Brief history of slow earthquake research in the Japan Trench
In this section, we introduce important studies related to slow earthquakes along the Japan Trench in a chronological order, with an aim to help readers understand the history of slow earthquake research in the Japan Trench. The detailed spatiotemporal distribution of slip phenomena discovered by the studies introduced in this section is presented in Sect. 3.2.
Brief history prior to the 2011 Tohoku-Oki earthquake
A large transient aseismic slip was first observed in 1992 at the shallow plate interface (approximately 10–20 km depth) of the northern Japan Trench (39–40° N) by extensometers (Kawasaki et al. 1995; Kawasaki et al. 2001; the blue dashed rectangle in Fig. 1). This transient aseismic slip was also reported by Miura et al. (1993) and Miura et al. (1994), although these reports were not peer-reviewed journal papers. Kawasaki et al. (1995) named this aseismic slip event the 1992 Sanriku-Oki ultraslow earthquake. This event can be regarded as the first-ever reported SSE at a subduction zone plate boundary. However, the duration of this event was 1 day, and the moment magnitude was approximately Mw 7.3–7.7, making it a strange event—in that it deviates significantly from the linear moment–duration scaling relation of slow earthquakes (M0 \(\propto\) T) (Ide et al. 2007a; the magenta circle labeled SR in Fig. 3a). Furthermore, the event occurred immediately after a Mw 6.9 interplate earthquake (the 1992 Mw 6.9 Sanriku-Oki earthquake) and can be interpreted as a huge afterslip. However, this event cannot be considered a typical afterslip (Alwahedi and Hawthorne 2019) because the seismic moment of this event is extremely large, ranging from four to 16 times the seismic moment of the preceding Mw 6.9 earthquake. Similar to the 1992 Sanriku-Oki ultraslow earthquake in the northern Japan Trench, an aseismic slip event with characteristics intermediate between an SSE and an afterslip has been reported in the Peru Trench (Villegas-Lanza et al. 2016). This event also had an extremely large moment, and thus, it cannot be a typical afterslip (10 times or more the seismic moment of the preceding Mw 5.8 earthquake). Villegas-Lanza et al. (2016) interpreted this event as a slow slip “helped” (or triggered) by an earthquake rather than a classical afterslip. Following the classification of Villegas-Lanza et al. (2016), we also regard the 1992 Sanriku-Oki ultraslow earthquake in the northern Japan Trench as a slow slip triggered by an earthquake.
Subsequently, in the late 1990s and 2000s, afterslip following major earthquakes, rather than SSEs, became the focus of much research. A continuous GNSS observation system (GEONET) installed by the Geospatial Information Authority of Japan (GSI) recorded the afterslip following the 1994 Mw 7.7 Sanriku-Oki earthquake (Heki et al. 1997), which ruptured the plate interface (approximately 20–30 km depth) of the northern Japan Trench (40–40.5° N). The afterslip took about a year after the mainshock to release a seismic moment greater than the mainshock. Yagi et al. (2003) found that the coseismic slip of the 1994 Mw 7.7 Sanriku-Oki earthquake and its afterslip were spatially complementary in distribution. In addition, Miura et al. (2006) reported a large afterslip following the 2005 Mw 7.1 Miyagi-Ken-Oki earthquake, which ruptured the deeper part of the central Japan Trench (37.8–38.5° N). This afterslip released a seismic moment comparable to that of the mainshock over a period of approximately one year after the mainshock.
In the 2000s, the GEONET also revealed the interplate slip deficit distribution along the Japan Trench (Ito et al. 2000; Nishimura et al. 2000, 2004; Suwa et al. 2006; Hashimoto et al. 2009; Loveless and Meade 2010). These studies consistently suggested the existence of a huge interplate locked zone in the central Japan Trench (37–39° N). This locked zone was subsequently ruptured by the March 11, 2011, Mw 9.0 Tohoku-Oki earthquake (thick red contours in the Japan Trench in Fig. 1).
In 2002, small repeating earthquakes (e.g., Nadeau and Johnson 1998; Nadeau and McEvilly 1999; Sect. 2.3) were first reported along the Japan Trench (Matsuzawa et al. 2002; small red points in Fig. 1). They were used to estimate the amount of steady (e.g., Igarashi et al. 2003; Igarashi 2010) and transient aseismic slip (e.g., Uchida et al. 2004; Matsuzawa et al. 2004). Igarashi et al. (2003) detected repeating earthquakes by a method based on waveform similarity and estimated the rate of interplate steady slip along the Japan Trench from the recurrence intervals of the repeating earthquakes. Furthermore, they found that few continuous-type small repeating earthquakes (i.e., repeating earthquakes that repeat at almost regular intervals and are not clustered in time) were distributed within the huge locked zone of the central Japan Trench (37–39° N), discovered by the GNSS observations (Ito et al. 2000; Nishimura et al. 2000). This is probably due to the absence of aseismic loading inside the locked region (see Sect. 2.3). Uchida et al. (2004) analyzed repeating earthquake activity prior to interplate large earthquakes in the northern Japan Trench and investigated preseismic changes in the interplate slip rate. They suggested the acceleration of interplate aseismic slip 6 days, 2 days, and 8 months before the 1989 Mw 7.4, 1992 Mw 6.9, and 1994 Mw 7.7 Sanriku-Oki earthquakes, respectively. These preseismic slow slips might have concentrated stress at the source regions of the large earthquakes.
