Regional Variation of Source Properties for Middle Earthquakes in a Subduction Region

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  • プレートの沈み込み帯における中小規模地震の震源スペクトルの地域性
  • プレート ノ シズミコミタイ ニ オケル チュウ ショウキボ ジシン ノ シン

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Abstract

The purpose of this study is to examine regional variation of source properties of middle earthquakes for the estimation of irregularities of the coupling along plate boundary, which is effective to the prediction of strong ground motions due to large interplate earthquakes. Strong motion records of acceleration and velocity have been observed at 5 stations since 1983 for earthquakes along the southern part of the Japan trench (off Fukushima prefecture). The magnitude Mj of these events ranges from 4.2 to 6.7 in the JMA scale. Classification of source properties, such as high-, medium-, and low-frequency earthquakes, is made by using the residual values for each earthquake from averaged acceleration response spectra S(T) in the period range shorter than 1s. S(T) is expressed as<br>logS(T)=a(T)MJ-(b(T)X+logX)+Ci(T)<br>where a(T), b(I), and Ci(T) are regression coefficients. Ci(T) is determined for each station. X is hypocentral distance. Judging from the dominant period of seismograms used for the magnitude determination, the JMA magnitude shows amplitude of surce spectrum at medium period. It is found that the high-frequency earthquakes mainly occur in the northwestern part of the subduction region, while the low-frequency earthquakes in the southeastern part. The stress drop of high-frequency events is about 300bar, which is meaningfully larger than the average stress drop over the fault plane of large events off Fukushima prefecture. In this region, a series of large earthquakes occurred in 1938. We classify 27 events of MJ≥6 from 1926 to 1938 including the events of the 1938 series into high-, medium-, and low-frequency earthquakes, based on the difference between MJ and MI MI is a magnitude scale determined from the data of seismic intensity, which shows amplitude of source spectrum at higher frequency. Regional variation of source properties of the old events is consistent with that of the recent earthquakes. This indicates that the spatial variation of source properties has not changed at least for the recent 50 years and suggests that the high frequency events show the distribution of sustained asperities along the plate boundary.

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