sP Depth Phase Research Articles

The Adriatic Thrust Fault of the 2021 Seismic Sequence Estimated from Accurate Earthquake Locations Using sP Depth Phases

ABSTRACT An earthquake sequence occurred in the Central Adriatic region during March–June 2021. This sequence started on 27 March with a mainshock of moment magnitude (Mw) 5.2 occurring at 13:47 coordinated universal time (UTC). No foreshock was observed before this mainshock. The sequence lasted approximately three months, until the end of June 2021. Approximately 200 seismic events were recorded by the regional seismic network during this time, including four M ≥ 4.0 earthquakes. The 27 March 2021 earthquake was one of the strongest instrumentally recorded events in the area bounded approximately by the Ancona–Zadar line to the north and the Gargano–Dubrovnik line to the south. The mainshock originated at a focal depth of 9.9 km. The seismicity spread from the mainshock up-dip and down-dip along a northeast-dipping plane. Here, we investigate the geometry of the fault activated by this seismic sequence by using sP depth phases. We aim to significantly reduce the large uncertainties associated with the hypocentral locations of offshore earthquakes beneath the Adriatic Sea—an area that plays a fundamental role in the geodynamics of the Mediterranean. These refined earthquake locations also allow us to make inferences with regards to the seismotectonic context responsible for the analyzed seismicity, thus identifying a structure (here referred to as the Mid-Adriatic fault) consisting of a northwest–southeast-striking thrust fault with a ∼35° northeast-dipping plane. The use of depth-phase arrival times to constrain off-network event locations is of particular interest in Italy due to both the peculiar shape of the peninsula and the extreme scarcity of seafloor stations, the cost and management of which are very expensive and complex. Here, we present the first attempt to apply this off-network locating technique to the Italian offshore seismicity research with the aim of improving hazard estimations in these hard-to-monitor regions.

Earthquake Depths, Focal Mechanisms, and Stress in the Lower St. Lawrence Seismic Zone

AbstractWe examine earthquake hypocenters, focal mechanisms, and the state of tectonic stress in the Lower St. Lawrence Seismic Zone (LSZ), a paleorift zone in eastern Canada. The largest earthquake recorded in the region is the 1999 Côte–Nord MN 5.1, which was followed by ∼80 aftershocks of MN>1. It is not known if the region is capable of producing hazardous Mw>6 earthquakes, similar to the Charlevoix Seismic Zone ∼250 km upriver. Focusing on 2015–2020, we apply a machine-learning-based phase picker to detect 72 earthquakes in addition to the 150 catalog earthquakes in the same region over this time span. We produce an updated 1D, gradient velocity model via a Monte Carlo search using a uniform VP/VS=1.77, which we computed with the Wadati method. We refine hypocenter estimates using the triple-difference method, with sP depth phases as additional constraints on earthquake depth. We estimate focal mechanisms for >100 earthquakes with automatically picked P-wave first motions and absolute value P-SV-SH amplitude ratios, and we use the focal mechanisms to invert for the state of tectonic stress. Grid searches and Bayesian analysis allow for robust uncertainty estimates of focal mechanisms, which in turn allow for uncertainty estimates of the stress tensor. The recovered west-northwest–east-southeast σ1 is consistent with previous estimates and with a stress tensor controlled by glacial isostatic adjustment, although a contrast between deep and shallow focal mechanisms suggests that these stresses may be concentrated in the lower crust. Epicenter lineations up to ∼40 km long may be indicative of sizable faults in the LSZ capable of generating Mw>6 earthquakes, but hypocenter and focal mechanism uncertainties are too high to say so definitively, thus pointing to a need for denser station coverage, including ocean-bottom seismometers.

