Historical Seismograms - Evidence from the AD 2000 Izu Islands ...

270 T. R. Toppozada, C. R. Real, and D. L. ParkTable 1. ContinuedNo. Date (Greenwich) Lat.’N Long.’W Quality I M53. 1948/12/04 33.93 116.38 VII 6.554. 1952/07/21 35.0 119.0 XI 7.2 & 6.455. 1952/07/23 35.37 118.6 V 6.156. 1952/07/29 35.4 118.75 VII 6.157. 1952/11/22 35.73 121.2 VII 6+58. 1954/03/19 33.3 116.2 VI 6.259. 1954/12/21 40.8 123.9 VII 6.5GO. 1966/09/12 39.42 120.15 VII 6+61. 1968/04/09 33.2 116.13 VII 6.462. 1971/02/09 34.4 118.4 XI 6.463. 1979/10/15 32.63 115.32 IX 6.464. 1980/05/25 37.6 118.83 VII 6.3 & 6.465. 1980/05/27 37.48 118.82 VI 6.366. 1980/11/08 41.0 124.64 VII 6.967. 1983/05/02 36.2 120.3 VIII 6.468. 1984/04/24 37.32 121.7 VII 6.2Notes:“No.” is also Identifying Number on the maps.Quality of pre-1900 epicenters: A = fault rupture identified, or uncertainty less than25 km; B = uncertainty up to 50 km; C = uncertainty up to 100 km.Quality of post-1900 epicenters is generally “A” by the above criteria.I = Maximum reported Modified Mercalli intensity.case is the great 1872 earthquake (event 13), for which 100 km of surface faultingwas reported on the Sierra Nevada fault system. Even though the epicenter couldhave been anywhere on this rupture, the quality is A because faulting was identified.Figure 1 shows the areas damaged by the 11 earthquakes that occurred from 1800to 1868. Earthquakes 1, 2, and 3 were damaging at the Missions on the southernCalifornia coast, and earthquakes 4 and 5 were damaging around San FtanciscoBay. Event 6 was damaging near Fort Yuma. As stated previously, before the 1849gold rush, records of earthquake damage were restricted to the coast southwardfrom San Fkancisco. During the 1850’s, newspaper coverage spread northeastwardfrom San Francisco into the gold country east of Sacramento, providing reports forevent 9. Event 7 (area from Agnew and Sieh, 1978) was a great earthquake onthe San Andreas fault having no obvious foreshocks or aftershocks, although twopossible foreshocks of M 5 6 occurred in the preceding two hours (Sieh, 1978).Events of M 5 6.5 may not have been identified in central California in 1857. Bycontrast, the increase in seismicity in San Francisco Bay area that started in 1858and culminated in 1868 is well substantiated (events 8, 10, and 11).Figure 2 shows the areas damaged by earthquakes that occurred from 1869 to1906. Two great earthquakes occurred in this period. The 1872 Sierra Nevadaearthquake had identifiable aftershocks that progressed northward (events 13, 14,15, 16). No obvious foreshocks were identified, although events smaller than M6.5could have been missed in eastern California. The 1906 San Francisco earthquakewas preceded by a remarkable increase in damaging earthquakes starting in 1892(events 22, 23, 25, 26, 27). No significantly damaging aftershocks of the 1906 eventwere noted.