1969 Santa Rosa earthquakes
UTC time | Doublet earthquake: |
---|---|
1969-10-02 04:56 | |
1969-10-02 06:19 | |
ISC event | |
803001 | |
803006 | |
USGS-ANSS | ComCat |
ComCat | |
Local date | October 1, 1969 |
Magnitude | 5.6 and 5.7 Mw |
Depth | 9.6 km and 10.4 km |
Epicenter | 38°28′01″N 122°41′31″W / 38.467°N 122.692°W[1] |
Areas affected | United States (Santa Rosa) |
Max. intensity | MMI VII (Very strong) MMI VIII (Severe) |
Aftershocks | Yes |
Casualties | 1 dead |
At 04:56 and 06:19 UTC on October 2, 1969 (21:56 and 23:19
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
Earthquake
The two earthquakes occurred about a kilometer apart, a few kilometers north of Santa Rosa, close to the trace of the Healdsburg Fault.[6] 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.[7]
Earthquake epicenter | Coordinates |
---|---|
First earthquake | 38°28.4′N 122°43.0′W / 38.4733°N 122.7167°W |
Second earthquake | 38°27.9′N 122°43.1′W / 38.4650°N 122.7183°W |
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.[8] 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.[9]
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.[8]
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.[10]
See also
- List of earthquakes in 1969
- List of earthquakes in California
- List of earthquakes in the United States
References
- doi:10.7289/V5TD9V7K. Retrieved 22 March 2024.
- ^ Bergen, K.J.; Fletcher J.B.; Boatwright J. & Langeheim V. (2007). "Site Response in the Northern San Francisco Bay Area, California" (PDF). USGS. Archived from the original (PDF) on 2012-03-05. Retrieved 5 September 2011.
- ^ USGS. "Santa Rosa, Sonoma County, California 1969 October 02 04:46 and 06:19 UTC (local October 1) Magnitude 5.6 and 5.7". Historic Earthquakes. USGS. Archived from the original on 2016-12-01. Retrieved 6 September 2011.
- ^ a b Wong, I.G.; Bott J.D.J. (1995). "A new look back at the 1969 Santa Rosa, California, earthquakes". Bulletin of the Seismological Society of America. 85 (1): 334–341. Archived from the original on 3 April 2010. Retrieved 5 September 2011.
- ^ . Retrieved 5 September 2011.
- ^ McLaughlin, R.J.; Langenheim V.E.; Sarna-Wojcicki A.M.; Fleck R.J.; McPhee D.K.; Roberts C.W.; McCabe C.A. & Wan E. (2008). "Open-File Report 2008-1009 Sheet 2 of 3" (PDF). Geologic and Geophysical Framework of the Santa Rosa 7.5' Quadrangle, Sonoma County, California. USGS. Retrieved 8 September 2011.
- ^ Cloud, W.K.; Hill D.M.; Huffman M.E.; Jennings C.W.; McEvilly T.V.; Nason R.D.; Steinbrugge K.V.; Tocher D.; Unger J.D. & Youd T.L. (1970). "The Santa Rosa earthquakes of October, 1969" (PDF). California Geology. 23 (3). Archived from the original (PDF) on 19 March 2012. Retrieved 6 September 2011.
- ^ ISBN 978-0-8137-0007-6. Retrieved 5 September 2011.
- . Retrieved 5 September 2011.
- ISBN 978-0-87262-237-1. Retrieved 5 September 2011.
External links
- The International Seismological Centre has a bibliography and/or authoritative data for this event.