and 

nearby springs. The increases approximate an order-ofmagnitude step rise and were followed by an exponential 

recession with time constants of from 37 to 59 days. The 

flow recession pattern of Waddell Creek was obscured by 

heavy rains after about 50 days; however, the nearby 

springs maintained an exponential recession with minimal 

rain interference for periods of several time constants, then 

ceased flowing abruptly. Spring flow cessations progressed 

systematically from higher to lower altitudes, suggesting a 

descending water table. A comparison of the pre- and 

post-earthquake chemical signature of one of the springs 

shows only minor changes in water chemistry, indicating a 

common source for pre- and post-earthquake waters. Increased permeability of the subsurface material due to the 

earthquake appears to be the most probable cause of the 

hydrologic changes. Postearthquake restoration to preearthquake permeability has not been observed or inferred. 

INTRODUCTION 

Waddell Creek, a perennial central California coastal 

stream, discharges into the Pacific Ocean about 38 km 

from the epicenter of the Loma Prieta earthquake. Temporary increases in surface water flow throughout its 48-km2 

watershed followed the seismic disturbance. 

Figure 1 shows the location of the Waddell Creek valley 

with respect to Loma Prieta and other geographic features. 

Figure 2 maps the Waddell Creek watershed and identifies 

specific points of interest. To locate the points more precisely, 

their Metric Universal Transverse Mercator grid coordinates 

from the U.S. Geological Survey maps (U.S. Geological 

Survey, 1955) are cited in the text. 

Hydrologic changes similar to those reported herein were 

observed elsewhere in the coastal area impacted by the Loma 

Prieta earthquake (Rojstaczer and Wolf, 1992), and a variety 

of hydrologic anomalies were reported following the 1906 

San Francisco earthquake (Lawson, 1908). The 1906 reports 

include increased discharge of springs and streams similar to 

those reported here; however, other phenomena attendant to 

the 1906 event, such as the emergence of geysers and hot 

springs and the appearance of turbidity, salt, and elevated 

temperatures in spring and well water, were not observed in 

the Waddell valley relative to the Lorna Prieta earthquake. 

PHYSICAL EFFECTS OF THE 

EARTHQUAKE IN WADDELL VALLEY 

Although residents of the Waddell valley reported vigorous ground motion during the earthquake, very little residual physical change resulted. Except for failure of two 
t7m HYDROLOGIC DISTURBANCES 

aged unreinforced brick chimneys, there was no earthquake-related damage to the six residential structures or to 

the several miles of gravel and dirt roads, two bridges, and 

several miles of underground water pipes. A landslide of 

moderate scale occurred in a historically slide-prone area 

outside the valley on the coastal slope of the western 

ridge, (fig. 2) but no slides or ground cracks were evident 

in the valley. However, immediately after the earthquake 

the air in the valley was filled with dust, which, in the 

absence of major physical changes, suggests numerous 

small slides. 

The physical effects of the earthquake in the Waddell 

valley are consistent with a modified Mercalli intensity of 

7, which agrees with U.S. Geological Survey contours in 

figure 1 (Plafker and Galloway, 1989). 

HYDROLOGIC EFFECTS OF THE 

EARTHQUAKE IN WADDELL VALLEY 

sion. Numerous new springs appeared, and springs which 

had been inactive for many years resumed flow. Discharge 

measurements were made of the creek for which considerable historical flow data are available and two springs for 

which some preearthquake information is on record. 

HYDROLOGIC CHARACTER OF 

WADDELL CREEK 

A study conducted by the California State Department of 

Fish and Game between 1933 and 1942 provides continuous 

records of Waddell Creek discharge for that period (Shapovalov and Taft, 1954). Since 1942 flow has not been continuously monitored. However, since 1975 spot data have been 

recorded frequently by the author, the California State Department of Fish andGame, the County of Santa Cruz Planning 

Department and the Department of Geology, University of 

California, Santa ~ruz.' These data were the basis of a hyThe most evident aftereffect of the earthquake in the 

Waddell valley was an abrupt increase in the discharge of 

creeks and springs followed by an exponential flow reces- 

'~urin~ the summer of 1977 flow measurements of Waddell Creek 

were made by the author using a 9-inch Parshall flume. All other creek 

data were measured by current meter surveys in open stream. 

I 

3 4 5 miles 

Figure 1.-Map of central California coastal area showing location of Waddell valley, Loma Prieta, and other 

points of reference and displaying U.S. Geological Survey modified Medli intensity contours (Plafker and 

Galloway, 1989). 
EFFECTS OF THE EARTHQUAKE ON SURFACE WATERS IN WADDELL VALLEY E23 

drologic study sponsored by the California State Coastal Conservancy wherein monthly flow duration curves for Waddell 

Creek were derived (Robert Coats, written commun., 1988). 

Shapovalov and Taft (1954) report greater summer discharge during the 1930's than is observed currently for similar meteorologic patterns. The change cannot be attributed 

to increased water usage from the creek or aquifer, because 

agricultural diversion above the points of measurement has 

never been practiced and there is very little domestic usage 

in the watershed. The reduction in summertime flow may be 

due to reforestation of the watershed, which was heavily 

impacted. by timbering and fires prior to the 1930's. In recent 

years the watershed has been protected, resulting in prolific 

regrowth of redwood and douglas fir. This result of reforestation on watershed yield has been reported for areas with 

similar histories (Dunne and Leopold, 1978). Coats' monthly 

flow duration curves are a more accurate reference for present 

creek behavior than the Shapovalov and Taft (1954) records. 

Waddell Creek's typical summer flow decreases from the 

beginning of the dry season (early summer) until the first 

major rain the following fall. Figure 3 presents the measured 

flow from March 27, 1989, to March 1, 1990, together with 

I - 

Pacific 

Ocean 

Figure 2.-Map of Waddell Creek drainage basin showing locations mentioned in text: a, Waddell Creek flow measurement station, b, Brown House Spring, c, Casa Spring, d. Entry Road Spring, e, site of earthquake-induced slide. 
E24 HYDROLOGIC DISTURBANCES 

the 50th percentile values from the monthly flow duration 

curves. The discharge during the summer of 1989 closely 

approximates the 50th percentile until the October 17 earthquake, when discharge increased dramatically. 

EARTHQUAKE-RELATED INCREASE IN FLOW 

OF WADDELL CREEK 

Discharge near the mouth of Waddell Creek (4,106,730 

meters N., 564,900 meters E.) showed an eightfold postseismic increase and a further rise to 12.5 times the preearthquake discharge, followed by a gradual recession 

which was obscured by rain runoff beginning in mid-January (fig. 4). Before it was obscured, the recession approximated a straight line in semilog space and is empirically 

described by the following exponential equation, with the 

time constant (the time required to reach 1/e of initial value) equal to 59 days: 

where 

t2 = time 2 

Q, = discharge at time 1 

Q2 = discharge at time 2 

T = time constant 

e = base of natural log. 

The last flow measurement prior to the October 17 

earthquake was 43.6 Us on October 12. Flow was 359 Us 

two days after the earthquake on October 19 and peaked at 

671 Us on October 24. No measurements were made between October 12 and 19 or between 19 and 24, so the 

character of the rise in discharge following the earthquake 

is not known. However, continuous records in several 

nearby streams (Rojstaczer and Wolf, 1992) show increasing discharge for several days following the seismic event 

and it is probable that Waddell Creek behaved similarly. 

Figure 3.-Discharge of Waddell Creek from March 27, 1989 to March 1, 1990, showing the discontinuity caused by the October 

17, 1989, Lorna Prieta earthquake together with the 50th percentile probable discharge of Waddell Creek as determined from 

monthly flow duration curves by Coats (written commun., 1988). 
EFFECTS OF THE EARTHQUAKE ON SURFACE WATERS IN WADDELL VALLEY E25 

The season's second measurable rain (October 21 

through 24, peaking on October 23) totaled 3.9 cm and 

accounts for the sharp spike early in the postseismic discharge2 (fig. 4). Rains following a dry season normally 

have only a short-term effect on Waddell Creek discharge 

and flow returns quickly to near its pre-rain ba~eline.~ 

Note the rapid recession of the early-season 2.5 cm of precipitation on September 17 (fig. 3). The October 23 precipitation appears to have a similarly brief impact on creek 

discharge, not significantly distorting the more slowly receding earthquake-induced transient. 

2~ainfall is measured near the location of the creek gaging station. 

Greater precipitation is frequently observed in the higher elevations of 

the watershed, but records at these locations are unavailable. 

3~he Waddell Creek runoff pattern of an early season isolated rain was 

carefully recorded by the author following a 4.5-cm rainfall on October 

25-26, 1991. It approximated an exponential decay with a time constant 

of 3.8 days. 

1 OOC 

Although it has not been rigorously determined, Waddell Creek appears to have resumed its typical annual 

cycle of discharge for the water year 1990-91. For that 

year the 63 cm of rainfall was 80 percent of the 100-year 

average (78.7 cm), with the majority of precipitation occurring toward the end of the season. Monthly flow duration curves place the 1990-91 summer discharge at the 

55th percentile. 

INCREASED YIELD FROM 

BROWN HOUSE SPRING 

Brown House Spring is located about 2.2 km upstream 

from the mouth of Waddell Creek (4,107,500 meters N., 

565,280 meters E.) at about 60 meters above sea level. 

The spring, which has reliably supplied domestic water 

to several residences since about 1940, has been observed frequently by the author since 1970 and gaged on 

19 Oct. -. First postearthquake measurement *.... *-*.. *..., l..- .... .I.. 

12 Oct. - Last preearthquake measurement 

Figure 4.-Discharge of Waddell Creek prior to and after the earthquake showing steep rise followed by gradual recession, until obscured by rain 

runoff. Dotted line is an exponential function with a time constant of 59 days, which empirically simulates the postearthquake creek flow recession 

pattern. A bar chart displays daily precipitation. 
HYDROLOGIC DISTURBA~JCES 

infrequent, random occasions since 1977.~ Its lowest recorded discharge was 0.126 L/s on October 10, 1977, 

following a severe drought and the greatest was 0.788 

Us on August 31, 1983, following a rain year with 152 

cm of precipitation (about twice the 100 year average). 

Before the earthquake, discharge of this spring was within its normal range: 0.252 Lls on January 29 and 0.189 

Us September 21. The earthquake-induced behavior of 

Brown House Spring is graphed in figure 5. 

On October 20, 1989, three days after the earthquake, 

the first postearthquake measurement was 2.37 Us, a 

12.5-fold increase from preearthquake discharge and three 

times the highest value previously recorded. Discharge 

then receded exponentially with a time constant of 37 

days. The recession pattern, which was unaffected by the 

January and February rains, continued its exponential 

decay for 152 days. The discharge then deviated sharply 

'spring discharges were determined by clocking the fill time of a calibrated vessel. 

downward, ceased within a few days, and had not resumed as of March 1992. 

CHEMICAL SIGNATURE OF 

BROWN HOUSE SPRING 

A comparison of the pre- and post-earthquake chemistry 

of Brown House Spring (which is the only source under 

observation for which a preearthquake chemical analysis is 

available) is presented in table 1. Analysis of samples 

taken July 23, 1981 (Kent, 1981), and November 9, 1989 

(Galloway, 1989), are sufficiently similar to indicate that 

the waters are from the same source. 

The only significant difference between the two samples 

is the reported postearthquake increase in sulfate concentration. However, the preearthquake sulfate concentration 

of 7.6 mg/L may be an error. A value of 76 mg/L yields a 

better electrical balance and is more consistent with typical values of sulfate in water sources of the area. The error 

cannot be confirmed since original laboratory notes and 

records no longer exist; however, it would not affect conclusions drawn from the data. 

1 0- *... - 

20 Oct - First postearthquake measurement 

- 

- 

1- 

- 

- 

- 

- 

- 

- 

Estimated flow 

Last measurement - 21 Sept. - 

- 

- 

- 

Figure 5.-Discharge of Brown House Spring prior to and after the earthquake, showing a steep rise followed by a gradual recession. 

Dotted line is an exponential function with a time constant of 37 days, closely simulating the flow recession, 
Table 1 .-Pre-and post-earthquake chemical analysis of Brown House Spring 

[Chemical concentrations expressed in parts per million] 

July 23, 1981 Nov. 9, 1989 

pH value (units) 

Conductivity (micromhos/cm) 

Carbonate alkalinity (as CaCOi) 

Bicarbonate alkalinity (as CaCO,) 

Total alkalinity (as CaCO,) 

Total hardness (as CaC0,) 

Total dissolved solids (as NO,) 

Nitrate 

Chloride (Cl) 

Sulfate (SO4) 

~luoride (F) 

Calcium (Ca) 

Magnesium (Mg) 

Potassium (K) 

Sodium (Na) 

Total iron (Fe) 

Manganese (Mn) 

RESUMPTION OF FLOW IN 

PREVIOUSLY DORMANT SPRING 

Several springs that had been inactive for many years 

resumed discharge after the earthquake and then returned to 

dormancy after several months of activity. One such source is 

Casa Spring, about 1.6 krn from the mouth of Waddell Creek 

at about 37 meters above sea level (4,106,650 meters N., 

565,040 meters E.). According to the recollection of residents 

and owners of the property, it was used as a domestic water 

source from 1914 until about 1925, when it was abandoned 

because of reduced productivity. Its flow ceased completely 

in about 1970, and (based on frequent observations by the 

author) it remained dormant until it was reactivated by the 

earthquake. Its postearthquake record is presented in figure 6. 

On October 20, 1989, flow was observed from Casa 

Spring, with gas bubbles rising through the water. A gas 

sample taken on December 6, 1989, consisted of 91 percent nitrogen, 2 percent oxygen, and 4 percent carbon dioxide, with traces of methane and other gases. The gas 

chemistry suggests that the ground water issuing from the 

spring contained air which was depleted of oxygen along 

its flow path. The gas discharge rate was not measured but 

appeared to diminish as the water discharge receded. 

Measurements of water discharge were begun on October 

28, 1989, and at that time a flow of 1.58 L/s was measured. The discharge recession approximates an exponential 

function with a time constant of 49 days and responds to 

the mid-January precipitation. Measurements were discontinued in mid-April and discharge ceased in early May of 

1990. As of March 1992 the spring remains dormant. 

APPEARANCE OF NEW SPRINGS 

Numerous new springs appeared after the earthquake. 

One such spring, designated Entry Road Spring, was first 

seen by the author within 30 minutes after the earthquake. 

It emerged in the middle of a gravel road near the mouth 

of Waddell Creek at an altitude of 11 meters above sea 

level (4,105,825 meters N., 564,760 meters E.). Row rate 

was not measured, but it was estimated by the author (visually comparing the discharge with other sources whose 

flow is known) at 1.5-2 L/s. This spring expanded over the 

next several days to form a quicksand-like material, making the road unusable. The water was diverted on October 

22, 1989, by installation of a drain which continued a substantial discharge for approximately 365 days. 

Similar new, temporary sources were observed at many 

other locations throughout the watershed, but none were 

quantitatively recorded and all ceased flowing within the 

first postearthquake year. 
HYDROLOGIC DISTURBANCES 

POSTEARTHQUAKE WATER-TABLE DROP 

The mathematical model describing the postearthquake 

flow recession of springs and creek also predicts an exponential descent of the water-table height concurrent with 

the flow recession. Anecdotal reports indicate that the 

water level of several wells on the east ridge of the Waddell watershed fell after the earthquake. A number of wells 

were deepened, abandoned, or replaced by deeper wells. 

