1

HEIGHT CHANGES IN THE EPICENTRAL REGION

PRECEDING THE JANUARY 17, 1994 NORTHRIDGE

EARTHQUAKE

By Robert O. Castle, Robert F. Packard, and Laura B. Dinitz

A characterization of the vertical-displacement field

in both space and time during

the period 1978-1989

OPEN-FILE REPORT 00-002

2000

This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey

editorial standards or with the North American Stratigraphic Code. Any use of trade, firm, or

product names is for descriptive purposes only and does not imply endorsement by the U.S.

Government

U.S. DEPARTMENT OF THE INTERIOR

U.S. GEOLOGICAL SURVEY
2

HEIGHT CHANGES IN THE EPICENTRAL REGION PRECEDING THE

 JANUARY 17, 1994 NORTHRIDGE EARTHQUAKE

By

Robert O. Castle, Robert F. Packard, and Laura B. Dinitz

Abstract. Analysis of the results of repeated levelings through the epicentral region of the Mw6.7,

1994 Northridge earthquake has disclosed the occurrence of differential uplift in this area that

preceded the earthquake. Although the distribution of the relevant vertical-control data is

somewhat sparse, in both space and time, those data that we have recovered indicate that this uplift

exceeded 0.10 m and peaked 20-25 km west of the 1994 epicenter. While our data also indicate that

this deformational event must have occurred during the period 1978-1989, evidence based on the

character and magnitude of misclosures developed from 1987 and 1989 surveys argue that the

deformation occurred largely during the period 1987-1989. The preseismic vertical-displacement

field that preceded the Northridge earthquake is similar in form and, less certainly, magnitude to that

which preceded the Mw6.7, 1971 San Fernando earthquake; other possible, but less significant

southern California analogues include the deformational events that preceded the Mw5.3, 1973

Point Mugu and Mw5.9, 1987 Whittier Narrows earthquakes. The small but growing number of

recognized aseismic deformational episodes that preceded small to moderate magnitude earthquakes

in southern California suggests that the deployment of the dense array of continuously recording

GPS receivers planned for southern California can be expected to detect and more accurately

describe such events than was heretofore possible. Moreover, if the relation between the duration

of these deformational anomalies and the magnitudes of any ensuing earthquakes can be much more

clearly established, the near perfect temporal control on position afforded by GPS suggests that we

may be on the threshold of a realistic earthquake warning system.

Introduction

Owing to severe funding constraints

experienced by a number of local jurisdictions,

geodetic control work had virtually ceased in

southern California by 1979. It was, in fact, not

until the late 1980s, and owing largely to the

efforts of the National Geodetic Survey to bring

to completion the new North American Vertical

Datum, that any significant number of higher

order vertical-control surveys were again

carried out in this area. Several of these

vertical-control lines fell within the

subsequently defined epicentral region of the

Mw6.7 Northridge earthquake. Analysis and

comparisons of these surveys, though sparsely

distributed, against the results of still earlier

levelings has disclosed compelling evidence of

an intriguing preseismic deformational event,

especially when viewed as an indication of

what can be expected to emerge with the

anticipated installation of a dense array of GPS

observatories in southern California, where this

array is integrated in turn with interferometric

synthetic aperature radar (InSAR) imagery in

this region.

While a fairly clearly defined

association between a regional deformational

event and subsequent earthquake activity has

been recognized in southern California (Castle

and Bernknopf, 1996), localized preseismic

deformational anomalies, such as the one

described here, are seemingly rare, whether in

southern California or elsewhere. However,

their detection has previously depended on the

spatial and temporal distribution of those

surveys that led to their recognition, surveys

not generally deployed with geophysical needs

in mind. Thus uncertainty surrounding the

frequency of occurrence of these preseismic

events, as well, of course, as similar events that

show no demonstrable associations with

subsequent seismicity, remains unresolved.

Discrimination between these separable types

of deformational events is especially

perplexing and probably will remain so until

our data reservoir is vastly enlarged.

Nevertheless, current uncertainties should
3

gradually diminish with increasing density

and accuracy of space-based observations,

particularly those based on arrays of

continuously recording GPS receivers, such as

that now being deployed in southern California

that occurred within the epicentral region of

the Northridge earthquake between 1978 and

1989--changes that probably occurred over the

much shorter interval 1987-1989. We have

made no comparisons with pre-1978 work here,

LOS ANGELES

SAN PEDRO (TIDAL 8)

TOPANGA

BEACH

POINT

MUGU

MONTALVO

SANTA PAULA

MOORPARK

CHATSWORTH

SAUGUS

SAN FERNANDO

119¡15' 119¡00' 118¡45' 118¡30' 118¡15'

34¡30'

34¡15'

34¡00'

34¡00'

118¡30'

119¡15' 119¡00'

34¡00'

34¡15'

34¡30'

GLENDALE

PACIFIC OCEAN

1973

1978

1987

1973

1978

1989

1973

1978

1987

1989

1989

0 10 20

KILOMETERS

1994 EPICENTER

1978

1989

1978

1989

1978

1989

1978

1989

1974

1989

338 Reset

Figure 1. Index map showing routes and dates of levelings used in this report.

or the 1000-station operating network in Japan

(Thatcher, 1999).

Vertical-Control Data Utilized

in this Report

The surveys on which this report is

based are limited in number (fig. 1). They are,

nonetheless, of sufficient density that we have

been able to reconstruct those height changes

other than for demonstration purposes, chiefly

because this area is known to have sustained

major regional deformation sometime during

the interval 1974-1978 (Castle and Gilmore,

1992; Castle and Bernknopf, 1996). All of the

elevations, and hence all of the elevation or

height changes, have been reconstructed with

respect to bench mark Tidal 8, San Pedro. This

control point traditionally has been utilized for

these purposes because of its proximity to the

San Pedro tide station. Because sea-level has
4

been falling at San Pedro relative to other

California tide stations (Hicks and Shofnos,

1965; Hicks and Crosby, 1974), height changes

referred to Tidal 8 tend to bias these changes

against the recognition of uplift and towards

the recognition of subsidence.

