MAPS SHOWING QUATERNARY GEOLOGY AND 
LIQUEFACTION SUSCEPTIBILITY, NAPA, CALIFORNIA, 
1:100,000 QUADRANGLE: A DIGITAL DATABASE

by 
Janet M. Sowers, Jay S. Noller, and William R. Lettis
William Lettis & Associates, Inc.


TEXT DERIVED FROM U.S. GEOLOGICAL SURVEY OPEN FILE REPORT 98-460




I.  PURPOSE OF THE PROJECT

Earthquake-induced ground failures such as liquefaction have historically brought loss of life and 
damage to property and infrastructure.  Observations of the effects of historical large-magnitude 
earthquakes show that the distribution of liquefaction phenomena is not random.  Liquefaction is 
restricted to areas underlain by loose, cohesionless sands and silts that are saturated with water.  
These areas can be delineated on the basis of thorough geologic, geomorphic, and hydrologic 
mapping and map analysis (Tinsley and Holzer, 1990; Youd and Perkins, 1987).  Once potential 
liquefaction zones are delineated, appropriate public and private agencies can prepare for and 
mitigate seismic hazard in these zones.

In this study, we create a liquefaction susceptibility map of the Napa 1:100,000 quadrangle using 
Quaternary geologic mapping, analysis of historical liquefaction information, groundwater data, 
and data from other studies.  The study is patterned after state-of-the-art studies by Youd (1973) 
Dupr and Tinsley (1980) and Dupr (1990) in the Monterey-Santa Cruz area, Tinsley and others 
(1985) in the Los Angeles area, and Youd and Perkins (1987) in San Mateo County, California. 

The study area comprises the northern San Francisco Metropolitan Area, including the cities of 
Santa Rosa, Vallejo, Napa, Novato, Martinez, and Fairfield (Figure 1).   Holocene estuarine 
deposits, Holocene stream deposits, eolian sands, and artificial fill are widely present in the region 
(Helley and Lajoie, 1979) and are the geologic materials of greatest concern.  Six major faults 
capable of producing large earthquakes cross the study area, including the San Andreas, Rodgers 
Creek, Hayward, West Napa, Concord, and Green Valley faults (Figure 1).





   





	Figure 1.  Location of the Napa 1:100,000 quadrangle in the San Francisco Bay area, 
showing active faults. Faults shown are those along which Holocene or historical displacement has 
occurred (Jennings, C. W., 1992).



II.  BACKGROUND

	A.  Liquefaction and liquefaction-induced ground failures

Liquefaction is the transformation of a granular material from a solid state into a liquefied state as a 
consequence of increased pore-pressure and decreased effective stress (Youd, 1973).  Increased 
pore pressures in unconsolidated sediments, especially on a regional scale in western California, 
are most likely seismically induced.  Observed types of ground failure resulting from liquefaction 
can include sand boils, lateral spreads, ground settlement, ground cracking and ground warping.

	B.  Liquefaction potential, susceptibility, and opportunity

The potential for liquefaction to occur depends on both the susceptibility of near-surface deposits to 
liquefaction, and the likelihood of ground motions to exceed a specified threshold level, or 
opportunity.  A liquefaction susceptibility map is based on the physical properties of near-surface 
deposits and depth to groundwater.  The liquefaction opportunity map is based on proximity to and 
activity of seismic sources.  If liquefaction opportunity is approximately uniform over the region 
the liquefaction susceptibility map essentially illustrates liquefaction potential.  The liquefaction 
susceptibility map of the Napa quadrangle presented here could be viewed as a liquefaction 
potential map.  Uniform liquefaction opportunity can be justified on the following grounds: (1) 
active faults capable of generating large-magnitude earthquakes are distributed throughout the study 
area (Figure 1); (2) no site is more than 30 km from an active fault capable of generating a 
magnitude 6.5 or larger earthquake; most sites are within 15 km of a fault; and (3) an earthquake 
on the San Andreas fault (Peninsula segment, 22% probability in 30 years), Hayward fault 
(northern segment, 28% probability), and Rodgers Creek fault (23% probability), will produce 
long duration ground motions in excess of 0.2g over most of the study area (USGS WGEP, 
1990).

Although we recognize that ground response is highly dependent on site specific variations in the 
duration (cycles), strength, and frequency (especially potential for amplified low frequencies) of 
ground motion, the assumption of uniform liquefaction opportunity is conservative and valid 
because most of these site dependent variations in ground motion tend to enhance liquefaction.  
Analysis of historical data by Tinsley and others (1985) shows that liquefaction has occurred up to 
20 km from the epicenter of a M 6.5 earthquake, and up to 50 km in a M 7.0 earthquake.  The 
1989 Loma Prieta earthquake dramatically illustrated the potential for liquefaction at great distances 
from the epicenter given appropriate geologic and hydrologic conditions and amplification of 
ground motion at lower frequencies.  The assumption of relatively uniform liquefaction 
opportunity was also made by Tinsley and others (1985) for the Los Angeles region. 

