Source Scaling Relations and Along-Strike Segmentation of Slow Slip Events in a 3-D Subduction Fault Model

Abstract:

Scaling between slip event duration and equivalent moment is diagnostic of the underlying physics of rupture process. For episodic slow slip events (SSEs) in subduction zones, their moment-duration relation is not clearly defined and shows considerable variation among individual margins where multiple episodes of SSEs are geodetically detected. Here I set up a 3D planar thrust fault model within the framework of rate-state friction to study the spatiotemporal evolution of slip and stress during SSEs. SSE source properties, including along-strike length, duration, equivalent moment and stress drop, are quantified and their scaling relations are compared to observations. Modeled SSEs have nearly constant stress drops of 0.001 to 0.01 MPa, due to near-lithostatic pore pressure at the SSE depths. The modeled moment-duration scaling is between 1 (linear) and 2 (squared). When multiple SSEs appear simultaneously along the strike, the stress interaction between approaching slip fronts results in higher average propagation speeds for longer lengths, which leads to a scaling close to 2. When SSEs are well offset in space and time, stress interaction is negligible and the scaling is close to 1. Two types of SSE along-strike segmentation gaps are identified from model results. The ``primary" gaps are persistent segmentation boundaries due to along-strike variation of effective normal stress, while the ``secondary" gaps evolve through SSE cycles and reflect the stress condition within a ``primary" segment. This implies that some geodetically inferred SSE segmentation boundaries may result from slower slip velocities below the resolution limits of geodetic inversion models.