However, quantitative data defining these water level 

changes are unavailable. 

At lower altitudes in the watershed, a systematic watertable drop is suggested in the sequence of spring flow terminations (fig. 7). The termination times progress from the 

highest altitude source to the lowest in a pattern consistent 

with an exponentially descending water table. However, 

the significance of this observation is uncertain since it is 

supported by only three data points from sources separated 

horizontally by nearly 2 km. 

PROBABLE CAUSE OF 

HYDROLOGIC ANOMALIES 

The increase in Waddell Creek discharge coincident 

with the earthquake cannot be attributed to release of surface stored water, since there are no significant manmade 

or natural ponds or impoundments on the creek or in the 

watershed. 

The similarity of the postearthquake and pqeearthquake 

chemical signature of Brown House Spring (indicating that 

the waters are from the same source), together with the postearthquake water-table drop, suggests that the step increase 

in surface discharge associated with the earthquake resulted 

from increased permeability of the subsurface material, allowing more rapid release of ground water from storage. 

A simple model consisting of a finite reservoir discharging through a restricted aperture yields an exponential 

flow recession versus time, which conforms with the flow 

recession patterns of the springs and creek. This model 

'4 28 Oct. - First measurement 

Figure 6.-Discharge of Casa Spring (dormant since 1970) after the earthquake. Dotted line is an exponential function with a time constant of 

49 days, which closely simulates the recession until the mid-January rain introduces a perturbation. 
Figure 7.-Times of flow cessation of three springs, showing systematic sequence from higher to lower altitudes suggesting a descending 

water table. 

also requires an exponential drop of the water table, which ACKNOWLEDGMENT 

is evident in the sequence of spring flow terminations. 

The 1990-91 creek discharge is apparently near normal. 

With the water-table relief, this supply could maintain the 

preearthquake volume of creek discharge only if the 

ground permeability remains relatively high. The pattern 

of continued dormancy of intermediate altitude springs together with normal flow of Waddell Creek suggests that 

restoration to pre-event permeability has not occurred. 

CONCLUSION 

When or whether the permeability will return to preearthquake values is not apparent. However, it is of practical as well as scientific interest to understand the full cycle 

of the earthquake-induced hydrologic changes. 

Perhaps an insight to changes attendant to previous seismic events could be found in natural historic records. For^ 

example, indications of the pattern of water-table perturbations coincident with historical or prehistorical earthquakes may exist in such records as tree ring patterns. 

The author is grateful to Dr. Luna B. Leopold for his 

encouragement and counseling on this and other studies of 

Waddell Creek hydrology; to Anne W. Briggs and Dr. 

David G. Willis, who have contributed to the observations 

and interpretations presented in this paper; and to Evelyn 

Roeloffs and William C. Evans for the gas sampling and 

analysis. 

REFERENCES 

Dunne, Thomas, and Leopold, L.B., 1978, Water in environmental planning: New York, W.H. Freeman and Company, 81 8 p. 

Galloway, Ken, 1989, Certified Analytical Report No. 83388-1-1 12: Soil 

Control Lab., 42 Hangar Way, Watsonville, Calif., unpublished report, 1 p. 

Kent, J.S., 1981, Certified Analytical Report No. 3WH9203: Soil Control Lab., 1234 Highway 1, Watsonville, Calif., unpublished report, 

1 P. 

Lawson, A.C., chairman, 1908, The California earthquake of April 18, 

1906, Report of the State Earthquake Investigations Commission: 

Carnegie Institution of Washington, Publication 87.45 1 p. 

THE LOMA PRIETA, CALIFORNIA, EARTHQUAKE OF OCTOBER 17,1989: 

STRONG GROUND MOTION AND GROUND FAILURE 

HYDROLOGIC DISTURBANCES 

SOURCES AND MAGNITUDES OF INCREASED STREAMFLOW 

IN THE SANTA CRUZ MOUNTAINS FOR THE 1990 WATER YEAR 

AFTER THE EARTHQUAKE 

By Robert R. Curry, Brett A. Emery and Tom Gentry Kidwell, 

University of California, Santa Cruz 

CONTENTS 

Drought .................................................... Flow increases ---------- - .................................. Methods of investigation ---- .............................. - ---- 

Individual cooperator program --------- ------------------ Agency cooperator program----------------------------- 

Estimation of augmented streamflow ------------------ Chemical database ------ ----- - ................................. -- Results of chemical analyses -------------- .................... Discussion ....................................................... 

Hypotheses of origin of augmented flows------------- 

Mechanisms for increased streamflow ------------------ Acknowledgments -----me---- ------------ ..................... References cited -------------- ---- ............................. 

Supplemental informationÃ‘database compiled ----------- 

However, there were no significant differences in cationcation and cation-anion ratios that could be attributed to 

the earthquake directly. Thus the water chemistry data support an interpretation of shallow ground-water origin for 

the augmented postearthquake flows based upon the observation of no significant changes in chemical signatures between pre- and post-earthquake water in shallow aquifers 

and springs and in streams that drain them. Water quality 

chemical data for over 1,500 ground water and over 1,500 

surface-water samples were compiled and reviewed to assess trends and background variation. The patterns of 

changes in flow, ground-water levels, and water chemistry 

all support a hypothesis of simple strain-induced increased 

permeability of near-surface (above sea level) bedrock 

units as the source of increased surface-water flows. However, the intermediate and far-field hydrologic changes do 

not preclude a hypothesis of coseismic strain-release forced 

dewatering of shallow water-bearing rocks. The existing 

San Francisco peninsula stream gauging network and 

record appear adequate to record shallow strain-induced 

changes in rock permeability. 

INTRODUCTION 

ABSTRACT 

PURPOSE AND SCOPE 

The Loma Prieta earthquake caused the release of about 

87 hm3 of stored, shallow ground water from the Santa 

Cruz Mountains on the San Francisco peninsula in the 

1990 water year. This release of ground water was manifest primarily by increased base flow of surface streams. 

About 33 percent of the augmented flows were estimated 

from gauge records of the U.S. Geological Survey, and the 

remainder were estimated in proportion to their ungauged 

watershed areas. 

Chemical analyses of about 100 surface-water and 

ground-water samples demonstrated modest increases in 

cation concentration in some but not all sampled waters. 

Significant streamflow increases were observed throughout the San Francisco peninsula immediately after the 

Lorna Prieta main shock. This study was conceived to explore the geochemistry of surface and ground waters to try 

to determine the origins of the increased flows. Through 

analysis of sequential changes in chemistry of base flow 

and ground waters at fixed stations, it was postulated that. 

the part of the chemical signature of the waters that decayed in proportion to postearthquake base flow decreases 

would indicate origins of that component of the flow. 

Sources of increased base flow can also be explored 
E32 HYDROLOGIC DISTURBANCES 

through analysis of the timing of this peak and other hydrograph analyses. 

The earthquake occurred during a time of prolonged regional drought. For the full water year after the earthquake, 

flows increased from 1 to 24 percent of mean annual longterm runoff, with up to 70 percent increase in total flow for 

some gauging stations estimated for the drought year itself 

(see table 1). Questions about the expected duration of 

those increases and impacts of those increases upon future 

surface-water flow volumes were raised. If pore or void 

volumes have decreased to provide the increased yields 

from bedrock or upper watershed surface materials, that 

decreased storage capacity could lead to decreased future 

base flow for surface-water supply sources. If void volumes have increased or become more permeable, other future water supply effects may be anticipated. 

The basic questions addressed in this report are as follows: What are the origins of the increased spring and 

streamflows associated with the earthquake, and how do 

these origins affect prognoses for surface-water base flow 

and ground-water availabilities after this and potential future seismic events? Secondary questions include those associated with perceived and real changes in water quality, 

times, and volumes of recharge required to replenish 

ground-water resources, mechanisms of changes in permeability and porosity of water bearing rocks, and long-term 

recovery times for restoration of preearthquake hydrologic 

conditions. 

PREVIOUS WORK 

Postevent increases in surface-water flows have been 

long recognized at places such as the 1952 M 7.1 ArvinTehachapi earthquake (Briggs and Troxel, 1955), the Alaska Good Friday earthquake (M 8.6, 1964; Waller, 1966), 

and Borah Peak, Idaho (M 7.0, 1983; Whitehead and others, 1985). Rojstaczer and Wolf (199 1) summarize the proposed mechanisms for inducing such changes as reported 

in the literature. Postevent changes in ground-water levels 

and conditions have also been long reported (Lawson, 

1908; LaRocque, 1941). Prior investigations support a 

very wide range of near-surface and deeper causal mechanisms for observed changes (Wood and others, 1985; 

Wood, 1985; Swenson, 1964; Sibson, 1981; Nur, 1974; 

Bell and Katzer, 1987). Multiple hypotheses for the origins of coseisrnic and postevent hydrologic change is fully 

supportable since such changes are observed in a wide variety of geologic conditions with both confined and unconfined aquifers and with both shallow and deep epicentral 

foci. Muir-Wood and King (1991a, 1991 b) have explored 

the general subject of earthquake related changes in hydrology in crustal rocks. 

Rojstaczer and Wolf (this volume) have released preliminary analyses of the hydrologic changes associated with 

the earthquake, with specific focus on the San Lorenzo 

and Pescadero Creek drainage basins. Their work demonstrates similar but lesser magnitude responses associated 

with the Lake Elsman earthquake of August 8, 1989 (M 

5.2), which has been considered a foreshock to the Lorna 

Prieta M 7.1 event (Lisowski and others, 1990). Rojstaczer 

and Wolf conclude that the origin of the increased surface 

flows is primarily from shallow ground waters released 

through a "wi e scale increase in near-surface permeabili- ^ ty" (p. 16). This is in accord with our findings. 

Prior investigations of coseismic and postevent changes in 

geochemistry of shallow and surface waters have largely 

focused on use of such observations as precursory tools 

(King, 198 1; Wesson, 198 1 ; Wang, 1985; Varshal and others, 

1985). The general subject of chemical change associated 

with earthquakes has led to the field of seismogeochernical 

research in China (Li and others, 1985). These investigations 

have been conducted extensively in Japan (Wakita, 1981) 

and China (Jiang and Li, 1981; Li and others, 1985). Chemical changes, other than those simply associated with outgassing of volatile rock components (Varshal and others, 

1985), have been investigated on the San Andreas fault, 

where a chloride brine is found below -300 m and waters 

saturated in calcite may originate in the depth range of 50 

to 280 m (Stierrnan and Williams, 1985). Finkel (198 1) 

reports marked changes in uranium concentrations and activity along the San Andreas 70 km from the October 1979 

M 6.6 Imperial Valley earthquake. 

HISTORICAL INFORMATION 

There are well-founded reports of hydrologic changes similar to those observed in 1989 that occurred in the Santa Cruz 

Mountains after the October 8, 1865, Mk6.5 earthquake on 

the Santa Cruz segment of the San Andreas fault as well as 

the major April 18, 1906, San Andreas fault event. These 

data have been summarized in an unpublished report (Jeff 

Marshall, University of California, Santa Cruz, Earth Sciences Board, 1990). Local newspaper accounts report (Santa 

Cruz Sentinel, 14 Oct. 1865, p. 3): 

All the mountain streams, immediately after the earthquake, commenced rising. The Pajaro has half more water; 

Coralitas, twice as much, and the Soquel near the same. 

The flour mill of Hames and Daubenbiss is now running 

night and day; before last Sunday the water only allowed 

eight hours' work. The San Lorenzo is considerably higher, 

also the streams north of this, up the coast. 

Three months after that 1865 event, a Santa Cruz Sentinel commentator (Dec. 9, 1865, p. 1) notes: 

But we intended, at the outset, to give some facts as connected with the great rise of mountain streams noted after 

the earthquake of October 9th: These streams have re- 
SOURCES AND MAGNITUDES OF INCREASED STREAMFLOW, SANTA CRUZ MOUNTAINS, 1990 WATER YEAR E33 

mained full of running water, ever since, up to the time of 

the recent rains, without diminishing in volume. 

SETTING 

The 1906 MÂ± event again caused significant hydrologic 

change locally, and these changes were documented by 

Lawson (1908), as well as by newspaper reports: "At a 

sawmill near Boulder Creek, water stops running from a 

hitherto permanent spring, but another in the neighborhood was flowing more freely than before" (Lawson, 

1908, p. 408). Of the Wrights area, near the Summit, Lawson also states: "Most of the springs are running with a 

greater flow since the earthquake; but the water in [a] well 

on top of the ridge sank rapidly to the level it usually 

holds in August***. The well at the summit, from which 

the Summit Hotel obtains it water, has its bottom on solid 

rock. After the shock the level of water in the well rose 

12 feet." 

In 1906, at the Pajaro River, the southern limit of clear 

hydrologic effects of the 1989 earthquake, it was noted 

that: "The flow of the Pajaro River suddenly increased 

with the earthquake. Some running wells went dry, whereas some old dry wells sprang to life" (Watsonville Register-Pajaronian, July 2, 1952, p. 57). Although lack of 

mention in newspaper articles may not indicate lesser effects, changes in streamflow associated with the 1906 

earthquake did not receive the attention that they did in 

1865. More attention was noted for ground-water changes 

than for surface water in 1906. This may be due to the fact 

that earthquake occurred during a period of above normal 

rainfall, in the spring, during the rain season. Lawson 

(1908, p. 399) notes: "At Boulder Creek, in Santa Cruz 

County, 55.70 inches of rain fell during January, February, 

and March alone, and 16 inches fell during the four 

months preceding." It is thus reasonable to assume that 

immediate changes in base flow were masked in April of 

1906. The rainfall of the 1906 water year occurred after a 

period of modest drought (fig. 1). Although Lawson's 

statements suggest significant departures, in fact the 1905 

water year (July 1, 1904-June 30, 1905 for precipitation 

data at the time) for Santa Cmz was 91.14 cm, and for 

1906 was 81.94 cm, with a long-term (1 15-year) average 

of 70.90 cm. With hindsight, we see that the 1906 regional 

precipitation, as indexed by the Santa Cruz station, was 

about 116 percent of normal. 

Careful review of newspapers and other historical sources 

did not provide any clues to the recovery times for displaced 

ground water, dewatered wells, or surplus streamflow. We 

know, from the 1865 semiquantitative notes on increased 

streamflow, that conditions then were approximately comparable to those in 1989 (see Rojstaczer, this volume). Since 

accounts of 1866 and 1867 no longer noted the "permanent" 

changes associated with the 1865 event, it is possible that 

subsequent precipitation and streamflow decay periods comparable to those of 1989 (50-400 days) restored preearthquake hydrologic conditions within 1 to 2 water years. 

The runoff hydrology of the Santa Cmz Mountains has 

been summarized by Rantz (1974), and by Blodgett and 

Poeschel (1984). About 85 percent of the annual precipitation occurs from November through March, with mean annual precipitation from 35 cm at San Jose (on the lee side 

of the highest peaks) to over 147 cm at Boulder Creek in 

the San Lorenzo basin on the southwest side of the range. 

