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In this paper, we explore the Western Pacific subduction impact on the geodynamics of the Asian continent. The data on migration of slow strain and earthquakes from the Nankai, Japan and Kuril-Kamchatka segments of the Western Pacific subduction zone deep into mainland Asia are analyzed. The calculations performed on five profiles, crossing the Kuril Islands, the Japanese Archipelago and Sakhalin Island toward the Asian continent, have revealed the transverse migration of earthquakes from the Japan–Kuril-Kamchatka subduction zone. The velocities of hypocenter migration of M ≥ 4.5 earthquakes from the Kuril-Kamchatka Trench via northern and central Sakhalin vary from 6 to 17 km/year, on average, at different depths. The profiles crossing the islands of Hokkaido and Sakhalin show the M ≥ 4.earthquake migration from the Kuril and Japan trenches at velocities of 8–27 km/year. [ABSTRACT FROM AUTHOR]

A modified k-nearest-neighbors method is introduced that provides an efficient nonlinear estimate of the intensity of a point process (field) based on locations of events. The method is applied to perform a detailed statistical analysis of the spatial structure of the seismic intensity field (in the context of this paper, the term "intensity" refers to the intensity of the seismic field, and should not be mistaken for the intensity of ground motion caused by an earthquake). The proposed method requires neither a preliminarily delineated area nor a normalization procedure for the estimates. In contrast to many interpolation methods, estimates based on the proposed method are statistically justified. The "uncertainty relation" between the spatial smoothing effect and random errors is established for the method in an explicit form. A procedure for choosing the number of nearest neighbors k* that controls the effective radius of smoothing is suggested. The proposed method is applied to analyze the seismic intensity field in two seismogenic regions surrounding the Kuril Islands and Japan during the period from 1904 to 2011. Spots of increased seismic activity in these regions are localized by the new method, and some quantitative statistical characteristics of these spots are determined and discussed. [ABSTRACT FROM AUTHOR]

Dense strong motion observation networks provided us with valuable data for studying strong motion generation from large earthquakes. From kinematic waveform inversion of seismic data, the slip distribution on the fault surface of large earthquakes is known to be spatially heterogeneous. Because heterogeneities in the slip and stress drop distributions control the generation of near-source ground motion, it is important to characterize these heterogeneities for past earthquakes in constructing a source model for reliable prediction of strong ground motion. The stress changes during large earthquakes on the faults recently occurring in Japan are estimated from the detailed slip models obtained by the kinematic waveform inversion. The stress drops on and off asperities are summarized on the basis of the stress change distributions obtained here. In this paper, we define the asperity to be a rectangular area whose slip is 1.5 or more times larger than the average slip over the fault according to the previous study for inland crustal earthquakes. The average static stress drops on the asperities of the earthquakes studied here are in the range 6-23 MPa, whereas those off the asperities are below 3 MPa. We compiled the stress drop on the asperities together with a data set from previous studies of other inland earthquakes in Japan and elsewhere. The static stress drop on the asperity depends on its depth, and we obtained an empirical relationship between the static stress drop and the asperity's depth. Moreover, surface-breaking asperities seemed to have smaller stress drops than buried asperities. Simple ground motion simulations using the characterized asperity source models reveal that deep asperities generate larger ground motion than shallow asperities, because of the different stress drops of the asperities. These characteristics can be used for advanced source modeling in strong ground motion prediction for inland crustal earthquakes. [ABSTRACT FROM AUTHOR]

Among the schemes for earthquake forecasting, the search for semi-periodicity during large earthquakes in a given seismogenic region plays an important role. When considering earthquake forecasts based on semi-periodic sequence identification, the Bayesian formalism is a useful tool for: (1) assessing how well a given earthquake satisfies a previously made forecast; (2) re-evaluating the semi-periodic sequence probability; and (3) testing other prior estimations of the sequence probability. A comparison of Bayesian estimates with updated estimates of semi-periodic sequences that incorporate new data not used in the original estimates shows extremely good agreement, indicating that: (1) the probability that a semi-periodic sequence is not due to chance is an appropriate estimate for the prior sequence probability estimate; and (2) the Bayesian formalism does a very good job of estimating corrected semi-periodicity probabilities, using slightly less data than that used for updated estimates. The Bayesian approach is exemplified explicitly by its application to the Parkfield semi-periodic forecast, and results are given for its application to other forecasts in Japan and Venezuela. [ABSTRACT FROM AUTHOR]

This paper presents an analysis of the distribution of earthquake magnitudes for the period 1990-1998 in a broad area surrounding the epicenter of the 1995 Kobe earthquake. The frequency-magnitude distribution analysis is performed in a nonextensive statistical physics context. The nonextensive parameter q, which is related to the frequency-magnitude distribution, reflects the existence of long-range correlations and is used as an index of the physical state of the studied area. Examination of the possible variations of q values is performed during the period 1990-1998. A significant increase of q occurs some months before the strong earthquake on April 9, 1994 indicating the start of a preparation phase prior to the Kobe earthquake. It should be noted that this increase coincides with the occurrence of six seismic events. Each of these events had a magnitude M = 4.1. The evolution of seismicity along with the increase of q indicate the system's transition away from equilibrium and its preparation for energy release. It seems that the variations of q values reflect rather well the physical evolution towards the 1995 Kobe earthquake. [ABSTRACT FROM AUTHOR]

