Detailed observations of California foreshock sequences: Implications for the earthquake initiation process

Abstract

Abstract. We find that foreshocks provide clear evidence for an extended nucleation process before some earthquakes. In this study, we examine in detail the evolution of six California foreshock sequences, the 1986 Mount Lewis (ML = 5.5), the 1986 Chalfant (ML = 6.4), the 1986 Stone Canyon (ML = 4.7), the 1990 Upland (ML = 5.2), the 1992 Joshua Tree (Mw= 6.1), and the 1992 Landers (Mw = 7.3) sequence. Typically, uncertainties in hypocentral parameters are too large to establish the geometry of foreshock sequences and hence to understand their evolution. However, the similarity of location and focal mechanisms for the events in these sequences leads to similar foreshock waveforms that we cross correlate to obtain extremely accurate relative locations. We use these results to identify small-scale fault zone structures that could influence nucleation and to determine the stress evolution leading up to the mainshock. In general, these foreshock sequences are not compatible with a cascading failure nucleation model in which the foreshocks all occur on a single fault plane and trigger the mainshock by static stress transfer. Instead, the foreshocks seem to concentrate near structural discontinuities in the fault and may themselves be a product of an aseismic nucleation process. Fault zone heterogeneity may also be important in controlling the number of foreshocks, i.e., the stronger the heterogeneity, the greater the number of foreshocks. The size of the nucleation region, as measured by the extent of the foreshock sequence, appears to scale with mainshock moment in the same manner as determined independently by measurements of the seismic nucleation phase. We also find evidence for slip localization as predicted by some models of earthquake nucleation.

An 8 month slow slip event triggers progressive nucleation of the 2014 Chile megathrust

“…Two concurrent models have been proposed to explain the initiation of seismic rupture [Dodge et al, 1996]. A first model assumes that the accelerated moment release observed before large earthquakes [Bowman and King, 2001] is triggered by a slow slip event on the fault interface [Bouchon et al, 2013;Ruiz et al, 2014;Dodge et al, 1996].…”

“…A first model assumes that the accelerated moment release observed before large earthquakes [Bowman and King, 2001] is triggered by a slow slip event on the fault interface [Bouchon et al, 2013;Ruiz et al, 2014;Dodge et al, 1996]. Alternatively a slow cascade of failures eventually may trigger the main shock [Dodge et al, 1996].…”

Mainshocks are aftershocks of conditional foreshocks: How do foreshock statistical properties emerge from aftershock laws

“…Foreshocks and aftershocks may result also from the dynamics of stress distribution on pre-existing hierarchical structures of faults or tectonic blocks [Huang et al, 1998;Gabrielov et al, 2000a,b;Narteau et al, 2000], when assuming that the scale over which stress redistribution occurs is controlled by the level of the hierarchy (cell size in a hierarchical cellular automaton model). Dodge et al [1996] argue that foreshocks are a byproduct of an aseismic nucleation process of a mainshock. Other possible mechanisms for both aftershocks and foreshocks are based on the visco-elastic response of the crust and on delayed transfer of fluids in and out of fault structures [Hainzl et al, 1999;Pelletier, 2000].…”

Adaptive Smoothing of Seismicity in Time, Space, and Magnitude for Time-Dependent Earthquake Forecasts for California

“…The predicted rate for the days of the Landers and El Mayor-Cucapah mainshocks is low for all models. The foreshock activity reported by Dodge et al (1996) and Hauksson et al (2010) did not increase the rate significantly by midnight of the day preceding these mainshocks (i.e., by the time forecasts are issued). This illustrates that larger probability gains can be achieved by reducing the forecast horizon to periods smaller than one day.…”