Thrust-type Earthquakes Research Articles

Episodic Strain Accumulation in Southern California

Reexamination of horizontal geodetic data in the region of recently discovered aseismic uplift has demonstrated that equally unusual horizontal crustal deformation accompanied the development of the uplift. During this time interval compressive strains were oriented roughly normal to the San Andreas fault, suggesting that the uplift produced little shear strain accumulation across this fault. On the other hand, the orientation of the anomalous shear straining is consistent with strain accumulation across northdipping range-front thrusts like the San Fernando fault. Accordingly, the horizontal and vertical crustal deformation disclosed by geodetic observation is interpreted as a short epoch of rapid strain accumulation on these frontal faults. If this interpretation is correct, thrust-type earthquakes will eventually release the accumulated strains, but the geodetic data examined here cannot be used to estimate when these events might occur. However, observation of an unusual sequence of tilts prior to 1971 on a level line lying to the north of the magnitude 6.4 San Fernando earthquake offers some promise for precursor monitoring. The data are adequately explained by a simple model of up-dip aseismic slip propagation toward the 1971 epicentral region. These observations and the simple model that accounts for them suggest a conceptually straightforward monitoring scheme to search for similar uplift and tilt precursors within the uplifted region. Such premonitory effects could be detected by a combination of frequenlty repeated short (30 to 70 km in length) level line measurements, precise gravity traverses, and continuously recording gravimeters sited to the north of the active frontal thrust faults. Once identified, such precursors could be closely followed in space and time, and might then provide effective warnings of impending potentially destructive earth-quakes.

Premonitory changes in ts/tp due to anisotropy and migration of hypocenters

Abstract The anisotropic characteristics of stress-induced cracks in dilatant rock can significantly influence the observed values of ts/tp obtained using the Wadati diagram. Systematic vertical migration of hypocenters of foreshocks within a region of thrust tectonics can lead to variations in both ts/tp and orientation of the compressive stress axis of the same type as observed before several thrust-type earthquakes. In this model, the seismic velocities become considerably anisotropic due to preferential alignment of open cracks, but it is not essential to assume pre-earthquake flow of groundwater into the dilated zone. Observations of ts/tp versus depth of focus in the Garm region, USSR and some other results appear to support this possibility.

Distribution of seismicity before large strike slip and thrust-type earthquakes

Distribution of seismicity before large strike slip and thrust-type earthquakes

Pre-seismic crustal deformation caused by an underthrusting oceanic plate, in eastern Hokkaido, Japan

Eastern Hokkaido is being considered as one of the prime candidates for a future great earthquake in Japan. Geodetic work and seismological data suggest that the continental plate is being compressed and dragged down into the asthenosphere by the underthrusting Pacific plate. A quantitative examination of this idea was carried out by an application of the finite-element method to a static-field problem of crustal deformation. The effect of heterogeneous medium with Poisson's ratio differing from 0.25 is significant. An inverse problem was solved by applying a quadratic programming method to the observed vertical displacement. A model, which assumes a nearly uniform tangential displacement along the interface of the oceanic and continental lithospheres down to a depth of 100 km, explains a gravity-change as well as the general features of crustal deformation observed in eastern Hokkaido. The displacement rate is estimated to be 2.7 cm/year. It indicates that a rebound of the continental lithosphere now brings about a thrust-type earthquake whose reverse slip is comparable to that estimated for great shallow earthquakes along the Kuril trench. A possibility of aseismic rebound is suggested at the lower part of the interface.

Postseismic Viscoelastic Rebound

The sudden appearance of a dislocation, representing an earthquake, in an elastic layer (the lithosphere) overriding a viscoelastic half space (the asthenosphere) is followed by time-dependent surface deformation, which is very similar to in situ postseismic deformation. The spectacular postseismic deformation following the large Nankaido earthquake of 1946 yields for the asthenosphere a viscosity of 5 x 10(19) poise and a 50 percent relaxation of the shear modulus. Large thrust type earthquakes may provide, in the future, a new method for exploring the rheology of the earth's upper mantle.