III.  METHODS

We assess liquefaction susceptibility on the basis of three factors: (1) presence of loose, 
cohesionless, sandy or silty deposits within 50 feet of the surface (depth threshold defined by 
Tinsley and others (1985)), (2) presence of groundwater which saturates these deposits, and (3) 
historical records of liquefaction during previous earthquakes (data compiled by Youd and Hoose 
(1978)).  Our procedure for assessing these factors is shown on Figure 2.



	Procedure 
	
	1.  Map surficial deposits on the basis of age and depositional environment.

	2.  Determine groundwater levels.

	3.  Examine historical liquefaction occurrences.

	4.  Develop criteria matrix to classify all potential combinations of type and age of deposit 	
		with groundwater depth.  Calibrate with historical data and comparison with criteria 
		matrix of Tinsley and others (1985).

	5.  Assign liquefaction susceptibility categories to map units.


	Figure 2.  Procedure for developing liquefaction susceptibility map



Previous studies document a correlation between the origin and age of a deposit and its tendency to 
liquefy (Tinsley and others, 1985; Tinsley and Holzer, 1990).  Age is important because deposits 
become more consolidated, weathered, and cemented with age.  Depositional environment is 
important because each environment is characterized by deposits with different sorting, bedding 
and grain-size characteristics.  For example, river channel deposits are likely to contain sand and 
silt, and young (i.e., late Holocene age) deposits are likely to be loose and cohesionless.  
Categories of age and environment distinguished in this study are listed in Figure 3 and map units 
are correlated in Figure 4.

In mapping the Quaternary geology we first interpreted aerial photographs and topographic maps to 
determine the depositional environment and to estimate relative age by evaluating landforms and 
geomorphic relationships.  Second, we interpreted published soil survey data to further assess the 
character and the age of near-surface deposits.  The soil surveys, published by the Soil 
Conservation Service (SCS), provide information on composition, soil profile development, and 
available moisture (e.g., depth to groundwater) of the surface deposits (Bates, 1977, Lambert and 
Kashiwagi, 1978, Kashiwagi, 1985; Miller, 1972; and Welch, 1977).  Soil profile development 
was used to estimate the ages of soils and surficial deposits.  Geologic unit descriptions contain 
information on soils characteristics of each unit (Table 1).  Additional data on the distribution of 
Quaternary alluvial and flatland deposits (including marine and estuarine mud) were compiled from 
published sources (Dibblee, 1981a and 1981b; Huffman and Armstrong, 1980; Helley and Lajoie, 
1979; Galloway, 1978, Sedway and Cooke, 1977; Frizzel and others, 1974; Fox and others, 
1973; Sims and others, 1973; Nichols and Wright, 1971; Kunkel and Upson, 1960; Thomasson 
and others, 1956 and 1960; and Cardwell, 1958).



	Age						Depositional environments

	Latest Holocene (<1000 yrs) (Qh, Qhi)	Stream channel and terrace (t)
	Earlier Holocene (Qh)				Alluvial fan (f)
	Late Pleistocene to Holocene (Q)		Alluvial basin (b)
	Late Pleistocene (Qp)				Undifferentiated alluvial (a)
	Early to middle Pleistocene (Qo)		Estuary (r)
							Beach and dune (s)
							Marine terrace (m)
							Artificial fill (af)


	Figure 3.  Categories of age and depositional environment distinguished in Quaternary 	
	geologic mapping.Abbreviations used in unit designations are shown in parentheses.


Age
Unit

Historical

af
Qhbm
Qhay
Qhay
/Qhb











Holocene





Qhs
Qha
Qht
Qhf
Qhb






Late 
Pliocene





Qpmt
Qpa
Qpt
Qpf

Qs
Qa
Qt
Qf
Qb

Early to 
middle 
Pleistocene





Qomt
Qoa









Pre-
Quaternary


br














	Figure 4.  Correlation of geologic map units

Data on the depth to groundwater were acquired in two ways.  Most data were taken from boring 
logs recorded for geotechnical studies.  These logs were obtained from the California Department 
of Transportation, and from reports on file with county and city governments in the study area.  
The remainder of the data were collected in the field during spring 1992 by measuring the depth to 
the water surface in streams, creeks, and drainage ditches below the adjacent terrace or fan surface.  
In making these measurements, we assumed that the stream level was representative of the level of 
shallow groundwater in the local area.  Data from water wells were not utilized because these wells 
tap deep, in many places artesian aquifers, thus observed water levels represent the potentiometric 
surface of the aquifer, not the depth below which the ground is saturated.

