Carla Musumeci

Stress Directions and Shear-Wave Anisotropy: Observations from Local Earthquakes in Southeastern Sicily, Italy

The spatial distribution of 414 earthquakes (1.0 M L 4.6), recorded from 1994 to 2002 in southeastern Sicily (Italy), has been analyzed. The seismicity generally coincides with mapped Plio-Quaternary faults, including the north-north- west-south-southeast striking offshore fault system, which is the most important tectonic structure of the area. For the best located events, we computed 70 focal mechanisms by combining P-wave polarities with S-wave polarizations. A predom- inance of strike slip and normal faults was observed. Focal mechanisms were then inverted for stress tensor parameters by using the algorithm of Gephart and Forsyth. The results highlighted a region governed mainly by a north-northwest-south-south- east to northwest-southeast compressional stress regime. Moreover, anisotropy anal- ysis of shear waves showed a polarization of fast S waves coherently aligned with this stress direction. A finer-scale analysis of the stress tensor evidenced three regions characterized by slightly differing orientation of the greatest principal stress axis, r1. The eastern sector displays a nearly horizontal r1 trending northwest-southeast; the central sector is affected by a low-dip north-northwest-south-southeast r1; whereas, in the western sector, a north-northwest-south-southeast-oriented r1 with a higher dip angle, was detected. Finally, the comparison of the spatial distribution of seismicity occurring during 1994-2002, with locations of previous instrumental earthquakes and larger (M 5.0) historical events showed that the seismicity patterns are persistent.

Attenuation of Short-Period P Waves at Mount St. Helens

An increase of seismicity occurred at Mount St. Helens between May and July 1998, with more than 900 events (Md 2.2) recorded by the Pacific North- west Seismograph Network. This article describes an attempt to estimate the temporal and spatial variation in seismic attenuation using 200 microearthquakes that occurred before (January 1995-April 1998), during, and after (August-December 1998) the period of higher seismicity and recorded on three different one-component seismic stations. Epicentral distances of the studied events ranged between 0.5 and 15 km. We performed the analysis distinguishing the deep (depth 5.5 km) from shallow (depth 5.5 km) events. We used the frequency decay method to estimate the quality factor Q and station site correction S for P waves in the frequency bands 2-7 and 18-30 Hz. The results show that the attenuation varies from site to site systematically and decreases with depth. Lower Qp values are obtained for focal depths less than 5.5 km. Moreover, the spatial variations of Qp show minimum values (30) in the crater area that are interpreted as due to a low-density mass distribution under the crater. We find that Qp was 30% higher before the period of high seismicity than after. This change may be attributed to an increase of pressure in the magma chamber producing new cracks and/or reopening of pre-existing cracks, which are the most viable mechanisms for increasing attenuation.

Structural health monitoring and Earthquake Early Warning: preliminary studies for application in eastern Sicily

In this work, the reduction of seismic hazard in eastern Sicily is addressed by both studying the fundamental resonant frequency of strategic buildings through low cost geophysical investigation techniques, and exploring a practical approach to earthquake early warning (EEW) system. The fundamental period and the corresponding amplification of some selected strategic buildings has been estimated using ambient vibration and earthquake data. We analyzed the basic dynamic parameters of buildings chosen as target, by using continuous vibration measurements at different floors. The dynamic behavior of structures was evaluated considering both small strains induced by ambient vibrations and larger excitation levels due to the earthquakes occurrence. A practical approach to earthquake early warning in the investigated area was dealt with by using empirical relationships between parameters measured on the initial portion of seismic recordings and related to the earthquake magnitude and peak ground motion. In particular, we performed the first preliminary tests by using empirical relationships calibrated for the considered area and taking into account the geometry of the existing permanent seismic network deployed in the eastern Sicily. The estimated relationships have been used to provide onsite warning around a given seismic station and evaluate the potential damaging effects. The joint of EEW system and geophysical investigation shown in this work may be deemed a useful guide for the future implementation of the in real time seismic monitoring in the region.

Estimation of Earthquake Early Warning Parameters for Eastern Sicily

Abstract Earthquake early warning systems (EEWSs) are becoming a suitable instrument for seismic risk management in real time. In fact, they are implemented or are undergoing testing in many countries around the world because EEWSs represent an effective approach to mitigating seismic risk on a short timescale. EEWSs are based on the use of relationships between some parameters measured on the initial portion of seismic signal after the onsets. Here, we address the first approach to the implementation of EEWS in eastern Sicily, a region that has been hit by several destructive earthquakes. We estimated the peak displacement amplitude of the first portion of P and S waves P d , the ground‐motion period parameter τ c , and the peak ground velocity (PGV) from earthquakes with M L ≥2.8 recorded by the broadband stations operated by the Istituto Nazionale di Geofisica e Vulcanologia. We found that the P d is correlated with the size of the earthquake and may be used to compute the magnitude for an EEWS in this area. We also derived the relationships between τ c and M L , and between P d and PGV, which can be used to provide on‐site warning in the area around a given station and to evaluate the potential damaging effects. These relationships may be deemed a useful guide for future implementation of the EEWS in the region.