In 2004, Yamanaka and Kikuchi (2004) investigated the slip distribution of Mw 7–8 class megathrust earthquakes from 1931 to 1994 along the northern and central Japan Trench by means of waveform inversion (see thin red contours in the Japan Trench in Fig. 1). They termed large slip areas of megathrust earthquakes asperities. Specifically, they regarded the area where the amount of slip is half the maximum slip or larger as an asperity. They found that in the northern Japan Trench, the asperities of several megathrust earthquakes significantly overlap. That is, the same area on the plate interface has been repeatedly ruptured by the megathrust earthquakes. Furthermore, in the central Japan Trench (38–39° N), the asperity of the 1981 Mw 7.1 earthquake, which is located at approximately 20–30 km depth (thin red contours in the Tohoku-Oki earthquake rupture in Fig. 1), slipped again during the largest foreshock (Mw 7.3) 2 days before the March 11, 2011, Mw 9.0 Tohoku-Oki earthquake (Ohta et al. 2012). These observations indicate the persistent nature of asperities. Note that there are various definitions of asperity, as summarized in Ide (2014). To avoid ambiguity, we follow Yamanaka and Kikuchi’s definition in the subsequent sections.
Brief history after the 2011 Tohoku-Oki earthquake
The Mw 9.0 Tohoku-Oki earthquake occurred on March 11, 2011 (the thick red contours in the Japan Trench in Fig. 1). After the Tohoku-Oki earthquake, aseismic slips that preceded the 2011 Tohoku earthquake were investigated in detail. Seismicity analyses (Ando and Imanishi 2011; Kato et al. 2012) and ocean-bottom pressure gauge observations (Ohta et al. 2012) revealed that afterslip had occurred on the plate interface after the largest foreshock (Mw 7.3) 2 days before the Tohoku-Oki earthquake. Ando and Imanishi (2011) suggested that the afterslip following the largest foreshock had migrated toward the rupture initiation point of the Tohoku-Oki earthquake. Furthermore, a repeating earthquake analysis (Kato et al. 2012), ocean-bottom pressure gauge observations (Ito et al. 2013), and ocean-bottom seismometer observations (Ito et al. 2015; Katakami et al. 2018) suggested that a Mw 7.0 SSE accompanied by a Japan Meteorological Agency magnitude (Mj) 5 class earthquake swarm and tectonic tremors had been occurring on the shallow plate interface (approximately 10–20 km depth) of the central Japan Trench during the month prior to the Tohoku-Oki earthquake (a green rectangle within the Tohoku-Oki earthquake rupture in Fig. 1). The source region of the preseismic slow earthquakes was ruptured by the Tohoku-Oki earthquake and slipped tens of meters (e.g., Ide et al. 2011; Iinuma et al. 2012), indicating that the identical area of the plate interface hosted both aseismic and huge seismic slips. However, we note that the signals of the preseismic slow earthquakes were observed at a small number of ocean-bottom stations (Ito et al. 2013, 2015; Katakami et al. 2018), and there should be large uncertainty in their source locations.
In 2014, GNSS data from the GEONET revealed that aseismic slip had been accelerating on the deeper plate interface (approximately 30–60 km depth) of the central and southern parts (36–39° N) of the Japan Trench in the decade prior to the Tohoku-Oki earthquake (Mavrommatis et al. 2014). A subsequent analysis by Yokota and Koketsu (2015) also supported the acceleration of the aseismic slip from 2002 to the 2011 Tohoku-Oki earthquake and regarded this transient slip as a fault slip phenomenon similar to long-term SSEs on the deeper plate interface of the Nankai Trough (Hirose et al. 1999; Ozawa et al. 2002; Fig. 1). This aseismic transient may have stressed locked parts of the Japan Trench megathrust (Yokota and Koketsu 2015). Furthermore, the decade-long aseismic transient may also have induced slips on the shallow plate interface (10–30 km depth) in the central and southern Japan, partially overlapping with the rupture area of the Tohoku-Oki earthquake, as suggested by repeating earthquake data (Mavrommatis et al. 2015).
In the southern end of the Kuril Trench (41–43° N), which is located just north of the Japan Trench, VLFEs were first observed in 2008 (Asano et al. 2008). In 2015, the F-net broadband seismograph network, which is an onshore seismograph network operated by the National Research Institute for Earth Science and Disaster Resilience (NIED) (NIED 2019b), also observed VLFEs in the Japan Trench (Matsuzawa et al. 2015). VLFEs were detected using a method based on a matched filter technique (Shelly et al. 2007). However, this detection was not sufficient to provide a full picture of the VLFE distribution along the Japan Trench because their template VLFEs were detected by visual inspection and were not spatially complete (Matsuzawa et al. 2015).
In 2016, temporal changes in interplate slip rate estimated from repeating earthquakes (Sect. 2.3) suggested that 1–6 year periodic SSEs are widespread in the Japan Trench (Uchida et al. 2016). Seismicity of Mj 5 or greater was active during the high slip rate periods of the SSEs. Uchida et al. (2016) suggested that the periodic SSEs induce periodic stress perturbations and modulate the activity of medium-to-large earthquakes in the Japan Trench. Furthermore, Nomura et al. (2016) developed a Bayesian statistical method to estimate the detailed space–time distribution of interplate slip rates from the recurrence intervals of repeating earthquakes and confirmed the quasi-periodic slip rate acceleration in the northern Japan Trench.
In 2019, pop-up-type ocean-bottom seismometers (Ohta et al. 2019) and the Seafloor Observation Network for Earthquakes and Tsunamis along the Japan Trench (S-net) (Tanaka et al. 2019; Nishikawa et al. 2019) observed clear signals of tectonic tremors on the shallow plate interface (approximately 10–20 km depth) of the Japan Trench (small green squares in the Japan Trench in Fig. 1). The S-net is an ocean-bottom seismic and pressure observation network that NIED has been operating since 2016 (NIED 2019a). These studies revealed detailed tectonic tremor activity from 2016. The tremors were widely distributed and active on the shallow plate interface of the northern and southern Japan Trench, roughly consistent with the VLFE distribution presented by Matsuzawa et al. (2015). Furthermore, Nishikawa et al. (2019) discovered an approximately 200-km-long tremor gap in the central Japan Trench (37–39° N) (Fig. 1), which corresponds well with the large slip area (10 m or larger) of the Tohoku-Oki earthquake (Iinuma et al. 2012; the thick red contours in the Japan Trench in Fig. 1).
