Seismogenic Expression of Three Dimensional Strain within a Single Earthquake Sequence

Abstract:

Tectonic plate boundary shear zones commonly initiate along preexisting weaknesses oriented obliquely to the direction of relative plate motion. Slip on multiple fault sets with diverse orientations and slip directions is required to accommodate the three dimensional strain of such plate margins. However, little is known about how this is accomplished by faults that rupture in individual earthquakes and specifically whether permutations in stress state are required to drive slip on multiple fault sets. The 2010 Mw 7.2 El Mayor-Cucapah (EMC) earthquake ruptured a complex network of high- and low-angle faults that record systematic, incremental changes in kinematics with fault orientation. Here we show that stress inversions of coseismic surface rupture and aftershock focal mechanisms define two coaxial, but permuted stress states. The maximum and intermediate principal stress axes are close in magnitude, but switch orientations due to topography- and density-controlled gradients in lithostatic load. Both states of nearly uniaxial stress permitted a wide variety of fault orientations to slip at once, accomplishing three-dimensional strain in a single earthquake sequence. Stress at seismogenic depth significantly exceeded the minimum failure strength of optimally oriented faults demonstrating the interlocking nature of the complex fault network. Rupture initiated on a keystone fault that required the greatest differential stress for failure and spontaneously propagated to faults with more favorable orientations. We conclude that tectonically generated stress within plate boundary shear zones is likely to be heterogeneous, with high-stress regions associated with interlocking fault networks that have longer recurrence intervals and generate earthquakes significantly larger than would be expected from any one isolated, constituent fault. High differential stress and dynamically reduced friction facilitates seismogenic slip on faults with low slip tendency such as on the low-angle normal faults of the EMC rupture.