A moderate earthquake with local magnitude 5.2 (M_L) occurred off the eastern coast of South Korea on May 29, 2004. This earthquake is one of the largest events near the Korean Peninsula since 1978 when instrumental observations began by the Korea Meteorological Administration. We carried out modeling of regional waveforms and deconvolutions using empirical Green functions to estimate source parameters. The results of the regional waveform modeling show a reverse fault mechanism with a hypocentral depth of 8 km and seismic moment of 5 × 10¹⁶ N·m, which is equivalent to Mw 5.1. Performing the empirical Green function deconvolution, we estimated a fault radius of 1.1 to 1.4 km and an average stress drop of 13.5 MPa, assuming an east dipping fault plane. Assuming a west dipping fault plane, we estimated a fault radius of 1.3 to 2.9 km and an average stress drop of 6.6 MPa.

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A new recursive inverse scheme is applied to a currently popular problem named seismic travel-time tomography, in order to enhance the efficiency and reliability in obtaining a new velocity model if a small number of new data are added to a large data set in the past. In comparison with conventional inverse schemes in seismic tomography, either least-squares or iterative types, this scheme does not require large amounts of matrix-type computations but utilizes the amount of modification in model parameters responsible for each new data set. We also introduce the computation of a collocation travel time (i.e., from a given station to every grid point) for the reference velocity model inverted by the data for all the past events, using a ray tracing scheme called the Huygens' method (Saito, 2001), suitable to computations prior to a new event. Combining the above information already stored with the recursive inverse scheme, we can obtain a new or updated velocity model immediately after a new event takes place, because a temporal interval between two events is usually very long in a given local area. Since the model is revised at each recursive step, we perform ray tracings with the updated reference model to get more accurate ray paths and travel times than the conventional inversion schemes that use all the ray tracings for the same reference model. We first showed the validity and stability of the proposed method with synthetic data. We then applied the new approach to the P-wave travel-time data recorded in the Hidaka, south-central Hokkaido, Japan, region, and compared our result with other previous results. Our result shares the overall feature with the previous ones. In addition, a new low-velocity zone is detected in the east of the Hidaka mountains at the depth of 10 km, corresponding to the collision zone of two arcs, due to the use of the updated reference velocity model at each recursive step. We also confirmed that the order of data does not affect the final result, so that the present approach is shown as an appropriate tool for so-called real-time seismic tomography: a updated velocity model is immediately obtained at each time that a new event takes place, in order to monitor temporal variations of model parameters such as velocity structure on the real-time basis.

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We have examined temporal changes in seismic velocity of the crust through repeated active seismic experiments at Iwate volcano, Japan, where a significant volcanic activity and an M6.1 earthquake were observed in 1998. We apply a cross spectrum moving window technique to seismic data recorded at eight stations for the six explosions detonated from1998 to 2003. The seismic velocity at the frequency range of 3-9 Hz decreased by about 1% during the three months including the occurrence of M6.1 earthquake. The seismic velocity gradually increased, and about one third of the decrease was recovered by 2002. Then, the seismic velocity decreased again in 2003. Spatio-temporal changes in the volumetric strains predicted from the M6.1 fault mechanism and the volcanic pressure sources are well correlated with the seismic velocity changes observed in 1998. However, the predicted stress fields are not completely matched with the observed velocity changes from 1998 to 2003. This inconsistency may be due to unknown regional tectonic stress field and/or localized stress fields induced by volcanic pressure sources. It should be noted that the observed velocity changes indicate frequency dependent characteristics although the mechanism is not yet understood.

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Transient crustal deformation occurred in the regions of Kanto and Tokai during the seismo-volcanic event in the northern Izu Islands in 2000. In our investigation of the observed deformation, we constructed an optimumsource model of the event between Miyake and Kozu Islands. We then made an inversion analysis of the differences between the observed displacement field and the calculated displacement field from the optimum model, assuming that the differences were caused by the changes in the interplate coupling beneath the Tokai region. From the inversion analysis of data for each of three-month periods, May to August, June to September, and July to October, we found decreased interplate coupling in the early stages of the 2000 event. In the first stage, either a slow slip or a temporary suspension of the plate subduction occurred in the focal region of the anticipated Tokai earthquake. The area then extended to the west and, finally, a slow slip exceeded the secular convergence velocity on the plate interface near Lake Hamana in the fall of 2000. We believe this ongoing slow slip began in August or early September 2000.

