5 − 1. Structural controls on the earthquake swarm location
The most striking feature of the tomographic results is the presence of a northward inclined, three-layered structure above the PHS slab at 10–40 km depth, which consists of a distinct, inclined high-velocity anomaly immediately above the PHS slab, an overlying inclined low-velocity anomaly beneath the Wakayama swarm, and a high-velocity anomaly beneath the Osaka Bay (Fig. 10). Uehara et al. (2005) revealed that a high-density, high-resistivity zone exists above the PHS slab in the southwestern part of the Kii peninsula and interpreted this zone to be a pluton of acidic rocks. We infer that the high-density, high-resistivity body in Uehara et al. (2005) corresponds to the inclined high-velocity anomaly imaged above the PHS slab. We interpret the high-density, high-resistivity, and high-velocity zone to be a body of impermeable and rigid materials (areas enclosed by the green curves in Fig. 11a and 11b). Although it has been suggested that the mantle-wedge beneath the Kii peninsula has been serpentinized (e.g., Kato et al. 2014), our tomographic results do not support a serpentinized mantle wedge because the high-velocity anomaly and moderate Vp/Vs values are not indicative of serpentinization (e.g., Christensen 1996; Watanabe et al. 2007). The presence of the impermeable, rigid materials above the PHS slab is consistent with the interpretation of Nakajima and Hasegawa (2016), whereby impermeable materials act as a cap layer and prevent effective drainage from the subducting slab to the overlying plate. The tectonic LFEs along the megathrust boundary are probably enhanced immediately below the impermeable materials, where pore-fluid pressures along the megathrust boundary can be elevated to near-lithostatic values.
The high-velocity zone beneath the Osaka Bay is not distinct from the inclined zone that we interpreted to be the impermeable, rigid materials (profiles B–D in Fig. 10). This high-velocity zone possesses similar characteristics, namely a high velocity and low to moderate Vp/Vs values that are comparable to those observed in the inclined high-velocity zone. Therefore, we infer that the high-velocity zone beneath the Osaka Bay represents unmetamorphosed, rigid materials that have been minimally influenced by fluids or thermal anomalies (areas enclosed by green dashed curves in Fig. 11a and 11b). The inferred rigid materials are limited to the Kii peninsula and Osaka Bay and appear to bound both the Wakayama swarm and the deep LFE areas. This unique situation probably controls the upward migration of fluids to the surface, thereby forcing the fluids to localize along an inherent permeable zone that is sandwiched by the two above-mentioned impermeable materials. We interpret that this permeable zone is imaged as a distinct, inclined low-velocity zone, and that large amounts of fluids can migrate toward the Wakayama swarm through this permeable zone. This hypothesis strongly suggests that the location of the Wakayama swarm is controlled by the structural heterogeneities in the crust and the uppermost mantle.
Although the origin of the rigid materials above the PHS slab remains elusive, we infer that the rigid materials correspond to old, stagnant rocks in the continental plate that have not been heavily altered by slab-derive fluids, despite the subduction of the PHS slab over at least the last 15 Myr. Notably, the extent of these inferred rigid materials coincides with that of the steep, valley-shaped subduction of the PHS slab (Fig. 11a). Therefore, we suggest that the existence of the rigid, stagnant materials in the continental crust may have blocked the gently dipping subduction of the PHS slab, which is observed on both sides of the Kii peninsula, with these rigid materials causing the steeply dipping subduction of the PHS slab. This hypothesis can explain the valley-shaped geometry of the PHS slab that is locally developed beneath the Kii peninsula (Fig. 1).
5 − 2. High 3He/4He ratios, LFEs, and Arima-type hot springs
There are several anomalous geochemical and geophysical observations around the Kii peninsula. Sano and Wakita (1985) revealed that anomalously high 3He/4He ratios of up to 8 Ra (Ra, the atmospheric 3He/4He ratio) are observed across the Kii peninsula. Subsequently, it was confirmed that the observation of such high 3He/4He ratios is not limited to the Kii peninsula but is also observed along an NNW–SSE-oriented elongated area that extends from the southern part of the Kii peninsula to Arima hot spring (Fig. 11c) (Matsumoto et al. 2003; Umeda et al. 2007; Sano et al. 2009; Morikawa et al. 2016). Another notable observation is an Arima-type brine, whose oxygen and hydrogen isotope compositions are similar to those found in magmatic/metamorphic fluids (e.g.,Matsubaya et al. 1973). The origin of this Arima hot spring has been long argued since the Arima hot spring is far from any volcanic sources. Furthermore, the Osaka Bay is an exceptional area in Japan where isolated, deep LFEs intensively occur in non-volcanic environment (Figs. 9 and 11). Aso et al. (2011) showed that the overall pattern of LFE spectrum beneath Osaka Bay is similar to that of volcanic LFEs, thereby suggesting that the Osaka Bay LFEs are not distinct from volcanic LFEs and related to fluid accumulation near the Moho.