The construction of 3-D basin velocity structures is ongoing in many regions of Japan. The structure models are constructed mainly for the prediction of long-period ground motions from future large earthquakes. In this paper, we validate the 3-D velocity structure model of the Tokachi basin, a deep sedimentary basin located in eastern Hokkaido, Japan, based on 3-D simulation of long-period (2-20 s) ground motions from three nearby intermediate-depth earthquakes; this model was constructed by the National Research Institute for Earth Science and Disaster Prevention (NIED). We make comparisons between the observed and synthetic long-period ground motions for the basin-induced surface waves as well as the direct S-wave. We also try to revise the 3-D velocity structure in the western part of the Tokachi basin based on 1-D velocity structures estimated using long-period S-wave modeling and the microtremor survey method. We then perform the 3-D simulation again to validate the revised model. Based on quantitative comparisons of the long-period ground motions from these simulations with those observed, we conclude that the NIED and revised velocity structure models are generally good at the central basin sites, but that both models require modification at the basin edges to explain the details of the observed basin-induced surface waves. [ABSTRACT FROM AUTHOR]

It is an acquired result that, in order to enable realistic earthquake hazard assessment and reliable estimation of the ground motion response to an earthquake, three-dimensional velocity models have to be considered. In this paper we present a new analytical procedure for determining the seismic wavefield in a 3D anelastic model based on the combination of the ray theory with the modal summation method. The proposed procedure has been validated by considering the three-dimensional model of the Kanto basin (Japan) available in the literature. Three-dimensional simulations were performed for the 1990 Odawara earthquake. The results obtained were compared both with recorded signals and with simulations available in the literature for several stations located within the Kanto basin. Besides the advantage of being a useful tool for assessment of seismic hazard and seismic risk reduction, it is characterized by high efficiency, in fact, once the study region is identified and the 3D model is constructed, the computation, at each station, of the three components of the synthetic signal (displacement, velocity, and acceleration) takes less than 3 h on a 2 GHz CPU. [ABSTRACT FROM AUTHOR]

Aki (1969) first modeled coda waves of a local earthquake as a superposition of scattered surface waves. This paper attempts to clarify the constituents of surface-wave coda at long periods at very long lapse times. For a large earthquake of magnitude 7 or larger, vertical component oscillation in periods from 90 to 180 s persists for more than 20 hours from the earthquake origin time. Although the early portion of the coda envelope is successfully modeled by assuming incoherent scattered Rayleigh waves by heterogeneities distributed all over the Earth, the later potion of the observed coda envelope (roughly later than 35,000 s) has systematically larger amplitude than theoretical prediction. To clarify the cause of this discrepancy, we studied the constituents of vertical-component seismograms of three large earthquakes recorded by the F-net in Japan using the f- k power spectral analysis. We found that the direct and scattered fundamental-mode Rayleigh waves of velocity about 3.7 km/s are dominant in the earlier part of each envelope. It justifies the use of a scattering model of the fundamental Rayleigh waves for synthesizing the envelope. At lapse times later than 20,000 s–35,000 s, higher modes with phase velocities around 20 km/s become dominant. The transition time to the dominance of higher modes is found to become earlier for a deeper focus earthquake. The small coda attenuation factor from (1.90±0.23) × 10−3 to (2.38±0.32) × 10−3 estimated from later coda envelopes recorded at IRIS stations distributed worldwide also agrees with the attenuation factor of spheroidal modes according to PREM. We may interpret that higher mode waves are uniformly distributed at large lapse time due to large velocity dispersion and/or scattering and they dominate over the fundamental mode waves because of smaller attenuation in the lower mantle. The coda attenuation measurement proposed by Aki is found to be useful even for long periods and at very large lapse times. [ABSTRACT FROM AUTHOR]

This study seeks to construct a hazard function for earthquake probabilities based on potential foreshocks. Earthquakes of magnitude 6.5 and larger that occurred between 1976 and 2000 in an offshore area of the Tohoku region of northeast Japan were selected as events for estimating probabilities. Later occurrences of multiple events and aftershocks were omitted from targets. As a result, a total of 14 earthquakes were employed in the assessment of models. The study volume spans 300 km (East-West) × 660 km (North-South) × 60 km in depth. The probability of a target earthquake occurring at a certain point in time-space depends on the number of small earthquakes that occurred per unit volume in that vicinity. In this study, we assume that the hazard function increases geometrically with the number of potential foreshocks within a constrained space-time window. The parameters for defining potential foreshocks are magnitude, spatial extent and lead time to the point of assessment. The time parameter is studied in ranges of 1 to 5 days (1-day steps), and spatial parameters in 20 to 100 km (20-km steps). The model parameters of the hazard function are determined by the maximum likelihood method. The most effective hazard function examined was the following case: When an earthquake of magnitude 4.5 to 6.5 occurs, the hazard for a large event is increased significantly for one day within a 20 km radius surrounding the earthquake. If two or more such earthquakes are observed, the model expects a 20,000 times greater probability of an earthquake of magnitude 6.5 or greater than in the absence of such events. [ABSTRACT FROM AUTHOR]

