Improving Epicentral and Magnitude Estimation of Earthquakes from T Phases by Considering the Excitation Function

Abstract

A standard technique for locating events with T phases is to pick the peak energy of T phases as the arrival time, then proceed as if it was an unscattered phase originating at the epicenter. The peak energy arrival time, however, can shift to different parts of the wave train due to incoherent scattering. We show that a 50% reduction in variance relative to picks of peak arrival times can be achieved by fitting an assumed functional shape to the log of the entire envelope of the T phase. We test the stability of this approach by comparing relative event locations based on this method with those determined by cross-correlating waveforms of Rayleigh and Love surface waves recorded teleseismically using a swarm of earthquakes at the northern end of the Easter microplate as an example. Relative event locations show that there is no systematic bias in T-phase locations. The T-phase location and detection can be extended to much smaller events than are detectable by surface waves. We estimate T-phase maximum amplitudes of about 50 events within the swarm from the amplitude of the fitted functions rather than directly from the seismograms. Twenty-four of these events are large enough to estimate their magnitudes by surfacewave analysis. The empirical analysis shows the log of T-phase maximum amplitude and surface-wave magnitude (M S) exhibit a relatively uniform linear relationship with much less scatter than in previously published studies of T-phase amplitude versus magnitude. The reduction in scatter is due to both the stability of the functional estimate of amplitude and the use of a swarm of earthquakes with similar depth, mechanism, and source-receiver geometry. The observed T-phase coda for shallow earthquakes can be synthesized using a simple model of multiple-reverberation seafloor scattering. The results show that scattering of energy at a rough seafloor from multiple reverberations in the water column is significant in T-phase generation even where the water depths are relatively uniform.

Oceanic lithosphere magnetization : marine magnetic investigations of crustal accretion and tectonic processes in mid-ocean ridge environments

“…As a result, the T-phase location does not coincide with a point on the seafloor directly above the earthquake hypocenter. There have been several efforts to derive a relationship between seismic magnitude and acoustic magnitude [Dziak et al, 1997;Dziak, 2001;Fox et al, 2001;Bohnenstiehl, 2001;Pulli and Upton, 2002;Yang and Forsyth, 2003;Pan andDziewonski, 2005]. There are several problems, however, because in the past large magnitude earthquakes were clipped by the hydrophone sensors (mb > 4.7 [Fox et al, 2001]) making the estimate of T-phase magnitude unreliable (this problem is being eliminated with the larger dynamic range in more recent hydrophone systems).…”

“…The source of the T-phase therefore may be a large area of the seafloor that includes water depths above and below the critical depth (a broad area of insonification is also discussed by de Groot-Hedlin andOrcutt [1999, 2001] and Yang and Forsyth [2003]). Until the mechanisms of seismic energy coupling into the water column and conversion to acoustic energy are understood (whether it can be explained by ray or wave theory) this problem will remain.…”

“…We quantify the onset time following the curve fitting method of Yang and Forsyth [2003] (details can be found in Appendix C). The onset time was only calculated for those events with a signal-to-noise ratio (SNR) >2 and with latitude and longitude errors <0.030 (27 events at Atlantis and 17 events at Kane) so as to exclude those events with anomalously high ambient noise levels.…”

“…Appendix C: Curve Fitting [74] We quantify the onset time following the curve fitting method of Yang and Forsyth [2003]. Before computing the complex envelope, the time series is band pass filtered in an octave centered at 16 Hz.…”

Observations of Earthquake-Generated T-Waves in the South China Sea: Possible Applications for Regional Seismic Monitoring

“…Consequently, these new studies have recorded important data regarding natural earthquakegenerated T-waves, which has led to a wide variety of novel seismic applications for these acoustic phases propagating through seawater. For example, T-phases have been used to locate remote earthquake epicenters that are situated far out in the deep ocean with a high degree of accuracy (Yang and Forsyth 2003). In another study, T-phases recorded using a small hydroacoustic array were used to study the pattern of rupture propagation and the geographic extent of the source rupture for the M w = 9.0, 2004 Sumatra earthquake (Guilbert et al 2005).…”