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We have used Monte Carlo simulations of O⁺ velocity distributions in the high latitude F- region to improve the calculation of incoherent radar spectra in auroral ionosphere. The Monte Carlo simulation includes ion-neutral, O⁺-O collisions (resonant charge exchange and polarization interaction) as well as O⁺-O⁺ Coulomb self-collisions. At high altitudes, atomic oxygen O and atomic oxygen ion O⁺ dominate the composition of the auroral ionosphere and consequently, the influence of O⁺-O⁺ Coulomb collisions becomes significant. In this study we consider the effect of O⁺-O⁺ Coulomb collisions on the incoherent radar spectra in the presence of large electric field (100 mVm^-1). As altitude increases (i.e. the ion-to-neutral density ratio increases) the role of O⁺-O⁺ Coulomb self-collisions becomes significant, therefore, the one-dimensional, 1-D, O⁺ ion velocity distribution function becomes more Maxwellian and the features of the radar spectrum corresponding to non-Maxwellian ion velocity distribution (e.g. baby bottle and triple hump shapes) evolve to Maxwellian ion velocity distribution (single and double hump shapes). Therefore, O⁺-O⁺ Coulomb self-collisions act to isotropize the 1-D O⁺ velocity distribution by transferring thermal energy from the perpendicular direction to the parallel direction, however the convection electric field acts to drive the O⁺ ions away from equilibrium and consequently, non-Maxwellian O⁺ ion velocity distributions appeared. Therefore, neglecting O⁺-O⁺ Coulomb self-collisions overestimates the effect of convection electric field.

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The 2004 Mid Niigata Prefecture earthquake sequence produced a nearly 1-km-long, N-S to NNW-SSE-striking, west-side-up surface rupture along a previously unmapped fault at Obiro, Uonuma City, eastern margin of the epicentral region. The maximum vertical displacement is 20 cm. The topographic and geometric features of the surface rupture are indicative of reverse faulting with dip to the west, which is consistent with focal mechanism solutions and aftershock distribution. Along the major active faults and folds that form the framework of landforms in the epicentral region, systematic surface ruptures were not found. Exceptionally small rupture dimension and offset amount of the surface rupture compared with those of the modeled fault suggest that the fault slip appears to have been accommodated by both internal deformation in thick Neogene-Quaternary deposits and growth of the folds and blind thrusts around the epicentral region.

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The 2004 Mid Niigata Prefecture earthquake (37.289°N, 138.870°E, 13.1 km, M_JMA 6.8; JMA), also known as the 2004 Niigata Prefecture Chuetsu earthquake, was a thrust type earthquake that occurred on October 23, 2004 at 17:56 (JST). Strong ground motions of PGA 800-1700 cm/s² and PGV 60-130 cm/s were observed at stations located immediately above the source region. We deduced the rupture process of this earthquake with a multi-time-window linear waveform inversion procedure. We used near-fault strong ground motion data observed at nine K-NET and KiK-net stations within 50 km from the epicenter. In order to obtain appropriate Green's functions for the waveform inversion, we constructed two velocity structure models for stations on the hanging wall and one structure model for stations on the footwall. The estimated total slip distribution contains three asperities: (a) around the hypocenter, (b) in the upper-middle section of the fault plane, and (c) southwest of the hypocenter. The maximum slip is 3.8 m at the hypocenter and the total seismic moment is 1.2×10¹⁹ Nm, which corresponds to Mw=6.7. The moment rate functions in asperities (a) and (c) have a short rise time, while those in asperity (b) have a longer rise time.

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A preliminary source model composed of asperities for the 2004 Mid Niigata Prefecture, Japan, earthquake (M_JMA=6.8) was estimated by the empirical Green's function method. The source parameters for two asperities located on the fault plane were determined from the comparisons of the synthesized broad-band ground motions with the observed ones at several stations including near source. Furthermore, we performed the preliminary nonlinear analysis of the sedimentary soils to reproduce the observed ground motions at NIG019 of K-NET. Resultantly, we pointed out the need of more asperity in northern part on the fault plane and the importance of the quantitative analysis of the nonlinearity of the sedimentary soils at K-NET stations.

