A broadband seismological investigation of the 1989 Loma Prieta, California, earthquake: Evidence for deep slow slip?

• DOI: 10.1785/bssa0810051622
• Publication Date: 1991-10-01
• Authors: Terry C. Wallace; A. A. Velasco; Jiajun Zhang; Thorne Lay
• Keywords: Microseism; Rayleigh Wave; Focal mechanism

Analysis of regional and teleseismic broadband body waves long-period surface from the 1989 Loma Prieta earthquake demonstrates that a fairly simple average fault model explains most characteristics seismic wave radiation over broad period range (1 to 300 sec), but there are some systematic period-dependent not yet understood. Determinations preferred point source focal mechanism consistent for different waves, as long optimal source-time functions depths chosen each wavetype. The major double couple orientation compatible with entire has strike = 128 +_ 3 °, dip 67 _+ 5 ° , rake 133 8% While this consistency is encouraging indicates process quite simple, discrepancies in parameter estimates waves. duration estimated principal body-wave ground motions 8 + 2 sec, near-field strong-motion vibrations, indicate total durations 18 30 depending on earth selected computing propagation corrections. dependence surface-wave results suggests it may be possible reconcile improved models, discrepancy large. There complexity function, comprised three subevents at depths, only manifested short-period signals does explain discrepancy. Changes geometry or slip direction during rupture, suggested by recent finite-fault inversions, unlikely cause bias estimates. centroid depth determined our analysis 10 12 km, which rupture extending km. Surface-wave (15 km) vary substantially choices global Q lithospheric structure region, they do tend deeper than results. It again clear whether due inadequate models. moment also fully consistent; inversions 2.4 0.3 x 1019 N-m, while gives 3.2 0.5 N-m. known reasonable changes models can bring into full agreement. duration, depth, moment, despite orientation, raise possibility deep, relatively slow co-seismic component, trade-offs modeling assumptions preclude us confirming hypothesis present.