At 04:56 and 06:19 UTC on October 2, 1969 (21:56 and 23:19 PDT October 1), a pair of earthquakes of magnitude 5.6 and 5.7, respectively, struck the city of

Santa Rosa Santa Rosa is the Italian, Portuguese and Spanish name for Saint Rose. Santa Rosa may also refer to: Places Argentina *Santa Rosa, Mendoza, a city * Santa Rosa, Tinogasta, Catamarca * Santa Rosa, Valle Viejo, Catamarca *Santa Rosa, La Pampa * Sa ...

, California, killing one person and damaging buildings. The maximum felt intensity for the two events was VII (''Very strong'') and VIII (''Severe'') respectively on the

Mercalli intensity scale The Modified Mercalli intensity scale (MM, MMI, or MCS), developed from Giuseppe Mercalli's Mercalli intensity scale of 1902, is a seismic intensity scale used for measuring the intensity of shaking produced by an earthquake. It measures the eff ...

. 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 At 05:12 Pacific Standard Time on Wednesday, April 18, 1906, the coast of Northern California was struck by a major earthquake with an estimated moment magnitude of 7.9 and a maximum Mercalli intensity of XI (''Extreme''). High-intensity sha ...

Tectonic setting

The current tectonics of northern California are mainly controlled by the San Andreas Fault system, the zone of dextral strike-slip faulting that accommodates displacement between the North American Plate and the

Pacific Plate The Pacific Plate is an oceanic tectonic plate that lies beneath the Pacific Ocean. At , it is the largest tectonic plate. The plate first came into existence 190 million years ago, at the triple junction between the Farallon, Phoenix, and Iza ...

along this transform boundary. In addition to the San Andreas Fault itself there are numerous other sub-parallel dextral strike-slip faults that take up some of the plate boundary motion distributed through a zone more than 100 km wide. The 1969 earthquakes and most of their aftershocks occurred along the

Healdsburg Fault 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 tra ...

, which lies between the Maacama Fault and the

Rodgers Creek Fault The Hayward Fault Zone is a right-lateral strike-slip geologic fault zone capable of generating destructive earthquakes. This fault is about long, situated mainly along the western base of the hills on the east side of San Francisco Bay. It runs ...

, linked to them by right step-overs. These faults form a continuation of the Hayward Fault Zone north of

San Francisco Bay San Francisco Bay is a large tidal estuary in the U.S. state of California, and gives its name to the San Francisco Bay Area. It is dominated by the big cities of San Francisco, San Jose, and Oakland. San Francisco Bay drains water from a ...

Earthquake

The two earthquakes occurred about a kilometer apart, a few kilometers north of Santa Rosa, close to the trace of the Healdsburg Fault. The focal depths of the two mainshocks were 9.6 km and 10.4 km respectively. The first shock was described as "violent or explosive" by residents and the main shaking lasted for 15 seconds.

Damage

The earthquake caused severe damage in some areas of Santa Rosa, affecting many buildings, bringing down chimneys, causing sidewalks to buckle and rupturing underground pipes. The lack of casualties and the low number of injuries is ascribed to the earthquakes being in the evening, when there were few pedestrians in the streets. The degree of damage associated with the earthquakes was greater than expected for their magnitude, with many older building left beyond repair. Gravity data have been used to show the presence of two sedimentary basins, separated by the Trenton ridge, which is associated with the northeast-dipping Trenton thrust fault. Modeling of the ground motion for the 1906 and 1969 earthquakes shows increased shaking at the northeastern edge of the southern basin, where Santa Rosa is situated.

Aftermath

Following the relatively high level of damage caused by the 1969 earthquakes, the city set about retrofitting buildings to improve their earthquake resistance by adding for instance cross-bracing to unreinforced masonry structures. The first buildings to be retrofitted were the St. Rose Church and Parish hall due to their high level of occupancy and lack of reinforcing. The safety standards adopted in Santa Rosa in 1970 provided a model for other cities in California when the state brought in requirements for retrofitting. An analysis of the effects on structures in Santa Rosa showed that it was necessary to design buildings using dynamic principles rather than just static, even in the case of a moderate intensity earthquake.

References

External links

* {{Earthquakes in the United States Santa Rosa, 1969

1969 This year is notable for Apollo 11's first landing on the moon. Events January * January 4 – The Government of Spain hands over Ifni to Morocco. * January 5 **Ariana Afghan Airlines Flight 701 crashes into a house on its approach to ...

History of Sonoma County, California History of Santa Rosa, California

Earthquakes An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth's lithosphere that creates seismic waves. Earthquakes can range in intensity, from ...