Big mantle wedge, anisotropy, slabs and earthquakes beneath the Japan Sea

The Japan Sea is a part of the western Pacific trench-arc-backarc system and has a complex bathymetry and intense seismic activities in the crust and upper mantle. Local seismic tomography revealed strong lateral heterogeneities in the crust and uppermost mantle beneath the eastern margin of the Japan Sea, which was determined using P and S wave arrival times of suboceanic earthquakes relocated precisely with sP depth phases. Ambient-noise tomography revealed a thin crust and a thin lithosphere beneath the Japan Sea and significant low-velocity (low-V) anomalies in the shallow mantle beneath the western and eastern margins of the Japan Sea. Observations with ocean-bottom seismometers and electromagnetometers revealed low-V and high-conductivity anomalies at depths of 200–300km in the big mantle wedge (BMW) above the subducting Pacific slab, and the anomalies are connected with the low-V zone in the normal mantle wedge beneath NE Japan, suggesting that both shallow and deep slab dehydrations occur and contribute to the arc and back-arc magmatism. The Pacific slab has a simple geometry beneath the Japan Sea, and earthquakes occur actively in the slab down to a depth of ∼600km beneath the NE Asian margin. Teleseismic P and S wave tomography has revealed that the Philippine Sea plate has subducted aseismically down to the mantle transition zone (MTZ, 410–660km) depths beneath the southern Japan Sea and the Tsushima Strait, and a slab window is revealed within the aseismic Philippine Sea slab. Seismic anisotropy tomography revealed a NW-SE fast-velocity direction in the BMW, which reflects corner flows induced by the fast deep subduction of the Pacific slab. Large deep earthquakes (M>7.0; depth>500km) occur frequently beneath the Japan Sea western margin, which may be related to the formation of the Changbai and Ulleung intraplate volcanoes. A metastable olivine wedge is revealed within the cold core of the Pacific slab at the MTZ depth, which may be related to the deep seismicity. However, many of these results are still preliminary, due to the lack of seismic stations in the Japan Sea. The key to resolving these critical geoscientific issues is seismic instrumentation in the Japan Sea, for which international cooperation of geoscience communities in the East Asian countries is necessary.

P and S wave attenuation tomography of the Japan subduction zone

AbstractWe determine the first high‐resolution P and S wave attenuation (Q) tomography beneath the entire Japan Islands using a large number of high‐quality data collected from P and S wave velocity spectra of 4222 local shallow and intermediate‐depth earthquakes. The suboceanic earthquakes used in this study are relocated precisely using sP depth phases. Significant landward dipping high‐Q zones are revealed clearly, which reflect the subducting Pacific slab beneath Hokkaido and Tohoku, and the subducting Philippine Sea (PHS) slab beneath SW Japan. Prominent low‐Q zones are visible in the crust and mantle wedge beneath the active arc volcanoes in Hokkaido, Tohoku, and Kyushu, which reflect source zones of arc magmatism caused by fluids from the slab dehydration and corner flow in the mantle wedge. Our results also show that nonvolcanic low‐frequency earthquakes (LFEs) in SW Japan mainly occur in the transition zone between a narrow low‐Q belt and its adjacent high‐Q zones right above the flat segment of the PHS slab. This feature suggests that the nonvolcanic LFEs are caused by not only fluid‐affected slab interface but also specific conditions such as high pore pressure which is influenced by the overriding plate.

Seismic attenuation tomography of the Southwest Japan arc: new insight into subduction dynamics

We determined the first high-resolution P- and S-wave attenuation (Qp and Qs) tomography of the crust and upper mantle under the entire Nankai subduction zone from the Nankai Trough to the Japan Sea using a large number of high-quality t* data measured from P- and S-wave spectra of local earthquakes. The suboceanic earthquakes used in this study were relocated precisely using sP depth phases and ocean-bottom-seismometer data. The overall pattern of the obtained Q models is similar to that of velocity models of the study region. Our present results show that high-Q (i.e. weak attenuation) anomalies in the upper crust generally correspond to plutonic rocks widely exposed in the Nankai arc. Some of the low-Q (i.e. strong attenuation) anomalies in the upper crust along the Pacific coast are associated with the Cretaceous–Cenozoic accretionary wedge. Obvious low-Q anomalies exist in the crust under the active arc volcanoes. Most of the large inland crustal earthquakes are located in or around the low-Q zones in the crust. The subducting Philippine Sea slab is imaged clearly as a landward dipping high-Q zone. Prominent low-Q anomalies are revealed in the mantle wedge under the volcanic front and backarc area, which reflect the source zone of arc magmatism caused by slab dehydration and corner flow in the mantle wedge. Significant low-Q anomalies exist in the forearc mantle wedge, which reflects a highly hydrated and serpentinized forearc mantle wedge due to abundant fluids released from dehydration of the young and warm Philippine Sea slab.

