Double Seismic Zone Research Articles

A slab fragment wedged under Tokyo and its tectonic and seismic implications

A three-dimensional evaluation of earthquake hypocentres beneath the Kanto basin in Japan reveals the presence of a distinct, 25-km-thick and 100-km-wide body. Its fast seismic velocity and the presence of a double seismic zone suggest that it is a fragment of the Pacific slab, rather than an extension of the Philippine Sea slab. This implies that the penetration of the Philippine Sea slab is much shallower beneath the Kanto basin than was previously thought. The Philippine Sea and Pacific plate slabs both subduct beneath Tokyo, and so their configuration and seismic potential have been subject to intensive study1,2,3,4,5,6,7,8. Previous work suggests that the Philippine Sea slab extends up to 100 km northwest of Tokyo and subducts to a depth of 90 km beneath the Kanto basin, where it is folded against the underlying Pacific slab. Here we evaluate seismic data in three dimensions and delineate a distinct 25-km-thick and 100-km-wide body beneath the Kanto basin that has hitherto been considered to be part of the Philippine Sea slab. We find that several of its characteristics—such as its high seismic velocity and the presence of a double seismic zone—point instead to an affinity with the Pacific slab, implying that the Philippine Sea slab penetrates to depths of only 35–40 km beneath the Kanto basin. We propose that the body is a fragment of Pacific lithosphere that formed by the collision of two intersecting seamount chains with the Japan Trench 2–3 million years ago. We suggest that such slab fragments may not be uncommon, especially where seamount subduction deforms trenches and near triple junctions.

Double seismic zone of the Nazca plate in northern Chile: High‐resolution velocity structure, petrological implications, and thermomechanical modeling

This paper presents an interdisciplinary study of the northern Chile double seismic zone. First, a high‐resolution velocity structure of the subducting Nazca plate has been obtained by the tomoDD double‐difference tomography method. The double seismic zone (DSZ) is observed between 80 and 140 km depth, and the two seismic planes is 20 km apart. Then, the chemical and petrologic characteristics of the oceanic lithosphere associated with this DSZ are deduced by using current thermal‐petrological‐seismological models and are compared to pressure‐temperature conditions provided by a numerical thermomechanical model. Our results agree with the common hypothesis that seismicity in both upper and lower planes is related to fluid releases associated with metamorphic dehydration reactions. In the seismic upper plane located within the upper crust, these reactions would affect material of basaltic (MORB) composition and document different metamorphic reactions occurring within high‐P (>2.4 GPa) and low‐T (<570°C) jadeite‐lawsonite blueschists and, at greater depth (>130 km), lawsonite‐amphibole eclogite conditions. The lower plane lying in the oceanic mantle can be associated with serpentinite dehydration reactions. The Vp and Vs characteristics of the region in between both planes are consistent with a partially (∼25–30 vol % antigorite, ∼0–10% vol % brucite, and ∼4–10 vol % chlorite) hydrated harzburgitic material. Discrepancies persist that we attribute to complexities inherent to heterogeneous structural compositions. While various geophysical indicators evidence particularly cold conditions in both the descending Nazca plate and the continental fore arc, thermomechanical models indicate that both seismic planes delimit the inner slab compressional zone around the 400°C (±50°C) isotherm. Lower plane earthquakes are predicted to occur in the slab's flexural neutral plane, where fluids released from surrounding metamorphic reactions could accumulate and trigger seismicity. Fluids migrating upward from the tensile zone below could be blocked in their ascension by the compressive zone above this plane, thus producing a sheeted layer of free fluids, or a serpentinized layer. Therefore earthquakes may present either downdip compression and downdip tensile characteristics. Numerical tests indicate that the slab's thermal structure is not the only factor that controls the occurrence of inner slab compression. (1) A weak ductile subduction channel and (2) a cold mantle fore arc both favor inner slab compression by facilitating transmission of compressional stresses from the continental lithosphere into the slab. (3) Decreasing the radius of curvature of the slab broadens the depth of inner slab compression, whereas (4) decreasing upper plate convergence diminishes its intensity. All these factors indicate that if DSZs indeed contour inner slab compression, they cannot be linked only to slab unbending, but also to the transmission of high compressional stresses from the upper plate into the slab.