In 2020, Baba et al. (2020) conducted a comprehensive detection of VLFEs using data recorded by the F-net broadband seismograph network. They used synthetic waveforms of low-angle reverse faulting on the plate interface as templates of the matched filter technique (Shelly et al. 2007) and revealed the spatiotemporal distribution of VLFEs along the entire Japan Trench from 2003 to 2018. The distribution of VLFEs corresponded well with the distribution of tectonic tremors revealed by the S-net (Tanaka et al. 2019; Nishikawa et al. 2019), as expected from the fact that tectonic tremors and VLFEs are seismic waves emanating from the identical slow interplate slip phenomenon (Ide et al. 2007a; Kaneko et al. 2018; Masuda et al. 2020; Sect. 2.2.2). As with the tectonic tremors (Nishikawa et al. 2019), Baba et al. (2020) discovered an approximately 200-km-long VLFE gap in the central Japan Trench, which corresponded well with the large slip area of the Tohoku-Oki earthquake. Furthermore, this VLFE gap is a feature observed both before and after the 2011 Tohoku-Oki earthquake (Baba et al. 2020). This observation indicates that the Tohoku-Oki earthquake ruptured the gap of seismic slow earthquakes.
In 2021, Nishimura (2021) carried out a systematic and comprehensive short-term SSE detection using GNSS data from the GEONET in the southern Japan Trench. Nishimura (2021) discovered a bimodal depth distribution of SSEs (Mw 5.8–6.9) on the shallow (10–30 km depth) and deeper (40–60 km depth) parts of the plate interface (Fig. 1). SSEs avoid a depth of 30–40 km, where past megathrust earthquakes have repeatedly occurred (e.g., Mochizuki et al. 2008; Kubo et al. 2013). The bimodal depth distribution of SSEs is similar to that of shallow and deep slow earthquakes (shallower than 10 km depth and 25–40 km depth, respectively) in the Nankai Trough (Fig. 1).
Research on the relationship between slow and fast earthquakes along the Japan Trench has also made progress in recent years. In 2020, Kubo and Nishikawa (2020) revealed the along-dip complementary distribution of Mw 7 class megathrust earthquake ruptures (30–40 km depth) (e.g., Yamanaga and Kikuchi, 2004) and tectonic tremors (10–20 km depth) (e.g., Nishikawa et al. 2019) in the northern and southern Japan Trench. They suggested that the shallow slow-earthquake-genic regions in the northern and southern Japan Trench impede rupture propagation of megathrust earthquakes. In 2021, synchronous phenomena of tectonic tremors and swarms of small interplate earthquakes were observed in the shallow part of the southern Japan Trench (Obana et al. 2021). This observation implies that SSEs too small for onshore geodetic detection often trigger both tectonic tremors and earthquake swarms on the shallow plate interface of the Japan Trench.
Observational studies on slow earthquakes in the Japan Trench
In this section, we present the detailed slow earthquake distribution revealed by observational studies in the Japan Trench. We first present the spatiotemporal distribution of slow earthquakes from August 2016 (Sect. 3.2.1) because it is much more clearly resolved than before August 2016 (Sect. 3.2.2); the details of the spatiotemporal distribution of slow earthquakes before August 2016 are still enigmatic. We believe that a clear picture presented in Sect. 3.2.1 facilitates understanding of the subsequent Sects. (3.2.2 and 3.2.3). In the last part of this section, we review studies on repeating earthquakes, earthquake swarms, and foreshocks because the studies are closely related to slow earthquake observations in the Japan Trench (Sects. 3.2.1 and 3.2.2) and provide insights into slow earthquake activity before March 2011.
Spatiotemporal distribution of slow earthquakes from August 2016
The detailed tectonic tremor activity has been revealed by the S-net since August 2016. Figure 5 shows the spatiotemporal distribution of tectonic tremors (Nishikawa et al. 2019), VLFEs (Baba et al. 2020), and short-term SSEs (Nishimura 2021) from August 2016 to December 2021 along the Japan Trench. Nishikawa et al. (2019) detected the tectonic tremors until August 2018 by applying the envelope correlation method (Obara 2002; Ide 2010) to the S-net seismograms. We updated their tremor catalog until December 2021 using the same method as Nishikawa et al. (2019). Baba et al. (2020) detected VLFEs from January 2003 until July 2018 by a matched filter technique using synthetic waveforms of low-angle reverse faulting on the plate interface as templates. The templates were spaced approximately 40 km apart in the latitudinal and longitudinal directions. The SSE distribution was based on Nishimura (2021), who detected SSEs using the GNSS data of the GEONET until December 2019 in the southern Japan Trench. We updated the catalog until September 2021 using the same method as Nishimura (2021).
We observed good correspondence between tectonic tremors and VLFEs in both their epicenters and origin times (Fig. 5b, d, and e). This is not surprising because tectonic tremors and VLFEs originate from the identical slow interplate slip phenomenon (Ide et al. 2007a; Kaneko et al. 2018; Masuda et al. 2020; Sect. 2.2.2; Fig. 3b). Based on this fact, hereafter, we assume that, as with the VLFEs, the detected tectonic tremors are located along the plate interface, although the source depths of the tectonic tremors are not well constrained (Nishikawa et al. 2019).