Liquefaction susceptibility units were designated on the basis of a criteria matrix that correlates the 
geologic unit (type and age of the deposit) with groundwater levels (Table 3).   Liquefaction 
susceptibility units reflect the probable presence of saturated, loose, unconsolidated, sandy 
materials, within 50 feet of the surface.  The matrix is calibrated using Tinsley and others (1985) 
quantitative analysis of borehole data in the Los Angeles basin, and historical liquefaction data for 
the study area (Table 2).  Tinsley and others (1985) analyzed blow count data and lithologic logs 
for approximately 4000 borings to develop a criteria matrix for loose, cohesionless alluvium of 
latest Holocene, earlier Holocene, and late Pleistocene ages as a function of groundwater levels 
(Figure 2).   Extensive borehole analysis was beyond the scope of this study, thus we rely on the 
analysis of Tinsley and others (1985) as a general guide.  Geologic units known to have liquefied 
in previous earthquakes are as a rule assigned to the "Very High" liquefaction susceptibility unit.  
Historical occurrences of liquefaction in the study area are shown in Table 2, together with the 
geologic map unit that appears to have liquefied.

IV.  DATA

	A.  Quaternary geology

Quaternary deposits in the study area (Figures 3 and 4, Plate 1) occur in four general settings: (1) 
large northwest-trending valleys, (2) small intermontane valleys, (3) estuaries, and (4) coastal 
environments (Plate 1).  The large northwest trending valleys, such as Napa Valley, Sonoma 
Valley, and Santa Rosa-Petaluma Valley, and the somewhat smaller Olema, Vaca, and Suisun 
valleys, contain sediments deposited by streams on flood plains, alluvial fans, and basins.  
Sedimentary deposits in small intermontane valleys are similar in type to those of the larger valleys 
but are thinner, and smaller in areal extent.  Estuaries contain both sediments dumped by streams 
as they enter bays or the ocean, and fine-grained sediments distributed by slow currents at the 
margins of bays.  Coastal depositional environments include beaches, eolian dunes, and marine 
terraces. 

	B.  Groundwater

The depth to saturated deposits in areas underlain by Quaternary alluvial, estuarine, and beach 
sediments is generally less than 10 feet throughout most of the study area. The free groundwater 
table is at sea level near the bays and ocean, and increases gradually with distance up the valleys.  
Within a valley, groundwater depth is generally shallowest near the axis of the valley and deepens 
gradually toward the flanks.  A sharp increase in depth occurs near the contact between valley 
alluvium and bedrock along the valley margins.  Small alluviated valleys and pockets within the 
bedrock hills also appear to have fairly shallow groundwater, generally less than 10 feet.  Soils 
characteristic of wet environments are mapped in many of these valleys, and the few data available 
on depth to groundwater indicate shallow groundwater levels.

Marine terraces and dune sands commonly have significantly greater depths of groundwater than 
other Quaternary deposits in the study area.  Groundwater beneath marine terraces is generally 
deeper than 40 feet, the minimum elevation of these terraces, except where water is perched.   
Groundwater beneath coastal dunes which form or mantle hills can be as deep as 50 to 100 feet, 
the elevation of the hills.


	C.  Historical liquefaction

Records of liquefaction in the study area are available for four earthquakes: the 1906 San Francisco 
earthquake (m=8.3), the 1892 Winters earthquake (m=6) (Unruh and Moores, 1992), the 1898 
Mare Island earthquake (m=6.6) (Topozada, 1992), and the 1969 Santa Rosa earthquake (m=5.7).  
These records chronicle observations and contain data on ground failures consistent with 
liquefaction, which are used as calibration points for this study.  Data were compiled primarily 
from Youd and Hoose (1978).  Individual sites of known liquefaction are tabulated in Table 2 and 
are shown on Plate 2 differentiated as to type of failure.

Historical liquefaction most commonly occurred in Holocene estuarine sediments (11 reports + 2 
questionable).  These sediments are young, saturated, and contain silt and sand, especially near the 
mouths of rivers.  The second most common deposit to liquefy was artificial fill (7 reports + 1 
questionable); especially fill overlying estuarine sediments.  It is not known to what extent the 
failures were in the fill itself or in the estuarine deposits beneath the fill.  Third most common was 
latest Holocene alluvial deposits (3 reports + 1 questionable), and fourth was Holocene terrace 
deposits (2 reports + 3 questionable).  Less than 2 incidents each were reported in Holocene fan 
deposits (Qhf), late Pleistocene to Holocene alluvium (Qa), Holocene sand dunes (Qhs), and 
Holocene basin sediments (Qhb).


	D.  Liquefaction susceptibility units

Tinsley and others (1985) defined liquefaction susceptibility units based on quantitative evaluation 
of SPT data from boreholes in the Los Angeles area.  Deposits marked Very High and High are 
expected to liquefy in an earthquake of magnitude 6.5 or greater.  Deposits marked Moderate are 
expected to liquefy in a magnitude 8 event but not a magnitude 6.5 event, and deposits marked 
Low or Very Low will not liquefy, even in a magnitude 8 earthquake.  Our map units, because 
they are based in part on the criteria matrix developed by Tinsley and others in the Los Angeles 
study, can be interpreted in a similar manner.