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Aftershock observations of the 2004 Mid Niigata Prefecture earthquake were conducted in the central part of Ojiya city, the Niigata prefecture, central Japan, to investigate local site effects. We installed eight accelerographs in the vicinity of the K-NET and JMA stations in the area. The stations of the aftershock observations are situated under different geological conditions including one installed in a mountainous area on Tertiary layers to serve as a reference site. We examined the ground-motion characteristics of the records for a Mj 6.1 aftershock focusing on local site effects. The amplification, at a period of less than 1 sec, is the largest in the vicinity of the K-NET station. The amplification at periods longer than 2 sec is larger in the western part of the city than those in the east. We estimated the S-wave velocity structure in the sediments above the basement with an S-wave velocity of 3.4 km/sec from the inversion of phase velocities measured by the array observations of vertical microtremors. We discuss the amplification factors using the S-wave velocity profile and show that shallow soils over the layer with an S-wave velocity of 0.49 km/sec are responsible for the amplification at periods shorter than 0.4 sec. Deeper sedimentary layers are needed to explain amplification at periods of 1 sec.

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The 2004 Mid Niigata Prefecture Earthquake (Mj = 6.8) occurred on 23 October 2004 in the northeastern part of the Niigata-Kobe Tectonic Zone where large contraction rates were observed. The mainshock was followed by an anomalously intense aftershock activity that included nine Mj ≥ 5.5 aftershocks. We deployed three temporary online seismic stations in the aftershock area from 27 October, combined data from the temporary stations with those from permanent stations located around the aftershock area, and determined the hypocenters of the mainshock and aftershocks with a joint hypocenter determination (JHD) technique. The resulting aftershock distribution showed that major events such as the mainshock, the largest aftershock (Mj = 6.5), the aftershock on 27 October (Mj = 6.1), etc. occurred on different fault planes that were located nearly parallel or perpendicular to each other. This might be due to heterogeneous structure in the source region. The strain energy was considered to have been enough accumulated on the individual fault planes. These features are probably a cause of the anomalous intensity of the aftershock activity.

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We deployed 56 temporary seismic stations within approximately a month after the occurrence of the 2004 mid-Niigata prefecture earthquake. Using manually-picked arrival data obtained from the temporary and surrounding permanent seismic stations, 1056 aftershocks have been relocated. Based on the spatiotemporal variations in the relocated aftershocks, the cluster activities associated with the mainshock and some large aftershock events are identified. The aftershocks associated with the mainshock, the largest occurred on the two steep west-dipping planes at an angle of 60° and approximately 5 km away. In contrast, the aftershocks following the event on Oct. 27 are aligned on east-dipping plane at a low angle of 25°. It is further observed that the aftershock area extended in both northeastward and southwestward directions at a later stage. The triggered seismicity around the northeast edge was more significant than that around the southwest edge. This difference could be understood by the discrepancy in the shear stress level accumulated at the dynamic shear rupture due to the mainshock.

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An S-wave reflector is considered to relate to the existence of liquid in the seismogenic zone of the crust, which plays an important role in understanding the mechanism of earthquakes. We studied a distribution of S-wave reflectors in and around the hypocentral zone of the 2004 mid Niigata Prefecture Earthquake (M6.8). The earthquake was followed by several aftershocks that were greater than M6. Moreover, the aftershocks were not only located on the fault plane of the main shock but also on conjugate fault planes and on a parallel plane to that of the main shock. In order to discuss the relationship between this complex activity and the crustal heterogeneities, we analyzed the seismograms observed at the seismic stations in this region. Normal moveout processing (NMO) was applied to the data of the aftershock. Several S-wave reflectors could be identified from the NMO sections for every station. In particular, relatively strong S-wave reflectors exist in the lower crust at a depth of approximately 20-25 km in the middle part of the aftershock region. Additionally, reflectors were found beneath the fault planes of the main shock and the largest aftershock. This suggests a possibility of the correlation of the crustal heterogeneities to the occurrence of an earthquake.

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Nonlinear evolution of Alfvén turbulence is discussed within the framework of the derivative nonlinear Schroedinger equation (DNLS), a subset of the hall-MHD equation set, which includes quasi-parallel propagating right- and left-hand polarized Alfvén wave modes. By numerically time integrating the equation with periodic boundary conditions, we discuss relationship between generation of wave phase coherence and self-organization of the system due to birth of Alfvén solitons.

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