In the Japan Trench, tectonic tremors and VLFEs are distributed along the depth contours of 10–20 km of the plate interface (Ohta et al. 2019; Tanaka et al. 2019; Nishikawa et al. 2019; Baba et al. 2020). However, the tremors and VLFEs avoid the central Japan Trench (37–39° N), making the region an approximately 200-km-long gap of seismic slow earthquakes (Fig. 5a). This gap corresponds well with the large slip area of the 2011 Tohoku-Oki earthquake (e.g., Iinuma et al. 2012; the thick red contours in the Japan Trench in Fig. 1). This correspondence is the most prominent feature of the slow earthquake distribution in the Japan Trench.
The tectonic tremor activity along the Japan Trench and the southern end of the Kuril Trench has some common features with tectonic tremor activities in other subduction zones, implying that the slow-earthquake-genic regions in the Japan and Kuril trenches and other subduction zones share basically similar frictional and rheological properties. There are three prominent common features. The first one is tectonic tremor migration. In the southern end of the Kuril Trench, which is located to the north of 41° N, along-strike tremor migration episodes with a speed of ~ 10 km/day are observed approximately once a year (Tanaka et al. 2019; Nishikawa et al. 2019; Fig. 5b and c). Similar along-strike migration episodes with comparable speeds have been observed at the deeper (approximately 10 km/day; Obara 2002) and shallower (approximately 10–60 km/day; Yamashita et al. 2015; Annoura et al. 2017) parts of the Nankai Trough and at the deeper part of the Cascadia subduction zone (approximately 10 km/day; Rogers and Dragert 2003; Houston et al. 2011).
The second common feature is the simultaneous occurrence of tectonic tremors and short-term SSEs (Nishikawa et al. 2019; Fig. 5f and h). This composite fault slip phenomenon, called ETS (Sect. 2.2.1), has also been observed at the deeper part of the Cascadia subduction zone (Rogers and Dragert 2003) and at the deeper and shallower parts (Obara et al. 2004; Araki et al. 2017) of the Nankai Trough. However, in the Japan Trench, many tremor bursts occur without detectable SSEs (Nishimura 2021; Fig. 5b). This is probably because the SSE catalog (Nishimura 2021) is not complete near the trench axis. It is generally difficult to detect small (Mw \(\le\) 6) SSEs near the trench axis using only onshore GNSS observations. Furthermore, it is also worth noting that deep ETS has not been reported in the Japan Trench. Although SSEs exhibit a bimodal depth distribution (10–30 km and 40–60 km) (Nishimura 2021; Fig. 5a), tectonic tremors have been observed only on the shallower plate interface.
The third common feature is the correlation between the recurrence intervals of tremor bursts and their seismic energy rates. Yabe et al. (2021) compared the recurrence intervals of tremor bursts with the energy rates of their 2–8 Hz bandpass-filtered seismic waves to find a correlation between them. In the southern end of the Kuril Trench (north of 41° N), tectonic tremor bursts repeated approximately every 0.5–1 year (Fig. 5b), and their median seismic energy rate was high (1700 J/s). In the northern Japan Trench, tremor episodes repeated approximately every 1–2 months (Fig. 5b), and their median seismic energy rate was low (830 J/s). The southern Japan Trench was characterized by medium recurrence intervals of approximately 3 months and medium median seismic energy rates (approximately 1400 J/s). The correlation between recurrence intervals of tremor bursts and their seismic energy rates has also been observed for deep tremors in the Cascadia subduction zone in both the along-strike and along-dip directions (Wech and Creager 2011; Idehara et al. 2014; Yabe and Ide 2014). Yabe et al. (2021) suggested that differences in the frictional strength of faults producing tectonic tremors cause the correlation. A stronger fault may endure stress loading for a longer period and produce more energetic tremors.
The tectonic tremor activity along the Japan Trench also has a feature distinct from that in the other subduction zones: We observed a coincidence of tectonic tremors, short-term SSEs, and a swarm of moderate interplate earthquakes in the Japan Trench. In July and August 2021, a Mw 6.6 short-term SSE and a burst of tectonic tremors occurred in the southern Japan Trench (35.5–36.5° N) (Fig. 5h). In early August, a swarm of moderate interplate earthquakes, with a maximum magnitude of Mw 5.8, occurred in an adjacent region. The fault model of the SSE was located to the north (around 36.4° N) of the earthquake swarm (around 36.2° N), and the tremor burst occurred to the south (around 35.8° N). We also observed a similar coincidence of tectonic tremors, short-term SSEs, and three Mj \(\ge\) 4 interplate earthquakes in November and December 2018 in the same region (Fig. 5g). Composite fault slip phenomena of tectonic tremors, short-term SSEs, and a swarm of moderate interplate earthquakes have rarely been observed in subduction zones other than the Japan Trench. In the Nankai Trough and Cascadia subduction zone, ETSs on the deeper plate interface are not accompanied by swarms of interplate earthquakes (Rogers and Dragert 2003; Obara et al. 2004). Furthermore, moderate interplate seismicity (M \(\ge\) 4) is very rare in the Nankai Trough and Cascadia subduction zone (Ide 2013). However, we note that a recent study (Yamamoto et al. 2022) has reported the simultaneous occurrence of a short-term SSE, VLFEs, and a swarm of interplate microearthquakes (M \(\le\) 3) on the shallow plate interface (10 km depth or shallower) of the Nankai Trough. At the Boso Peninsula in the Sagami Trough, eastern Japan, swarms of moderate interplate earthquakes accompany short-term SSEs (Ozawa et al. 2003; Sagiya 2004), but concurrent tectonic tremors have not been observed. These differences in composite fault slip phenomena between subduction zones may reflect differences in interplate frictional properties of slow-earthquake-genic regions between subduction zones. Note, however, that each subduction zone has different observation conditions, such as station density and noise level, and that such differences might lead to apparent differences in composite fault slip phenomena.