Measurement Accuracy. All of the utilized

levelings meet either class I or class II firstorder standards. All have been corrected for

rod error, thermal expansion of the rods, and

instrument collimation error. Staff settlement,

which can produce a commonly overlooked

source of systematic error, is tightly controlled

in first-order work by requiring that the

direction of running be both regularly and

frequently reversed. The modeled refraction

corrections of the National Geodetic Survey

have not been incorporated in any of the lines

utilized in our analysis simply because they

tend to degrade the survey data (Castle and

others, 1985; Mark and others, 1987; Castle and

Gilmore, 1992; Castle and others, 1994)--see

below. While a degree of systematic error

surely has crept into these measurements,

because those corrections that we have

accepted are based on the most modern (late

20th-century) calibration techniques, the

known range of thermal expansion values for

invar identified with particular types of rods,

and very restrictive field procedures, whatever

systematic error remains no more than competes

with, and, in the general case, probably is a

good deal less than the estimated random error.

The standard deviation (σ) in the

measured elevation difference between any two

marks (virtually interchangeable here with

random error) is adequately described by an

expression of the form αL1/2, where α is in units

of mm/km1/2 and L is the distance in kilometers

between the indicated marks. The 1978 base

line leveling was double run to current firstorder, class I standards, where the standard

deviation has been derived through statistical

examination of the section closures. The

standard deviations for the five 1978 lines

utilized here range between ±0.66 and ±0.73 mm

for a 1-km section (the α term in our standard

deviation expression), for an average of ±0.70

mm. The subsequent 1987 and 1989 levelings met

first-order, class II standards, but all were

single run; the equipment and procedures were

otherwise identical to those currently in force

for double-run first-order leveling (Federal

Geodetic Control Committee, 1984, p. 3.6-3.8).

Thus acceptance of the square root rule

indicates that the standard deviation for a 1-

km section obtained from the 1987 and 1989

leveling should be about 0.7 x √2 or

approximately 1.00 mm. Accordingly, an

average value for α based on a comparision of

the results of the later 1987 and 1989 levelings

against those of the 1978 work would be about

1.22 mm/km1/2. Because the greatest length of

any of these levelings referred to Tidal 8 is

about 220 km, the estimated random error

associated with the height changes reported

here reaches a maximum of about 18 mm. 

In spite of the rigorous standards and

procedures that have been adopted in order to

minimize systematic error in any geodetic

leveling, particularly those embraced during

the last three or four decades, a good deal of

doubt still attaches to the accuracy of geodetic

leveling relative to the described signals. This

doubt stems largely from the conviction on the

part of a number of geophysicists that

significant height (rod)- or slope (refraction)-

dependent error (at or above various or

virtually all the signals described here and

elsewhere) has seriously contaminated the

data (Jackson and others, 1980; Reilinger and

Brown, 1981; Strange, 1981; Holdahl, 1982;

Stein and others, 1986; and Geller, 1997).

Disbelief in the accuracy of the measurement

system attributable to disagreement between

heights or height changes obtained from steric

leveling or relative changes in sea level along

selected coasts is thought to have largely

dissipated (Castle and Elliott, 1982).

Similarly, significant residual rod error (above

that accounted for through normal calibration

procedures) has rarely, if at all, been cited in

recent years as a significant contaminant in

geodetic leveling (see, for example, Mark and

others, 1981; Stein, 1981; Craymer and others,

1995). This is not the case, however, with

respect to the so-called unequal-refraction error

(URE). While, as we note above, such evidence

as we have before us indicates that the URE

simply does not accumulate to values that even

begin to match the estimated random error--

owing to both the procedural constraints and

observer experience identified with any

geodetic leveling--we have examined the
5

effect of the modeled refraction corrections of

the National Geodetic Survey on two lines

utilized here that should be especially

vulnerable to any URE. Both of these lines are

characterized by relatively shallow slopes

over significant height differences (300 m over

65 km in the first case and 370 m over 55 km in

the second case). The magnitudes of the

modeled refraction corrections applied to both

of the comparative levelings are such that

they increase the height changes along one of

these lines by a maximum of about 4 mm and

decrease the height changes along the other by

about 2 mm. Accordingly, even though we have

not incorporated refraction corrections here,

had we chosen to do so it would have had little

impact on this analysis.

Although, as we imply in the preceding

paragraph, geodetic leveling is now thought to

be about as accurate as it was believed to be

prior to the late 1970's, this conclusion is

qualified by the fact that the worst systematic

error ever recognized in any terrestrially based

geodetic measurement system could have

compromised our analysis. Specifically, the

1978 leveling was based in part on the

utilization of a particular automatic level, the

Zeiss Ni l, that was subject to a potentially

large magnetic deflection error. This error is

design dependent, instrument specific, azimuth

dependent, and time dependent, such that

hindcast corrections could be expected to be no

more than approximate. Nevertheless,

following the recognition of this error in the

early 1980's, the National Geodetic Survey

developed an empirical correction (Holdahl

and others, 1987) that successfully diminished

or even removed this error in varying degree.

The only 1978 line used here that incorporates

any Ni l leveling is the one that runs along the

coast between Tidal 8 and Montalvo (fig. 1).

The difference between the empirically

corrected and uncorrected levelings between

Tidal 8 and Topanga Beach is 4 mm; that

between Tidal 8 and Montalvo is 7 mm. Thus,

while it would have made little difference

whether we did or did not incorporate this

correction, both the generalized azimuth of

this line and our own experience with the NGS

empirical corrections indicate that utilization

of this correction is probably the safest

procedure to follow. We have tested the

effectiveness of this correction along the

coastal route by backing into our Los Angeles

starting mark, 338 Reset, via 1978 leveling

through Montalvo and Saugus (fig. 1). This

procedure produces a height for this mark of

103.4210 m, a value that falls within

millimeters of the 1974 and 1989 heights for 338

Reset (103.4192 m and 103.4171 m,

respectively)--an admittedly fallible, but

certainly supportive indication of the accuracy

of the magnetically corrected 1978 leveling.

