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![Hypocenter distribution of earthquakes occurring as part of the Yamagata-Fukushima border earthquake swarm activity. The dots indicate the hypocenters of earthquakes listed in the JMA unified catalogue for the period of 800 days from the beginning of the swarm activity. The elapsed time after the 2011 Tohoku-Oki earthquake is indicated by the color scale. The broken line denotes the border between Yamagata and Fukushima prefectures. The black line denotes the rim of the Ohtoge caldera [Kanisawa et al., 2006]. (a) Map view. (b) E-W vertical cross-section view. (c) N-S vertical cross-section view.](profile/Keisuke-Yoshida-5/publication/306932495/figure/fig11/AS:399353995907073@1472224820537/Hypocenter-distribution-of-earthquakes-occurring-as-part-of-the-Yamagata-Fukushima-border.png)
Hypocenter distribution of earthquakes occurring as part of the Yamagata-Fukushima border earthquake swarm activity. The dots indicate the hypocenters of earthquakes listed in the JMA unified catalogue for the period of 800 days from the beginning of the swarm activity. The elapsed time after the 2011 Tohoku-Oki earthquake is indicated by the color scale. The broken line denotes the border between Yamagata and Fukushima prefectures. The black line denotes the rim of the Ohtoge caldera [Kanisawa et al., 2006]. (a) Map view. (b) E-W vertical cross-section view. (c) N-S vertical cross-section view.
Source publication
Temporal variations of the fault frictional strength was investigated based on the diversity of focal mechanisms in the source area of the Yamagata-Fukushima border earthquake swarm, a significant earthquake swarm that occurred in central Tohoku, NE Japan, which started just after the 2011 M9.0 Tohoku-Oki earthquake. The focal mechanisms of events...
Contexts in source publication
Context 1
... remarkable earthquake swarm activity that occurred near the border between Yamagata and Fukushima Prefectures in central Tohoku (Figure 2) is one example of such seismicity, which is characterized by reversefault focal mechanisms with a P axis oriented EW or WNW-ESE, despite the reduction of the EW or WNW-ESE compressional stress by the static stress change of the 2011 Tohoku-Oki event. The seismic activity of this event was very substantial (Figures 2 and 3). ...
Context 2
... remarkable earthquake swarm activity that occurred near the border between Yamagata and Fukushima Prefectures in central Tohoku (Figure 2) is one example of such seismicity, which is characterized by reversefault focal mechanisms with a P axis oriented EW or WNW-ESE, despite the reduction of the EW or WNW-ESE compressional stress by the static stress change of the 2011 Tohoku-Oki event. The seismic activity of this event was very substantial (Figures 2 and 3). Terakawa et al. [2013] suggested that this swarm occurred in response to the increase in pore pressure due to the upwelling fluids facilitated by the EW extension associated with the Tohoku-Oki earthquake. ...
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Citations
... Studies have revealed that fluid movement seems to play a significant role in natural earthquake swarms that occur in the overriding plates of subduction zones (Bianco et al., 2004;Iio et al., 2002;Okada et al., 2015;Yoshida et al., 2016;Yukutake et al., 2011). Recent research (Kato, 2024;Nakajima, 2022;Nishimura et al., 2023;Yoshida et al., 2023) has also suggested that the upward fluid flow in the crust could drive large earthquakes and long-lasting earthquake swarms in the northeastern tip of the Noto Peninsula. ...
Plain Language Summary
A Mw7.45 earthquake hit the northeastern tip of the Noto Peninsula on 1 January 2024. This region has hosted several long earthquake swarms. This unusual earthquake provides a good opportunity to explore the fault behaviors following a major earthquake. The coseismic and early postseismic deformation have been well recorded at the GNSS stations with unprecedentedly high spatial and temporal resolutions. We inverted the coseismic and early postseismic slip distributions from the GNSS data. The coseismic slip distributes mostly in two patches and reaches up to approximately 4 m. The 19‐day postseismic slip distributes mainly in the coseismic slip region as well as to the north of it. The postseismic slip in the rupture area overlapped with the aftershocks. The result suggests that these aftershocks were likely triggered by the afterslip. Additionally, the pattern of poroelastic rebound implies that fluid flow may play a role in triggering these aftershocks. This study helps advance our understanding of earthquake‐triggering mechanisms and fault behaviors following large earthquakes.
