Significant seismic streaks corresponding to lithological contrasts in Mesozoic and Paleozoic accretion units, southwest Japan

Abstract:

While high-density seismic streaks are commonly seen in aftershocks of a strike-slip earthquake, significant linear clusters in background seismicity are rare. A typical example of such sustained streaks is observed along the creeping section of the San Andreas fault, in which strain localization associated with frictional heterogeneity takes a responsibility. Here we show other examples from Tamba and Wakayama regions, around Osaka-Kyoto district, where a number of several-to-20-km-long seismic streaks are observed. We explore the role of geologic heterogeneity into the seismicity comparing spatial distribution of earthquakes with geologic structure in both regions, where a significant high background rate of seismicity has been continuously recorded since the mid-1900. Epicenters of numerous small earthquakes are located mainly on the Mesozoic metamorphic rocks and Mesozoic-Paleozoic accretion units, whereas low seismicity is characterized in granite and ultra-mafic rocks. Within the Wakayama seismic zone, in particular, we observe many E-W and ENE-WSW trending dense seismic clusters in hypocenters of Japan Meteorological Agency (JMA) data relocated with the hypo DD algorithm of Waldhauser and Ellsworth (2000). Most of the E-W trending seismic clusters possibly correspond to the E-W trending local scale geologic faults, folds, bedding planes, and schistosity. However, well-determined fault plane solutions by JMA and the National Research Institute for Earth Science and Disaster Prevention (NIED) show NS-trending reverse faults corresponding to EW compression. We further sought the focal mechanisms for smaller earthquakes using waveform data recorded in the SATARN seismic network system of DPRI, Kyoto University. As a result, among the many reverse fault mechanisms, we found some amounts of strike-slip ones, which may associate with the visible EW-trending seismic clusters.