Shoichi Yoshioka

Interplate coupling in northeast Japan deduced from inversion analysis of GPS data

Abstract The spatial distribution of the strength of interplate coupling between the subducting Pacific plate and the overlying continental plate in northeast Japan was investigated through an inversion analysis of GPS data, employing Akaike’s Bayesian Information Criterion. The GPS data used for the inversion analysis are rates of baseline length changes and vertical displacement rates of crustal movements during the period from April 6, 1996 to March 20, 1998, which were obtained at 161 continuous GPS observation stations operated by the Geographical Survey Institute of Japan. The result shows the average drag rates of the overriding plate on the model source regions off the Tohoku and Hokkaido districts are 6.7 cm/yr and 6.8 cm/yr, respectively, which are 0.85 and 0.87 as interplate coupling ratio. These values are much higher than those in previous studies. The average directions of drag rates are oriented N73°W±15° and N78°W±13° on the model source regions off the Tohoku and Hokkaido districts, respectively. These directions agree well with the direction of coseismic slip of the foot-wall side (N75°W) of the 1978 Miyagiken–Oki earthquake ( M 7.4), but differ slightly from the direction estimated from the plate motion model (N65°W) [Seno et al., J. Geophys. Res. 101 (1996) 11305–11315]. We also found afterslip of 5.5 cm/yr associated with the 1994 Sanriku–Haruka–Oki earthquake ( M 7.5) off Aomori prefecture. Interestingly enough, large afterslip occurred on the plate boundary even when more than a year had passed after the Sanriku–Haruka–Oki earthquake. The amount of drag rate becomes dramatically small at depths greater than about 50 km on the model source regions, indicating weak interplate coupling there.

Three-dimensional numerical modeling of detachment of subducted lithosphere

Recent seismological studies suggest that slab detachment has occurred in the Mediterranean and the New Hebrides subduction zones. Subducted slabs in these regions are recognized to be torn at depths ranging from 100 to 300 km, presumably caused by the lateral migration of the tear along the strike of the slab. To investigate the physical mechanism of the slab detachment and in particular its migration, we constructed a viscoelastic three-dimensional finite element model and introduced a small initial tear from one side of the slab. We investigated spatio-temporal variations in the state of stress within the slab, as a function of tear length, rheology, and a variety of force distribution. Our results show that an area of high shear stress concentration of the order of several hundred megapascals forms near the tip of the tear inside the slab, which is probably sufficient to cause further lateral migration of the tear. The stress concentration increases with the length of the tear and lower viscosity values of the surrounding mantle and increases with downdip tension. From our modeling, we conclude that favorable conditions for slab detachment are characterized by a high interplate frictional force at a subduction zone and a low convergence rate, forming in-plate tensional stress at intermediate depths. Such a condition is indeed observed in the Dinarides/Hellenic and the New Hebrides subduction zone.

Interplate coupling in southwest Japan deduced from inversion analysis of GPS data

Abstract Recently, the Geographical Survey Institute of Japan completed the installation of a GPS continuous observation network in Japan, which has enabled us to investigate real-time crustal movements. In this study, we attempt to obtain spatial distribution of interplate coupling and relative plate motion between subducting and overriding plates in southwest Japan, using horizontal and vertical deformation rates, which were observed at 247 GPS observation stations during the period from April 6, 1996 to March 20, 1998. For this purpose, we carried out an inversion analysis of geodetic data, incorporating Akaikes Bayesian Information Criterion (ABIC). As a result, strong interplate coupling was found off Shikoku and Kumanonada regions, which corresponds well with the fault regions of the 1946 Nankai (M 8.1) and the 1944 Tonankai (M 8.0) earthquakes, respectively. We also found that interplate coupling becomes weak at depths deeper than about 30 to 40 km beneath the Shikoku and Kii peninsula. The recurrence time of great trench-type earthquakes was roughly estimated as 107 years, which is consistent with previous research. The direction of relative plate motion is oriented N53°W, which is close to the direction predicted from the plate motion model. On the other hand, a large forward slip was found in the Hyuganada region off southeast of Kyushu. Since the coseismic displacements associated with the two 1996 Hyuganada earthquakes (M 6.6, M 6.6) are removed from the GPS data, this suggests that after-slip occurred near the source region and/or that Kyushu moves southeastward stationarily due to other tectonic forces.

