Deep Seismic Zone Research Articles

Deep magma storage during the 2021 La Palma eruption.

The 2021 La Palma eruption provided an unpreceded opportunity to test the relationship between earthquake hypocenters and the location of magma reservoirs. We performed density measurements on CO2-rich fluid inclusions (FIs) hosted in olivine crystals that are highly sensitive to pressure via calibrated Raman spectroscopy. This technique can revolutionize our knowledge of magma storage and transport during an ongoing eruption, given that it can produce precise magma storage depth constraints in near real time with minimal sample preparation. Our FIs have CO2 recorded densities from 0.73 to 0.98g/cm3, translating into depths of 15 to 27km, which falls within the reported deep seismic zone recording the main melt storage reservoir.

Focal depth estimates of earthquakes in the Himalayan-Tibetan region from teleseismic waveform modeling

We estimate the focal depths and fault plane solutions of 46 moderate earthquakes in the Himalayan-Tibetan region by modeling the broadband waveforms of teleseismic P waves. The depths of 38 of these earthquakes range between 0–40 km, with a peak at ∼5 km. One earthquake is located within the lower crust of the Indian shield. The remaining eight earthquakes occurred between depths of 80–120 km and are all located in the Pamir-Hindu Kush and the Indo-Myanmar deep seismic zones. None of the earthquakes outside these deep seismic zones are located in the mantle. Global centroid moment tensor (CMT) solutions indicate that most earthquakes in northern Tibet and northern India had thrust-faulting mechanisms and that normal and strike-slip faulting earthquakes occurred primarily in central Tibet. These mechanisms are consistent with the predominantly NNW-SSE compression in the direction of current Himalayan-Tibetan continental collision.

日本海溝周辺における太平洋プレート内の地震発生機構

A double-planed shallow seismic zone has been found in the northeastern Japan forearc region. However, the characterizations of the focal mechanisms of earthquakes in this zone, especially the lower plane events, cannot be carried out adequately due to low seismicity and poor station coverage on the focal sphere of the onshore P-wave polarity data. In this study, we determine the focal depth using the sP depth phase and the focal mechanisms using the P-wave initial motions observed by the ocean bottom cabled seismic stations and temporary autonomous ocean bottom seismic networks off Miyagi, as well as the onshore seismic networks. Seven focal mechanism solutions of events were precisely determined. Using the classification based on the dip angles of the T, B and P axes, we classified 21 weakly constrained focal mechanisms of other events having poor station coverage. All the determined solutions were of the thrust-faulting type regardless of the focal depth. The offshore observations helped considerably in constraining the focal mechanisms of these far-offshore earthquakes, especially their rake angles. Although previous studies had reported that the focal mechanisms of the upper and lower plane events show predominantly normal and reverse faulting respectively, our result showed that the thrust events in the upper plane seem to occur on the plate boundary. We found that the focal mechanisms change from normal faulting in the upper plane to reverse faulting in the lower plane at a depth of 15km from the plate boundary, possibly defining the depth of the neutral plane in the northeastern Japan forearc region included in the source area of the 1933 Mw8.4 Sanriku earthquake. The western edge of the normal faulting events along the upper plane is located about 70km inward from the trench axis. These results suggest that the earthquake-generating stress field in the double-planed shallow and deep seismic zone in the northeastern Japan arc can be explained by the bending-unbending model of the subducting Pacific plate.

Mantle structure and dynamics under East Russia and adjacent regions

We present seismic images of the mantle beneath East Russia and adjacent regions and discuss geodynamic implications. Our mantle tomography shows that the subducting Pacific slab becomes stagnant in the mantle transition zone under Western Alaska, Bering Sea, Sea of Okhotsk, Japan Sea, and Northeast Asia. Many intraplate volcanoes exist in these areas, which are located above the low-velocity zones in the upper mantle above the stagnant slab, suggesting that the intraplate volcanoes are related to the dynamic processes in the big mantle wedge above the stagnant slab and the deep slab dehydration. Teleseismic tomography revealed a low-velocity zone extending down to 660 km depth beneath the Baikal rift zone, which may represent a mantle plume. The bottom depths of the Wadati–Benioff deep seismic zone and the Pacific slab itself become shallower toward the north under Kamchatka Peninsula, and the slab disappears under the northernmost Kamchatka. The slab loss is considered to be caused by the friction between the slab and the surrounding asthenosphere as the Pacific plate rotated clockwise at about 30 Ma ago, and then the slab loss was enlarged by the slab-edge pinch-off by the hot asthenospheric flow and the presence of Meiji seamounts.

