Modeling and imaging asperities on a fault plane and characterizing spatial and temporal patterns of precursory seismicity.

摘要

First, quasi-static models were used to explore the failure of a strong stuck asperity on an otherwise creeping fault plane. They produced the temporal scaling observed at Parkfield. However such scaling requires the constant density of unit asperities within the cluster. It rules out the Cantor dust fractal model suggested at Parkfield. Although the average stress drop for asperity models decreases with earthquake size, it is significantly lower over the entire rupture area, equivalent to that estimated from spectral analysis. The fracture energy is estimated to be G ≥ 10 7 J/m2, near the upper limit of estimates on the San Andreas Fault.; Next, the rupture process of the 1999 Chi-Chi, Taiwan earthquake was explored from high-frequency near-field strong-motion seismograms. The entire mainshock was resolved into a sequence of distinct bursts in high frequency, each of which corresponds to a sub-event from an asperity. Their origin times, locations and magnitudes were determined from a pre-determined Chelungpu fault. The first appeared sub-events follow the Chelungpu rupture propagation at a velocity of 2.0 km/s. Later sub-events can be interpreted as aftershocks that begin before the rupture has terminated. These sub-events have the Gutenberg-Richter b-value of 1.0. Spatially, the larger sub-events are located at greater depth, while the small sub-events are only located at shallower depths. Overall, they accord with results of waveform inversions.; Finally, the Gutenberg-Richter a- and b-values, cluster statistics, and the migration of seismicity were measured as functions of magnitude, space, and time before the 1992 Landers California earthquake using raw and declustered catalogs. The pronounced increase in a-value for distances less than 120km to the Landers as well as its un-correlation with changes in b-value indicate an increase of events at all magnitudes. More and larger clusters were formed with time before the Landers mainshock, which reflected smoother and more spatially correlated regional stress fields before a large event. Foreshock migration towards the Landers mainshock was observed in the active stress lobes defined in the stress recovery model, but not in randomly selected regions.