Intrinsic Qp at Mt. Etna from the inversion of rise times of 2002 microearthquake sequence

S De Lorenzo S De Lorenzo,D Patanè D Patanè,E Giampiccolo E Giampiccolo,M Filippucci M Filippucci

doi:10.4401/ag-3101

Abstract

About three-hundred microearthquakes, preceeding and accompanying the 2002-2003 Mt. Etna flank eruption, were considered in this study. On the high-quality velocity seismograms, measurements of the first half cycle of the wave, the so-called rise time τ, were carried out. By using the rise time method, these data were inverted to infer an estimate of the intrinsic quality factor Qp of P waves and of the source rise time τ0 of the events, which represents an estimate of the duration of the rupture process. Two kind of inversions were carried out. In the first inversion τ0 was derived from the magnitude duration of the events, assuming a constant stress drop and Qp was inferred from the inversion of reduced rise times τ−τ0. In the second inversion both τ0 and Qp were inferred from the inversion of rise times. To determine the model parameters that realize the compromise between model sim- plicity and quality of the fit, the corrected Akaike information criterion was used. After this analysis we obtained Qp=57 ±42. The correlation among the inferred τ0 and Qp, which is caused by some events which concomitant- ly have high τ0 (>30 ms) and high Qp (>100) indicates that the technique used is able to model rise time versus travel time trend only for source dimensions less than about 80 m.

Highlights

The intrinsic quality factor Qp of the compressional body waves is considered one of the geophysical parameters best correlated to the physical state of the rocks
We focus on the 2002-2003 flank eruption which started in the night between October 26 and 27, 2002 with a seismic swarm in the central and upper part of the volcano
In this way we were able to infer that, for 59 events the above choice of retrieving source rise times assuming a constant stress drop produces a better fit of data, whereas for the remaining 68 events the source rise times are better constrained from the inversion of rise times

Summary

Introduction

The intrinsic quality factor Qp of the compressional body waves is considered one of the geophysical parameters best correlated to the physical state of the rocks. Pezzo et al, 2001) It has been shown (e.g., Liu et al, 1994) that the methods based on the inversion of pulse widths of first P waves are more appropriate to estimate the intrinsic Qp, in that only the start of the signal, which is relatively free of complicated effects, such as complexities at the source and/or scattering from heterogeneities, is used. Based on these reasonings, this article presents the results of a first study aimed to obtain a first estimate of the average intrinsic Qp at Mt. Etna by using the classic rise time method (Gladwin and Stacey, 1974). (3.1) can be considered as the minimum Qp estimated from pulse width inversion (de Lorenzo et al, 2004)

Technique

Data analysis

Results

Qp estimates under the assumption of a constant stress drop

Joint estimation of τ0 and Qp

Discussion and conclusions