1978 Ni 1 leveling was also carried out between

Tidal 8 and 338 Reset. However, based on a

comparison between the empirically corrected

and uncorrected values over this reach, the

magnetic error accumulated to at least 50 mm;

because the resulting, corrected height falls

several centimeters below the 1974 and 1989

heights for this mark, it probably

underestimates the actual error. Thus, the 1978

survey between Tidal 8 and Los Angeles has

been rejected for use in our analysis. 

Finally, survey blunders or "busts"

constitute a separate category of leveling error

and are considered here under a separate

heading (see "Results").

Results

The chief results of our analysis are

summarized as profiles of height changes,

together with their corresponding terrain

profiles (fig. 2). Comparison of these height

changes against terrain provides a qualified

procedure for searching for either height- or

slope-dependent error (although obviously not

for magnetic error nor staff settlement). Even

though none of the profiles start at the San

Pedro tide station, all of the height changes

have been reconstructed with respect to bench

mark Tidal 8 as fixed in height. The 1978-1989

height change shown at San Fernando (P 1296)

(fig. 2A) is based on a 1989 tie between

Chatsworth (C 1452) and San Fernando that

does not relevel any earlier (1978 or later)

survey over this route. Accordingly, the 1989

tie between C 1452 and P 1296 is based on the

assumptions that: (1) the orthometric correction

applicable to the leveling between Chatsworth

and San Fernando can be taken as zero; and (2)

the difference in the orthometric corrections in

the levelings northward into C 1452 and P 1296
6

1978

1989

1987/89

1987/89

1978

1978

1974

5 0

5 0

5 0

100

500

D H(mm) HEIGHT (m) -50

-50

0

0

0

0

LOS ANGELES(338Reset)

(V 768)GLENDALE(N 1141)(09 01799) 

(P 1296)SAN FERNANDO

(J 52)SAUGUS

(Q 970)

A

1878

5 0

-100

100

500

D H(mm) HEIGHT (m) -50

0

0

-150

(B 1218)MONTALVO

(T 1293)(Laguna 2 AZ)POINT MUGU

(W 1051)

(W 1051)

SANTA MONICA

(P 99 Reset)TOPANGA BEACH

(T 767)

B

Oxnard Plain

1989

DISTANCE (Km)

0 4 0 6 0 8 0 100
7

1978

1989

1989

5 0

-100

500

D H(mm) HEIGHT (m) -50

0

0

-150

(B 1218)MONTALVO

(L 1218)

(Z 1293)

(096 067330)

(Q 970)SAUGUS

(V 1218)

FILLMORE

D

DISTANCE (Km)

0 20 40 6 0 8 0 100

1978

5 0

-100

100

500

D H(mm) HEIGHT (m) -50

0

0

-150

Santa Monica Mountains(56 12)

(A 1141)(C 1452)CHATSWORTH

(Y 1134)

(I 31 Reset)

(K 1134)

MOORPARK(D 1134)

(P 1295)

(T 1293)POINT MUGU

C

1989 1989

TOPANGA BEACH(P 99 Reset)

Figure 2. Profiles showing terrain and height changes (Æh) with respect to bench mark Tidal 8 along four lines within or

adjacent to the epicentral region of the 1994 Northridge earthquake. Height-change segments devoid of comparative

heights shown by dotted line. (A) Los Angeles to Saugus route; the 1974-1978 comparison provides a small sample of

the very broadly defined regional deformation sustained over much of southern California during this interval. (B)

Santa Monica to Montalvo route. (C) Topanga Beach to Point Mugu via Chatsworth route. (D) Montalvo to Saugus

route.
8

from Tidal 8 is negligible. Owing to the

roughly comparable heights identified with

these marks (316 and 380 m) and the very short

reach (~25 km ) and approximately east-west

trend between them, we view these as

conservative assumptions--assumptions

supported by the generally small orthometric

closures around all of the circuits within the

epicentral region (Burford and Gilmore, 1984, p.

2-5)

Examination of the profiled

comparisons discloses two significant

discrepancies in the represented height

changes. The most obvious is that obtained by

comparing the height changes at Q 970, near

Saugus, derived from the results of levelings

eastward through Fillmore to Saugus (fig. 2D),

against those extending northwestward out of

Los Angeles (fig. 2A). The 1987/89 Los AngelesSaugus survey was constructed through joining

the results of 1987 leveling between Glendale

and Saugus with a 1989 line extending

northward from Tidal 8 to Glendale ( fig. 1).

Nonetheless, because the 1987 elevation

differences over the ~3-4-km reach between V

768 and N 1141 (fig. 2A) are very nearly an

exact overlay of those based on 1989 leveling

(an end-to-end difference of 0 mm with a

maximum departure from 0--midway along this

comparative reach--of only 2 mm), a reasonable

case can be made that even if the 1987 leveling

had begun at Tidal 8, the stability of 338 Reset

and uplift at Q 970 would have been about the

same as that shown here (fig. 2A). The height

change at Q 970 based on surveys northward

from Tidal 8 along the coast and thence

eastward from Montalvo, indicates that Q 970

subsided 29 mm during the period 1978-1989

(fig. 2D), whereas the 1978-1987/89 height

change obtained from levelings northwestward

from Los Angeles indicates that Q 970 rose by 47

mm. The second discrepancy of 55 mm emerges

from the results of levelings along the coast into

bench mark T 1293 (fig. 2B) and those over the

route between Topanga Beach, via Chatsworth

and Moorpark, into this same mark (fig. 2C).

While no two sets of levelings are apt to

produce exactly the same height changes a t

any indicated junction point, differences in

height changes of the order of millimeters or,

conceivably, a centimeter or two are more

reasonably anticipated over an area as limited

as that with which we are dealing here.

If the described discrepancies are well

above those expected with first-order work, we

are left with providing a reasonable

explanation for these discrepancies. Errors in

the reconstruction of the observed elevations

with respect to Tidal 8 leap to mind. We have

reexamined these reconstructions with

considerable care and have found no basis to

seize upon this explanation. The second

possible explanation is measurement error,

especially systematic error. However, as we

show above, neither rod nor refraction error can

reasonably explain the discrepancies in the

height changes at these two points. Systematic

accumulation of a magnetic deflection error

well above the empirically hindcast error in

the 1978 leveling along the coast is clearly a

possibility (see, for example, Castle and

Gilmore, 1994). Nevertheless, because the

surveys between Topanga Beach and Point Mugu

follow a line that is nearly orthogonal to

magnetic north, where any magnetic error

would have to accumulate in order to explain

the second discrepancy in particular, this

possibility is especially unlikely. Random

error seems an equally unlikely explanation for

either of these discrepancies. The estimated

random error in the signal at Q 970, based on

repeated levelings over the two separate routes

into this mark out of Tidal 8, is about ±22 mm.