The 2016 Kumamoto earthquake occurred on April 16, 2016, and induced the complex distribution of surface ruptures and fault displacements, which revealed the difficulty of evaluating off-fault displacement hazards. The evaluation of off-fault displacements is necessary for the site investigation of an important facility, such as nuclear installations, but is difficult because off-fault displacements are thought to occur on unmapped or small-scale structures. A probabilistic fault displacement hazard analysis (PFDHA) assesses an off-fault displacement hazard as well as an on-fault one. Several PFDHA models have been developed from fault displacement datasets parsed by global-specific and Japanese-various faulting types. We developed a procedure to assess off-fault displacement hazards based on probabilistic and numerical simulation approaches. In the probabilistic approach, we constructed strike-slip and reverse-slip PFDHA models from a dataset of Japanese fault displacements. The off-fault components in PFDHA had a probability of non-zero off-fault displacement and an attenuation relationship. In the case of evaluating the site of an important facility, detailed field surveys were performed to collect numerous site-specific information. There is little site-specific data of off-fault displacements in Japan. The trench surveys were performed at off-fault displacement sites from the 2016 Kumamoto earthquake. The two trench sites were selected based on a linear low-coherence zone derived from the InSAR analysis, which emphasizes small-scale displacements. The excavation survey result at one site suggests a shorter recurrence interval than that of the active fault long-term evaluation. We also performed a probabilistic fault hazard analysis (PFDHA) of the trench sites by using the global strike-slip model and Japanese PFDHA models with and without the site-specific data. For the PFDHA results without site-specific data, the difference in the annual rates of exceedance between the global strike-slip and Japanese models increased with increasing fault displacements. This is because of the reduced decrease in the attenuation relationship of Japanese models. The differences in the annual rates of exceedance constrained with the site-specific data are independent of fault displacements due to variation in the probability of non-zero off-fault displacement between the global strike-slip and Japanese PFDHA models. These results suggest that the off-fault PFDHA models have high variability. In the case of PFDHA evaluation with several models, the individual PFDHA components are carefully selected by considering the individual effects. The site-specific short recurrence interval revealed by the trench survey leads to higher annual rates of exceedance. [ABSTRACT FROM AUTHOR]

Asperity models for ground motion prediction is widely used in Japan. Here we expand the application of these asperity models to predict fault displacement caused by surface rupture. The proposed approach is rather simple and practical for the use in fault displacement hazard analysis in nuclear installations and other critical infrastructures, as it is the emphasis of the current topical issue. The proposed method mainly consists of two steps. The first step consists in the characterization of asperities at the seismogenic zone based on the kinematic asperity source model following Irikura's recipe (Irikura and Miyake in Pure Applied Geophysics, 168:85–104, 2011) for strong ground motion prediction. Since the kinematic model does not take into account surface rupture mechanism, this first step assumes that the fault is buried, and then by trial and error procedure the stress drop on the asperities are estimated, so that the average slip at each asperity be consistent with the ones from the kinematic model. At this stage, the dynamic model predicts strong ground motion consistent with those from the kinematic model. In the second step, the surface rupture is included by calibrating the shallow layer (SL) with stress drop, strength excess and critical slip distance, so that the final fault displacement along the fault be consistent with observations. The 2010 Mw 7.0 Darfield (New Zealand) earthquake is used to test the proposed method. Surface-rupturing was observed in several sites along the main fault reaching values of fault displacement larger than 5 m. The main fault of this earthquake is strike-slip, almost vertical. Therefore, a simplified planar fault asperity model to capture the main features of the fault displacement is assumed. The fault dimensions are assumed to have a length of 60 km and a width of 24 km with three asperities. The preferred model of the first step predicts average slip for each asperity of 2.7, 2.7, and 2 m corresponding, respectively, to stress drops of 6.0 MPa, 8.5 MPa, and 7.0 MPa. In the second step, the surface rupture is calibrated assuming a SL zone of 3 km depth. We found that negative stress drop is not necessary in the SL, because this strongly inhibits surface rupturing. Our preferred model produces fault displacement distribution closer to the observed ones, but average slip at each asperity increases to 3.4 m, 3.2 and 2.8 m. This increase in average slip is due to the contribution of surface rupturing. Ground motion differences between fault-surface rupturing and buried models are negligible, except at the very near-source. These differences are attributed to the SL rupture that mainly affect the ground motion at the very near-source. Overall, our simple asperity model captures the main features of the observed fault displacement and near-source ground motion, proving that the proposed simple and practical two step-procedure provides meaningful estimate of fault displacement and near-source ground motion consistent with observations, as such, this method has the potential to be used in practical fault displacement hazard analysis. [ABSTRACT FROM AUTHOR]

We examined the scaling relationships of inland crustal earthquakes occurring in tectonically active regions such as the Japanese Islands. Two important points related to reliable scaling relationships are discussed empirically: the objective estimation of source parameters such as the seismic moment and rupture area, and the regionality of the scaling. Rupture areas are objectively estimated by trimming heterogeneous slip distributions obtained from waveform inversion. Two trimming procedures have been proposed to determine effective rupture areas, by Somerville et al. (Seismol Res Lett 70:59–80, 1999) and Thingbaijam and Mai (Bull Seismol Soc Am 106:1802–1816, 2016). We confirmed that both trimmed rupture areas agree well with each other when applied to the same dataset. The source scaling characteristics are known to depend on the seismotectonic regime. We investigate the source scaling relationships of inland crustal earthquakes in Japan using source parameters obtained from 36 slip models for 22 recent events (Mw 5.4–7.1) by waveform inversion analysis. We confirmed that the source scaling relationships consisting of two stages were almost the same as those obtained for crustal earthquakes on the US West Coast by Hanks and Bakun (Bull Seismol Soc Am 92:1841–1846, 2002). For larger earthquakes (Mw > 7.4) in seismically active regions, Tajima et al. (ZISIN2; J Seismol Soc Jpn 66:31–45, 2013) found saturation of fault displacements showing the third stage. Combining the earthquake data of Mw 5.4–7.1 in this study with those of Mw 7.0–8.0 compiled by Murotani et al. (Pure Appl Geophys 172:1371–1381, 2015), we conclude that the scaling relationship for rupture area versus seismic moment coincides with the three-stage source scaling relationship using source parameters extracted from waveform inversions. [ABSTRACT FROM AUTHOR]

