SD marks 150th anniversary of biggest quake
Written by Gary Robbins - San Diego Union-Tribune8:16 p.m., May 26, 2012 - Updated 8:21 p.m.
Most earthquakes that shake San Diego originate far beyond the city. But on May 27, 1862, a 5.9-.6.0 quake that appears to have started on or near North Island let loose, causing shaking felt as far north as Los Angeles. We were reminded of the event by Mark Legg, a Huntington Beach geophysicist who co-authored a scientific paper about the quake with Duncan Carr Agnew when they were both at the Scripps Institution of Oceanography.
Legg says, "I believe the epicenter was likely within the Rose Canyon fault zone and possibly along the Spanish Bight Fault (considering the wave observed in the Bay).
Here is the scientific paper they published on the quake in 1979:
Geophysicist Mark Legg Mark Legg Geophysical
The earthquake of May 27, 1862 is of special importance in making seismic risk estimates for San Diego. Most earthquakes that have caused damage in San Diego have been located in the Imperial Valley or northern Baja California. Although its location is not completely determinable, the 1862 earthquake seems to have been closer to the San Diego metropolitan area. This earthquake serves as a good example of the fragmentary nature of the historical record of California earthquakes: despite its size, it has received almost no mention in earthquake catalogs. The purpose of this paper is to give a brief description of the earthquake, based on contemporary documents.
The main shock occurred at about noon on May 27, 1862. Two accounts say that there were two shocks separated by some minutes, the second being stronger. In San Diego (the present Old Town) this shock stopped clocks and upset bottles and tumblers so that "many sets of crockery were demolished." The bell at the Army depot was set ringing. Understandably enough, all the people ran outside in fright, and many slept outside because they feared further earthquakes.
There apparently were no injuries, and no buildings were destroyed. However, many of the accounts mention damage to buildings, primarily cracking; it should be remembered that in 1862 most buildings in San Diego were either adobe or poor masonry. The newspaper reports say that "various adobe houses" were "cracked through and through." Some specific examples were the Pico adobe, which sustained several cracks, one passing through the wall; the Bandini adobe; and the 2-story Fitch adobe, which was "much sprung on its side wall." Of masonry buildings, the Whaley house cracked in several places, and the lighthouse tower was not thrown out of adjustment, nor was any glass broken. Some frame buildings were racked so that windows and doors were loosened n their frames, and windows and door hinges were broken. Several accounts mention cracks in low ground near the San Diego River, which washed over its banks. At La Playa (on Point Loma), cracks formed on the beach, water came out of the sand on the tidal flats, and a piling that had just been driven into the mud was shaken loose. Some bluff banks on the east side of Point Loma collapsed.
This shock was felt in Los Angeles, where it was termed "light" and in Anaheim. At Temecula and Aguanga it rattled plates on the shelves; at Aguanga it also caused a pile of sacks to fall over. At Mesa Grande it seemed to last about 10 seconds, and caused the building roof to creak. It was also felt at Lake Henshaw, El Cajon, Carlsbad, Rincon del Diablo, Vallecito, San Luis Rey, San Dieguito, San Felipe, and the Cuyamacas. It was not felt at Fort Yuma.
There are two lists of aftershocks. These show that earthquakes were felt every day at San Diego up to June 8, 12 days after the first shock, and relatively frequently for the rest of June. An aftershock at 10 a.m. on May 29 was described as "violent" at San Diego, and was felt at San Ysidro and Anaheim. One in the afternoon of May 31 was felt at San Diego and Temecula. A relatively large aftershock occurred at 10:30 a.m. on June 13; it was strong at San Diego and also felt at Penasquitos, but was not generally felt in Los Angeles or San Bernardino.
Based on the descriptions given here, we estimate that this earthquake caused shaking in San Diego on about intensity VI to VII on the Modified Mercali Scale. The upsetting of small objects and the extent of building damage (cracking but no serious damage) both suggest intensity VI, although the damage seems to have been greater that that associated with intensity VIII, but this is certainly too high, judging by the effects on buildings. It is possible that this reflects higher intensity on marshy ground. There is not enough information to estimate intensities elsewhere, except that they were lower, possibly IV-V, in the Temecular-Aguanga area, and lower still in Los Angeles.
The distribution of intensities suggests that San Diego was closer to the epicenter than any other place for which we have reports. This seems to be confirmed by the aftershock records, though there is an obvious bias because San Diego is the only place for which there is a continuous record. That the earthquake was not felt at Fort Yuma would seem to rule out a source in the Imperial Valley. On the whole, a location south or west of San Diego seems most likely; in the absence of more information we can say little else.
Source: http://www.utsandiego.com/news/2012/may/26/sd-marks-150th-anniversary-its-biggest-quake/--
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