By Suzanne Hecker and Carolyn E. Randolph Loar

Pamphlet to accompany

Scientific Investigations Map 3410

2018

U.S. Department of the Interior

U.S. Geological Survey

Map of Recently Active Traces of the Rodgers Creek Fault, 

Sonoma County, California
U.S. Department of the Interior

RYAN K. ZINKE, Secretary

U.S. Geological Survey

James F. Reilly II, Director

U.S. Geological Survey, Reston, Virginia: 2018 

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must be secured from the copyright owner.

Suggested citation: 

Hecker, S., and Randolph Loar, C.E., 2018, Map of recently active traces of the Rodgers Creek Fault, Sonoma County, 

California: U.S. Geological Survey Scientific Investigations Map 3410, 7 p., 1 sheet, https://doi.org/10.3133/sim3410.

ISSN 2329-132X (online) 
iii

Figures

1. Shaded relief map of the San Francisco Bay area, California, showing the Rodgers

 Creek Fault and other significant faults..................................................................map sheet

2. Maps of the the Rodgers Creek Fault, California, showing relation to regional

 topography and the main sources of prior fault mapping used in this study...map sheet 

3. Hillshade images derived from 2007 Earthscope lidar dataset illustrating types of

 geomorphic features used to identify recently active fault traces....................map sheet 

Tables

1. Fault-strand database attribute fields.......................................................................................3

2. Fault strands trenched for purpose of paleoseismic study...................................................3

Contents

Introduction.....................................................................................................................................................1

Fault Nomenclature.......................................................................................................................................1

Approach and Scope.....................................................................................................................................1

Database Structure and File Formats.........................................................................................................2

Notable Aspects of Revised Mapping........................................................................................................4

Acknowledgments.........................................................................................................................................5

References Cited (Text and Database).......................................................................................................5

Introduction

The accompanying map and digital data identify 

recently active strands of the Rodgers Creek Fault in 

Sonoma County, California, interpreted primarily from 

the geomorphic expression of recent faulting on aerial 

photography and hillshade imagery derived from airborne 

light detection and ranging (lidar) data. A recently active 

fault strand is defined here as having evidence consistent 

with slip during the Holocene epoch (approximately the 

past 11,700 years). The purpose of the map is to update the 

fundamental fault dataset for characterizing surface-rupture 

hazard, siting slip-rate and paleoseismic studies, and studying the geometry and evolution of slip. To serve a range of 

users, the map is presented in several formats: as an image 

map, as a digital database for use within a geographic information system (GIS), and as a KML file for visualizing the 

fault using virtual globe software.

Important outcomes of this mapping revision include 

the following: (1) a northward 17-km increase in the known 

length of Holocene-active faulting to include most of the 

Healdsburg Fault, a structural continuation of the Rodgers 

Creek Fault northwest of a bend in the fault at Santa Rosa; 

(2) first-time identification of fault strands across the Santa

Rosa Creek floodplain in central Santa Rosa (Hecker and others, 2016); (3) increases in the known width and complexity

of faulting; and (4) identification of fault splays that project

toward the Bennett Valley-Maacama Fault system to the east

and toward an active extension of the Hayward Fault to the

south beneath San Pablo Bay, recently mapped by Watt and

others (2016) and previously inferred from shallow microseismicity (Lienkaemper and others, 2012).

The Rodgers Creek Fault is a principal strand of the San 

Andreas Fault system north of San Francisco Bay (fig. 1, on 

map sheet) that accommodates 6–10 millimeters per year 

(mm/yr) of plate-boundary motion (Schwartz and others, 

1992). The 30-year mean probability of a magnitude (M)>6.7 

earthquake on the combined Rodgers Creek-Hayward Fault, 

estimated at 33 percent, is the highest among faults in the 

region (Field and others, 2015; Aagaard and others, 2016). 

The most recent surface-rupturing earthquake on the Rodgers 

Creek Fault was likely between 1715 and 1776 (Hecker and 

others, 2005), indicating that the elapsed time may have 

reached or exceeded the poorly constrained average recurrence 

of large earthquakes on the fault, estimated to be 130–370 

years (Schwartz and others, 1992). More research is needed 

to clarify other aspects of the fault’s behavior, such as timing 

of earlier events, lengths of earthquake ruptures, and amounts 

and distribution of displacement through time. Aseismic 

shallow fault slip, or creep, a phenomenon documented along 

many parts of the San Andreas Fault system in northern 

California, has been recognized along the Rodgers Creek 

Fault, from Santa Rosa northward, only relatively recently 

(Funning and others, 2007; Lienkaemper and others, 2014; 

McFarland and others, 2016; Jin and Funning, 2017).

