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

268 T. R. Toppozada, C. R. Real, and D. L. Park3. Method of AnalysisCatalogs of felt earthquakes were prepared by Townley and Allen (1939) through1927, and by the U.S. Department of Commerce (U.S. Earthquakes) starting in1928. Since 1950, U.S. Earthquakes has also included isoseismal maps showing theareal distribution of the reported intensities. For events prior to 1950, we preparedisoseismal maps for 140 damaging earthquakes dating back to 1812 (Toppozadaet al., 1981, 1982). In addition to using the intensity information available fromthe catalogs, we also used information from original sources such as newspapers.Some 14,000 newspaper issues were searched, and about a quarter had earthquakereports. Obtaining information from the original sources is preferable because itavoids any previous misinterpretations.Damaging earthquakes were identified from the existing catalogs, and newspaperswere searched for the surrounding area and the days following the earthquake. Thereported effects were interpreted in terms of the Modified Mercalli (MM) intensityscale and isoseismal maps were drawn showing the localities reporting the earthquakeand the intensity at each locality. The resulting isoseismal maps were usedto determine the approximate epicentral locations and magnitudes of the earthquakes.The epicentral location was estimated to fall at or near the center of thezone of highest intensity and to be near any locations reporting foreshocks or aftershocks.The magnitude was estimated by comparing the size of the areas shaken atvarious levels of intensity to those of modern earthquakes in the same region thathave known instrumental magnitudes (Toppozada, 1975). For this comparison tobe most meaningful, the isoseismal maps of the instrumental and pre-instrumentalearthquakes should be prepared in the same way. The interpretation of reportedeffects in terms of the intensity scale can vary, and it is important that a givenvalue of intensity be based consistently on the same reported effects. For example,people’s reactions and ground failure are less reliable indicators of intensity thanare permanent effects on buildings and their contents.4. Seismicity, M > 6Some 70 earthquakes of M > 6 are known to have occurred since 1800 in Californiaand within 50 km of the state borders. These occurrences are listed in Table 1and are numbered sequentially. The locations of these earthquakes are shown inFigures 1, 2, and 3 and are identified by the numbers used in Table 1. The areasdamaged (MM intensity VII or greater) are outlined. For earthquakes not damaginga definable area, the epicentral location is plotted as a square.The largest earthquakes of about Ma or greater occurred in 1857, 1872, and1906. The only one of these to clearly affect the seismicity before it occurredwas the 1906 San Fkancisco earthquake. Any increase in local seismicity precedingthe 1857 and 1872 earthquakes would have been difficult to detect because theyoccurred in remote and sparsely populated areas. In the decade or two before the1906 earthquake, there was a remarkable increase of damaging earthquakes withinabout 50 km of the zone that was to rupture. That level of seismicity is the highestfor the Bay area, particularly the north Bay, during the century and a half of record.For pre-1900 earthquakes, the uncertainty of the epicentral location is indicatedunder “Quality” in Table 1. Quality “A” indicates either an epicentral uncertaintyless than 25 km, or that surface faulting was identified. An example of the latter