Three-dimensional P- and S-wave velocity structures beneath the Japan Islands obtained by high-density seismic stations by seismic tomography

We construct fine-scale 3D P- and S-wave velocity structures of the crust and upper mantle beneath the whole Japan Islands with a unified resolution, where the Pacific (PAC) and Philippine Sea (PHS) plates subduct beneath the Eurasian (EUR) plate. We can detect the low-velocity (low-V) oceanic crust of the PAC and PHS plates at their uppermost part beneath almost all the Japan Islands. The depth limit of the imaged oceanic crust varies with the regions. High-V P/V S zones are widely distributed in the lower crust especially beneath the volcanic front, and the high strain rate zones are located at the edge of the extremely high-V P/V S zone; however, V P/V S at the top of the mantle wedge is not so high. Beneath northern Japan, we can image the high-V subducting PAC plate using the tomographic method without any assumption of velocity discontinuities. We also imaged the heterogeneous structure in the PAC plate, such as the low-V zone considered as the old seamount or the highly seismic zone within the double seismic zone where the seismic fault ruptured by the earthquake connects the upper and lower layer of the double seismic zone. Beneath central Japan, thrust-type small repeating earthquakes occur at the boundary between the EUR and PHS plates and are located at the upper part of the low-V layer that is considered to be the oceanic crust of the PHS plate. In addition to the low-V oceanic crust, the subducting high-V PAC plate is clearly imaged to depths of approximately 250 km and the subducting high-V PHS zone to depths of approximately 180 km is considered to be the PHS plate. Beneath southwestern Japan, the iso-depth lines of the Moho discontinuity in the PHS plate derived by the receiver function method divide the upper low-V layer and lower high-V layer of our model at depths of 30–50 km. Beneath Kyushu, the steeply subducting PHS plate is clearly imaged to depths of approximately 250 km with high velocities. The high-V P/V S zone is considered as the lower crust of the EUR plate or the oceanic crust of the PHS plate at depths of 25–35 km and the partially serpentinized mantle wedge of the EUR plate at depths of 30–45 km beneath southwestern Japan. The deep low-frequency nonvolcanic tremors occur at all parts of the high-V P/V S zone—within the zone, the seaward side, and the landward side where the PHS plate encounters the mantle wedge of the EUR plate. We prove that we can objectively obtain the fine-scale 3D structure with simple constraints such as only 1D initial velocity model with no velocity discontinuity.

Seismic anisotropy in the wedge above the Philippine Sea slab beneath Kanto and southwest Japan derived from shear wave splitting

We conduct shear wave splitting measurements on waveform data from the Hi-net and the broadband F-net seismic stations in Kanto and SW Japan generated by shallow and intermediate-depth earthquakes occurring in the subducting Philippine Sea and Pacific slabs. We obtain 1115 shear wave splitting parameter pairs. The results are divided into those from the shallow (depth < 50 km) and the deep (depth > 50 km) events. The deep events beneath Kanto are further divided into PHS1 and PHS2 (upper and lower planes of the double seismic zone in the Philippine Sea slab, respectively), PAC1 and PAC2 (western and eastern Pacific slab, respectively) events. The results from the shallow events represent the crustal anisotropy, and their fast directions are more or less aligned in the σ Hmax directions, implying that the anisotropy is produced by the alignment of the vertical cracks in the crust induced by the compressive stresses. In Kanto, Kii Peninsula and Kyushu regions, the results from the deep events suggest a contribution from the mantle wedge anisotropy. Events from all groups beneath Kanto show NW, NE and EW fast directions. This complex pattern seems to be produced by the corner flows induced by both the WNW PAC plate subduction and the oblique NNW PHS slab subduction with the associated olivine lattice-preferred orientations (LPOs), and the anisotropy frozen in the PHS slab. The deep events beneath Kii Peninsula show NE and NW fast directions and may be produced by the corner flow produced by the NNW PHS slab subduction with the associated olivine LPOs. The NE directions might also be produced by the segregated melts in the thin layers parallel to the PHS slab subduction. The deep events beneath N Kyushu show NNW fast directions, which may result from the southeastward flow in the upper mantle inferred from the stresses in the upper plate. Results from the deep events beneath middle-south Kyushu show dominantly E–W fast directions, in both the fore- and back-arcs. They may be produced by the corner flow of the westward PHS slab subduction with the olivine LPOs. Because the source regions with multiple fast directions are not resolved in this study, further detailed analyses of shear wave splitting are necessary for a better understanding of the stress state, the induced mantle flow, and the melt-segregation processes.