Fault Nomenclature

The nomenclature of the Rodgers Creek Fault system 

has varied over time and with point of view. The fault north 

and south of Santa Rosa is shown on early fault-activity 

and geologic maps (for example, Brown, 1970; Huffman 

and Armstrong, 1980) as separate faults (the Healdsburg 

Fault and Rodgers Creek Fault, respectively), reflecting 

discontinuities in the geometry of faulting across the Santa 

Rosa Creek floodplain. However, other fault-activity maps 

and more recent publications have regarded the southern 

part of the Healdsburg Fault (southward from the vicinity 

of Windsor; fig. 2A, on map sheet) as part of the modern 

Rodgers Creek Fault because of the continuous nature 

of Holocene faulting (Herd and Helley, 1977; Jennings, 

1994; Hart, 1998a, b; Working Group on Earthquake 

Probabilities, 2003; U.S. Geological Survey and California 

Geological Survey, 2006). Our revised mapping indicates 

that Holocene faulting is indeed continuous across the 

Santa Rosa Creek floodplain (Hecker and others, 2016) and 

extends along most (at least 80 percent) of the length of the 

Healdsburg Fault, as well as along the Rodgers Creek Fault 

south of Santa Rosa, suggesting that the two faults operate 

as a single, integrated seismic source. To reflect this new 

understanding, we herein refer to the Holocene-active fault 

in its entirety simply as the Rodgers Creek Fault and adapt 

the original nomenclature to refer to the sections of the 

fault north and south of Santa Rosa.

Approach and Scope

This publication builds upon prior mapping of recently 

active traces of the Rodgers Creek Fault by Hart (1982, 1992) 

and Bryant (1982), prepared under California’s Alquist-Priolo 

program of fault-rupture hazard zoning (fig. 2B, on map 

Map of Recently Active Traces of the Rodgers Creek Fault, 

Sonoma County, California

By Suzanne Hecker1 and Carolyn E. Randolph Loar2

1

U.S. Geological Survey.

2

Stantec Consulting Services, Inc.
2

sheet). These maps, prepared at a scale of 1:24,000, utilized 

existing fault mapping, interpretation of fault geomorphology 

from aerial photography, and limited field inspection to identify fault strands that are sufficiently active and well defined 

to meet the criteria of the Alquist-Priolo regulatory zones. We 

also consulted earlier (1970–1971) unpublished 1:24,000-

scale mapping of recently active traces from the vicinity of 

Santa Rosa southward by R.D. Brown of the U.S. Geological 

Survey, as reproduced in Hart (1982, fig. 5A, B).

Similar to previous efforts, our mapping of the Rodgers 

Creek Fault relies largely on the interpretation of fault 

traces from remote sensing imagery. An aerial perspective is 

advantageous for identifying geomorphic features indicative 

of active strike-slip faulting, such as scarps, tonal lineaments, linear troughs and drainages, hillside benches, offset 

drainages, and closed depressions (fig. 3, on map sheet; also 

Wallace, 1990), and for judging the continuity of these features. For this latest effort, we checked, revised, and extended 

the existing mapping of the Rodgers Creek Fault primarily 

using hillshade imagery generated from a 3-ft bare-earth 

digital elevation model (DEM) derived from a high-density 

(13.7 points per square meter) 2013 airborne lidar survey of 

Sonoma County.1

 This dataset provided the best view of the 

geomorphology along the fault. We also used (1) 0.5- and 

1.0-m-resolution hillshade imagery generated from airborne 

lidar collected in 2007 within an approximately 1-km-wide 

strip along the fault;2 (2) stereographic inspection of low-elevation (1:6,000-scale), low sun angle, color aerial photography flown along the Rodgers Creek and southern Healdsburg 

sections of the fault for the U.S. Geological Survey (1991); 

(3) inspection of smaller scale (≤1:20,000-scale) aerial

photography, including low sun angle, color photography

(U.S. Geological Survey, 1973, 1974; California Division of

Mines and Geology, 1976) that was utilized by Hart (1982

and 1992); (4) three-dimensional aerial imagery provided by

Google Earth; and (5) field mapping along the southern quarter of the Rodgers Creek section of the fault (Randolph Loar,

2002; fig. 2B, on map sheet). Although field observations are

important for verifying the faulting origin of some features,

we were able to field check remotely mapped fault strands

along only a few stretches of the fault. Future improvements

to mapping will come from additional on-the-ground inspection of less-confidently identified features and incorporation

of exploratory trench data from unpublished geotechnical

reports. We generally limited the mapping area to the 1-kmwide swath of the 2007 lidar survey, as we found that this

footprint includes most of the zone of distributed shear.