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

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

Stress fields associated with metastable phase transitions in descending slabs and deep-focus earthquakes

Inside subducting slabs the interaction of different metastable phase transformations (α-olivine → β-spinel, β-spinel → γ-spinel, γ-spinel → perovskite + magnesiowustite) associated with the latent heat release and absorption for these transitions can result in a complex thermal slab structure. Thermokinetic coupling processes cause thermal anomalies and buoyancy contrasts between slab and mantle that strongly influence the stress field within subducting plates. We have described the thermal field within the downgoing slab in a Lagrangian framework by a self-consistent, high-resolution, two-dimensional thermokinetic coupling model, which also incorporates the effects of nucleation site saturation. Using an adaptive method, the threshold for the nucleation site saturation is found to be less than 0.1% transformation degree in the cold slab interior. This can significantly shift the metastable wedge of α-olivine deeper, as suggested from previous thermokinetic slab models. However, this effect is compensated by the latent heat release. During the α-olivine → β-spinel and β-spinel → γ-spinel phase transitions, latent heat release produce very sharp phase boundaries in the slab. For fast slabs the phase boundaries reveal significant metastable perturbations from the equilibrium state in the cold interior. However, the strong thermal interaction between both ongoing phase transitions results in sharp overlapping phase boundaries with dramatic consequences for the slab stress field. We have investigated the detailed physical properties and the state of stress in the deeper portion of a subducting plate by using up-to-date temperature field and physical properties determined from recent high P-T experiments. We find that the slab is denser up to 100 kg m−3 than the surrounding mantle. Remarkable denser portions up to 260 kg m−3 are located just below the uplifted α → β and β → γ equilibrium phase transitions, while lighter portions up to −210 kg m−3 can be found just above the depressed γ-spinel to perovskite + magnesiowustite transition and for the metastable wedge of α-olivine. These density differences effectively act as a body force and produce a significant stress field, which we calculated with a two-dimensional finite-element code. The results show that the buoyancy-induced forces produce maximum shear stress up to 23 MPa along the metastable wedge and deeper portion just above the depressed last phase transition involving γ-spinel. For the latter, the dominant state of stress is down-dip compression. The calculated P-T dependent state of stress is very similar to the depth distribution of deep-focus

A dike intrusion model in and around Miyakejima, Niijima and Kozushima in 2000

From June 26, 2000, an earthquake swarm started in Miyakejima, about 50 km south off Honshu, central Japan. Eruptions of Miyakejima and five large earthquakes with magnitudes 6.0 and above occurred for the following 2 months together with a large number of smaller earthquakes over 100,000. In this study, we focused on spatio-temporal crustal deformation observed at GPS stations in this area. In order to explain the seismicity and the crustal deformation, we considered a dike intrusion model placed between Miyakejima and Kozushima. Then, we attempted to obtain spatio-temporal distributions, the amount of the dike intrusion, deflation beneath Miyakejima, and fault slips of the seven large events by geodetic data inversion. For this purpose, we divided the time series of GPS data from June 12 to August 27 into 10 periods, which are related to significant events. In order to find location, depth, strike and dip of the dike plane and a depth of deflation beneath Miyakejima, we used the Monte Carlo method and tested 10,000 models for each period. As a result, we estimated that the total amount of the dike intrusion and the deflation beneath Miyakejima reached about 1.1 � 10 9 and 5.4 � 10 8 m 3 , respectively. The maximum amount of the dike intrusion and the deflation beneath Miyakejima reached over 3.5 � 10 8 m 3 during the period from July 20 to July 28, 2000, and over 1.7 � 10 8 m 3 during the period from June 15 to June 28, 2000, respectively. Temporal change of the amount of the dike intrusion corresponds well to that of the deflation beneath Miyakejima until the middle of July. However, since large amount of the dike intrusion from July 20 to 28 did not correspond to that of the deflation beneath Miyakejima, we deduced that the magma source changed from Miyakejima to Kozushima and the magma might come from sub-crustal magma pockets from the middle of July. If we assume the open crack associated with the dike intrusion and the deflation beneath Miyakejima are filled with magma with a density of 2500 kg/m 3 , the mass would be about 2.75 � 10 9 and 1.35 � 10 9 tons, respectively. We deduced that at least 1.4 � 10 9 tons of magma came from sub-crustal magma pockets. D 2002 Elsevier Science B.V. All rights reserved.

