Tectonic, volcanic, and semi-volcanic deep low-frequency earthquakes in western Japan
Highlights
► We compare the activities of tectonic, volcanic, and isolated intraplate LFEs. ► We examine tidal sensitivity, magnitude distribution, and hypocentral distribution. ► Isolated intraplate LFEs have similar characteristics to the volcanic LFEs. ► Isolated intraplate LFEs are suggested to be generated by fluid movements. ► We propose that isolated intraplate LFEs be called “semi-volcanic LFEs”.
Introduction
In subduction zones at convergent margins like Japan, various types of earthquakes are recognized: interplate earthquakes, inland earthquakes, intraslab earthquakes, outer-rise earthquakes, and shallow volcanic earthquakes. In contrast to these well-known types of earthquake, there are also small earthquakes of M < 2 that occur at depths around 30 km and radiate seismic waves dominantly in the 2–8 Hz frequency band, lower than the characteristic frequency of ordinary earthquakes of similar magnitude. Such events are called deep low-frequency earthquakes (LFEs).
The study of LFEs in Japan has shown great advances during the last decade, mainly as a result of the high-sensitivity seismograph network “Hi-net” in Japan, which was deployed by the National Research Institute for Earth Science and Disaster Prevention (NIED) after the large impact of the 1995 Kobe earthquake. Since about 2000, the Japan Meteorological Agency (JMA) has detected and located many LFEs as part of its routine monitoring by using the same procedure for ordinary earthquakes. In the JMA hypocenter catalog (JMA catalog), 31,252 events are marked as LFEs from June 2000 to December 2011, while about 1,600,000 ordinary events were detected in the same period.
Most of these developments concern tectonic LFEs that occur on plate boundaries in a belt-like distribution in western Japan; however, another kind of LFE is the volcanic LFE that occurs beneath an active volcano, as widely recognized since the 1980s, much earlier than the tectonic LFEs. There are also isolated intraplate LFEs that occur far from either plate boundaries or active volcanoes.
As introduced here, LFEs are categorized in three major types based on their locations. Fig. 1 shows the distribution of LFEs in western Japan in a map view, and Fig. 2 shows a schematic cross-section. The basic features of the three types are listed in Table 1. Here, we will explain each type of LFE separately: tectonic LFEs in Section 1.2, volcanic LFEs in Section 1.3, and isolated intraplate LFEs in Section 1.4.
After Nishide et al. (2000) reported tectonic LFEs by using Hi-net data, JMA began to detect and locate LFEs as part of their routine monitoring. At almost the same time, Obara (2002) recognized a tectonic tremor occurring for several days, and Rogers and Dragert (2003) demonstrated an episodic tremor and slip (ETS), which is a slow slip on the plate boundary accompanied simultaneously by a tremor. For several years after their individual discoveries, tectonic LFEs and ETS were thought to be different phenomena, but later, Shelly et al., 2006, Shelly et al., 2007a demonstrated that tectonic LFEs are a constituent of ETS on the plate boundary. The tectonic LFEs occur in a belt-like distribution along an isodepth contour of 30–40 km on the subducting plate (Ide, 2012, Obara, 2002). Ide et al. (2007b) analyzed the mechanisms of tectonic LFEs in western Shikoku, and obtained a low-angle thrust fault dipping to the northwest, consistent with a shear slip on the plate interface. Remarkable phenomena involving activities such as spatial migrations (Obara, 2002, Shelly et al., 2007b) and tidal responses (Nakata et al., 2008) are also recognized. The waveforms of tectonic LFEs show a broad spectral peak (Fig. 3a,b). The existence of fluid is thought to be related to the generation of tremor (Katsumata and Kamaya, 2003, Shelly et al., 2006). Many other low-frequency events have recently been found as well as LFEs or ETS: long-term slow slip events (Hirose et al., 1999), shallow very-low-frequency (VLF) earthquakes (Ishihara, 2003), and deep VLF earthquakes (Ito et al., 2007). The relationships among these low-frequency events and interplate megathrust earthquakes are attracting attention among those trying to understand the subduction system (Obara, 2011, Rubinstein et al., 2010).
In volcanic regions, various types of earthquake shallower than 10 km are known (Minakami, 1960, Nishimura and Iguchi, 2006). There are not only high-frequency events caused by shear fracture, but also low-frequency events that often contain a considerable non-double-couple component, and various mechanisms and models have been suggested for them (e.g., Fujita and Ida, 2003, Julian, 1994). The continuous events are called tremor. These low-frequency events and tremor are regarded as critically important because they often precede and accompany volcanic eruptions (Chouet et al., 1994, Ishihara and Iguchi, 1989).
The shallow low-frequency earthquakes are poorly understood, but the low-frequency earthquakes at depths greater than 10 km beneath active volcanoes are even less known. We call these deep low-frequency earthquakes beneath active volcanoes “volcanic LFEs”, and they are the target of this study. Here, we define active volcanoes as “volcanoes which have erupted within 10,000 years, or volcanoes with vigorous fumarolic activity”, and this is the definition given by JMA.
