Cascading Rupture of Patches of High Seismic Energy Release Controls the Growth Process of Episodic Tremor and Slip Events

Abstract

Slip phenomena on plate interfaces reflect the heterogeneous physical properties of the slip plane and thus exhibit a wide variety of slip velocities and rupture propagation behaviors. Recent findings on slow earthquakes reveal similarities and differences between slow and regular earthquakes. Episodic tremor and slip (ETS) events, a type of slow earthquake widely observed in subduction zones, likewise show diverse activity. We investigated the growth of 17 ETS events beneath the Kii Peninsula in the Nankai subduction zone, Japan. Analyses of waveform data recorded by a seismic array enabled us to locate tremor hypocenters and estimate the migration patterns and spatial distribution of the energy release of tremor events. Here we describe three major features in the growth of ETS events. First, independent of their start point and migration pattern, ETS events exhibit patches of high seismic energy release on the up-dip part of the ETS zone, suggesting that the location of these patches is controlled by inherent physical or frictional properties of the plate interface. Second, ETS events usually start outside the high-energy patches, and their final extent depends on whether the patches participate in the rupture. Third, we recognize no size dependence in the initiation phase of ETS events of different sizes with comparable start points. These features demonstrate that the cascading rupture of high-energy patches governs the growth of ETS events, just as the cascading rupture of asperities govern the growth of regular earthquakes.

Extension of Aseismic Slip Propagation Theory to Slow Earthquake Migration

“…In Equation 9, if the stress drop for RTR is the same as ETS at Δ τ = 10 kPa, aσ ETS = 1.3 kPa (e.g., Nakata et al., 2008), and V prop RTR / V prop ETS = 20–50 observed in Cascadia (Rubin & Armbruster, 2013), I obtain aσ RTR = 0.86–0.93 kPa. In case of V prop RTR / V prop ETS = 16.7 observed in Nankai trough (Nakamoto et al., 2021), aσ RTR = 0.95 kPa. If I assume that the value of frictional parameter a is the same between RTR and ETS, the reduction ratio of effective normal stress in the locally temporal area of pore water migration ( aσ RTR / aσ ETS ) is 0.66–0.73.…”

“…The migration speed of SSF, including RTR, is variable (e.g., Behr & Burgmann, 2021). For example, RTR has propagation speed 20‐50 times faster than the main ETS migration in Cascadia (Houston et al., 2011; Rubin & Armbruster, 2013) and 17 times faster in Nankai trough (Nakamoto et al., 2021). If the faster migration speed of SSF/RTR is due to the faster slip velocity (e.g., Rubin & Armbruster, 2013), It is possible to explain SSF/RTR from the linear relationship between V prop and $${V}_{slip}^{init}$$ in Equation 9.…”

“…Secondary slip fronts (SSF) also occur within ETS, including rapid tremor reversals (RTR) and along‐dip migrating tremor swarms with faster propagation than the main ETS process, have been observed in the Cascadia (Bletery et al., 2017; Ghosh et al., 2010) and Nankai subduction zones for both the shallower and deeper extensions (Nakamoto et al., 2021; Obara et al., 2012; Yamamoto et al., 2022). Rubin and Armbruster (2013) suggest that different migration speeds can be explained by different slip velocity.…”