Focal depths and moment tensor representations of shallow earthquakes associated with the Great Sumba earthquake

Journal Article · Sat Oct 10 04:00:00 EDT 1981 · J. Geophys. Res.; (United States)

DOI:https://doi.org/10.1029/JB086iB10p09357· OSTI ID:5543200

Fitch, T J; North, R G; Shields, M W

Focal depths in the range 8 to 24 km below the seafloor were determined for seven of the larger shocks associated with the Great Sumba earthquake of August 19, 1977. Depth control was achieved by modeling digitized long-period P wave recorded by the World-Wide Standard Seismic Network. If the base of the rupture at the time of the mainshock were no greater than the depth of the deepest aftershock, the stress drop would be nearly 500 bars. The inversion of Rayleigh and Love spectra computed from digital seismograms recorded by the Seismic Research Observatories (SRO) and the Abbreviated Seismic Research Observatories (ASRO) broadened the source study to include earthquakes with moments as small as 10/sup 24/ dyn cm: too small for either traditional first motion solutions or P wave modeling. The propagation phase for the Love and the Rayleigh waves was calibrated with signals from two earthquakes for which the source phase was inferred from the P wave solutions.Normal faulting along an east to northeasterly strike characterizes the mechanisms in the principal zone of aftershock activity. Strike slip mechanisms consistent with horizontal compression normal to the arc prevail in the delayed aftershock zone located 200 km northwest of the mainshock. Of the earthquakes subjected to this surface wave analysis the foreshock stood out as having felt a significantly higher apparent stress, perhaps as much as one half an order of magnitude higher than the others. These results suggest that accurate source parameters for moderate to small magnitude earthquakes can be determined from the surface waves recorded by the SRO and ASRO. In particular, the mechanisms of small magnitude foreshocks could be obtained in this way.

• Research Organization: Applied Seismology Group, MIT Lincoln Laboratory, Cambridge, Massachusetts 02142
• OSTI ID: 5543200
• Journal Information: J. Geophys. Res.; (United States), Journal Name: J. Geophys. Res.; (United States) Vol. 86:B10; ISSN JGREA
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
580201* -- Geophysics-- Seismology & Tectonics-- (1980-1989)
AFTERSHOCKS
AMPLITUDES
EARTHQUAKE FOCI
EARTHQUAKE MAGNITUDE
EIGENVALUES
FORESHOCKS
INVERSE SCATTERING PROBLEM
SEISMIC P WAVES
SEISMIC SURFACE WAVES
SEISMIC WAVES
WAVE PROPAGATION