Showing papers by "Domenico Giardini published in 1994"

Isolated deep earthquakes and the fate of subduction in the mantle

Abstract: The locations of earthquakes with depth in the mantle provide direct evidence of the trajectory and state of stress of subducting lithosphere. The occurrence over the last decade of isolated large deep earthquakes outside the main Wadati-Benioff zones in the Kurile, Izu-Bonin, and Chile deep seismic areas (in addition to the observed flat distributions of deep seismicity beneath the Banda Sea and Fiji Plateau areas and the large number of deep earthquakes located away from the main slab in the Tonga arc) supports recent tomographic and other seismological studies, leading to a reconsideration of slabs as simple planar structures sinking undeformed into the lower mantle. The focal geometries of these isolated earthquakes, which often exhibit near vertical compressional stresses, are distinct from adjacent events in the main Wadati-Benioff zone. They are also different in orientation from events in the deflecting toe of the slab and from the stress orientation expected if the deflected slab acts as a stress guide. These isolated mantle earthquakes are interpreted to take place in subducted lithosphere that has deflected to a horizontal posture at the base of the upper mantle. Conversely, the deep seismicity in the adjacent Wadati-Benioff zone is associated with the deflection of the subducting lithosphere.

Broad-band moment tensor inversion from single station, regional surface waves for the 1990, NW-Iran earthquake sequence

Abstract: e present a method for the inversion of complete waveforms in the 5-30 mHz frequency band for moment tensor determination. The method is based on the calibration of phase and group velocity dispersion curves for Rayleigh and Love fundamental modes to account for heterogeneous lithospheric structure, and is applied to the analysis of single station records of the VSL MEDNET station for the 1990 NW Iran earthquake sequence (the events of June 20, 21 and 24). The revised seismic moment of the June 20, 1990 Iranian earthquake is Mo = 1.56 x 1027 dyne-cm, corresponding to Mw = 7.4. The method proves to be a very robust tool for the analysis of moderate and large earthquakes at regional distances, producing consistent moment tensor solutions trom single station inversions in narrow (2-4 mHz) and wide (up to 20 mHz) frequency bands across the whole band of interest.