... However, the mechanical properties of rock materials are closely related to lithology, confining pressures (McBeck et al., 2023;Wang et al., 2020), brittleness (Altındağ & Güney, 2010;Hucka & Das, 1974;Mikaeil et al., 2011;Wang et al., 2014), fluid saturation (Verwer et al., 2010;Yoshida et al., 2016), and pore conditions (e.g., porosity, permeability; Fossen, 2016;Gu et al., 2016;Kaproth et al., 2016;X. Li et al., 2017). ...
Plain Language Summary
Shear strength of a fault damage zone inform us its ability to withstand shear stress before surface rupture occurs. This information often provides insights into the earthquakes rupture hazards near the Earth's surface. Natural earthquakes provide a perfect opportunity for us to address the question of “what is the shear strength of a fault damage zone.” To this end, we require a set of comparative references: the utmost shear stress that a damage zone can withstand without surface rupture and the minimal one necessary for the zone to generate surface rupture. In this investigation, we studied the 2021 Maduo earthquake because it had multiple distinct surface rupture segments in some places but not in others. By comparing shear stress in these segments, we could figure out how strong the damage zones were. We used real observations and numerical simulations to estimate how much stress these damage zones can withstand. Our results indicate that, for damage zones embedded in intact sandstone with a Young's modulus of 15 GPa, its strength ranges from 7 to 17 MPa. Significantly, our study is the first to reveal the shear strength of damage zone from a nutural earthquake using geodetic data.
... A similar subsidence at active volcanoes, associated with mega-thrust earthquakes, was also reported in Chile (Pritchard et al. 2013). There have been reports on the activation of seismic swarms in several regions of northeastern Japan due to spatiotemporal variation of pore fluid pressure triggered by the 2011 earthquake (Okada et al. 2011;Yoshida et al. 2016). However, no notable anomalous seismic activity around Azuma volcano from background level was reported. ...
Inflations at active volcanoes are indicators of overpressure in the subsurface, which is known to be a phenomenon that precedes eruptions. Volcanic overpressure is induced by the injection of magmatic fluids, accumulated magma, or heat supply from greater depths. Azuma volcano (Japan) has experienced several episodes of volcanic unrest with increases in seismicity at the depth of the hydrothermal system, implying a potential increase in phreatic eruptions. The time series of interferometric synthetic aperture radar data, associated with the unrest episodes occurring in 2014–2015 and 2018–2019, revealed spatiotemporal variations of inflation episodes, centered around Oana crater, the most active fumarole of Azuma volcano. The modeled best-fit geometry of the elongated pressure source for the local deformation has the same dip as the overlying topographic slope direction and angle around Oana crater, suggesting the existence of topography-correlated layered structures within the hydrothermal system. In contrast, the broader deformation associated with the 2014–2015 unrest was explained by the overpressure of a horizontal flat source at 360–1500 m below sea level, showing the similar depth of the top as the conductive low-resistivity or low-viscosity body suggested by previous studies. The unrest episodes were thus interpreted as resulting mainly from the supply of magmatic fluids, or the transfer of heat from greater depths. Our study helps in understanding the shallow structure of this volcanic system and contributes to evaluating the potential for forthcoming eruptions in Azuma volcano.
Graphical Abstract
... Recently natural earthquake swarm studies discovered that fluid migration from a deeper part of the crust played a very important role in swarm activity (e.g., [56][57][58][59]. The phenomena of natural earthquake swarms and injection-induced seismicity have a lot of similarities, such as migration of hypocenters and interaction with pore pressure. ...
... A number of examples of crustal seismic swarms related to upwelling fluids have been observed in non-volcanic areas, including Matsushiro (Japan) 38 and L'aquila (Italy) 39 . Several earthquake swarms after the 2011 M w 9.0 Tohoku-oki earthquake in northeastern Japan were likely triggered by an increase in pore fluid pressure 40,41 . However, transient deformation associated with seismic swarms is not usually detected geodetically except in the vicinity of volcanoes and major faults. ...