We compared the attenuation characteristics of peak ground accelerations (PGAs) and velocities (PGVs) of strong motion from shallow, small earthquakes that occurred in Japan with those predicted by the equations of Si and Midorikawa (J Struct Constr Eng 523:63-70, 1999). The observed PGAs and PGVs at stations far from the seismic source decayed more rapidly than the predicted ones. The same tendencies have been reported for deep, moderate, and large earthquakes, but not for shallow, moderate, and large earthquakes. This indicates that the peak values of ground motion from shallow, small earthquakes attenuate more steeply than those from shallow, moderate or large earthquakes. To investigate the reason for this difference, we numerically simulated strong ground motion for point sources of M 4 and 6 earthquakes using a 2D finite difference method. The analyses of the synthetic waveforms suggested that the above differences are caused by surface waves, which are predominant at stations far from the seismic source for shallow, moderate earthquakes but not for shallow, small earthquakes. Thus, although loss due to reflection at the boundaries of the discontinuous Earth structure occurs in all shallow earthquakes, the apparent attenuation rate for a moderate or large earthquake is essentially the same as that of body waves propagating in a homogeneous medium due to the dominance of surface waves. [ABSTRACT FROM AUTHOR]

Mechanisms are suggested that could explain anomalously high PGAs (peak ground accelerations) exceeding 1 g recorded during the 2011 Tohoku earthquake ( M = 9.0). In my previous research, I studied soil behavior during the Tohoku earthquake based on KiK-net vertical array records and revealed its 'atypical' pattern: instead of being reduced in the near-source zones as usually observed during strong earthquakes, shear moduli in soil layers increased, indicating soil hardening, and reached their maxima at the moments of the highest intensity of strong motion, then reduced. We could explain this assuming that the soils experienced some additional compression. The observed changes in the shapes of acceleration time histories with distance from the source, such as a decrease of the duration and an increase of the intensity of strong motion, indicate phenomena similar to overlapping of seismic waves and a shock wave generation, which led to the compression of soils. The phenomena reach their maximum in the vicinity of stations FKSH10, TCGH16, and IBRH11, where the highest PGAs were recorded; at larger epicentral distances, PGAs sharply fall. Thus, the occurrence of anomalously high PGAs on the surface can result from the combination of the overlapping of seismic waves at the bottoms of soil layers and their increased amplification by the pre-compressed soils. [ABSTRACT FROM AUTHOR]

Early tsunami warning critically hinges on rapid determination of the tsunami hazard potential in real-time, before waves inundate critical coastlines. Tsunami energy can quickly characterize the destructive potential of generated waves. Traditional seismic analysis is inadequate to accurately predict a tsunami's energy. Recently, two independent approaches have been proposed to determine tsunami source energy: one inverted from the Deep-ocean Assessment and Reporting of Tsunamis (DART) data during the tsunami propagation, and the other derived from the land-based coastal global positioning system (GPS) during tsunami generation. Here, we focus on assessing these two approaches with data from the March 11, 2011 Japanese tsunami. While the GPS approach takes into consideration the dynamic earthquake process, the DART inversion approach provides the actual tsunami energy estimation of the propagating tsunami waves; both approaches lead to consistent energy scales for previously studied tsunamis. Encouraged by these promising results, we examined a real-time approach to determine tsunami source energy by combining these two methods: first, determine the tsunami source from the globally expanding GPS network immediately after an earthquake for near-field early warnings; and then to refine the tsunami energy estimate from nearby DART measurements for improving forecast accuracy and early cancelations. The combination of these two real-time networks may offer an appealing opportunity for: early determination of the tsunami threat for the purpose of saving more lives, and early cancelation of tsunami warnings to avoid unnecessary false alarms. [ABSTRACT FROM AUTHOR]

Properties of seismoelectric waves in relation to natural earthquakes have been investigated. The electromagnetic disturbances were analyzed to test the hypothesis that pulse-like electric variations are directly related to microcracks as source. Because variation is very difficult to detect, there have been few quantitative field investigations. We used selected events with clear S and P phases from the data catalog obtained before the Tohoku earthquake in 2011. The electric strength of the fast P wave ( P), S wave ( S), and electromagnetic wave (EM) associated with formation of cracks of tensile mode were estimated. The co-seismic electric signal accompanied by the S wave has the largest strength, well above the noise level, and the EM wave has the lowest strength. Analytical estimation of the ratio of the strengths of the P and EM phases to that of the S phase by use of Pride's equations gave results partially in agreement with observation (the order was Ap > A > A). The strength of the observed electromagnetic mode is approximately two orders of magnitude larger than that estimated from the theory. We suggest this greater strength can be attributed to the converted modes at layer contracts or to the effect of the boundary between free atmosphere and crust. Overall agreement between observations and theoretical estimates suggests that electromagnetic anomalies, crustal deformation, and groundwater changes can be investigated on the basis of the unified equations for the coupled electromagnetics, acoustics, and hydrodynamics of porous media. [ABSTRACT FROM AUTHOR]