Ground-water hydrology for parts of the range has been 

summarized by Muir and Johnson (1 979), Johnson (1 980), 

qnd by Akers and Jackson (1977). Clark's (198 1) mapping 

of the central range has been supplemented by a newly 

compiled synthesis of bedrock geology for Santa Cruz 

County (Brabb, 1989). Drought conditions and the effects 

of the earthquake on water supplies in Santa Cruz County 

are summarized by Goddard and Laclergue (1991). 

DROUGHT 

The October 17, 1989, earthquake occurred near the beginning of the 1990 water year and before any significant 

precipitation in that year. The 1989 water year was the 

third of a series of drought years with significant cumulative departure from long-term normal precipitation. Using 

the 1879-1991 long-term Santa Cmz city precipitation 

record, for which a clean complete record exists from July 

1, 1878, to the present without urban influence or inversion effect changes through time, the precipitation-year 

(July 1 of prior year through June 30 of named-year) data 

show that the earthquake occurred at the point of the second-most-severe drought of record (fig. 1). 

The peak drought for 113 years of record culminated in 

1931, with a 4-year cumulative departure from long-term 

normal of -25.18 cm. The 1990 precipitation water year, 

during which the earthquake occurred, had a cumulative 

departure for that and the preceding three years of -23.35 

cm. Unlike the drought of the 1920's and 1930's, the drought 

during the earthquake was preceded by a period of significant 

excess precipitation, with the second largest positive 4-year 

cumulative departure of the period of record. That precipitation excess persisted until 1985-86 (100.43 cmlyr at Santa 

Cruz; cumulative departure +20.5 cm). The years surrounding the earthquake (1987- 199 1) all had significantly below 

normal regional precipitation as follows: 1986-87 (44.7 cm, 

87 percent of mean for period of record, -0.55 cm cumulative 

departure); 1987-88 (47.04 cm, 65 percent, -5.38 cm); 1988- 

89 (59.1 cm, 82 percent, -9.35 cm); 1989-90 (44.42 cm, 62 

percent, -23.35 cm); 1990-9 1 (5 1.13 cm, 7 1 percent, -2 1.74 

cm). Rojsticzer (oral commun., 1992) finds some headwater 

precipitation stations with slightly higher precipitation in the 

1990 water year than in 1989. For the San Lorenzo watershed 

(six stations), and for those reviewed for regional trends 
City of Boulder Creek 

1889 - 1991 

Cumulative departure of 4-ye% running 

350 

300 

250 

s 

u 

z 

H 200 

z0 ' 150 < ^ 

H 

CL 

H 

u 100 

K 

On 

50 

0 

-50 

means 

- 

- 

- 

- 

- 

- 

- 

- 

- 

for 

Figure 1 .-Sank3 Cruz regional area long-term precipitation and cumulative departures from 4-year running means. Annual data are presented for city of Santa Cruz (bar chart) and 

city of Boulder Creek (lines) in the upper San Lorenzo Creek watershed. The two data sets are parallel, although higher values of mountain station (Boulder Creek) best reflect 

runoff. Cumulative departure data (filled bars) for longer continuous Santa Cruz record best reflect three years of antecedent precipitation that control water table elevations and 

ground-water-fed base flow. Last year of record is July 1, 1990, to June 30, 1991. Year of the earthquake is that designated 1990. Santa Cruz data are for July 1, 1878, through June 

30, 1991, for precipitation water years (July 1 of the previous year through June 30 of the named year). 

Santa 
SOURCES AND MAGNITUDES OF INCREASED STREAMFLOW, SANTA CRUZ MOUNTAINS, 1990 WATER YEAR E35 

Table 1.-Summary of selected Santa Cruz Mountain stream records from October 1, 1989, to September 30, 1990 

[Column A (mean annual precipitation) is based on isoheyet-weighted basinwide average; column B (distance to epicenter) is measured to the area-weighted watershed basin center, 

columns J and K (mean annual runoff) are based on Rantz (1974); all other values are based on hydrograph disaggregations detailed in this paper (see fig. 2, Soquel Creek, for 

example). NA, not applicable] 

Station 

Name 

Mean Dist- Water- Post Total Total Inaease Inaease InaeasedMean Mean Inaeasedlncreased Increase 

annual ance to shed earth- quantity quantity in total in yield annual annual yield yield in mean 

predp epi- area quake minus with quantity baseflowperunit runoff runoff annual 

itation center change in storms storms area runoff 

Baseflow 

(cm) (km) (km2) (acre- (acre-ft) (acre-ft) (percent) (percent) (an) (an) (acre-ft) (m3/km2) (hm3/yr) (percent) 

i ft/yr) 

Pajaro R near Chittenden NA 

Corralitos Ck at Freedom 96.5 

Soquel Ck nr Soquel 101.6 

San Lorenzo R at Big Trees 121.9 

San Lorenzo R nr Boulder Ck2 140 

Boulder Ck nr Boulder Ck2 147 

Bear Ck at Boulder Ck2 137 

Zayante Ck nr Zayante 114.3 

Bean Ck nr Scotts Valley 99.1 

Pescadero Ck nr Pescadero 104.1 

San Gregorio Ck near 81 49.5 131.80 2,768 3,206 5,219 53 86 2.59 25.55 27,310 25,901 3.41 10.14 

San Gregorio2 

Pilardtos Ck at Half 94.0 67 70.40 405 637 1,725 23 64 0.71 18.65 10,650 7,093 0.50 3.80 

Moon Bay 

Redwood Ck at Redwood City 61.0 56.5 4.71 24 40 205 12 60 0.62 21.44 819 6,196 0.03 2.89 

SaratogaCknrSaratoga 114.3 29 23.88 614 829 1,345 46 74 3.17 37.80 7,320 31,726 0.76 8.39 

Assumes 1,080 acre-ft/yr of base flow between October 17,1989 and July 19,1990 is all attributable 

to earthquake; in fact, it was not possible to differentiate multiple sources of 

base flow. Value includes interbasin waste discharges. 

Estimated from Rantz (1971); all other basinwide values (Cols A, C, J and K) from Rantz (1974). 

(seven stations), the above values and those illustrated in 

figure 1 are representative. 

The earthquake occurred at a time when water levels in 

regional shallow open aquifers in unpumped areas were 

close to or slightly above long-term normal. This estimation is based on the observation that streamflow records 

for the period following the 1973 and 1983 high-precipitation water years in the Santa Cruz Mountains indicate that 

about 4 years are required to drain the "excess" stored 

water after periods of positive cumulative precipitation departures. "Normal" is here considered the low point in a 

water year annual cycle of variation for any one point. 

Thus, ground-water levels just prior to the earthquake 

were at their normal annual low point but were not significantly below the levels expected at the same time of year 

within a series of water years with about average precipitation. This point is illustrated by an observation well in 

the Pescadero basin cited by Rojstaczer and Wolf (1991, 

fig. 8, p. 13). 

Since ground-water supplies base flow for streams and 

springs in the Santa Cruz Mountains and normal October 

base flow conditions existed at the time of the earthquake, 

it is reasonable to assume that streams and springs were at 

about their normal lowest annual flow levels at the time of 

the earthquake. This is verified by analysis of successive 

annual stream hydrographs for the gauging stations in the 

region. However, Rojstaczer and Wolf (1991) show that 

after the August 8, 1989, M 5.2 Lake Elsrnan foreshock, 

the base flow levels in both the San Lorenzo and Pescadero basins were somewhat above normal compared to the 

prior year. This minor effect can be discerned in most of 

the gauging records in the region. Without the drought 

conditions of this period of years, it is doubtful if such a 

clear hydrologic signature of the foreshock~or of the regional strain manifest at the time of the foreshock~could 

be observed. 

FLOW INCREASES 

Table 1 summarizes information on increases in flow for 

14 selected stations on the San Francisco peninsula and 

Santa Cmz Mountains. Row changes in springs were anecdotally reported as far as Sonoma County, 200 km north 

of the epicenter. Verified changes in stream discharge as 

recorded by recording gauges were noted as far away as 

the city of San Francisco, 88 km from the epicenter. Similar gauges also recorded flow increases outside the area of 

the Santa Cruz Mountains and San Francisco peninsula, 

but only at a distance of about 60 km from the epicenter 
1 E36 HYDROLOGIC DISTURBANCES 

Table 2.Ã‘Strea gauging stations reviewed within the study area, including lag time in days after October 17 to the time of 

gauged peak earthquake-induced base flow peak discharge 

I [- - -, not discernible] 

uses 

gauge 

11 143200 

11152000 

11159000 

1 1 159200 

11160000 

11160020 

1 1 160060 

1 11 60070 

1 ll6O3OO 

11160430 

1 1 160500 

11161000 

11161300 

11161500 

11162500 

11 162570 

11 162600 

11162630 

11 162720 

11162800 

1 1 164500 

11166000 

11166480 

1 1 1 67980 

11169000 

11169500 

Station name 

Carmel River at Robles del Rio 

Arroyo Seco near Soledad 

Pajaro R. at Chittenden 

Corralitos Creek 

Soquel Ck. at Soquel 

San Lorenzo R. nr Boulder Ck. 

Bear Ck. at Boulder Ck. 

Boulder Ck. at Boulder Ck. 

Zayante Creek at Zayante 

Bean Ck. at Scotts Valley 

San Lorenzo R. at Big Trees 

San Lorenzo at Santa Cruz 

Carbonera Ck. at Scotts Valley 

Branciforte Ck. 

Pescadero Ck. nr Pescadero 

San Gregorio Ck. at San Gregorio 

Purisima Ck. 

Pilarcitos Ck. 

Colma Ck. nr Colma 

Redwood Ck. at Redwood City 

San Franscisco Ck. at Stanford 

Matadero Ck. at Palo Alto 

Stevens Ck. 

Los Gatos Ck. at Los Gatos 

Guadalupe River at San Jose 

Saratoga Ck at Saratoga 

STREAM GAUGING 

Area 

(km2) 

500 

632 

3,070 

72 

104.1 

275 

41.4 

29.3 

28.7 

22.8 

275 

298 

9.3 

46.8 

118.9 

131.8 

21.6 

70.4 

28.0 

4.71 

96.9 

18.8 

44.8 

95.8 

373 

23.9 

STATIONS - Study Area 

Ã Lag 

(days) 

--- 

-- - 

37 

2 1 

22 

'5 and 24 

l4 

'4 and 13 

(9 

'2 

l4 

l4 

--- 

--- 

'6 

0 

--- 

5 and 23 

--- 

4 

--- 

--- 

- - - 

- - - 

--- 

0 

Comment 

- - 

South of area 

South of area 

South of area, >3/4alhvial 

SW-most station, alluvial 

West side, alluvial 

West side, bedrock 

West side, bedrock 

West side, <'I4 alluvial 

West side, bedrock 

West side, <I/, alluvial 

West side, <'I4 alluvial 

West side, alluvial 

West side 

West side 

West side, bedrock 

West side, alluvial 

West side 

West side, <I/. alluvial 

NE-most station 

East side 

East side 

East side 

East side 

East side 

East side 

East side, bedrock 

Peak obscured by rainfall of October 21-25 in Santa Cruz County, and 21-26 in San Mateo County. 

Time to maximum flow decrease immediately post-earthquake. 

in Monterey County, and near the Hayward fault in southern Alameda and Santa Clara Counties. 

Although minor changes in flow are recorded, for example, in the Carmel River about 60 krn south of the epicenter, the marked increases in flow that persisted for months 

after the earthquake were restricted to the Santa Cruz 

Mountains and San Francisco peninsula. It is probable that 

flow changes recorded beyond the area of focus of this 

study were due to mechanisms different from those investigated in more detail here. A total of 26 recording gauging stations were reviewed in detail for this study 

(table 2). The initial selection was based upon long-period 

high-quality records and the magnitude of the seismogenic 

signature recorded on the gauges. Stations with marked 

annual variations due to pumping or release of interbasintransfer water and treated effluents were eliminated. The 

Pajaro River station at Chittenden is included in the table 

1 tabulation as the largest end of the scale of watershed 

size, but it cannot be interpreted in the same fashion as 

can the other stations because it does receive treated effluents and irrigation flows that distort the base flow regression and is fed by major alluvial aquifers. 

The base flow estimates of table 1 differ markedly from 

those reported by Rojstaczer and Wolf (1991). Their estimates of excess base flow were based upon comparisons 

with the prior water year (1988-89), which started only 

two years after the second largest precipitation excess of 

110 years of record, and was itself a year with 82 percent 

of normal precipitation. That year may thus have had 

somewhat excess base flow volume, despite a negative cumulative 4-year precipitation departure of -9.35 cm. The 

base flow was certainly higher than that of the subsequent 

year without the seismogenic excess, since the year of the 

earthquake had a -23.35 cm cumulative precipitation deficit and only 62 percent of normal precipitation. 

The estimates by Rojstaczer and Wolf (1991, p. 5-7) of 

seismically augmented flows are therefore smaller than our 

estimates, despite similar straight-line hydrograph separations, because we used a straight-line method to separate 

both background base flow and storm-recharged seasonal 
I 

SOURCES AND MAGNITUDES OF INCREASED STREAMFLQW, SANTA CRUZ MOUNTAINS, 1990 WATER YEAR E37 

base flow (fig. 2, Sequel Creek), whereas they subtracted storms and assume that storm-induced base flow recession 

these values for the prior year to calculate flow excess. continues after the storm surface runoff ends. Our different 

~~th methods acknowledged base flow increases during methods provide bounds for the range of possible flow 

1~'1'11'11'"11~11'~11~ 

Pajaro River at Chittenden 

I I I I I I M I San Gregorio Creek S 

at San Gregorio 

Lag Obscured in - 10 

I I I I I I I I I I I 

Bear Creek at 1 

Boulder Creek - 10 

Lag Obscured in First 

Storm Peak 

- 0 

e Regional Rainfall 

Index 

.. ...... " .L.- 0 

0 30 60 90 120 150 180 210 240 270 300 330 360 

Days after Oct. 1, 1989 

Figure 2.-Hydrographs of mean daily flow for 1990 water year for Pajaro, Sequel, Corralitos, San Gregorio, and 

Bear Creek gauging stations (details provided in table 1). In general, lag in increase in base flow of earthquake origin 

becomes progressively delayed from stations at bottom of this figure to those at top. Rainfall index is the sum of 

daily totals for San Bruno Mountain, Skyline College, and Watsonville Water Works. Hydrograph disaggregation is 

diagrammatically illustrated for Sequel Creek station where volume of water enclosed in dotted polygon is assumed 

to be earthquake-augmented base flow. 
E38 HYDROLOGIC DISTURBANCES 

augmentation, and the actual values for base flow increase 

probably lie between the estimates of Rojstaczer and Wolf 

and those of table 1. 

Table 1 illustrates base flow increases from about 20 

percent to 100 percent. This increase translates into increased yields from zero to almost 9 cm, with the highest 

values being in the upper San Lorenzo, Pescadero, and Soquel watersheds. In terms of. mean annual runoff, the excesses released owing to the earthquake are small, ranging 

from 1 to 23 percent of total mean annual flows. Except 

where otherwise noted, all average values in this table are 

based upon Rantz (1974) for the periods 1931-70. 