Structural control on the nucleation of megathrust earthquakes in the Nankai subduction zone

AbstractTo clarify the causal mechanisms of megathrust earthquakes, we studied the detailed three‐dimensional P and S wave velocities (V), attenuation (Q), and Poisson's ratio (σ) structures of the Nankai subduction zone in southwest Japan, using a large number of high‐quality arrival time and t* data measured precisely from seismograms of local earthquakes. The suboceanic earthquakes used are relocated precisely using sP depth phase and ocean bottom seismometer data. Our results show the existence of two prominent high‐V, high‐Q, and low‐σ patches separated by low‐V, low‐Q, and high‐σ anomalies in the Nankai megathrust zone. Megathrust earthquakes during 1900 to 2013 nucleated in or around the high‐V, high‐Q, and low‐σ patches, which may represent strongly coupled areas (i.e., asperities) in the megathrust zone. Our results indicate that structural heterogeneities in the megathrust zone, such as the subducting seafloor topography and compositional variations, control the nucleation of the Nankai megathrust earthquakes.

Seismic attenuation tomography of the Northeast Japan arc: Insight into the 2011 Tohoku earthquake (Mw9.0) and subduction dynamics

Detailed three-dimensional (3-D) P and S wave attenuation (Qp and Qs) models of the crust and upper mantle under the entire Northeast Japan (Tohoku) arc from the Japan Trench to the Japan Sea coast are determined, for the first time, using a large number of high-quality t* data measured precisely from P and S wave spectra of local earthquakes. The suboceanic earthquakes used in this work are relocated precisely using sP depth phases. Our results reveal a prominent landward dipping high-Q zone representing the subducting Pacific slab, a landward dipping intermediate- to high-Q zone in the mantle wedge between the Pacific coast and the volcanic front, and significant low-Q anomalies in the crust and mantle wedge between the volcanic front and the Japan Sea coast. Prominent high-Q patches surrounded by low-Q anomalies are revealed in the interplate megathrust zone under the Tohoku fore arc where the great 2011 Tohoku-oki earthquake (Mw 9.0) occurred. The high-Q patches in the megathrust zone generally exhibit large coseismic slips of megathrust earthquakes and large slip deficit on the plate interface. We think that these high-Q patches represent asperities in the megathrust zone, whereas the low-Q anomalies reflect weakly coupled areas. We also find that the hypocenters of the 2011 Tohoku-oki interplate earthquakes (Mw > 7.0) are located in areas where Qp, Qs, and Qp/Qs change abruptly. These results suggest that structural heterogeneities in the megathrust zone control the interplate seismic coupling and the nucleation of megathrust earthquakes.

Seismic imaging of the Southwest Japan arc from the Nankai trough to the Japan Sea

Detailed three-dimensional P- and S-wave velocity (Vp and Vs) models of the entire Southwest Japan arc from the Nankai trough to the Japan Sea are determined for the first time using a large number of high-quality arrival-time data from local earthquakes. The suboceanic earthquakes used in the tomographic inversion were relocated precisely using sP depth phase data. Our results show that strong lateral heterogeneities exist in the interplate megathrust zone under the Nankai forearc. Large interplate earthquakes mainly occurred in or around high-velocity (high-V) patches in the megathrust zone. These high-V patches may represent asperities formed by the subducted oceanic ridges and seamounts. Low-velocity (low-V) zones in the megathrust zone may contain sediments and fluids associated with slab dehydration and so become weakly coupled areas. Our results also show that the coseismic slip distributions of some megathrust earthquakes are not limited in the high-V patches (asperities) where the ruptures initiated. Because of the weak interplate coupling in the low-V areas, the rupture of an interplate earthquake could unimpededly pass through the low-V anomalies and so result in a great megathrust earthquake.

