Localized slip and distributed deformation in oblique settings: the example of the Denali fault system, Alaska

Abstract

In most fault systems the direction of the relative plate motion is oblique to the azimuth of the existing faults. Hence, during earthquakes the displacement may be partitioned between several faults that accommodate different components of the total motion. Here, we quantify the effect of the obliquity of the fault system relatively to the plate-motion direction on the distribution of the deformation in the fault system, during distinct periods of the seismic cycle. The 2002 November, M w 7.9, Denali strike-slip earthquake ruptured 341 km of the Denali fault. The azimuth of the fault varies by more than 50 • over the total rupture length, making the Denali fault an ideal system to test the effect of obliquity. From west to east, thrust dominates the first part of the rupture while strike-slip dominates the central and eastern sections. Using a kinematic model that considers the obliquity of the plate-motion direction relative to the local fault azimuth, we explored how much of the far-field tectonic loading is accommodated on the main strike-slip fault during the earthquake, and how much is accommodated by distributed deformation off the main fault, on secondary structures. Using a dataset of 735 focal mechanisms, we represent the deformation using strain rosettes and we compare seismological data with model results using the areal strain. Then we developed the parameter Ca, the coefficient of accommodation, which allows a direct quantification of the efficiency of a fault to accommodate oblique motion. Using these indicators, we show that in oblique setting, such as in the Denali case, the aftershocks and the background seismicity are organized to accommodate a significant part of the deformation that is not taken on the Denali strike-slip fault during the main earthquakes. The westward increase of the obliquity actually increases the amount of such deformation accommodated through distributed thrust faults, leading to the westward widening of the Alaska Range, located north of the Denali fault. Therefore, the strain partitioning between localized slip on pre-established major faults and distributed deformation accommodated through aftershocks and background seismicity on smaller faults (highlighted here by the longer-term topography) seems to be needed during the seismic cycle to accommodate the boundary conditions in such oblique settings.