Regional and teleseismic double‐difference earthquake relocation using waveform cross‐correlation and global bulletin data

We have developed a double‐difference algorithm to relocate earthquakes recorded at global seismic networks, using differential arrival times for first and later arriving regional and global phases to invert for the vectors connecting the hypocenters. Differential times are formed from global seismic bulletins and are accurately measured on similar seismograms by time domain waveform cross correlation. We evaluate the performance of this spherical, multiphase double‐difference algorithm using three‐dimensional regional‐scale synthetic data and two sets of earthquake data in different tectonic settings. The first includes 3783 intermediate depth earthquakes that occurred between 1964 and 2000 in the subducting Nazca plate beneath northern Chile, where the relocated seismicity confirms a narrowly spaced double seismic zone previously imaged with temporary local seismic data. Residual statistics and comparison with accurately known locations indicate mean relative location errors at the 90% confidence level of 2.4 km laterally and 1.8 km vertically. Later events typically constrained by cross‐correlation data have errors of 1.6 km laterally and 1.4 km vertically. The second data set includes 75 crustal earthquakes in the 1999 Izmit and Düzce, Turkey, aftershock sequences, where the double‐difference solutions image orientation and dip of individual fault segments that are consistent with focal mechanisms and near‐surface information. Fault complexity likely causes a low level of waveform similarity in this aftershock sequence and thus generates fewer correlated events compared to the Chile earthquakes. Differences between the double‐difference locations and corresponding locations in global seismicity catalogs (Earthquake Data Report, EDR; International Seismological Centre, ISC; Engdahl‐Hilst‐Buland, EHB) are typically greater than 10 km. We evaluate the potential of cross‐correlation and double‐difference methods to improve hypocenter locations on a global scale.

P and S velocity structure of the crust and the upper mantle beneath central Java from local tomography inversion

Here we present the results of local source tomographic inversion beneath central Java. The data set was collected by a temporary seismic network. More than 100 stations were operated for almost half a year. About 13,000 P and S arrival times from 292 events were used to obtain three‐dimensional (3‐D) Vp, Vs, and Vp/Vs models of the crust and the mantle wedge beneath central Java. Source location and determination of the 3‐D velocity models were performed simultaneously based on a new iterative tomographic algorithm, LOTOS‐06. Final event locations clearly image the shape of the subduction zone beneath central Java. The dipping angle of the slab increases gradually from almost horizontal to about 70°. A double seismic zone is observed in the slab between 80 and 150 km depth. The most striking feature of the resulting P and S models is a pronounced low‐velocity anomaly in the crust, just north of the volcanic arc (Merapi‐Lawu anomaly (MLA)). An algorithm for estimation of the amplitude value, which is presented in the paper, shows that the difference between the fore arc and MLA velocities at a depth of 10 km reaches 30% and 36% in P and S models, respectively. The value of the Vp/Vs ratio inside the MLA is more than 1.9. This shows a probable high content of fluids and partial melts within the crust. In the upper mantle we observe an inclined low‐velocity anomaly which links the cluster of seismicity at 100 km depth with MLA. This anomaly might reflect ascending paths of fluids released from the slab. The reliability of all these patterns was tested thoroughly.

Earthquake mechanics and deformation in the Tonga‐Kermadec subduction zone from fault plane orientations of intermediate‐ and deep‐focus earthquakes

