Joint hypocentral determination and the detection of low-velocity
anomalies. An example from the Phlegraean Fields earthquakes
Bulletin of the Seismological Society of America, 80 (1):
129--139 (Feb 1, 1990)Abstract
Arrival time data from the Phlegraean Fields (Italy) earthquake swarm
recorded by the University of Wisconsin array in 1983 to 1984 were
reanalyzed using a joint hypocentral determination (JHD) technique.
The P- and S-wave station corrections computed as part of the JHD
analysis show a circular pattern of central positive values surrounded
by negative values whose magnitudes increase with distance from the
center of the pattern. This center roughly coincides with the point
of the maximum uplift (almost 2 m) associated with the swarm. Corrections
range from -0.85 to 0.10 sec for P-wave arrivals and from -1.09 to
0.70 sec for S-wave arrivals. We interpret these patterns of corrections
as caused by a localized low-velocity anomaly in the epicentral area,
which agrees with the results of a previous 3-D velocity inversion
of the same data set. The relocated (JHD) epicenters show less scatter
than the epicenters obtained in the velocity inversion, and move
more of the seismic activity to the vicinity of the only presently
active fumarolic feature. The capability of the JHD technique to
detect low-velocity anomalies and at the same time to give reliable
locations, particularly epicenters, was verified using synthetic
data generated for a 3-D velocity model roughly resembling the model
obtained by velocity inversion.