Occurrence Apparent Velocities for Identification and Quantification of Space-Time Clustering Precursory to a Large Earthquake. Application to Large (M >7.0) Earthquakes in Southern California and Northern Baja California

Space-time seismic clusters, localized bursts of seismic activity, are a feature of background seismicity before the occurrence of large earthquakes, a feature that agrees with observations of diminishing Gutenberg-Richter b-value, fractal dimension, and entropy, and is therefore suggestive of high stress. However, identification and quantification of these space-time clusters, particularly when they are small, is not an easy task and requires a priori assumptions. A novel method for space-time cluster identification, based on an extension of the concept of apparent velocities, is proposed because space-time clusters in the background seismicity have a particular signature in the apparent velocity domain. The contents of histogram peaks due to clusters in the apparent velocity histogram can be used to quantify the cluster activity compared with null hypothesis levels. Identification of the earthquakes corresponding to the apparent velocities in the peaks allows identification of cluster activity in time and space. Apparent velocity peaks do appear in real catalog data for southern California and northern Baja California before the Landers 1992 M = 7.3, Hector Mine 1999 M = 7.1, El Mayor-Cucapah 2010 M = 7.2, and Ridgecrest 2019 M = 7.1 earthquakes, and they appear only within 15 to 25 years before the occurrence of large earthquakes. They are not observed either long before the large earthquakes or after them, and hence could be related to high local states of stress and be of value as a possible precursory observable.