Earthquake mechanisms from linear-programming inversion of seismic-wave amplitude ratios

• Main Authors: Bruce R. Julian, G. R. Foulger
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Published: 1996
• Subjects: Grensdalur; Hengill; Iceland
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.548.3113; http://community.dur.ac.uk/g.r.foulger/Offprints/JulianFoulgerBSSA1996.pdf

Abstract The amplitudes of radiated seismic waves contain far more information about earthquake source mechanisms than do first-motion polarities, but amplitudes are severely distorted by the effects of heterogeneity n the Earth. This distortion can be reduced greatly by using the ratios of amplitudes of appropriately chosen seismic phases, rather than simple amplitudes, but existing methods for inverting amplitude ratios are severely nonlinear and require computationally intensive searching methods to ensure that solutions are globally optimal. Searching methods are particularly costly if general (moment tensor) mechanisms are allowed. Efficient linear-program-ming methods, which do not suffer from these problems, have previously been ap-plied to inverting polarities and wave amplitudes. We extend these methods to am-plitude ratios, in which formulation on inequality constraint for an amplitude ratio takes the same mathematical form as a polarity observation. Three-component digital data for an earthquake at the Hengill-Grensdalur geothermal rea in southwestern Iceland illustrate the power of the method. Polarities of P, SH, and SV waves, unu-sually well distributed on the focal sphere, cannot distinguish between diverse mech-anisms, including a double couple. Amplitude ratios, on the other hand, clearly rule out the double-couple solution and require a large explosive isotropic omponent.