3-D ATTENUATION STRUCTURE FROM THE INVERSION OF MICROEARTHQUAKE PULSE WIDTH DATA: Inferences on the thermal state of the Campi Flegrei Caldera Aldo Zollo (1) and Salvatore de Lorenzo (2) (1) Dipartimento di Scienze Fisiche, Università di Napoli (2) Istituto di Geologia e Geofisica, Università di Bari

Research objectives 3-D attenuation structure of Campi Flegrei caldera from microearthquake data Relation between Q and T at macroscopic scale Present thermal state of the volcano

Contents C. Flegrei caldera: volcan. & geophys. setting Source and attenuation model The inversion method Resolution tests Results 1: source parameters Results 2: Qp images T vs Qp relationship Qp-inferred temperature field Conclusions Geo Setting Model The inversion method Synthetic tests Source parameters Qp images T vs Qp relation Thermal field Conclusions

Volcanological Setting Geo Setting Model The inversion method Synthetic tests Source parameters Qp images T vs Qp relation Thermal field Conclusions

Geophysical setting Geo Setting Model The inversion method Synthetic tests Source parameters Qp images T vs Qp relation Thermal field Conclusions

Unrest phenomena meters Geo Setting Model The inversion method Synthetic tests Source parameters Qp images T vs Qp relation Thermal field Conclusions

Local earthquake tomography: 3-D velocity model (Aster & Meyer,1988) Geo Setting Model The inversion method Synthetic tests Source parameters Qp images T vs Qp relation Thermal field Conclusions

Source model : Sato & Hirasawa(1973) circular crack constant Vr, stress drop Abrupt stop Attenuation model : Azimi et al.(1968) near-constant Q Geo Setting Model The inversion method Synthetic tests Source parameters Qp images T vs Qp relation Thermal field Conclusions Far-field velocity waveform U(t)=S SH (t) * Q AZ (t)

Rise-time & pulse width Geo Setting Model The inversion method Synthetic tests Source parameters Qp images T vs Qp relation Thermal field Conclusions

Rise-time Pulse width  o, source radius c, rupture velocity , take-off angle  o, rise-time for Q=   T o, pulse width for Q=  T, travel time Numerically-built relationships between rise-time, pulse width and model parameters Geo Setting Model The inversion method Synthetic tests Source parameters Qp images T vs Qp relation Thermal field Conclusions

Numerically-built relationships between rise-time, pulse width and model parameters Rise-time Pulse width Geo Setting Model The inversion method Synthetic tests Source parameters Qp images T vs Qp relation Thermal field Conclusions

Inversion scheme Model parameters: source radii, fault angles, Q in each block of a 3-D medium Data: rise-times, pulse widths from M eqs recorded by N stations Non-linear Method: search for a minimum of a cost function by Downhill Simplex Strategy: iterative loop on source and Q parameters Geo Setting Model The inversion method Synthetic tests Source parameters Qp images T vs Qp relation Thermal field Conclusions

The acquisition lay-out Geo Setting Model The inversion method Synthetic tests Source parameters Qp images T vs Qp relation Thermal field Conclusions

Estimate of source parameters 1 Geo Setting Model The inversion method Synthetic tests Source parameters Qp images T vs Qp relation Thermal field Conclusions

Estimate of source parameters 2 Geo Setting Model The inversion method Synthetic tests Source parameters Qp images T vs Qp relation Thermal field Conclusions

Estimate of attenuation parameters: chess-board test Geo Setting Model The inversion method Synthetic tests Source parameters Qp images T vs Qp relation Thermal field Conclusions

Estimate of attenuation parameters: “fixed-geometry” test 1 Geo Setting Model The inversion method Synthetic tests Source parameters Qp images T vs Qp relation Thermal field Conclusions

Estimate of attenuation parameters: “fixed-geometry” test 2 Geo Setting Model The inversion method Synthetic tests Source parameters Qp images T vs Qp relation Thermal field Conclusions

Example of P-pulse recordings Geo Setting Model The inversion method Synthetic tests Source parameters Qp images T vs Qp relation Thermal field Conclusions

Pulse width & rise-time measurements Geo Setting Model The inversion method Synthetic tests Source parameters Qp images T vs Qp relation Thermal field Conclusions

Residuals vs travel time Geo Setting Model The inversion method Synthetic tests Source parameters Qp images T vs Qp relation Thermal field Conclusions

Estimates of Source radii and stress drops Geo Setting Model The inversion method Synthetic tests Source parameters Qp images T vs Qp relation Thermal field Conclusions 1E-12E-1

Estimates of fault angles: Comparison with fault plane solutions Geo Setting Model The inversion method Synthetic tests Source parameters Qp images T vs Qp relation Thermal field Conclusions

P-wave attenuation spatial distribution 0<Z<1 1<Z<2 2<Z<3 Geo Setting Model The inversion method Synthetic tests Source parameters Qp images T vs Qp relation Thermal field Conclusions

Comparison between velocity and attenuation images Geo Setting Model The inversion method Synthetic tests Source parameters Qp images T vs Qp relation Thermal field Conclusions

Geothermal gradient: high pass filtered map Geo Setting Model The inversion method Synthetic tests Source parameters Qp images T vs Qp relation Thermal field Conclusions

Temperature measurements in boreholes Geo Setting Model The inversion method Synthetic tests Source parameters Qp images T vs Qp relation Thermal field Conclusions

Local calibration of T(Qp) relationship Geo Setting Model The inversion method Synthetic tests Source parameters Qp images T vs Qp relation Thermal field Conclusions

Qp-inferred temperature field Geo Setting Model The inversion method Synthetic tests Source parameters Qp images T vs Qp relation Thermal field Conclusions

3-D Heat conduction modeling of Qp-inferred temperature field: A simple two-blocks model Geo Setting Model The inversion method Synthetic tests Source parameters Qp images T vs Qp relation Thermal field Conclusions

3-D Heat conduction modeling of Qp-inferred temperature field: Results Geo Setting Model The inversion method Synthetic tests Source parameters Qp images T vs Qp relation Thermal field Conclusions

Low stress drops (0.1-1 bar ) Variable fault geometries from pulse width (only few events), consistent with fault plane solutions from P-polarities Local calibration of T(Qp) relationship Shallow (0-1 km) Qp-anomaly pattern correlates with areas of intense hot fluid circulation Deep (>2 km) low Qp- high T anomaly, correlates with low Vs. In this area the most recent eruptive activity and present hydrothermalism are concentrated A magma chamber top at 4-5 km depth is consistent with the Qp-inferred temperature field, assuming a simple model of heat conduction cooling of molten bodies. Geo Setting Model The inversion method Synthetic tests Source parameters Qp images T vs Qp relation Thermal field Conclusions

T HE E ND

Volcanological Setting Geo Setting Model The inversion method Synthetic tests Source parameters Qp images T vs Qp relation Thermal field Conclusions