Geophysical properties and seismotectonics of the Tohoku forearc region

Abstract We investigated the detailed three-dimensional (3-D) isotropic and anisotropic structures of the crust and upper mantle under the NE Japan forearc region using a large number of P and S wave arrival-time data from onshore and offshore earthquakes. The suboceanic earthquakes used in this study are well relocated using the sP depth phases. We also determined the 3-D distribution of Poisson’s ratio, crack density and saturation rate using the 3-D P and S wave velocity model obtained by this study. The relatively complex anisotropic structures in the megathrust zone may reflect the complex geological structures, lithological variations and fluids in the accretional prism under the forearc region. The tomographic images reflect strong lateral heterogeneities in the megathrust zone under the Tohoku forearc. Areas with low velocity, high Poisson’s ratio, high crack density and high saturation rate may be due to entrapment of fluid-filled, unsolidated sediments on the plate interface close to the Japan Trench. Most of the large megathrust earthquakes since 1900 (M ⩾ 6.0) and the large 2011 Tohoku-oki earthquakes (M 6.0–9.0) are located in areas with high velocity, high Poisson’s ratio, low crack density and high saturation rate, which may represent strongly-coupled asperities in the megathrust zone resulting from the subducted oceanic ridges and/or seamounts. In contrast, the areas with high Poisson’s ratio may indicate that the fluids have infiltrated into the strongly coupled patches. We think that the great Tohoku-oki earthquakes were caused by not only the stress concentration but also the in situ structural heterogeneities in the megathrust zone.

日本海溝周辺における太平洋プレート内の地震発生機構

A double-planed shallow seismic zone has been found in the northeastern Japan forearc region. However, the characterizations of the focal mechanisms of earthquakes in this zone, especially the lower plane events, cannot be carried out adequately due to low seismicity and poor station coverage on the focal sphere of the onshore P-wave polarity data. In this study, we determine the focal depth using the sP depth phase and the focal mechanisms using the P-wave initial motions observed by the ocean bottom cabled seismic stations and temporary autonomous ocean bottom seismic networks off Miyagi, as well as the onshore seismic networks. Seven focal mechanism solutions of events were precisely determined. Using the classification based on the dip angles of the T, B and P axes, we classified 21 weakly constrained focal mechanisms of other events having poor station coverage. All the determined solutions were of the thrust-faulting type regardless of the focal depth. The offshore observations helped considerably in constraining the focal mechanisms of these far-offshore earthquakes, especially their rake angles. Although previous studies had reported that the focal mechanisms of the upper and lower plane events show predominantly normal and reverse faulting respectively, our result showed that the thrust events in the upper plane seem to occur on the plate boundary. We found that the focal mechanisms change from normal faulting in the upper plane to reverse faulting in the lower plane at a depth of 15km from the plate boundary, possibly defining the depth of the neutral plane in the northeastern Japan forearc region included in the source area of the 1933 Mw8.4 Sanriku earthquake. The western edge of the normal faulting events along the upper plane is located about 70km inward from the trench axis. These results suggest that the earthquake-generating stress field in the double-planed shallow and deep seismic zone in the northeastern Japan arc can be explained by the bending-unbending model of the subducting Pacific plate.

Aftershock distribution of the February 27, 2010 Okinawa-honto Kinkai earthquake (Japan) using sP depth phase

A major earthquake (moment magnitude; Mw = 7.0) occurred on February 27, 2010 in the central Ryukyu Trench, southwestern Japan. A slab-parallel extension-type mainshock and its aftershocks occurred on a strike-slip fault, whose extension axis was parallel to the dip of the Philippine Sea plate. The epicenters of the mainshock and the aftershocks were located in the cluster of slab-parallel compression-type earthquakes, whose compression axes were parallel to the dip of the Philippine Sea plate. To clarify the relationship between the slab-parallel extension-type mainshock and aftershocks and the slab-parallel compression-type earthquakes, I determined the depths of the hypocenters of 114 aftershocks (M ≥ 3.0) of the mainshock and 29 background earthquakes (M ≥ 3.5) from 2006 to 2009 near the central Ryukyu Trench using the arrival times of the depth phase. Then, I determined the distribution of the aftershocks using the double-difference method. The relocated aftershock distribution extends along the ENE–WSW direction. This suggests that the mainshock fault lies on a left-lateral strike-slip fault extending along ENE–WSW direction. On the basis of the relocated hypocenters, it is found that reverse-fault-type (slab-parallel compression-type) earthquakes occur in the oceanic crust of the subducted Philippine Sea plate, and strike-slip fault-type (slab-parallel extension-type) earthquakes occur in the mantle of the subducted Philippine Sea plate. This suggests that a double seismic structure exists locally in the shallow part of the subducted Philippine Sea plate of the central Ryukyu Trench.