Spatiotemporal distribution of slow earthquakes before August 2016
With respect to slow earthquake activity prior to the start of the S-net observations in August 2016, the VLFE activity along the entire Japan Trench (Baba et al. 2020) and SSE activity in the southern Japan Trench (Nishimura 2021) were revealed by the F-net and GEONET, respectively. Systematic and comprehensive detection of SSEs in the central and northern Japan Trench has not yet been conducted. However, in the central Japan Trench, ocean-bottom pressure gauge observations detected short-term SSEs in November 2008 and from the end of January 2011 to just before the largest foreshock (Mw 7.3) of the March 11, 2011, Tohoku-Oki earthquake (Ito et al. 2013). We note that one should be careful of the treatment of the SSE preceding the Tohoku-Oki earthquake, especially when discussing the slow earthquake distribution prior to the Tohoku-Oki earthquake. It is natural that the period just prior to and the region close to the epicenter of the Tohoku-Oki earthquake be analyzed in greater detail than the other periods and regions, given their scientific importance. This causes a spatiotemporal bias in the detectability of SSEs. Moreover, several SSEs similar to the SSE preceding the Tohoku-Oki earthquake (i.e., SSEs accompanied by earthquake swarms and/or tectonic tremors) have recently been reported in the southern Japan Trench (Nishimura 2021; Fig. 5h), implying that such SSEs are more common in the Japan Trench than previously thought.
Figure 6 shows the slow earthquake activity before and after the March 2011 Tohoku-Oki earthquake. VLFEs frequently occurred in the northern and southern Japan Trench both before and after the 2011 Tohoku-Oki earthquake (Fig. 6b and c). In the gap of tectonic tremors and VLFEs that we identified in Fig. 5a (37–39° N), VLFE activity was low even before the Tohoku-Oki earthquake (Fig. 6a and b). The VLFEs before the Tohoku-Oki earthquake appear to have avoided the huge interplate locked zone in the central Japan Trench (Suwa et al. 2006; the red shaded area in Fig. 6a), although ocean-bottom pressure gauge and seismometer observations suggested that a short-term SSE and tectonic tremors preceding the March 2011 Tohoku-Oki earthquake had occurred in the VLFE gap (Ito et al. 2013; Ito et al. 2015; Katakami et al. 2018; a green rectangle within the Tohoku-Oki earthquake rupture in Fig. 6a and b).
As shown in Fig. 6b, short-term SSEs and VLFEs sometimes occur simultaneously in close proximity. These coincidences are potential ETSs (Sect. 2.2.1). Here, if the distance between a VLFE epicenter and an SSE fault model was within 40 km and the VLFE occurred within 2 weeks before or after the short-term SSE period, we considered the event as a potential ETS. As a result, we identified six potential ETSs before the March 2011 Tohoku-Oki earthquake (Fig. 6b). However, Fig. 6b shows that most of the VLFEs were not accompanied by detectable short-term SSEs (Nishimura 2021). This may be due to the incompleteness of the SSE catalog near the trench axis, as mentioned in Sect. 3.2.1 Furthermore, the uncertainty in location of the VLFEs and SSEs may also make the correspondence between the two slow earthquakes worse.
Figure 6 also shows two slow fault slip phenomena similar to SSEs. One is the decade-long acceleration of aseismic slip on the deeper plate interface (approximately 30–60 km depth) of the central Japan Trench, which was discovered by the GEONET (Ozawa et al. 2012; Mavrommatis et al. 2014; Yokota and Koketsu 2015; Sect. 3.1.2; the yellow shaded area in Fig. 6a and b). This transient lasted from around 2002 to just before the 2011 Tohoku-Oki earthquake. Note that this decade-long transient may also have induced slips on the shallower plate interface (approximately 10–30 km depth), as suggested by repeating earthquake data (Mavrommatis et al. 2015). This transient slip resembles long-term SSEs (Yokota and Koketsu 2015). However, the duration of this transient slip (9 years) is longer than that of typical long-term SSEs (several months to several years), and the magnitude of this transient slip (Mw 7.7) is also larger than that of typical long-term SSEs (Mw 6.5–7.5) (Fig. 3a). Furthermore, it is unclear whether this transient slip recurs like the deep long-term SSEs in the Nankai Trough (e.g., Kobayashi 2014; Takagi et al. 2019), as it was observed only once in the decade prior to the 2011 Tohoku-Oki earthquake. Mavrommatis et al. (2014) suggested that this transient slip might be an aseismic slip intruding from the deep plate interface into the inside of the locked region prior to the coseismic rupture. A definite conclusion has not yet been reached as to the identity of this aseismic transient slip.