In a previous publication, the seismicity of Japan from 1 January 1984 to 11 March 2011 (the time of the $$M9$$ Tohoku earthquake occurrence) has been analyzed in a time domain called natural time $$\chi.$$ The order parameter of seismicity in this time domain is the variance of $$\chi$$ weighted for normalized energy of each earthquake. It was found that the fluctuations of the order parameter of seismicity exhibit 15 distinct minima-deeper than a certain threshold-1 to around 3 months before the occurrence of large earthquakes that occurred in Japan during 1984-2011. Six (out of 15) of these minima were followed by all the shallow earthquakes of magnitude 7.6 or larger during the whole period studied. Here, we show that the probability to achieve the latter result by chance is of the order of $$10^{-5}$$ . This conclusion is strengthened by employing also the receiver operating characteristics technique. [ABSTRACT FROM AUTHOR]

We estimate corner frequencies and stress drops for 298 events ranging from M 3.2-7.0 in 17 inland crustal earthquake sequences in Japan to investigate the source scaling and variation in stress drops. We obtain the source spectral ratio from observed records by the S-wave coda spectral ratio method. The advantage of using the S-wave coda is in obtaining much more stable source spectral ratios than using direct S-waves. We carefully examine the common shape of the decay of coda envelopes between event pair records. The corner frequency and stress drop are estimated by modeling the observed source spectral ratio with the omega-square source spectral model. We investigate the dependences of stress drops on some tectonic effects such as regionality, focal mechanism, and source depth. The principal findings are as follows: (1) a break in self-similar source scaling is found in our dataset. Events larger than M 4.5 show larger stress drops than those of smaller events. (2) Stress drops of aftershocks are mostly smaller than those of mainshocks in each sequence. (3) There are no systematic differences between stress drops of events occurring inside and outside the Niigata-Kobe Tectonic Zone in Japan. (4) Clear dependence of the faulting type on stress drops cannot be seen. (5) Stress drops of aftershocks depend on their source depth. (6) The crack size obtained from the corner frequency corresponds to the total rupture area of heterogeneous slip models for large events. [ABSTRACT FROM AUTHOR]

Finite difference simulations of seismic wave propagation are performed in the Niigata area, Japan, for the 2007 Mw 6.6 Niigata-ken Chuetsu-Oki earthquake at low frequencies. We test three 3D structural models built independently in various studies. First aftershock simulations are carried out. The model based on 3D tomography yields correct body waves in the near field, but later phases are imperfectly reproduced due to the lack of shallow sediment layers; other models based on various 1D/2D profiles and geological interpretation provide good site responses but generate seismic phases that may be shifted from those actually observed. Next, for the mainshock simulations, we adopt two different finite source models that differ in the near-field ground motion, especially above the fault plane (but under the sea) and then along the coastline. Each model is found to be calibrated differently for the given stations. For engineering purposes, the variations observed in simulated ground motion are significant, but for seismological purposes, additional parameter calibrations would be possible for such a complex 3D case. [ABSTRACT FROM AUTHOR]

We investigated the development of a distinct later phase observed at stations near the Japan Trench associated with shallow, outer-rise earthquakes off the coast of Sanriku, northern Japan based on the analysis of three-component broadband seismograms and FDM simulations of seismic wave propagation using a heterogeneous structural model of the Japan Trench subduction zone. Snapshots of seismic wave propagation obtained through these simulations clearly demonstrate the complicated seismic wavefield constructed by a coupling of the ocean acoustic waves and the Rayleigh waves propagating within seawater and below the sea bottom by multiple reflections associated with shallow subduction zone earthquakes. We demonstrated that the conversion to the Rayleigh wave from the coupled ocean acoustic waves and the Rayleigh wave as they propagate upward along the slope of seafloor near the coast is the primary cause of the arrival of the distinct later phase at the station near the coast. Through a sequence of simulations using different structural models of the Japan Trench subduction zone, we determined that the thick layer of seawater along the trench and the suddenly rising sea bottom onshore of the Japanese island are the major causes of the distinct later phase. The results of the present study indicate that for realistic modeling of seismic wave propagation from the subduction zone earthquakes, a high-resolution bathymetry model is very crucial, although most current simulations do not include a water column in their simulation models. [ABSTRACT FROM AUTHOR]

Records of the Niigata Chuetsu-Oki earthquake (July, 16, 2007, M = 6.6, depth ~17 km) and its aftershocks from seismic vertical arrays deployed at the territory of the Kashiwazaki-Kariwa nuclear power plant (~15 km from the fault) are used to study the soil behavior down to ~250 m during strong ground motion. Nonlinear models of soil behavior during the main shock and six aftershocks are constructed, and stresses and strains induced by the strong motion in the soil layers at various depths are estimated. The data are processed using the method developed by P avlenko and I rikura (Bull Seismol Soc Am 96(6): 2131-2145, ) and previously applied for studying the soil behavior in near-fault zones during the 1995 Kobe and 2000 Tottori earthquakes. A rather good agreement between the recorded and simulated acceleration time histories testifies to the validity of the obtained vertical distributions of stresses and strains in soil layers. In the upper, softer layers (~45 m) at the territory of the plant, the shear moduli were reduced by ~30-35% during the main shock and by ~1.5-3% during the aftershocks. The constructed models of soil behavior can be used in scenario earthquake shaking maps of Japan where, based on source modeling parameters, the level of strong motion can be evaluated for the territory of the power plant in future earthquakes with various magnitudes and fault planes. Using methods of stochastic finite-fault modeling of ground motions from the Chuetsu-Oki earthquake, we estimated input motion to the soil layers during the main shock and found that it differs from the imposed motion (recorded by the deepest sensor of the vertical array) by slightly decreased (by a factor of ~1.2) low-frequency ( f < 10 Hz) spectral components. [ABSTRACT FROM AUTHOR]