Offshore double-planed shallow seismic zone in the NE Japan forearc region revealed by sP depth phases recorded by regional networks

SUMMARY We detected the sP depth phase at small epicentral distances of about 150 km or more in the seismograms of shallow earthquakes in the NE Japan forearc region. The focal depths of 1078 M > 3 earthquakes that occurred from 2000 to 2006 were precisely determined using the time delay of the sP phase from the initial P-wave arrival. The distribution of relocated hypocentres clearly shows the configuration of a double-planed shallow seismic zone beneath the Pacific Ocean. The upper plane has a low dip angle near the Japan Trench, increasing gradually to ∼30° at approximately 100 km landward of the Japan Trench. The lower plane is approximately parallel to the upper plane, and appears to be the near-trench counterpart of the lower plane of the double-planed deep seismic zone beneath the land area. The distance between the upper and lower planes is 28–32 km, which is approximately the same as or slightly smaller than that of the double-planed deep seismic zone beneath the land area. Focal mechanism solutions of the relocated earthquakes are determined from P-wave initial motion data. Although P-wave initial motion data for these offshore events are not ideally distributed on the focal sphere, we found that the upper-plane events that occur near the Japan Trench are characterized by normal faulting, whereas lower-plane events are characterized by thrust faulting. This focal mechanism distribution is the opposite to that of the double-planed deep seismic zone beneath the land area. The characteristics of these focal mechanisms for the shallow and deep doubled-planed seismic zones can be explained by a bending–unbending model of the subducting Pacific plate. Some of relocated earthquakes took place in the source area of the 1933 Mw8.4 Sanriku earthquake at depths of 10–23 km. The available focal mechanisms for these events are characterized by normal faulting. Given that the 1933 event was a large normal-fault event that occurred along a fault plane dipping landward, the earthquakes that currently occur just beneath or oceanwards of the Japan Trench are probably its aftershocks, suggesting that aftershock activity continues to the present day, 70 years after the main shock.

Stress due to the interseismic back slip and its relation with the focal mechanisms of earthquakes occurring in the Kuril and northeastern Japan arcs

AbstractAssuming a uniform and inhomogeneous distribution of back slip, based on a realistic configuration of the plate boundary that was newly determined from the distribution of earthquakes, we have investigated if the stress due to the interseismic back slip can explain the focal mechanisms of earthquakes occurring in the Kuril and northeastern Japan arcs. The inhomogeneous back-slip model was taken from the distribution of slip deficit estimated from GPS data collected in recent years. We examined the change in the Coulomb Failure Function (ΔCFF) for several target faults to evaluate the fitness of the models to the observed focal mechanisms. The results of ΔCFF are clearly different in the two back-slip models near the down-dip end of the locked zone. Despite this difference, the models can generally explain the earthquake focal mechanisms in the overriding plate as well as those on the megathrust plate boundary. However, they cannot explain the focal mechanisms of intermediate-depth earthquakes in the double-planed deep seismic zone. Neither can they fully explain the focal mechanisms with the pressure axis parallel to the trench that are observed at the junction of the Kuril and northeastern Japan arcs. Other tectonic processes, such as the transcurrent movement of the forearc sliver along the Kuril arc, may be involved in producing the stress anomaly.

Isolated deep earthquakes beneath the North Island of New Zealand

Seismicity shallows towards the south along the Tonga-Kermadec-Hikurangi margin, deep and intermediate seismicity being absent altogether in the South Island of New Zealand. Beneath the Taranaki region of the North Island the maximum depth of the main seismicity is 250 km, but very rare events occur directly below at 600 km. These could be associated with a detached slab or a vertical, aseismic continuation of the subducted Pacific Plate. Six small events that occurred in the 1990s were recorded extensively by digital instruments of the New Zealand National Network (NZNN) and temporary deployments. We relocate these events by a joint hypocentre determination (JHD) method and find their focal mechanisms using first motions and relative amplitudes of P and S arrivals. The earthquakes relocate to a remarkably uniform depth of 603 +/- 3 kmrelative error (+/- 10 km absolute error) in a line 30- km long orientated 40 NE, roughly parallel to the strike of the intermediate- depth seismicity. The only consistent component of the focal mechanisms is the tension axis: all lie close to horizontal and tend to align with the line of hypocentres. We interpret this deep seismic zone as a detached sliver of plate lying horizontally with the same orientation as the main subducted plate above. Volume change caused by a phase change controlled by the pressure at 600 km and temperature in the sliver produces a pattern of strain that places the sliver under tension along its length

