Search

We present the surface wave dispersion results of the application of the ambient noise method to broad-band data recorded at 114 stations from the Istituto Nazionale di Geofisica e Vulcanologia (INGV) national broad-band network, some stations of the Mediterranean Very Broadband Seismographic Network (MedNet) and of the Austrian Central Institute for Meteorology and Geodynamics (ZAMG). Vertical-component ambient noise data from 2005 October to 2007 March have been cross-correlated for station-pairs to estimate fundamental mode Rayleigh wave Green's functions. Cross-correlations are calculated in 1-hr segments, stacked over periods varying between 3 months and 1.5 yr. Rayleigh wave group dispersion curves at periods from 8 to 44 s were determined using the multiple-filter analysis technique. The study region was divided into a grid to invert for group velocity distributions. Checkerboard tests were first carried out, and the lateral resolution was estimated to be about 0.6°. The resulting group velocity maps from 8 to 36 s show the significant difference of the crustal structure and good correlations with known geological and tectonic features in the study region. The Po Plain and the Southern Alps evidence lower group velocities due to soft alluvial deposits, and thick terrigenous sediments. Our results also clearly showed that the Tyrrhenian Sea is characterized with much higher velocities below 8 km than the Italian peninsula and the Adriatic Sea which indicates a thin oceanic crust beneath the Tyrrhenian Sea. [ABSTRACT FROM AUTHOR]

We present a duration–amplitude procedure for rapid determination of a moment magnitude, Mwpd, for large earthquakes using P-wave recordings at teleseismic distances. Mwpd can be obtained within 20 min or less after the event origin time as the required data are currently available in near real time. The procedure determines apparent source durations, T0, from high-frequency, P-wave records, and estimates moments through integration of broad-band displacement waveforms over the interval tP to tP+ T0, where tP is the P-arrival time. We apply the duration–amplitude methodology to 79 recent, large earthquakes (global centroid-moment-tensor magnitude, MCMTw, 6.6–9.3) with diverse source types. The results show that a scaling of the moment estimates for interplate thrust and possibly tsunami earthquakes is necessary to best match MCMTw. With this scaling, Mwpd matches MCMTw typically within ±0.2 magnitude units, with a standard deviation of σ= 0.11, equaling or outperforming other approaches to rapid magnitude determination. Furthermore, Mwpd does not exhibit saturation; that is, for the largest events, Mwpd does not systematically underestimate MCMTw. The obtained durations and duration–amplitude moments allow rapid estimation of an energy-to-moment parameter Θ* used for identification of tsunami earthquakes. Our results show that Θ*≤−5.7 is an appropriate cut-off for this identification, but also show that neither Θ* nor Mw is a good indicator for tsunamigenic events in general. For these events, we find that a reliable indicator is simply that the duration T0 is greater than about 50 s. The explicit use of the source duration for integration of displacement seismograms, the moment scaling and other characteristics of the duration–amplitude methodology make it an extension of the widely used, Mwp, rapid magnitude procedure. The need for a moment scaling for interplate thrust and possibly tsunami earthquakes may have important implications for the source physics of these events. [ABSTRACT FROM AUTHOR]