Estimation of the 3-D velocity structure of the Sorachi–Yezo and Oshima Belts in the western part of Hokkaido, Japan (original) (raw)
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Three-dimensional velocity structure beneath the Kanto district, Japan
Journal of Physics of the Earth, 1982
Three-dimensional velocity structure under the Kanto district, Japan, is determined by inversion of the P-wave arrival time data from the earthquakes occurring under this district using the method originally due to AKI and LEE (1976). It is found that the crustal structure (0-32km depth) is dominated by a low velocity zone centered in Tokyo Bay, which coincides with the low Bouguer anomaly as well as the thick surficial layer. In the mantle just beneath the crust (32-65km depth), the velocity is high in northeastern and southwestern Kanto and is low in northwestern Kanto and the northeast part of Chiba prefecture. The velocity variation in this depth range is estimated to be 6-7% At depths of 32-65km, the northeast high velocity zone corresponds to the subduction of the Pacific plate. The southwest high velocity zone correlates with the seismic zone inclining from the Sagami trough toward the northeast. It has been suggested that this seismic zone corresponds to the subduction of the Philippine Sea plate from studies on hypocenter distribution, seismic intensity distribution and focal mechanism solutions. The high velocity characteristic found in the present study confirms this suggestion. The low velocity zone beneath the northeast part of Chiba prefecture roughly coincides with the low-Q zone found in the spectral study of shear waves. We suggest that this low-V, low-Q zone is caused by the downward bending of the Pacific plate.
Journal of Physics of the Earth, 1992
We determined the three-dimensional Vp and Vs structures beneath Japan by applying seismic tomography to a large number of arrival times recorded at temporary stations in the Japan Sea and the Pacific Ocean, as well as those at permanent stations on the Japan Islands. As a result, we obtained more precise seismic images than previous studies. In the crust and the uppermost mantle, southwestern Honshu exhibited weaker heterogeneity than the other areas in Japan, corresponding to the distribution of active volcanoes. Stripe-like heterogeneities exist in the subducting Pacific slab. Relatively low-velocity zones correspond to low-seismicity areas in the Pacific slab, suggesting that the slab is possibly torn or thin around the areas. The fact that nonvolcanic deep tremors associated with the subducting Philippine Sea slab beneath Shikoku, Kii, and Tokai do not occur in zones of high Vp, high Vs, and low Vp/Vs ratio may reflect the existence of fluids generated by the dehydration processes of the slab. Prominent and wide low Vp and Vs zones exist beneath central Honshu at the depth range of 30-60 km, where the volcanic front related to the subducting Pacific plate is located and seismicity around the Philippine Sea plate is very low. This condition may exist because magma genesis processes related to the subducting Pacific plate activate the same processes around the Philippine Sea plate.
Earth, Planets and Space, 2008
The velocity structure and accurate aftershock distributions of the Noto Hanto Earthquake in 2007 (thrust type) are elucidated by inverting the arrival times from 917 aftershocks using double-difference tomography. P-wave velocity (V p) of the hanging wall in the southeast appears to be higher than that of the footwall in the northwest, and the high-V p body of the hanging wall has a relatively high V p /V s ratio. Conversely, the low-V p body in the footwall appears to have a low V p /V s ratio at depths greater than 3 km. Aftershocks associated with the mainshock fault are roughly distributed along this velocity boundary between the hanging wall and footwall. Near-surface thin layers with significantly low V p and high V p /V s are imaged in a northwest direction from the mainshock epicenter. A likely explanation is that the mainshock fault plane was reactivated as a reverse fault in terms of the inversion tectonics due to the crustal shortening which initiated from the late Miocene. Both the mainshock hypocenter and the vertical alignment of aftershocks beneath it are located in the low-V p and low-V p /V s zones, indicating the potential presence of water-filled pores. Crustal stretching and shortening in and around the Noto Peninsula have created complex structures, including weak high-dip angle faults, almost vertical faults, and low velocity zones, which can potentially affect the seismic activities around the source region.
Tomographic imaging of P and S wave velocity structure beneath northeastern Japan
Journal of Geophysical Research, 1992
The seismic body wave tomography method has been improved and extended to adapt to a general velocity structure with a number of complexly shaped seismic velocity discominuities (SVDs) and with three-dimensional variations in the velocities in the modeling space. An efficiem three dimensional ray tracing algorithm which iteratively uses the pseudobending technique and Snell's law is developed. The large and sparse system of observation equations is solved by using the LSQR algorithm. This method is applied to 18,679 arrival times from 470 shallow and intermediate-depth earthquakes in order to study P and S wave tomographic images beneath northeastern Japan. In addition to first P and S wave arrivals, clear later arrivals of SP waves converted at the Moho and PS and SP waves converted at the upper boundary of the subducted Pacific plate (UBPP) are also used in the inversion. The UBPP, Conrad and Moho are taken as three SVDs, and their depth distributions obtained by previous studies are used. Highresolution P and S wave tomographic images down to a depth of 200 km have been determined. Large velocity variations amouming to 6% for P wave and 10% for S wave are revealed in the crust and upper mamle. In the crust low-velocity (low-V) zones exist beneath active volcanoes. In the upper mamle the 1ow-V zones dip toward the west from the volcanic from. A high-velocity (high-V) zone corresponding to the subducted Pacific plate is clearly delineated. Most earthquakes in the lower plane of the double-planed deep seismic zone are found to occur in relatively high-V areas. The obtained tomographic images are also found to explain other seismological observations well.
