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 rn 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 300-60 ø (with an average uncertainty of 12ø); the fault strikes mainly range between -60 ø and 60 ø and seem to define preferential directions oriented radially from the synm•etry 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 threedimensional Qv 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 1ow-Qp zone and low-V• (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-Qv 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 (450ø-500øC) 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 Q•, images has been interpreted by using a three-dimensional thermal conduction model assuming an extended heat source (initial temperature of 800øC) located underneath the attenuation anomalous region. The results indicate that the Qv-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.
Abstract. We propose a new method to determine source parameters and attenuation structure of a three-dimensional medium based on first P rise time and total pulse width measurements from microearthquake data. The effects of fault finiteness on seismic radiation are taken into account by assuming the rupture model for a circular crack of Sato and Hirasawa (1973). Ray theory synthetic seismograms in a constant Q anelastic medium are computed to derive a set of nonlinear equations which relate the source and attenuation parameters (fault radius, orientation of the fault plane, and quality factor) to the pulse width data (half and total duration of the P waveforms).The numerically built relationships are used to compute the direct problem in the framework of a nonlinear inversion scheme, based on the modified downhill Simplex method. The validity and robustness of the inversion method are tested by synthetic simulations by assuming the sources and receivers configuration of the seismic passive experiment conducted in the Campi Flegrei caldera (southern Italy) during the last microearthquake crisis (1982)(1983)(1984). Different heterogeneous Q models have been considered in order to assess the uncertainty and resolution of source and attenuation parameters for the given acquisition layout. The results of this simulation study indicate that first pulse width data from a local network permit retrieval with sufficient accuracy of the heterogeneous Q structure and fault radii. A rather dense azimuthal coverage of the sources is instead needed to recover the angles (in particular, the fault strike) which define the fault orientation.
The present heat flow in the southern Tyrrhenian Sea appears as a transient thermal wave that has migrated eastward in time. The higher heat flow in the south‐eastern side of the basin confirms the suggestion of an eastward‐migrating rift. Punctuation of the Tyrrhenian backarc extension in lithospheric boudins is accompanied by a concentrated increase in heat flow generated by asthenospheric intrusions and related magmatism progressively moving eastward. The migration of the asthenosphere in the same direction could explain these phenomena.
Historical seismic catalogs report that the Gargano Promontory (southern Italy) was affected in the past by earthquakes with medium to high estimated magnitude. From the instrumental seismicity, it can be identified that the most energetic Apulian sequence occurred in 1995 with a main shock of MW = 5.2 followed by about 200 aftershocks with a maximum magnitude of 3.7. The most energetic earthquakes of the past are attributed to right-lateral strike-slip faults, while there is evidence that the present-day seismicity occur on thrust or thrust-strike faults. In this article, we show a detailed study on focal mechanisms and stress field obtained by micro-seismicity recorded from April 2013 until the present time in the Gargano Promontory and surrounding regions. Seismic waveforms are collected from the OTRIONS Seismic Network (OSN), from the Italian National Seismic Network (RSN), and integrated with data from the Italian National Accelerometric Network (RAN) in order to provide a robust dataset of earthquake localizations and focal mechanisms. The effect of uncertainties of the velocity model on fault plane solutions (FPS) has been also evaluated indicating the robustness of the results. The computed stress field indicates a deep compressive faulting with maximum horizontal compressive stress, SHmax, trending NW-SE. The seismicity pattern analysis indicates that the whole crust is seismically involved up to a depth of 40 km and indicates the presence of a low-angle seismogenic surface trending SW-NE and dipping SE-NW, similar to the Gargano–Dubrovnik lineament. Shallower events, along the eastern sector of the Mattinata Fault (MF), are W-E dextral strike-slip fault. Therefore, we hypothesized that the seismicity is locally facilitated by preexisting multidirectional fractures, confirmed by the heterogeneity of focal mechanisms, and explained by the different reactivation processes in opposite directions over the time, involving the Mattinata shear zone.
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