Dehydration and earthquakes in the subducting slab: empirical link in intermediate and deep seismic zones

In this study, a close link between the depth-distribution of subduction-zone earthquakes and dehydration events in the hydrated slab-peridotite is established. A phase diagram up to 27 GPa in the MgO–Al 2O 3–SiO 2–H 2O (MASH) system was constructed using a combination of thermodynamic calculation and Schreinemakers analysis on previous experimental data. At intermediate-depths (<250 km), dehydration of antigorite, talc, brucite, clinochlore, and Mg-sursassite occur. In the 200–500 km depth-range, dehydrations of Mg-sursassite and brucite+phase D are possible. Dehydration of brucite and dense hydrous Mg-silicates (phase E, D and superhydrous phase B) occurs in the mantle-transition zone. Since conditions for dehydration are dependent on slab temperature, we constructed diagrams illustrating the distribution patterns of dehydration events at several model slab-temperatures. In parallel to the mineralogical study, the depth-distribution of global subduction-zone earthquakes are compiled into a thermal parameter-depth diagram and compared with the dehydration event distributions determined previously. The distribution of dehydration events and earthquakes are comparable in terms of: (1) non-linear correlation between the maximum depth of earthquake and temperature of the slab, (2) an aseismic zone at 300–600 km depth in medium- T subduction zones, and (3) lack of deep-earthquakes in young subduction-zones. Considering those links, we propose an extended dehydration-induced earthquake (EDIE) hypothesis that considers any dehydration event has a potential to induce earthquakes in the subducting slab peridotite.

A Long-Lasting Relaxation of Seismicity at Aswan Reservoir, Egypt, 1982-2001

The Aswan reservoir seismicity is accepted as an example of reservoir- induced seismicity with the M 5.4 event of 1981, which occurred 15 years after the reservoir impoundment started. During the 1982-2001 period, the Aswan seismicity separates into shallow and deep seismic zones, between 0 and 15 and 15 and 30 km, respectively. These two seismic zones behave differently over time, as indicated by the seismicity rate, depth migration, b-value, and spatial clustering. For the deep events, the rate decreases with time, the depth remains at a constant or increasing depth, and the b-value is lower than for the shallow events, which migrate toward the surface with a close to constant seismicity rate. Of all the previous parameters, only the seismicity rate correlates with seasonal variations of the lake level. This is positive evidence for the Aswan reservoir seismicity to be reservoir triggered in the 1982-2001 period. Clustering over time and space expresses that numerous after- shock sequences are activated, that is, a minimum of 35% of the seismicity, similarly to tectonic seismicity. The observed decrease of the b-value with depth is also a property suggested for natural events. Our results suggest that the Aswan seismicity emerges both from the water-level loading and the interplay between induced earth- quakes themselves through numerous standard aftershock sequences. This latter pro- cess induces stochastic fluctuations in the seismicity patterns that inhibit a recovery of a simple seismic response to water-level changes. Even though aftershock se- quences dominate the temporal fluctuations of seismicity, this does not negate the importance of influence of the reservoir in sustaining this unusual long-lasting seis- mic episode. During the 1982-2001 period, the Aswan seismicity appears as a power- law relaxation response to the 1981 M 5.4 shock, this response being decorated by long-term (larger than 100 days) water-level changes.

Newly imaged shape of the deep seismic zone within the subducting Pacific plate beneath the Hokkaido corner, Japan‐Kurile arc‐arc junction

The geometry of the deep seismic zone associated with the subducting Pacific plate beneath the North American plate in the Hokkaido corner, Japan‐Kurile arc‐arc junction has been investigated based on the hypocenters accurately relocated by a dense local seismographic network with three‐dimensional P and S wave velocity structures. The model suggests that the lateral changes in the dip of the subduction zone are more rapid than previously believed. We found that the deep seismic zone in the Pacific plate dips at 20° to 30° on the side of the Japan arc, the dip angle increased in a narrow region around 143°–144°E by 10° to 20°, and the dip was 40° to 50° on the side of the Kurile arc. In the transitional zone around 143°–144°E an unusual distribution of microearthquakes has been discovered within the Pacific plate, which was located on a near‐vertical plane with an area of 50 × 100 km2 with strike that was perpendicular to the Kurile Trench axis. This feature was named the Tokachi‐Oki slab‐cracking zone. We suggest that this is a slab‐cracking zone torn by the extensional stress due to the rapid lateral changes in the dip of the subduction zone. The slab‐cracking zone may possibly grow into the large disruption or tear of the slab in the future.