We expect that 80 percent of future liquefaction will take place in areas marked High or Very High.  
The experience of Dupr and Tinsley (1990) in the Monterey area is that the areas that liquefied in 
the 1906 San Francisco earthquake are the same areas that liquefied in the 1989 Loma Prieta 
earthquake.  We expect that 20 percent of future liquefaction will take place in areas marked 
Moderate, Moderate-High, Low-High, and Moderate-Low, and that less than 1 percent will take 
place in areas marked Low or Very Low.


V.  SUMMARY

A liquefaction susceptibility map was created for the Napa 1:100,000 quadrangle on the basis of 
Quaternary geologic mapping, groundwater levels, and historical occurrences of liquefaction.   The 
Quaternary map differentiates deposits on the basis of age and depositional environment -- two of 
the criteria necessary to locate loose cohesionless sands and silts.  Age is evaluated because 
deposits become more compact (dense) and cemented with age.  Depositional environment is 
evaluated because each is characterized by deposits with different sorting, bedding, and grain 
sizes. 

A criteria matrix was developed that matches age and type of deposit with groundwater levels to 
yield relative liquefaction susceptibility.  The criteria matrix is calibrated with data of Tinsley and 
others (1985) and with historical liquefaction occurrences.  Sediments most susceptible to 
liquefaction in the study area are estuarine deposits, artificial fill emplaced over estuarine 
sediments, and latest Holocene stream deposits.  Other susceptible deposits include Holocene 
stream terrace deposits, Holocene beach and dune sands, Holocene undifferentiated alluvium, 
Holocene basin deposits, and undifferentiated late Pleistocene to Holocene alluvium.

This map shows general conditions in the region for planning purposes only and cannot be used to 
assess the presence or absence of liquefiable sediments for specific sites.  Site-specific geotechnical 
investigations must be conducted to make that assessment.  Communities within or immediately 
adjacent to areas over 1 km2 in size having high or very high liquefaction susceptibility include 
Petaluma, Santa Rosa, Sebastopol, Novato, Napa, Yountville, Pinole, Benicia, Martinez, Vallejo, 
Fairfield, Cordelia, Suisun City.  Communities within or immediately adjacent to areas over 1 km2 
in size having moderate to high liquefaction susceptibility Santa Rosa, Cotati, and Rohnert Park.


ACKNOWLEDGEMENTS

We thank the various people and agencies who provided access to borehole logs and other 
information on local geology.  These include the California Department of Transportation, Wayne 
Wirick of the City of Sonoma, Sue Kelley of the City of Sebastopol, Roland Brust and Joe 
Gaffney of the City of Rohnert Park, Maurice Magsamem of the City of Santa Rosa, Janette Alm 
of CH2MHill, Anthony Quicho of the City of Martinez, Albert Hurd of the City of Fairfield, 
Gilbert Yau of the City of Vallejo, and Don Cardwell of the City of Napa.

Assistance in field work and map compilation was provided by Jeff Unruh, Meg Schram, Steve 
Thompson, Tina Wickens, Raymond Huang, and Colleen Haraden of William Lettis & Associates, 
Inc.  We thank Edward Helley for assistance in obtaining reports, maps, and photography of the 
study area, and John Tinsley and Chuck Real for helpful discussions.  Earl Brabb provided 
guidance during the review process and made the USGS publication of this map possible.  John 
Tinsley and Edward Helley reviewed the manuscript.

This research was supported by the U. S. Geological Survey (USGS), Department of the Interior, 
under USGS award number 14-08-0001-G2129.  The views and conclusions contained in this 
document are those of the authors and should not be interpreted as necessarily representing the 
official policies, either expressed or implied, of the U. S. Government.  


Table 1.  Description of geologic units

  Map 
 symbol	Unit name and description



   af	Artificial fill.  Material constructed or deposited by humans.  Fill may be 
engineered or non-engineered material.  Most artificial fill in the study area overlies 
estuarine sediments and forms new land, levees, or dikes near sea level (Goldman, 
1969).  Thickness of the fill overlying estuarine sediments is typically 5-20 feet 
thick.  Other fill includes large highway embankments, consisting of engineered fill 
up to approximately 100 feet thick.
	Liquefaction susceptibility may be very high to very low depending on (1) the 
nature and thickness of the fill materials, (2) whether the fill was engineered or non-
engineered, and (3) its depth of saturation.  Most fill emplaced in the last few 
decades is engineered; older fill is less likely to be engineered.  Many of the reports 
of damage in the 1906 San Francisco earthquake involved failures in fill that 
probably was not engineered.  We judged that parcels of fill located within estuaries 
might have very high liquefaction susceptibility because water levels are close to the 
surface, there is a significant possibility fill might not be engineered, and the fill 
may be a relatively thin cover over very susceptible estuarine sediments.  Artificial 
fill comprising the large highway embankments mapped in the Vallejo area is 
presumed to be engineered, fairly thick, and have groundwater below 30 feet, thus 
to have very low susceptibility to liquefaction.  Site specific studies should be 
conducted to determine the condition and liquefaction susceptibility of any artificial 
fill.

