津村 紀子

A seismic array observation across the slow-earthquake source region in the eastern Kii Peninsula, southwest Japan, reveal the detailed structure and seismicity along the upper boundary of the subducted Philippine Sea Plate. Tomography analysis and earthquake reflection imaging clarify the geometry of the upper boundary of the Philippine Sea Plate with a subduction angle of similar to 15 degrees at a depth range of 22-30 km. We observe intraslab low-frequency earthquakes (LFEs) in and around the low-Vp and high-Vp/Vs zones, which are located above the intraslab earthquakes that are controlled by the dehydration embrittlement of serpentine. The intraslab LFE activity may be related to fluid movement. Fluids, which are derived from both dehydration of the serpentinized oceanic mantle and crustal eclogitization, may control the fault slip behavior. Therefore, the fluid behavior around the subducting plate boundary has an important role in controlling the downdip limit of the seismogenic zone.

The tectonic stress field was investigated in and around the aftershock area of the Hokkaido Eastern Iburi earthquake (M-JMA = 6.7) occurred on 6 September 2018. We deployed 26 temporary seismic stations in the aftershock area for approximately 2 months and located 1785 aftershocks precisely. Among these aftershocks, 894 focal mechanism solutions were determined using the first-motion polarity of P wave from the temporary observation and the permanent seismic networks of Hokkaido University, Japan Meteorological Agency (JMA), and High Sensitivity Seismograph Network Japan (Hi-net). We found that (1) the reverse faulting and the strike-slip faulting are dominant in the aftershock area, (2) the average trend of P- and T-axes is 78 degrees +/- 33 degrees and 352 degrees +/- 51 degrees, respectively, and (3) the average plunge of P- and T-axes is 25 degrees +/- 16 degrees and 44 degrees +/- 20 degrees, respectively: the P-axis is close to be horizontal and the T-axis is more vertical than the average of the P-axes. We applied a stress inversion method to the focal mechanism solutions to estimate a stress field in the aftershock area. As a result, we found that the reverse fault type stress field is dominant in the aftershock area. An axis of the maximum principal stress (sigma(1)) has the trend of 72 degrees +/- 7 degrees and the dipping eastward of 19 degrees +/- 4 degrees and an axis of the intermediate principal stress (sigma(2)) has the trend of 131 degrees +/- 73 degrees and the dipping southward of 10 degrees +/- 9 degrees, indicating that both of sigma(1)- and sigma(2)-axes are close to be horizontal. An axis of the minimum principal stress (sigma(3)) has the dipping westward of 67 degrees +/- 6 degrees that is close to be vertical. The results strongly suggest that the reverse-fault-type stress field is predominant as an average over the aftershock area which is in the western boundary of the Hidaka Collision Zone. The average of the stress ratio R = (sigma(1) - sigma(2))/(sigma(1) - sigma(3)) is 0.61 +/- 0.13 in the whole aftershock area. Although not statistically significant, we suggest that R decreases systematically as the depth is getting deep, which is modeled by a quadratic polynomial of depth.

In the original publication of this article (Iwasaki et al. 2019), the author name ‘Matsubara Makoto’ in author list is not correct. The correct name should be ‘Makoto Matsubara’.

After the occurrence of the 2011 magnitude 9 Tohoku earthquake, seismicity in the overriding plate changed. This chapter considers some possible evidence of the influence of crustal fluid/water on the occurrence of the triggered seismicity after the 2011 Tohoku-Oki earthquake. It estimates the permeability from the observed hypocenter diffusion. Shallow seismic activity and crustal deformation are strongly affected by the water dehydrated and upwelling from the subducting Pacific plate. Spatiotemporal expansion of hypocenter areas of some earthquake swarms can be attributed to fluid diffusion. In the upper crust, the earthquakes seem to be distributed in seismic high-velocity areas rather than in seismic low-velocity areas. In the lower crust, the seismic low-velocity areas appear to be elongated along N-S or NE-SW, the strike of the island arc. The chapter also estimates three-dimensional seismic velocity structure using the double-difference tomography method.