The other slow fault slip phenomenon similar to SSEs is a transient aseismic slip in the northern part (39.2–40.2° N) of the Japan Trench in early 2015 (Honsho et al. 2019; the blue dashed square in Fig. 6a). Honsho et al. (2019) detected this transient slip (approximately Mw 7.3) based on GNSS-acoustic observations and repeating earthquake activity. Fujiwara et al. (2022) also detected the probably identical transient slip as Honsho et al. (2019) using onshore GNSS observations. Although Honsho et al. (2019) considered this transient slip as a shallow SSE, it may not be a typical SSE. This is because the transient slip followed a large interplate earthquake of Mw 6.7 (Fujiwara et al. 2022). Therefore, one might consider the transient slip as a huge afterslip of the Mw 6.7 interplate earthquake, although it is too large to be a typical afterslip (Alwahedi and Hawthorne 2019). We found that the region where the transient slip was observed significantly overlaps with the region where Kawasaki et al. (1995) observed the 1992 Sanriku-Oki ultraslow earthquake, a transient aseismic slip of Mw 7.3–7.7 that followed a Mw 6.9 interplate earthquake (Sect. 3.1.1 and the blue dashed rectangle in Fig. 1). Due to the similarity and proximity between the 1992 and 2015 transient slip events, we interpret the 2015 event as a slow slip triggered by an earthquake as well as the 1992 event (Sect. 3.1.1). A transient aseismic slip with characteristics intermediate between SSEs and afterslip (Villegas-Lanza et al. 2016) may be a typical slip behavior on the shallow plate interface of the northern part (39–40° N) of the Japan Trench.
Repeating earthquakes, earthquake swarms, and foreshocks before March 2011
Slow earthquake activity along the Japan Trench prior to the 2011 Tohoku-Oki earthquake has been inferred from fast earthquake activity potentially indicative of slow earthquake occurrence (i.e., repeating earthquakes, earthquake swarms, and swarms of foreshocks) (Sect. 2.3). In the Japan Trench, repeating earthquakes have been vigorously studied since the first report by Matsuzawa et al. (2002). The spatiotemporal distribution of repeating earthquakes has been revealed in detail (Igarashi et al. 2003; Igarashi 2010; Uchida and Matsuzawa 2013; Igarashi 2020; red points in Fig. 7). As already described in Sect. 3.1.1, Igarashi et al. (2003) found that few continuous-type small repeating earthquakes (i.e., repeating earthquakes that repeat at almost regular intervals and are not clustered in time) were distributed within the huge locked zone in the central Japan Trench (Ito et al. 2000; Nishimura et al. 2000). Consistent with Igarashi et al. (2003), repeating earthquakes detected by Uchida and Matsuzawa (2013) are sparse within the large slip area of the Tohoku-Oki earthquake in the central Japan Trench (37–39° N) in comparison with the adjacent regions in the northern and southern Japan Trench (39–40° N and 36–37° N) (Fig. 7).
Repeating earthquakes have been used to detect transient aseismic slip on the plate interface in the Japan Trench (Matsuzawa et al. 2004; Uchida et al. 2004, 2016; Kato et al. 2012; Uchida and Matsuzawa 2013; Khoshmanesh et al. 2020). A Mj 5 class earthquake swarm including repeating earthquakes occurred on the shallow plate interface in the central Japan Trench a month before the March 2011 Tohoku earthquake, with their epicenters migrating toward the rupture initiation point of the Tohoku-Oki earthquake (Kato et al. 2012). This observation is a representative example of inferring the occurrence of a propagating SSE from repeating earthquake activity (Sect. 2.3).
Uchida et al. (2016) analyzed repeating earthquake activity from 1984 to 2011 along the entire Japan Trench and found that the interplate slip rates estimated from the repeating earthquake change with periods of 1–6 years. The periodic slip rate changes inferred from repeating earthquakes were widespread in the Japan Trench. They were also suggested by temporal changes in spatial gradients of the surface displacement rate field (Iinuma 2018). Furthermore, Uchida et al. (2016) showed that the occurrence rate of Mj 5 or greater earthquakes was positively correlated with the interplate slip rate. Based on these observations, Uchida et al. (2016) suggested that periodic SSEs widespread in the Japan Trench had stressed the surrounding regions and triggered Mj 5 or greater earthquakes.
Uchida et al. (2016) identified the northern part (39–40.4° N) of the Japan Trench (the black squares in Fig. 7a) as an area of pronounced 3-year periodicity. In and near this region, a repeating earthquake analysis by Uchida et al. (2004) also suggested SSEs preceding the 1989 Mw 7.4, 1992 Mw 6.9, and 1994 Mw 7.7 Sanriku-Oki earthquakes (Sect. 3.1.1). Furthermore, the far offshore region of the pronounced 3-year periodicity significantly overlaps with an area where SSEs triggered by earthquakes were geodetically observed in 1992 (Kawasaki et al. 1995; Kawasaki et al. 2001; Sect. 3.1.1; the blue dashed square in Fig. 1) and 2015 (Honsho et al. 2019; Fujiwara et al. 2022; Sect. 3.2.2; the blue dashed square in Fig. 6a). Uchida et al. (2016) probably captured the recurrence of these SSEs. In fact, the 1992 event (i.e., the 1992 Sanriku-Oki ultraslow earthquake) (Kawasaki et al. 1995, 2001) corresponds with a peak of the periodic interplate slip rate in Uchida et al. (2016).