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.

Stress State, Seismicity and Subduction Geometries of the Descending Lithosphere Below the Hindukush and Pamir

The paper presents an analysis of spatial distribution of 6600 earthquake events which occurred during the period 1964 to 1999 between latitude 34 to 40°N and longitude 68 to 76°E. This large volume event is reported in the International Seismological Centre (ISC) catalog. In addition to this a total of 248 focal mechanism solutions are considered to derive a generalised predominant stress prevailing in the descending lithosphere below the Hindukush and Pamir regions. The analysis of spatial distribution shows that the epicentres of the events at shallow level (depth<70 km) are sparsely distributed throughout except for a cluster at the northern end of both the Hindukush and Pamir. The concentration of epicentres at intermediate-depth level between 71 and 170 km below the Hindukush takes a strip-like pattern. It trends along SW-NE, and narrows at the northeastern end of the Hindukush. At deeper level (depth>170 km) the epicentres below the Hindukush are mainly concentrated in a triangular-shaped zone, and the mean points of concentration of the epicentres appear to be shifted towards southwest at increasing depth. The distribution of epicentres at the intermediate and deeper layers of the Pamir is observed to be diffused except a cluster of few events in each layer appears to be shifted towards south-southeast at increasing depth. The distribution of hypocentres changes its concentration from lesser to considerably higher at about 70 km depth, and further takes a minimum at about 170 km depth below the Hindukush and Pamir. The present study further involves in analysing the composite/group effect of stresses associated with the descending lithosphere below the Hindukush and Pamir after deriving the best-fit generalized predominant directions of stresses. It shows that the intermediate-depth seismic zone below the Hindukush is acted upon by maximum compressive stresses (P axes) from two directions while the deeper-depth zone from three directions, and may convincingly be correlated with the changing shape of the respective seismic zones. Another interesting phenomenon observed here is the change in direction of maximum compressive stresses in clockwise fashion from intermediate to deep seismic zones below the Hindukush. At shallow depths below the Pamir the maximum and minimum (T axes) compressive stresses are acting almost along NNW-SSE and ENE-WSW and are oriented horizontally. T-axes for few events at these depths show almost vertical orientation. The observed down-dip extension is predominantly parallel with the descending lithosphere below the Hindukush. The entire analysis along with the observed scattering of P- and T-axes of some events at intermediate-depths might be indicating a slight contortion of the middle layer below the Hindukush. The spatial distribution of seismicity and the generalised stress pattern of both the regions infer the existence of two-isolated subducting lithosphere. It perhaps has created the eastward expulsion or lateral extrusion of Tibet along the major strike-slip faults like Karakorum, Altyn-Tagh, Kunlun and Red River. Finally, the whole analysis confirms the existence of shield-like continental rigid slab at depths greater than 170 km below the Hindukush.

2-D viscoelastic FEM simulation on stress state in the deep part of a subducted slab

The characteristics of the stress fields in deep subducting slabs are studied using viscoelastic plain strain finite element method. When introducing the new rheology structure given by Karato, et al into our computation, there emerge two regions with great shear stress just below the olivine-spinel phase transition zone, which encompass the low viscosity zone below the lower tip of the metastable wedge. Further, the directions of the main compressional stress of these two regions are all along the dip direction of the slab. These are in accordance with the seismic observations that there are two deep seismic zones in a slab and the directions of the main compressional stress in these two seismic zones are along the dip direction of the slab. Smaller effective viscosity probably caused by smaller grain size in the phase transformation zone does not have great influence on the stress state in the deep part of the slab. There is the maximum of shear stress at the transition region from olivine to spinel and the direction of the main compressional stress in this region is roughly perpendicular to the trend of the phase boundary no matter whether there exists metastable wedge, which nevertheless do not correspond to some well-known seismic observations.