The Kanto Basin, the largest lowland in Japan, developed by flexure as a result of (1) the subduction of the Philippine Sea (PHS) and the Pacific (PAC) plates and (2) the repeated collision of the Izu-Bonin arc fragments with the Japanese island arc. Geomorphological, geological, and thermochronological data on vertical movements over the last 1 My suggest that subsidence initially affected the entire basin after which the area of subsidence gradually narrowed until, finally, the basin began to experience uplift. In this study, we modeled the tectonic evolution of the Kanto Basin following the method of Matsu'ura and Sato (1989) for a kinematic subduction model with dislocations, in order to quantitatively assess the effects of PHS and PAC subduction. We include the steady slip-rate deficit (permanent locking rate at the plate interface) in our model to account for collision process. We explore how the latest collision of the Izu Peninsula block has been affected by a westerly shift in the PHS plate motion vector with respect to the Eurasian plate, thought to have occurred between 1.0-0.5 Ma, using long-term vertical deformation data to constrain extent of the locked zone on the plate interface. We evaluated the change in vertical deformation rate for two scenarios: (1) a synchronous shift in the orientation of the locked zone as PHS plate motion shifts and (2) a delayed shift in the orientation of the locked zone following the shift in plate motion. Observed changes in the uplift/subsidence pattern are better explained by scenario (2), suggesting that recent (<1 My) deformation in the Kanto Basin shows a lag in crustal response to the plate motion shift. We also calculated stress accumulation rates and found a good match with observed earthquake mechanisms, which shows that intraplate earthquakes serve to release stress accumulated through long-term plate interactions. (C) 2016 Elsevier B.V. All rights reserved.

In the southern Kanto region of Japan, where the Philippine Sea plate is descending at the Sagami trough, two different types of large interplate earthquakes have occurred repeatedly. The 1923 (Taisho) and 1703 (Genroku) Kanto earthquakes characterize the first and second types, respectively. A reliable source model has been obtained for the 1923 event from seismological and geodetical data, but not for the 1703 event because we have only historical records and paleo-shoreline data about it. We developed an inversion method to estimate fault slip distribution of interplate repeating earthquakes from paleo-shoreline data on the idea of crustal deformation cycles associated with subduction-zone earthquakes. By applying the inversion method to the present heights of the Genroku and Holocene marine terraces developed along the coasts of the southern Boso and Miura peninsulas, we estimated the fault slip distribution of the 1703 Genroku earthquake as follows. The source region extends along the Sagami trough from the Miura peninsula to the offing of the southern Boso peninsula, which covers the southern two thirds of the source region of the 1923 Kanto earthquake. The coseismic slip takes the maximum of 20 m at the southern tip of the Boso peninsula, and the moment magnitude (Mw) is calculated as 8.2. From the interseismic slip-deficit rates at the plate interface obtained by GPS data inversion, assuming that the total slip deficit is compensated by coseismic slip, we can roughly estimate the average recurrence interval as 350 years for large interplate events of any type and 1400 years for the Genroku-type events.

 地震動評価のための深部地下構造の把握を目的として，近地地震のP直達波からS直達波までのPコーダ波に対する地震波干渉法解析の適用性について検討した。地下構造に関する情報が豊富に存在する千葉県成田市において，稠密地震計アレイを用いて2010年12月から2011年4月までの約4ヶ月間の自然地震連続観測を行い，近地地震Pコーダ波の地震波干渉法解析により地殻構造のイメージングを試みた。データ解析では，まず，気象庁一元化震源リストを基に選定条件を満たす近地地震の記録を連続観測記録から切り出し，P波とS波の初動走時を手動により読み取りつつ品質確認を行った後に437個の良質な近地地震記録を選別した。次に，上下動成分についてP波初動とその多重反射波が含まれるPコーダ波部分を解析対象として，干渉法解析を行って仮想震源記録を合成した。最後に，合成された擬似反射波記録に対して反射法データ処理を施してP波反射波による地下構造断面を得た。解析の結果，基盤からの反射波を明瞭に識別できる擬似反射法記録が合成され，展開長1kmの稠密展開測線では高分解能なP波反射波断面が得られ，受振点間隔の粗い展開長5kmの測線では比較的深部の反射波と考えられる波群を確認することができた。地震毎に震源関数の異なる近地地震を解析する際には，震源関数の影響が抑制されるデコンボリューション型干渉法が適していることを示した。また，解析に利用する地震イベント数や震央の方位分布について反射波合成への影響を評価し適用性を確認した。本研究により，稠密な地震計アレイを用いた近地地震のPコーダ波の干渉法解析は，地殻内の堆積構造や基盤形状など深部地下構造の調査法として有効であることが示された。<br>

