A threshold-based earthquake early warning using dense accelerometer networks (original) (raw)
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Bulletin of the Seismological Society of America, 2011
The Mw 6.3 L'Aquila earthquake, central Italy, on April 6, 2009 has been recorded by the Irpinia Seismic Network (ISNet) about 250 km south-east of the epicenter. Up to 19 three-component accelerometer stations could be used to infer the main source parameters with different seismological methods. We obtained an approximate location of the event from arrival times and array-based backazimuth measurements and estimated the local magnitude (6.1) from an attenuation relation for southern Italy. Assuming an omega-square spectral model we inverted S-wave displacement spectra for moment magnitude (6.3), corner frequency (0.33 Hz), stress drop (2.5 MPa) and apparent stress (1.6 MPa). Waveform modeling using a point source and an extended source model provided consistent moment tensors with a centroid depth around 6 km and a prevalently normal fault plane solution with a dominant directivity toward south-east. The relatively high corner frequency and an overestimated moment magnitude of 6.4 from moment tensor inversions are attributed to the rupture directivity effect. To image the rupture geometry we implemented a beamforming technique that back-projects the recorded direct P-wave amplitudes into the earthquake source region. A NW-SE striking rupture of 17 km length is imaged, propagating with an average velocity up to 3 km/s. This value is significantly higher than our estimate of 2.2 km/s from S-wave spectra. Our case study demonstrates that the use of array techniques and a dense accelerometer network can provide quick and robust estimates of source parameters of moderate-size earthquakes located outside the network.