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; Nishikawa and 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. 2.3) as earthquake swarms and detected such seismicity rate increases in the central and southern Japan Trench using a statistical seismicity model called the epidemic-type aftershock-sequence (ETAS) model (e.g., Ogata 1988; Zhuang et al. 2002). The ETAS model usually expresses the seismicity rate as the summation of a constant background seismicity rate and aftershock rates derived from Omori–Utsu’s aftershock law. Therefore, increases in the seismicity rate that do not obey the Omori–Utsu’s aftershock law appear as anomalous increases in the seismicity rate in the ETAS model analysis (e.g., Llenos et al. 2009; Okutani and Ide 2011). As shown in Fig. 7, Mj \(\ge\) 3 earthquake swarms frequently occur in the southern Japan Trench. Furthermore, Nishikawa and Ide (2018) found that earthquake swarms including repeating earthquakes had repeatedly occurred in a region around 36.2° N, 142° E (Fig. 7a). The earthquake swarm activity was the most active during the weeks prior to the 1982 and 2008 Mj 7 Ibaraki-Oki earthquakes (Matsumura 2010; Nishikawa and Ide 2018), which ruptured the plate interface downdip of the recurrent earthquake swarms. Nishikawa and Ide (2018) inferred from these observations that SSEs recur in the region around 36.2° N, 142° E and that the SSEs preceding the Mj 7 Ibaraki-Oki earthquakes were possibly aseismic slip acceleration in the nucleation phase of the Ibaraki-Oki earthquakes (e.g., Dieteich, 1992; Ohnaka 1992; McLaskey 2019). Recently, seismic slow earthquakes (i.e., tectonic tremors and VLFEs) were observed in the region around 36.2° N, 142° E (Nishikawa et al. 2019; Baba et al. 2020; Fig. 6a). The coincidence of a short-term SSE, burst of tectonic tremors, and earthquake swarms was also observed in July and August of 2021 in the same region (Fig. 5h). 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 (Mj 5 or greater) of Mj \(\ge\) 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. 7a). In these areas, 38% of the Mj \(\ge\) 6 mainshock earthquakes were preceded by a swarm of foreshocks (three or more Mj \(\ge\) 5 earthquakes within 10 days before the mainshock) (Maeda and Hirose 2016). These foreshock-prone areas are close to areas where tectonic tremors and VLFEs occur (Figs. 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. 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.
Although repeating earthquakes and earthquake swarms have been used to infer the occurrence of SSEs in the Japan Trench (e.g., Kato et al. 2012; Marsan et al. 2013; Uchida et al. 2016; Nishikawa and Ide 2018), they have hardly ever been compared with independently observed SSEs or VLFEs in the Japan Trench. Here, we compared the catalogs of small repeating earthquakes (Uchida and Matsuzawa 2013; Nishikawa et al. 2019), Mj \(\ge\) 3 earthquake swarms (Nishikawa and Ide 2018; Nishikawa et al. 2019), SSEs (Nishimura 2021), and VLFEs (Baba et al. 2020). In Fig. 8a and b, we counted repeating earthquakes and earthquake swarm events around SSE fault models (inside the rectangular faults or within 20 km of the fault models) within 80 days before or after the central dates of the SSE occurrence periods. Here, we excluded SSEs associated with Mw \(\ge\) 5.8 fast earthquakes (Nishimura 2021) because not SSEs but afterslip following the Mw \(\ge\) 5.8 earthquakes may have triggered repeating earthquakes and earthquake swarms.
Figure 8a and b shows that the counts of repeating earthquakes and earthquake swarm events have a maximum within 10 days before or after the central dates of the SSE occurrence periods. This is consistent with the idea that SSEs trigger repeating and earthquake swarms in their vicinities (e.g., Kato et al. 2012; Marsan et al. 2013; Uchida et al. 2016; Nishikawa and Ide 2018). However, with respect to earthquake swarms, the sample size (18 earthquake swarm events) was too small to make a statistical argument. As for repeating earthquakes, the tendency that repeating earthquakes are more likely to occur within 10 days before or after the SSE central dates than in the other periods is statistically significant (p = 1%). Here, we conducted a statistical test with a null hypothesis that repeating earthquakes are equally likely to occur on the dates within 80 days before or after the SSE central dates and used a significance level of 5%. However, in Fig. 8a, the repeating earthquakes also occurred on days not close to the central dates of the SSEs. This is probably because repeating earthquakes are triggered by not only SSEs but also interplate steady slip (e.g., Nadeau and Johnson 1998; Nadeau and McEvilly 1999; Matsuzawa et al. 2002; Igarashi et al. 2003). Furthermore, as mentioned in Sect. 3.2.2, the incompleteness of the SSE catalog near the trench axis may have affected the results in Fig. 8a and b.
Figure 8c shows the spatial distribution of coincident events of earthquake swarms and SSEs or earthquake swarms and VLFEs. From January 1995 to March 2011, we observed six coincident events of earthquake swarms and SSEs and nine coincident events of earthquake swarms and VLFEs (Fig. 7b). Here, if an earthquake swarm event occurs within 2 weeks before or after an SSE period inside the SSE rectangular fault or within 20 km of the fault, we regard it as a coincident event. Similarly, if an earthquake swarm event occurs within 2 weeks before or after a VLFE within 40 km of the VLFE epicenter, we regard it as a coincident event. Small repeating earthquakes included in earthquake swarm activity are indicated by colored stars in Fig. 8c. We found that the coincident events had repeatedly occurred at the same locations (Fig. 8c). Specifically, they recurred in regions around 36.3° N, 142.5° E, 36.2° N, 142.0° E, and 35.1° N, 141.4° E. The region around 36.3° N, 142.5° E corresponds with the area where Obana et al. (2021) observed coinciding events of tectonic tremors and swarms of interplate microearthquakes in March and June of 2017 (Sect. 3.1.2). Furthermore, in the region around 36.2° N, 142.0° E, the coincidence of a short-term SSE, burst of tectonic tremors, and an interplate earthquake swarm was also observed in July and August of 2021 (Fig. 5h). These observations imply that coincident events of slow and fast earthquakes are common on the shallow plate interface of the southern Japan Trench.