1

Data provided by NASA Grant NNX13AP69G, the University of 

Maryland, and the Sonoma Vegetation Mapping and Lidar Program 

(https://doi.org/10.5069/G9G73BM1); processing services provided by the 

OpenTopography Facility with support from the National Science Foundation 

under NSF Award Numbers 1226353 and 1225810.

2

This material is from the EarthScope Northern California Lidar Project, 2007 

(https://doi.org/10.5069/G9057CV2) and is based on services provided to the 

Plate Boundary Observatory by NCALM (http://www.ncalm.org). PBO is operated by UNAVCO for EarthScope (http://www.earthscope.org) and supported by 

the National Science Foundation (No. EAR-0350028 and EAR-0732947).

For this revision, we also incorporated faults identified 

from geologic mapping (fig. 2B, on map sheet) that are within 

or near the zone mapped as recently active. Geologically 

mapped faults provide a longer-term context for recent surface 

faulting and, despite lacking geomorphic evidence of recency, 

may have been active during the Holocene, as demonstrated 

by the 2014 M6 South Napa earthquake (DeLong and others, 

2016). In places where the geomorphically mapped active 

traces lie in the vicinity of these framework faults, we generally show the latter only where they are farther than a few tens 

of meters from the more conspicuously active traces, although 

we note the spatial correspondence in the database attribute 

records of the mapped strands. 

Database Structure and File Formats

The mapping data for this study is being released as GIS 

map layers to be viewed and queried using analysis software 

such as Esri ArcGIS or ArcReader. As with all GIS data, the 

reader must keep in mind the resolution limitations associated with the sources of data. For less technical applications, we are providing a KMZ (zipped Keyhole Markup 

Language) file of fault traces and data that may be viewed 

interactively in Google Earth and other similar software and 

a fixed image of the map at a scale of 1:36,000. 

The basic structure of the digital database derives from 

guidelines developed for a prototype community fault map 

database for the San Francisco Bay area (Graymer, personal 

communication, 2004). The database design for the Rodgers 

Creek Fault includes attribute fields that record information 

related to fault-strand age and rank, geomorphic expression, 

location certainty, and mapping sources (table 1). We have not 

included site-specific data such as locations of fault-trench 

exposures. However, we identify in the attribute records the 

few fault strands that have been trenched for paleoseismic 

research (table 2) and some strands that have been trenched as 

part of the Alquist-Priolo program (mainly in Healdsburg).

Fault strands interpreted from geomorphic evidence of 

recent activity are assumed to be Holocene in age; geologically mapped faults that lack sufficient evidence of activity are 

regarded as potentially having been active in the Holocene. 

Fault strands described as accurately located in this study are 

estimated to be within 10 m of their true locations; mapped 

strands that are approximately located (the majority) are judged 

to be within 50 m of their true locations, although sections of 

these strands may be more accurately located. Holocene faults 

are reported as inferred where they are projected between 

geomorphically identified strands. In some places, a single 

mapped fault represents multiple strands visible on imagery but 

too closely spaced to feasibly map separately. Where this is the 

case, it is commonly noted in the comment field for location 

certainty (table 1). All of the geologically mapped faults not 

identified as recently activity are portrayed as approximately 

located (or inferred) to reflect uncertainty in the location of possible rupture during the Holocene. The lengths of digitized fault 
3

Table 1. Fault-strand database attribute fields.

