ALBERT

Description: SUMMARY A high-resolution P -wave tomography of the crust and mantle down to 700 km depth beneath the Japan Islands is determined using a large number of high-quality arrival-time data from local earthquakes and teleseismic events simultaneously. The tomography shows that the Philippine Sea slab is subducting aseismically down to 430 km depth under southwest Japan, though the seismicity within the slab ends at 180 km depth. A low-velocity (low-V) zone in the mantle wedge under Tohoku and Kyushu is found to extend westward from the volcanic front to the backarc under the Japan Sea and East China Sea. Significant low-V anomalies are revealed in the deep portion of the mantle wedge (400–500 km depth) above the Pacific slab under southwest Japan, which may reflect hot mantle upwelling associated with fluids from the deep dehydration of the Pacific slab. Low-V anomalies appear at 420–700 km depths beneath the Pacific slab under eastern Japan, which may reflect hot mantle upwelling associated with the deep subduction of the Pacific slab and its collapsing down to the lower mantle.

Description: SUMMARY 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 H 2 O 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.

Description: SUMMARY We determined a detailed 3-D crustal model in the 1995 Kobe earthquake ( M 7.2) area in southwest Japan using both finite-frequency and ray tomography methods. Our finite-frequency tomography technique is based on the single-scattering theory. The finite-frequency sensitivity kernel derived in this study reflects correctly the sensitivity of the heterogeneity off the geometrical ray path and the existence of Fresnel volume, and the kernel depends on the dominant frequency of the observed wave. The dominant frequency is estimated directly from the earthquake magnitude based on a relation that is obtained by regressively analyzing the displacement spectra of 20 earthquakes in the study area. We used 141 118 P -wave and 133 648 S -wave high-quality arrival-time data from 2813 Kobe aftershocks and 3140 other local earthquakes during 1995–2010. The tomographic images obtained with the finite-frequency and ray tomography methods show a high level of similarity, which is verified quantitatively by adopting the structural similarity index. Our results show that the Kobe main shock hypocentre is located in a distinctive zone characterized by a high Poisson's ratio and a low product V P × V S of P - and S -wave velocities, which is interpreted as a fluid-filled, fractured rock matrix that may have triggered the 1995 Kobe earthquake.

Description: SUMMARY We determined P- and S -wave tomography and P -wave anisotropic structure of the Alaska subduction zone using 259 283 P - and 73 817 S -wave arrival times from 7268 local shallow and intermediate-depth earthquakes recorded by more than 400 seismic stations. The results show strong velocity heterogeneities in the crust and upper mantle. Low-velocity anomalies are revealed in the mantle wedge with significant along-arc variations under the active volcanoes. In the mantle wedge, the low-velocity zone extends down to 100–150 km depth under the backarc. The results indicate that H 2 O and fluids brought downwards by the subducting Pacific slab are released to the mantle wedge by dehydration and they are subsequently transported to the surface by the upwelling flow in the mantle wedge. Significant P -wave anisotropic anomalies are revealed under Alaska. The predominant fast velocity direction (FVD) is trench-parallel in the shallow part of the mantle wedge (〈90 km depth) and in the subslab mantle, whereas the FVD is trench-normal within the subducting Pacific slab. The trench-parallel FVDs in the mantle wedge and subslab mantle may be caused by 3-D mantle flow that is induced by the complex geometry and strong curvature of the Pacific slab under Alaska. The flat and oblique subduction of the Pacific slab may play a key role in forming the trench-parallel FVD under the slab. The trench-normal FVD in the subducting Pacific slab may reflect the original fossil anisotropy when the Pacific Plate was produced at the mid-ocean ridge.

Description: Seismic anisotropy evidence for dehydration embrittlement triggering intermediate-depth earthquakes Scientific Reports, Published online: 1 June 2017; doi:10.1038/s41598-017-02563-w

Description: 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 MidAdriatic 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.

Description: Published

Description: 480–493

Description: OST3 Vicino alla faglia

Description: JCR Journal

Description: A new 3-D ray tracing technique is developed to determine seismic raypaths and theoretical travel times in weak radial anisotropic media. Anisotropic schemes of the pseudo-bending technique and Snell's law are derived and used iteratively to find the fastest ray trajectory. Many synthetic tests are performed to evaluate the accuracy of our algorithm and investigate the raypath differences between the 3-D isotropic and anisotropic rays. The test results show that the travel time and raypath differences are smaller than 0.1 s and 10 km, respectively. Applying our technique to a model of radial anisotropy tomography of the Japan subduction zone, we find visible travel time and raypath differences due to variations of the 3-D isotropic velocity and radial anisotropy, but the travel time difference caused by the radial anisotropy is generally 〈~0.1 s and the raypath difference is generally 〈20 km. Two models of Vp radial anisotropy tomography under Japan are obtained using the isotropic and anisotropic ray tracing codes. The two models are very similar to each other, and they have only small differences (〈0.5%) in the amplitudes of isotropic velocity and radial anisotropy. Considering the limited resolution of the current anisotropic tomography models and the effects of damping and smoothing regularizations, the use of the conventional isotropic ray tracing techniques is acceptable at this stage. However, for high-resolution models of anisotropic tomography in the near future, the 3-D anisotropic ray tracing will be necessary. ©2018. American Geophysical Union. All Rights Reserved.