Automated Identification of Teleseismically Recorded Depth Phases with Application to Improving Subduction Zone Earthquake Locat

Automated Identification of Teleseismically Recorded Depth Phases with Application to Improving Subduction Zone Earthquake Locations Figure 1:
Example of the automated depth phase identification procedure applied to a Mw 5.5 earthquake occurring at 40 km depth in the Sunda subduction zone. (top) Velocity and displacement waveforms recorded at GSN station AAK. The theoretical arrival times for P, pP and sP (red) and the new reported arrivals (purple) following the automated procedure are shown. (middle, bottom) The automated method uses abrupt changes the gradient of the power spectral density function (PSD) of both the velocity and displacement time series, termed triggers (green), and associates triggers with likely phases. The initial identifications may be dynamically reidentified during the EHB relocation process.
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A large portion of the seismogenic megathrust of most subduction zones lies beneath the ocean, which ultimately limits the amount of local seismic data available for high precision earthquake location studies. Standard single-event teleseismic earthquake locations are limited in their ability to provide the level of precision necessary to properly evaluate spatio-temporal aftershock variability, to constrain subducting plate geometry, or to investigate temporal strain release. Earthquake locations in global catalogs can be in error by 10's of kilometers in location and depth due to the use of imprecise arrival time picks, phase misidentification, poor station coverage, etc. Improvements to teleseismic event location, especially in depth, can be accomplished using more accurate depth phase arrival times. We have developed a new method to identify depth phases (pP, pwP, sP) and improve P onset times that takes advantage of the high quality IRIS Global Seismic Network waveforms available through the IRIS Data Management Center. The picker uses abrupt amplitude changes of the power spectral density (PSD) function calculated at optimized frequencies for each waveform. The technique identifies depth phases not reported in the standard catalogs and works over a wide-range of depths. The additional depth phases are incorporated with the ISS, ISC, and NEIC phase catalogs and relocated using the EHB teleseismic location approach [Engdahl et al., 1998]. The automatically identified depth phases and revised EHB catalogs are being used for two subduction zone studies: 1) Teleseismic double-difference relocation and tomography along the Andaman-Sunda subduction zone, which takes advantage of the extensive global recordings of the 2004, 2005, and 2007 great (M>8) Sumatra earthquakes [i.e. Pesicek et al., 2010]; 2) A global study characterizing earthquake source durations in subduction zones, with a focus on identifying unusually slow source processes such as those associated with tsunami earthquakes [i.e. Bisrat et al., 2009]. To date, we have provided nearly 29,000 new phases to the EHB catalog for ~1250 earthquakes in the Sumatra, Java, Kurile, Japan, Peru, and Alaska subduction zones. Epicentral changes following relocation using additional depth phases are generally small (<5 km). Changes in depth may be on the order of 5-20 km for some events, while the standard deviation of depth changes within each subduction zone is ~5 km.
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References
</p><p>Bisrat, S.T., H.R. DeShon, E.R. Engdahl, S.L. Bilek (2009), Improved Teleseismic Locations of Shallow Subduction Zone Earthquakes, Eos Trans. AGU, 90(52), Fall Meet. Suppl., Abstract T23B-1913.
</p><p>Engdahl, R.E, R. Hilst and R. Buland (1998), Global tele- seismic earthquake relocation with improved travel times and procedures for depth determination. Bull. Seismol. Soc. Amer., 88, 722-743.
</p><p>Pesicek, J.D., C.H. Thurber, H. Zhang, H.R. DeShon, and E.R. Engdahl (2010), Teleseismic Double-difference Relocation of Earthquakes along the Sumatra-Andaman Subduction Zone using a Three-Dimensional Model, J. Geophys. Res., (in press).
</p><p>Acknowledgements: We gratefully acknowledge NSF EAR and OCE support (EAR0608988, OCE0841022 to HRD; EAR0609613, OCE0841040 to ERE; OCE0941077 to SLB; EAR0608988 to CT).</p>

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