Cascading rupture of patches of high seismic energy release controls the growth process of episodic tremor and slip events

Keita Nakamoto,Yoshihiro Hiramatsu,Kazutoshi Imanishi,Takahiko Uchide

doi:10.1186/s40623-021-01384-6

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.

Highlights

Earthquakes feature heterogeneous slip distributions on fault planes that are characterized by one to several areas of large slip, or asperities
We show that persistent patches of relatively high seismic energy release are located on the plate interface in the Episodic tremor and slip (ETS) zone, and that the growth process of ETS events is controlled by cascading ruptures of these patches
The start and end points of an ETS event were defined as the centroids of the first and last clusters, and these points in turn defined the duration of an ETS event (Table 2)

Summary

Introduction

Earthquakes feature heterogeneous slip distributions on fault planes that are characterized by one to several areas of large slip, or asperities. Earthquake hypocenters are located outside of asperities (e.g., Yamanaka and Kikuchi 2004; Mai et al 2005). Recent findings about slow earthquakes have broadened our understanding of subduction dynamics and the generation of megathrust earthquakes (e.g., Obara and Kato 2016). Strain on the plate interface is released by slow slips in slow earthquakes and high-speed ruptures in regular earthquakes. The spatial distributions of slow and regular earthquakes are complementary: slow earthquakes are distributed around and outside the locked zones that are the source areas of megathrust earthquakes, both along dip (Obara and Kato 2016) and along strike (Nishikawa et al 2019). A temporal relationship between slow and regular earthquakes has been proposed. Kato et al (2012) reported the occurrence of two slow slip events (SSEs) from earthquake activities that migrated toward the future hypocenter of the 2011 Tohoku earthquake before

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Conclusion