We make use of rupture directivity to analyze 82 deep earthquakes (≥100 km depth) in the Tonga‐Kermadec subduction zone. Identifying the fault planes for 25 of them, we are able to place new constraints on both the physical mechanism of intermediate‐ and deep‐focus earthquakes and deformation within the subducting slab. We find that half of deep earthquakes with MW ≥ 6 have detectable directivity. We compare the obtained fault orientations with those expected for the reactivation of outer‐rise normal faults and with those expected for the creation of new faults in response to the ambient stress field. Earthquakes &gt;300 km depth match the patterns expected for the creation of a new system of faults: we observe both subhorizontal and subvertical fault planes consistent with a downdip‐compressional stress field. Slip along these faults causes the slab to thicken. Rupture propagation shows no systematic directional pattern. In contrast, at intermediate depths (100–300 km), all ruptures propagate subhorizontally and all identified fault planes, whether in the upper or lower region of the double seismic zone, are subhorizontal. Rupture propagation tends to be directed away from the top surface of the slab. After accounting for the angle of subduction, the subhorizontal fault plane orientation is inconsistent with the orientation of outer‐rise normal faults, allowing us to rule out mechanisms that require the reactivation of these large surface faults. Subhorizontal faults are consistent with only one of the two failure planes expected from the slab stress field, suggesting that isobaric rupture processes or preexisting slab structures may also influence the fault plane orientation. If all deformation takes place on these subhorizontal faults, it would cause the slab to thin. Assuming the slab is incompressible, this implies that the slab is also lengthening and suggests that slab pull rather than unbending is the primary force controlling slab seismicity at intermediate depths.

Existence of a seismic belt in the upper plane of the double seismic zone extending in the along‐arc direction at depths of 70–100 km beneath NE Japan

We relocated microearthquakes using data obtained via a dense seismic network and systematically detected the characteristic distribution of the upper seismic plane seismicity within the Pacific slab beneath NE Japan. We found a seismic “belt” parallel to the iso‐depth contour of the plate interface that is beneath the forearc area at depths of 70–100 km, indicating that the distribution of the upper plane earthquakes is non‐uniform. The location of the deeper limits of this belt and seismicity of the upper seismic plane appear to correspond respectively to two facies boundaries where H2O contents change in the slab crust. Events in the upper seismic plane have mainly down‐dip compression‐type focal mechanisms but several events have normal fault‐type (NF‐type) versions, whose spatial distribution appears to correspond to these boundaries. These NF events might be induced by the tensional stress field that is caused by volume reduction due to dehydration reactions.

Tarapacá intermediate‐depth earthquake (Mw 7.7, 2005, northern Chile): A slab‐pull event with horizontal fault plane constrained from seismologic and geodetic observations

A large (Mw 7.7) intermediate‐depth earthquake occurred on 13 June 2005 in the Tarapacá region of the northern Chile seismic gap. Source parameters are inferred from teleseismic broadbands, strong motions, GPS and InSAR data. Relocated hypocenter is found at ∼98 km depth within the subducting slab. The 21‐days aftershock distribution, constrained by a postseismic temporary array, indicates a sub‐horizontal fault plane lying between the planes of the double seismic zone and an upper bound of the rupture area of 60 km × 30 km. Teleseismic inversion shows a slab‐pull down dip extension mechanism on a nearly horizontal plane. Total seismic and geodetic moments are ∼5.5 × 1020 N.m, with an averaged slip of 6.5 m from geodesy. The earthquake rupture is peculiar in that the effective velocity is slow, 3.5 Km.s−1 for a high stress‐drop, 21–30 MPa. We propose that rupture was due to the reactivation by hydraulic embrittlement of a inherited major lithospheric fault within the subducting plate. The stress‐drop suggests that the region of the slab between planes of the double seismic zone can sustain high stresses.

The Taiwan‐Ryukyu subduction‐collision complex: Folding of a viscoelastic slab and the double seismic zone

The termination of the Ryukyu trench against Eurasia and the oblique subduction of the Philippine Sea plate create a subduction‐collision complex offshore Taiwan, which has not previously been elucidated in detail. We combine traveltime data from the seismic networks in Taiwan and Japan to better illuminate how the subducting Ryukyu slab deforms in this subduction‐collision zone. More than 5000 events recorded by both networks were relocated with the double‐difference method using an optimal regional one‐dimensional velocity model. The offshore seismicity indicates that the double seismic zone, with a gap of 15–20 km, exists in the subducting slab in the depth range of 40–80 km. Focal mechanisms suggest that the double seismic zone is caused by east‐west compression resulting from oblique convergence. The improved hypocentral locations for the first time reveal folding of the slab into a horizontal curvature larger in magnitude than and opposite in sign to that of the Ryukyu trench in the depth range 50–100 km. The anomalous curvature, together with the focal mechanisms, suggests that the slab folds against the Eurasian lithosphere and that this deformation cannot be fully elastic. We model this deformation mode as the developing instability of a viscoelastic Maxwell layer embedded in a viscous medium. The characteristic wavelength of the instability, i.e., ∼250 km, is consistent with folding of a slab whose viscosity is 100 times higher than that of the surrounding mantle for an along‐strike elastic membrane strain as small as 0.01, or more than 3 orders of magnitude higher if 5% elastic strain is allowed.

