A M6.3 earthquake occurred on March 2nd 1987 beneath the Rangitaiki Plain at the NE end of the Central Volcanic Zone, North Island, New Zealand. Surface faulting and regional subsidence of up to 2 m are consistent with shallow depth (about 12 km) extensional normal faulting. The most prominent rupture developed along a previously unrecognized active fault trace over a distance of 7km with maximum vertical displacement of 1.5 m and extension of about 1.2 m. The vertical movement almost doubled the height of the pre-earthquake scarp. Compressional features developed on either side of the main rupture and many sand fountains were erupted over the area of regional subsidence where the water table was near the surface. Continuity of stratigraphy across the main fault which contains tephra layers and pumice alluvium indicates that possibly two faulting events took place prior to 1.8 Ky B.P. with the same sense of movement as the March 2nd earthquake. Buried tephras and 6.5 Ky shells indicate an average subsidence rate of about 2 mm/yr over this period for the same area that underwent subsidence during the latest event. The effects of the March 2nd earthquake and previous earthquakes are consistent with the extension that is taking place across the Central Volcanic Zone.

View full abstract

In order to elucidate the generating mechanism of microearthquakes, we analyzed three earthquake groups around the Yamasaki fault system in southwest Japan. It has been revealed that the distribution of hypocenters in an earthquake group consists of several "clusters." Events in each cluster have the common fault plane with linear dimension of about 100 to 200 m. The fault plane is considered to be a small unit of heterogeneous structure in the source region, and it imposes restrictions on the fracture process of microearthquake sequences. Events with M≤2.5 are fractures within one fault plane. On the other hand, events with M≥2.5-3.0 are found to be multiple shocks composed of two or three subevents. It was made clear that one of the multiple shocks fractured two fault planes of clusters successively. In a series of fractures on the fault plane of a cluster, the migration of hypocenters and the increase of stress drops are observed before the final complete fracture. This can be explained by small strong patches on the fault plane of the cluster.

View full abstract

We propose a kinematic model of plate convergence at the Nankai trough, a convergent boundary between the Philippine Sea and Asian plates. By using a two-dimensional finite element technique, we take account of some structural inhomogeneities of the crust and upper mantle; the Philippine Sea plate is subducting into the asthenosphere underlying the anomalously thin Asian plate, where the plate boundary is coupled tightly in the shallower portion but loosely in the deeper. The model is inferred from preseismic, coseismic, and postseismic changes in surface elevation associated with the 1946 Nankaido earthquake (M=8.2). These changes are precisely estimated from the first-order leveling data (1890-1980) by using an epoch reduction method. The preseismic seaward tilt can be interpreted by a steady state subduction of the Philippine Sea plate with a convergence rate of 4.5 cm/yr. The coseismic surface elevation change is well explained by a low-angle thrust faulting with an average shear stress drop of 2.0 MPa; although the faulting propagates along a lower portion of the plate boundary, it extends upward to the earth's surface branching away from the boundary at a depth of 22 km. The postseismic surface movements are interpreted by superposition of the viscoelastic response to the coseismic faulting and the effect of the steady state subduction of the Philippine Sea plate. In particular, the viscoelastic response of the loosely coupled part of the plate boundary is the underlying mechanism of the postseismic uplift localized in the coseismically subsided region. If the steady state subduction of the Philippine Sea plate continues for 150 yr, the shear stress averaged over the fault plane is found to amount to 2.0 MPa, which is sufficiently high to cause a major subduction earthquake. Hence, we can predict that the cyclic process of stress accumulation and release at the Nankai trough can be repeated every 150 yr.

View full abstract

The directions of the polarizations of the leading shear-wave, recorded by the United Kingdom seismometer network (UKNET) of the British Geological Survey, have been analyzed to investigate the anisotropic structure beneath the United Kingdom. Fifty-nine short-period and seventeen long-period records from 15 intermediate and deep earthquakes, mainly from the northwestern Pacific area, were used to identify the polarization directions. The short-period shear-waves show polarization directions primarily scattered around the polarization directions of long-period (about 20 s) shear-waves. Since such long-period shear-waves can preserve the polarization directions radiated at the source fixed by the geometry and orientation of the focal mechanism, the short-period polarization directions are primarily the same as the source polarization directions. This can be interpreted as indicating that anisotropy beneath the UK, if it exists, is weak and not uniformly aligned, and is localized possibly into several blocks.

View full abstract