Characteristics of Foreshocks Revealed by an Earthquake Forecasting Method Based on Precursory Swarm Activity

Abstract

We have developed an empirical earthquake forecast method, Maeda's method, based on the statistical features of precursory seismic swarm activity, that is foreshocks, which sometimes appear before a mainshock, and issuing an alert of a mainshock occurrence within a certain period of time. In this study, we investigated the effectiveness of earthquake forecast of Maeda's method by applying it to seismicity under various tectonic environments of Japan such as regions characterized by interplate seismic activity, a tectonic fault line (concentrated deformation zone), and an island arc area of seismic and volcanic activity. As a result, we confirmed that Maeda's method yielded generally higher scores than a forecast model based on a stationary space‐time epidemic‐type aftershock sequence (ETAS) model. We also found that foreshocks detected along the Japan Trench were distributed along the edges of low‐velocity anomalies and among areas with background swarms related to slow slip events (SSEs). The foreshocks may have been caused by a heterogeneous stress distribution associated with the existence of a plate‐bending axis and a subducted seamount. Foreshocks off Iwate prefecture, in particular, were excited by periodic SSEs. In an inland tectonic zone and an island arc, swarm activity associated with magmatic or fluid activity related to low‐velocity anomalies tended to be followed by a mainshock. Maeda's method is a simple and efficient counting‐number‐based earthquake forecast model and may capture characteristics of foreshocks that reflect a physical phenomenon, such as a nucleation process involving precursory slip, which the stationary ETAS model is not able to represent.

A review on slow earthquakes in the Japan Trench

“…These recent observations are consistent with the inference made by Nishikawa and Ide (2018). Maeda and Hirose (2016) and Hirose et al (2021) systematically surveyed foreshock activity (M j 5 or greater) of M j ≥ 6 earthquakes in the Japan Trench from 1961 to 2010. They identified areas of pronounced foreshock activity far offshore in the Japan Trench (the orange shaded areas in Fig.…”

“…These results suggest that the foreshock activity in the Japan Trench cannot be explained by earthquake-to-earthquake triggering and that transient aseismic phenomena such as slow earthquakes may be involved in the triggering of the foreshocks. Moreover, Hirose et al (2021) found that foreshock activity in the northern Japan Trench had been synchronized with the 3-year periodic SSEs reported by Uchida et al (2016) (the far offshore black square in Fig. 7a) and suggested that the periodic SSEs had excited the foreshock activity.…”

“…In the Japan Trench, there have been a number of attempts to infer the occurrence of SSEs from earthquake swarm activity (Marsan et al 2013;Ide 2017, 2018;Nishikawa et al 2019) and activity of swarms of foreshocks (i.e., earthquake swarms followed by megathrust earthquakes) (Matsumura 2010;Kato et al 2012;Maeda and Hirose 2016;Nishikawa and Ide 2018;Kubo and Nishikawa 2020;Hirose et al 2021). Nishikawa and Ide (2018) considered increases in the seismicity rate that do not obey the Omori-Utsu's aftershock law (Utsu 1957;Utsu et al 1995;Sect.…”

“…6a and 7a). Furthermore, Hirose et al (2021) showed that the occurrence rate of foreshock activity increases as it approaches the time of mainshock occurrence and that this foreshock rate acceleration cannot be fully reproduced by the ETAS model (Ogata 1988;Zhuang et al 2002), which describes the earthquake-to-earthquake triggering. These results suggest that the foreshock activity in the Japan Trench cannot be explained by earthquake-to-earthquake triggering and that transient aseismic phenomena such as slow earthquakes may be involved in the triggering of the foreshocks.…”

Spatiotemporal Distribution of Shallow Tremors Along the Nankai Trough, Southwest Japan, as Determined From Waveform Amplitudes and Cross‐Correlations

“…In the Japan Trench, shallow slow slip is known to have preceded the 2011 M w 9.0 Tohoku earthquake (Ito et al, 2013). Hirose et al (2021) suggested that episodic SSEs along the Japan Trench excite foreshock activity based on analysis of precursory swarm activity. Therefore, it is also important to investigate the relationship between slow and ordinary earthquakes, as it may contribute to our understanding of the nucleation process of major earthquakes.…”