robust numerical model to simulate propagation and runup of tsunami waves in the framework of non-linear shallow water theory is developed. The numerical code adopts a staggered leapfrog finite-difference scheme to solve the shallow water equations formulated for depth-averaged water fluxes in spherical coordinates. A temporal position of the shoreline is calculated using a free-surface moving boundary algorithm. For large scale problems, the developed algorithm is efficiently parallelized employing a domain decomposition technique. The developed numerical model is benchmarked in an exhaustive series of tests suggested by NOAA. We conducted analytical and laboratory benchmarking for the cases of solitary wave runup on simple beaches, runup of a solitary wave on a conically-shaped island, and the runup in the Monai Valley, Okushiri Island, Japan, during the 1993 Hokkaido-Nansei-Oki tsunami. In all conducted tests the calculated numerical solution is within an accuracy recommended by NOAA standards. We summarize results of numerical benchmarking of the model, its strengths and limits with regards to reproduction of fundamental features of coastal inundation, and also illustrate some possible improvements. [ABSTRACT FROM AUTHOR]

conglomerate appears on a rocky coast called 'Tsubutega-ura Coast', located on the southwestern coast near the southern tip of the Chita Peninsula, Aichi Prefecture, central Japan. The conglomerate belongs to Miocene sedimentary rocks termed the Morozaki Group. The conglomerate includes meter-scale boulders, indicating that it was formed by an extraordinary event. In the geological investigation, we observed that the conglomerate shows alternate changes of paleocurrent directions between seaward and landward. This feature is supposed to be formed by tsunami currents. In the hydrodynamical investigation, we obtained following results: (1) the lowest limit of a current velocity to move a boulder of about 3 m in diameter would be about 2-3 m/s, (2) the speed of tsunami currents reproduced by tsunami simulation exceeds 3 m/s at 300 m in depth when the tsunami is generated by a gigantic earthquake with magnitude 9.0 or more, (3) the transport distance of the boulder would be several hundred meters to several kilometers by one tsunami event caused by a gigantic earthquake. We conclude that tsunamis best explain the formation of the conglomerate deposited in upper bathyal environments about 200-400 m depth, both from geological and hydrodynamical viewpoints. [ABSTRACT FROM AUTHOR]

In this study we present a characteristic kinematic rupture model for an intraslab earthquake of the type of the 2001 Geiyo earthquake. The procedure for developing the characteristic rupture model follows the recipe developed by I rikura and M iyake (Proceedings of the 8th US National Conference on Earthquake Engineering, San Francisco, ) with a few modifications. We used the functional form of N akamura and M iyatake [Zisin (J Seism Soc Jpn), 53, 1-9, ] to model the spatial and temporal variability of the slip rate function. In our procedure the earthquake source is specified by the kinematic description of the fault model that incorporates spatial heterogeneity in slip and rise time and constant rupture velocity. In the proposed characteristic model the asperity locations and rupture initiation point were the only parameters that were constrained by the available earthquake rupture models of the target earthquake. The quality of the characteristic rupture model was assessed by comparing recorded and synthetic ground motion time histories from the 2001 Geiyo earthquake calculated with a standard broadband (0.1-10 Hz) ground motion simulation technique. Our analyses show that the characteristic rupture model performs well in reproducing the recorded ground motion, in spite of its simple representation of the fault geometry and kinematic rupture complexity. The analyses of ground motion sensitivity to relative location of asperities, and their stress drop contrast suggest that the overall ground motion goodness of fit remains the same at short periods, but decreases slightly at periods longer than 1 s. We suspect that, due to the depth of the intraslab source (40-50 km), the higher-frequency ground motions are relatively insensitive to the details of the slip distribution and slip contrast. On the other hand, even for intraslab events, the lower-frequency motions are influenced by rupture directivity effects and slip distribution. [ABSTRACT FROM AUTHOR]

Ground motions of the 1923 Kanto Earthquake inside the Kanto Basin are numerically simulated in a wide frequency range (0-10 Hz) based on new knowledge of the earthquake's source processes, the sedimentary structure of the basin, and techniques for generating broadband source models of great earthquakes. The Kanto Earthquake remains one of the most important exemplars for ground motion prediction in Japan due to its size, faulting geometry, and location beneath the densely populated Kanto sedimentary basin. We reconstruct a broadband source model of the 1923 Kanto Earthquake from inversion results by introducing small-scale heterogeneities. The corresponding ground motions are simulated using a hybrid technique comprising the following four calculations: (1) low-frequency ground motion of the engineering basement, modeled using a finite difference method; (2) high-frequency ground motion of the engineering basement, modeled using a stochastic Green's function method; (3) total ground motion of the engineering basement (i.e. 1 + 2); and (4) ground motion at the surface in response to the total basement ground motion. We employ a recently developed three-dimensional (3D) velocity structure model of the Kanto Basin that incorporates prospecting data, microtremor observations and measurements derived from strong ground motion records. Our calculations reveal peak ground velocities (PGV) exceeding 50 cm/s in the area above the fault plane: to the south, where the fault plane is shallowest, PGV reaches 150-200 cm/s at the engineering basement and 200-250 cm/s at the surface. Intensity 7, the maximum value in the Japan Meteorological Agency's intensity scale, is calculated to have occurred widely in Sagami Bay, which corresponds well with observed house-collapse rates due to the 1923 event. The modeling reveals a pronounced forward directivity effect for the area lying above the southern, shallow part of the fault plane. The high PGV and intensity seen above the southeastern corner of the fault plane and further east are largely due to this effect. Waveforms above the fault plane contain both short- and long-period components, but the short-period components are not observed further afield. Away from the fault, long-period waves (>2 s) dominate the ground motion, and in areas where the base of the third layer is relatively deep, the predominant period is >5 s. Levels of long-period ground motion in the southern part of the study area, around Sagami Bay and the southern parts of Boso Peninsula and Tokyo Bay, exceed that recorded at Tomakomai during the 2003 Tokachi-oki earthquake, when large oil storage tanks collapsed in response to sloshing generated by strong long-period motions. [ABSTRACT FROM AUTHOR]