東北日本弧下の太平洋プレート内部に発生する稍深発地震活動の時空間変化

In order to interpret various seismic activities in relation to the subduction process of the Pacific plate beneath the northeastern Japan arc, information about the spatio-temporal stress change in the slab is indispensable. In this study, we estimated the stress change by analyzing the activity and focal mechanisms of the intermediate-depth earthquakes in detail. Predominant focal mechanism is of down-dip compression type in the upper plane, while down-dip extension type dominates in the lower plane of the double-planed deep seismic zone. If the overall down-dip compressional stress in the plate increases, it is expected that the upper plane seismicity becomes active while the lower plane activity is suppressed. Thus, we can qualitatively estimate the stress change in the plate from the seismicity. For this purpose, we analyzed Z-value which indicated the spatio-temporal change in the seismicity, and R-value which indicated the ratio of earthquake occurrence frequencies between upper plane and lower plane. Both analyses revealed that the relative frequency of earthquakes in the upper plarle to that in the lower plane increased after large earthquakes that occurred on the plate boundary. Z-value distribution indicates that the upper plane seismicity had been active in the westward extension of the after-slip region of the 1994 Far-off Sanriku earthquake during the period of about 1.5 years after the event. Moreover, normal fault type earthquakes near the slab surface disappeared in the same period. These phenomena suggest that down-dip compressional stress in the plate increased by the large earthquake on the plate boundary.

Seismological constraints on the mechanism of deep earthquakes: temperature dependence of deep earthquake source properties

Seismological observations are of vital importance for understanding the mechanism of deep earthquakes. Most of the seismological observables for earthquakes deeper than 300 km are similar to shallow earthquakes. Deep earthquakes clearly represent shear failure on a planar surface as shown by their double couple mechanisms. Deep earthquake aftershock sequences show temporal decay rates and magnitude–frequency relations ( b-values) that are similar to shallow earthquakes, with the aftershocks occurring preferentially along the mainshock fault planes. In addition, deep earthquake rupture velocities are similar to those of shallow earthquakes. However, there are also some observations that are clearly distinctive relative to shallow earthquakes. Stress drops of deep earthquakes show a large variation but are larger, on average, than shallow earthquakes. Different deep seismic zones show very different b-values, in contrast to shallow earthquakes, which show similar b-values worldwide. In addition, deep earthquakes show fewer aftershocks than shallow earthquakes. A variety of observations suggest that deep earthquakes are highly sensitive to the temperature of the slab. Both deep earthquake b-values and the rate of deep earthquake aftershock occurrence are inversely correlated with the temperature of the deep slabs, suggesting that these factors are temperature-controlled to an extent much greater than with shallow earthquakes. Large deep earthquakes in warm slabs show slower rupture velocities, larger stress drops, and lower seismic efficiencies than similar earthquakes in cold slabs. The width of deep seismic zones is also probably temperature controlled, but deep earthquake rupture can propagate outside the normal limits of Benioff zone seismicity. Simple thermal models for the Tonga slab near the 1994 deep earthquake suggest that the temperature at the rupture termination point was at least 200°C warmer than the temperature that limits smaller earthquakes in the slab. These observations can be used to evaluate physical models for deep earthquakes, including brittle slip along fluid-weakened faults, transformational faulting, and thermal (and perhaps melt lubricated) shear instabilities. It is difficult to explain the large fault widths of some deep earthquakes using a fluid weakening model, since hydrated materials are expected in only a narrow depth zone at the top of the slab, and the fault planes do not have the expected orientations for reactivated faults. The lateral extent of the largest deep earthquakes cast doubt on the transformational faulting model, in which events should be confined within a narrow metastable wedge. Seismological studies have also failed to find evidence for the existence of metastable olivine in slabs. The temperature dependence of deep earthquakes argues in favor of a temperature-activated phenomenon, such as thermal shear instabilities and perhaps fault zone melting.

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.

Three-dimensional attenuation structure beneath the northeastern Japan arc estimated from spectra of small earthquakes

A fine-scale three-dimensional attenuation structure beneath the northeastern Japan arc 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-arc 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.

