Norihito Umino

Double-planed structure of the deep seismic zone in the northeastern Japan arc

Abstract A double-planed structure of deep seismic zone has been found over a wide area more then 300 km × 200 km in the Tohoku District, the northeastern part of Honshu, Japan. This prominent feature of the configuration of the deep seismic zone has been ascertained through a precise determination of the microearthquake hypocenters by using the data from the seismic network of Tohoku University. The two planes are nearly parallel to each other, the distance between the two planes being from 30 to 40 km. Composite focal mechanism solutions are derived from the superposition of the distribution of first motions of P waves, and the different fault types are obtained for the two groups of earthquakes; the earthquakes which occurred in the upper plane are characterized by reverse faulting, some of them by down-dip compressional stresses, and those in the lower plane by down-dip extensional stresses. The evidence obtained here provides valuable information for the definition of the type of mechanism producing the plate motion beneath the island arc.

Seismic structure of the northeastern Japan convergent margin: A synthesis

Many studies recently made on the basis of seismic observations have revealed a detailed structure of the crust and the upper mantle beneath the northeastern Japan arc and its relationship to seismic and volcanic activity. Spatial distributions of the depths to the Conrad and the Moho discontinuities, estimated from shallow earthquake data and seismic explosion data, show that both discontinuities are deep in the middle of the land area and shallow toward the coastlines of the Japan Sea and the Pacific Ocean. The Pn velocity has a lateral variation; it is as low as ∼7.5 km/s beneath the land area, while that beneath the Japan Sea and the Pacific Ocean is 8.0–8.2 km/s. It changes abruptly at the transition zones, which are located along the coastlines. Precise structure and location of the subducted Pacific plate beneath the land area is inferred from converted or reflected seismic waves at the top or bottom of the plate. The Pacific plate is composed of a thin (∼5 km) low-velocity upper layer and a thick high-velocity lower layer, its total thickness being 80–90 km. The upper plane seismicity of the double seismic zone is confined to the thin low-velocity upper layer, which probably corresponds to the subducted former oceanic crust. The lower plane seismicity lies at the middle of the high-velocity lower layer, and the lower half of the plate below it is incapable of generating earthquakes. The shallower portion of the upper surface of the plate beneath the Pacific Ocean, along which major seismicity with low-angle thrust faultings is actually occurring, is also located by seismic observations on land and in the sea. The Pacific plate subducts at an extremely low angle of ∼5° for the first ∼25-km depth, and then the dip steepens rather abruptly to ∼30°. Normal-fault type events at the top of the plate have not been detected in the portion where the downward-bending is the largest, but have been ditected near the trench axis, where it is rather small. Tomographic inversions for seismic velocity structure clearly delineate the inclined high-velocity Pacific plate with a thickness of 80–90 km and low-velocity zones in the crust and the mantle wedge beneath active volcanoes. Seismic attenuation tomography also shows similar zones of low-Q value beneath active volcanoes, although its spatial resolution is much lower. The low-velocity zones with 2–6% velocity lows are continuously distributed from the upper crust just beneath active volcanoes to a depth of 100–150 km in the mantle wedge, and are approximately parallel to the dip of the underlying Pacific plate. These low-velocity zones probably reflect the pathway of magma ascent from a depth in the mantle wedge to the Earths surface, corresponding to a portion of the subduction-induced secondary mantle wedge flow.

Structural heterogeneity in the megathrust zone and mechanism of the 2011 Tohoku-oki earthquake (Mw 9.0)

The great 2011 Tohoku-oki earthquake (Mw 9.0) and its 339 foreshocks and 5,609 aftershocks (9–27 March 2011) were relocated using a three-dimensional seismic velocity model and local P and S wave arrival times. The distribution of relocated hypocenters was compared with a tomographic image of the Northeast Japan forearc. The comparison indicates that the rupture nucleation of the largest events in the Tohoku-oki sequence, including the mainshock, was controlled by structural heterogeneities in the megathrust zone.