   Qhr	Estuarine deposits (bay mud).  Sediments deposited in a tidal marsh, estuary, 
delta, or lagoon.  The water table is very high, usually at the surface.  Sediments 
are silts, fine sands, peats, and clays and include materials mapped by Helley and 
Lajoie (1979) as bay mud.  The limits of this unit were compiled chiefly from 
mapping of historical tidal marshlands by Nichols and Wright (1971).  Soils are 
histosols, aquic entisols or mollisols.  Age is Holocene with most areas being late 
Holocene and subject to modern deposition and flooding.  Some areas are diked 
and drained for farming; the water table in these areas is artificially lowered to 
several feet below the surface.
	Liquefaction susceptibility is very high.  Historical reports (Table 2) indicate that 
liquefaction of these sediments was widespread during past earthquakes.  Estuarine 
sediments near the mouths of major streams (Napa River, Petaluma River, Suisun 
Creek, Olema Creek, and Sonoma Creek) are probably the most susceptible 
because the streams regularly deliver large volumes of sand and silt to the estuary.  
Three miles of train track reportedly sank in the marsh near the mouth of Suisun 
Creek (Table 2, location 254) during the 1906 San Francisco earthquake.

   Qhr/af	Estuarine deposits with areas of artificial fill.  Complex of estuarine 
sediments and bodies of artificial fill that are too small to delineate at map scale.  
Liquefaction susceptibility is very high.

   Qhi	Latest Holocene alluvial deposits.  Fluvial sediments judged to have been 
deposited in the last approximately 1000 years.  Includes modern flood plains, 
active stream channels, active alluvial fans, flood-prone areas, and areas known to 
have flooded historically.  Does not include flood-prone areas characterized by 
erosional topography.  These areas, though occasionally flooded, are eroding faster 
than aggrading, thus would not be underlain by significant thicknesses of recent 
flood deposits.  Sediments are loose sands, gravels, silts and clays.  Typical soils 
developed on these sediments are fluvents.  This mapping unit does not include 
basin or estuarine sediments, which tend to be more fine-grained and cohesive. 
	Liquefaction susceptibility is high to very high.  Tinsley and others (1985) present 
an analysis of borehole data in the Los Angeles area which shows that 76-81 % of 
boreholes in latest Holocene alluvium contain liquefiable materials, assuming water 
levels at the surface, compared to 34-54 % of boreholes in earlier Holocene 
alluvium.  Deposits are designated "Very High" if historical liquefaction has taken 
place, e.g. the flood plain deposits of Olema Creek, Petaluma Creek, and Putah 
Creek (Table 2).  

   Qhi/Qhb	Latest Holocene flood plain and basin deposits along Laguna de Santa 
Rosa.  The area is a complex of flood plain, basin, and marsh depositional 
environments.  Water draining from the Cotati-Rohnert Park basin and Santa Rosa 
Creek collects in this area and moves northward to the Russian River.  The water 
table is close to the surface.  Liquefaction susceptibility is high.

   Qhs	Holocene dune and beach sand.  Beach and beach-derived dune sand in 
predominantly coastal environments.  Beach sediments are well-sorted fine to 
coarse sands with some gravel.  Where the beach is adjacent to a seacliff, beach 
sediments probably form a veneer 1 to 10 feet thick over a bedrock platform.  Dune 
sands are very well-sorted fine to medium sands.
	In areas of high groundwater or perched water conditions, liquefaction 
susceptibility is high to very high.  Where sand boils occurred in Holocene sand 
dunes near Dillon Beach during the 1906 earthquake (Table 2, location 280), 
deposits are designated "very high".  Sand of Limantour spit, where sands are 
probably thicker than sands veneering a bedrock platform, are also designated "very 
high".

   Qhb	Holocene basin deposits.   Sediments of late Holocene age deposited in 
topographic lows.  These areas have a high water table and are subject to flooding.  
Sediments are more fine-grained than flood plain and fan sediments because basins 
collect standing water allowing clays to deposit.  Typical soils are vertisols and 
aquic mollisols.
	Liquefaction susceptibility is moderate to high.  Although these sediments contain 
abundant clay, they may also contain layers of sand and silt.  For example, the 
Cotati-Rohnert Park basin is covered over most of its area by a layer of black clay 
several feet thick.  Shallow boreholes show, however, that sands and silts are 
present intermittently below the black clay.  Of the 32 borehole logs available, 10 
record sands, silty sands, or sandy silts that are at least 3 feet thick, saturated, and 
have blow counts under 20 blows per foot.  In a fluvial environment we expect the 
distribution of sands to be intermittent and seemingly random.  Therefore we 
assume that layers of liquefiable materials could be present anywhere in the basin.  
Their relative abundance should be less in this basin environment than in a flood 
plain environment because of the greater proportion of clays in basin sediments, 
therefore these and other basin sediments are assigned moderate to high liquefaction 
susceptibility, instead of high susceptibility.

