Fore-arc deformation controls frequency-size distribution of megathrust earthquakes in subduction zones

摘要

Seismotectonic deformation in subduction zones seems to follow rather simple spatiotemporal patterns with fore-arc basins overlying the areas of large slip during quasiperiodic megathrust earthquakes. To study the possible coupling between long-term deformation and earthquake behavior, we use compressive granular wedges overlying a rate- and state-dependent frictional interface as analogue models of subduction zone fore arcs overlying a seismogenic megathrust. For different seismogenic zone geometries, we analyze deformation time series with respect to the accumulation of permanent strain and frequency-size distributions of episodic slip events equivalent to great (M > 8) earthquakes. We observe that permanent deformation in the wedges localizes at the periphery of unstable slip at depth over tens of simulated seismic cycles. For updip-limited seismogenic zone models, this leads to structural wedge segmentation characterized by an elastic domain overlying the zone of unstable basal slip. Along with the evolution of segmentation the frequency-size distributions of episodic slip events develop from more random, Gutenberg-Richter-like events (b value 0.6) toward more periodic, characteristic events (b value <0.1). Corresponding coefficients of variation (C_v) of recurrence intervals decrease from C_v 0.6 in deforming wedges to C_v 0.3 in segmented wedges. From the experiments we thus infer a positive feedback between fore-arc tectonics and megathrust seismogenesis which brings the system from a stochastic to a more deterministic state. Our experimental observations imply that the quasiperiodic recurrence of great subduction earthquakes evident from existing earthquake records is a long-term feature intrinsically related to the seismotectonic segmentation of the fore-arc wedges.