Spatial distribution of focal mechanisms for interplate and intraplate earthquakes associated with the subducting Pacific plate beneath the northeastern Japan arc: A triple‐planed deep seismic zone

The northeastern Japan arc is located in one of the most seismically active subduction zones in the world. In this study, we relocated hypocenters and determined focal mechanisms of small earthquakes (M≤5) beneath the arc in order to investigate in detail the stress distribution in and around the descending oceanic plate. In the hypocenter relocation we adopted a “source region station correction method” in which station corrections vary with hypocenter locations. We have developed a new focal mechanism determination technique named “master solution method,” which is analogous to the “master event method” in hypocenter determination. We applied the method to P and SH wave amplitude data to obtain 1106 focal mechanism solutions. From the new mechanism solutions and relocated hypocenters we found that there occur both low-angle thrust fault (LT) type and downdip compression (DC) type earthquakes at depths from 40 to 70 km near the aseismic front; the DC type events are underlying the LT-type events. Almost all the earthquake clusters are composed of LT-type events. The western limit of the region where LT-type events have occurred is subparallel to the trench axis, although it undulates considerably; it delineates the westernend of the active region of interplate seismicity. Furthermore, we found that normal fault (NF) type events also occur at depths from 70 km in the upper plane of the double-planed deep seismic zone, which is characterized mainly by DC-type event. These NF-type events are distributed only in a thin uppermost portion of the slab close to the plate boundary. Below these, in the lower plane, are downdip extension (DE) type events. This result indicates that the deep seismic zone in the northeastern Japan arc is not double-planed but triple-planed, even beneath the land area, which cannot be explained by any simple models.

Extensional structure in Northern Honshu Arc as inferred from seismic refraction/wide‐angle reflection profiling

A recent extensive seismic wide-angle experiment revealed a new image of crustal and uppermantle structure across Northern Honshu Arc, Japan. The western part of the arc recorded the crustal deformation by the Miocene back arc spreading of the Sea of Japan. The crust is composed of highly deformed Tertiary sedimentary layers, a relatively low velocity (5.75–5.9 km/s) crystalline basement and a 15-km thick lower crust with a velocity of 6.6–7.0 km/s. Clear westward crustal thinning from 32 to 27 km represents the extensional deformation by the backarc spreading. The crust attains the maximum thickness (32–35km) east of the backbone range for which the magmatic intrusion/underplating since 10–15 Ma is a predominant factor. The eastern part of the arc has a less deformed upper crust and a reflective middle/lower crust, probably remaining a stable block since the time of the backarc spreading.

Tomography of northeast Japan forearc and its implications for interplate seismic coupling

[1] We determined, for the first time, a 3-D seismic structure of P and S wave velocity (Vp, Vs) in the forearc region of northeast (NE) Japan from the Pacific coast to the Japan Trench using a large number of high-quality arrival times from sub-oceanic earthquakes that are well located with sP depth phase data. Strong lateral heterogeneities in Vp and Vs are revealed. A majority (95%) of great interplate earthquakes are located outside the low-V zones while a few (5%) of them are located inside the low-V zones. This may suggest a different degree of interplate seismic coupling at the plate interface parallel to Japan Trench. The low-V zones probably contain fluids and may represent decoupled areas. We propose that this new approach of investigating 3-D seismic structure can be applied to other forearcs of the world for a better understanding of interplate coupling and its influence on big earthquakes. INDEX TERMS: 6982 Radio Science: Tomography and imaging; 7200 Seismology; 7209 Seismology: Earthquake dynamics and mechanics; KEYWORDS: Tomography, Forearc, Asperities, Seismic coupling, Fluids. Citation: Mishra, O. P., D. Zhao, N. Umino, and A. Hasegawa, Tomography of northeast Japan forearc and its implications for interplate seismic coupling, Geophys. Res. Lett., 30(16), 1850, doi:10.1029/2003GL017736, 2003.