Since November 30, 2020, an intense seismic swarm and transient deformation have been continuously observed in the Noto Peninsula, central Japan, which is a non-volcanic/geothermal area far from major plate boundaries. We modeled transient deformation based on a combined analysis of multiple Global Navigation Satellite System (GNSS) observation networks, including one operated by a private sector company (SoftBank Corp.), relocated earthquake hypocenters, and tectonic settings. Our analysis showed a total displacement pattern over 2 years shows horizontal inflation and uplift of up to ~ 70 mm around the source of the earthquake swarm. In the first 3 months, the opening of the shallow-dipping tensile crack had an estimated volumetric increase of ~ 1.4 × 10⁷ m³ at a depth of ~ 16 km. Over the next 15 months, the observed deformation was well reproduced by shear-tensile sources, which represent an aseismic reverse-type slip and the opening of a southeast-dipping fault zone at a depth of 14–16 km. We suggest that the upwelling fluid spread at a depth of ~ 16 km through an existing shallow-dipping permeable fault zone and then diffused into the fault zone, triggering a long-lasting sub-meter aseismic slip below the seismogenic depth. The aseismic slip further triggered intense earthquake swarms at the updip.
... The presently obtained features are remarkably similar to those observed in earthquake swarms that became active near volcanoes or paleo-caldera in northeastern Japan after the 2011 M9 Tohoku earthquake (Yoshida, Hasegawa, et al., 2019). The Tohoku swarms were inferred to have been caused by fluid movement associated with the crustal deformation of the Tohoku earthquake (Terakawa et al., 2013;Yoshida et al., 2016). Indeed, these earthquake swarms share a marked tendency for earthquake locations to move from deep to shallow via multiple planes, which can be well explained by fluid migration (Kosuga, ...
This study describes an ongoing intense earthquake swarm in the crust of the northeastern Noto Peninsula, Japan, that began around the end of 2019. Fluid movement related to volcanic activity is often involved in earthquake swarms in the crust. However, no volcanic activity has occurred in this region since the Middle Miocene (15.6 Ma). This study investigates the cause of this earthquake swarm based on the spatiotemporal evolution of earthquake hypocenters and seismic reflectors. The hypocenter relocation of 10,940 earthquakes (M > 1) reveals that they are all crustal and migrated upward, activating a complex network of faults at depths shallower than 20 km. The initiation of this earthquake swarm occurred at a locally deep depth (z = 17 km), and the local hypocenter distribution shows a characteristic circular pattern. We find a distinctive S‐wave reflector in the immediate vicinity. A low‐velocity anomaly exists below the reflector, and a high helium isotope ratio and a low‐gravity anomaly were observed at the surface. These observations suggest that the current seismicity is associated with fluids released by ancient or possibly unrecognized modern magmatic activity, although no volcanic activity has been documented in this area for 15 million years. Significant crustal deformation was observed during this swarm and is probably related to the fluid movement and aseismic deformation that contributed to this earthquake swarm. The strongest M5.4 earthquake (as of October 2022) occurred near the migration front on the largest planar structure, leaving the shallow extension unruptured.
... A similar subsidence, associated with mega-thrust earthquakes, was reported in Chile (Pritchard et al. 2013). There have been reports on the activation of seismic swarms in several regions of northeastern Japan due to spatiotemporal variation of pore uid pressure triggered by the 2011 earthquake (Okada et al. 2011;Yoshida et al. 2016). However, no notable anomaly from background level was reported, except for the above described subsidence. ...
Inflations at active volcanoes are indicators of overpressure in the subsurface, which is known to be a phenomenon that precedes eruptions. Volcanic overpressure is induced by the injection of magmatic fluids, accumulated magma, or heat supply from greater depths. Azuma volcano (Japan) has experienced several episodes of volcanic unrest with increase in seismicity at the depth of the hydrothermal system, implying a potential increase of phreatic eruptions. The time-series of interferometric synthetic aperture radar data, associated with the unrest episodes occurring in 2014–2015 and 2018–2019, revealed spatiotemporal variations of inflation episodes, centered around Oana crater, the most active fumarole of Azuma volcano. The modeled best-fit geometry of the elongated pressure source for the local deformation has the same dip as the overlying corresponded with the topographic slope direction and angle around Oana crater, suggesting the existence of topography-correlated layered structures within the hydrothermal system. In contrast, the broader deformation associated with the 2014–2015 unrest was explained by the overpressure of a horizontal flat source at 500 m below sea level, showing similar depth of the top as the conductive low-resistivity or low-viscous body suggested by previous studies. The unrest episodes were thus interpreted as mainly resulting from the supply of magmatic fluids or the volumetric thermal expansion of the hydrothermal system, caused by heat transfer from greater depths. Our study aids in understanding the shallow structure of this volcanic system and contributes towards evaluating the potential for forthcoming eruptions in Azuma volcano.