Interplate coupling in the Kanto district, central Japan, deduced from geodetic data inversion and its tectonic implications

Abstract Interplate coupling between the Philippine Sea (PHS), the North American (NAM), and the Pacific (PAC) plates in the Kanto district, central Japan, has been investigated through the inversion analysis of geodetic data using Akaikes Bayesian Information Criterion (ABIC). The data used for the analysis are annual rates of level changes (1972–1985) and horizontal length changes (1973–1990), which presumably represent average crustal movements during the interseismic period. Disregarding effects of steady plate subduction, we may ascribe the observed crustal movements to the effects of locking or faulting on some parts of the plate boundaries. In our model, the effect of locking is represented by back slip on the plate boundary, and that of faulting by forward slip. The results of the inversion analysis show the existence of a strongly coupled area on the southwestern, shallower part of the NAM-PHS plate boundary, which almost coincides with the faulting area of the 1923 Kanto earthquake ( M 7.9), and a notable faulting area on the northeastern, deeper part of the PHS-PAC plate boundary. In the southwestern strongly coupled area, the rate of back slip reaches 3.5 cm/yr, and its direction is oriented N33°W, which is almost opposite to the direction of fault slip (S29°E) at the time of the 1923 event. This suggests that tectonic stress accumulation for the next large event is effectively proceeding there, and its recurrence time is roughly estimated as 245 years. The strongly coupled region is also recognized as corresponding to a recent seismic gap. The inverted forward-slip distribution on the PHS-PAC plate boundary, which has the average rate of 1.9 cm/yr and the average direction of N56°E, may be explained by aseismic northeastward slip in relation to the process to form a peculiar configuration of downward bend of the PHS plate beneath the Kanto district.

Source parameters controlling the generation and propagation of potential local tsunamis along the cascadia margin

The largest uncertainty in assessing hazards from local tsunamis along the Cascadia margin is estimating the possible earthquake source parameters. We investigate which source parameters exert the largest influence on tsunami generation and determine how each parameter affects the amplitude of the local tsunami. The following source parameters were analyzed: (1) type of faulting characteristic of the Cascadia subduction zone, (2) amount of slip during rupture, (3) slip orientation, (4) duration of rupture, (5) physical properties of the accretionary wedge, and (6) influence of secondary faulting. The effect of each of these source parameters on the quasi-static displacement of the ocean floor is determined by using elastic three-dimensional, finite-element models. The propagation of the resulting tsunami is modeled both near the coastline using the two-dimensional (x-t) Peregrine equations that includes the effects of dispersion and near the source using the three-dimensional (x-y-t) linear long-wave equations. The source parameters that have the largest influence on local tsunami excitation are the shallowness of rupture and the amount of slip. In addition, the orientation of slip has a large effect on the directivity of the tsunami, especially for shallow dipping faults, which consequently has a direct influence on the length of coastline inundated by the tsunami. Duration of rupture, physical properties of the accretionary wedge, and secondary faulting all affect the excitation of tsunamis but to a lesser extent than the shallowness of rupture and the amount and orientation of slip. Assessment of the severity of the local tsunami hazard should take into account that relatively large tsunamis can be generated from anomalous ‘tsunami earthquakes’ that rupture within the accretionary wedge in comparison to interplate thrust earthquakes of similar magnitude.

