Two complementary stress release processes based on departures from Omori’s law

Abstract

Three aftershock series in Southern California, associated with the mainshocks of Landers (1992), Northridge (1994) and Hector Mine (1999) are interpreted as the superposition of a long stress relaxation process, in agreement with the Omori’s law, and a number of short episodes with sudden stress release. These short episodes are detected as departures from the expected Omori’s law. Aftershocks following a long stress relaxation process are designed as leading aftershocks (LA) and their generation rate fits well to the modified Omori’s law (MOL). The rest of aftershocks correspond to the different episodes of sudden stress release, every one designed as a cascade (CA). Cascades are found to be characterized by four basic properties. First, although the number of aftershocks NC belonging to every cascade fluctuates greatly along the stress relaxation process, a clear positive trend of NC with the elapsed time since the mainshock is observed in the initial phase of the aftershock sequence. After a critical elapsed time, very short in comparison with the aftershock sequence length, this positive trend diminishes significantly or even becomes negative. Second, the aftershock generation rate, GR, for every cascade, is almost constant. Third, GR for the successive CAs, represented as a function of its starting time measured since the mainshock, decreases according to a power-law. Four, the validity of the Gutenberg-Richter law is preserved for the sequence of all aftershocks belonging to cascades, with values of the b-parameter quite similar to those deduced for the complete aftershock series. Additionally, some statistical and fractal (self-affine) properties of CAs are analyzed. Given that the number of aftershocks with minimum magnitudes assuring catalogue completeness ranges, for the three seismic crises, from 6,000 up to 20,000, properties concerning LA and CA can be established with a high degree of confidence. As a conclusion, a single stress relaxation process following the Omori’s law should be discarded. A physical explanation, based on the complex spatial patterns of the stress field and tectonic fractures, as well as on the two proposed relaxation processes, is qualitatively discussed.