Kinetics of antigorite dehydration: A real-time X-ray diffraction study

Antigorite dehydration has been studied in situ using time-resolved X-ray diffraction (XRD) in the 1.1–5.5 GPa pressure range. The sequence of XRD spectra reveals that the high-temperature stability limit of antigorite is decreased by 50–100 °C under H 2O-unsaturated conditions. The decomposition of antigorite into the high-temperature products, forsterite + enstatite, proceeds via an intermediate assemblage of forsterite and a “talc-like” phase, observed within a temperature interval of 130 ± 20 °C. The analysis of the transformation-time data using the Avrami model suggests that the breakdown of antigorite and the “talc-like” phase is kinetically controlled by surface growth processes at the edges of grains. The overall transformation rates are 10 to 100 times faster than those observed in water-saturated experiments, and show that the H 2O activity is a strong driving force for dehydration. The breakdown of antigorite in subducting oceanic mantle with such kinetics would result in a fluid discharge rate of the order of 10 − 6 to 10 − 8 s − 1 . This is faster than the viscous relaxation of serpentinites, and could lead to brittle failure or weakening of pre-existing faults. This provides an explanation for the seismicity in the lower plane of double seismic zones.

Hypocenter distribution of plate boundary zone off Fukushima, Japan, derived from ocean bottom seismometer data

Abstract Microearthquake observation using Ocean Bottom Seismometers (OBSs) was carried out to obtain a detailed distribution of microearthquakes beneath the area off Fukushima, in the middle section of the Japan Trench in the summer of 1997. The observation period spanned approximately one month. Almost all of the well-determined hypocenters occurred in the vicinity of the plate boundary in this region (approximately 100 km landward of the trench axis), while seismicity is markedly lower between this area and the trench itself. The seaward limit of the high seismicity region is close to the western end of the zone of direct contact between the oceanic crust and the overriding landward crust. Twenty-nine earthquakes were recorded within the overriding landward plate. Twelve earthquakes were recorded about 30 km below the plate boundary, and form a landward dipping plane that appears to be an up-dip continuation of the lower plane of the double seismic zone. The microseismicity characteristics of the plate boundary region are interpreted to be controlled by the geometry and physical properties of the plate interface. The seismic activity in the lower seismic plane near the trench is considered to relate to bending of the subducting plate and to dehydration of serpentinized Pacific Plate mantle.

Characteristic seismic activity in the subducting plate boundary zone off Kamaishi, northeastern Japan, revealed by precise hypocenter distribution analysis using ocean‐bottom seismometers

High seismic activity prevails along the plate boundary to the east of northeastern Japan. To understand how this seismic activity is related to subduction process, hypocenter locations are re‐determined using data obtained over 6 years by fiber‐cabled permanent ocean bottom seismometers off Kamaishi. The double‐difference method is adopted to obtain the relative location in more detail. As a result of this analysis, a number of seismic clusters including small repeating earthquakes are identified along the plate boundary. This observation supports the hypothesis that seismic coupling is very low in this region and there only small asperities are distributed along the plate boundary off Kamaishi. A number of events with down‐dip compressional focal mechanisms are identified, corresponding to the upper plane seismicity of the double seismic zone in this area. Although most of these events are located in the crust of the subducting Pacific Plate, some are located in the mantle of the slab.

Imaging of three-dimensional small-scale heterogeneities in the Hidaka, Japan region: coda spectral analysis