To determine crustal structure of the Neodani Fault zone, where the largest inland Nobi earthquake occurred in 1891, an active-source seismic experiment was carried out in the northern Mino region, central Japan. The shots were recorded by seismic array stations settled across the Neodani Fault Zone and we applied seismic reflection analysis to the data and obtained reflection profiles of the crust. Then we estimated crustal velocity by forward modeling using travel times of first break and reflected waves. We found significant reflections with duration of 2 s around 10 s two way travel time. Based on comparisons with other reflection profiles, we interpreted the events to come from the laminated lower crust within the overriding plate. The depth of the laminated crust varies across the Neodani Fault Zone, which is shallower in the southwest than in the northeast, implying that displacement along the Neodani faults extends to the deep crust. In the southwestern part of study area, the top of the Philippine Sea plate (PSP) estimated from travel time tomography is shallower than the lower limit of the laminated lower crust. We thus suggest that the subducting PSP may contact with the bottom of the overriding crust beneath the northern Mino district.

We estimated seismic velocity structures for both <i>V</i><sub>p</sub> and <i>V</i><sub>s</sub> in the crust of the Tokai region, central Japan, with observations of seismic ACROSS signals. "ACROSS" is the abbreviation of the terms "Accurately Controlled, Routinely Operated, Signal System". The seismic ACROSS aims at highly-stable monitoring of the Earth's crust using continuous elastic waves. This study is the first investigation of deep crustal structure with the use of the seismic ACROSS. In the Tokai region, three seismic ACROSS vibrators are constantly transmitting seismic signals for monitoring the focal region of impending Tokai earthquake. We deployed a seismic linear array consisting of 81 seismometers with 120 km aperture from April to August in 2008 in order to receive ACROSS signals from two transmitting stations named TOKI (in Gifu Pref.) and MORI (in Shizuoka Pref.). The ACROSS signals were stacked for four months to obtain the 6-components Green's functions in time domain. We observed clear arrivals of seismic phases of both P and S waves along the linear array. Some phases were interpreted as reflections originated from deep crust and the upper surface of the Philippine Sea Plate. We estimated a seismic velocity model of the crust beneath the array that can explain these major seismic phases using a ray tracing method. Structural features of the estimated model, such as downward bending of the island-arc Moho and uplift of the upper surface of the plate, are consistent with the result of previous structure survey in the region.

Anisotropy is an important feature of elastic wave propagation in the Earth and can arise from a variety of ordered architectures such as fractures with preferential alignments or preferred crystal orientations. We studied the regional variations in shear wave anisotropy around a deep Low-Frequency Earthquake (LFE) zone beneath the Kii Peninsula, SW Japan, using waveforms of local earthquakes observed by a dense linear array along the LFE zone. The fast directions of polarization are subparallel to the strike of the margin for both crustal and intraslab earthquakes. The delay time of the split shear waves in intraslab earthquakes is larger than that in crustal earthquakes and shows a down-dip variation across the LFE zone. This indicates that anisotropy exists in the mantle wedge and in the lower crust and/or oceanic slab. We explain the observed delay time of 0.015-0.045 s by suggesting that the mantle wedge consists of a deformed, 1-15 km thick serpentine layer if the mantle wedge is completely serpentinized. In addition to high-fluid pressures within the oceanic crust, the sheared serpentine layer may be a key factor driving LFEs in subduction zones.

Intense swarm-like seismicity associated with shallow normal faulting was induced in Ibaraki and Fukushima prefectures, Japan, following the 2011 Tohoku-Oki earthquake. This seismicity shows a systematic spatiotemporal evolution, but little is known of the heterogeneity in crustal structure in this region, or its influence on the evolution of the seismicity. Here, we elucidate a high-resolution model of crustal structure in this region and determine precise hypocenter locations. Hypocenters in Ibaraki Prefecture reveal a planar earthquake alignment dipping SW at similar to 45 degrees, whereas those in Fukushima Prefecture show a more complex distribution, consisting of conjugate sets of aligned small earthquakes. On the north of the hypocenter of the largest earthquake in the sequence (the M7.0 Iwaki earthquake), we imaged a high-velocity body at shallow depths that lacks aftershock seismicity. Based on fault source models, the large-slip region of the Iwaki earthquake is situated along a zone that roughly coincides with this high-velocity body. We delineated a separate low-velocity anomaly directly beneath the hypocenter of the Iwaki earthquake, indicating crustal fluids in this region. We hypothesize that strong crust underwent structural failure due to the infiltration of crustal fluids into the seismogenic zone from deeper levels, causing the Iwaki earthquake. Citation: Kato, A., et al. (2013), Imaging the source regions of normal faulting sequences induced by the 2011 M9.0 Tohoku-Oki, Geophys. Res. Lett., 40, 273-278, doi: 10.1002/grl.50104.