Different stress directions in the aftershock focal mechanisms of the Kushiro-Oki earthquake of Jan. 15, 1993, SE Hokkaido, Japan, and horizontal rupture in the double seismic zone

On 15 January, 1993, a large intermediate-depth earthquake with a magnitude of M=7.8, shook the eastern part of the island of Hokkaido, Japan, close to the northeastern Japan trench and to the Kuril trench; this earthquake is now referred to as the Kushiro-Oki earthquake. The main shock of the Kushiro-Oki earthquake showed a down-dip extension mechanism having horizontal and vertical nodal planes. The hypocenter was located at a depth of 107 km in the lower plane of the double seismic zone in the subducting Pacific plate. After the mainshock, more than 300 aftershocks were recorded by three seismograph networks. From these data we were able to determine the aftershock hypocenters and 63 focal mechanism solutions for the aftershocks. The aftershock region extended horizontally in a northerly direction from the lower plane towards the upper plane of the double seismic zone in the subducting plate; it did not, however, reach the upper plane. We estimated the maximum and minimum compressional stress directions along the deep seismic zone by converting the mechanism solutions from horizontal plane projections to an oblique plane parallel to the deep seismic zone. Thirty-eight mechanisms were found to have maximum and minimum compressional stress directions very similar to those of the mainshock, but 25 aftershock mechanisms had almost opposite directions to those of the mainshock. The hypocenters of these aftershocks were spatially mixed. This suggests that the stress state is strongly inhomogeneous in the aftershock region, and that horizontal movement occurs by the down-dip extensional stress which is stored in the region between two planes of the deep seismic zone. The remanent down-dip extensional stress was released as the consistent group of aftershocks. High aftershock activity may be attributed to the inhomogeneity of the material in the region. The previous four intermediate-depth earthquakes beneath Hokkaido Island as well as aftershock distributions of two of them have been examined for comparison with the Kushiro-Oki earthquake. The mainshocks of the four large intermediate-depth earthquakes are also located on the lower plane with a similar mechanism to that of the Kushiro-Oki earthquake. The aftershock regions of two of these earthquakes are also distributed horizontally. They indicate dominantly horizontal movement, with down-dip extension mechanisms at an intermediate depth in the subducting Pacific plate beneath Hokkaido Island.

Tomographic inversion for P wave velocity structure beneath the northeastern Japan arc using local and teleseismic data

We have determined three‐dimensional P wave velocity structure up to a depth of 296 km beneath northern Tohoku, Honshu, Japan, by inverting arrival times of local and teleseismic events simultaneously. Velocities in the upper crustal layers correlate well with the surface geology and Bouguer gravity anomalies. Low‐velocity zones are ubiquitous in the crust and the uppermost mantle beneath the Quarternary volcanic areas. A well‐known feature of a pronounced low‐velocity mantle wedge underlain by the dipping high‐velocity (high‐V) Pacific plate is generally recovered over the study area. Strong heterogeneity within the high‐velocity Pacific plate is suggested by the presence of a pronounced low‐velocity anomaly confined to the depths of 130–200 km. The decreased seismicity of intermediate‐focus events may be correlated with the low‐velocity zone. A two‐dimensional inversion scheme was invoked to determine stably the average mantle structure perpendicular to the island arc. In the depths of 50–150 km, the inclined high‐V zone has a thickness of 80–100 km with its top coinciding with the upper plane of the double‐planed deep seismic zone. Below the depth of 200 km, the high‐V zone is shifted toward the bottom of the 80‐km‐thick reference slab. In the intermediate‐depth layer (150–200 km), the high‐V zone is not clearly visible within the 80‐km‐thick reference slab.

A double‐planed seismic zone in Kamchatka from local and teleseismic data

The fine structure of a double‐planed deep seismic zone is studied over a wide area of the Kamchatka peninsula. This prominent feature of deep seismic zone configuration is ascertained through the analysis of microearthquake hypocenters from the local seismic network of the Institute of Volcanology of Kamchatka and 22 focal mechanism solutions from the formal inversion of long‐period P and SH waves for events with mb≥5.5. Additionally, 11 focal mechanism solutions estimated from the first motion of P‐waves and 12 centroid moment tensor solutions of Harvard University are used. The maximum depth of the double seismic zone is 170–180 km. The two planes of seismicity are separated by 40 km at a depth of 50 km, and by 10–15 km at 180 km depth. The focal mechanism solutions of shallow earthquakes show an abrupt change from the thrust events to down‐dip compressional events at approximately 60 km depth at the upper boundary of the descending slab. Within the descending slab, the earthquakes with down‐dip tensional axis form the lower plane of the double‐planed deep seismic zone. Several earthquakes with down‐dip tensional axis are discovered in a narrow area of the upper seismic zone at the depth of about 50 km. The double seismic zone is revealed clearly in the area between ∼52°N to ∼54°N and probably extends up to ∼56°N.