The estimated random error in the signal at T

1293, based on repeated levelings over the two

alternative routes into this mark, again with

respect to Tidal 8 , is about ±20 mm

If neither systematic nor random error

afford particularly satisfactory explanations

for the discrepancies at Q 970 and T 1293, there

is always the possibility that these

discrepancies are attributable to simple

blunders or "busts" in the levelings leading into

these two marks. Or, as Schomaker and Berry

(1981, p. 3.5) correctly caution, "no unit is so

experienced [in geodetic leveling] that the

work performed by the group can be

automatically considered free of blunders."

Blunders are divided into "random"

and "systematic" categories by Schomaker and

Berry (1981, p. 3.6). Random blunders include:
9

(1) mistakes in reading or recording scale

values; (2) observing the rods in the wrong

order; (3) improper placement or leveling of the

instrument; (4) gross movement of the

instrument or turning points during the setups ;

(5) improper placement or plumbing of the rods;

and (6) gross movement of the forward point

between setups. Schomaker and Berry note that

the first four of their listed random blunders

may be detected and eliminated by using

double-scale rods in conjunction with a parallel

plate (optical) micrometer and computer

recording equipment, as was done in connection

with both the 1987 and 1989 leveling utilized

here. The blunders included under items 5 and 6

can be detected in hindsight only by comparing

repeated levelings over the same line.

Blunders of this type are disclosed as offsets

that carry through the remainder of the

compared line segment from the point of the

blundered observation. Because the repeated

levelings out of Tidal 8 into both Los Angeles

and Topanga Beach close within millimeters

(fig. 2), the likelihood of any blunders over

these line segments can be dismissed.

Similarly, comparisons of the repeated

levelings over the several profiles examined

here (fig. 2) disclose no detectable offsets.

Spikes or other indications of displacement

that "come back on datum" are clearly not in

the category of perpetuated offsets along the

compared lines.

"Systematic" blunders are, in effect,

one-time blunders that produce systematic

effects. They are attributable primarily or

entirely to procedural failures and include such

things as improperly performed collimation or

compensation checks. Systematic blunders are

not easily detected, although their effects may

be minimized by still other procedural

constraints. Barring collective incompetence,

systematic blunders are probably rare, since

they are apt to be detected by more than one

member of the survey party.

The misclosures around the two circuits

defined by the 1987 and 1989 levelings (fig. 3)

could be interpreted as evidence of survey

blunders. The misclosure around circuit A is

only slightly below limits for first-order, class

2 leveling (90.4 mm allowed for a 327-km

circuit) and that around circuit B again pushes

the allowed limit of 60.6 mm over a 147-km

circuit. Indeed, the existence of survey errors

(blunders) can always be asserted without the

possibility of direct disproof. Nevertheless,

because the instrumental and procedural

requirements built into both the 1987 and 1989

levelings are so restrictive, and because

independent evidence of their occurrence is

otherwise unrecognized, blunders are considered

very unlikey explanations for either of these

two misclosures.

There is, however, an alternative

explanation for both the cited discrepancies

and the indicated misclosures (fig. 3) that does

not fall back on survey error. Specifically,

while geodetic leveling is an inherently

accurate measurement system, its value is

compromised by one significant consideration: i t

is time consumptive. Should any movement

occur along the survey route during the course of

otherwise flawless leveling, it would be

expressed as a misclosure proportional to the

magnitude of the postulated movement.

 Accepting the plausibility of

intrasurvey movement during any given

leveling, we can easily explain the discrepancy

at Q 970 as deformation (differential

subsidence) northward from Glendale during

the the two-year interval 1987-1989. We can

also, but much less easily, explain the

discrepancy at T 1293 in much the same way.

That is, there was a three- to four-week delay

(late spring to early summer) between the

initiation of leveling at T 1293 that extended

eastward through Moorpark and Chatsworth,

and subsequent leveling into this mark along

the coast. In other words, we argue here that

during this very short interval T 1293 subsided

about 50 mm. Moreover, accepting this

explanation, it is very unlikely that this is an

expression of compaction-induced subsidence,

since the compaction rate at this mark would

have to have exceeded the previously

recognized maximum rate in the center of the

Oxnard Plain (Castle and others, 1977, p. 222)

by a factor of more than 3. While any

suggestion that aseismic, tectonically induced

vertical displacements operate with this

rapidity invites immediate skepticism, there

is growing evidence that such displacements do

occur, and, indeed, that the verticaldisplacement field in southern California is

both sharply episodic and oscillatory (see, for
1 0

LOS ANGELES

SAN PEDRO

TOPANGA

BEACH

TOPANGA

BEACH

POINT

MAGU

POINT

MUGU

MONTALVO

SANTA PAULA

MOORPARK

MOORPARK

CHATSWORTH

CHATSWORTH

SAUGUS

SAN FERNANDO

119¡15' 119¡00' 118¡45' 118¡30' 118¡15'

34¡30'

34¡15'

34¡00'

34¡00'

118¡30'

34¡15'

34¡30'

GLENDALE

T 1293

T 1293

1989

1989

1989

1987

1989

1989

V 768

N 1141

1989

Q970

A

B

PACIFIC OCEAN

TIDAL 8

1987

1989

0 10 20

KILOMETERS

1994 EPICENTER

- 71.0 mm

- 03.1 mm

- 74.1 mm

- 41.0 mm

- 01.2 mm

- 42.2 mm

Figure 3. Misclosures around selected circuits based on 1987/89 and 1989 levelings within or adjacent to the

epicentral region of the1994 Northridge earthquake. Upper figure identified with each misclosure is summation of

corrected observed-elevation differences around the circuit; figure on second line is summation of orthometric

corrections around the same circuit, where orthometric corrections are based on observed gravity. The sum of the two

preceding values, below the solid line, represents the orthometrically corrected misclosure around the indicated circuit

A. Combined 1987 and 1989 levelings (see Figure 1). B. 1989 levelings only.

example, Castle and Gilmore, 1992). By way of

illustration, the first of the two major

increments of uplift associated with the growth

of the southern California uplift, about 0.18 m,

had to have occurred sometime between the

late spring and early autumn months of 1961

(Castle and others, 1984). 