SUMMARY While large-scale heterogeneities have been studied intensively by seismic tomography with traveltime data, small-scale heterogeneities are not fully investigated yet. We obtained the 3D spatial distribution of small-scale heterogeneities as relative scattering coefficients in the Hidaka, Japan, region using the spatial variation of high-frequency coda power spectra. We analysed 530 seismograms recorded at 62 stations for nine local earthquakes in the frequency range of 1‐32 Hz. After correcting source, station and propagation effects, we assigned the spatial and temporal variations of observed power spectra of coda waves into the 3-D distribution of scattering coefficient, based on single and isotropic scattering models. Small-scale heterogeneities are distributed not only in a non-uniform manner but also differently among scale lengths estimated from their frequency dependence. An area of large relative scattering coefficient in 2 and 16 Hz is located at a depth shallower than 45 km in the southwest of the Hidaka Mountains, correlating well with a highly active area of microearthquakes. At a depth of 60‐120 km, two dipping zones of large scattering coefficient are imaged beneath the summit of the Hidaka Mountains. These zones may imply two layers of strong seismic-wave reflection, consistent with the upper and lower planes of the double seismic zone in the subducting Pacific Plate, as determined by the hypocentral distribution in this region.

New constraints on seismicity in the Wellington region of New Zealand from relocated earthquake hypocentres

The Wellington region of New Zealand overlies a strongly coupled portion of the shallow subduction interface between the Pacific and Australian plates. To better understand the active deformation in the area, we perform a double-difference (DD) relocation of 6825 local earthquakes that occurred between 1990 and 2001. Using both cross-correlation (CC) and bispectrum (BS) methods, we calculate high-quality waveform-based differential times (WBDTs) for event pairs. We manage to more than triple the S-wave differential time measurements after reasonably estimating many of the unpicked S arrivals. After relocation, the image of the double seismic zone beneath this region is greatly sharpened. We find several northeast-striking linear seismic features near Lake Wairarapa on the North Island, which may result from slip along normal faults within the subducting Pacific Plate. We further search for repeating events that might define creeping patches along the interplate thrust. Using a 20-s data window for CC, we find 287 event clusters with very high waveform similarities. Most of these clusters contain just a doublet that occurred within a short time period. The failure to find repeating earthquakes along the plate boundary in this 12-yr time period is consistent with the hypothesized strong coupling between the Pacific and Australian plates under the Wellington region.

Stresses along the metastable wedge of olivine in a subducting slab: possible explanation for the Tonga double seismic layer

A numerical calculation of the stresses associated with changes in volume during phase transitions of olivine in a descending slab results in a double layer of high shear stress along the metastable olivine wedge in the depth range 350–460 km. Stress in the upper layer is in-plane tensional and stress in the lower layer is down-dip compressional. The modeled stress field agrees with observations of stress in the Tonga double seismic zone. High shear stress also exists in the slab at depths below the metastable wedge. This stress distribution involves down-dip compression and trench-parallel tension, which agrees with about half of the focal mechanisms in the Tonga slab at depths of 460–690 km. The model supports the idea that at least two possible stress release mechanisms for deep earthquakes may act in the Tonga subducting slab. One, transformational faulting, is restricted to the metastable wedge while the other one acts below the metastable wedge.

震源分布からみた伊勢湾から四国西部にかけてのフィリピン海スラブの形状

We inferred minutely the geometry of the upper surface of the seismic Philippine Sea (PHS) slab beneath the region from Ise Bay to western Shikoku, southwest Japan, based on hypocentral distribution. Despite the importance of the slab geometry for the subduction zone seismotectonics as well as for the assessment and forecasting of destructive interplate and slab earthquakes, the shape of the PHS slab has been obscure and controversial. We selected well-determined hypocenters during the period from Oct. 1997 through Dec. 2002 from the integrated hypocenter database prepared by the Japan Meteorological Agency. At first, we composed depth data set of the slab upper surface from vertical cross sections of hypocentral distribution along longitude and latitude of every 0.1 degree, and drew first-trial isodepth contours of the upper surface of the PHS slab assuming that the upper boundary of slab earthquake distribution roughly coincides with the slab upper surface. Then, in many areas we made vertical cross sections of hypocenter distribution in many directions including inferred dip-direction and strike of the slab, and revised isodepth contours. The main results are as follows: (1) In the region to the east of Lake Biwa, northwestward gently-dipping PHS slab reaches around the depth of 35km, in which disastrous slab earthquakes presumably took place. (2) To the northeast it continues to the slab beneath the central Tokai district, but to the southwest it is obscure whether it continues to the slab beneath the eastern Kii Peninsula or it is separated from the latter. (3) The slab beneath the middle part of the Kii Peninsula may be torn into two parts, with the southwestern part underlying the northeastern part. (4) There are double seismic zones beneath the southern Kii Peninsula. (5) In the west part of the Kii Peninsula no slab earthquake occurs in deeper part, whereas just in this area a remarkable shallow crustal swarm earthquake activity exists. (6) Beneath Shikoku the slab is being subducted with gentle dip angle and forms a broad ridge. The slab looks continuous from Shikoku to Kyusyu. Recently the PHS slab geometry has been investigated by seismic refraction/reflection profilings and receiver function analyses. Those results are shallower than our result in eastern Shikoku suggesting that slab earthquakes in this region occur in the oceanic mantle. We relocated slab earthquakes in this region using a velocity structure model consistent with the result of seismic profiling and found that the shallower main activity lays in the slab mantle of the assumed model with deeper remarkable events beneath it. In order to clarify the true slab geometry it is essentially important to know in which part of the slab, oceanic crust or mantle, slab earthquakes are actually taking place in each region.

