Empirical Green's functions: A comparison between pulse width measurementsand deconvolution by spectral division

Data from a microearthquake cluster in northern Switzerland and synthetic s eismograms simulating the observed signals are used to compare two differen t techniques of obtaining information ai;out earthquake source-time functio ns. Comparisons between the observed P-wave velocity pulse widths and the r ise times of far-field displacement pulses obtained from empirical Green's function (EGF) deconvolutions show significant discrepancies. Whereas the o bserved velocity pulse widths of the larger events scale with seismic momen t over a broad range, this scaling is practically lost in the deconvolution s. The reason is that velocity pulse widths are usually measured at high tr ace magnifications from the first break to the firstly zero crossing. At lo wer magnifications, these pulse: widths are seen to include an emergent ons et, which can be attributed to an initial phase of gradual rupture accelera tion and whose duration scales with moment, Synthetic simulations, based on a source model of a circular crack with constant stress;drop and rupture p ropagating outward from the center with a gradually increasing velocity, co rrectly reproduce these emergent onsets. Deconvolutions using the synthetic signals show that the slow initial phase is masked by the noise amplificat ion and stabilizing measures inherent in the deconvolution. Therefore, desp ite the uncertainties in the necessary corrections for attenuation and scat tering along the path, relative pulse width measurements are more reliable and provide better resolution for small earthquakes than rise-time measurem ents on far-field displacement pulses obtained from EGF deconvolutions by s pectral division.