[Information includes fault-section name, fault-strand age estimate, location certainty, geomorphic 

expression, relative importance in accommodating slip, and source(s) of mapping]

Field name Field description

FID Unique feature identifier*

Shape Feature geometry*

FltName Fault section name

QFltID Fault section identification number**

LocCert Location certainty description (accurately located, approximately 

located, or inferred)

LocCertCom Comment on location certainty

FltRank Fault-strand rank [either part of (1) principal, (2) distributed, or (3) 

long-term displacement zone, or (4) questionable]

FltRankCom Comment on fault-strand rank

FltAge Age estimate (Holocene or potentially Holocene)

FltAgeCom Comment on age estimate

FltMappBy Citation for fault-strand mapping

FltMappCom Comment on fault-trand mapping

FtrMapped Geomorphic evidence of recent activity

FtrMappBy Citation for geomorphic evidence

FtrMappHow Method of observing geomorphic evidence

FtrMappCom Comment on geomorphic evidence

*Internal to Esri ArcGIS

**Adapted from U.S. Geological Survey and California Geological Survey, 2006

Table 2. Fault strands trenched for purpose of paleoseismic study

Database fault-strand 

identifier (FID) Study reference

Rodgers Creek section

114 Budding and others (1991); Schwartz and others (1992)

186 Randolph Loar and others (2004)

192 Randolph Loar (2002)

205 Schwartz and others (1992); Hecker and others (2005)

429 Givler and Baldwin (2016)

613 Givler and Baldwin (2016)

Healdsburg section

12 Swan and others (2006)
4

strands in the database are somewhat arbitrary or are inherited 

from source datasets.

A field for displacement hierarchy is included in the database (fault-strand rank; table 1). This parameter is intended 

to convey relative importance in accommodating slip and 

differentiates between strands that appear to be part of the 

principal displacement zone and those that form the remainder of the distributed displacement zone. We surmise that the 

latter may include older principal strands, as well as secondary strands and discrete off-fault deformation. Geologically 

mapped strands that lack geomorphic evidence of recency are 

indicated as being part of the long-term displacement zone. 

Strands mapped from features that are fault-like in appearance but whose origins are equivocal are given a rank of 

“questionable.” Such equivocal features may be produced by 

ridge-top spreading (sackungen) or landsliding, by differential 

erosion along older faults or bedrock contacts, or by grading 

for roads. Because landslide scarps may resemble fault scarps, 

fault strands mapped through landslide terrain are commonly 

flagged as such in the comment field. Additional database 

documentation can be found in the metadata of the GIS files.

Notable Aspects of Revised Mapping

The detail that can be observed using high-resolution 

lidar topography allowed us to map fault traces at a finer scale 

than had been attempted previously and was indispensable for 

identifying geomorphic evidence of active faulting in forested 

and urban environments (see, for example, fig. 3, on map 

sheet; Hecker and others, 2016). The improved topographic 

resolution afforded by lidar reveals the Rodgers Creek Fault 

to be wider (locally >1 km in width) and more structurally 

complex than previously thought. This led us to differentiate 

strands that are relatively well developed and that we interpret 

to be part of the principal, through-going zone of displacement 

from strands that make up the broader zone of deformation. 

We caution that interpretation of discrete fault traces 

involves significant uncertainty in some areas owing to one 

or more of the following factors: (1) faulting is complex and 

distributed; (2) faulting is obscured by land development or 

by landsliding or other surficial processes; (3) in steep terrain, landslide scarps, especially those related to sackungen, 

may be mistaken for faults; and (4) landforms indicative of 

faulting are discontinuous or near the limit of image resolution. These conditions give rise to mapping inaccuracies and 

to a non-unique representation of the pattern of faulting. This 

uncertainty is reflected in part by the “approximately located” 

designation assigned to most fault strands in the dataset.

The revised map increases the known extent of Holocene 

rupture by 17 km, with strands identified as far north as the hills 

southwest of Geyserville, northwest of Healdsburg (fig. 2A, on 

map sheet), for a total Holocene fault length of at least 73 km. 

This result substantiates the work of early mappers who cited 

evidence consistent with geologically young surface rupture 

to delineate the Healdsburg Fault as active in the Quaternary 

(for example, Gealey, 1951; Huffman and Armstrong, 1980). 

Evidence of Holocene activity along essentially the entire length 

of the Healdsburg Fault section is consistent with the results 

of interferometric synthetic aperture radar (InSAR) studies 

(Funning and others, 2007; Jin and Funning, 2017) showing that 

shallow creep, at preferred rates in the range of 1.9–6.7 mm/yr, 

is occurring along the fault between Santa Rosa and Healdsburg. 

