Article,
Source parameters and three-dimensional attenuation structure from inversion of microearthquake pulse width data: Qp imaging and interferences on the thermal state of the Campi Flegrei caldera, (southern Italy)
Journal of Geophysical Research, 106 (B8): 16265--16286 (August 2001)
DOI: 10.1029/2000JB900462
Abstract
The three-dimensional P wave attenuation structure of the Campi Flegrei
caldera and the estimate of source parameters for 87 local microearthquakes
is obtained by the nonlinear inversion of pulse width and rise time
measurements by using the method described by Zollo and de Lorenzo
(this issue). Source radii represent the better resolved parameters
with values ranging from 70 m to 230 m; the dip and strike angles
defining fault orientations are usually affected by larger uncertainties
and are well constrained only for 11 events. The dip fault is usually
confined in the range 30-60 deg (with an average uncertainty of 12
deg); the fault strikes mainly range between -60 deg and 60 deg and
seem to define preferential directions oriented radially from the
symmetry axis of the ground deformation. Stress drop estimates indicate
rather low values (0.01-1 MPa) which suggest low strength properties
of the incoherent and brittle materials filling the caldera (primarily
yellow tuffs). The three-dimensional Qp images obtained from the
inversion of P pulse durations show two significant low-Qp anomalies
between 0 and 1 km of depth, in the north-eastern sector and at 2-3
km of depth in the central eastern sector of the caldera. The high
degree of spatial correlation of the low-Qp zone and low-Vs (as inferred
by Aster and Meyer (1988)) at 0-1 km in depth and other geophysical
and geochemical observations suggest that this anomaly can be related
to the presence of densely fractured, porous, and fluid-filled rocks
in the NE sector of the caldera. The deeper low-Qp anomaly is interpreted
as being related to a dominant thermal effect. We used the surface
and deep borehole temperature measurements available in the area
to obtain a local calibration curve to convert Qp in temperature
at Campi Flegrei. The retrieved T(Qp) map shows a high thermal deep
disturbance (450-500C) at depths between 2 and 3 km in the eastern
sector of the caldera, where the most recent eruptive activity is
concentrated. The present-day temperature field retrieved by Qp images
has been interpreted by using a three-dimensional thermal conduction
model assuming an extended heat source (initial temperature of 800C)
located underneath the attenuation anomalous region. The results
indicate that the Qp-inferred temperature field can be related to
the heat conduction effect of one or more molten bodies whose top
should be at about 4-km depth, consistent with recent seismic estimates
of the magma chamber top at Campi Flegrei (Ferrucci et al., 1992).
This study suggests that the present thermal state and rock rheology
of the inner caldera could be controlled by the cooling of molten
bodies that originally intruded at depths of 1.4-1.6 km, during one
or more recent (time of <10 kyr) eruptive events.
