Change In Dip Angle Research Articles

稠密余震観測データを用いて推定した２００４年新潟県中越地震震源域の応力場

We have investigated a stress field in the source region of the 2004 Mid-Niigata Prefecture (Niigata-Chuetsu) earthquake (M=6.8) by using P-wave polarity data obtained by a dense temporal seismic observation. The earthquake took place in central Japan near the eastern border of the Niigata-Kobe Tectonic Zone characterized by E-W contraction. The purpose of this study is to get insight into the stress heterogeneities in the source area including quite complex fault system with two parallel, westerly dipping fault planes and one conjugate plane. About two-thirds of well-constrained 514 focal mechanisms are reverse-fault type with WNW-ESE trending P-axes, which is consistent with the regional stress field in the area. Minor strike-slip events are distributed in the northern and central part of aftershock zone. Ten normal-fault events that are rare in the northeastern Japan are scattered mostly outside the major earthquake faults and occurred in a short period, suggesting that they reflect local stress field. Stress tensors derived from the focal mechanisms exhibit the compressional stress field characterized by nearly horizontal maximum principal stress that is consistent with the average direction of P-axes in the whole aftershock zone. Horizontal rotation of maximum principal stress axis is evident from WNW-ESE in the northeastern part of aftershock zone to E-W in the southwestern part. The direction of intermediate principle stress axis also changes from NE-SW in the northeastern part to NNE-SSW in the southwestern part. The fold axes in the area show similar strike variation. An interesting feature of stress distribution found in this study is the change in dip angle of the maximum principle stress axes. The axes through the aftershock zone dip gently to WNW, but those for earthquake clusters of mainshock fault dip steeply in a similar direction. Judging from the stress ratio, the horizontal compression is dominant near the mainshock hypocenter while the state in the surrounding areas is relatively close to isotropic stress. Though the difference in dip angle may not significant by considering the estimation error of stress axes, the combined anomalies with the stress ratio suggest that the mainshock occurred in an area of local stress inhomogeneity. We further examined the spatial distribution of earthquakes that have inconsistent focal mechanisms with the estimated stress tensors. Such earthquakes are distributed in the northeastern edge of the aftershock zone, in the southern edge of easterly dipping fault plane, and in a shallow earthquake cluster that is located between the faults of mainshock and the largest aftershock. The location of these events indicates small-scale stress heterogeneities in the source area superposed on the larger-scale stress variation mentioned above.

Mantle wedge deformation by subducting and rotating slab and its possible implication

Abstract We have constructed a simple model of the deformation of the mantle wedge caused by the subducting and rotating slab based on corner flow model. By applying this model to the geological settings of the Somuncura plateau volcanic region, northern Patagonia, which is located far from the volcanic front, we constrain the mechanical aspect of the hypothesis that the volcanisms of the Somuncura region are triggered by the dehydration-induced melting of the up-warped transition zone which may contain more water than other regions do. Assuming that the water concentrates in the transition zone under the Somuncura plateau, we find that the above scenario may be possible, if the speed of the subducting slab is less than ∼2 cm/yr or the dip angle changes significantly (∼50 degrees within ∼8 Myr).

Simulating acoustic emissions in bonded-particle models of rock

Longyou Grottoes, the underground projects with a rich history of thousands of years, are one of the key cultural relics to be protected in China and are actively preparing to apply for a world cultural heritage site. However, due to the shallow buried argillaceous siltstone with low-strength bedding planes, some cavern roofs experienced apparent breakages or large-scale collapses. Clarifying the roof deformation and failure mechanism is essential for the long-term protection of the historical relic and sustainable development of tourism. Productive previous work proved that three-point bending tests of rock samples could reproduce stress states of cavern roofs. Therefore, three-point bending numerical models with various bedding plane structures for Longyou argillaceous siltstone are created in this study to investigate the rock mechanical behaviors. The simulation results show that as bedding mechanical properties decrease, the bending strength (BS) has a slow to fast decrease, indicating that timely protection in Longyou Grottoes before the BS drops sharply is conducive to preventing roof failures. This declining trend is further accelerated when the bedding dip angle changes from horizontal to vertical. Dip angles, mechanical properties, and spacings of bedding planes control fracture modes of models. Moreover, the fitted Gutenberg–Richter-type relationships between acoustic emission (AE) moment magnitudes and logarithmic AE events present high average activity levels and high proportions of large AE events in both bedding planes and rock matrix for models with strong, dispersedly distributed, and near-horizontal bedding planes. In summary, the simulation results reveal the impacts of transverse isotropy in surrounding rocks on roof bending failures, which may provide a valuable reference for the reinforcement and protection of Longyou Grottoes.