Experimental studies on slow earthquakes in the Japan Trench
Experimental studies on slow earthquakes in the Japan Trench have been focused on the slip behavior of the shallow plate interface (Ikari et al. 2015; Ito and Ikari 2015; Ito et al. 2017; Ikari and Kopf 2017; Sawai et al. 2017). These studies were motivated by the observational findings that an SSE preceding the Tohoku-Oki earthquake occurred on the shallow plate interface of the central Japan Trench and that the shallow plate interface subsequently underwent a huge coseismic slip breaching the trench axis (Kato et al. 2012; Ito et al. 2013). Before the 2011 Tohoku-Oki earthquake, the shallow subduction plate interface was thought to exclusively host aseismic creep (e.g., Nishizawa et al. 1992; Ikari et al. 2015). This is consistent with the standard model of the megathrust slip behavior at that time (Scholz 1998), in which the shallowest part of the plate interface is characterized by rate-strengthening behavior and stable sliding. However, this view was challenged by the aforementioned observations implying the unstable nature of the shallow plate interface. With this background, the above experimental studies reconsidered the frictional behavior of the shallow plate interface using core samples from the shallow plate boundary of the Japan Trench.
Ikari et al. (2015) conducted an experiment in which a core sample retrieved from the shallow plate boundary fault zone (~ 7 km landward of the trench axis and 822 m below sea floor) by the Japan Trench Fast Drilling Project (JFAST) (Chester et al. 2013) was sheared at a rate comparable to the plate convergence rate (8.5 cm/year). They found that the clay-rich sample exhibited rate-weakening behavior, which is suitable for coseismic rupture propagation, and observed spontaneous slow strength perturbations similar to SSEs. Ikari et al. (2015) proposed that these experimental results explain both the preseismic SSE and coseismic rupture of the Tohoku-Oki earthquake. Furthermore, Ikari and Kopf (2017) showed that weak clay-rich fault samples from shallow parts of several subduction zones also exhibit rate-weakening behavior and produce slow strength perturbations when sheared at a rate comparable to plate velocities. They suggested that the observed unstable nature of the shallow fault zone materials may facilitate the propagation of coseismic rupture and slip at shallow depths in subduction zones.
Ito et al. (2017) also used a core sample retrieved by JFAST and showed that an increase in the slip rate can induce a change from slip-strengthening friction to slip-weakening friction. They found that velocity steps with initial slip rates consistent with afterslip following the largest (Mw 7.3) foreshock of the Tohoku-Oki earthquake exhibit significant slip weakening. Based on this experiment, they suggested that the accelerated interplate aseismic slip due to the afterslip following the largest foreshock (Ando and Imanishi 2011; Kato et al. 2012; Ohta et al. 2012; Sect. 3.1.2) might have been a favorable initial condition for the huge coseismic slip of the Tohoku-Oki earthquake.
Numerical simulations on slow earthquakes in the Japan Trench
There are several simulation studies that consider slow earthquakes (especially SSEs) in the Japan Trench (Mitsui et al. 2012; Ohtani et al. 2014; Shibazaki et al. 2019; Barbot 2020; Nakata et al. 2021). These are earthquake cycle simulations based on rate-and-state-dependent friction (Dieterich 1979; Sect. 2.4). Some studies reproduced SSEs in the Japan Trench by assuming large characteristic slip distances (Ohtani et al. 2014; Nakata et al. 2021; Sect. 2.4), and others reproduced the SSEs by assuming friction that exhibits rate-weakening behavior at low slip rates but switches to rate-strengthening behavior at high slip rates (Shibazaki et al. 2019; Sect. 2.4).
Ohtani et al. (2014) assumed that regions outside asperities show rate-weakening behavior but have a large characteristic slip distance. Asperities here mean large slip areas of past megathrust earthquakes (Yamanaka and Kikuchi 2004), as described in Sect. 3.1.1. Ohtani et al. (2014) found that recurrent SSEs occur on the shallow plate interface in the southern Japan Trench and pointed out that these SSEs might correspond to a decrease in the interplate coupling in the southern part (~ 37° N) of the Japan Trench prior to the 2011 Tohoku-Oki earthquake detected by GNSS observations (Geospatial Information Authority of Japan GSI 2011). Shibazaki et al. (2019) assumed that the shallow plate interface near the trench axis has a frictional property that exhibits rate-weakening behavior at low slip rates but switches to rate-strengthening behavior at high slip rates. Their model reproduced the preseismic SSE, the largest foreshock, and coseismic rupture of the Tohoku-Oki earthquake, although the location of the simulated SSE (the shallow part of the northern Japan Trench) was different from that of the observed SSE (the shallow part of the central Japan Trench) (Ito et al. 2013).
No previous simulation studies have reproduced or considered the complex slow earthquake distribution along the Japan Trench (Figs. 5 and 6). However, Nakata et al. (2021) attempted to reproduce along-strike changes in the slip behavior of the Japan Trench. They assumed rate-weakening behavior with a large characteristic slip distance in the southern and northern Japan Trench and rate-weakening behavior with a small characteristic slip distance in the central Japan Trench, considering the along-strike distribution of interplate sedimentary units revealed by multichannel seismic reflection surveys (Tsuru et al. 2002; Sect. 5.1.1). They reproduced the coseismic rupture of the Tohoku-Oki earthquake in the central Japan Trench (e.g., Ide et al. 2011; Iinuma et al. 2012; Lay 2018) and the large afterslip in the shallow part of the southern Japan Trench (Uchida and Matsuzawa 2013; Sun and Wang 2015; Iinuma et al. 2016; Tomita et al. 2017; Honsho et al. 2019; Tomita et al. 2020; Watanabe et al. 2021; the blue contours in Fig. 6a). The simulation results of Nakata et al. (2021) also show that transient slow fault slips repeatedly occur in the southern Japan Trench during interseismic periods. These transient aseismic slips might correspond to the slow earthquakes observed in the southern Japan Trench (Nishikawa et al. 2019; Baba et al. 2020; Nishimura 2021; Figs. 5 and 6), although the characteristic time scale of the transient slips (tens of years) and that of the observed SSEs (tens of days) are different.