ALBERT

Description: We address a classic problem in observational seismology, namely the precise estimation of the focal depth of small to moderate earthquakes. The data available are three-component records including Pn onsets and their coda. We believe that the Pn coda contains P -type phases that originate in the source area and lag behind due to longer travel paths. Phases such as pP and sP are generally not included in seismic bulletins, and even cepstrum and array f - k techniques have not been successful in extracting such arrivals. We use recent developments in polarization analysis ( Roberts and Christoffersson, 1990 ; Fedorenko et al. , 2008 ) to extract secondary P phases in local and regional records from five earthquakes. Phase characteristics are typically slowness and lag time relative to the preceding Pn phase. However, information is incomplete for confidently deciding whether an arriving wavelet should be classed as pP , sP , or something else. This validation problem is resolved by picking several coda arrivals that can uniquely be fitted to travel-time curves for suites of Pn -coda waves. Focal-depth estimates, even for Ground Truth 05 events, are seldom more accurate than to 5 km, and therefore we compared our results with those from moment tensor analysis. Differences in depth estimates were less than 5 km and for H 〈12 km the first secondary Pn arrival is generally sPn . The Dt ( Pn – pPn ) lag time for shallow events is 2.0 s or less and hence difficult to detect in the coda. Most techniques for focal-depth estimation are not user friendly. In contrast, the polarization schemes tested are easy to use and depth estimates are more accurate than those derived from ground truth studies based exclusively on P -wave arrivals.

Description: We address a classic problem in observational seismology, namely the precise estimation of the focal depth of small to moderate earthquakes. The data available are three-component records including Pn onsets and their coda. We believe that the Pn coda contains P-type phases that originate in the source area and lag behind due to longer travel paths. Phases such as pP and sP are generally not included in seismic bulletins, and even cepstrum and array f-k techniques have not been successful in extracting such arrivals. We use recent developments in polarization analysis (Roberts and Christoffersson, 1990; Fedorenko et al., 2008) to extract secondary P phases in local and regional records from five earthquakes. Phase characteristics are typically slowness and lag time relative to the preceding Pn phase. However, information is incomplete for confidently deciding whether an arriving wavelet should be classed as pP, sP, or something else. This validation problem is resolved by picking several coda arrivals that can uniquely be fitted to travel-time curves for suites of Pn-coda waves. Focal-depth estimates, even for Ground Truth 05 events, are seldom more accurate than to 5 km, and therefore we compared our results with those from moment tensor analysis. Differences in depth estimates were less than 5 km and for H〈12 km the first secondary Pn arrival is generally sPn. The Dt(Pn-pPn) lag time for shallow events is 2.0 s or less and hence difficult to detect in the coda. Most techniques for focal-depth estimation are not user friendly. In contrast, the polarization schemes tested are easy to use and depth estimates are more accurate than those derived from ground truth studies based exclusively on P-wave arrivals.

Description: Lg waves are generally the most prominent phase in seismic records at local and regional distances. They are dispersive and have complex waveforms. Their propagation characteristics, such as phase and group velocity, are not as easily related to formal crustal model features, as are P and S velocities. However, slowly propagating Lg waves may provide important travel-time observations in the context of epicenter location, but this potential has not been realized. The main reason is that Lg phases arrive in the coda of S waves and that the original waveforms are complex. The latter was resolved by replacing waveforms with their envelopes using the short-term average (STA) transform. We address the Lg arrival-time picking problem by associating time picks with the Lg amplitude maxima (Z component) in the record envelope. Validations of Lg onsets are not easy due to the lack of accurate Lg travel-time curves. Instead Lg times are converted to group velocities and station-wise consistency of velocities for closely spaced earthquake sources is required. With rare exceptions these group velocities agree within Formula . We also tested the potential of our Lg arrival times for the epicenters location using the grid search scheme of Rodi (2006). On a stand alone basis the locations were almost identical to the International Seismological Centre (ISC) and the National Earthquake Information Center (NEIC) epicenter solutions. A practical problem with introducing joint P-Lg locations is that these phases do not have similar wave paths, and besides Lg travel times are not focal depth dependent. The latter result stems from our Lg synthetic record analysis. We conclude that the Lg arrival-time picking procedure developed is robust and convenient for near real time analysis. Because of its slow propagation velocities between 2.8 and 3.7 km/s, use of Lg arrival times can enhance focal parameter estimates reported by seismological centers around the world.