Healdsburg Fault

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The Healdsburg Fault is a seismically active geological feature associated with the Santa Rosa Plain and the Alexander Valley, in Sonoma County, California, United States. The eastern sides of these floodplains 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 magnitude scale. [1]

Sonoma County, California County in California, United States

Sonoma County is a county in the U.S. state of California. As of the 2010 United States Census, its population was 483,878. Its county seat and largest city is Santa Rosa. It is to the north of Marin County and the south of Mendocino County. It is west of Napa County and Lake County.

Floodplain Land adjacent to a stream or river which is flooded during periods of high discharge

A floodplain or flood plain is an area of land adjacent to a stream or river which stretches from the banks of its channel to the base of the enclosing valley walls, and which experiences flooding during periods of high discharge. The soils usually consist of levees, silts, and sands deposited during floods. Levees are the heaviest materials and they are deposited first; silts and sands are finer materials.

The so-called Richter magnitude scale – more accurately, Richter's magnitude scale, or just Richter magnitude – for measuring the strength ("size") of earthquakes refers to the original "magnitude scale" developed by Charles F. Richter and presented in his landmark 1935 paper, and later revised and renamed the Local magnitude scale, denoted as "ML" or "ML". Because of various shortcomings of the ML scale most seismological authorities now use other scales, such as the moment magnitude scale (Mw ), to report earthquake magnitudes, but much of the news media still refers to these as "Richter" magnitudes. All magnitude scales retain the logarithmic character of the original, and are scaled to have roughly comparable numeric values.

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The last major event in Sonoma County were the 1969 Santa Rosa earthquakes, a magnitude 5.6 and 5.7 doublet that occurred on this fault. The county anticipates similar events every 20–30 years. [2]

1969 Santa Rosa earthquakes pair of earthquakes that struck Santa Rosa, California

At 04:56 and 06:19 UTC on October 2, 1969, a pair of earthquakes of magnitude 5.6 and 5.7, respectively, struck the city of Santa Rosa, California, killing one person and damaging buildings. The maximum felt intensity for the two events was VII and VIII (Severe) respectively on the Mercalli intensity scale. The $8.35 million cost of the earthquakes went mostly to replace buildings damaged beyond repair. There were at least 200 aftershocks after the initial pair of earthquakes. At the time they were the largest earthquakes to affect the northern San Francisco Bay Area since the 1906 San Francisco earthquake.

Doublet earthquakes – and more generally, multiplet earthquakes – were originally identified as multiple earthquakes with nearly identical waveforms originating from the same location. They are now characterized as single earthquakes having two main shocks of similar magnitude, sometimes occurring within tens of seconds, but sometimes separated by years. The similarity of magnitude – often within four-tenths of a unit of magnitude – distinguishes multiplet events from aftershocks, which decrease in magnitude and frequency according to known laws.

The Rodgers Creek Fault passes through Sonoma Mountain and its north end terminates within Santa Rosa. The Healdsburg Fault continues along the east side of the Cotati Valley north of Santa Rosa, and the Healdsburg Fault is actually a step-over from the Rogers Creek fault.

Sonoma Mountain mountain in Sonoma County, California

Sonoma Mountain is a prominent landform within the Sonoma Mountains of southern Sonoma County, California. At elevation of 2,463 ft (751 m), Sonoma Mountain offers expansive views of the Pacific Ocean to the west and the Sonoma Valley to the east. In fact, the viticultural area extends in isolated patches up the eastern slopes of Sonoma Mountain to almost 1,700 feet (520 m) in elevation.

Santa Rosa, California City in California, United States

Santa Rosa is a city in and the county seat of Sonoma County, in California's Wine Country. Its estimated 2016 population was 175,155. Santa Rosa is the largest city in California's Redwood Empire, Wine Country and the North Bay; the fifth most populous city in the San Francisco Bay Area after San Jose, San Francisco, Oakland, and Fremont; and the 28th most populous city in California.

Cotati, California City in California, United States

Cotati is an incorporated city in Sonoma County, California, U.S., located about 45 mi (70 km) north of San Francisco in the 101 corridor between Rohnert Park and Petaluma.

The upper soil surface above the Santa Rosa Plain is composed of Plio-Pleistocene alluvial fan deposits. Further north, one encounters Quaternary alluvium from the Russian River and its tributaries. North and east of Healdsburg, the Healdsburg fault cuts through Lower Cretaceous marine rocks with stringers of Mesozoic ultrabasic intrusive volcanic rocks. [3]

The Pleistocene is the geological epoch which lasted from about 2,588,000 to 11,700 years ago, spanning the world's most recent period of repeated glaciations. The end of the Pleistocene corresponds with the end of the last glacial period and also with the end of the Paleolithic age used in archaeology.

Russian River (California) river in California

The Russian River is a southward-flowing river that drains 1,485 sq mi (3,850 km2) of Sonoma and Mendocino counties in Northern California. With an annual average discharge of approximately 1,600,000 acre feet (2.0 km3), it is the second-largest river flowing through the nine-county Greater San Francisco Bay Area, with a mainstem 110 mi (180 km) long.

The Cretaceous is a geologic period and system that spans 79 million years from the end of the Jurassic Period 145 million years ago (mya) to the beginning of the Paleogene Period 66 mya. It is the last period of the Mesozoic Era, and the longest period of the Phanerozoic Eon. The Cretaceous Period is usually abbreviated K, for its German translation Kreide.

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References

  1. ↑ C.Michael Hogan, John Torrey, Brian McElroy et al., Environmental Impact Report, Southeast Santa Rosa Annexation 2-88, Earth Metrics Inc., Report 7941, California State Clearinghouse, Sacramento, Ca., March 1990
  2. ↑ "Sonoma County General Plan: Seismic Hazards" . Retrieved 2008-01-17.
  3. ↑ Geology of Sonoma County

Coordinates: 38°41′53″N122°56′59″W / 38.69797°N 122.94981°W / 38.69797; -122.94981

Geographic coordinate system Coordinate system

A geographic coordinate system is a coordinate system that enables every location on Earth to be specified by a set of numbers, letters or symbols. The coordinates are often chosen such that one of the numbers represents a vertical position and two or three of the numbers represent a horizontal position; alternatively, a geographic position may be expressed in a combined three-dimensional Cartesian vector. A common choice of coordinates is latitude, longitude and elevation. To specify a location on a plane requires a map projection.


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