Seismic imaging of the Amur–Okhotsk plate boundary zone in the Japan Sea

We determined high-resolution images of P and S wave velocity and Poisson’s ratio under the Japan Sea off Honshu Island using a large number of arrival-time data from local earthquakes to understand the interaction of the Amur and Okhotsk plates. Our new data collected from the crustal earthquakes under the Japan Sea relocated precisely with sP depth phase are crucial to make this work possible. Our results show that strong lateral heterogeneities exist in the crust and upper mantle under the Amur–Okhotsk plate boundary zone, which may have affected the seismotectonics in the region. The crustal velocity variations under the Japan Sea may reflect the complicated geologic structures that were produced during the opening of the Japan Sea and the present compressional stage of the Honshu arc associated with the collision of the Amur plate with the Okhotsk plate. Low-velocity zones in the mantle wedge are found to extend westward under the Japan Sea, rather than just confined under Honshu Island, suggesting that the back-arc magmatism and tectonics are part of the complex geodynamic system in East Asia.

Crustal heterogeneity in the 2007 Noto-Hanto earthquake area and its geodynamical implications

To understand the genesis of the 25 March 2007 Noto-Hanto earthquake (Mw 6.6), we determined high-resolution 3-D seismic velocity (Vp and Vs) and Poisson's ratio (σ) images in the epicenter area using 24,698 P-wave and 22,927 S-wave arrival times from 809 earthquakes and 265 sP depth-phase data collected from seismograms of 162 earthquakes beneath the Japan Sea. The sub-oceanic events are relocated accurately by using P-, S- and sP depth-phase arrival time data jointly. Our results demonstrate that the seismic velocity (Vp and Vs) and Poisson's ratio (σ) vary markedly in the source area. In and around the main shock hypocenter, low-velocity and high-σ anomalies are revealed, which are apt for accumulation of differential strain to bring the brittle failure. We infer that the zone with pronounced low-velocity and high Poisson's ratio beneath the source area reflects fluid-related anomalies derived either from the dehydration of the subducting Pacific slab or from permeation of sea-water through deep-seated active faults or both to the main shock hypocenter, which in turn may have facilitated the rupture initiation of the main shock and its aftershock sequence by weakening the overlying seismogenic layer beneath the region. Our study reinforces the concept of fluid-driven earthquakes in the peninsular regions, elsewhere in the world.

Seismic heterogeneity and anisotropy of the Honshu arc from the Japan Trench to the Japan Sea

We determined P- and S-wave tomography and P-wave anisotropic structure under the Honshu arc from the Japan Trench to the backarc area under the Japan Sea using 310 749 P- and 150 563 S-wave arrivals from 4655 local earthquakes recorded by 982 seismograph stations. Arrival times from 1451 suboceanic earthquakes relocated with sP depth phases enable us to determine the structures under the Pacific Ocean and Japan Sea, which expand the study region from the land area to the whole arc from the Japan Trench to the Japan Sea with a width of more than 500 km. The results show strong heterogeneities above the subducting Pacific slab under the Pacific Ocean and most large thrust-type earthquakes occurred in the high-velocity areas where the Pacific slab and the overriding continental plate may be strongly coupled. Low-velocity (low-V) zones are imaged in the mantle wedge with significant along-arc variations under the volcanic front. The mantle-wedge low-V zone extends westwards under the Japan Sea and it is connected with the subducting Pacific slab at depths of 150–200 km under the backarc. The results indicate that the H2O and fluids brought downwards by the subducting Pacific slab are released into the mantle wedge by dehydration and are subsequently transported to the surface by the upwelling flow in the mantle wedge. Significant P-wave anisotropic anomalies are revealed under the Honshu arc. The predominant fast velocity direction (FVD) is E–W in the mantle wedge while it is N–S in the subducting Pacific slab. The anisotropy in the mantle wedge is the result of deformation caused by the subduction of the Pacific plate and the induced mantle-wedge convection, while the FVD pattern in the middle of the mantle wedge argues for the 3-D mantle flow or the specific alignment of the olivine in the partial-melting mantle. The N–S (trench-parallel) FVD in the subducting Pacific slab represents either the original fossil anisotropy when the Pacific plate formed or the trench-parallel crystallographic and shaped preferred orientation in the subducting slab due to the slab bending. The present results shed new light on the structural heterogeneities and seismic anisotropy under the Honshu arc, which may improve our understanding of the dynamic processes of subduction zones.