Geophysical Journal International, 2009
The seismic velocity structure in and around the source area of the 2004 mid-Niigata earthquake, which featured complicated heterogeneities, was investigated by combining waveform modelling and traveltime tomography inversion using low-frequency (0.05 ≤ f ≤ 0.2 Hz) and high-frequency (f ≥ ∼1 Hz) data, respectively. On the footwall of the main shock that includes multiplanar faults, 3-D finite-difference waveform modelling using only a previously proposed tomography model was not sufficient to synthesize the observed waveforms in 0.05 ≤ f ≤ 0.2 Hz at most stations. Thus, we derived a final 3-D model 3DM-28, examining body wave amplitudes, phases and traveltimes. The image produced by model 3DM-28 shows a clearer contrast between low-and high-velocities than that seen in the original tomography models due to the higher velocity on the footwall. This increase in velocity, particularly in the seismogenic zone, also indicates that short-wavelength low-velocity anomalies revealed in the revised tomography image may be more localized in the vicinity of the multiplanar faults than that shown in the original one. Moreover, the low-velocity anomaly zone within a depth range of 15-20 km beneath the seismogenic zone (lower crust) appears to be associated with the short-wavelength low velocity anomalies at shallower depths (upper crust). These characteristics may support the hypothesis of infiltration of pressurized fluids from the lower crust into the multiplanar fault system. The volumes or distributions of such fluids may be clarified by further examination of 'unsatisfactory fit waveforms' recorded at stations along the strikes of the major faults. We suggest that the clear velocity contrasts between the hangingwall and footwall and the upper crust and lower crust, including the effects of fluids, all seem to be essential characteristics of the seismogenic conditions in this earthquake sequence.
Earth, Planets and Space, 2013
The Yamasaki fault zone in southwestern Japan currently has a high potential for producing a large damaging earthquake. We carried out a seismic tomographic study to determine detailed crustal structures for the region. The velocity model clearly images a low-velocity and high V p /V s (high Poisson's ratio) anomaly in the lower crust beneath the Yamasaki fault zone at a depth of ∼15-20 km. This anomaly may be associated with the existence of partially-melted minerals. The existence of this anomaly below the fault zone may contribute to changing the long-term stress concentration in the seismogenic zone.
Three-dimensional P- and S-wave velocity structures beneath Japan
Physics of The Earth and Planetary Interiors, 2008
We determined the three-dimensional Vp and Vs structures beneath Japan by applying seismic tomography to a large number of arrival times recorded at temporary stations in the Japan Sea and the Pacific Ocean, as well as those at permanent stations on the Japan Islands. As a result, we obtained more precise seismic images than previous studies. In the crust and the uppermost mantle, southwestern Honshu exhibited weaker heterogeneity than the other areas in Japan, corresponding to the distribution of active volcanoes. Stripe-like heterogeneities exist in the subducting Pacific slab. Relatively low-velocity zones correspond to low-seismicity areas in the Pacific slab, suggesting that the slab is possibly torn or thin around the areas. The fact that nonvolcanic deep tremors associated with the subducting Philippine Sea slab beneath Shikoku, Kii, and Tokai do not occur in zones of high Vp, high Vs, and low Vp/Vs ratio may reflect the existence of fluids generated by the dehydration processes of the slab. Prominent and wide low Vp and Vs zones exist beneath central Honshu at the depth range of 30-60 km, where the volcanic front related to the subducting Pacific plate is located and seismicity around the Philippine Sea plate is very low. This condition may exist because magma genesis processes related to the subducting Pacific plate activate the same processes around the Philippine Sea plate.
Journal of Physics of the Earth, 1984
We examined the spatial distribution of earthquake hypocenters and the three-dimensional velocity structure of P-wave for the Kanto-Tokai District, central Japan, in order to investigate their relation. About 5,000 earthquakes which occurred during the period from January 1980 to December 1981 were relocated. The double-planed structure of the intermediate-depth seismic zone is observed clearly beneath the Kanto District. The seismic zone underthrusting from Suruga trough toward the west and from Sagami trough toward the east were clearly traced throughout the troughs. Their dip angles are found to be steeper at the northern end compared with the southern part. These inclined seismic zones, however, could not be traced through the inland region to the north of the Izu Peninsula. The three-dimensional P-wave velocity structure was investigated by high velocity zone with the inclined seismic zone is found under Sagami trough but such a correlation is not found under Suruga trough owing presumably to poor resolution.