Tomographic Imaging outside a Seismic Network: Application to the Northeast Japan Arc

The conventional local tomography can determine the 3D seismic velocity structure right beneath a seismic network, but it cannot determine the 3D structure outside a seismic network. In this study we show that such a limitation of tomography can be overcome if many earthquakes occur outside a seismic network. In the northeast Japan arc earthquakes occur actively from the Japan Trench to the Pacific coast. We detected and used sP -depth phase to relocate the suboceanic earthquakes accurately, and then used P - and S -wave arrival times from the relocated suboceanic events and the earthquakes under the northeast Japan land area to determine the 3D P - and S -wave velocity and Poisson’s ratio structures of the entire northeast Japan arc from the Japan Trench to the Japan Sea coast. Our results exhibit strong lateral heterogeneities under the forearc region. The mainshock hypocenters of large interplate earthquakes ( M 7.0–8.2) mainly cluster near the boundaries of high and low velocity and Poisson’s ratio above the subducting Pacific slab. A few large earthquakes are located in areas with high velocity and low Poisson’s ratio. These results suggest that lateral heterogeneities on the slab boundary can affect the rupture nucleation of large thrust earthquakes and the degree of interplate seismic coupling.

Seismic activity and deformation process of the overriding plate in the northeastern Japan subduction zone

Abstract We estimated the temperature distribution within the crust of the northeastern Japan arc from P wave velocity perturbations obtained by travel time tomography. By comparing the estimated temperature distribution with the focal depth distribution of shallow, precisely relocated microearthquakes, we found that the brittle to ductile or stick-slip to stable-sliding transition occurs at the ∼400°C isotherm and that the transition depth has considerable lateral variations. The brittle seismogenic zone, the upper portion of the crust, becomes locally thin in the P wave low-velocity areas, where the temperature is estimated to be relatively high. Concentration of shallow microearthquakes, high topography and relatively large contractile deformation of the crust are also observed in these low-velocity areas. Active faults are not distributed in the low-velocity areas but lie just along the edge of those areas or outside them. All these observations suggest that earthquake occurrence and deformation within the crust is governed, to a considerable degree, by the thermal regime of this volcanic arc, which is characterized by a horizontally inhomogeneous distribution of temperature.

Seismic attenuation beneath northeastern Japan: Constraints on mantle dynamics and arc magmatism

We apply a three-step approach to estimate three-dimensional (3-D) P wave attenuation (Qp−1) structure beneath northeastern Japan. First, corner frequencies of earthquakes are determined using the spectral-ratio method for S-coda waves. Then, whole-path attenuation terms, t*, and site-amplification factors are simultaneously estimated by a joint inversion. The set of t* is finally inverted for 3-D attenuation structure. The results show that the mantle wedge has low attenuation in the fore arc and high attenuation in the back arc. A depth profile of Qp−1 in the back-arc mantle is explained by attenuation expected for a two-dimensional (2-D) thermal model with Qp/Qs = 2 and grain sizes of 1 and 3 cm. However, an inclined high-attenuation zone observed in the back-arc mantle wedge, which is interpreted as an upwelling flow, shows higher attenuation than that calculated from the 2-D thermal model. The higher seismic attenuation is probably caused by the concentration of partial melt in the upwelling flow. A combined interpretation of seismic attenuation and velocity structures further suggests that the degree of partial melt in the upwelling flow varies along the arc and that volcanoes are clustered transverse to the arc, below which the upwelling flow contains a higher degree of melt. These observations indicate that magmatism is controlled by a mantle-wedge process that depends strongly on spatial variations in the degree of partial melt in the upwelling flow. Our results further imply the breakdown of hydrous minerals in a hydrous layer above the Pacific plate at a depth of ~120 km.

Coseismic fault model of the 2008 Iwate-Miyagi Nairiku earthquake deduced by a dense GPS network

A large earthquake of Mj 7.2 occurred on June 14, 2008, beneath the border between Iwate and Miyagi prefectures in northeastern Japan. We propose a simple rectangular fault model based on a dense GPS network, including continuous GPS sites run by four agencies, to describe the coseismic deformation. The coseismic displacements are estimated by kinematic PPP (precise point positioning) analysis. Near the hypocenter, colocated independent instruments (integrated accelerogram and kinematic PPP) measure the same large displacement caused by the mainshock. The fault model explains the observations well and reproduces the observed complex spatial pattern, especially around the northern part of the focal area, which is the focus of a debate on whether or not the coseismic slip occurred on the Dedana fault system. Our results show that no major slip on the Dedana fault system occurred. The estimated amount of moment release was equivalent to Mw 6.9, and the maximum slip reached 3.5 m on the southern sub-fault.