Long term slow slip (LTSS) and non-volcanic low frequency earthquakes (LFEs) were reported in the central part of the Tokai district, central Japan. Such LTSS and LFE events are considered to take place at a transition zone of frictional property from stick-slip to stable sliding on the top of subducting Philippine Sea plate. To clarify the spatial variation of physical properties in this region, we estimated a three dimensional seismic attenuation structure using a joint inversion method. In the shallow depths from the surface to 5km, we found a lower <I>Q</I> zone located along the Median Tectonic Line which divides the southwestern Japan into two parts; an old geologic belt and a new accretionary belt. In the lower crust of the land plate at the depths of 17 to 25km, a very high <I>Q</I> zone (about 2000) exists just above the region where large slip rates were observed during the LTSS between 2001 and 2005. Since very few earthquakes occur in this high <I>Q</I> zone, that portion might consist of harder rocks than surroundings. On the contrary, the region just beneath the large slip zone has lower <I>Q</I> values than those of surrounding area. Comparing our results with a seismic velocity structure derived from travel time tomography, we found the high <I>Q</I> zone approximately coincides with a zone of relatively high velocities and the lower <I>Q</I> zone corresponds to a zone of relatively low velocities and high <I>V</I><SUB>P</SUB>/<I>V</I><SUB>S</SUB> values. A low <I>Q</I> zone with low velocities and high <I>V</I><SUB>P</SUB>/<I>V</I><SUB>S</SUB> can be interpreted as the zone which involves high-pressure fluid. Probably the high <I>Q</I> zone above the large slip zone works as a cap rock and prevents the fluid from moving toward the shallow part, and then the fluid pressure becomes high and it affects the occurrence of slow slip in this region.

We have elucidated depth variations in the stress field associated with the 2007 Noto Hanto, Japan, earthquake by stress tensor inversion using high-quality aftershock data obtained by a dense seismic network. Aftershocks that occurred above 4 km in depth indicated a strike-slip stress regime. By contrast, aftershocks in deeper parts indicated a thrust faulting stress regime. This depth variation in the stress regime correlates well with that in the slip direction derived from a finite source model using geodetic data. Furthermore, the maximum principal stress (sigma(1)) axis was stably oriented approximately W20 degrees N down to the depth of the mainshock hypocenter, largely in agreement with the regional stress field, but, below that depth, the s1 axis had no definite orientation, indicating horizontally isotropic stress. One likely cause of these drastic changes in the stress regime with depth is the buoyant force of a fluid reservoir localized beneath the seismogenic zone. Citation: Kato, A., et al. (2011), Anomalous depth dependency of the stress field in the 2007 Noto Hanto, Japan, earthquake: Potential involvement of a deep fluid reservoir, Geophys. Res. Lett., 38, L06306, doi:10.1029/2010GL046413.

地下の3次元的な地震波減衰構造を推定するため, 複数の観測点で得られた複数の地震の波形スペクトルから震源パラメター, Q構造, 観測点近傍の地盤増幅特性の同時推定を行うインバージョン法を紹介する。また, その方法を2004年に発生した中越地震の余震データに適用し, 震源域近傍の詳細なQ構造を求めたので報告する。<br> 中越地震余震の波形データから推定されたQ値の分布は, 本震断層面を境として西 (上盤) 側が相対的に低く, 東 (下盤) 側が相対的に高い。西側の低Q値領域は地表の堆積層分布とほぼ一致し, 西に傾いて深くなる傾向が見られた。東側の高Q値領域はこの地域の基盤に対応すると考えられる。本震断層と平行し, 約5km 下に位置すると考えられる余震の断層面や東下がりの断層面などの周辺では周囲よりQ値が低下する部分が存在する。他の地球物理学的データを比較すると, これらの減衰の特徴は日本海拡大時のリフト構造に関連した堆積構造および基盤の形状を表していると考えられ, また高Q値領域内でのQ値低下は本震の震源断層周辺での複雑な応力分布に影響を与える堆積岩や基盤岩内の弱い領域を示している可能性がある。<br>