   Qht	Holocene terrace deposits  Stream-terrace deposits of early Holocene to 
present age deposited as point bar and overbank deposits by major streams such as 
the Napa River.  Although terrace deposits are also found along smaller streams, 
these are generally too small in extent to be shown at this map scale and therefore 
are included in Qa and Qha mapping units.  Terrace sediments include sand, gravel, 
silt, with minor clay, moderately to well-sorted, and moderately to well-bedded.  
Soils are typically entisols, inceptisols, and mollisols.
	Liquefaction susceptibility is high where groundwater is within 10 feet of the 
surface, and moderate where it is between 10 and 30 feet of the surface.  Three 
areas have very high liquefaction susceptibility because they have liquefied 
historically (Table 2, locations 266, 282, and 283).  All are located along coastal 
streams within 2 km of the estuary boundary.

   Qhf	Holocene fan deposits.  Holocene alluvial fan sediments, deposited by streams 
emanating from the mountains as debris flows, hyperconcentrated mudflows, or 
braided stream flows.  Sediments include sand, gravel, silt, and clay, that are 
moderately to poorly sorted, and moderately to poorly bedded.    Some Holocene 
fans exhibit levee/interlevee topography, particularly the Suisun Creek and Santa 
Rosa Creek fans.  The levees are identified as long, low ridges oriented down fan, 
and contain coarser material than adjoining interlevee areas. 
	Liquefaction susceptibility is moderate where groundwater is within 10 feet of the 
surface, and low to very low elsewhere.  Fan deposits are judged to be less 
susceptible than terrace deposits (Qht) of the same age due to poorer sorting and 
coarser grain size.  
	This unit includes active stream channels which are too narrow to show separately 
at this map scale.  Sediments in the stream channel are dominantly gravel and sand.  
Liquefaction susceptibility in these channels, especially point bar deposits, is high.

	The City of Santa Rosa sits on the Santa Rosa Creek fan; one of the larger 
Holocene fans in the study area.  The fan forms a wedge of sediment overlying the 
older alluvium (Qoa) identified as the Glen Ellen Formation (Cardwell, 1958), and 
exhibits strong levee/interlevee topography.  Santa Rosa Creek is at present incised 
as much as 25 feet into fan sediments near the fan apex, thus sediments are judged 
to be pre-historical.  
	Liquefaction susceptibility is moderate, however, local areas may have greater 
susceptibility.  The downtown Santa Rosa area sustained considerable damage in 
the 1969 Santa Rosa earthquake.  Eugene Miller (in Steinbrugge and others, 1970) 
attributes the severity of the damage partly to amplification of ground motions by 
soft alluvial materials underlying central Santa Rosa.  Reports of ground cracking, 
settlement, along Santa Rosa and Matanzas Creek and in other parts of Santa Rosa 
in the 1969 Santa Rosa and 1906 San Francisco earthquakes (Table 2) suggest that 
liquefaction hazards in the Santa Rosa area should be further evaluated.

   Qha	Holocene alluvium, undifferentiated.  Alluvium of Holocene age, deposited 
in fan, terrace, or basin environments.  Unit is mapped where separate types of 
deposits could not be delineated either due to complex interfingering of depositional 
environments or the limited size of the area.
	Liquefaction susceptibility is moderate to high, where groundwater is within 10 feet 
of the surface.  This assignment is based on a combination of the susceptibilities of 
fan, terrace, and basin sediments (Qhf, Qht, and Qhb)

   Qs	Late Pleistocene to Holocene dune sands.  Very well-sorted fine to medium 
eolian sands derived from beach sediments in coastal environments.  Holocene 
sands discontinuously overlie late Pleistocene sands, both of which form a mantle 
of varying thickness over marine terraces and bedrock hills.  Depth to free 
groundwater varies, primarily with the height of the dune, but is generally greater 
than ten feet.
	Liquefaction susceptibility is generally low, but may be high locally where 
groundwater is shallow and sands are Holocene in age.

   Qt	Late Pleistocene to Holocene terrace deposits.  This unit is mapped on 
relatively flat undissected stream terraces where late Pleistocene or Holocene age 
was uncertain or where the deposits of different age interfingered such that they 
could not be delineated at the map scale.  Soils are typically inceptisols, mollisols, 
and alfisols.  Groundwater depth is variable, but is generally less than 30 feet.  
Liquefaction susceptibility is low to high, where groundwater is within 10 feet of 
the surface.  The range in susceptibility is a reflection of the range or uncertainty in 
age of the terrace deposits.
	This unit includes active stream channels which are too narrow to show separately 
at this map scale.  Sediments in the stream channel are dominantly gravel and sand.  
Liquefaction susceptibility in these channels, especially in point bar deposits, is 
high.

