Slant-stack velocity analysis for one-dimensional upper mantle structure using short-period data from MAJO (original) (raw)
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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.
Chinese Science Bulletin, 2005
A detailed 3-D P-wave velocity model of the crust and uppermost mantle under the capital region is determined with a spatial resolution of 25 km in the horizontal direction and 4-17 km in depth. We used 48750 precise P-wave arrival time data from 2973 events of local crustal earthquakes, controlled seismic explosions and quarry blasts. These events were recorded by 123 seismic stations. The data are analyzed by using a 3-D seismic tomography method. Our tomographic model provides new information on the geological structure and complex seismotectonics of this region. Different patterns of velocity structures show up in the North China Basin, the Taihangshan and the Yanshan Mountainous areas. The velocity images of the upper crust reflect well the surface geological, topographic and lithological features. In the North China Basin, the depression and uplift areas are imaged as slow and fast velocity belts, respectively, which are oriented in NE-SW direction. The trend of velocity anomalies is the same as that of major structure and tectonics. Paleozoic strata and Pre-Cambrian basement rocks outcrop widely in the Taihangshan and Yanshan uplift areas, which exhibit strong and broad high-velocity anomalies in our tomographic images, while the Quaternary intermountain basins show up as small low-velocity anomalies. Most of large earthquakes, such as the 1976 Tangshan earthquake (M 7.8) and the 1679 Sanhe earthquake (M 8.0), generally occurred in high-velocity areas in the upper to middle crust. However, in the lower crust to the uppermost mantle under the source zones of the large earthquakes, low-velocity and high-conductivity anomalies exist, which are considered to be associated with fluids, just like the 1995 Kobe earthquake (M 7.2) and the 2001 Indian Bhuj earthquake (M 7.8). The fluids in the lower crust may cause the weakening of the seismogenic layer in the upper and middle crust and thus contribute to the initiation of the large crustal earthquakes.
Analysis of earthquakes in the distance range 40–70° and inferred lower mantle structure
Physics of the Earth and Planetary Interiors, 1982
. Analysis of earthquakes in the distance range 40-70°and inferred lower mantle structure. Phys. Earth Planet. Inter., 28: 242-250. About fifty earthquakes in the distance range 40-70°and azimuthal range 45-120°from the Celebes, Philippines, Mariana and Kurile Island regions, and recorded at Gauribidanur seismic array in southern India, were used in the present study. Measurements on slowness and apparent azimuths were made on the first 30 s of the short period P-wave trains using an adaptive processing technique. Analysis of this data set has revealed no strong evidence for any triplications in the travel-time curve over the ranges in question. The P-wave velocity increases continuously with an almost uniform gradient below 1000 km depth range and is in very close agreement with the JB model. Almost all the observed slowness values of the events were anomalously low and consistent suggesting that they are caused by some azimuthal dependent structure near the array.
Gondwana Research, 2009
We constructed vertical cross-sections of depth-converted receiver function images to estimate the seismic velocity structure of the crust and uppermost mantle beneath the Kanto district, central Japan. Repeating earthquake data for the plate boundary were also used to estimate geometries of the subducting Philippine Sea plate and the subducting Pacific plate. As a result, we present images of some major seismic discontinuities. The upper boundary of the Pacific plate dips to the northwest in northern Kanto and to the west-southwest in southern Kanto with some undulations. On the other hand, the upper boundary of the Philippine Sea plate as a whole dips to the northwest. However, it is concave to the northeast in the southern Boso peninsula. We suggest that the low-velocity mantle wedge may be indicated on the top of both subducting plates. Plate thickness gradually decreases to the northeast. The northeastern end of the Philippine Sea plate is interpreted to be at depths of 45-90 km. The Moho discontinuity in the overriding plate is deeper than 25 km in the northern Kanto. It contacts the subducting Philippine Sea plate in the southwestern part near 35.8°N.
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.
Seismic velocity structure of the subducting Pacific Plate in the Izu-Bonin Region
Journal of Geophysical Research, 1992
Although numerous studies have been made on the contrast between subducting slabs and surrounding mantle, little is known about the internal structure of the oceanic lithosphere after it descends into the mantle at subduction zones. The velocity structure within the slab can be expected to be heterogeneous, based on thermal and petrological considerations. In this study, observed travel time residual data from a spatially dense seismic network above deep earthquakes in the Izu-Bonin region are compared with three-dimensional ray tracing calculations. The data are inconsistent with a homogeneous slab model and consistent with a heterogeneous slab model with regional velocity variations. The residual data can be explained by a model that has a velocity gradient within the slab. In the subducting Pacific plate, the velocity near the center of the slab is faster than that near the upper boundary, and gradually decreases toward the bottom of the plate. A model with a velocity decrease of 3%, as predicted by a thermal profile, explains the observed data. The subducting plate is heated by the surrounding mantle after subduction at the trench. Thermal models of the subducting plate [e.g., McKenzie, 1969; Hsui and Toksoz, 1979] show that the temperature within the slab increases from the central part toward both sides, resulting in a heterogeneous thermal distribution. The velocity structure within the slab can thus be expected to be heterogeneous. Heterogeneous velocity models of deep slabs have been developed using thermal models and observed travel time and amplitude data [Sleep, 1973; Creager and Jordan, 1984, 1986; Engdahl and Gubbins, 1987; Vidale, 1987; Fischer et al., 1988; Vidale and Gonzalez, 1988; Cormier, 1989; Iidaka et al., 1989c; Weber, 1990]. However, there have been few studies of heterogeneous velocity structure within the slab, due to the lack of dense seismic stations with suitable ray paths. However, the Japan arc is an exception because of the spatially dense local network. Beneath the Japan region, the heterogeneous velocity structure within the slab has been revealed by many seismological studies, including travel time residual analysis and converted phase analysis [Suyehiro and Sacks, 1979; Matsuzawa et al., 1986; Ando et al., 1989; Iidaka and Mizoue, 1991].
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.
Geophysical Journal International, 2011
We investigated the upper crustal P-wave velocity structure down to 5 km in the Kii Peninsula, SW Japan by a tomographic inversion of the first-arrival traveltimes from the seismic refraction experiment performed by the Research Group for Explosion Seismology (RGES) in 1988. The observations were carried out on a profile running in the N-S direction from Kawachinagano, Osaka Prefecture to Kiwa, Mie Prefecture. The profile extends for about 65 km across the major geological zones, which characterize the geological features of SW Japan. Six shots were fired and the generated seismic waves were recorded at 86 temporary observation sites. The inversion scheme was applied to 359 first-arrival times to delineate the velocity structure along the profile. In the tomographic scheme, velocity estimation was achieved by an iterative, linearized least-squares inversion. The Jacobian matrix was constructed via a finite-difference approximation by perturbing the slownesses of the cells instead of performing ray tracing. Traveltime calculations were carried out by using a fast finite-difference eikonal solver. Velocity updates were obtained by a matrix inversion algorithm using a conjugate gradient least-squares scheme. In addition, model covariance and model resolution matrices were obtained to assess the velocity image. Two low-velocity zones observed in the northern and southern parts of the profile are the most prominent features of the tomogram. The P-wave velocity structure is generally consistent with the surface geology, and the fault zones associated with the Median (MTL) and Gobo-Hagi (GHTL) tectonic lines across the peninsula.