The total volume of ground water that may have been 

released by the earthquake from the 14 selected gauged 

basins and from other ungauged basins is on the order of 

84 hm3. This figure represents the excess base flow component from the gauged watersheds on the San Francisco 

peninsula and Santa Cruz Mountains north of the Pajaro 

River (excluding the Pajaro River basin completely except 

for Corralitos Creek) to which is added an estimate of the 

proportion of excess base flow runoff expected from ungauged basins and from those for which reliable disaggregation of gauging records was not possible owing 

diversions, pumping, and waste-flow releases within the 

same geographic area. 

Excess base flow discharge for the 1989 water year was 

estimated by comparing the ratios of watershed areas supplying excess gauged flows to hydrographically and geologically similar areas of ungauged upland watersheds. 

Only the areas of the three before-mentioned counties 

within the study area that represented fractured bedrock 

above the levels of the lowest coastal marine terrace (230 

m elevation), and outside of areas of alluvial basin fill 

were tabulated. The basin fill mapping was based upon 

generalized maps of hydrographic basins after Rantz 

(1972a, fig. 2) and the geologic maps of Brabb (1989), 

and Clark (1981). The area represented by gauged flows 

records of sufficient accuracy for this work was 823.5 

km2, which represented 33 percent of the total upland area 

assumed to contribute excess ground-water flows. The 

County of San Francisco (236 km2) was considered not to 

contribute any appreciable naturally recharged ground water. As pointed out by Rojstaczer and Wolf (1991) it is 

likely that most Santa Cruz Mountains ground water is released into the deeply incised watercourses, and that little 

upland bedrock basin ground-water flows directly into the 

ocean. However, postevent observation of rapid rises in 

ground water in lowland alluvial basins in Santa Clara 

Valley (J. Berklund, oral commun., 1990, reports wells becoming artesian) suggests that not all the estimated excess 

flows entered watercourses. 

The data in table 1 were evaluated to determine if there 

were patterns of hydrologic change related to epicentral 

distance, geologic parent material, or elevation indices of 

basins. No statistically significant trends could be discerned for watersheds normalized with respect to size (table 1, column I). Watersheds with only a few percent 

increase in mean annual runoff (for example, Zayante 

Creek) were closer to the epicenter than those with the 

highest increases per unit area (for example, San Lorenzo 

at Boulder Creek, 8.75 percent; Pescadero Creek, 5.1 1 percent). Particularly when anecdotal observations of changes 

in spring flow and water table rise and fall are plotted and 

compared to the gauge records, we find that there is great 

heterogeneity in observed distribution of hydrologic responses. The Pescadero area, and particularly the coastal 

San Mateo County areas around the San Gregorio-Hosgri 

fault, showed disproportionately great increases in strearnflow and in rising water tables at distances of 30 to 50 krn 

from the epicenter. Areas south of the epicenter, both 

southeast and southwest, together demonstrated lesser hydrologic responses than did those north of the epicenter. 

A lag in peak hydrologic response was noted in many 

stations (table 2). Not all stations had a seismically induced 

peak flow that was masked by the October 21 storm period. 

In general, it was found that gauging stations in streams that 

were not flowing in alluvial valleys and those that lacked 

alluvial terraces above the gauging stations responded more 

rapidly to increased base flow discharge. This was found to 

be the case wherever the gauging station was located in a 

bedrock-controlled reach in an inner gorge, regardless of the 

presence or absence of potential alluvial storage upstream 

that could release additional base flow into the channel. On 

the other hand, where the gauging station was located in an 

alluvial reach, and where significant alluvial storage was 

found above the gauge, peak flow was not observed until 

well after the October 17 event. This effect is illustrated in 

figure 2, for annual hydrographs of daily means flows for 

Bear Creek at Boulder Creek, located in bedrock close to 

the summit zone of maximum surface rupture, San Gregorio 

Creek far from the epicenter in a valley with very little 

alluvium, Corralitos Creek in an alluvial valley very close 

to the epicenter, Soquel Creek that drains the epicenter region 

itself and is characterized by a significant lower alluvial reach 

extending several kilometers above the gauge and bedrock 

above that, and the Pajaro River gauge at Chittenden that 

drains a very large watershed with low-gradient alluvial 

sources supplying its minimal base flow. In general, the lag 

until initiation of rise in base flow was not correlated with 

distance to epicenter. Lag times were shortest for the lower 

San Lorenzo River, and north coast stations such as Pescadero 

and San Gregorio, where the bedrock adjacent to the gauges 

supplied increased base flow. In more alluvial channels such 

as Corralitos Creek only 6.5 km from the epicenter, the onset 

of seismically induced flow was masked by the October 21 

storm, 4 days after the earthquake, but the base flow peak 

did not occur until 21 days after the earthquake, during a 

time of no precipitation. Bear Creek may have peaked during 

the first rain of October 21, and then peaked again about 22 

days after the initial shock (fig. 2). At San Gregorio aug- 
SOURCES AND MAGNITUDES OF INCREASED STREAMFLOW, SANTA CRUZ MOUNTAINS, 1990 WATER YEAR 1339 

mented base flow peaked about 11 days after the event, 

whereas Soquel Creek did not peak for 22 days, during a 

period of minimal rainfall. The Pajaro River record is complicated by possible releases of urban storm and sewage 

effluents, as well as irrigation return flows in season. However, if the first primary base flow peak is of seismogenic 

origin, tlie Pajaro peaks at the end of November, 37 days 

after the earthquake. Base flow for the Pajaro River is controlled by a major component of rainfall-induced increased 

ground-water flow over its 3,070 km2 watershed, and its flow 

record cannot be disaggregated into base flow and storm flow 

components ,with the facility of those smaller watersheds in 

the Santa Cmz Mountains. 

Details of epicentral distances from the gauges and watershed areas may be found in tables 1 and 2. The regional 

rainfall index, illustrated in figure 2, is based upon the sums 

of daily values for San Bruno Mountain in South San Francisco at the north end of the study area. Skyline College in 

the east-central part of the area. and Watsonville Water Works 

at the extreme south end of the region. In general, precipitation at these three stations is not highly correlated internally, 

since storm patterns differ. An aggregation of the sum of the 

three stations provides an index of regional precipitation, but 

the presence of measurable precipitation for any given day 

does not imply that all watersheds received rain. General 

decreases in precipitation from north to south tend to weight 

the value of the index toward northern watersheds. 

METHODS OF INVESTIGATION 

Initial sampling was directed to surface and groundwater stations that could be used for sequential collection 

of samples through several months of postearthquake base 

flow decline. These point-sampling stations were later augmented with three other strategies that included individual 

cooperator point sampling, agency cooperator data collection, and detailed assessment of the pre- and post-earthquake chemical analyses made by Santa Cruz County. The 

first point sampling station was established in November 

1989, approximately 6 weeks after the earthquake. Such 

stations were equipped with staff plates and gauged at 

times of sampling for water chemistry. An array of precipitation gauging and sampling stations was also begun. 

After completion of chemical analyses on samples collected during the 1990 water year, further analyses of gauging 

station and precipitation records were used to set limits on 

the volumes of water released by the earthquake. 

INDIVIDUAL COOPERATOR PROGRAM 

Through public outreach, we sought sources of preearthquake water that could be compared with postearthquake 

samples, where such sampling could reasonably be associated with observed changes in well levels, spring flow 

characteristics, or surface-water flow changes. From about 

300 public responses and offers of cooperation we selected 

51 sites for resampling that had preearthquake water samples or chemical analyses. These sites were visited in 

March, April, and June 1990 to collect preearthquake 

water and to resample from the same supply systems. In 

total, 107 ground water (wells and springs) and 100 surface-water (streamflow) samples were ultimately selected 

for analysis out of about 350 collected samples. All samples were analyzed for Ca, Na, Mg, and K using a Perkin 

Elmer 2380 Atomic Absorption Spectrophotometer, using 

standard methods. Selection of samples for analysis was 

based primarily upon reproducibility of sampling conditions for pre- and post-earthquake water, and elimination 

of possibilities of contamination or postsampling treatment. We sought raw unfiltered preearthquake samples 

stored in cool dark nonreactive conditions. Some samples 

frozen before the earthquake remained so 5 months or 

more after the event. 

Postearthquake sampling was conducted so as to minimize potential sources of chemical water variation when 

compared to preearthquake samples. With the exception of 

ice-cubes in metal refrigerator trays, every attempt was 

made to collect samples through the same distribution system, from the same tap, or hose, or dipped from the same 

spring, as the preearthquake sample. Where preearthquake 

samples were not available, but postearthquake differences 

were deemed potentially significant chemically, added 

sampling was done over a period of several months. 

At two sites, one near the headwaters of Stevens Creek 

in Santa Clara County, east of the San Andreas fault, and 

another on Zayante Creek in Santa Cruz County, just 

below the USGS gauging station, sequential observation 

stations were established with staff plates and local observers. At these sites detailed observations of flow changes and water chemistry were correlated with water 

sampling programs for streamflow. One hundred samples 

were collected and analyzed through this program to assess sequential changes and to explore possibilities for associating post6arthquake water chemistry changes with 

aftershock time series. In the Zayante Creek site, our observer reported marked episodic changes i~ sulfur odors. 

Samples were thus collected to assess possible changes in 

sulfur compounds. 

All sampling was done in triple-acid washed bottles suitable for trace metal analyses. Samples were refrigerated 

until analyzed. Originally frozen preearthquake samples 

were maintained frozen until moved to the analytical lab 

for buffering and dilution. In the lab all samples were 

buffered with cesium chloride and diluted to give readings 

in the 1-t0 ppm range. Selection of Atomic Absorption 

Spectrophotometric tubes and instrument settings was developed to establish linear response over the range of concentrations measured. 
E40 HYDROLOGIC DISTURBANCES 

AGENCY COOPERATOR PROGRAM 

In addition to individual and small water association cooperators, 16 public agencies and 16 water associations 

were selected for data exchange. The most valuable data 

were found in the county planning and water agencies. 

Santa Clara County provided extensive data on flow and 

water level changes. In 1975 Santa Cruz County Planning 

Department established a water quality laboratory and sampling program. By the time of the earthquake, this lab had 

compiled a carefully organized database of multielement 

analyses of 1,667 ground-water samples from 116 sample 

locations throughout the county. About 30 of these stations 

were sampled regularly at least once a year. Analyses included temperature, electrical conductivity, depth to water, 

lab pH, alkalinity, total dissolved solids, chloride, sulfate, 

orthophosphorus, calcium, magnesium, sodium, potassium, 

iron, manganese, copper, zinc, lead, chromium, and cadmium. Silica, boron, and fluoride were tested occasionally. 

Santa Cruz County preearthquake surface-water data 

were compiled for 1,432 samples at 205 stations beginning 

in 1975. In addition to the same suites of major and trace 

elements that were assessed for ground-water samples, 

surface-water data sets included gauge height and discharge (where available), sulfate, nitrate, nitrite, orthophosphate, total phosphate, chloride, alkalinity, lab pH, 

turbidity, ammonia, total phosphorus, chemical oxygen demand, organic nitrogen, and organic carbon. Twenty-nine 

surface-water stations were part of the long-term core 

group sampled twice a year before the earthquake. U.S. 

Geological Survey WATSTOR data were searched for 

analyses that would supplement the Santa Cruz County 

surface and ground-water data for the late 1970's aqd 

early 1980's. About 240 samples not in the county database were found with the major element suite of Mg, Na, 

K and Ca, and about 80 trace-element samples with data 

for Cd, Cr, and Pb. 

ESTIMATION OF AUGMENTED STREAMFLOW 

To determine the amount of water released by the earthquake, hydrographs for the full 1990 water year (Oct. 1- 

Sept. 30 of named year) were disaggregated to estimate 

base flow volumes. A conservative manual separation 

technique was used to separate storm hydrographs from 

base flow. We used the assumption that base flow responds 

to the storm hydrograph concurrently with surface runoff 

and that no base flow recession continues after the time 

storm surface runoff ends. This model is best suited only 

for mountainous upland watercourses without significant 

bank or alluvial storage. Thus, the volumes estimated for 

each storm period are separated by drawing a straight line 

from the point of first storm flow inflection on the rising 

storm hydrograph to the point of intersection of base flow 

recession with the tail of the storm peak (see Nathan and 

McMahon, 1990, Sklash and Farvolden, 1979). 

The storm peak flows thus tend to be overestimated because no allowance is made for rise in base flow during 

storm times. Thus, the estimates of increase in base flow 

in table 1 may be a few percent low, but this error is compensated by assumptions about the constancy of base flow. 

In general, ground water can be expected to contribute a 

significant portion of runoff from streams such as those of 

the Santa Cruz Mountains that flow in deep bedrock gorges and canyons without significant alluvial storage above 

the gauging stations (Pinder and Jones, 1969). There were 

only 7 to 8 periods of precipitation-induced runoff in most 

studied watersheds during the water year, and these were 

well separated by dry periods; consequently, storm hydrograph peaks were not compounded so as to render manual 

base flow separation techniques difficult. 

The single atypical condition that required judgment for 

hydrograph separation was the occurrence of a storm period which began on October 21 during the general period 

of rise in volume of water released by the earthquake in 

most but not all watersheds. In this instance, it was necessary to assume an exponential decay in the rate of increase 

of earthquake-induced base flow through the period of the 

initial storm of the water year. 

The base-flow index (Rantz, 1972b) is defined as the percentage of total annual runoff that occurs in August and 

September, the months of lowest precipitation input in 

streams in this region. The base flow index for all watersheds 

studied in this region was less than 1.3 percent. Groundwater pumping decreases the base flow in the Pajaro basin, 

yielding a distorted base-flow index of 0-0.05 percent only, 

but the other study basins average 0.3 to 0.5 percent, with 

the upper and lower San Lorenzo stations yielding the highest 

values of 1.1 and 1.3 percent, respectively. 

Once storm peaks were removed from the annual hydrographs, an estimation of the contribution of the earthquake 

to the base flow was made by projecting the October 1-17 

base flow horizontally to the point of subsequent intersection with the base flow recession. In most watersheds, this 

occurred within 8-10 months. The exceptions were the 

smaller watersheds (S30 km2) where the duration of the 

seismogenic water was on the order of 4 months or less 

and Pescadero Creek near Pescadero (118.9 km2) where 

the effect persisted beyond the end of the 1990 water year. 

Several months after the main shock, we learned of an 

apparently anomalous site on the Zayante Creek tributary 

of the San Lorenzo River where flow depths were highly 

variable and reported to be linked to aftershock activity. 

This site was only a few hundred meters below a continuous recording gauge, but that gauge did not reflect much 

of the observed variation. At that Zayante Creek site, the 

stream was on fractured bedrock and residents reported 

highly variable spring-flow activity within and immediately adjacent to the channel. Subsequent instrumentation 
SOURCES AND MAGNITUDES OF INCREASED STREAMFLOW, SANTA CRUZ MOUNTAINS, 1990 WATER YEAR w1 

verified that the primary source of augmented flows for 

that middle reach of the Zayante originated in a short 

(150 m) reach near where that creek cuts the axis of the 

Scotts Valley syncline. Subsequent investigations revealed 

several other similar sites on other watercourses, including 

the Pajaro, Stevens Creek, Big Creek tributary to Scott 

Creek, Butano Creek tributary to Pescadero, the lower San 

Lorenzo, and others. There were not always obvious structural or bedrock controls to explain the sources of the inchannel variable springs, but we learned to recognize them 

readily by growth of distinctive white and black algae-like 

mats and filaments that appear to be associated with flows 

of oxygen-depleted ground-waters and reduced sulfur and 

iron compounds. 