Displacement fields due to the 1946 Nankaido earthquake in a laterally inhomogeneous structure with the subducting Philippine Sea plate—a three-dimensional finite element approach

Abstract The displacement fields associated with the 1946 Nankaido earthquake have been investigated in detail, taking into account the configuration of the Philippine Sea plate subducting beneath southwest Japan. We used a three-dimensional finite element method for the elastic problem to examine the effects of three-dimensionally inhomogeneous structure. Some differences in the displacement patterns between homogeneous and inhomogeneous media have been identified. The amounts of displacements are large in low-rigidity portions such as in the crust and mantle wedge, and are small in a high-rigidity portion composed of the subducting plate. The inhomogeneity affects horizontal displacements at the Earths surface more than vertical displacements. If the rigidity and the Poisson ratio given here to southwest Japan are appropriate, the fault parameters of the earthquake, which have been estimated from simple dislocation models in a homogeneous semi-infinite medium, need to be modified somewhat. The present computations indicate that larger dip angles and smaller fault slips are more appropriate fault parameters when structural inhomogeneity is taken into account. This suggests that the discrepancy in the obtained total seismic moment between the geodetic and seismic models of the Nankaido earthquake could be reduced.

The interplate coupling and stress accumulation process of large earthquakes along the Nankai trough, southwest Japan, derived from geodetic and seismic data

Abstract The interplate coupling and stress accumulation process associated with the subduction of the Philippine Sea plate beneath the Eurasian plate along the Nankai trough, where large earthquakes have occurred periodically, have been investigated in detail, taking into account the three-dimensional complicated structure of the Philippine Sea plate and its motion relative to the Eurasian plate. For this purpose, we mainly used extensive geodetic data in southwest Japan, such as levelling, tide gauge and trilateration, which presumably are effective means of revealing coupling properties, and applied a three-dimensional finite element method. Here, two seismic periods associated with the 1944 Tonankai and 1946 Nankaido earthquakes were considered—interseismic and pre-seismic stages. Comparing the coupled regions obtained for the two cases, a possible explanation of its temporal change was proposed: it might gradually spread from a shallower to deeper portion, with transition from the interseismic to pre-seismic stage. A comparison of the coupling properties between southwest Japan and northeast Japan reveals the following properties. In southwest Japan, the coupling is generally strong, and its region is limited in a relatively shallow portion. The pre-seismic coupled regions coincide with the co-seismic and post-seismic fault zone, suggesting that the accumulated stress is released at the time of a large earthquake. In northeast Japan, on the other hand, the coupling is weak and its extent appears to reach a depth of about 100 km, and the co-seismic and post-seismic regions are limited to the upper half of the coupled region. The weakly coupled region in southwest Japan seems to correspond well to the region with high seismic activity, a high dip angle subducting slab and the possible existence of seamounts, in contrast to the strongly coupled region correlated with low seismicity, low dip angle subduction and no seamounts. On the western coupled region off Shikoku, an anomalously strongly coupled region, corresponding to a convergence rate of 7–8 cm year −1 , was identified. This may be caused by geomorphological properties, such as the low dip angle of the subducting plate and smooth plate boundary, resulting in a large amount of stress concentration there. The stress field associated with the interplate coupling was also calculated. There is fair agreement between the observations and calculations near the plate boundary beneath the southern Kii peninsula and Shikoku. Although the effects of the interplate coupling may have some influence on strike-slip faulting with the N-S compressive axes beneath Shikoku, other earthquake generating stress, such as a two-layered double-buckling, would be dominant there.