   Qf	Late Pleistocene to Holocene fan deposits.  This unit is mapped on gently 
sloping, fan-shaped, relatively undissected alluvial surfaces where late Pleistocene 
vs Holocene age was uncertain or where the deposits of different age interfingered 
such that they could not be delineated at the map scale.  Sediments include sand, 
gravel, silt, and clay, that are moderately to poorly sorted, and moderately to poorly 
bedded.  Soils are typically inceptisols, mollisols, and alfisols.  Groundwater depth 
is variable, but is generally less than 30 feet.  Liquefaction susceptibility is low-to-
moderate where groundwater is within ten feet of the surface.
	This unit includes active stream channels which are too narrow to show separately 
at this map scale.  Sediments in the stream channel are dominantly gravel and sand.  
Liquefaction susceptibility in these channels, especially in point bar deposits, is 
high.

   Qa	Late Pleistocene to Holocene alluvium, undifferentiated.  This unit is 
typically mapped in small valleys where separate fan, basin, and terrace units could 
not be delineated at the map scale, and where Holocene or Pleistocene age was 
uncertain.  The unit includes flat, relatively undissected fan, terrace, and basin 
deposits, and small active stream channels.  Groundwater depth is variable, but is 
generally less than 30 feet.  Liquefaction susceptibility is low-to-high, where 
groundwater is within ten feet of the surface.  The wide range in liquefaction 
susceptibility is a reflection of uncertainties and local variability in the both the 
nature and age of the deposits.

   Qpt	Late Pleistocene terrace deposits.  This unit is mapped on relatively flat 
undissected to slightly dissected stream terraces where a late Pleistocene age is 
indicated by the development of alfisols.  For example, the extensive terraces 
bordering Sonoma Creek are capped by alfisols and are incised such that they no 
longer aggrade.  Liquefaction susceptibility is moderate where groundwater is 
within ten feet of the surface, and low to very low elsewhere.  
	Small inset terraces and the active channel deposits of Sonoma Creek are also 
included in this unit because they are too narrow to delineate at the map scale.  
These deposits are Holocene to present in age and consist of unconsolidated sand 
and gravel, thus have high liquefaction suceptibility.

   Qpf	Late Pleistocene fan deposits  This unit is mapped on gently sloping, fan-
shaped alluvial surfaces where late Pleistocene age is indicated by slight dissection 
and/or the development of alfisols. 
	Liquefaction susceptibility is low where groundwater is within ten feet of the 
surface, and is very low elsewhere.

   Qpa	Late Pleistocene alluvium, undifferentiated  This unit is mapped on gently 
sloping to level alluvial fan or terrace surfaces where a late Pleistocene age is 
indicated by slight dissection and/or the development of alfisols. 
	Liquefaction susceptibility is moderate-to-low where groundwater is within ten feet 
of the surface, and is low to very low elsewhere.

   Qpm	Late Pleistocene marine terrace deposits.  Deposits on uplifted marine 
abrasion platforms along the Pacific coast and the margin of Tomales Bay.  Some 
terraces along the margin of Tomales Bay may be partly alluvial in origin (Grove 
and Neimi, 1992).  The terraces are 40-160 ft above sea level.  Sediment veneer on 
the platform is typically greater than 10 feet thick and consists of moderately to 
well-sorted, moderately to well-bedded sands and gravels, which may locally be 
fossiliferous.  Groundwater is generally deeper than 20 feet, though areas may have 
perched groundwater where the marine sediments overlie relatively impermeable 
bedrock.  Liquefaction susceptibility is generally very low over most of the terraces 
but is designated low in this study to account for areas of perched groundwater.

   Qoa	Early or middle Pleistocene fan or terrace deposits.  Moderately to 
deeply dissected alluvial deposits capped by alfisols, ultisols, or soils containing a 
silica or calcic hardpan.  Unit includes areas of the Llano de Santa Rosa mapped by 
Cardwell (1958) as the Glen Ellen Formation of late Tertiary to early Pleistocene 
age.  Liquefaction susceptibility is very low. 

   Qom	Early to middle Pleistocene marine terrace deposits.  Gently undulating, 
dissected marine terraces near Point Reyes Station.  Additional older terraces, not 
mapped in this study, are also present along the western flank of Bolinas Ridge 
(Keenan, J. L., 1976).  Both mapped and unmapped terraces may be partly alluvial 
in origin (Grove and Neimi, 1992).  Liquefaction susceptibility is very low.

   br	Pre-Quaternary deposits and bedrock, undifferentiated.  Primarily 
Jurassic to Pliocene sedimentary and volcanic rock and sediments.  Unit also 
includes landslides, other bodies of colluvium, and small stream channels in 
bedrock that could not be delineated at the map scale.  Liquefaction susceptibility is 
very low, except in stream channels where it may be low to high. 

   w	Water.  This unit includes lakes, reservoirs, bays, ponds, and sea.