The anomalous geochemical and geophysical observations in this non-volcanic area have been interpreted as indicators of either by the upward migration of PHS slab-derived fluids (Matsumoto et al., 2003; Umeda et al., 2006), fluid supply from the Pacific (PAC) slab through a fissure in the PHS slab (Nakajima and Hasegawa 2007; Sano and Nakajima 2008), or fluids expulsion from a shallow magma body or solidified magmas (Sano and Wakita, 1985; Kato et al., 2014). The results of this study suggest that aqueous fluids, rather than magma or partial melts, are likely to exist in the upper crust beneath the Wakayama swarm because the observed low-velocity anomaly and low Vp/Vs values (< 1.7) cannot be explained by the presence of magma or melts (e.g., Nakajima et al. 2001; Takei 2002). Conversely, the low-velocity anomaly and higher Vp/Vs values (> 1.8) observed at 20–40 km depth can be attributed to either magma (partial melts) or aqueous fluids (e.g., Takei, 2002). Although we cannot discriminate the cause of this low-velocity anomaly from seismological observations alone, the Wakayama swarm and Arima hot spring are both in non-volcanic areas (100–150 km away from the nearest Quaternary volcano), and the terrestrial heat flow in this region is not as high as that in volcanic areas (Tanaka et al. 2004). Therefore, we infer that the low-velocity anomaly and higher Vp/Vs values in the lower crust represent the existence of fluids rather than that of magma or partial melts.
Kusuda et al. (2014) suggested that the high salinity and solute concentrations of the Arima-type hot springs can be explained by dehydration of the PHS slab because the estimated 87Sr/86Sr ratio of the deep brine is closer to the estimated ratio of PHS slab-derived fluids rather than that of PAC slab-derived fluids. Crustal dehydration reactions of the PHS slab have been inferred to occur at either 15–50 km (Yamasaki and Seno, 2003), 32–44 km (Yoshioka et al. 2008), or ~ 60 km (Peacock 2009) depth, whereas the mantle dehydration reactions are expected at 25–75 km depth (Yamasaki and Seno, 2003). Although the inferred depth ranges of these dehydration reactions are less constrained due to large uncertainties in the thermal structure of the PHS slab, it is clear that PHS slab-derived fluids are potentially expelled to the overlying plate over a broad depth range where intraslab earthquakes occur (e.g., Hasegawa and Nakajima 2017).
We infer that PHS slab-derived fluids are supplied to the uppermost mantle beneath the Osaka Bay and are the direct cause of the anomalous geophysical and geochemical observations across the Kii peninsula. Our observations suggest that slab-derived fluids first trigger deep LFEs beneath the Osaka Bay, then they migrate upward toward the Wakayama area along the permeable zone, eventually facilitating the Wakayama earthquake swarm in the seismogenic upper crust and producing the high 3He/4He ratios across the Kii peninsula. It is noted that the impermeable materials that exist above the PHS slab may prevent the slab-derived fluids from migrating directly upward to the southern part of the Kii peninsula where high 3He/4He ratios are observed (Fig. 11c). We speculate that the fluids that have accumulated beneath the Wakayama swarm are being supplied to the southern part of the peninsula through small-scale permeable pathways above the impermeable materials, even though this study cannot resolve such fine-scale structures. The location of Arima hot spring is consistent with the location of a branch of the permeable zone, which facilitates upward fluid migration toward Arima hot spring, as illustrated in Fig. 11(b). The high salinity and solute concentrations of the Arima-type hot springs and high 3He/4He ratios observed near Arima hot spring are likely a result of the supply of the slab-derived fluids. Therefore, we conclude that the anomalous geophysical and geochemical signatures that have been observed across the Kii peninsula arise from a common source, and the observed variations are caused by different manifestations of the frictional and hydrological processes that are related to the concentrated supply and upward migration of slab-derived fluids.
