The Healdsburg Fault is a seismically active geological feature associated with the Santa Rosa Plain and the Alexander Valley

Alexander Valley

The Alexander Valley is a Californian American Viticultural Area just north of Healdsburg in Sonoma County. It is home to many wineries and vineyards, as well as the city of Cloverdale. It is the largest and most fully planted wine region in Sonoma. Highway 101 runs through the valley, and the...

, in Sonoma County, California

Sonoma County, California

Sonoma County, located on the northern coast of the U.S. state of California, is the largest and northernmost of the nine San Francisco Bay Area counties. Its population at the 2010 census was 483,878. Its largest city and county seat is Santa Rosa....

, USA. The eastern sides of these floodplain

Floodplain

A floodplain, or flood plain, is a flat or nearly flat land adjacent a stream or river that stretches from the banks of its channel to the base of the enclosing valley walls and experiences flooding during periods of high discharge...

s are bounded by strike-slip or transform faults. The maximum credible earthquake expected to be generated from the Healdsburg Fault is estimated to be about 7.5 on the Richter Scale.

The last major earthquake in Sonoma County was a magnitude 5.7 event which occurred in 1969 on this fault. The county anticipates similar events every 20-30 years.

The Rodgers Creek Fault passes through Sonoma Mountain

Sonoma Mountain

Sonoma Mountain is a prominent landform within the Sonoma Mountains of southern Sonoma County, California. At elevation of , Sonoma Mountain offers expansive views of the Pacific Ocean to the west and the Sonoma Valley to the east...

 and its north end terminates within Santa Rosa

Santa Rosa, California

Santa Rosa is the county seat of Sonoma County, California, United States. The 2010 census reported a population of 167,815. Santa Rosa is the largest city in California's Wine Country and fifth largest city in the San Francisco Bay Area, after San Jose, San Francisco, Oakland, and Fremont and 26th...

. The Healdsburg Fault continues along the east side of the Cotati

Cotati, California

Cotati is an incorporated city in Sonoma County, California, U.S.A., located about north of San Francisco in the 101 corridor between Rohnert Park and Petaluma....

 Valley north of Santa Rosa, and the Healdsburg Fault is actually a step-over from the Rogers Creek fault.

The upper soil surface above the Santa Rosa Plain is composed of Plio-Pleistocene

Pleistocene

The Pleistocene is the epoch from 2,588,000 to 11,700 years BP that spans the world's recent period of repeated glaciations. The name pleistocene is derived from the Greek and ....

 alluvial fan deposits. Further north, one encounters Quaternary alluvium from the Russian River

Russian River (California)

The Russian River, a southward-flowing river, drains of Sonoma and Mendocino counties in Northern California. With an annual average discharge of approximately , it is the second largest river flowing through the nine county Greater San Francisco Bay Area with a mainstem 110 miles ...

 and its tributaries. North and east of Healdsburg, the Healdsburg fault cuts through Lower Cretaceous

Cretaceous

The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...

 marine rocks with stringers of Mesozoic

Mesozoic

The Mesozoic era is an interval of geological time from about 250 million years ago to about 65 million years ago. It is often referred to as the age of reptiles because reptiles, namely dinosaurs, were the dominant terrestrial and marine vertebrates of the time...

 ultrabasic intrusive volcanic rocks.

External links

• Discussion of Healdsburg Fault: Seismic Safety Element of the Sonoma County General Plan

38.69797°N 122.94981°W