Estimates of stress drop and crustal tectonic stress from the 27 February 2010 Maule, Chile, earthquake: Implications for fault strength

Abstract

[1] The great 27 February 2010 M w 8.8 earthquake off the coast of southern Chile ruptured a ∼600 km length of subduction zone. In this paper, we make two independent estimates of shear stress in the crust in the region of the Chile earthquake. First, we use a coseismic slip model constrained by geodetic observations from interferometric synthetic aperture radar (InSAR) and GPS to derive a spatially variable estimate of the change in static shear stress along the ruptured fault. Second, we use a static force balance model to constrain the crustal shear stress required to simultaneously support observed fore-arc topography and the stress orientation indicated by the earthquake focal mechanism. This includes the derivation of a semianalytic solution for the stress field exerted by surface and Moho topography loading the crust. We find that the deviatoric stress exerted by topography is minimized in the limit when the crust is considered an incompressible elastic solid, with a Poisson ratio of 0.5, and is independent of Young's modulus. This places a strict lower bound on the critical stress state maintained by the crust supporting plastically deformed accretionary wedge topography. We estimate the coseismic shear stress change from the Maule event ranged from −6 MPa (stress increase) to 17 MPa (stress drop), with a maximum depth-averaged crustal shear-stress drop of 4 MPa. We separately estimate that the plate-driving forces acting in the region, regardless of their exact mechanism, must contribute at least 27 MPa trench-perpendicular compression and 15 MPa trench-parallel compression. This corresponds to a depth-averaged shear stress of at least 7 MPa. The comparable magnitude of these two independent shear stress estimates is consistent with the interpretation that the section of the megathrust fault ruptured in the Maule earthquake is weak, with the seismic cycle relieving much of the total sustained shear stress in the crust.

Patterns and mechanisms of coseismic and postseismic slips of the 2011 M W 7.1 Van (Turkey) earthquake revealed by multi-platform synthetic aperture radar interferometry

“…The largest shear stress drop located in the earthquake zone reaches up to -12 MPa (decreased) (Figure 9 (a)), which is compatible with the stress drop of 17 MPa during the 2010 MW 8.8 earthquake (Luttrell et al, 2011). The upward shear stress increases along the fault plane and reaches a maximum of 5 MPa that is also nearly equivalent in magnitude with the Mule earthquake.…”

How weak is the subduction zone interface?

“…In this scenario, to match the observed age-independent subduction velocity in nature (Figure 2a), the subduction zone interfacial shear stresses must increase with age in order to counteract the increasing available driving force. However, such an age-dependent and increasing interfacial shear stress contradicts worldwide observations of heat flux, focal mechanisms, GPS, and in situ stress measurements, which all indicate very low values of interfacial shear stresses (<20 MPa) regardless of plate age [Bird, 1978;Magee and Zoback, 1993;Wang et al, 1995;Springer, 1999;Currie et al, 2002;Grevemeyer et al, 2003;Luttrell et al, 2011]. The notion of an age-dependent and increasing interfacial shear stress also disagrees with numerical modeling results that imply the opposite scenario, in which interfacial stresses should decrease with age due to strain weakening [Gerya et al, 2002;Gerya and Meilick, 2011].…”

“…This hypothesis has been challenged by recent calculations, which take into account the nonneutral state of stress in the volcanic arc [e.g., Richardson and Coblentz, 1994] and show that interfacial stresses along the Andean plate interface are in the range of~14-16 MPa [Seno, 2009]. Likewise, observations have implied low shear stresses (<20 MPa) at subduction interfaces worldwide [Bird, 1978;Magee and Zoback, 1993;Wang et al, 1995;Springer, 1999;Currie et al, 2002;Grevemeyer et al, 2003;Luttrell et al, 2011]. Higher estimates of shear stress have also been suggested: 15-30 MPa [Zhong and Gurnis, 1994], 14-43 MPa [Tichelaar and Ruff, 1993], an average of~40 ± 17 MPa with values up to 50-100 MPa [Honda, 1985;Von Herzen et al 2001],~100 MPa [Molnar and England, 1990;Alcock et al, 2005], and 200-300 MPa [Turcotte and Schubert, 1973;Sleep, 1975].…”

Testing the Coulomb stress triggering hypothesis for three recent megathrust earthquakes

“…1). In order to examine the sensitivity of ΔCFF computation to slip models, we also use the variable-slip models proposed by Rhie et al (2007) and Ammon et al (2005) for the Sumatra-Andaman earthquake, by Delouis et al (2010) and Luttrell et al (2011) for the Maule earthquake, and by Yokota et al (2011) and Gusman et al (2012) for the Tohoku-Oki earthquake (Additional file 1: Figure S1). For the case of the Sumatra-Andaman earthquake, we include the ΔCFF due to the Mw 8.5 Nias earthquake that occurred in March 2005 southeast of the 2004 source region using the fault-slip model obtained from teleseismic body-wave inversion (Konca et al 2007).…”