Earthquake distribution in the subduction zone off eastern Hokkaido, Japan, deduced from ocean-bottom seismographic and land observations

SUMMARY In 1986, observations of microearthquakes were conducted with the use of five ocean-bottom seismometers (OBSs) on the continental slope east of the Kushiro Submarine Canyon (KSC) off Hokkaido Island, Japan. The observation area is located about 50-100 km landward of the southern Kuril Trench beneath which the Pacific Plate is subducted toward Hokkaido Island. The OBS data were analysed together with land data on Hokkaido Island to determine the precise earthquake distribution and the geometry of the subducted plate. The OBS data revealed for the first time the seaward seismicity east of KSC. High and shallow seismicity is concentrated in an area between the coast line of Hokkaido Island and the 2000 m isobath. A region between the 2000 and 4000 m isobaths corresponds to the source area of an interplate earthquake occurring in 1973 (the Nemuro Hanto-Oki earthquake, M= 7.4). The OBS array detected more than 10 events in this region. Most of them, however, are classified as earthquakes occurring within the subducted plate, and the remarkable aftershock activity of the 1973 event is not recognized. The seismic activity is extremely low between the 4000 and 6000 m isobaths as is commonly seen in subduction zones. The seismicity is also very low beneath the trench, in contrast with the adjacent trench areas. In a vertical section taken perpendicular to the trench axis, most of the located events form a seismic plane dipping landward. The dip-angle of the plane is 20d-25d beneath the continental slope, while 35d-40d under Hokkaido Island. The change in dip angle occurs 150 km landward of the trench axis. This seismic plane is located 30-40 km below the plate boundary defined from the aftershock distribution of the Nemuro Hanto-Oki event. The seismicity on the plate boundary is correlated with the geometry of the subducted plate. The high activity occurs in a restricted region where the subducted plate shows a remarkable bend. The seismic activity obtained is different from those in the adjacent regions along the southern Kuril Trench. In particular, the regional difference in seismicity is remarkable between the western and eastern sides of KSC. Our result suggests that the subduction zone along the Kuril Trench is divided into several tectonic blocks and KSC is one of their boundaries.

San Fernando 地震の断層モデル

Simple dislocation fault models of the San Fernando, California, earthquake of 1971 as deduced from leveling data have been presented with emphasis on an abrupt change in dip angle of fault plane at a shallow depth, and geologic and geomorphic implications of the models have been discussed.Model-1 is a basic fault model ignoring the effects of sporadically distributed surface breaks. This model consists of two rectangular fault segments: the deeper (F1) and the shallower (F2). The geometry of F1 was fixed with the constraints that it should fit the focal mechanism and that the hypocenter should lie on F1. The geometry of F2 and the offsets on F1 and F2 were determined by trial and error. In Model-2, a subsidiary fault segment (F3) that represents the surface breaks was added in order to make displacement pattern near the surface breaks better fit.The results obtained are as follows:(1) The dip angle of F1 and F2 are 52° and 13° respectively. The dip of F2 is very low angle, so that the fault plane bends abruptly at a depth of about 2.5km.(2) The offsets on F1 and F2 are 1.5m and 5.5m respectively. The offset on F1 is notably smaller, so that no noticeable uplift appears in the San Gabriel Mountains which has been uplifted due to thrust faulting during late Cenozoic time as suggested by geological evidence. If we assume uniform offset on entire depths of fault plane, it becomes easier to avoid this contradiction. Therefore it seems likely that the deeper part of the fault had slipped seismically or possibly aseismically prior to the earthquake of 1971.(3) The geometry of fault models, together with some geological evidence, permits us to describe a probable history of structural evolution in the San Fernando Valley-San Gabriel Mountains border region: The Santa Susana fault, which is the projection of F1, had been active and thick basin deposit had been deposited on the down-thrown side of the fault until early Pleistocene time. In middle Pleistocene time, F2 was formed in, or on the base of, the basin deposit, and the Santa Susana fault became inactive. Consequently the location of surface faulting shifted several km south of the Santa Susana fault, and the up-thrown side of F2 has been uplifted and intensely deformed scince middle Pleistocene time.(4) Although the mechanism is unknown, the bending of fault plane can be explained in terms of an effect of “accretion” by the analogy of that in subduction zones. Such a bending model of fault plane may be applicable to some thrust fault systems in Japan which have very similar geologic and geomorphic settings to the San Fernando Valley-San Gabriel Mountains border region.

Variation of VLF propagation characteristics of the earth-ionosphere waveguide with dip angle

The variation of VLF propagation characteristics of the earth-ionosphere waveguide with change in dip angle is examined in the light of mode coupling and the penetration of the electromagnetic fields into the ionosphere. The above information is obtained as a function of the dip angle by solving coupled vacuum-mode equations for a cylindrical model of the earth-ionosphere waveguide. Propagation in the West-East (WE) and East-West (EW) directions for data corresponding to the first two modes is considered. The range of dip angles <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">20\deg-35\deg</tex> in which the solution of the modal equation is discontinuous is found to be characterized by very low reflection coefficients for the parallel polarized waves, and a deep penetration of the fields into the ionosphere. A comparison of the characteristics of the electromagnetic fields between WE and EW directions of propagations is made. Of the two modes which are predominantly TE modes, the WE mode is seen to have a deeper penetration of the fields, while the TM mode has a deeper penetration for the two modes in the EW direction. Plots of upgoing and downcoming components of electric fields in the ionosphere corresponding to the first two modes for WE and EW directions of propagation are given for various dip angles.