From these observations and the analyses of lags, it appears to us that the augmented flows originate at multiple 

diffuse or discrete sites along the bedrock channels. As 

pointed out by Rojstaczer and Wolf (1991, p. 7) some 

streamflow increases were preceded by an apparent decrease in flow. This immediately postevent decrease was 

noted by us in Soquel Creek, Bean Creek, and San Francisquito Creek, as well as in the reported incidence in 

Bear Creek. 

CHEMICAL DATABASE 

Analyses of major cations (Ca, Na, Mg, K) from the initial samples of pre- and post-earthquake water demonstrated somewhat increased ionic concentrations for postevent 

water, but consistent ratios. This was found in all geographic areas for which we had been able to locate preearthquake water samples and where it was possible to 

subsequently sample from the same sources through essentially the same distribution systems. Review of field samples, both in the near field and at distant points 30 krn or 

more from the epicenter, revealed that there were no significant differences in the patterns of change for the major 

cations. To attempt to learn more about the preearthquake 

variability of both surface and ground-water chemistry, we 

reviewed the Santa Cruz County data base of over 3,000 

samples developed from 1975 to the time of the earthquake. The lab data base is updated at least twice a year 

for 29 surface water and annually for 30 ground-water stations. The record is nearly continuous for these primary 

stations (with the exception of a break in the 1982 water 

year), but U.S. Environmental Protection Agency-approved analytical methods have changed in some instances. The break in the record for 198 1-82 was associated 

with a change in funding, personnel, and some analytical 

equipment and procedures. This raised some possible 

questions associated with the sometimes-significant changes seen before and after that hiatus. 

It was necessary to select a limited subset of analytical 

values where there were adequate numbers of samples from 

the same sites at a range of times but with reasonably regular seasonal sampling and consistent analytical procedures. 

Because of uncertainties that arose when the entire period 

of record was reviewed for certain constituents in certain 

aquifers (for example, Na, K, and sulfate in the Santa Margarita Sandstone in the Scotts Valley area, see fig. 9 for 

Ca:Na ratio), we chose to restrict detailed analyses to the 

records for the 1984 and subsequent water years. We also 

chose to evaluate only records for sites where there were 

equal numbers of samples for all years of record and where 

such record was continuous from 1984 to and including the 

1990 water year. Thus, data here reviewed and presented 

include two samples for each surface-water station and one 

for each well or spring for each year from 1984 through the 

1990 water year. A full record of the databases is available 

from the U.S. Geological Survey on CD-ROM as explained 

in the section "Supplemental Information" at the end of this 

chapter for both the cleaned and verified subset used here 

and the larger base record. 

RESULTS OF CHEMICAL ANALYSES 

Analysis of the chemical data revealed that well waters 

collected between 3 and 90 days after the main shock 

demonstrated increases in concentrations of calcium from 

0 to 5 times, with an average doubling of concentration 

(n=12, near field). As discussed by Rojstaczer and Wolf 

(1991, fig. 7), the regional waters are dominated by calcium and bicarbonate in the mountainous areas. We looked 

at ratios of Ca:Na, Ca:Mg, Na:Ca+Mg, and Ca:sulfate to 

try to determine any possible changes in sources of ground 

waters (Hem, 1985, p. 166 ff). We found that, in general, 

cation ratios remained constant within the limits of one 

standard deviation about the mean for pre- and post-earthquake water samples. 

Analyses were conducted to evaluate ionic concentrations and ratios by watershed, by individual wells, spring, 

and stream sampling points, and by township. All preearthquake samples taken between October 1 and the time 

of the earthquake during the 1990 water year were assigned to the 1989 water year for the purposes of plotting, 

to insure that the points for the 1990 water year all represented values after the earthquake. 

Examples of summary plots of surface-water chemistry 

in the near field of south-central Santa Cruz County are 

represented by figures 3 and 4. For Aptos Creek, directly 

downstream from the epicenter, a slight increase in Na 

concentration without a similar change in Ca concentration 

changed the Ca:Na ratio distinctively for the average of 

the two postearthquake samples for the 1990 water year. 

However, for nearby Soquel Creek, differences before and 

after the earthquake are not apparent. The minor changes 

in Na and chloride concentration in these watersheds nearest the epicenter is seen in figure 4. There is a general 
HYDROLOGIC DISTURBANCES 

Water Year 

0 Alkalinity E3 Chloride (ID Magnesium 

Calcium Sodium Potassium 

Figure 3.-Average concentrations of potassium, magnesium, sodium, chloride, calcium, and alkalinity for Soquel Creek station at Soquel Bridge, 1984 to 1990. This is one of three primary Soquel Creek sampling stations. 

Numbers at top of columns are the number of samples per year. 

4 Figure 4.Ã‘Concentration of 

selected ions averaged for Aptos 

Creek, Soquel Creek, and Schwan 

Lake, all within 6 km of the epicenter in south-central Santa Cruz 

County, 1984 to 1990. Schwan 

Lake station receives urban runoff, 

but all these stations show homogeneous responses for 1990 water 

year after earthquake. Numbers at 

top of columns are the number of 

samples per year. 

Water Year 

Alkalinity Chloride 

Calcium Sodium 

(ID Magnesium 

Potassium 
SOURCES AND MAGNITUDES OF INCREASED STREAMFLOW, SANTA CRUZ MOUNTAINS, 1990 WATER YEAR m3 

trend in Na and chloride concentrations upward through 

time since 1984. The anomalously low values of the 1989 

water year may be primarily related to dilution by the 

anomalously higher base-flow discharge during the 1989 

sampling times. As pointed out by Rojstaczer and Wolf 

(1991) and by Sylvester and Covay (1978), this slight inverse correlation between ionic concentrations and discharge is attributable to dilution by interflow. 

Figures 5 through 8 explore variations in surface water 

observed in the far field in watersheds in northwestern Santa 

Cruz County. These are in sites of only very minor human 

habitation with forested parklands as the primary watershed 

cover. Statistically significant changes occur in most cation 

concentrations. In most cases, the single postearthquake ionic 

concentration analysis falls above one standard deviation or 

more above the value for all previous analyses, and there is 

no statistically significant upward trend prior to the earthquake (fig. 7). The increase in Na and Cl concentrations is 

dramatic in these surface waters, collected above marine 

water influence. Since (hey are matched by similar increases 

in other major cations, the most logical explanation is simply 

that these coastal watersheds have significant concentrations 

of NaCl in their meteoric-origin ground waters. These are 

released by the seismic activity. ) 

Shelton and others (1991) present a figure plotting total 

dissolved solids for the Pajaro River in southern Santa Cruz 

County before and after the earthquake on a monthly basis. 

Their data illustrate that the postearthquake ionic concentrations, whereas higher than the 1974-89 long term means, do 

not exceed the maximum range recorded before the earthquake. Our data for all southern Santa Cruz County watersheds served primarily by the open alluvial and Aromas and 

Purisima sand aquifers also show slight increases for the 

1990 water year of about the same orders of increase of 1 

to 2 times. Chloride is the most variable ion, but this may 

reflect in part deeper ground-water pumping with more agricultural return flows to surface streams in this region of 

rapidly increasing salt water intrusion. However, chloride 

concentrations are also seen to rise in the San Lorenzo watershed, as well as other ions by lesser amounts. For the same 

Pajaro River station, we indicate increased cation concentrations and decreased sulfate concentration. 

Overall, surface waters in Santa Cruz County seem to have 

the largest noted changes after the earthquake in the geographically isolated extreme southern part of the county south 

of the epicenter, and in the northwestern part of the county, 

farthest from the epicenter. The differences have more to do 

with rock type and ground-water chemistry than proximity 

Water Year 

0 Alkalinity Chloride 1 :$& Conductivity 

Calcium 0 Potassium 

Figure 5.-Alkalinity, calcium, potassium, chloride and conductivity values for single surface-water station on 

Waddell Creek draining granitic rocks of Ben Lomond Mountain in northwestern Santa Cruz County, 1984 to 

1990. Postearthquake increases in streamflow in this region were associated with increased ionic concentrations. Numbers at top of columns are the number of samples per year. 
HYDROLOGIC DISTURBANCES 

60 J 

Water Year 

All stations Scott Creek 

Waddell Creek IS Big Creek 

Figure 6.-Surface-water chloride concentration for three watersheds in northwestern coastal Santa Cruz 

County draining granitic Ben Lomond Mountain, 1984 to 1990. This was a region of marked increases in 

ionic concentrations. 

' 

84 ' 85 ' 86 ' 87 89 90 

Water Year 

HAlkalinity HChloride 

Calcium Sodium 

Magnesium 

Potassium 

4 Figure 7.-Averaged values 

for magnesium, alkalinity, chloride, 

potassium, sodium, and calcium 

concentrations for the same area 

and dates of northwestern coastal 

Santa Cruz County as in figures 5 

and 6. 
SOURCES AND MAGNITUDES OF INCREASED STREAMFLOW, SANTA CRUZ MOUNTAINS, 1990 WATER YEAR R45 

to the epicenter or to the zones of greatest surface fracturing 

and ground acceleration. 

Ground-water chemistry reflects the same patterns as 

shown in surface-water sampling. Aggregating all samples 

from 1976 to the present by major aquifers (fig. 9) illustrates 

little discernible change after the earthquake in these diverse 

sites. Although there is an apparent increase in ionic concentrations in some geographic locales after the earthquake, 

this may be at least in part an artifact of the lower (more 

dilute) concentrations noted in 1989. Taken as a whole, the 

ground-water data suggest slightly increased concentrations 

in the far field, but not beyond the range of one standard 

deviation higher than preearthquake values (fig. 10). 

DISCUSSION 

HYPOTHESES OF ORIGIN 

OF AUGMENTED FLOWS 

The following multiple working hypotheses were originally to be explored: (1) Increased surface-water discharge 

is due primarily to increased effective permeability due to 

new or reactivated bedrock fracture systems, allowing the 

drainage of previously perched water tables or facilitating 

increased rates of lateral flow and resulting decreased gradients on ground-water effluent to surface streams and 

springs. (2) Increased surface-water discharge is due to regional compressional forces that decrease pore volume or 

void volumes in the phreatic zone and thus increase piezometric head and hence surface flow. In this model, which 

was suggested for explanation of hydrologic change associated with Idaho's Borah Peak earthquake, extensional 

and compressional conditions in adjacent quadrants may 

be anticipated with resulting decreases in well water levels 

or spring flows in some areas and rising water levels in 

alternate quadrants (Wood, 1985). (3) Increased flow is the 

result of deep old ground water in the vicinity of the hypocenter (18.29 km depth in this case) that is displaced 

upward, probably as the result of changes in the storage 

coefficient at depth (Wood and others, 1985). Water from 

1 km depth emerged after the Borah Peak event. (4) Flows 

are the result of settlement and compaction of partly saturated and saturated surface soils and rock forcing water 

previously stored in fracture voids in bedrock, soils, the 

capillary fringe, and massive landslides to the surface. 

Combinations of the above and modifications to this preliminary list were explored through this study. Discovery 

of quantities of colloidal materials after the earthquake in 

headwater streams and wells led to an alternate favored 

hypothesis. In this model iron-rich anaerobic organo-metallic colloids may serve to slowly decrease permeability 

Water Year 

Alkalinity Chloride 

Calcium Sodium 

Figure 8.-Field temperature (in degrees Celsius), alkalinity, chloride, calcium, and sodium values averaged 

for all long-term surface-water stations in northwestern Santa Cruz County, 1984 to 1990. 
E46 HYDROLOGIC DISTURBANCES 

of shallow bedrock units such as sandstones. Pressure 

transients associated with seismic energy release force 

those colloids through the pores that they previously 

blocked, thus temporarily restoring unimpeded permeability. After a major seismic event, the colloids may rebuild 

and gradients on water tables may again increase gradually 

to preearthquake levels. 

MECHANISMS FOR INCREASED STREAMFLOW 

Surface-water runoff in the study area is dominated by 

ground water, and thus ground-water chemistry and surface-water chemistry are found to be very similar. McDonnell and others (1991) have recently completed a study in 

a small headwater catchment that clearly shows that the 

ionic concentrations in runoff may be explained primarily 

by the chemistry of the shallow ground water and soil 

water. Our results verify this. Increased ionic concentrations observed by us are attributed to longer residence 

times but not to differing or deeper sources. The constancy 

of ionic ratios indicate that earthquake-induced excess 

flows originated locally in shallow ground-water storage 

sites above the topographic levels of the receiving streams 

and springs. The primary origin of increased ionic concentrations is the release of waters that were previously inaccessible because of lack of readily interconnected pore 

space. 

Rojstaczer and Wolf (1991, p. 17 ff) have summarized 

the possible causes of the seismically triggered increased 

surface flows and observed ground-water changes. On the 

basis of fundamental ground-water hydrologic flow-path 

limits, they dismiss the possibilities of transport of overpressured fluid from hypocentral depths (18.3 krn for the 

initial event). The response time of surface-water stream 

gauges throughout the near and far field is simply too 

rapid to allow diffusive pore-pressure transfer upward for 

distances of 5-18 km within several days, let alone minutes. The shallow ground-water chemical responses to 

such a mechanism might not differ from those observed 

within the first few months since this mechanism postulated a pore pressure transient traveling much faster than actual pore water, but the observation of increased stream 

and spring flow at the base of upland areas where concomitant dropping of ground-water levels is observed argues 

against such an origin. The chemical signature of such a 

Water Year 

* Alluvium Granitic Rocks 

0 Santa Margarita Sandstone * P urisima Formation 

Aromas Sand * Terrace deposits 

Figure 9.-Ratio of calcium to sodium for ground waters drawn from known aquifers throughout Santa Cruz County, 1976 to 1990. 

No analyses were performed in 1983, and some sampling points changed after that time. Data are not comparable for the two separate 

time series. Over 100 ground-water sampling stations are reflected in these averaged data. Individual consistently sampled stations 

reflect these general trends for Ca:Mg, Ca:Na, Na:Ca+Mg and Ca:sulfate ratios. 
SOURCES AND MAGNITUDES OF INCREASED STREAMFLOW, SANTA CRUZ MOUNTAINS, 1990 WATER YEAR p47 

pore-pressure transient may be expected to ultimately 

bring deeper water to the surface, yet the limited sequences of postearthquake water monitoring performed at two 

daily sampling sites preliminarily suggest that ionic concentrations rapidly return to preearthquake levels in a matter of months without any evidence of admixture by 

deeper waters. 