Table 2.  Historical occurrences of liquefaction in the study area correlated with 
probable geologic units affectedGeologic units are queried where uncertainty in the exact 
location of the liquefaction occurrence led to considerable uncertainty identifying the geologic unit 
affected.  Locations of occurrences are shown on Plate 2.  Location numbers are from Youd and 
Hoose (1978).  See Youd and Hoose (1978) and Steinbrugge and others (1970) for historical 
quotations which further describe each location and the nature of failure.  

Location 
# (plate 
2)
Year of 
earthquake
Geologic unit(s)
(probable)
Nature of failure





250

1906

Qhr/af

ground settlement


251
1906, 1898
af over Qhr
miscellaneous effects


252
1906
Qhr, af over Qhr
lateral spread


254
1906
Qhr
ground settlement


255**
1892
Qha? Qhf? Qf? Qa?
ground cracks


256**
1892
Qhi, Qht
Qha?
sand boils, ground cracks, disturbed 
wells, streambank landslides


260
1906
Qhr
lateral spread


264
1906
Qht
ground settlement


266
1906
Qhi, Qhr, af over 
Qhr*, Qht
ground cracks, lateral spread, ground 
settlement


267
1906
Qhr, af over Qhi*
ground cracks, lateral spread, ground 
settlement


269
1906
Qhr, af over Qhr*
ground settlement


270
1906
Qhr
miscellaneous effects


271
1906
Qhr
lateral spread, ground cracks


277
1906
af* over Qhs or Qs
ground settlement


279
1906
Qhr
ground settlement


280
1906
Qhs
sand boils


282
1906
Qhr? Qht? Qha?
lateral spread


283
1906
Qht? Qhr?
lateral spread


284
1906
Qha, Qhr
lateral spread


287
1906
Qhi
ground settlement, sand boils


288
1906
Qhb?
ground cracks


289
1906
Qhb, Qhf, Qoa
no cracks found


293
1906
Qhi/Qhb? br? 
lateral spread


295
1906
Qa, marshy or low 
areas*
ground settlement, miscellaneous 
effects, ground cracks


295
1969
Qa, af over marshy 
substrata*

Qhf
ground cracks 


ground cracks


296
1906
Qa, marshy area*
ground settlement,
stream bank slip


Note:		* Unit or sub-unit not delineated on map.  Body of material which failed is too 	
		   small to show at map scale and is mapped as part of a larger unit.
		** Located in the Winters-Vacaville area, east of the map boundary.


Table 3.  Criteria matrix for assigning liquefaction susceptibility unitsUnits indicate 
relative susceptibility of deposits to liquefaction as a function of groundwater depth within that 
deposit.  VH = very high, and includes areas of historical liquefaction, H = high, M = moderate, L 
= low, and VL = very low to none.  A groundwater depth of less than 10 feet was assumed for 
nearly all localities, so most of the categories with greater depths are hypothetical.

Map 
symbol

Age and Type of Deposit

Depth to Groundwater (ft)



<10
10-30
30-50
>50

Qhr

Holocene estuarine deposits
VH
H
L
VL

af

Artificial fill 
L-VH
L-H
VL-L
VL

Qhi

Late Holocene flood plain deposits
VH, H
H
L
VL

Qhs

Holocene dune and beach sands
VH, H
H
L
VL

Qhb

Holocene basin deposits
M-H
L-M
VL
VL

Qht

Holocene stream terrace deposits
H
M
L
VL

Qhf

Holocene alluvial fan deposits
M
L
VL
VL

Qha

Holocene alluvium, undifferentiated
M-H
L-M
VL-L
VL

Qs
Late Pleistocene to Holocene dune 
sand
L-H
L-M
VL-L
VL

Qt
Late Pleistocene to Holocene 
stream terrace deposits
L-H
L-M
VL-L
VL

Qf
Late Pleistocene to Holocene 
alluvial fan deposits
L-M
VL-L
VL
VL

Qa
Late Pleistocene to Holocene 
alluvium, undifferentiated
L-H
L-M
VL-L
VL

Qpt
Late Pleistocene stream terrace 
deposits
L-M
L
VL
VL

Qpf
Late Pleistocene alluvial fan 
deposits
L
VL
VL
VL

Qpa
Late Pleistocene alluvium, 
undifferentiated
L
L
VL
VL

Qpm
Late Pleistocene marine terrace 
deposits
L
L
VL
VL

Qoa
Older alluvial fan and stream terrace 
deposits
VL
VL
VL
VL

Qom

Older marine terrace deposits
VL
VL
VL
VL

br

Bedrock
VL
VL
VL
VL

Note:  Separation of unit designations by a dash indicates that the susceptibility in the mapped area could range from 
the lower to the higher designation.  This range reflects uncertainty or variability in the geologic nature (age, texture, 
cohesion) or groundwater conditions of the unit.  The range may be narrowed in a specific area if additional 
information is available.


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