If aseismic slip occurs on a fault or its deeper extension, both seismicity and crustal deformation around the source would be affected. Anomalous phenomena of this kind are revealed from earthquake occurrence data and geodetic records during a period of 10 years leading up to the March 2005 M 7.0 earthquake west of Fukuoka that occurred off the northern coast of Kyushu, Japan. Seismicity rate anomalies (quiescence and activation) took place relative to the rates expected by the ETAS model in a number of seismic zones in and around the Kyushu District. The seismic zone of the relative quiescence and activation consistently corresponds to the zone of the negative and positive ΔCFS (Coulomb failure stress change), respectively, assuming the precursory aseismic slips on the M 7.0 source fault. In addition, we consider the time series of geodetic baseline distances between permanent GPS stations in the Kyushu District for the same period, which also supports the possible precursory slips rather than the known slow slips beneath the Bungo Straight, off the eastern coast of Kyushu. [ABSTRACT FROM AUTHOR]

We construct a single hazard function from multiple predictive parameters independently developed for moderate earthquakes in Kanto, Japan, during a learning period from 1990 to 1999, and applied to a testing period from 2000 to 2005. Here, we consider as predictive parameters the a and b values of the Gutenberg–Richter relation, the ν value (change in b value), and the Every Earthquake a Precursor According to Scale (EEPAS) model rate. To study the correlations among the parameters, we prepare two groups of space–time coordinate sets for assessment, namely the background and conditional groups selected from the learning period. The background group contains ten thousand sets of coordinates randomly selected from the space–time volume of our study. The conditional group contains 33 sets of space–time coordinates corresponding to the epicenters of the target earthquakes ( M ≥ 5.0) just before their times of occurrence. Each parameter for the background group is transformed so that its distribution conforms to the standard Normal function. The mean and variance of the conditional distribution is then estimated after applying the same transformation to the conditional group. Using the means and variances of b values, ν values and EEPAS rates and the correlation matrices in the background and conditional distributions, we construct a combined hazard function following the procedure developed for normally distributed parameters. The information gain per event ( IGpe) of the new hazard function is 0.26 and 0.3 units larger than that of the EEPAS rate for the learning and testing period, respectively. The R-test confirms the statistical significance of the difference in the IGpe value for the testing period. [ABSTRACT FROM AUTHOR]

The 1995 Kobe (Hyogo-ken Nanbu) earthquake, M = 7.2, ruptured the Nojima fault in southwest Japan. We have studied core samples taken from two scientific drillholes that crossed the fault zone SW of the epicentral region on Awaji Island. The shallower hole, drilled by the Geological Survey of Japan (GSJ), was started 75 m to the SE of the surface trace of the Nojima fault and crossed the fault at a depth of 624 m. A deeper hole, drilled by the National Research Institute for Earth Science and Disaster Prevention (NIED) was started 302 m to the SE of the fault and crossed fault strands below a depth of 1140 m. We have measured strength and matrix permeability of core samples taken from these two drillholes. We find a strong correlation between permeability and proximity to the fault zone shear axes. The half-width of the high permeability zone (approximately 15 to 25 m) is in good agreement with the fault zone width inferred from trapped seismic wave analysis and other evidence. The fault zone core or shear axis contains clays with permeabilities of approximately 0.1 to 1 microdarcy at 50 MPa effective confining pressure (10 to 30 microdarcy at in situ pressures). Within a few meters of the fault zone core, the rock is highly fractured but has sustained little net shear. Matrix permeability of this zone is approximately 30 to 60 microdarcy at 50 MPa effective confining pressure (300 to 1000 microdarcy at in situ pressures). Outside this damage zone, matrix permeability drops below 0.01 microdarcy. The clay-rich core material has the lowest strength with a coefficient of friction of approximately 0.55. Shear strength increases with distance from the shear axis. These permeability and strength observations reveal a simple fault zone structure with a relatively weak fine-grained core surrounded by a damage zone of fractured rock. In this case, the damage zone will act as a high-permeability conduit for vertical and horizontal flow in the plane of the fault. The fine-grained core region, however, will impede fluid flow across the fault. [ABSTRACT FROM AUTHOR]

Linear and nonlinear responses of ten well-type tide gauge stations on the Japan Sea coast of central Japan were estimated by in situ measurements. We poured water into the well or drained water from the well by using a pump to make an artificial water level difference between the outer sea and the well, then measured the recovery of water level in the well. At three tide gauge stations, Awashima, Iwafune, and Himekawa, the sea-level change of the outer sea is transmitted to the tide well instantaneously. However, at seven tide gauge stations, Nezugaseki, Ryotsu, Ogi, Teradomari, Banjin, Kujiranami, and Naoetsu, the sea-level change of the outer sea is not always transmitted to the tide well instantaneously. At these stations, the recorded tsunami waveforms are not assured to follow the actual tsunami waveforms. Tsunami waveforms from the Niigataken Chuetsu-oki Earthquake in 2007 recorded at these stations were corrected by using the measured tide gauge responses. The corrected amplitudes of the first and second waves were larger than the uncorrected ones, and the corrected peaks are a few minutes earlier than the uncorrected ones at Banjin, Kujiranami, and Ogi. At Banjin, the correction was significant; the corrected amplitudes of the first and second upward motion are +103 cm and +114 cm, respectively, while the uncorrected amplitudes were +96 cm and +88 cm. At other tide gauge stations, the differences between the uncorrected and corrected tsunami waveforms were insignificant. [ABSTRACT FROM AUTHOR]

