Seismology TowArd Research innovation with data of Earthquake Project (STAR-E Project)「New developments in space-time earthquake forecasting and monitoring: from long-term to real-time」

Essay on: "Review on slow earthquakes in the Japan Trench"

• Essay author: Tomoaki Nishikawa
• Article authors: Tomoaki Nishikawa¹, Satoshi Ide² and Takuya Nishimura¹

• ¹DPRI, Kyoto University
• ²The University of Tokyo

Slow earthquakes are episodic slow fault slips. They form a fundamental component of interplate deformation processes, along with fast, regular earthquakes (Ide et al., 2007). Recent seismological and geodetic observations (e.g., Nishikawa et al., 2019) have revealed detailed slow earthquake activity along the Japan Trench—the subduction zone where the March 11, 2011, moment magnitude 9.0 Tohoku-Oki earthquake occurred. Based on the recent research advance, we presented the first review on slow earthquake research in the Japan Trench and its history (Nishikawa et al., 2023).

By compiling the observations of slow earthquakes (e.g., tectonic tremors, very-low-frequency earthquakes, and slow slip events) and related fault slip phenomena (e.g., small repeating earthquakes, earthquake swarms, and foreshocks of large interplate earthquakes), we drew an integrated slow earthquake map along the Japan Trench (Figure 1). We found that slow and megathrust earthquakes are spatially complementary in distribution, and that slow earthquakes sometimes trigger fast earthquakes in their vicinities. An approximately 200-km-long along-strike gap of seismic slow earthquakes (i.e., tectonic tremors and very-low-frequency earthquakes) (Figure 1) corresponds with the huge interplate locked zone of the central Japan Trench (e.g., Nishimura et al., 2004). The Tohoku-Oki earthquake ruptured this locked zone, but the rupture terminated without propagating deep into the slow-earthquake-genic regions in the northern and southern Japan Trench. Given the above observation and the fact that slow earthquakes were observed during the month preceding the Tohoku-Oki earthquake near its rupture initiation point, slow earthquakes were probably involved in both the rupture initiation and termination processes of the Tohoku-Oki earthquake.

Figure 1. Distribution of slow and fast earthquakes in the Japan Trench and Nankai Trough. Blue and red symbols indicate slow and fast earthquakes, respectively

We then compared the integrated slow earthquake distribution with seismicity parameters (Gutenberg–Richter relationship’s b-value, the tidal response of seismicity, and the p-value of Omori–Utsu’s aftershock law), the crustal structure (e.g., interplate sedimentary units, subducting seamounts, petit-spot volcanoes, horst and graben structures, residual gravity, seismic velocity structure, and plate boundary reflection intensity), and the geological environment (e.g., water sources, pressure–temperature conditions, and metamorphism). The results show that Omori–Utsu’s p-value (Ogata, 2011) is particularly high (approximately 1.4) near the slow-earthquake-genic regions in the shallow part of the Japan Trench. This implies that aftershock activity rapidly decays in time around the regions. Furthermore, the shallow slow-earthquake-genic regions are found to be located near fluid-rich sedimentary units (Tsuru et al., 2002), subducting seamounts that entrain sediments (Mochizuki et al., 2008), interplate areas of strong seismic reflection intensity (e.g., Fujie et al., 2001), and the depth at which minerals in sediments (opal and smectite) dehydrate (Hyndman & Peacock, 2003), implying that the distribution of water at the plate boundary corresponds to the distribution of slow earthquakes.

Finally, we discussed the use of slow earthquake observations for forecasting fast earthquake activity. In the Japan Trench, slow earthquakes triggering large earthquakes and swarms of small-to-moderate earthquakes have been reported (e.g., Uchida et al., 2016). Slow earthquakes accompanied by a swarm of foreshocks were also observed just before the Tohoku-Oki earthquake (e.g., Ito et al., 2013). In light of these observations, slow earthquake observations are expected to be useful in forecasting interplate seismicity. For example, real-time monitoring of slow earthquake activity and accompanying earthquake swarms may enable us to detect foreshock activity of impending large earthquakes. Furthermore, incorporating the seismicity-triggering effects of slow earthquakes into statistical (Ogata, 1988; Zhuang et al., 2002) and physical (Dieterich, 1994) seismicity models could improve the forecasts of fast earthquakes. It is also important for short-term forecasts of large earthquakes to clarify the presence/absence of features specific to slow earthquakes that precede megathrust earthquakes.

The Japan Trench has a surprisingly complex distribution of slow and fast earthquakes, and these earthquakes interact (Figure 2). Further investigation of the factors that govern this complex distribution and the interaction between the two types of earthquakes is expected to provide important insights into the physics of slow earthquakes and improve forecasts of fast earthquakes.

Figure 2. Schematic of the megathrust slip behavior of the Japan Trench.

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