We show fine-scale variations of seismic velocities and converted teleseismic waves that reveal the presence of zones of high-pressure fluids released by progressive metamorphic dehydration reactions in the subducting Philippine Sea plate in Tokai district, Japan. These zones have a strong correlation with the distribution of slow earthquakes, including long-term slow slip (LTSS) and low-frequency earthquakes (LFEs). Overpressured fluids in the LTSS region appear to be trapped within the oceanic crust by an impermeable cap rock in the fore-arc, and impede intraslab earthquakes therein. In contrast, fluid pressures are reduced in the LFE zone, which is deeper than the centroid of the LTSS, because there fluids are able to infiltrate into the narrow corner of the mantle wedge, leading to mantle serpentinization. The combination of fluids released from the subducting oceanic crust with heterogeneous fluid transport properties in the hanging wall generates variations of fluid pressures along the downgoing plate boundary, which in turn control the occurrence of slow earthquakes. Citation: Kato, A., et al. (2010), Variations of fluid pressure within the subducting oceanic crust and slow earthquakes, Geophys. Res. Lett., 37, L14310, doi:10.1029/2010GL043723.

To reveal a subsurface structure beneath the southern part of the Boso Peninsula, Japan, where the Philippine Sea plate is subducting and great interplate earthquakes associated with the subduction occur repeatedly, we conducted a new seismic reflection survey from March to April 2005 (Boso05). We also reanalyzed old multi-channel seismic (MCS) survey data that had been collected off the Boso Peninsula in 1978 (SK78). We found clear strong reflectors beneath the southern coast of the Boso Peninsula. Since common mid points (CMPs) were distributed widely beneath the study area owing to the design of the receiver and shot lines of Boso05, we selected appropriate directions of stacking lines to get the best image of the dipping reflectors by optimum azimuth search (OAS) processing. We carefully checked the seismic profiles at the intersections of the survey lines to confirm the NNE-dipping configuration of the strong reflectors. These strong reflectors were interpreted as the upper surface of the subducting PHS plate from their locations and the estimated velocities beneath the reflectors. Furthermore, these reflectors revealed a topographic high (bump) beneath the southern coast of the Boso Peninsula where the source fault of the Genroku earthquake of 1703 is thought to be located. (C) 2008 Elsevier B.V. All rights reserved.

A multi-purpose seismic experiment named the 2002 integrated seismic experiment Southwest Japan was conducted in 2002 along a more-than-240-km-long line across southwest Japan from the Pacific coast to the Japan Sea coast. Its profile provides the first crustal-scale cross section across the Japanese island arc, which highlights a number of significant points related to the structural development of the arc. Major outstanding points are that the Japanese island arc is composed of two completely different crusts juxtaposed by the Median Tectonic Line (MTL), and that the MTL started its activity associated with lower crustal thinning and formation of an upper crustal-scale half-graben in Late Cretaceous. (C) 2008 Elsevier B.V. All rights reserved.

This paper describes a rockfall event in the Daisekkei Valley of Mount Shirouma-dake (2,932 m), the northern Japanese Alps. The rockfall occurred on a steep cliff comprising well-jointed felsites and produced debris of >= 8,000 m(3). Most debris was deposited on an elongated snowpatch located immediately beneath the cliff, and it caused casualties among people who were trekking along a trail on the snowpatch. Additionally, a large rock block slipped 1 km on the snowpatch. The rockfall could have been due to the differential retreat of the rockwall, which contains areas of high- and low-density joints. Seasonal and diurnal freeze-thaw activities and snow avalanches and wash appear to be important factors responsible for the retreat. Although some rock blocks that can collapse further remain on the rockwall, the position of the mountain trail in the Daisekkei Valley is fixed. Fundamental reform of tourism systems for climbers, including education on natural hazards, is required.

白馬大雪渓で発生した落石事故について，現地調査を行い，落石の原因となる地質状況，落石の量，気象条件，地震動の解析により求められる落石発生時刻など，落石についての基礎的な情報を報告した．現地調査を担当．

