Hi All. I hope your not getting tired of my harping on the Sonoma County area, but I do have this very strong feeling it is the area to watch.

This special report was first printed in 1980, and was reprinted in 2000. Take Care…Don in creepy town.

Seven potentially active faults or fault zones have been identified within the boundaries of Sonoma County: Burdell Mountain, Chianti, Healdsburg, Maacama, Rodgers Creek, San Andreas, and Tolay. Most of the faults are thought to be steeply dipping, right- lateral, strike-slip faults. All have been identified on aerial photographs. In Sonoma County, the Chianti, Healdsburg, Maacama, Rodgers Creek, San Andreas, and Tolay faults are included within Special Studies Zones. Faults in Sonoma County are scheduled to be re-evaluated for Special Studies Zones in 1980. These faults exhibit features suggestive (but not conclusive) of potential activity. Further studies would be necessary to determine their degree of activity.

Potentially Active Faults

BURDELL MOUNTAIN FAULT

This fault is considered to be a complex reverse fault with topographic evidence of geologically young surface displacements.

CHIANTI FAULT

Noteworthy evidence of Quanternary movement on the Chinante fault are the right-lateral deflection of Gill Creek and the ponded alluvium immediately to the southeast. The right-lateral deflection of Big Sulphur Creek near its intersection with the fault probably also was controlled by faulting (Gealey, 1951, p. 32, 35).
North of Big Sulphur Creek the fault trace is apparently obscured by landslides. Preston Lake, on the projection of the fault near the County boundary, may be of fault rupture origin (Gealey, 1951, p. 35). Structural extension of the Chianti fault zone in the Kelseyville 15-minute quadrangle was mapped by McNitt ( 1968) , and supplemental information was added by Blake and Wright (Blake and others, 1971). These authors did not ascribe activity to the fault, however. Inference of potential activity is based on the work of Gealey (1951) and aerial photo interpretation by Huffman. The potentially active fault shown on Plate 3 is a composite of the cited sources supplemented by aerial photographic interpretation during this study. The Chianti fault may be continuous with the possibly active Alexander Valley fault described below.

HEALDSBURG FAULT

Epicenters of the October 1, 1969, Santa Rosa earth- quakes (magnitude 5.6, 5.7) and aftershocks are located near identified surface fault traces of the Healdsburg fault (plate 4) .The quakes are inferred to have originated on this fault. No surface fault rupture occurred during these events, however.
The fault zone is complex, with numerous, often disconnected, parallel and divergent breaks. Its traces can be projected through the City of Santa Rosa, and a link with the Rodgers Creek fault to the south is possible. However, the nature of the linkage, if any, between these faults is uncertain. The fault zone merges with the Alexander fault zone of Gealey (1951, p. 36), who traced the combined faults north of the latitude of Cloverdale. This northward extension was not identified during the present study. The geology at the south end of the Healdsburg fault zone was mapped in detail by C.W. Jennings (1970) and this information was incorporated in the regional map by Fox and others (1973).
The queried fault segments on Plate 3A were shown on USGS-HUD Basic Data Contribution 12. However, limited aerial photo and field checking during this study and site investigations by consulting geologists have failed to confirm their presence.
Ponded alluvium, indicative of Quaternary surface rupture, was mapped by Gealey (1951, plate 1, p. 32, 36) north of Healdsburg and along branches of Windsor Creek northeast of Windsor (plate 3A) .A fault contact between deformed Quaternary terrace alluvium and serpentine is well exposed in recently constructed road cuts on Dutcher Creek Road 0.9 miles south of its intersection with Highway 101, and is continuous with the northern segment of Gealey's Alexander fault. Recently active fault traces were mapped by R.D. Brown Jr. (Blake and others, 1971; Fox and others, 1973). Potentially active traces shown on Plate 3A are derived largely from this source, supplemented by aerial photographic interpretation and minor field examination.

MAACAMA FAULT

The westernmost break of the fault is well exposed in a road cut on Highway 128, where Quaternary terrace alluvium is faulted against tuff on the Glen Ellen Formation. Dorothy Radbruch-Hall of the U.S. Geological Survey has observed evidence of historic creep along a break of the fault approximately I 1/4 miles southeast of Big Sulphur Creek (oral communication, 1974). Here, two fence lines are offset right laterally about 6 inches along the fault trace.
Strong geomorphic evidence of Quaternary surface movement also includes: ponded alluvium between Sausal and Gird Creeks and at the head of Crocker Creek (plate 3A); well-expressed, systematic, right-Lateral diversion of drainages between Franz and Sausal Creeks; and tilting of the large Quaternary terrace south of the intersection of Maacama Creek and the fault (Gealey, 1951, p. 39).
The fault zone was\ mapped by Gealey (1951) in the Healdsburg 15-minute quadrangle and by McNitt ( 1968) in the Kelseyville 15-minute quadrangle. These data were compiled by Blake and others ( 1971) .McNitt and Blake did not ascribe activity to the fault, however. The potentially active traces shown on Plate 3A are largely a product of aerial photographic interpretation by Huffman, guided by information cited above.