Postulated compressional forces collapsing open fractures and reducing near-surface pore volume, as posed for 

explanation of the Borah Peak, Idaho, event (Wood and 

others, 1985; Wood, 1985) cannot explain the distribution 

of changes in flow and water table levels observed in the 

Santa Cruz event. Compressional collapse should cause 

water table levels to rise in those areas subject to rnaximum compressional distortion. Anecdotal evidence suggests rises in water tables primarily in the lowland areas 

and basins adjacent to mountainous areas, widely distributed to the west, south, and east of the epicenter and of 

the areas of maximum surface rupture and upthrusting of 

0.5 m or more. Although rising water tables were locally 

reported in some upland sites, most high-elevation sites 

adjacent to lowlands with observed surface-water flows, 

experienced steadily dropping water tables. Chemical signatures of water expelled from compressionally collapsed 

dilatant fractures would be expected to show significant 

increases in ionic concentrations of materials derived from 

the bedrock minerals crushed in that collapse. Although 

we do see increased ionic concentrations, the magnitude of 

most are on the order of 2X, and many sites show changes 

of only a few percent (figs. 3, 10). Cokeisrnic strains much 

larger than those estimated from surface changes that were 

observed (Lisowski and others, 1990; Anderson, 1990) 

would have to have occurred in both the near and far field 

to account for compressionally induced simultaneous increases in flows over regions of many thousands of square 

kilometers. Further, one would expect such a mechanism 

to result in increased flows in the areas of maximum compression. These would be the northwest and southeast 

quadrants for right-lateral strike-slip motion and the southwestern block for high-angle reverse motion. Such regional differences were not discerned. Robert Muir-Wood 

(written commun., 1992) notes that the flow augmentation 

figures derived in our work are fully typical of a M 7 

oblique reverse strike-slip event and are in accord with 

their strain-modeling approach. 

Coseismic hydrologic response mechanisms that increase 

the effective permeability of near-surface rocks to release 

stored ground waters from a wide geographic area are needed 

Water Year 

I Magnesium 

Figure 10.-Averaged values for calcium, sodium, and magnesium concentrations for three consistently sampled wells drawing from Pleistocene Aromas alluvial and aeolian sands, 1984 to 1990. Bars show mean, minimum, and maximum values, and ticks represent one standard deviation. 
E48 HYDROLOGIC DISTURBANCES 

to explain the observed physical and chemical changes. One 

such mechanism may be a pressure transfer process that 

increases the fracture permeability of shallow surface bedrock. Such a microfracturing process is postulated as a possible mechanism by Rojstaczer and Wolf (1991). This 

mechanism would have to work in both brittle rocks such as 

the granitic rocks and Tertiary sandstones, and in the unconsolidated or loosely consolidated Pliocene to late Pleistocene 

marine and dune sands. Particularly difficult is the propagation mechanism that creates increased fracture permeability 

in an isolated topographic hill of poorly consolidated sand, 

such as the Aromas Sand hills west of Corralitos. In sites 

such as those, observations of rapid rises in ground water 

adjacent to such hills and mobilization of saturated debris 

slides and seep zones around the base of the hills suggest 

much increased hydraulic conductivity of the sand hills. The 

strain-induced permeability model is the most logical of the 

various options considered, but it must be modified to make 

it independent of fracture permeability. 

Chemical data suggest a modification of the stress-induced 

fracture-permeability hypothesis. Among the most striking 

changes recognized in postearthquake waters by some respondents was a marked increase of an order of magnitude or 

more in iron compounds in ground waters and springs. 

Springs were reported to fill with gelatinous colloidal brown 

material despite higher flows. Filamentous and gelatinous 

masses of brown, black, and white algal-like growths were 

seen associated with at least 30 percent of the reported springs 

and headwater streams that increased in flow following the 

earthquake, and these effects persisted for on the order of 8 

months or more. At one site along the Pajaro River, a series of 

drainage pipes placed into the Mt. Pajaro shale by theHighway Department began discharging greatly increased volumes of red-brown, black, and white-coated flow streams, 

each color from a different immediately adjacent pipe. All 

drained an apparently homogeneous bedrock mass and the 

different color effluent systems were no more than 30 m apart. 

In another instance in Zayante Creek, the smell of reduced 

sulfur compounds (sulphur and sulfite) was reported to be 

associated with increased flows from bedrock fractures on the 

bed of the shallow stream, with red-brown, black and white 

filamentous growths associated with nearby distinct bubbling 

streambed springs. Careful sampling and analysis for sulfate, 

sulfide, and trace elements from the Zayante "fissures" 

showed no significant trends of variations (R. Golling, oral 

commun.). However, dissolved oxygen and temperature variations suggest that microbial populations may be playing an 

important role that we were not able to evaluate. 

We postulate that shallow surface aquifers, to depths of 

100-200 m, become plugged with colloidal organo-metallic compounds, particularly those associated with iron and 

sulfur. This colloidal coating and plugging of pore voids 

would slowly reduce permeabilities of near-surface aquifers. Mineral cements such as silicates and calcite may 

also effect similar progressive reductions in permeability. 

During earthquakes, that permeability changes dynamically as shear strain or pressure transients shake loose or 

force saturated voids to dislodge their colloidal plugs. 

Microbially mediated stress-induced transient permeability changes are well beyond the focus of this study. The 

inverse relationships between sulfate and metal cations 

found to characterize some sites of postearthquake water 

change (for example, Pajaro River) suggest that oxygen 

concentration differences may play an important role in 

mediating variable rock permeability (see Elder, 1988). 

Direct microbial interaction of chemo-autotrophic organisms with iron, sulfide, and carbon-dioxide in ground 

water can form substances that effectively plug smaller 

voids and dynamically change rock permeability (Olson, 

1983). These are little-studied phenomena, especially as 

associated with seismogenic ground-water flow changes. 

Whether due to dislodging of mineral cementing agents 

or organo-metallic colloids, or both, coseisrnic strain appears to directly affect aquifer permeability in an almost 

instantaneous fashion. Much work remains to be done 

around the concept dynamic permeability. Seismogenic 

flow increases in 1865 and 1906 evidently allowed permeabilities to eventually return to preearthquake levels and 

water tables to again equilibrate with greater heads and 

steeper flowpath gradients. This suggests that the mechanisms of permeability change may be deduced by observing the recovery times for the Loma Prieta hydrology. 

Rapid restoration of ground-water gradients within 1 to 2 

years may favor a biogeochemical process, and restoration 

within periods of decades may suggest mineral cementation origins. Both mechanisms will presumably work 

equally well in unconsolidated isolated sand hills and in 

fracture-porosity bearing brittle bedrock. 

ACKNOWLEDGMENTS 

We wish to thank the several hundred respondents to our 

public information pleas for earthquake-related water information. Particular thanks must go to Barbara Gilwood 

and Kathy Bowers, the two cooperators who maintained 

detailed records of staff plate readings, filled refrigerators 

with sample bottles, and pressed their families to sample 

and gauge in difficult conditions. Santa Cmz, Santa Clara, 

and San Mateo County Planning Departments all contributed considerable staff time and effort. Particular gratitude 

is expressed to Bob Golling of the Santa Cmz County 

Planning Department Water Lab for his review, encouragement, and personal interest. Finally, Kathy Hall and Steve 

Ryder, University of California Santa Cruz undergraduate 

students who volunteered much time, helped to keep this 

project organized and on course. Special acknowledgment 

is made to Tom McMannus, of TowSoft, Inc. for his extraordinary personal programming contribution of several 

months of full-time work. 
SOURCES AND MAGNITUDES OF INCREASED STREAMFLOW, SANTA CRUZ MOUNTAINS, 1990 WATER YEAR UC) 

REFERENCES CITED 

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western Santa Cruz County, California, with particular emphasis on 

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earthquake on spring and streamflows, in Oakeshott, G.B., ed, 

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Clark, J.C., 1981, Stratigraphy, paleontology, and geology of the central 

Santa Cruz Mountains, California coast ranges: U.S. Geological 

Survey Professional Paper 1 168, 5 1 p. 

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1013,43 p. 

Finkel, R.C., 1981, Uranium concentrations and ^u/^u activity ratios 

in fault-associated ground water as possible earthquake precursors: 

Geophysical Research Letters, v. 8, no. 5, p. 453-456. 

Goddard, T.C., and Laclergue, B., 1991, Drought Conditions in Santa 

Cruz County: Santa Cruz County Flood Control and Water Conservation District report of May 1991.64 p. 

Hem, J.D., 1985, Study and interpretation of the chemical characteristics 

of natural water (3d ed.): U.S. Geological Survey Water-Supply 

Paper 2254,264 p. 

Jiang, F-I, and Li, G-r, 1981, Experimental studies of the mechanisms of 

seismo-geochemical precursors: Geophysical Research Letters, v. 8, 

no. 5, p. 473-476. 

Johnson, M.J., 1980, Geology and ground water in north-central Santa 

Cruz County, California: U.S. Geological Survey Water Resources 

Investigations. 80-26, 33p. 

King, C-Y, 1981, A special collection of reports on earthquake prediction: Hydrologic and geochemical studies: Geophysical Research 

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Angeles basin, California, during 5 strong earthquakes, 1933- 1940: 

American Geophysical Union Transactions, v. 22, p. 374-386. 

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1906; Report of the State Earthquake Investigation Commission: 

Carnegie Institute of Washington, Publication 87, v. 1 and 2, 600 p. 

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Lisowski, M., Prescott, W.H., Savage, J.C., and Johnson, M.J., 1990, 

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p. 1437-1440. 

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investigation of earthquake-related changes in crustal hydrology: 

Proceedings, Second International High-Level Radioactive Waste 

Management Conference, Las Vegas, Nev. 

1991b, Hydrologic signatures of earthquake strain (abstract): Eos, 

v. 72, p. 232. 

Muir, K.S., and Johnson, M.J., 1979, Classification of ground-water recharge potential in three parts of Santa Cruz County, California: 

U.S. Geological Survey Water Resources Investigations Open-File 

Report 79-1065,4 p. 

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,i York, Academic Press, p. 201-229. 

Pinder, G., and Jones, J.F., 1969, Determination of the ground-water 

component of peak discharge from the chemistry of total runoff: 

Water Resources Research, v. 5, no. 2, p. 438-445. 

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Francisco Bay region, California: U.S. Geological Survey Open-File 

Report, 41 p. 

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Rojstaczer, S., and Wolf, S., 1991, Hydrologic changes associated with the 

Loma Prieta Earthquake in the San Lorenzo and Pescadero Drainage 

Basins: U.S. Geological Survey Open-File Report 91-567, 21 p. 

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Water Resources Data for California-Water Year 1990, v. 2: Geological Survey Water-Data Report CA-90-2. 

Sibson, R.H., 1981, Fluid flow accompanying faulting: field evidence and 

models, in Simpson, D.W., and Richards, P.G., eds. Earthquake prediction: Maurice Ewing Series 4, p. 593-603. 

Sklash, M.G., and Farvolden, R.N., 1979, The role of ground water in 

storm runoff: Journal of Hydrology, v. 43, p. 45-65. 

Stierman, D.J., and Williams, A.E., 1985, Hydrologic and geochemical 

properties of the San Andreas Fault at the Stone Canyon Well: Pure 

and Applied Geophysics, v. 122, p. 403-423. 

Swenson, F.A., 1964, Ground-water phenomena associated with the Hebgen Lake earthquake: U.S. Geological Survey Professional Paper 

434, p. 159-165. 

Sylvester, M.A., and Covay, K.J., 1978, Stream quality in the San Lorenzo River basin, Santa Cruz County, California: U.S. Geological Survey Water Resources Investigations 78-19, 61 p. 

Varshal, G.M., and others, 1985, Separation of volatile components from 

rocks under mechanical loading as the source of hydrogeochemical 

anomalies preceding earthquakes (results of large-scale model experiment): Pure and Applied Geophysics, v. 122, p. 475-477. 

Wakita, H., 1981, Changes in ground water level and chemical composition, in Asada, T., and Ohnuki, M., eds., Earthquake prediction techniques-Their application in Japan: University of Tokyo Press, 

p. 175-216. 

Waller, R.M., 1966, The Alaska earthquake March 27, 1964Ã‘Effect on 

hydrologic regimen, Anchorage area (544-B) and south-central 

Alaska (544-A): U.S. Geological Survey Professional Paper 544, 

46 p. 

Wang, C., 1985, Ground-water studies for earthquake prediction in 

China: Pure and Applied Geophysics, v. 122, p. 215-217. 

Wesson, R.L., 1981, Interpretation of changes in water level accompanying fault creep and implications for earthquake prediction: Journal 

of Geophysical Research, v. 86, no. B10, p. 9259-9267. 
E50 HYDROLOGIC DISTURBANCES 

Whitehead, R.L., Harper R.W., and Sisco, H.G., 1985, Hydrologic changes associated with the October 28, 1983 Idaho earthquake: Pure and 

Applied Geophysics, v. 122, p. 280-293. 

Wood, Spencer H., Wurts, C., Lane, T., Ballinger, N., Shaleen, M., and 

Totorica, D., 1985, The Borah Peak, Idaho earthquake of October 

28, 1983-Hydrologic effects: Earthquake Spectra, v. 2, no. 1, 

p. 1 27-1 50. 

Wood, S.H., 1985, Regional increase in ground water discharge after the 

1983 Idaho earthquake: coseismic strain release, tectonic tilt, and 

natural hydraulic fracturing, in Stein, R.S., and Bucknam, R.C., 

eds., Proceedings of Workshop XXVII3 on the Borah Peak Earthquake, Idaho: U.S. Geological Survey Open-File Report 85-290, 

p. 573-592. 

SUPPLEMENTAL INFORMATIONDATABASES COMPILED 

A series of data bases have been compiled in rnachinereadable formats. The Santa Cmz County water quality 

data base consists of 4 DBase-111 files separated into surface water, ground water, and location files for each. 

These are provided as straight ASCII comma-separated 

files for entry to spreadsheets, DBase, or other database 

management formats. 

These files, updated with aquifer information, and selectively reduced to emphasize continuous records, have been 

imported to Quattro-Pro and Lotus- 123 spreadsheet files 

for manipulation and graphic analysis. Eight files are 

available in uncompressed formats readable by most versions of these popular software packages. 

Additional Quattro-Pro and Lotus files of analyses completed for this project, sampling completed, WATSTOR 

data compiled from USGS records, and miscellaneous data 

are also available with required documentation. 

These files are available in CD-ROM format to accompany this and subsequent papers. Total size of the archive 

is on the order of 3.5 MB. Accompanying the database is 

a location map showing sampling sites on a regional geologic base. 
THE LOMA PRIETA, CALIFORNIA, EARTHQUAKE OF OCTOBER 17,1989: 

STRONG GROUND MOTION AND GROUND FAILURE 

HYDROLOGIC DISTURBANCES 

HYDROLOGIC CHANGES ASSOCIATED WITH THE EARTHQUAKE IN 

THE SAN LORENZO AND PESCADERO DRAINAGE BASINS 

By Stuart Rojstaczec Duke University 

and 

Stephen Wolf U.S. Geological Survey 

CONTENTS 

ABSTRACT 

The earthquake caused significant changes in the hydrology of the San Lorenzo and Pescadero drainage basins. 

Streamflow increased at most gaging stations within 15 

minutes after the earthquake. Ionic concentrations and the 

calcite saturation index of the streamwater also increased. 

Streamflow and solute concentrations decayed significantly 

over a period of several months after the earthquake. 

Ground-water levels in the highland parts of the basins 

were locally lowered by as much as 21 m within weeks to 

months after the earthquake. The spatial and temporal 

character of the hydrologic response suggests that the 

earthquake increased rock permeability and temporarily 

enhanced ground-water flow rates in the region. 

