The detectability of shallow slow earthquakes by the Dense Oceanfloor Network system for Earthquakes and Tsunamis (DONET) in Tonankai district, Japan

Abstract

In order to understand the characteristics of shallow very low-frequency (VLF) events as revealed by recent ocean-floor observation studies, we perform a trial simulation of earthquake cycles in the Tonankai district by taking the characteristics of the 1944 Tonankai earthquake and assuming that slow earthquakes occur on numerous small asperities. Our simulation results show that the increase of moment release rate of shallower VLF events in the pre-seismic stage of a megathrust earthquake is higher than that of deeper VLF events. This increase may make leveling change due to VLF swarms detectable at Dense Oceanfloor Network system for Earthquakes and Tsunamis (DONET). We also introduce the time series of hydraulic pressure data at DONET, comparing with the leveling change expected from our numerical simulation. Since leveling change due to shallower VLF swarms is so local as to be incoherent, removal of the moving-averaged data from the data stacked by four nearby observation points in the same node may be useful to detect the short-term local leveling change.

The 2016 Mw 5.9 earthquake off the southeastern coast of Mie Prefecture as an indicator of preparatory processes of the next Nankai Trough megathrust earthquake

“…These studies have mainly focused on reproducing the slip distributions, rupture segmentations, and recurrence intervals of historical earthquakes, based on realistic fault models and subduction zone structures obtained from seismic surveys. In contrast, simulations based on simplified fault models can provide us with insights into earthquake rupture patterns, cycles, and related phenomena such as slow slip events (SSEs) (e.g., Matsuzawa et al 2010;Noda et al 2013Noda et al , 2014Ariyoshi et al 2014). Hori and Miyazaki (2011) simulated occurrences of M7-8 earthquakes during M9 earthquake cycles to reproduce the occurrence of the 1978 M w 7.5 earthquake that occurred offshore Miyagi before the 2011 Tohoku-oki earthquake (M w 9.0).…”

A trial derivation of seismic plate coupling by focusing on the activity of shallow slow earthquakes

“…In Figure 5a,b,d, the migration of slow earthquakes is shown by streaks of warm colors (i.e., yellow through red; see the close-up in Figure 5e,f,g,h), which are mainly composed of VLFEs, as demonstrated by Ariyoshi et al (2012Ariyoshi et al ( , 2013. As confirmed by other trial simulations, if frictional instability in the small asperities is greaterthat is, the value of γ 3 and/or κ 2 (κ 3 ) is greater -slip events in the belt of the small asperities approach from slow earthquake to regular earthquake, which is consistent with results of recent simulations (e.g., Shibazaki and Shimamoto 2007;Matsuzawa et al 2010;Ariyoshi et al 2009;Ariyoshi et al 2012) and theoretical analysis of nucleation sizes L min and L ∞ (Ampuero and Rubin 2008).…”

“…Effect of a strong coupling model on the activity of shallow VLFEs Figure 3 shows the spatial distribution of the coseismic slip (defined to be faster than 3 cm/s, in accordance with Ariyoshi et al 2013) of the simulated megathrust earthquake that occurred in the large asperity, which is almost characteristic. The asymmetric distribution of the coseismic slip is due to the asymmetry at the edge of the small asperity belt, as shown in Figure 2.…”

“…The numerical simulation employed in the present study is similar to that described by Ariyoshi et al (2013). In our study, the motion equation for a subduction plate boundary is described by a quasi-dynamic equilibrium (e.g., Rice 1993) between the shear stress (due to reverse dip slip on the discretized faults) and the frictional stress based on an RSF law.…”

“…In this study, we chose two models: a strong coupling model (SCM) and a weak coupling model (WCM). The WCM is the model adopted by Ariyoshi et al (2013) for the 1944 Tonankai earthquake. The SCM is the same as the WCM, except that the frictional stability parameter γ for large asperity is about 2.6 times greater under the higher value of the frictional parameter b in the RSF law.…”