Strong ground motions recorded in central Tokyo during the 1944 Tonankai Mw8.1 earthquake occurring in the Nankai Trough demonstrate significant developments of very large (>10 cm) and prolonged (>10 min) shaking of long-period (T > 10–12 s) ground motions in the basin of Tokyo located over 400 km from the epicenter. In order to understand the process by which such long-period ground motions developed in central Tokyo and to mitigate possible future disasters arising from large earthquakes in the Nankai Trough, we analyzed waveform data from a dense nation wide strong-motion network (K-NET and KiK-net) deployed across Japan for the recent SE Off-Kii Peninsula (Mw 7.4) earthquake of 5 September 2004 that occurred in the Nankai Trough. The observational data and a corresponding computer simulation for the earthquake clearly demonstrate that such long-period ground motion is primarily developed as the wave propagating along the Nankai Trough due to the amplification and directional guidance of long-period surface waves within a thick sedimentary layer overlaid upon the shallowly descending Philippine Sea Plate below the Japanese Island. Then the significant resonance of the seismic waves within the thick cover of sedimentary rocks of the Kanto Basin developed large and prolonged long-period motions in the center of Tokyo. The simulation results and observed seismograms are in good agreement in terms of the main features of the long-period ground motions. Accordingly, we consider that the simulation model is capable of predicting the long-period ground motions that are expected to occur during future Nankai Trough M 8 earthquakes. [ABSTRACT FROM AUTHOR]

The 2004 Mid Niigata Prefecture earthquake ( M JMA 6.8) and its aftershock sequences generated complicated, i.e., several conjugate fault planes in their source region. In order to understand the generating process of these earthquakes, we estimated a 3-D distribution of relative scattering coefficients in the source region. The large slip area during the main shock rupture seems to be bounded by strong heterogeneous zones with larger scattering coefficients. Hypocenters of the main shock and major large aftershocks with M 5-6 classes tend to be located close to stronger scattering areas. We found that one of these strong heterogeneities already existed before the occurrence of the M 5.9 aftershock on November 8. We suppose that heterogeneous structures in the source region of this earthquake sequence affected the initiation and growth of ruptures of the main shock and major large aftershocks. [ABSTRACT FROM AUTHOR]

A probabilistic seismic hazard assessment was performed for the Japanese islands and surrounding areas. Seismic hazard parameters characteristic of the seismic history of the regions were obtained. The probability of occurrence of a large M ≥ 7 earthquake within a 10- and 50-year period was also calculated. Regions of very high levels of hazard occur where the Pacific, Phillipine and Eurasian Plates meet. High probabilities of occurrence of a large M ≥ 7 earthquake within a 10- and 50-year period occur within the region where the Pacific Plate subducts with the Eurasian Plate. [ABSTRACT FROM AUTHOR]

A seismic waveform inversion algorithm is proposed for the estimation of elastic soil properties using low amplitude, downhole array recordings. Based on a global optimization scheme in the wavelet domain, complemented by a local least-square's fit operator in the frequency domain, the hybrid scheme can efficiently identify the optimal solution vicinity in the stochastic search space, whereas the best-fit model detection is substantially accelerated through the local deterministic inversion. Results presented for selected aftershocks of the M w 7.0 Sanriku-Minami earthquake in Japan, recorded by the Kik-Net Strong Motion Network, illustrate robustness of the impedance structure estimation. By contrast, the attenuation structure is shown to be sensitive to the frequency content of seismic input data, attributed to the deterministic description of the continuum in the forward model that cannot simulate late arrivals of multiple-scattered energy. Sensitivity analyses illustrate that for the same forward model, results can be substantially different based on the definition of the objective function. It is concluded that even for engineering purposes, inversion should aim to decouple intrinsic and scattering attenuation mechanisms. [ABSTRACT FROM AUTHOR]

- I studied crustal deformation in the Kanto district, central Japan, based on continuous GPS data. Horizontal as well as vertical displacement rate demonstrate significant interaction between the landward Kanto block and the Philippine Sea plate. Although the subduction effect of the Pacific plate is not apparent, it is reasonable to consider the entire Kanto district is displaced westward due to the interaction with the Pacific plate. The GPS velocity data were inverted to estimate the slip deficit distribution on the Sagami Trough subduction zone. The result delineates a strongly coupled region on the plate interface, part of which corresponds to the 1923 Kanto earthquake. The strongly coupled region is located shallower than 20 km. In addition, the plate interaction is laterally heterogeneous even in the same depth range, implying thermal structure is not the only factor controlling interplate coupling. The GPS data also detected a silent earthquake event on the interface of the Philippine Sea slab east of the Boso Peninsula in the middle of May, 1996. The silent rupture propagated over a 50 km * 50 km wide area during about a week. The maximum slip was approximately 50 mm and the released seismic moment was 4.7*1018Nm (M w 6.4). There was a small seismicity triggered by this silent event. The silent slip was located in the peripheral of the strongly coupled area, suggesting that frictional properties and/or stress conditions are inhomogeneous on the plate boundary interface. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]