Abstract
The 6 April 2009 Mw 6.3 L'Aquila earthquake, central Italy, has been
recorded by the Irpinia Seismic Network (ISNet) about 250 km southeast
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 back-azimuth 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 the southeast. 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 northwest-southeast 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-sized earthquakes located outside
the network.
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