A fine-scale three-dimensional attenuation structure beneath the northeastern Japan are has been obtained using a joint inversion for source parameters, site response and Q values. Data used for this study are P wave spectra of microearthquakes occurring in the subducting Pacific plate. The subducting plate is found to have higher Q values than the overlying mantle. There is an along-are variation in the distribution of low-Q zones in the crust and the mantle wedge within the study area. For the southern part of the study area, low-Q zones are distributed in the crust beneath the volcanic front. They extend into the mantle wedge and become deep toward the west, that is, toward the backarc region. On the other hand, the northern part of the study area has low-Q zones at two depth ranges; at shallower depths just beneath the volcanic front and at the deeper part of the mantle wedge beneath the backarc region. The region without active volcanoes along the volcanic front has relatively high mantle Q values. Seismic activity in the lower plane of the double-planed deep seismic zone is high beneath this relatively high-Q region of the mantle wedge. Spatial distribution of low-Q zones found in the present study is nearly consistent with that of the low-velocity zones estimated from studies of travel times tomography. Partial melting zones in the mantle wedge, whose existence has been suggested from the study of seismic velocity structure and laboratory experiments, are located in the low Q zones obtained in the present study. These results suggest that spatial distribution of Q values is closely related to heterogeneous distribution of temperature in the mantle wedge. (C) 2000 Elsevier Science B.V. All rights reserved.

In the Hidaka Collision Zone of Hokkaido, northern Japan, the Kuril island are collides with the northeast Japan are. fn order to better understand the collision process, new high resolution information about lithospheric structure was obtained by means of a series of seismic reflection surveys. These seismic profiles reveal distinct zones of seismic lamination in the lower crust of the Kuril are. The upper portion of the lower crust is characterized by numerous east-dipping reflectors. In contrast, west-dipping reflectors dominate the lower part of the lower crust. From this reflector configuration, the lower crust of the Kuril are is interpreted to be delaminated by the collision. The geometry of delamination is consistent with other geophysical data, as well as the peak metamorphic grades exposed in the Hidaka mountains, As earthquakes indicate that delamination here is ongoing, the Hidaka Collision Zone represents an active model for continental growth by are accretion with delamination of lower are crust.

Since ray paths of direct P and S waves are largely distorted in a magma body, a seismic shadow zone is created. However, scattered or diffracted waves with very small amplitudes can arrive in the shadow zone because their ray paths are different from those of the direct waves. They become the first arrival in the shadow zone. More than 100 seismologists from many institutions in Japan joined together to perform an intensified seismic study in the Nikko-Ashio area, northern Kanto, Japan. It was suggested that there is a magma body beneath Mount Nikko-Shirane, an active volcano, that causes a seismic shadow zone, because P waves recorded by the joint observation are anomalously attenuated in cases when they propagate through a zone beneath the eastern part of the volcano. We developed a new method to estimate the shape of the anomalous attenuation zone by proposing the use of a new parameter, the energy ratio between directP or S waves and their coda waves, to discriminate whether or not each observation station is located inside or outside the shadow zone. We estimated the shape of the anomalous attenuation zone by dividing the study area into blocks with dimensions of 2 km x 2 km x 1 km and using the energy ratio data of P waves for local events. The obtained result shows the existence of an anomalous attenuation zone in the area east of Nikko-Shirane volcano with a diameter of about 10 km at depths greater than 3 km. It was also found from plots of deep event seismograms recorded by the same observation that travel times are delayed by about 0.7 s for ray paths crossing the anomalous attenuation zone obtained by the energy ratio data. This value requires the P wave velocity in the anomalous zone to be less than that of the surrounding crustal material by at least about 30%, suggesting the existence of a huge magma reservoir in this area.

We have developed an inversion method for estimating Q structure by the use of a large number of waveform data from earthquakes. We divide an area to be analyzed into small blocks and simultaneously estimate the Q value in each block, source parameters for each event and site response spectrum for each station. The source spectrum is assumed to be expressed by the omega(2) model, and values of corner frequencies and low-frequency levels of source spectra are estimated. Amplitudes are calculated by taking the orientation bf focal mechanism solutions into account. A trade-off among estimated parameters is tested by numerical experiments using three sets of artificial data. In a case when artificial data include error of 15 degrees in the orientation of the P axis of focal mechanism solutions, the inverted result shows that the original checkerboard pattern of Q structure is well reconstructed, although estimated Q(-1) values are about 10% higher than those of the original one. The result without the corrections for radiation effect shows that the checkerboard pattern is only reconstructed in the blocks having a sufficient number of ray paths. We applied this inversion method to a real data set in the northeastern part of Honshu, Japan. Estimated Q(p) structure shows that low Q(p) regions are distributed beneath the volcanic front and they move towards the west with increasing depth. Previous studies of three-dimensional seismic velocity structure in this area show that P-wave low-velocity regions exist in the crust beneath the volcanic front and they are inclined towards the west in the mantle wedge. The low Q(p) regions estimated in this study coincide in general with the P-wave low-velocity regions.