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 governs 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 Δτ = 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 𝐴𝐴 𝐴𝐴 𝑖𝑖𝑖𝑖𝑖𝑖𝑖𝑖 𝑠𝑠𝑠𝑠𝑖𝑖𝑠𝑠 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.…”

Rogue serpentinite-breakdown waves: a source mechanism for major subduction earthquakes?

“…The observed correlation between the seismic and geodetically recorded signals leads to a range of important implications on the dynamics of megathrust events and was found to be universally related to subducting slabs where young and hence hot slabs are thrust below adjacent continents such as Cascadia, Mexico, Japan Trench and the Hikurangi subduction zone [5]. Although physics based models [6][7][8][9][10] have shown that the fluid-release hypothesis provides a plausible explanation for the ETS mechanism, the network forming feedbacks between correlated patches of ETS events and the equivalent seismogenic rupture of patches of asperities in locked seismogenic zone [11] has not yet been explored from a first-principle perspective.…”

Short‐Lived and Voluminous Fluid‐Flow in a Single Fracture Related to Seismic Events in the Middle Crust

“…Recent geophysical observations suggest that cyclic fluid infiltration is associated with slow‐slip events (Obara, 2002; Ohmi & Obara, 2002; Shelly et al., 2006). These include episodic tremor and slip (ETS) events and low‐frequency earthquakes (LFE), both of which occur within and outside of subduction zone (Ohmi & Obara, 2002; Rogers & Dragert, 2003) and are associated with surface displacement on the scale of millimeters, and can last for several hours (Bostock et al., 2015; Chestler & Creager, 2017; Nakamoto et al., 2021).…”