P-wave tomography, anisotropy and seismotectonics in the eastern margin of Japan Sea

To understand the seismotectonics in the eastern margin of the Japan Sea, we determined the first high-resolution 3-D P-wave velocity structure and azimuthal anisotropy under the Japan Sea off Northeast Japan using 175,425 high-precision P-wave arrival times from 2833 local earthquakes recorded by 330 seismograph stations. P-wave arrival times from 145 suboceanic earthquakes relocated with sP depth phase are crucial to determine the structure of the crust and uppermost mantle under the Japan Sea. Our results show that strong velocity variations exist in the crust and uppermost mantle under the eastern margin of the Japan Sea. Many large crustal earthquakes occurred in or around low-velocity zones which may represent weak sections of the seismogenic crust. The P-wave azimuthal anisotropy is complex under the Japan Sea, which may also indicate the complex crustal structures there. In the eastern margin of the Japan Sea, the strong heterogeneities in the crust and upper mantle revealed by seismic tomography may reflect the complicated geologic structures such as the alternate rift zones, ridges, basins, horsts, grabens, volcanics, and continental fragments which were produced during the back-arc spreading, opening of the Japan Sea and the present compressional stage of the Honshu arc.

Offshore double-planed shallow seismic zone in the NE Japan forearc region revealed by sP depth phases recorded by regional networks

SUMMARY We detected the sP depth phase at small epicentral distances of about 150 km or more in the seismograms of shallow earthquakes in the NE Japan forearc region. The focal depths of 1078 M > 3 earthquakes that occurred from 2000 to 2006 were precisely determined using the time delay of the sP phase from the initial P-wave arrival. The distribution of relocated hypocentres clearly shows the configuration of a double-planed shallow seismic zone beneath the Pacific Ocean. The upper plane has a low dip angle near the Japan Trench, increasing gradually to ∼30° at approximately 100 km landward of the Japan Trench. The lower plane is approximately parallel to the upper plane, and appears to be the near-trench counterpart of the lower plane of the double-planed deep seismic zone beneath the land area. The distance between the upper and lower planes is 28–32 km, which is approximately the same as or slightly smaller than that of the double-planed deep seismic zone beneath the land area. Focal mechanism solutions of the relocated earthquakes are determined from P-wave initial motion data. Although P-wave initial motion data for these offshore events are not ideally distributed on the focal sphere, we found that the upper-plane events that occur near the Japan Trench are characterized by normal faulting, whereas lower-plane events are characterized by thrust faulting. This focal mechanism distribution is the opposite to that of the double-planed deep seismic zone beneath the land area. The characteristics of these focal mechanisms for the shallow and deep doubled-planed seismic zones can be explained by a bending–unbending model of the subducting Pacific plate. Some of relocated earthquakes took place in the source area of the 1933 Mw8.4 Sanriku earthquake at depths of 10–23 km. The available focal mechanisms for these events are characterized by normal faulting. Given that the 1933 event was a large normal-fault event that occurred along a fault plane dipping landward, the earthquakes that currently occur just beneath or oceanwards of the Japan Trench are probably its aftershocks, suggesting that aftershock activity continues to the present day, 70 years after the main shock.

Mapping the mantle wedge and interplate thrust zone of the northeast Japan arc

We detected sP depth phases from three-component, short-period seismograms from the forearc earthquakes under the Pacific Ocean recorded by the dense seismic network on the Japan Islands. Then we use the sP depth-phase data together with P and S wave arrival times to locate the suboceanic earthquakes accurately. A large number of P and S wave arrival times from the relocated suboceanic events and earthquakes under the land area are used to determine the 3-D P and S wave velocity and Poisson's ratio structures under the entire NE Japan arc from the Japan Trench to the Japan Sea. Our results exhibit strong lateral heterogeneity under the forearc region and a clear correlation between the structural heterogeneity and distribution of large earthquakes (M 7.0–8.2) occurred in the interplate thrust zone. Prominent low-velocity and high Poisson's ratio zones are visible in the mantle wedge under the volcanic front and back-arc region, which show nicely the images of arc magma caused by the slab dehydration and corner flow in the mantle wedge.