... Interval between last great EQ (~1000 years) differential stress is approximately 20 MPa in this region 62 . We computed the critical pore pressure for a well-oriented fault to be 170.5 MPa, which is 100.5 MPa above the hydrostatic pressure at a depth of 7 km. ...
Slab-derived fluids control crustal dynamics in the subduction zone. However, the slab-derived fluid budget has never been quantified beyond a geophysical and geological spatiotemporal resolution. Here, we target an intense earthquake swarm associated with the M9 Tohoku earthquake, which represented the critical dynamic behavior of slab-derived fluid. The fluid volume involved has been quantified, with a plausible range of 10 ⁶ −10 ⁸ m ³ , by utilizing injection-induced seismicity insights. Comparisons with geological proxies suggest that the estimated fluid volume can be accumulated via supply from the lower crust within 10 ² –10 ⁴ y. Our study demonstrated such amount of aqueous fluid stored at the midcrustal level, which triggered consecutive swarm activity for ~2 y with the Tohoku earthquake, suggesting a possible link between earthquake swarms to M9 class earthquakes (10 ³ y cycle) and mineral veins and deposits. This study has shed light on the quantitative understanding of the dynamic slab-derived fluid budget.
... Earthquakes also frequently occur near paleo-volcanoes and paleo-calderas where volcanic activity has ceased in the past (more than a million years ago). For example, in Japan, intense earthquake swarms recently occurred near paleo-calderas at the Yamagata-Fukushima border area (Okada et al., 2015;Yoshida et al., 2016), Sendai-Okura area (Yoshida & Hasegawa, 2018b), and Kagoshima Bay area (Matsumoto et al., 2021). These earthquakes show similar characteristics to earthquake swarms in active volcanic regions, including hypocenter migration from deep to shallow areas and the presence of low-velocity regions and seismic reflectors beneath the source region (Kosuga, 2014;Okada et al., 2015;Yoshida et al., 2016;Yoshida & Hasegawa, 2018a and b;Matsumoto et al. 2021). ...
... For example, in Japan, intense earthquake swarms recently occurred near paleo-calderas at the Yamagata-Fukushima border area (Okada et al., 2015;Yoshida et al., 2016), Sendai-Okura area (Yoshida & Hasegawa, 2018b), and Kagoshima Bay area (Matsumoto et al., 2021). These earthquakes show similar characteristics to earthquake swarms in active volcanic regions, including hypocenter migration from deep to shallow areas and the presence of low-velocity regions and seismic reflectors beneath the source region (Kosuga, 2014;Okada et al., 2015;Yoshida et al., 2016;Yoshida & Hasegawa, 2018a and b;Matsumoto et al. 2021). These similarities suggest that the occurrence of earthquake swarms around paleo-calderas is also related to fluid behavior. ...
... The obtained features were remarkably similar to those observed in earthquake swarms that became active near volcanoes or paleo-caldera in northeastern Japan after the 2011 M9 Tohoku earthquake. Indeed, these earthquake swarms share a marked tendency for earthquake locations to move from deep to shallow using multiple planes, which can be explained by fluid migration (Kosuga, 2014;Yoshida & Hasegawa, 2018a and b;Yoshida et al., 2019) and were inferred to have been caused by fluid movement associated with the crustal deformation of the Tohoku earthquake, despite the decreased shear stress caused by the Tohoku earthquake (Terakawa et al., 2013;Yoshida et al., 2016). The similarity of the migration characteristics of the earthquake swarm in the northeastern Noto Peninsula and the above earthquake swarms suggests that the present activity was also influenced by upward fluid movement. ...
