Localization and coalescence of seismicity before large earthquakes

Yehuda Ben-Zion, & Ilya Zaliapin

Under Review March 15, 2020, SCEC Contribution #10048

We examine localization processes of low magnitude seismicity in relation to the occurrence of large earthquakes using three complementary analyses: (i) estimated production of rock damage by background events, (ii) evolving occupied fractional area of background seismicity, and (iii) progressive coalescence of individual earthquakes into clusters. The different techniques provide information on different time scales and the spatial extent of weakened damaged regions. Techniques (i) and (ii) employ declustered catalogs to avoid the occasional strong fluctuations associated with aftershock sequences, while technique (iii) examines developing clusters in entire catalog data. We analyze primarily earthquakes around large faults that are locked in the interseismic periods, and examine also as a contrasting example seismicity from the creeping Parkfield section of the San Andreas fault. Results of analysis (i) show that the M > 7 Landers 1992, Hector Mine 1999 and Ridgecrest 2019 earthquakes in Southern California were preceded in the previous decades by generation of rock damage around the eventual rupture zones. Analysis (ii) reveals localization (reduced fractional area) 2-3 yr before these M > 7 earthquakes, along with the M > 7 El Mayor-Cucapah 2010 earthquake in Baja CA and the D├╝zce 1999 earthquake in Turkey. Results with technique (iii) indicate that individual events tend to coalesce rapidly to large clusters in the final 1-2 yr before the mainshocks. Corresponding analyses of data from the Parkfield region show opposite delocalization patterns and decreasing clustering before the 2004 M6 earthquake. Continuing studies with these techniques, combined with analysis of geodetic data and insights from laboratory experiments and model simulations, can improve the ability to track preparation processes leading to large earthquakes.

Citation
Ben-Zion, Y., & Zaliapin, I. (2020). Localization and coalescence of seismicity before large earthquakes. Geophysical Journal International, (under review).