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

Historical Earthquakes of Thailand, Burma, and Indochina 261101" 104"EI 1KunmingEXPLANATION25'h22Figure 5. Seismic and tectonic structures of Yunnan Province, China and NorthVietnam. Fault information compiled from Fontaine and Workman (1978) and TectonicMap of China (1975, in Chinese).3. Magnitude-frequency AnalysisThe seismic data used for this magnitude-frequency analysis is taken from variousinternational seismological institutes such as USGS, ISC, and MOS, and from theThai Meteorological Department. The area of study is bounded by the coordinates5'N to 25'N latitude and 90'E to IlOOE longitude; the time period of seismic datais 1900 to 1983.The temporal variation of earthquake occurrence is shown in Figure 8. Afterthe year 1960, the total number of earthquakes increased due to installation of theWorld-wide Standardized Seismograph Network (WWSSN). The lack of data priorto the WWSSN era is due mainly to the absence of sufficient seismic instruments.Most of the earthquake observations prior to the instrumentation era were basedon human perceptions, therefore earthquakes having insufficient strength to be feltby an observer would not be recorded.Figure 9 shows the magnitude-frequency distribution. The range of magnitudevalue used was mb > 4.0. Figure 9 shows that earthquakes having magnitude 4.5