5 − 3. Earthquake triggering along pre-existing faults
Our observations have revealed that the hypocenters of the Wakayama swarm are aligned along well-defined planes that dip to the west at 30–45º (Fig. 8), which demonstrates that the earthquakes in this so-called earthquake swarm are occurring along individual fault planes. The focal mechanism solutions suggest that reverse faulting along N–S-oriented nodal planes under an E–W compressional stress regime is the predominant fault style in the Wakayama swarm, but the major geological units in this regoin are oriented almost in the E–W direction. Therefore, we infer that the seismicity of the Wakayama earthquake swarm is not related to the reactivation of the major E–W-trending geological units, but is rather triggered by shear slip of pre-existing weak faults that are preferably reactivated under the present-day E–W compressional stress regime (e.g., Terakawa and Matsu’ura 2010; Uchide et al. 2022). However, small earthquakes tend to show either strike-slip or normal faulting along primarily E–W-oriented nodal planes (Maeda et al., 2018), thereby suggesting that minor geological faults can be still active, and small earthquakes occur along these planes.
Yoshida et al. (2011) found that the annual rate of vertical movement is up to 10 mm/yr immediately to the east of the Wakayama swarm and proposed that the movement is caused by the intrusion of volcanic rocks beneath the eastern part of the swarm region (a purple ellipse in Fig. 11c). Maeda et al. (2018) argued that the observed depth-dependent stress regime of the Wakayama swarm, with shallow strike-slip-type and deep reverse-type stress regimes, is due to the thermal stress effect from a heat source beneath the swarm. However, our tomographic results suggest that a specific heat source, such as magmatic body or solidified magma, is unlikely to be present beneath the Wakayama swarm. Instead, we suggest that the migration of a large amount of the slab-derived fluids to the upper crust may have induced the observed vertical uplift of ~ 10 mm/year. The heat transport caused by fluid migration may result in the observed moderate terrestrial heat flow values across the Kii peninsula (Tanaka et al., 2004). The locally shallow hypocenters in the Wakayama swarm, which are 5–10 km shallower than those of the surrounding earthquakes (Fig. 3a), may be manifestations of the concentrated heat flux from the depth.
We propose, based on recent experiments and observations, that three essential processes must occur to generate the Wakayama earthquake swarm. First, a large volume of overpressurized fluids originating from slab-derived fluids migrate upward, and infiltrate into individual faults in the upper crust (Fig. 12). High pore-fluid pressures can then stabilize shear slip along the rate-strengthening part (creep areas) of each fault by increasing the friction parameter (a–b) and reducing the critical stiffness (kc) (e.g., Scholz 1998; Cappa et al. 2019; Bedford et al. 2021), which may simultaneously trigger aseismic slip along pre-existing faults. These high pore-fluid pressures can also cause a reduction in the shear strength of the frictionally locked asperity patches on each fault. The combination of enhanced aseismic slip and the reduced strength of asperity patches is an essential factor to trigger earthquakes. This hypothesis suggests that earthquake swarm is not district from regular earthquakes and the much larger volume of the fluid migration probably facilitates the persistent seismicity that is recognized as an earthquake swarm. Nakajima and Hasegawa (2022) have argued that this three-step process is probably essential for all the earthquakes, regardless of their tectonic regime, types of earthquake activity, and focal depths.
The Wakayama earthquake swarm is limited to a narrow (and shallow) depth range (5–10 km depth; Figs. 3 and 9), even though there is a continuous supply of slab-derived fluids to the seismogenic upper crust. Brittle deformation is fundamentally required for the generation of earthquakes (e.g., Albaric et al. 2009); however, the frictional properties of rocks are also an important factor for seismogenesis (e.g., Scholz 1998). Maeda et al. (2021) estimated that the thickness of the seismogenic layer is 4–6 km for pelitic and mafic rocks beneath the Wakayama swarm and concluded that earthquakes occur in a limited depth range where the friction parameter is negative, such that velocity-weakening behavior is expected.