ALBERT

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 [IMG]/medium/1341eq1.gif" ALT="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.