Modified frequency-wavenumber algorithm for regional seismograms using Filon's quadrature: modelling ofL_gwaves in eastern North America

Abstract

S U M M A R YThe main focus of this study is to develop an efficient frequency-wavenumber code to synthesize high-frequency (say, 10 Hz) regional seismograms (up to a distance of more than 1000 km) in a medium consisting of a large number of crustal layers, and investigate effects of regional waveguides on L, waves. In this newly developed code, the frequency-wavenumber response is evaluated by using the Filon's quadrature at high frequency and the polynomial approximation of Bessei's functions (implementing trapezoidal integration) at low frequency. This has reduced the computation time by several folds compared to the time needed for the trapezoidal integration. Using this algorithm, both broad-band seismograms at Harvard and short-period ECTN (Eastern Canadian Telemetered Network) seismograms from 1988 November 25 Saguenay earthquake were successfully modelled. A moment magnitude of 5.9 was used. Initially an average waveguide was calibrated to successfully excite the regional P,,, and S,,, waves at HRV, its near-surface structure constrained by matching the recorded frequency content. The crust-mantle transition zone was constrained by modelling the strength of the broad-band S,,, waves, consisting of S,,, sSmS,,, SmSSmS, and sSmSSmS phases, relative to the initial P,,, waves. The model was modified to consist of thin layers with an alternating highand low-velocity distribution to match the high-frequency ECTN Lg waves observed from 300 to 600 km away. In addition, the effect of a near-surface irregular receiver structure on the duration of L, waves was also investigated by convolving its response with the regional L, seismograms. Initial study suggests this method as viable for modelling the entire L, waves including the coda. Numerical studies suggest that the shapes and peak amplitudes on L, seismograms depend more strongly on the source depth, anelasticity and crustal waveguide than do the P,, seismograms.

Moment Tensor Inversions of Icequakes on Gornergletscher, Switzerland

“…Because this numerical implementation differs from the FKRPROG software (Saikia, 1994) used to generate Green's functions, variance reductions of waveform fits of synthetic data generated with the reflectivity method may not reach 100%. The synthetic seismograms were filtered with the same band-pass filters applied to the data.…”

“…As discussed in the following section, this is a good approximation to the velocity model of the glacier. The Green's functions were computed using the FKRPROG software developed by Chandan Saikia of URS (Saikia, 1994). The deviatoric-and full-moment tensor solutions were calculated by applying a corrected form (Minson et al, 2007) of the linear time domain moment tensor inversion scheme used by Dreger et al (2000) and Dreger and Woods (2002).…”

The 10 June 2012 MW 6.0 earthquake sequence in the easternmost end of the Hellenic arc.

“…A Laplacian-smoothing operator, slip positivity, and a scalar moment minimization constraint is applied to stabilize the inversions. Green's functions based on the Novotný et al (2001) velocity profile, shown to be effective in modelling regional wave propagation, were used to invert the seismic waveforms, and were calculated using the frequency-wavenumber integration method (Saikia, 1994) for 1-km intervals in distance and 1-km intervals in depth. The source model used is a single fault plane with constant rupture velocity and constant dislocation rise time.…”

“…Prior to the inversion, full broadband waveforms of the three recorded components were band-pass filtered between 0.02-0.08 Hz or 0.05-0.10 Hz depending on the magnitude of the event and the signal-to-noise ratio of the waveforms. Theoretical Green's functions required to model the propagation of the seismic waves were constructed with the method and code described by Saikia (1994) using the velocity model of Novotný et al (2001), which has proven to successfully describe low frequency wave propagation (e.g., Roumelioti et al,2010;Kiratzi, 2010, 2011 andreferences therein). In Figure 2 we show the moment tensor solution and the waveform fit for the mainshock, while the parameters of the computed focal mechanisms of the aftershocks are listed in …”

Source Parameters of Moderate and Strong Earthquakes in the Broader Area of Zakynthos Island (W. Greece) From Regional and Teleseismic Digital Recordings

“…From that moment it was quite easy to use the regional waveforms and the produced synthetic ones to calculate source parameters through a linear inversion procedure. One of the inversion methods is the frequency-wavenumber (F-K) integration method (Saikia, 1994;Zeng & Anderson, 1995;Bouchon, 2003). This method which is applied in the present study mainly for moderate earthquakes, calculates synthetic waves directly compared with the observed ones for a given velocity structure.…”