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

1. IntroductionEARTHQUAKE HISTORY OF CALIFORNIAT. R. Toppozada, C. R. Real, and D. L. ParkeCalifornia Department of ConservationDivision of Mines and Geology690 Bercut Drive, Sacramento, CA. 95814, USAThe history of earthquake occurrences is basic to assessing and mitigating earthquakehazards. Statewide seismographic coverage in California was established in1932. Before 1932, the record of earthquakes comes from the effects reported by thepopulation. Interpretation of the pre-instrumental record provides the approximatelocation and sizes of earthquakes. Because the pre-instrumental record covers alonger period than the instrumental record, it includes a major proportion of thelarge earthquakes, which are rare events.The earthquake history shows what the strongest earthquake effects have beenin various parts of the state. Critical structures such as nuclear power plants,dams, and hospitals are then designed to withstand at least these historical effects.The earthquake history also suggests the location of seismically active fault zones.If further geological study of these faults shows that they were active during theHolocene (last 10,000 years), they are zoned so that structures are not built onactive fault traces.The rate of occurrence of earthquakes in various fault zones, when combined withgeologic and geodetic rates of deformation, helps to define the recurrence interval ofdestructive earthquakes in these zones. The fault zones that are most likely to rupturein the near future are then selected for the development of emergency responseplans, and for the concentration of earthquake prediction monitoring efforts.2. Completeness of the RecordThe written history of damaging California earthquakes begins in 1800, whenthe first earthquake damage was reported at Mission San Juan Bautista. From thattime on, earthquakes were reported only if they were damaging (Modified Mercalliintensity VII or greater) at any of the missions, which were located along the coastin central and southern California. Thus, the record is probably complete backto 1800 only for earthquakes of about magnitude 7 (M7) or greater that occurredwithin 70 km of the coast south of San Francisco.Newspaper coverage came to California with the 1849 gold rush. Newspapersreported earthquake felt effects as well as damage effect. During the 1850’s, 1860’s,and 1870’s, newspaper coverage was concentrated around San Francisco Bay andin the gold mining area east of Sacramento, with limited coverage along the coast.Thus, from 1850 the record is probably complete for earthquakes of about A4 6or greater around San Francisco Bay and in the gold country. From 1850-1880,in other areas of California the record is probably complete only for M7 or largerearthquakes. After 1880, with the increase of newspaper coverage, the statewiderecord of earthquakes is probably complete for events of M6 or greater. Starting inthe early 1900’s, with the advent of statewide seismographic recording, the recordis complete for M6 or greater earthquakes.Historical Seismogramsand Earthquakes of the World267Copyright 1C IO8X by Academic Press, Inc.All rights of reproduction in any form reserved.