Vp and Vs tomography of Kyushu, Japan: New insight into arc magmatism and forearc seismotectonics

High-resolution 3D tomographic images of the crust and upper mantle are determined under the entire-arc region of Kyushu by inverting a large number of arrival time data of P-wave, S-wave and sP depth phases. The hypocenters of the offshore earthquakes are accurately located by applying a combined method that makes use of sP depth phases and double-difference of the P-wave and S-wave arrival times. Our tomographic results confirmed the major features delineated by previous studies and revealed some new features of structural heterogeneity related to arc magmatism beneath the Kyushu arc. High-velocity anomalies correspond to the subducting Philippine Sea slab whilst low-velocity zones in the mantle wedge are clearly imaged, coinciding well with the velocity heterogeneities determined by previous studies. Strong low-velocity anomalies are distributed extensively along the volcanic front and extend to the back-arc side in the crust and upper mantle. Our present seismic images together with the previous results suggest that the fluids supplied by the dehydration processes of the descending Philippine Sea slab and the hot upwelling materials related to the opening of the Okinawa Trough might have contributed to the Unzen volcanism. Under the forearc region of the Kyushu subduction zone, prominent low-velocity anomalies are visible at depths of 10–40 km along the upper boundary of the subducting Philippine Sea slab and large interplate earthquakes are located around the low-velocity zones, which is similar to the features revealed in the Northeast Japan subduction zone. The spatial distribution of large thrust earthquakes below the offshore region reflects the existence of weak coupled or decoupled portions on the interface of the subducting Philippine Sea slab.

Seismic evidence for the influence of fluids on the 2005 west off Fukuoka prefecture earthquake in southwest Japan

High-resolution 3-D seismic velocity ( V p and V s) and Poisson's ratio ( σ) structures in the epicentral area of the 2005 west off Fukuoka prefecture earthquake were determined by inverting a large number of high-quality P- and S-wave arrival times and sP depth phase data to better understand the genesis of the main-shock and its aftershock sequence. The hypocenters of the earthquakes used in this study were relocated with a combined method which enables to accurately relocate the earthquakes occurred outside the seismic network by using sP depth phase data together with double-difference travel time data. This new approach of earthquake location can be applied to other offshore regions of subduction zones where few seismic stations exist while numerous suboceanic earthquakes occur. Strong crustal velocity heterogeneities were revealed in the hypocentral area, which might reflect the existence of fluids, geometrical irregularities of the fault zone, and geothermal anomalies in the region. The main-shock hypocenter was located near a distinct zone characterized by low-velocity, high-Poisson's ratio and high-conductivity anomalies, being considered to be associated with the existence of fluids in and around the source area. Such fluids might have reduced the seismic velocity of the crustal materials around the main-shock hypocenter and weakened the mechanic strength of the fractured rock matrix in the source area, and thus triggered the 2005 west off Fukuoka prefecture earthquake.

Focal depth distribution using sP depth phase and implications for plate coupling in the Hyuganada region, Japan

The recurrence interval and magnitude of great interplate earthquakes and characteristics of crustal uplift in the Late Pleistocene indicate clear lateral variations of the plate interaction on the convergent boundary along the Nankai Trough, i.e. relatively less cohesive for the Hyuganada region than for off the Shikoku and Kii Peninsula. The Hyuganada region is characterized by both significant aseismic crustal uplift in the Late Pleistocene and negative free-air gravity anomalies with the maximum magnitude of ∼−130 mgal. In order to examine the relationship for these observations, we investigated the focal depth distribution in the Hyuganada region by using sP depth phases. The focal depth distribution obtained by sP depth phases indicates different seismicity patterns in the southern and northern regions of the forearc. That is, most events for the southern and northern areas are thrust type and normal fault type, respectively. In the central part of the Hyuganada region located in the central area of negative gravity anomalies, however, each type event occurs in clusters and there is a seismicity gap between each cluster. The hypocenters for thrust type events generally coincide with the plate boundary as usual cases. However, most normal fault type events, with hypocenters in the landward side for the thrust type events and T-axes parallel to the plate boundary, occur in the crust or around the plate boundary of the forearc and the epicenters coincide with the peak position of negative gravity anomalies. The correlation between the seismicity and gravity anomaly suggests that the buoyancy inferred from negative gravity anomalies may cause the observed tensile events and a few relatively small asperities of the observed M w 7 class earthquakes on the convergent boundary. This may be related to relatively less cohesive coupling around the Hyuganada region compared with that for off the Shikoku.