INTRODUCTION 

Hydrologic changes associated with moderate and large 

earthquakes have long been recognized (Lawson, 1908; La 

Rocque, 1941). While some changes are associated with the 

dilatational waves generated by earthquakes and are ephemeral (Baton and Takasaki, 1959; Cooper and others, 1965; Liu 

and others, 1989), other changes persist and have less obvious 

explanations. Postevent increases in spring flow and streamflow followed such events as the Arvin-Tehachapi earthquake 

(M 7.1) of 1952 (Briggs and Troxel, 1955), the Borah Peak 

earthquake (M 7.0) of 1983 (Whitehead and others, 1985), 

and the Matsushiro earthquake swarm of 1968 (Nur, 1974); 

the increases lasted from months to years. Postevent changes 

in ground-water level have also been observed (Waller, 1966; 

Bell and Katzer, 1987); these changes are often too large to 

be explained by the static compression or extension induced 

by the earthquake (Bower and Heaton, 1978). 

Many mechanisms have been postulated to explain these 

long-term changes in ground-water level and rate of surface 

discharge. Some studies suggested that these phenomena are 

intimately related to the earthquake cycle. They have been 

attributed to the expulsion of overpressured fluids in the 

seismogenic zone (Sibson, 1981) and to the collapse of a 

broad network of preearthquake-induced dilatant fractures 

(Nur, 1974). Other studies have suggested that these nearsurface changes strictly reflect near-surface processes. 

Streamflow and spring flow changes have been attributed to 

elastic compression of confined aquifers (Wood and others, 

1985). Changes of water level in wells may be due to seismically induced ground failure near the borehole (Bredehoeft and others, 1965). Streamflow and ground-water changes 

have also been related to permeability changes in near-surface materials (Waller, 1966; Bell and Katzer, 1987). 

If near-surface changes in hydrology are directly related 

to the state of midcrustal pore fluids or preearthquake instabilities, then hydrologic monitoring in areas of active 

seismicity may provide information on the role of fluids 

in earthquake generation. Hydrologic monitoring may also 

be used as an aid in earthquake prediction. If, on the other 

hand, the hydrologic changes reflect only shallow processes, 

then hydrologic monitoring tells us little about earthquake 

generation; however, it provides insight in the rheologic 

response of shallow earth materials to earthquakes. 
E52 HYDROLOGIC DISTURBANCES 

The Loma Prieta earthquake provided a unique opportunity to examine hydrologic changes associated with earthquakes. Long-term changes in both surface discharge and 

ground-water levels were observed in the region after the 

earthquake. Minor changes were also noted in response to 

the Lake Elsman earthquake (M 5.2, 8/8/89), an event 

which has been described as a foreshock to the Loma Prieta earthquake (Lisowski and others, 1990). Many of the 

changes were well documented and their possible origins 

can be examined in some detail. In this paper we examine 

surface water and ground water response of the Pescadero 

and San Lorenzo drainage basins to the Lorna Prieta earthquake. The Pescadero drainage basin is well outside the 

Loma Prieta rupture zone; most of the San Lorenzo drainage is a minimum of 10 km outside of the rupture zone 

(fig. 1). Although other nearby drainage basins were affected by the Loma Prieta earthquake, the San Lorenzo 

and Pescadero drainage basins are particularly worthy of 

examination because the postevent response of the groundwater system in parts of their recharge areas can be partially quantified. The San Lorenzo drainage basin was also 

worth examining because unlike other basins in the region, 

it contains numerous gaging stations that monitor streamflow (Markham and others, 1988). In this report, we examine the character of the streamflow response. We also 

examine the spatial and temporal response of groundwater levels in parts of the recharge areas of these basins. 

Finally, we attempt to relate the observed surface and subsurface hydrologic changes to extensive permeability 

changes within the basins. 

DESCRIPTION OF STUDY AREA 

The San Lorenzo and Pescadero drainage basins comprise a region greater than 600 krn2 in area and are located 

west of the ruptured segment of the San Andreas fault associated with the Lorna Prieta earthquake (fig. 1). The baPacific Ocean 

- 

Figure 1.-Location of study-area drainage basins (light dashed lines demarcate boundaries of basins) in relation to San Andreas 

fault, Lorna Prieta epicenter (triangle), and northern part of Lorna Prieta rupture zone (heavy dashed line). Stream gaging stations 

(circles): a, Big Trees; b, Zayante; c, Boulder; d, Bear; e, San Lorenzo Park; f, Pescadero. 
HYDROLOGIC CHANGES ASSOCIATED WITH THE EARTHQUAKE, SAN LORENZO AND PESCADERO DRAINAGE BASINS E53 

Table 1 .-Mean annual base flow from July 1, 1988, to July I, 1989, in relation to drainage area of six gaging 

stations in the study area 

Station Bear Big Boulder Pescadem San Lorenzo Zayante 

Trees Park 

Base flow (Lfs) 20 400 40 100 20 20 

sins are mountainous, with slopes commonly exceeding 30 

percent. Elevation increases to over 900 m along the 

boundary of the basins closest to the San Andreas fault. 

Stream gradients range from 0.003 to 0.2 m/m, with the 

steepest gradients occurring along small low-order tributaries in the extreme upper portions of the drainage basins 

(Nolan and others, 1984). These small channels are only 

slightly incised into the surrounding hillslopes, and bedrock exposures are common along such channels. Loworder channels usually contain relatively thin deposits of 

alluvium over the underlying bedrock. Larger intermediate-order channels in the middle parts of the watersheds 

are typically V-shaped and narrow and are incised into the 

surrounding landscape more than the lower order channels. The channels contain varying amounts of bedrock 

and characteristically have beds composed of sandy alluvium or boulders surrounded by sandy alluvium. Mean annual rainfall ranges from about 500 mm near the coast to 

about 1,500 mrn in the higher elevations (Rantz, 1971). 

Rainfall is generally absent from May through September, 

although fog is common during these months. 

Bedrock in the study area consists predominantly of Tertiary marine sandstone, mudstone, and shale, as well as 

some interbedded volcanic units (Clark, 1981). In the high 

elevations of the basins, the San Lorenzo Formation, Vaqueros Sandstone, and the Lambert Shale are the most 

common bedrock formations and aquifers. These formations are heavily fractured at the surface, and the degree to 

which they are permeable is highly variable and dependent 

on the degree to which they are fractured at depth 

(Johnson, 1980; Akers and Jackson, 1977). Stratigraphically beneath the Tertiary marine rocks are Cretaceous 

granitic and metasedimentary rocks which crop out along 

the southeastern margin of the San Lorenzo drainage basin. As with the Tertiary rocks, permeability in the Cretaceous units is variable and highly dependent on the degree 

of fracturing present. 

The region contains many extensive zones of structural 

weakness, both in the near surface and at depth. In the 

highlands, many ancient and active landslides can be 

found in the Tertiary formations (Hector, 1976). In addition to the San Andreas fault, major active faults in the 

basins include the San Gregorio, Butano, and perhaps 

parts of the Zayante (Clark, 1981). Most folds and faults 

in the area follow the trend of the San Andreas fault, 

which is northwest-southeast. 

STREAMFLOW RESPONSE 

Steamflow has been monitored in the region by the U.S. 

Geological Survey since the 1930's. The stream gaging 

stations shown in figure 1 have all been operating for at 

least 13 years. Regulation and diversion of these rivers 

and their tributaries is minor upstream of all the gaging 

stations (Markham and others, 1988). Mean annual base 

flow in water year 1989 (July 1, 1988 to July 1, 1989) at 

the six gaging stations ranged from 20 to 500 Lls, with the 

amount of base flow generally proportional to the drainage 

area associated with each gaging station (table 1). The hydrograph shown in figure 2 from the San Lorenzo Park 

station is indicative of the seasonal character of the discharge at all the gaging sites. During the dry summer 

months, streamflow is generally governed by the contribution of ground water. In the winter months, streamflow is 

greatly augmented by rainfall-induced runoff and interflow. The effects of the Loma Prieta earthquake can be 

seen as a rapid rise in water level during late October 

1989 superimposed on the normal seasonal pattern. 

In all drainage areas monitored, there was an increase in 

streamflow associated with the earthquake, indicating the 

greatly enhanced contribution of ground water to the 

streams (fig. 3). Except for the Bear Creek station, streamflow increases were observed at the first sampling after the 

earthquake (within about 15 minutes of the earthquake) at 

all stations which were not temporarily disabled by the 

ground motion. At Bear Creek, streamflow increases were 

preceded by a postevent decrease, which persisted for 22 

hours. The San Lorenzo Park station was not recording for 

a period of 70 hours after the earthquake. Streadow increases were monotonic for several days after the earthquake, but were masked by rain which began on October 

21. Peak streamflow due to the earthquake was generally an 

order of magnitude greater than preearthquake streamflow. 
E54 HYDROLOGIC DISTURBANCES 

At Big Trees and Boulder Creek, streamflow increases were 

more modest, reflecting that the magnitude of the response 

was not spatially uniform. There is no coherent relation 

between the magnitude of streamflow increases and proximity to the rupture area. 

Although there were large increases in streamflow due 

to the earthquake, the long-term postevent response indicates that these increases were generally short lived. We 

estimated the longer term effects of the earthquake on 

streamflow by comparing the base flow over the period 

after the earthquake with the base flow of the previous 

year. Base flow for each year was estimated by straightline hydrograph separation. The excess base flow produced 

by the earthquake was determined by calculating the difference between the postearthquake base flow and the base 

flow for 1 year prior at the same station. This base flow 

was adjusted by subtracting or adding any differences between the base flow on October 17, 1988, relative to the 

base flow on October 17, 1989. In determining excess 

base flow, we made the assumption that, independent of 

the earthquake, streamflow did not vary greatly, for the 

seasonal character and annual amount of precipitation for 

both years was very similar (72 cm for 1988 and 76 cm 

for 1989). For all stations except Boulder Creek, the difference between base flow on October 17, 1988, and October 17, 1989, was less than 10 percent of the amount of 

the excess peak base flow. The flow at three of the stations 

from July 1, 1988, to July 1, 1990, is shown in figure 4. 

The inferred excess flow produced by the earthquake is 

shown in figure 5. Excess base flow decays rapidly at all 

but the Boulder Creek station. Forty-five days after the 

earthquake, excess base flow is roughly one-half that of 

peak flow. After 150 days, excess flow is difficult to identify in the records. 

The increases in base flow due to the earthquake were 

generally small in comparison to the total annual flow for 

the 1989 water year. Figure 6 shows the relation between 

precipitation and total annual flow atall of the stations for 

5 years of similar total annual precipitation during the decade of the eighties (1985, 1987, 1988, 1989 and 1990). 

Despite the 1989 increased base flow, total streamflow decreased at three of the stations relative to the previous 

year; two other stations' streamflow increases were very 

small. Only San Lorenzo Park station showed a significant 

change in total annual flow, with an increase of roughly 1 

billion liters. This amount of increase is approximately 

equal to the total amount of inferred earthquake-induced 

base flow at this station during this time period. The absence of significant increase in total flow at most stations 

indicates that the earthquake did not greatly enhance the 

surface water supply in this region; for example, the 

amount of inferred earthquake-induced base flow is about 

20 percent of the total annual flow at Big Trees. 

Stream chemistry at two of the stream gaging stations in 

the San Lorenzo drainage basin (Big Trees and San Lorenzo Park) has been monitored on a biannual basis. The data 

LOMA PRIETA 

I I 

LAKE ELSMAN 1 E 

Figure 2.-Hydrograph showing discharge of San Lorenzo Park gaging station from July 1984 to July 1990, and accompanying record of daily 

precipitation (gray line). Dates of Loma Prieta and Lake Elsman earthquakes shown. 
HYDROLOGIC CHANGES ASSOCIATED WITH THE EARTHQUAKE. SAN LORENZO AND PESCADERO DRAINAGE BASINS E55 

-- BEAR 

- BOULDER 

- ZAYANTE 

0 PRECIPITATION 

Figure 3.-Streamflow discharge in response to earthquake at (A) Big Trees, Pescadero, and San Lorenzo Park, and (B) Bear, Boulder, 

and Zayante gaging stations. Streamflow values are daily means. Precipitation values are daily totals. 
HYDROLOGIC DISTURBANCES 

shown in figure 7 indicate that at both sites the water 

chemistry is dominated by calcium and bicarbonate. Hardness and overall ionic concentrations are slightly higher at 

the San Lorenzo Park gaging station. The differences in 

water quality between the two sites have been attributed to 

the geology of the southwestern part of the San Lorenzo 

drainage basin (Sylvester and Covay, 1978). The biannual 

analyses of major ions at the two sites prior to the earthquake suggests that overall concentrations are slightly inversely related to stream discharge rates. The inverse 

dependence has also been noted in other streams in the 

region and is likely due to the influence of surface runoff 

and interflow during winter storms (Steele, 1968; Sylvester and Covay, 1978). The waters are slightly oversaturated 

with respect to calcite (preearthquake saturation index was 

0.4 at both sites). Stream chemistry at these stations is 

similar to the chemistry of ground-water samples which 

tap the Vaqueros Sandstone and Lambert Shale in the 

highlands of the Soquel-Aptos drainage basin, the neighboring drainage basin to the east (Johnson, 1980). 

In response to the earthquake, stream chemistry increased markedly in terms of its overall ionic strength, but 

the overall proportions of the major ions were nearly the 

same as under preearthquake conditions. The increase in 

bicarbonate and calcium caused the calcite saturation 

index to increase to 0.8. Water temperature in early November 1989 was 7O C at both stations, nearly 4' cooler 

than any previous measurement either in spring or fall. By 

April 1990, the stream chemistry had begun to approach 

preearthquake conditions at both locations. The changes in 

temperature and chemistry suggest that the additional 

water caused by the earthquake was derived from ground 

water from the surrounding highlands. The source rock of 

the ground water likely contributed to streamflow before 

the earthquake, but to a smaller degree. 

The hydrologic response of the study area to the Loma 

Prieta earthquake was similar to the area's response to the 

Lake Elsman earthquake of August 8, 1989, but much 

larger in extent and magnitude. The Lake Elsman earthquake produced a two-fold increase in flow at the San 

Lorenzo Park and Pescadero stations; at the other stations, 

changes were too small to be detected. The Lake Elsman 

earthquake of June 27, 1988 (M 5.0), did not cause a detectable change at any of the gaging stations. 

Anecdotal reports suggest that for two ungaged streams 

in basins neighboring the study area, large increases in 

streamflow preceded the Loma Prieta earthquake by 

roughly an hour (D. Friend, oral cornmun.; K. Tarkuchi, 

oral commun.). At the stations examined in this report 

streamflow is monitored at 15-minute intervals, and preearthquake increases were not detectable in the data. Any 

precursory changes in streamflow which occurred in ungaged streams were local in extent and not representative 

of the hydrologic response of the region. 