The likelihood that the two described

discrepancies are the products of intrasurvey

movement, where the considered survey in one

case extended over a two-year period and that

in the second over no more than about a month,

is supported by the misclosures around the two

critical circuits (fig. 3). The misclosure (fig.

3A) that necessarily involves any vertical

displacment at Q 970 during the interval 1987-

1989 sums clockwise around this circuit to -74.1

mm, a figure that is consistent in sense and

magnitude with subsidence of 76 mm at this

mark, as measured with respect to Tidal 8,

during the 1978-1987/89 interval. This

consistency is not a tautology, since the -76-mm

figure is derivative from a comparison between

the results of two successive surveys, whereas

the -74.1-mm value depends on the misclosure

obtained from a single, albeit discontinuous

survey. The contrarian view, and one that

should not be dismissed out of hand, argues that

the signal (the 76-mm subsidence) is simply an
1 1

expression of survey error that is virtually

demanded by the misclosure (fig. 3A). If this

argument is pursued, however, it would also

require that over 97 percent of the defining

error (the signal equivalent) be attributed to

the later leveling--certainly a possibility, but

one that challenges believability. If we reject

the contrarian view, the 1987/89 misclosure

(fig. 3A) testifies to both: (1) the high

probability that the deformation described

here occurred largely during the period 1987-

1989; and (2) a high level of accuracy

identified with the appropriately corrected

1978 leveling--against which the subsequent

1989 and 1987/89 levelings produced subsidence

at Q 970 that almost precisely matches that

deduced from the misclosure (fig. 3A). While

this misclosure supports the accuracy of the

1978 leveling, it provides no independent

support for the accuracy of the subsequent

levelings, since, for example, any increase in

the value of the 1987 observed elevation

difference between Glendale and Saugus would

enlarge both the comparative-based subsidence

and the misclosure by exactly the same amount.

The same, but much less definitive argument

can be developed from the misclosure given in

figure 3B. That is, this misclosure again

accounts in both sense and magnitude for a large

fraction of the subsidence (42.2 mm versus 55

mm) observed at bench mark T 1293.

The profiled height changes (fig. 2)

can be more readily grasped when displayed in

map form (fig. 4). In transferring the data from

profiled to contoured height changes, we have

deliberately ignored the profiled spikes as

nothing more than indications of bench mark

disturbance. Similarly, we have disregarded

zones of compaction-induced subsidence, ranging

upward from small pockets that straddle no

more than a bench mark or two to domains as

large as that in the Glendale area (California

State Water Rights Board, 1962, pl. 31) or that

astride the Oxnard Plain (fig. 2). Thus the map

presentation (fig. 4) constitutes a relatively

generalized characterization of height

changes, a characterization that clearly

involves an element of subjectivity. For

example, a feature that occurs along the south

flank of the Santa Monica Mountains, between

bench marks 56 12 and A 1141 (fig. 2C), that

could easily have been interpreted as a tectonic

signal is, in fact, simply an expression of

apparently continuing or reactivated movement

along the west side of a large landslide (Yerkes

and Campbell, 1980). In any case, were we to

attempt to show all of the recognized height

changes--regardless of origin--on the map, i t

would certainly diminish, if not destroy, the

coherence of the map. While the accuracy of

the contoured height changes is, among other

things, proportional to the distance of a given

point from the indicated survey route (fig. 1),

the contours are, nonetheless, fairly tightly

constrained. Accordingly, what finally

emerges on this map (fig. 4) is the closest

representation that the data permit of the

1978-1989 (or, more likely, 1987-1989) verticaldisplacement field that can be attributed to

apparently aseismic tectonic deformation

within the epicentral region of the

subsequently defined Northridge earthquake.

Several points are illustrated on the

map (fig. 4) that are certainly more evident

than could be gleaned from a casual

examination of the profiled height changes.

For example, the contoured height changes

spread out southeastward from their high in

the Moorpark-Chatsworth area, whereas they

tend to bunch up along the north flank of the

uplift. This north-south asymmetry athwart

the uplift, moreover, is a virtually inescapable

consequence of however the profiled data

might be contoured.

Interpretation

Accepting the subjectivity in

transferring the data from the several profiles

(fig. 2) to the map showing contoured height

changes (fig. 4), it is difficult to reject the

conclusion that the indicated verticaldisplacement field is somehow related to the

crustal disturbance identified with the 1994

earthquake. The spatial association with the

epicentral region is remarkably well defined,

even though the modeled coseismic uplift given

by Hodgkinson and others, (1996, accompanying

map) peaks some 20-25 km east of the zone of

maximum uplift shown in figure 4. The

temporal isolation of the deformational event

shown here (fig. 4) is still uncertain, but as we

indicate above, while it clearly fell within the

interval 1978-1989, the evidence disclosed by

the misclosures, even though unique, indicates
1 2

that all or nearly all of the relative uplift

occurred within a much shorter period--

namely, sometime betweeen 1987 and 1989 or

roughly 5-7 years before the earthquake.

to the modeled uplift astride deep slip on a

thrust fault (see, for example, Savage and

Hastie, 1966 or Freund and Barnett, 1976). Thus

a possible explanation for the observed uplift

338 Reset P 99 Reset

T 767

- 40

0

0

0

20

20

- 20

20

20

20

- 20

0

60

60

40

40

-40

- 20

LOS ANGELES

SAN PEDRO

TOPANGA

BEACH

POINT

MUGU

MONTALVO

SANTA PAULA

MOORPARK

CHATSWORTH

SAUGUS

SAN FERNANDO

119¡15' 119¡00' 118¡45' 118¡30' 118¡15'

34¡30'

34¡15'

34¡00'

34¡00'

118¡30'

119¡15' 119¡00'

34¡00'

34¡15'

34¡30'

GLENDALE

EXPLANATION PACIFIC OCEAN

Isobase of equal height change

Contour interval 20mm

20

C 1452

B 1218

T 1293

P 1296

V768

Q 970

T 1293

Bench Mark

TIDAL 8

1994 EPICENTER

0 10 20

KILOMETERS

Figure 4. Map showing 1994 epicenter, focal mechanism, and contoured height changes with respect to bench mark

Tidal 8 within the epicentral region of the Northridge earthquake. Focal mechanism from Thio and Kanamori (1996).