Volcanic LFEs have been discovered beneath many active volcanoes worldwide, as in Hawaii (Aki and Koyanagi, 1981) and Japan (Ukawa and Ohtake, 1987). Most of the waveforms of volcanic LFEs show monochromatic or harmonic characteristics (Fig. 3c). They occur at depths of 20–40 km, around the Mohorovicic discontinuity (Okada and Hasegawa, 2000), although there are shallower events such as the LFEs beneath Mt. Fuji at depths of 11–16 km (Nakamichi et al., 2004). Since the rheological properties of rocks are expected to be within the ductile regime at a depth of about 30 km (Shimamoto, 1986), any rapid movement of magma or crustal fluid may relate to these seismic activities (e.g., Aki and Koyanagi, 1981, Hasegawa et al., 1991). Although Hasegawa and Yamamoto (1994) studied the seismicity of volcanic LFEs as well as the shallow seismicity of ordinary earthquakes, there are few studies on volcanic LFE mechanisms, probably because of their weak signals (Nakamichi et al., 2003, Nishidomi and Takeo, 1996). In spite of the fact that information on volcanic LFEs has been collected at many volcanoes since the early 1980s, they remain poorly understood (Chouet, 2003, McNutt, 2005).
Katsumata and Kamaya (2004) reported isolated LFEs that occurred away from volcanoes, and these included not only tectonic LFEs but also LFEs that did not fall in a recognized belt of tectonic activity. More recently, Takahashi and Miyamura (2009) have divided non-tectonic LFEs into about 100 regional clusters, and these include volcanic LFEs and other LFEs that are distant from active volcanoes. We tentatively refer to these latter LFEs, not associated with plate boundaries or active volcanoes, as “isolated intraplate LFEs”.
Although isolated intraplate LFEs are not beneath active volcanoes, most of them, in fact, are located near dormant Quaternary volcanoes. Examples are those observed in eastern Shimane (Ohmi and Obara, 2002, Ohmi et al., 2004), where they occur right beneath the Yokota Quaternary volcanic cluster, even though there is no active volcano within 50 km of their epicenters. Examples of isolated intraplate LFEs that are not located near a Quaternary volcano are those in Osaka Bay, some 100 km from the nearest volcano (Aso et al., 2011). In this study we will focus mainly on the LFEs in these two regions. The waveforms of isolated intraplate LFEs are monochromatic and show several distinct spectral peaks, and they resemble those of volcanic LFEs (Fig. 3d,e).
In both eastern Shimane and Osaka Bay, large inland strike-slip earthquakes have occurred. While LFEs in Osaka Bay are located 10–30 km away from the 50-km-long fault plane of the 1995 Kobe earthquake (Mw 6.8), LFEs in eastern Shimane are located just 10 km away from the 20-km-long fault plane of the 2000 western Tottori earthquake (Mw 6.6). Analogous to these two situations, most of the large crustal earthquakes in western Japan occur within 20 km of an active volcano, a Quaternary volcano, or an isolated intraplate LFE (Ide et al., 2010). Since volcanoes exist where water, produced by dehydration processes, generates mantle diapirs, and crustal earthquakes are affected by the presence of the fluids (e.g., Zhao et al., 1996), isolated intraplate LFEs may also be related to crustal fluid movements. However, little work has been done on isolated intraplate LFEs. The purpose of our study was to help rectify this situation, and in this paper we show that isolated intraplate LFEs are quite similar to volcanic LFEs, and for this reason we will propose a new name: “semi-volcanic LFEs”.
The basic characteristics of the three types of LFEs are listed in Table 1, and observations of the LFEs give rise to a main question: What are the common features among these three types of LFEs, and what are the crucial physical differences? To answer a portion of this question we have investigated the seismicity in five regions, always employing the same strategy of using high-quality large-quantity catalogs. This approach would further lead to the understanding of linkages between LFEs and large earthquakes or volcanic eruptions.
Section snippets
Methods
The JMA provided a seismicity catalog including LFEs in each area. Following routine procedure, officers picked clear arrivals manually and estimated hypocenters automatically. If an earthquake had low-frequency waveforms and occurs at deeper than 10 km, the earthquake was denoted as a low-frequency event. In this catalog, although almost all of the large events have been listed, but there is not sufficient number of data for statistical analysis. Therefore, to analyze the activities of LFEs, we
Data
The present study compares the seismicity of three types of LFEs in the five regions: tectonic LFEs in northern Kochi and central Ehime, volcanic LFEs in Sakurajima, and isolated intraplate (semi-volcanic) LFEs in Osaka Bay and eastern Shimane (Fig. 1). The band-like distribution of tectonic LFEs in the Shikoku region actually consists of many small clusters, and two of them are studied. Central Ehime is the well-studied region of tectonic tremor (Shelly et al., 2007b). On the other hand, we
Temporal distribution
We detected about 1000 new LFEs in each area except eastern Shimane and Sakurajima (Table 2). Many more events were detected in eastern Shimane, and fewer events in Sakurajima. The periodicity of the detected events was examined by evaluating a spectrum of the delta function sequence. We used the Fourier power spectrum of the Dirac delta function δ(t − tj) at each event origin time tj aswhere denotes Fourier
Discussion and conclusions
We detected about 1000 LFEs in each area. In the present study, the number of detected events, much larger than in the previous catalog, enabled the investigation of the activity spectrum and the magnitude statistics within each region.
From the activity spectrum, volcanic LFEs in Sakurajima and the isolated intraplate (semi-volcanic) LFEs in Osaka Bay and eastern Shimane show no evidence of tidal modulation. On the other hand, tectonic LFEs in northern Kochi and central Ehime are controlled by
Acknowledgments
We are grateful to Kazushige Obara, Minoru Takeo, and Emily E. Brodsky for constructive discussions. We also thank Takao Ohminato, Hitoshi Kawakatsu, and Gaku Kimura for valuable comments. We greatly appreciate Yoshiharu Kurihara for discussions about seismicity and for analyzing our LFE catalog with his original code for the discrete triggering model. Finally, we would like to thank the anonymous reviewers for pointing out unclear explanations.
We used the earthquake catalogs of JMA, and all
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