Figure 4.Ã‘Compariso of mean daily streamflow over the time periods July 1988 to July 1989 and July 1989 to July 1990 at 

(A) Big Trees, (B) Pescadero, and (Q San Lorenzo Park. 
HYDROLOGIC CHANGES ASSOCIATED WITH THE EARTHQUAKE, SAN LORENZO AND PESCADERO DRAINAGE BASINS E57 

GROUND- WATER RESPONSE is derived from local precipitation and moves downward 

through fracture networks (Akers and Jackson, 1977). A 

Ground-water flow in the study area is predominantly study of ground-water flow in the neighboring Soquel-Aptos 

controlled by fracture orientation and density. Ground water drainage basin indicated that 90 percent of all ground-water 

1 -Jul 

Figure 4.Ã‘Continued 
HYDROLOGIC DISTURBANCES 

+ BEAR 

v BIG TREES 

X BOULDER 

Ã‡ PESCADERO 

0 SAN LORENZO P. 

A ZAYANTE 

a MODEL FLUX 

Figure %-Base flow after the earthquake plotted against time at the gaging stations. Base flow at each gaging station is 

normalized by the peak excess base flow following the earthquake at each station: Bear, 110 Us; Big Trees 920 Us; 

Boulder 40 Us; Pescadero 690 Us; San Lorenzo Park, 170 Us; Zayante 1 10 Us. 

X BEAR 

+ BIG TREES 

BOULDER 

PESCADERO 

A SAN LORENZO P. 

54 59 64 69 74 79 84 

ANNUAL PRECIPITATION (cm) * ZAYANTE 

Figure 6.-Total annual flow plotted against precipitation at the six gaging stations for water years 1985 and 1987- 

1990 ('89 refers to the water year July 1988 to July 1989). 
HYDROLOGIC CHANGES ASSOCIATED WITH THE EARTHQUAKE, SAN LORENZO AND PESCADERO DRAINAGE BASINS E59 

discharges into local streams and springs (Essaid, 1990). ocean. Because the climate and geometry of the basins in 

Very little basin-derived ground water flows directly into the this study are similar to those of the Soquel-Aptos basin, it 

is- & 

s 8 is- ^ 

Discharge 

Figure 7.- Major-ion stream chemistry plotted against time and streamflow discharge at (A) San Lorenzo Park and (B) Big Trees. 
E60 HYDROLOGIC DISTURBANCES 

is likely that almost all ground-water flow in the San Lorenzo and Pescadero basins also discharges into local streams 

and springs. 

There were numerous anecdotal reports of earthquakerelated changes in water level and water quality of wells 

in the study area as well as reports of changes in spring 

flow. Exact measurements of preearthquake water levels 

are generally not available in the region. A well that taps 

an unconfined aquifer located in the eastern headwaters of 

the Pescadero Creek has been monitored weekly since 

1976 (fig. 8; location of well is northernmost well shown 

in fig. 9A) and has a strong semiannual cycle. The amplitude of the water-table-elevation cycle is highly dependent 

on the amount of rainfall but is on the order of 5 m during 

years of near-average rainfall. Although the effects of 

drought had an influence on the water-table elevation, the 

earthquake caused the water-table elevation to drop 4 m 

within several weeks after the earthquake. 

Although water-level changes in the recharge areas of 

the study area are generally difficult to quantify, we infer 

that drops in water-table elevation similar to that just described occurred in a significant part of the basins' highland area. Throughout the study area, numerous wells 

either went dry or underwent a significant reduction in 

their capacity to pump water within several weeks after 

the earthquake. We focus our attention on measured and 

inferred ground-water-level changes in two highland areas. 

One area, which includes the well that is noted above, is 

shown in figure 9A. This area is along the crest of the 

Santa Cmz Mountains and straddles the border of the 

Pescadero drainage basin. The other area is near the headwaters of the San Lorenzo River and is shown in figure 

9B. These areas were selected owing to their well density 

and the level of cooperation on the part of the landowners. 

The wells shown in figure 9 are used by single homes and 

range in depth from 40 to 140 m. 

Of the wells shown in figure 9A, roughly one-half experienced a reduced capacity to deliver water for home uses 

or were completely dry by January 1990. In general, the 

wells which were most affected were in the southern part 

of the area (where elevation is highest). In most of the 

wells which were adversely affected (wells which either 

went dry or no longer provided enough water for home 

use), changes were noted within several weeks after the 

earthquake. In other affected wells, the changes were gradual, and wells which became dry did so over a period of 2 

to 3 months. It is difficult to quantitatively relate the adverse impact of the wells to a water-table decline. Wells in 

the region which were not adversely affected generally 

have water levels which are in excess of 7 m above the 

well bottom. If earthquake-impacted wells had water-table 

elevations that were at this level above the well bottom, 

then the water-table declines produced by the earthquake 

would be of this order of magnitude or greater. 

In the San Lorenzo headwaters area (figure 9B), wells 

adversely affected were generally confined to two ridgetops. Wells along the northeast edge of the area and wells 

located near the valley floor were not adversely impacted. 

Although no extensive preearthquake water-level records 

exist for this area, there is limited anecdotal information 

Figure 8.-Water-table elevation (black line) plotted against precipitation (gray line) for 1976-90 in a well on the eastern 

edge of Pescadero basin. Well is northernmost well in figure 9A. 
Figure 9.-Effect of the earthquake on ground-water wells in the (A) Pescadero and (B) San Lorenzo headwaters. Symbols indicate that the well went dry (circle), the well 

developed a reduced ability to yield water for supply (square), or the well was unaffected (triangle). Elevation contours (light lines) are in meters. Dashed line is boundary 

of drainage basin. 
E62 HYDROLOGIC DISTURBANCES 

on preearthquake levels in some wells. In one well which 

became dry, the water level was 21 m above the bottom 

on October 11, 1989. In another well which became dry, 

the water level was 40 m above the bottom during February 1989. In two wells whose water levels began to be 

monitored after October 17, water levels dropped over 20 

m within a period of weeks to months after the earthquake. Subsequent measurements in the region of unaffected and impacted (but not dry) wells over the time 

period January through July 1990 indicated that water levels declined gradually in many wells (on the order of 1.5 

dmonth or less). The rates of decline during 1990 were 

too gradual to be uniquely identified with the Loma Prieta 

earthquake; more likely they were due to the effects of 

drought in the region. The water-level data suggest that 

the impact of the earthquake on ground-water levels in the 

northern tip of the San Lorenzo drainage basin had either 

greatly diminished or had essentially disappeared after 

several months following the earthquake. 

POSSIBLE CAUSES OF RESPONSE 

The observed hydrologic changes (temporary excess 

streamflow, dropping water table, changes in stream 

chemistry) were likely due to a common cause. The 

mechanism responsible produced changes which were 

spatially nonuniform. Streamflow increases ranged from a 

factor of 4 to 24 above preearthquake conditions and generally decayed rapidly. Water-table drops examined were 

common but patch-like in distribution. The stream chemistry changes indicate that the difference in the excess 

water was ionic strength, not ionic composition. The 

mechanism which most likely explains the postearthquake 

hydrologic observations is a permeability increase caused 

by seismically induced formation of fractures and microfractures. 

It is difficult to ascribe the observed hydrologic changes 

to processes occurring at mid-crustal depths. Transport of 

overpressured fluids from the mid-crust to the surface 

would require highly permeable vertical pathways at great 

depth. Streamflow increased at most of the monitoring 

stations within 15 minutes of the earthquake, indicating 

that any pore-pressure propagation due to a mid-crustal 

source would have to travel at a rate on the order of 10 

ds. Assuming that pore-pressure propagation was a diffusive process, the hydraulic diffusivity of the pathway 

would have to be impossibly high to allow for such rapid 

rates of pore-pressure propagation. If, for example, expulsion of overpressured fluids at depths of 5 krn were responsible for the response of the streams (a rise to peak 

flow within several days of the earthquake), then the hydraulic diffusivity of the pathway would be on the order 

of 1x10~ cm2/s. This value for hydraulic diffusivity is orders of magnitude above that which has been inferred or 

observed in the crust (Brace, 1980, 1984). The mechanism of transport of overpressured fluids is also incompatible with the water-table drops observed in the area 

and the lowered temperatures of the stream water. 

Collapse of near-surface dilatant fractures or squeezing 

of near-surface pore fluids owing to static compression induced by the earthquake are also unlikely mechanisms. 

They run counter to the water-table drops seen in the area. 

They also require an unrealistic amount of pore volume 

collapse to account for the amount of excess stream discharge. For example, consider the 1 billion liters of earthquake-related excess discharge at San Lorenzo Park. If we 

assume that this excess fluid has its source area in the near 

surface (to a depth of 200 m below the water table), then 

there would have to be compressional strains on the order 

of 3x10"~ in this region to account for the excess discharge. This amount of shortening is at least an order of 

magnitude greater than that which can be inferred from 

geodetic measurements of the displacements caused by the 

earthquake (Lisowski and others, 1990). 

The mechanism which accounts for both excess discharge and a lowered water table in the study area is widescale increase in near-surface permeability. If the fracture 

networks which control ground-water flow in the region 

were enhanced owing to the earthquake, we would expect 

to see many of the effects actually observed: (1) Groundwater flow rates would initially increase in proportion to 

the permeability increase; (2) the water table would drop 

because the ground-water system would be effectively 

drained by the increased discharge; (3) areas of high elevation would be most susceptible to water-table drops because they would tend to have the highest water-table 

elevations prior to the earthquake. 

The increased fractures and microfractures in the 

ground-water system would also be expected to temporarily alter the chemistry of the ground water. They would 

tend to expose previously near-stagnant water in small 

pores to enhanced ground-water flow paths. This nearstagnant pore water, because it would have a great deal of 

time to interact with rock-mineral surfaces, would have a 

relatively high concentration of solutes. As a result of the 

generation of new flow paths, a greater proportion of high 

solute concentration water would be expected to enter the 

major ground-water flow paths and the ionic strength of 

the exiting ground water would be increased. 

To account for the initial surge in discharge, the fracturing would have to effectively increase the permeability in 

parts of the aquifers and aquitards in the highlands by 

roughly an order of magnitude. Subsequently, streamflow 

would decay rapidly because the hydraulic gradient which 

drives fluid flow would decay as the water table dropped. 

The fall in water-table height would be expected to decline at a rate similar to the decline of streamflow. 

The relatively shallow depth of the water table in the 

highlands part of the study area suggests that permeability 
HYDROLOGIC CHANGES ASSOCIATED WITH THE EARTHQUAKE, SAN LORENZO AND PESCADERO DRAINAGE BASINS m3 

increases and concomitant water-table drops are temporary 

in nature. If they were permanent, the numerous historic 

and prehistoric earthquakes in the region would likely 

have lowered the water table to great depths. Apparently, 

between earthquakes, the fracture networks likely heal and 

the water table partly recovers to its preearthquake level. 

We propose that permeability in this region is a time-dependent parameter, increasing during times of seismicity 

and relaxing during interseismic periods. The temporal nature of permeability in response to shear strain in the region would be consistent with time-dependent variability 

in permeability of laboratory samples subjected to shear 

(Kranz and Blacic, 1984). In the laboratory work of Kranz 

and Blacic the cracks were sealed by silica cementation. 

Because the base flow is oversaturated with respect to calcite, we speculate that calcite cementation would be a likely mechanism for fracture healing and concomitant 

permeability reduction. The time constant for this inferred 

process, however, is not identifiable in this analysis of the 

data. 

A SIMPLE DIFFUSIONAL MODEL 

OF HYDROLOGIC RESPONSE 

In order to examine the theoretical response of streamflow and ground-water levels to permeability changes, we 

employ a very simple diffusional model of ground-water 

flow along a hillside. Prior to the earthquake, the watertable increases linearly with distance from the stream at 

a rate of 0.05 dm and flow rates in the ground-water 

system are constant. Preearthquake ground-water-flow 

rates can be estimated from the ratio of base flow to area 

given in table 1 and are on the order of 1x10" cds for 

all the drainage areas. Employing Darcy's law and assuming the aforementioned water-table gradient of 0.05 

yields a bulk preearthquake permeability for the drainage 

basins of 20 rnilliDarcies. This permeability estimate is 

obtained by assuming that the gradient of hydraulic head 

can be approximated by the gradient of the water table. 

Assuming that the earthquake increases permeability by 

an order of magnitude, the gradient of the water table will 

decline and the ground-water flow rate into the stream 

will initially increase by an order of magnitude. The governing equations and boundary conditions for this simple 

model are: 

where h is the hydraulic head, x is the horizontal distance, 

c is the hydraulic diffusivity, t is time, w is the maximum 

height of the water table relative to the stream, and L is the 

maximum length of the ground-water flow path. Solution of 

equation 1 and use of Darcy's law yields the ground-water 

flux, v, into the stream as a function of time, t: 

00 

v = [4kpgw/pL] [ (-1) "/ (2n + 1)] 

n=o 

exp [- (2n + 1) 2~2ct/ (4~~) ] (2) 

where k is the permeability, p is the fluid density, g is 

gravity, and p is the fluid viscosity. 

The fit of the model to the excess flow data is shown in 

figure 6. The fit is based upon a value for the hydraulic 

diffusivity, c, of 3,700 cm2/s, and a value for L of 2,000 m. 

The model is able to mimic the magnitude and decay characteristics of the streamflow. However, the model overestimates the magnitude of the ground-water response. The 

model would indicate that water-table drops in the high 

elevation regions would be 90 m rather than the observed 

drops of tens of meters. This discrepancy may reflect the 

inappropriateness of the assumed gradient or length scale, 

L. It also indicates that this model is too simplistic to provide 

for more than a first-order description of the hydrologic 

response. 

CONCLUSIONS 

This study has focused on the ground-water and surfacewater response of two basins to the Loma Prieta earthquake. Because streamflow in the San Lorenzo River is 

monitored extensively and because the Santa Cmz Mountains contain numerous land owners who utilize groundwater supplies, the hydrologic response can be examined 

in some detail. The signature of the hydrologic response is 

one which is consistent with earthquake-enhanced groundwater flow paths. 

The enhancement of ground-water flow paths may also 

be responsible for hydrologic changes seen in response to 

other earthquakes. This mechanism may explain why the 

general response of streams to earthquakes is one of increased flow. Because streams are usually the exit area for 

ground water flow, any enhanced ground water motion 

would be readily detected in the base flow signature of the 

stream. For one order-of-magnitude increase in permeability to produce identifiable streamflow response, base flow 

must be a significant contributor to the stream. In addition, 

the permeability increases must either be areally extensive 

or occur in key locations. 

The hydrologic response suggests that the shallow rnaterials in the highlands areas of the study area are in a state 

of incipient failure. Dynamic or static shear strains pro-) 

duced by both the Loma Prieta and Lake Elsman earthquakes are large enough to generate new cracks and 

microfractures in the upper 200 to 300 m of the crust. The 
E64 HYDROLOGIC DISTURBANCES 

fractures generated must be able to form a new continuous 

flow path or enhance an old continuous flow path in order 

to cause increases in ground-water flow rates. The weak 

nature of the near surface is evident by the numerous active and ancient landslides in the area; it can also be inferred from the influence that fracture permeability had on 

the preearthquake state of ground water flow in the region. 

This area has been subjected to repeated earthquakes, and 

it is likely that seismic events, both historic and prehistoric, have had a large impact on the geologic evolution of 

permeability and ground-water flow paths in the region. 

ACKNOWLEDGMENTS 

We thank the members of the South Skyline Homeowners Association for their cooperation. Robert Golling of 

the Santa Cmz Flood Control and Water Conservation 

District provided the stream chemistry data. The precipitation data was collected by the Santa Clara Valley Water 

District. John Hem, K. Michael Nolan, and Arthur 

Lachenbruch provided helpful comments and suggestions 

on an earlier version of this manuscript. 

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