Based on both the focal mechanism for

the main shock (Thio and Kanamori, 1996) and

the aftershock distribution (Hauksson and

others, 1995), the seismogenic fault that

produced the 1994 earthquake dips to the south

at an angle of about 42°. Had the displacement

on this fault propagated to the surface, i t

would have broken ground along the north

flank of the uplift, precisely where the

preseismic contours tend to steepen (fig. 4). The

resultant deformational asymmetry through

the waist of the uplift (fig. 4) is similar in form

is preseismic slip on a down-dip extension of

the south-dipping fault that produced the

main shock.

Comparisons with at least two other

southern California examples of localized

aseismic deformation, followed by earthquakes

of small to moderate magnitude, lend

credibility to the interpretation of this event

(fig. 4) as a preseismic (effectively, a

precursive) event and offers some support for its

probable origin. The best example of localized
1 3

aseismic deformation recognized in southern

California, and reasonably interpreted as

precursive deformation, is that which preceded

the Mw6.7 San Fernando earthquake of 1971

(Castle and others, 1974). The San Fernando

example is more revealing in terms of the

temporal nature of the preseismic deformation,

but somewhat less spatially constrained, since

it depended on repeated levelings along what

were effectively only two lines. Two well

defined episodes of localized uplift were

recognized, both of which peaked as close to

the subsequently recognized epicenter as the

levelings permitted. The first occurred between

1964 and and 1965 (see, also, Castle and others,

1984, pl. 8A) and the second occurred during the

period 1968-1969. Thus this deformation

occupied a time frame preceding the

earthquake of about the same duration as that

which preceded the Northridge shock. There

may have been a still earlier event, one which

occurred sometime between 1961 and 1964.

However, there is a good deal of uncertainty as

to whether this event should be associated

with activity on the adjacent Santa Susana

fault, rather than the fault that produced the

1971 San Fernando earthquake. The spatial

dimensions of the pre-San Fernando events were

about the same or slightly less than those

associated with the Northridge deformational

event--although that in the east-west

direction can only be inferred from its extent

eastward from Saugus and a presumption of

rough symmetry. A significant and possibly

pertinent difference in the earthquakes

themselves, however, is in their depths.

While the Northridge was identified with a

relatively deep focus of about 18.4 km, the main

shock of the San Fernando earthquake occurred

at less than half that depth--about 8.4 km.

A second, and certainly less definitive

example of aseismic uplift preceding a southern

California earthquake, was that which

preceded the Mw5.3 Point Mugu shock of

February 1973 (Castle and others, 1977). The

seismogenic fault that produced this

earthquake strikes about N80°E and probably

dips about 36° to the north (Stierman and

Ellsworth, 1976). The nodal plane solution,

moreover, suggests oblique slip, a possibly

significant consideration in the present context.

The horizontal dimensions of the preseismic

uplift at Point Mugu are known in only one

direction, ~WNW, over which it extended no

more than about 25 km. Measured uplift with

respect to a control point west of Ventura

exceeded 30 mm and persisted through at least

the period 1971-1973 or, conceivably, the much

longer period 1969-1973. Thus the duration of

this anomaly could have been as long as 3.5 or

as short as 1.0+ years. While the measured

amplitude of the uplift was only about 30 mm,

allowance for likely compaction-induced

subsidence suggests that the tectonic signal was

closer to 40-50 mm. Although both the

preseismic and coseismic uplift peaked W N W

of the epicenter, the former by about 10 km and

the latter by perhaps no more than 2-3 km, both

are consistent with the notion of oblique slip a t

depth.

A third, relatively equivocal example

of aseismic uplift preceding a subsequently

recognized earthquake in southern California is

that which may have preceded the Mw5.9

Whittier Narrows earthquake of 1987. While

it seems likely that there was indeed a degree

of preseismic differential uplift within the

epicentral region of this shock, persisting

questions remain regarding its magnitude and

form, both of which are related to uncertainties

in rod (and, perhaps, other) error(s) in the 1975

and 1986 preseismic profile levelings (less

accurate than 3rd-order) carried out by the Los

Angeles County Road Department (Lin and

Stein, 1989). These errors were certainly large,

but where the errors in one set of rods (those

used in the 1975 survey) can be corrected with

some confidence, the rod corrections applied to

those used in the 1986 survey remain in doubt.

Other confusing issues are: (1) the magnitude of

any subsidence associated with operations in

the Montebello and Santa Fe Springs oil fields;

and (2) the existence of an unexplained,

abruptly defined subsidence trough between the

Montebello and Santa Fe Springs fields, a

feature that cannot be easily dismissed as

measurement error (Lin and Stein, 1989, p. 9629-

9631). Comparisons of repeated levelings along

the north-south control line--which are

probably the most revealing indices of

deformation during the intervals of concern

(1975-1986 and 1986-1987), since this line is

roughly orthogonal to the strike of the

presumed seismogenic fault plane (Shaw and
1 4

Shearer, 1999)--produce perplexing results,

whether one uses either the observed or

corrected data (Lin and Stein, 1989, p. 9628).

Based on the corrected data, and granting the

persuasiveness of the argument advanced by Lin

and Stein (1989, p. 9629) in support of the

reality of what they call the "Pico Rivera

hole", there is at least some possibility of a

0.1-ft (0.0305-m) blunder in the 1986 leveling

between two marks about 20-22 km from the

starting point of the north-south control line as

projected on a N-S azimuth. The removal of

this postulated blunder would eliminate the

abrupt definition of the northern boundary of

the "Pico Rivera hole." Alternatively, the

uplift "doublet" that appears in the 1975-1986

comparison may have resulted from either

successive aseismic slip events on the Puente

Hills fault or movement on two separate faults.

