Seismogenic Characteristics and Dynamic Triggering in the Slab Penumbra

Date: 4/26/2019

Time: 09:15 AM

Room: Cascade II

Observations from recent earthquakes, such as the 2018 Tonga-Fiji deep events, provide crucial insight into the mechanics of slab earthquakes. The 19 August (M_w 8.2) earthquake represents a larger repeat of the 1994 Tonga (M_w 7.6) event, located 30 km away, with similar mechanism and rupture characteristics (McGuire et al., 1997; Fan et al., in press). Both events initiated near the center of the active slab, occurred along near-vertical faults at high angles to the slab, and ruptured into a region with no previous seismicity. This demonstrates that deep earthquake rupture can propagate well into the aseismic region surrounding the active slab.

The 6 September (Mw 7.9) event, in a previously aseismic region 270 km away, illustrates the dynamic triggering of deep earthquakes. We conduct a systematic study of dynamic triggering of slab earthquakes worldwide since 1964. The seismicity rates at distances of 80-400 km from earthquakes with M_w>6.5 greatly exceeds the background rates for the first 5 hours following the mainshock, and continues at an elevated rate for about 4 days, demonstrating dynamic triggering in that time period. Dynamic triggering is much more common for deep earthquakes than for intermediate depth earthquakes, particularly at depths greater than 600 km. Statistically, dynamically triggered earthquakes are much more likely than background events to occur in areas that are otherwise aseismic, generally deeper or laterally displaced from the active slab. Triggered events follow an Omori’s Law decay with time, but there is no triggered seismicity during the passage of waves. Thus, there must be a delay mechanism that stores the transient energy and triggers earthquakes with delays of minutes to a few days. Slabs in the transition zone must be surrounded by material that is critically stressed and warm enough to inhibit nucleation, but which can sustain rupture if a large shear failure propagates into the region. The passage of strong seismic waves can also initiate a runaway shear failure, which nucleates over a period of minutes to days.

Presenting Author: Douglas A. Wiens

Authors