TOLAY FAULT

The Tolay fault is concealed by San Pablo Bay to the south, where it has been projected by Koenig (1963) to the Hayward fault. To the north it has been projected by Koenig (1963) across Cotati Valley to connect with a fault offsetting Pliocene rocks. Fault related topographic features along the Tolay fault are less distinctive than those along the Rodgers Creek fault. However, several right-lateral offset streams and a large playa lake (Tolay Lake) suggest Pleistocene or later displacement. It is probable that the Tolay and Rodgers Creek fault zones are closely related, and photo and field evidence suggests that, to the south, a nearly continuous zone of fracture exists from one to the other.
Data sources used for the location of portions of this fault were Sims and others (1973), Blake and others (1974), and Koenig (1963). Field observations by Arm- strong (1974) form the basis for the location of the remainder of the fault and for the conclusion that it is potentially or possibly active.

RODGERS CREEK FAULT

The southern extension of the Rodgers Creek fault zone is concealed by San Pablo Bay and the northern extension possibly extends under Santa Rosa to join the Healdsburg fault zone. This is a Holocene fault zone exhibiting highly distinctive fault-related topographic features, including sag ponds, scarps, benches, linear ridges and troughs, and numerous stream channels offset right laterally. Along most of its length, the fault zone is an array of several (sometimes as many as ten) distinct strands. A coincidence of earthquake epicentres also strongly suggests that the Rodgers Creek fault is indeed an active fault.
Data sources used for this fault are Fox and others ( 1973) , Sims and others ( 1973) , Blake and others ( 1974) , Huffman (1971), and Armstrong (unpublished).

SAN ANDREAS FAULT

The San Andreas fault is concealed by the Pacific Ocean for half of its extent along the Sonoma County coastline. The northward trace of the fault zone can be observed on land at Bodega Bay, and from Fort Ross to Gualala.
Movement along the entire length of the visible portions of this fault within Sonoma County was recorded in 1906. All of the known and potentially active breaks have been delineated within an Alquist-Priolo Special Studies Zone.
Data from Blake and others ( 1971) were used to delineate the main fault traces. Some outlying and subsidiary traces are from Huffman (1972, 1973) and Koenig (1963).

BlACK MOUNTAIN FAULT

Possible Quaternary movement on the Black Mountain fault is indicated on aerial photos by lineaments crossing Quaternary terrace alluvium and by relatively young appearing ground disturbance along the trace in the vicinity of Maacama Creek.

Possibly Active Faults

DIANNA ROCK FAULT

McLaughlin ( 1974) mapped this fault and attributed Quaternary movement to it on the basis of Holocene terrace alluvium deformed against it (oral communication, 1975). In addition, Hamilton and Mumer (1972) record- ed micro earthquakes at The Geysers and concluded, "Most epicenters lie in a zone about 4 km long and 1 km wide passing through the geothermal field along a principal fault zone...A composite fault-plane solution indicates dextral strike-slip faulting on a NNW-striking plane sub- parallel with regional fault pattern." The fault shown may be the source of the microearthquake activity.

TOMBS CREEK FAULT

The Tombs Creek fault is an extension of the northern- most branch of the Porter Creek fault zone of Gealey ( 1951). Displacement of rock masses along it suggests right-lateral movement. The fault trace was identified northwest of the Healdsburg quadrangle by the alignment of saddles and canyons as seen on aerial photos. Right- lateral deflection of Osser Creek, Flat Ridge Creek, the Wheatfield Fork of the Gualala River, and Tombs Creek occurs at their intersections with the trace.

MT. JACKSON FAULT

Right-Lateral displacements of rock units and streams are common along the Mt. Jackson fault. The Santa Rosa Sheet of the Geologic Map of California (Koenig, 1963 ) well illustrates the larger features, notably the offsets of "The Cedars" serpentine mass and of the Merced Formation near Forestville, and the right-Lateral deflection of the Russian River. Systematic right-Lateral stream deflections are present near Devils Creek.
The Mt. Jackson fault zone may be projected southward through the Laguna de Santa Rosa, and it is probable that the latter is related to the Mt> Jackson fault structure.