Inversion for absolute deviatoric crustal stress using focal mechanisms and coseismic stress changes: The 2011 M9 Tohoku‐oki, Japan, earthquake

Abstract

[1] The absolute magnitude of stress in the crust and the shear strength of faults are poorly known, yet fundamental quantities, in lithospheric dynamics. While stress magnitude cannot be measured directly, deviatoric stress state can be inferred indirectly from focal mechanism solutions collected before and after an earthquake. We extend a standard stress inversion for normalized stresses to invert for the 3-D spatial distribution of absolute deviatoric stress and variation of fault strength with depth using focal mechanism solutions and coseismic stress changes produced by large earthquakes. We apply the method to the 2011 M9 Tohoku-oki, Japan, earthquake. The northern Japan fore-arc crust between 5 and 15 km depths appears to be weak with fault strength of 40-90 MPa, consistent with a coefficient of friction of 0.2-0.5. The M9 Tohoku-oki coseismic stress change was large enough, relative to the ambient stress, to rotate the principal stress directions typically~20°i n the upper 20 km of the crust. The data from Japan require a heterogeneous ambient deviatoric stress field with short-wavelength (~20-50 km) fluctuations in principal stress orientations.Citation: Yang, Y.-R., K. M. Johnson, and R. Y. Chuang (2013), Inversion for absolute deviatoric crustal stress using focal mechanisms and coseismic stress changes: The 2011 M9 Tohoku-oki, Japan, earthquake,

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 difference in orientations may indicate that the stress in the earthquake zone was decreased to a low level during the coseismic rupture, which is consistent with one seismicity analysis for this event (Görgün, 2013). If the focal mechanisms of a number of earthquakes before the mainshock can be collected, it would be possible to estimate deviatoric stress in the earthquake zone, as has been done for the 2011 Tohoku-Oki, Japan earthquake (Yang et al, 2013b). However, this analysis is far beyond what is possible using only InSAR-determined slip models.…”

Stress change before and after the 2011 M9 Tohoku-oki earthquake

“…In particular, rotations of inferred major compressive stress axes have been documented for the co-seismic effect near the fault zone of the Tohoku-oki event (Hasegawa et al, 2011;Hardebeck, 2012), and have been used in joint stress inversions (Yang et al, 2013). These studies suggested near-complete shear stress drop along the fault due to the M9, indicating ∼ 10 MPa pre-earthquake deviatoric stress levels close to the fault interface Hardebeck, 2012), and closer to ∼ 50 MPa in the upper crust (Yang et al, 2013). Yoshida et al (2012) studied the change in seismicity in the northern Honshu area before and right after the M9 and compared results with estimates from coseismic stress modeling.…”

The spatial distribution of earthquake stress rotations following large subduction zone earthquakes

“…Numerical inversion (Wesson and Boyd 2007;Yoshida et al 2012;Yang et al 2013) or forward modeling (e.g., Yoshida et al 2014Yoshida et al , 2015 can then be used to determine the background differential stress. This approach requires spatially mapping the stress rotation with a resolution that may not be possible for smaller or more poorly recorded events.…”

“…Rotations of the principal stress axes have been observed following recent great subduction zone earthquakes (e.g., Hasegawa et al 2011Hasegawa et al , 2012Hardebeck 2012;Chiba et al 2012;Yang et al 2013). These rotations usually take the form of a change in the plunge of the maximum compressive stress axis, σ1, and can also involve a horizontal rotation of the trend of the sub-horizontal σ1 axis.…”