Focal Mechanisms of Weak Earthquakes from Amplitude Spectra and Polarities

Abstract

Ð The ASPO method (Amplitude Spectra and POlarities) for the focal-mechanism retrieval from relatively weak events is based on a widely available instrumental setup: A few broadband stations within a denser short-period network. Collectively all stations provide the epicenter location. Complete records are taken from three-component broadband stations, without selecting a particular wave type, or picking amplitudes. It makes the method suitable for automated data processing, and enables studies of the interference crustal phases. Only the amplitude spectra are inverted. This is a robust feature which makes the method insensitive to any timing problems (such as those due to uncertain origin time, or due to technical failures). The ®rst-motion polarities serve as an additional constraint of the amplitude-spectra inversion; only few (clear) polarities are taken from the nearest stations, where they mostly belong to direct P waves. The method seeks ®ve parameters: The focal depth, scalar moment, strike, dip, and rake. Green's function, automatically including possible near-®eld eects and interference (e.g., surface) waves, is calculated by the discrete wavenumber method. ASPO works below the corner frequency, and the time function is not being retrieved. This feature not only minimizes the number of the inverted parameters, but also speeds up the calculation, because the lower the frequency, the faster the discrete wavenumber run. Instead of an exceedingly slow 5-parameter grid search, the inversion is organized in two steps: (i) the depth and moment determination with a coarse grid search of the strike, dip and rake, and (ii) a ®ne grid search of the three source angles. Uncertainty of the best-®tting solution is assessed from the minimum error value and from the scatter of the nodal lines (and/or P and T axes) between min and min + 10%. The method was tested on the clustered M » 3.5 earthquakes recorded by a temporary network of three CMG3-T broadband stations in western Corinth Gulf. A fundamental problem is that the broadband stations suer systematically from event-induced instabilities at horizontal components if earthquakes of the studied magnitudes occur at short distances, 10±30 km. Therefore, the ASPO method could not be applied below 0.1 Hz. As such, the results are sensitive with respect to unknown crustal structure details, and the focal mechanisms remain rather uncertain (minimum error higher than 0.34). Compared to synthetic tests with perturbed data, in which the error is lower than 0.2, it is concluded that the crustal model needs further improvement.

Moment magnitude – local magnitude relationship for the earthquakes of the Shillong-Mikir plateau, Northeastern India Region: a new perspective

“…Zahradnik et al (2001). Some of these focal mechanism solutions have been used by Angelier and Baruah (2009) The local magnitude M L estimates are provided by the IMD catalog.…”

“…The local moment tensor solutions presented here are determined using the code of Zahradnik et al (2001). In this method, the prime approach is to calculate Green's function for a trial depth by the discrete wave (DW) number of Bouchon (1981).…”

“…In this present study the moment tensor solutions of the local earthquakes 2.0 M L 5.0 have been estimated by the ASPO method (Amplitude Spectra and POlarities) (Zahradnik et al 2001). A sum of 106 moment tensor solutions is used to examine M W -M L relations; the results are highlighted here.…”

Solving seismological problems using SGRAPH program: I-source parameters and hypocentral location

“…Special thanks to D. Carroll for offering the micro-GA code, D. Helmberger for the GRT code, and J. Zahradník for the ASPO code [13] used in this program. Open source codes received from SAC [14], SEISAN [15], numerical recipes [8], and the Generic Mapping Tools (GMT) [12] are used.…”

Combined Inversion of Broadband and Short-Period Waveform Data for Regional Moment Tensors: A Case Study in the Alborz Mountains, Iran

“…Following the approach of Zahradník et al (2001), we used first-motion body-wave polarities as an additional constraint for the selection of the most reliable solution among all possible ones from the inversion. Moreover, we used all definable polarities from broadband and short-period data instead of only the very obvious ones.…”