We observed field evidence of creep on the Healdsburg section 

of the fault at a location in Santa Rosa (strand FID 518) and 

east of Healdsburg (strand FID 614). The new mapping also 

corroborates the findings of geotechnical consulting studies 

that indicate the presence of Holocene faulting within northern 

Healdsburg (for example, Hart, 1998b, and as noted in the ageestimate comments for some strands in the database).

Splays of the Rodgers Creek Fault (some of equivocal origin) project or continue beyond the eastern boundary of the map 

area in several locations and may accommodate transfer of slip 

to the Bennett Valley-Maacama Fault system to the east (fig. 2A, 

on map sheet). In particular, several fault strands on the southeast 

side of Taylor Mountain branch northward toward the Spring 

Valley fault strand, a section of the Bennett Valley Fault Zone that 

displaces latest Pleistocene and Holocene deposits (McLaughlin 

and others, 2008; Sowers and others, 2016; fig. 2A, on map 

sheet). Other strands that appear to connect the Rodgers Creek 

Fault with the Bennett Valley Fault Zone and associated unnamed 

faults (U.S. Geological Survey and California Geological Survey, 

2006; Hecker, unpublished mapping) lie within a broad restraining bend in the Rodgers Creek Fault along the southwest side 

of Sonoma Mountain (fig. 2A, on map sheet), as well as farther 

south where the Bennett Valley Fault Zone approaches to within 

1.5 km of the Rodgers Creek Fault. On the north side of Santa 

Rosa, several northward-trending strands connect to a broader 

zone of Quaternary faulting associated with the Rodgers Creek 

Fault (fig. 2A, on map sheet). Farther north, near Windsor, 

strands of the fault branch to the southeast toward a section of 

the Alexander-Redwood Hill Fault Zone identified as active in 

the latest Quaternary (U.S. Geological Survey and California 

Geological Survey, 2006; fig. 2A, on map sheet).

At its south end, the Rodgers Creek Fault bifurcates, and 

the eastern branch of the fault, expressed as pop-up features 

in the mudflats fringing San Pablo Bay, has been identified as 

the south end of the mapped trace (for example, Hart, 1992). 

The western branch has been interpreted as a late Quaternary 

strand (U.S. Geological Survey and California Geological 

Survey, 2006), or as primarily an erosional feature (Hart, 

1992). It strikes south-southeast and projects toward active 

strands of the Tolay fault in the area of Sears Point (Hecker, 

unpublished mapping) and toward a recently discovered strand 

of the Hayward Fault beneath San Pablo Bay (Lienkaemper 

and others, 2012; Watt and others, 2016).

 Our results indicate that the Rodgers Creek Fault is longer, 

wider, and more structurally complex than previously known 

and, therefore, presents a greater hazard to the built environment. Although the new map provides a substantial refinement 

in the representation of recently active traces, it also shows firstorder agreement with previous maps. In particular, the principal 

displacement zone that we’ve delineated based primarily on 

geomorphic expression interpreted from lidar imagery generally follows the main active traces identified by Hart (1982, 

1992), Bryant (1982), and earlier workers. This is evidence 

that, although in detail the configuration of surface faulting is 
5

complex and likely evolves through time, the Rodgers Creek 

Fault is fundamentally well developed, with a relatively stable 

locus of slip during the Holocene.

Acknowledgments

We thank Luke Blair for GIS technical support and review 

and thank Luke, Chris Crosby, Carrie Whitehill, and Jonathon 

Brown for creating initial bare-earth digital elevation models from 

the 2007 lidar acquisition. We are grateful to Robert McLaughlin 

for supplying annotated fault maps based on his geologic framework mapping, Craig McCabe for digitizing linework from early 

fault maps, and Marc Delattre for making available preliminary 

digital files of his quadrangle mapping at the northern end of the 

Rodgers Creek Fault. We thank Tim Dawson and the California 

Geological Survey for making available Earl Hart’s original airphoto mylar overlays with fault interpretations, Bill Bryant for 

facilitating access to fault investigation reports prepared under 

California’s Alquist-Priolo program, and Bill McCormick for 

access to consulting reports for Healdsburg by Kleinfelder, Inc. 

We are grateful to Jim Lienkaemper and Tim Dawson for their 

review comments, as well as Bob McLaughlin and Russ Graymer 

for reviewing an earlier draft of the report.

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