4.15 - Episodic Aseismic Slip at Plate Boundaries

Abstract

Slip in earthquakes accounts for only a fraction of plate tectonic displacements. Until recently, the aseismic component of plate motions has been assumed to take place primarily as steady-state creep, although no direct observations of creep at depth exist. This chapter summarizes the accumulating evidence that a large fraction of aseismic slip occurs episodically rather than continuously. Several types of episodic aseismic slip have been documented, distinguished by when they occur in the earthquake cycle (precursory, interseismic, or afterslip) and whether they represent deviations in slip rate from long-term rates or discrete slow-slip events (SSEs). Interseismic SSEs at convergent margins are the most commonly observed mode of episodic aseismic motion. SSEs in the northern Cascadia and southwestern Japan subduction zones correlate with seismic tremor, a signal that is commonly associated with active volcanoes. Given their correspondence, it is likely that slow slip and tremor are manifestations of the same phenomena. Aseismic slip events concentrate near the Earth’s surface, at the up- or down-dip edge of the seismogenic zone, or at complementary locations to strongly locked patches or coseismic asperities within the seismogenic zone. The nucleation of aseismic slip events at frictional transitions is supported by rate- and state-dependent frictional modeling. The fairly common occurrence of aseismic slip following moderate to large earthquakes (afterslip) suggests a relationship between higher-velocity ruptures (earthquakes) and slow slip. Although isolated episodes of interseismic slow slip can be related to nearby earthquake activity, a consistent triggering pattern is yet to emerge and further study is clearly warranted.