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 on the spatial extent of weakened damaged regions. Techniques (i) and (ii) use declustered catalogues to avoid the occasional strong fluctuations associated with aftershock sequences, while technique (iii) examines developing clusters in entire catalogue data. We analyze seismicity catalogs from Southern California, Parkfield section of the San Andreas Fault, Alaska, and region around the 1999 Izmit and Duzce earthquakes in Turkey.
We introduce a technique for quantifying relative localization (concentration) of spatial distributions (similar to the receiver operating characteristic) and apply it to the estimated intensity distributions of background earthquakes. Analysis within the rupture zones of large earthquakes indicate increase of localization prior to the Landers (1992, M7.3), El Mayor-Cucapah (2010, M7.2), Ridgecrest (2019, M7.1), Denali (2002, M7.9), and Duzce (1999, M7.2) mainshocks. We also observe ongoing damage production by the background seismicity around these rupture zones several years before their occurrences. In contrast, we observe decrease of localization prior to the Parkfield (2004, M6.0) mainshock in the creeping section of the San Andreas Fault.
The coalescence process is represented by a time-oriented graph that connects each earthquake in examined catalog to all earlier earthquakes with the earthquake nearest-neighbor proximity below a specified threshold. We examine the size of the clusters that correspond to low thresholds, and hence represent active clustering episodes. We document increase of the average cluster size prior to the Landers, El Mayor-Cucapah, Ridgecrest and Duzce mainshocks, and decrease of the average cluster size prior to the Parkfield mainshock. The results of our complementary localization and coalescent analyses consistently indicate progressive localization of damage prior to the largest earthquakes on non-creeping faults and de-localization on the creeping Parkfield fault. These findings are consistent with analysis of acoustic emission data. The study is a step towards developing methodology for analyzing the dynamics of seismicity in relation to preparation processes of large earthquakes, which is robust to spatio-temporal fluctuations associated with aftershock sequences, data incompleteness and common catalog errors.
 Ben-Zion, Y., & Zaliapin, I. (2020). Localization and coalescence of seismicity before large earthquakes. Geophysical Journal International, 223(1), 561-583. https://doi.org/10.1093/gji/ggaa315