Astronomy and Astrophysics – Astronomy
Scientific paper
Jun 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004geoji.157.1255n&link_type=abstract
Geophysical Journal International, Volume 157, Issue 3, pp. 1255-1268.
Astronomy and Astrophysics
Astronomy
3
Closely Located Events, Earthquake Source Region, Heterogeneous Structure, Waveform Correlation
Scientific paper
The way in which cross-correlation coefficients of seismograms for closely located events change with frequency and event separation was investigated by using records of aftershocks of the 1995 Hyogo-Ken Nanbu, Japan, earthquake (Mw= 6.9). For 11 master events (M between 2.9 and 3.5) on the mainshock fault plane and smaller events (M greater than 2.0) within 5 km of the master events, the relative hypocentre was determined with respect to each master event and cross-correlation coefficients of seismograms were calculated for a time window of 2 s including direct P waves for pairs of events in each cluster at two stations with an average hypocentral distance of 36 km and 42 km. Relative hypocentres were located within errors of several hundred metres for most of the earthquakes. Cross-correlation coefficients are high for frequencies below 8 Hz and decrease with increasing frequency above 8 Hz at both stations. It was also found that cross-correlation coefficients decay with increasing event separation. If the source-time function is assumed to be simple for small earthquakes, it may be said that dependence of cross-correlation coefficients on event separation below 8 Hz is mainly influenced by the heterogeneous structure around the earthquake source. We introduce a correlation distance where cross-correlation coefficients first become lower than 0.6 with increasing event separation. The correlation distance exhibits a clear regional difference: it is less than 4.5 km in the Awaji region and more than 4.5 km in the Kobe region. This result implies that heterogeneity of velocity structure around the earthquake source is stronger in the Awaji region than in the Kobe region. It is notable that both low- and high-frequency seismic waves radiated strongly from the Awaji region at the time of the mainshock. The correlation distance may be an indicator of heterogeneous structure around an earthquake source, which may have a close relationship to the rupture process itself.
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