Key Points (<140 characters) 1. An intense seismic swarm occurs in a non-volcanic region with hypocenters migrating from deep to shallow depths via many planes. 2. A deep S-wave reflector, suggesting a fluid source, is located near the seismicity initiation point and above the low-velocity anomaly. 3. A ring-fault near the reflector represents part of a hidden, ancient, or new magmatic system that caused the swarm by supplying fluids. Abstract (243 words < 250) An intense earthquake swarm is occurring in the crust of the northeastern Noto Peninsula, Japan. Fluid movement related to volcanic activity is often involved in earthquake swarms in the crust, but the last volcanic activity in this area occurred in the middle Miocene (15.6 Ma), and no volcanic activity has occurred since then. In this study, we investigated the cause of this earthquake swarm using spatiotemporal variation of earthquake hypocenters and seismic reflectors. Hypocenter relocation revealed that earthquakes moved from deep to shallow areas via many planes, similar to earthquake swarms in volcanic regions. The strongest M5.4 earthquake initiated near the migration front of the hypocenters. Moreover, it ruptured the seismic gap between the two different clusters. The initiation of this earthquake swarm occurred at a locally deep depth (z =~17 km), and we found a distinctive S-wave reflector, suggesting a fluid source in the immediate vicinity. The local hypocenter distribution revealed a characteristic ring-like structure similar to the ring dike that forms just above the magma reservoir and is associated with caldera collapse and/or magma intrusion. These observations suggest that the current seismic activity was impacted by fluids related to ancient or present hidden magmatic activity, although no volcanic activity was reported. Significant crustal deformation was observed during this 1 ESSOAr | https://doi.org/10.1002/essoar.10512876.1 | CC_BY_4.0 | First posted online: Sat, 19 Nov 2022 03:55:26 | This content has not been peer reviewed. earthquake swarm, which may also be related to fluid movement and contribute to earthquake occurrences. A seismic gap zone in the center of the swarm region may represent an area with aseismic deformation. Plain language summary (199 words < 200) An intense earthquake swarm is currently occurring in the crust of the northeastern Noto Peninsula, Japan. Fluid movement related to volcanic activity is often involved in earthquake swarms in the crust, but no volcanic activity has occurred in this region since the middle Miocene (15.6 Ma). We here investigated the cause of this earthquake swarm using the precisely-determined earthquake locations and seismic reflectors. We found that the earthquakes moved from deep to shallow areas via many planes, similar to seismicity induced by fluid-injection. The strongest M5.4 earthquake initiated near the upward migration front on the largest planar structure. Further earthquakes may be possible in the shallow part of this fault. We found a distinctive S-wave reflector, suggesting a fluid source, in the immediate vicinity of the initiation point of this swarm. The local hypocenters show a characteristic ring-like structure similar to the ring dike that forms above the magma reservoir. These observations suggest that the current seismic activity is being impacted by fluids related to ancient or new hidden magmatic activity. The present results suggest that hidden magma-induced structures and fluids can generate earthquakes even in areas where no volcanic activity has been observed for over 10 million years.
... Indeed, several studies estimated subsurface structures, suggesting the presence of fluid under the hypocenters of earthquake swarms as a low-seismic-velocity zone (Okada et al., 2015;Yukutake et al., 2015) or S-wave reflector/scatter zone (Kosuga, 2014;Umino et al., 2002). Moreover, the focal mechanisms of earthquake swarms exhibit a misoriented direction of the stress field (Terakawa et al., 2013;Yoshida et al., 2016). A high b-value and low static stress drop were estimated during the initial seismic sequence of an earthquake swarm (Yoshida et al., 2017). ...
Recent seismic and geodetic observations suggest the involvement of pressurized fluid and aseismic slip during earthquake swarms. However, the interaction between these two factors is not fully understood. In this study, we show geodetic signals induced by aseismic opening and shear dislocation on a fault, accompanied by hypocenter migration during an earthquake swarm in the Hakone volcano, central Japan. The hypocenters were concentrated on a vertical plane, and the focal mechanisms were strike‐slip faults whose nodal planes were consistent with the hypocenter distribution. Precursory seismicity occurred 1 week before the main swarm. We observed a complex pattern of hypocenter migration during the seismic sequence; namely, the lower limit of the seismicity appears to expand downward during the precursory seismicity, followed by an upward hypocenter diffusion. We also observed tilt changes that could be explained by opening and shear dislocation on the fault plane inferred from the planar hypocenter distribution. Furthermore, we identified several groups of repeating earthquakes. The average aseismic slip history inferred from the repeating earthquakes indicated a minor aseismic slip of 0.4 mm during the precursory seismic activity, which accelerated by up to 3 mm during the main swarm. The total cumulative slip was consistent with the geodetic model results. These observations may suggest that fluid intrusion caused the aseismic slip, and the complex migration of the hypocenters reflects the aseismic slip propagation. We hypothesize that aseismic deformation acts as the driving force for the earthquake swarms together with the intrusion of pressurized fluid.
