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

HISTORICAL EARTHQUAKES OF THAILAND,BURMA, AND INDOCHINAS. PrachuabMeteorological Department, Bangkok, ThailandABSTRACTHistorical earthquake data from 642 BC to 1983 has been analyzed andreported in this paper in order to understand the seismicity and associatedphenomena. From quantitative seismicity analysis, Thailand, Burma andIndochina have been divided into 12 seismic zones corresponding to theseismotectonic structure and geologic features in this region.Using the well-known =magnitude-frequency" relation of Gutenbergand Richter and a modification of this relation by Peter Welkner, thevalues of u and b are investigated for the period 1900 to 1983. The valuesof a and b obtained for the region are 6.654 and 0.75 respectively, whichare typical of moderately active seismic zones. The recurrence periodsfor earthquakes of magnitude 7.0 and 7.5 are calculated as 3 and 8 years,respectively.1. IntroductionA study of historical earthquakes in Thailand, Burma and Indochina providesnot only a basis for statistical analysis of earthquakes but also for the earthquakehazard studies in this area. The data used for analysis are taken from the reportSeismicity Data of Thailand and Adjacent Areas (Nutalaya and Sodsri, 1983).This earthquake report contains two main catalogs. The first catalog contains thehistorical earthquake data for the period 642 BC to 1900, accumulated from thehistorical texts, annuals, stone inscriptions and astrological documents, and fromthe Geological Survey of India Records and Memoirs. The second catalog containsthe instrumental earthquake data for the period 1900 to 1983, accumulated fromvarious international seismological centers (eg. USGS, ISC, MOS) and from theThai Meteorological Department. The seismicity map of the region between 5"N to25'N latitude and 90°E to llOoE longitude for the period 642 BC to 1983 is shownin Figure 1.Nutalaya and Sodsri (1984) divided Thailand, Burma and Indochina into 12seismic source zones, using quantitative seismicity analysis (Figure 2). Table 1 liststhe zone name, the number of seismic events, and the maximum magnitude eventfor each seismic source zone. These seismic source zones were identified by plottingthe historical epicenters along with the geological and tectonic information. Thesize and shape of each source zone reflects the geographic distribution of seismicitytogether with the regional tectonic style. Only zone C is characterized by seismicityassociated with individual active faults as documented by historical records.Magnitude-frequency analysis has been determined for the entire region for theperiod 1900 to 1983. The objective of this analysis is to investigate the earthquakesthat occurred during this century, the results of which indicate the statusof historical seismicity in the region.Historical Seismogramsand Earthquakes of the World253Copyright Q 1988 by Academic Press, Inc.All rights of reproduction in any form reserved.