The dimensions of the preseismic deformation

field seem to be of the order of 30-35 km; the

preseismic differential uplift near the

epicenter may have been as great as 40-50 mm.

Both of these generalizations, however, are

clearly questionable.

Finally, we hesitate to dismiss this

subject without mentioning two other aseismic

events that preceded (but were not necessarily

linked to) subsequent nearby earthquakes.

These two examples share a common

consideration: the observations that permitted

their detection were confined to the footwalls

of the identified seismogenic faults. The first,

and probably most widely cited of such events,

is the uplift that preceded the 1964 Niigata

earthquake in Japan (Tsubokawa and others,

(1968). Because recognition of the localized

uplift, which peaked at about 50 mm, was

confined to the footwall, it is reasonably

interpreted, as Thatcher (1976, p. 695) pointed

out, as a second-order effect associated with

downwarping of the footwall block. The second

event consists of tectonic subsidence of the order

of 0.3-0.4 m in the footwall block of the White

Wolf fault during the period 1926-1931 (Castle

and Elliott, 1982, p. 7002-7003, 7011; Castle and

Yerkes, 1983, p. 36-39; Castle and others, 1987,

p. 64-67). While the geodetic control that led

to the detection of this subsidence is generously

characterized as sparse, its apparently sharp

definition invites speculation that it was

precursive to the 1952, Mw7.5 Kern County

earthquake. The occurrence of this earthquake

more than 20 years after the indicated tectonic

subsidence challenges belief in any association

between the two events. Nevertheless, if there

is in fact some correlation between the time

delay following the onset of an aseismic

deformational event and the magnitude of the

the resultant earthquake, the described

deformation and the 1952 earthquake may be

mechanically related in some as yet obscure

way.

Conclusion

Recognition of the differential uplift

that preceded the 1994 Northridge earthquake

is especially valuable, in our judgment, as an

example of the type of localized deformational

anomaly that can be expected to be disclosed

and defined with the deployment of a well

designed, dense array of continuously recording

GPS receivers in southern California. Most of

the uncertainties that are described in our

reconstruction of this event, especially in the

temporal domain, will largely disappear with

the anticipated installation of this network.

Secondly, the recognition of this event,

its dimensions and its probable duration,

together with what we have learned from the

detection of still other localized aseismic

anomalies preceding earthquakes in southern

California, suggests that a viable earthquake

warning system may be closer at hand than any

of us could have anticipated a decade ago.

Continuous GPS observations should be capable

of not only identifying anomalous

deformational zones potentially vulnerable to

near-term seismic activity, but might also

provide some indication of what the magnitude

of any postulated seismic event would be were

it to occur at any given time in the history of

the evolving anomaly--provided that it can be

independently established that the duration of

the anomalous event could be (or should be)

related to the magnitude of any resultant

earthquake.
1 5

Acknowledgments

We are especially indebted to Kathy

Koepsell of the National Geodetic Survey for her

generous assistance in the acquisition of the survey

data utilized in this report. Edward Riddle, Los

Angeles County Surveyor, provided both advice

and assistance in searching the geodetic record in

the epicentral region of the Whittier Narrows

earthquake. The illustrations were prepared by

John K. Nakata. We also thank James C. Savage

and Arthur F. McGarr for their meticulous and

constructive reviews of earlier versions of this

report.

References

Burford, R.O., and Gilmore, T.D., 1984, Vertical

crustal movements in southern California, 1974-

1978: U.S. Geological Survey Circular 905, 22 p.

California State Water Rights Board, 1962, The City

of Los Angeles vs. City of San Fernando:

California State Water Rights Board, Report of

Referee no. 650079, v. II, various pages.

Castle, R.O., Alt, J.N., Savage, J.C., and Balazs, E.I.,

1974, Elevation changes preceding the San

Fernando Earthquake of February 9, 1971:

Geology, v. 2, p. 61-66.

Castle, R.O. and Bernknopf, R.L., 1996, The

southern California uplift and associated

earthquakes: Geophysical Research Letters, v. 23,

p. 3011-3014.

Castle, R.O., Church, J.P., Elliott, M.R., and

Savage, J.C.,1977, Preseismic and coseismic

elevation changes in the epicentral region of the

Point Mugu earthquake of February 21, 1973:

Bulletin of the Seismological Society of America,

v. 67, p. 219-231.

Castle, R.O. and Elliott, M.R., 1982, The sea slope

problem revisited: Journal of Geophysical

Research, v. 87, p. 6989-7024.

Castle, R.O., Elliott, M.R., Church, J.P., and Wood,

S.H., 1984, The evolution of the southern

California uplift, 1955 through 1976: U.S.

Geological Survey Professional Paper 1342, 136 p.

Castle, R.O., Elliott, M.R., and Gilmore, T.D., 1987,

An early-20th-century uplift in southern

California: U.S. Geological Survey Professional

Paper 1362, 70 p.

Castle, R.O. and Gilmore, T.D., 1992, A revised

configuration of the southern California uplift:

Geological Society of America Bulletin, v. 104, p.

1577-1591.

Castle, R.O. and Gilmore, T.D.,1994, The selective

salvation of Zeiss Ni 1 levelings: U.S. Geological

Survey Open-File Report 94-698, 29 p.

Castle, R.O., Gilmore, T.D., Mark, R.K., and Shaw,

R.H., 1985, Empirical estimates of cumulative

refraction errors associated with procedurally

constrained levelings based on the GaithersburgTucson refraction tests of the National Geodetic

Survey: Geophysical Research Letters, v. 12, p.

239-242.

Castle, R.O., Mark, R.K. and Shaw, R.H., 1994, An

empirical assessment of refraction error in

leveling as a function of survey order and

environment: U.S. Geological Survey Bulletin

2114, 50 p.

Castle, R.O. and Yerkes, R.F., 1983, Historical

surface deformation near Oildale, California:

U.S. Geological Survey Professional Paper 1245,

42 p.

