No aftershocks, fluid-driven aftershocks, and Omori’s Law

<p>Aftershock sequences follow three empirical laws; Gutenberg Richter, Omori, and Bath. Unless they don't. This raises the question as to why most earthquakes follow empirical laws, while other earthquakes generate few, if any, aftershocks. For example, a magnitude 7.1 earthquake in Mexico in 2017 and a magnitude 8 earthquake in Peru in 2019 generated no aftershocks, while a magnitude 7.1 earthquake in 2019 in California and a magnitude 6.4 earthquake in 2020 in Puerto Rico generated thousands of aftershocks. In this work, I show from numerical modelling and comparisons with data that the differing behaviours rests with the presence of high-pressure fluids at depth. Using a simple model of non-linear diffusion, I compare model results with well-located aftershocks from four Southern California earthquakes and show strong spatial correlation between measured hypocenters and calculated fluid pressure emanating from a high-pressure source. I also show that Omori's Law arises from permeability dynamics. That is, permeability: 1) is effective-stress dependent, 2) undergoes a co-seismic step-like increase, and 3) exponentially heals through either precipitation processes or tectonic re-compaction. I find excellent temporal correlation (Omori's Law) between the number of measured and modelled earthquakes from the strike-slip earthquakes of Joshua Tree and Landers (1992), the strike-slip Hector Mine earthquake (1999), and the thrust Northridge earthquake (1994). Finally, I demonstrate that the fit to the Omori-Utsu Law depends only on the rate of permeability recovery, and argue that all rich aftershock sequences are fluid-driven, while fluid-absent geodynamic settings produce few, if any, aftershocks. </p>

SOME PRELIMINARY RESULTS ON THE DISTRIBUTION OF AFTERSHOCK SEQUENCES IN JAPAN-KURIL ISLANDS AND KAMCHATKA

study on the aftershock sequences distributed along the subductions in Japan and Kuril islands, as well as in Kamchatka is undertaken. Aftershock sequences, having a main shock magnitude Mw >7.0, during the time period 1973-2013 are taken into account. The data used (mainshocks, aftershocks and foreshocks if there are any) are restricted in shallow focal depths. A large earthquake in Japan Trench (11 March 2011 / Mw=9.0) occurred and for this reason the investigated area is of particular interest. Our study is concentrated on the spatial distribution of some parameters [Mc, a, b (Gutenberg-Richter distribution) and p, c, k (Omori’s law)] closely associated with the seismic sequences statistics.