2003年5月26日宮城県沖地震 (&lt;i&gt;M&lt;/i&gt;7.1) と二重深発地震面上面の地震活動

2003年5月26日宮城県沖地震 (&lt;i&gt;M&lt;/i&gt;7.1) と二重深発地震面上面の地震活動

Focal Mechanisms of Intermediate-depth Earthquakes Beneath Southeastern Hokkaido, Japan Implications of the Double Seismic Zone

In this study, the focal mechanism solutions and spatial distributions of 100 intermediate-depth earthquakes are examined and discussed. These earthquakes occurred from 1985 to 1993 mostly in the upper and lower planes of the double seismic zone beneath the southeastern part of Hokkaido. The focal-mechanism solutions along with the spatial distribution of their epicenters as a function of depth and the minimum- and maximum-compressive stress directions confirm the following essential features; A double-planed deep seismic zone between the depths from 60 to 120 km has been clearly revealed in this region and the number of earthquakes located in the upper plane are less than those for the lower one. Low-angle thrust fault mechanism is dominant to a depth of about 55–60 km in the zone. At depths deeper than about 60 km focal mechanism solutions are mostly characterized by down-dip extension type or down-dip compression type in both lower and upper planes of the zone. Most of the earthquakes deeper than 60 km have dip-slip components and some of the solutions have significant strike-slip components, as well. Although the down-dip extensional-type events are dominant, seven events that are located on the lower plane are constrained as down-dip compression mechanism. The T axes of earthquake focal mechanisms on the lower plane show extension at about N40°E and N35°W, and the dip-angle between 25° and 70°, almost coincide with the dip of the subducting Pacific Plate in this region. We also investigate the statistical features of the stress directions of the earthquakes occurring both on the lower and upper plane after converting all the events onto the plane having the same strike and dip angle as the subducting lithosphere in the region. Seventy-two percent of the minimum compressive stress directions are oriented mainly in the NS or NNE- SSW directions (within 45° of the dip direction) at the plate surface of the lower plane and are almost parallel to the dip of the local slab in this region. The number of earthquakes located in the upper plane is less than that for the lower one. Those which occurred on the upper plane show various types of mechanism solutions. Weak systematic orientation is found in the direction distribution of stress in the upper plane.

Double seismic zone and dehydration embrittlement of the subducting slab

Dehydration embrittlement of metamorphosed oceanic crust and mantle in the subducting slab may be responsible for the occurrence of intermediate‐depth earthquakes. We explore the possibility that this hypothesis can explain the morphology of the double seismic zones observed in northeast Japan, southwest Japan, northeast Taiwan, northern Chile, Cape Mendocino, and eastern Aleutians. We calculate transient temperature structures of slabs based on geologically estimated subduction histories of these regions. We then determine dehydration loci of metamorphosed oceanic crust and serpentinized mantle using experimentally derived phase diagrams. The depth range of the dehydration loci of metamorphosed oceanic crust and serpentine is dependent on slab age. The dehydration loci of serpentine produce a double‐layered structure. Because the upper dehydration loci of serpentine are mostly located in the wedge mantle above the slab, we regard the upper plane seismicity representing dehydration embrittlement in the oceanic crust, and we fix the slab geometry so that the upper plane seismicity is just below the upper surface of the slab. We find that the lower plane seismicity is located at the lower dehydration loci of serpentine, which indicates that the morphology of the double seismic zones is consistent with the dehydration embrittlement.