Craymer, M.R., Vanicek, Petr, and Castle, R.O.,

1995, Estimation of rod scale errors in geodetic

leveling, Journal of Geophysical Research, v. 100,

p. 15,129-15,146.

Federal Geodetic Control Committee, 1984,

Standards and specifications for geodetic control

networks: National Geodetic Information Branch,

NOAA, Silver Spring, MD, various pages.

Freund, L.B. and Barnett, D.M., 1976, A twodimensional analysis of surface deformation due

to dip-slip faulting: Bulletin of the Seismological

Society of America, v. 66, p. 667-675.

Geller, R.J., 1997, Earthquake prediction: are

further efforts warranted?: Geophysical Journal

International, v. 131, p. 425-450.

Hauksson, Egill, Jones, L.M., and Hutton, Kate,

1995, The 1994 Northridge earthquake sequence

in California: seismological and tectonic aspects:

Journal of Geophysical Research, v. 100, p.

12,335-12,355.

Hicks, S.D. and Crosby, J.E., 1974, Trends and

variability of mean sea level 1893-1972: National

Oceanic and Atmospheric Administration

Technical Memorandum NOS 13, 14 p.

Hicks, S.D., and Shofnos, William, 1965, Yearly sea

level variations for the United States: American

Society of Civil Engineers, Proceedings,

Hydraulics Division Journal, v. 91, n. 5, p. 23-32.

Hodgkinson, K.M., Stein, R.S., Hudnut, K.W.,

Satalich, Jay, and Richards, J.H., 1996, Damage

and restoration of geodetic infrastructure caused

by the 1994 Northridge, California, earthquake:

U.S. Geological Survey Open-File Report 96-517,

70 p.
1 6

Holdahl, S.R., 1982, Recomputation of vertical

crustal motions near Palmdale, California, 1959-

1975: Journal of Geophysical Research, v. 87, p.

9374-9388.

Holdahl, S.R., Strange, W.E., and Harris R.V. 1987,

Empirical calibration of Zeiss Ni-1 level

instruments to account for magnetic errors:

Manuscripta Geodetica, v. 12, p. 28–39.

Jackson, D.D., Lee, W.B., and Liu Chi-Ching, 1980,

Aseismic uplift in southern California: An

alternative interpretation: Science, v. 210, p. 534-

536.

Lin, Jian and Stein, R.S., 1989, Coseismic folding,

earthquake recurrence, and the 1987 source

mechanism at Whittier Narrows, Los Angeles

basin, California: Journal of Geophysical

Research, v. 94, p. 9614-9632.

Mark, R.K., Gilmore,T.D., and Castle, R.O., 1987,

Evidence of suppression of the unequal refraction

error in geodetic leveling: Journal of Geophysical

Research, v. 92, p. 2767-2790.

Mark, R.K., Tinsley, J. C., III, Newman, E.B.,

Gilmore, T.D., and Castle, R.O., 1981, An

assessment of the accuracy of the geodetic

measurements that define the southern

California uplift: Journal of Geophysical

Research, v. 86, p. 2783-2808.

Reilinger, R.E. and Brown, L.D., 1981, Neotectonic

deformation, near-surface movements and

systematic errors in U. S. releveling

measurements: implications for earthquake

prediction, in Simpson, D.W. and Richards, P.G.,

eds., Earthquake prediction, an international

review (Maurice Ewing Series, v. 4): Washington,

American Geophysical Union, p. 422-440.

Savage, J.C. and Hastie, L.M., 1966, Surface

deformation associated with dip-slip faulting:

Journal of Geophysical Research, v. 71, p. 4897-

4904.

Schomaker, M.C., and Berry, R.M., 1981, Geodetic

leveling: NOAA Manual NOS NGS 3, National

Oceanic and Atmospheric Administration,

Washington, (1-1)-(5-27) p.

Shaw, J.H. and Shearer, P.M., 1999, An elusive

blind-thrust fault beneath metropolitan Los

Angles: Science, v. 283, p. 1516-1518.

Stein, R. S., 1981, Discrimination of tectonic

displacement from slope-dependent errors in

geodetic leveling from southern California, 1953-

1979, in Simpson, D.W. and Richards, P.G., eds.,

Earthquake prediction, an international review

(Maurice Ewing Series, v. 4): Washington,

American Geophysical Union, p. 441-456.

Stein, R.S., Whalen, C.T., Holdahl, S. R., Strange,

W.E., and Thatcher, W.R., 1986, SaugusPalmdale, California field test for refraction error

in historical leveling surveys: Journal of

Geophysical Research, v. 86. p. 9031-9044.

Steirman, D.J., and Ellsworth, W.L., 1976,

Aftershocks of the February 21, 1973 Point Mugu,

California earthquake: Bulletin of the

Seismological Society of American, v. 66, p. 1931-

1952.

Strange, W.E., 1981, The impact of refraction

correction on leveling interpretations in southern

California: Journal of Geophysical Research, v.

86, p. 2809-2824.

Thatcher, W.R., 1976, Episodic strain accumulation

in southern California: Science, v. 194, p. 691-695.

____________, 1999, New strategy needed in

earthquake, volcano monitoring: EOS,

Transactions of the American Geophysical

Union, v. 80, p. 330-331.

Thio, Hong Kie and Kanamori, Hiroo, 1994,

Source complexity of the 1994 Northridge

earthquake and its relation to aftershock

mechanisms in Ta-Liang and Keiti Aki, eds.,

Special issue on the Northridge California

earthquake of January 17, 1994: Bulletin of the

Seismological Society of America, v. 86, n. 1, pt.

B, p. S84-S92.

Tsubokawa, I., Dambora, T., and Okada, A., 1968,

Crustal movements before and after the Niigata

earthquake in Kawasumi, H.,.General Report on

the Niigata earthquake of 1964: Tokyo Electrical

Engineering College Press, Tokyo, p. 129-139.

Yerkes, R. F. and Campbell, R.H., 1980, Geological

map of east-central Santa Monica Mountains,

Los Angeles County, California: U.S. Geological

Survey Miscellaneous Investigation series Map

I-1146.