Geology of the earthquake source: an introduction

Abstract

Earthquakes arise from frictional 'stick-slip' instabilities as elastic strain is released by shear failure, almost always on a pre-existing fault. How the faulted rock responds to applied shear stress depends on its composition, environmental conditions (such as temperature and pressure), fluid presence and strain rate. These geological and physical variables determine the shear strength and frictional stability of a fault, and the dominant mineral deformation mechanism. To differing degrees, these effects ultimately control the partitioning between seismic and aseismic deformation, and are recorded by fault-rock textures. The scale-invariance of earthquake slip allows for extrapolation of geological and geophysical observations of earthquake-related deformation. Here we emphasize that the seismological character of a fault is highly dependent on fault geology, and that the high frequency of earthquakes observed by geophysical monitoring demands consideration of seismic slip as a major mechanism of finite fault displacement in the geological record.

Multi-scale imaging of a slow active fault zone: contribution for improved seismic hazard assessment in the Swiss Alpine foreland

“…b Estimated rupture surfaces for M 0 to M 6 earthquakes, using the same scaling as in a. Scaling relationships are excerpted from Bohnhoff et al (2010) and Fagereng and Toy (2011). c Regional tectonic map featuring the major fault systems in the Alpine foreland across the Jura belt and the western Swiss Molasse basin: V Vuache, SC St-Claude, A Arve, Mz Morez, M Mouthe, LS la Sarraz, P Pontarlier, LL la Lance, LF la Ferrière, F Fribourg (Fendringen and St. Sylvester); and RL, Rhenish Lineament [modified from Chevalier et al (2010)].…”

“…The sequences of 1987, 1995 and 1999 are projected using the relative location by Kastrup et al (2007). The scaled rupture size of the events is estimated following Bohnhoff et al (2010) and Fagereng and Toy (2011), and considering -1.5 \ M L \ 0.5 as M 0, 0.5 \ M L \ 1.5 as M 1, 1.5 \ M L \ 2.5 as M 2, 2.5 \ M L \ 3.5 as M 3 and 3.5 \ M L \ 4.5 as M 4 (Fig. 11b).…”

Geological evidence for shallow ductile-brittle transition zones along subduction interfaces: example from the Shimanto Belt, SW Japan

“…Recently, additional types of failure have been found along active subduction zones, such as slow slip events (e.g., Hirose et al, 1999;Kato et al, 2012;Ito et al, 2013), episodic tremor and slip (e.g., Rogers and Dragert, 2003;Obara et al, 2004, Ishida et al, 2013, very low frequency earthquakes (e.g., Ito and Obara, 2006;Sugioka et al, 2012), and dynamic overshoot to the trench axis (e.g., Ide et al, 2011). Studies have been conducted to connect geologic deformation features from on-land accretionary complexes with seismic and aseismic deformations or with the newly found failure styles (e.g., Ikesawa et al, 2003;Kimura et al, 2007;Meneghini et al, 2010;Fagereng and Toy, 2011;Saito et al, 2013). Pseudotachylyte and fluidized shear zones, which indicate dynamic weakening of the fault during displacement, were found in on-land accretionary complexes; this suggests that the geologic features of the seismogenic slips along subduction interfaces are visible in on-land accretionary complexes (e.g., Ikesawa et al, 2003;Kitamura et al, 2005;Ujiie et al, 2007;Meneghini et al, 2010;Hashimoto et al, 2012;Saito et al, 2013).…”

“…The evolution of deformation features and deformation mechanisms along subduction zones has been revealed from on-land geology in exhumed accretionary complexes, not only in the Shimanto Belt (e.g., summarized in Kimura et al, 2007) but also in other accretionary complexes (e.g., Fisher and Byrne, 1987;Fagereng and Toy, 2011;Rowe et al, 2013). By reviewing many previous studies, Fagereng and Toy (2011) pointed out that ductile flow by diffusion-precipitation creep and cataclastic deformation coexist as the major deformation mechanisms in seismogenic crust.…”

“…By reviewing many previous studies, Fagereng and Toy (2011) pointed out that ductile flow by diffusion-precipitation creep and cataclastic deformation coexist as the major deformation mechanisms in seismogenic crust. The spatiotemporal relation between plastic and brittle deformations, however, has been expressed as deformations in a heterogeneous mélange rheology (e.g., Fagereng and Sibson, 2010).…”