Changes in the Coda Decay Rate and Shear-Wave Splitting Parameters Associated with Seismic Swarms at Mt. Vesuvius, Italy

Abstract

We study the time changes of (1) the b-value of the Gutenberg–Richter distribution, (2) the inverse coda Q(QC-1 ), and (3) the shear-wave splitting parameters (i.e., the time delay Td between qS1 and qS2 phases and the polarization direction of the qS1 wave) for small-magnitude volcano-tectonic earthquakes of Mt. Vesuvius, Italy. We used for (1) the seismic catalog of Mt. Vesuvius seismicity starting from January 1994, for (2) a selected (on the basis of the best signal-to-noise ratio) set of data with hypocentral distances smaller than 4 km recorded at station BKE (analogical) with a 1-Hz vertical seismometer during the period from January 1994 until the present, and for (3) a set of data recorded at two digital, high dynamical range, portable short-period seismic stations. These stations (BKE and BKN) were in operation in two periods, BKE (digital) from January 1999 to the middle of 2000 and BKN from January 1999 to the end of 1999; the hypocentral distances were not greater than 4 km. We found evidence of time changes of QC-1 measured at high frequency (6, 12, and 18 Hz).The changes seem to be correlated with the occurrence of two swarms with largest magnitudes of 3.4 and 3.6, respectively in April 1996 and October 1999. The earthquake with the largest magnitude in the second swarm appears to be the largest event since the latest eruption in 1944. An increase in QC-1 starts after the occurrence of both swarms, reaching a maximum after more than 1 yr for the first swarm and after 6 months for the second swarm. These two changes were not accompained by any corresponding variation of the b-value, which shows an almost constant (inside the statistical uncertainty) pattern. The last swarm (M 3.6) was preceeded by an increase of Td at both stations, indicating a possible change of the stress state before the M 3.6 earthquake.The qS1 polarization direction also shows a variation in correspondence to the same earthquake, which was interpreted as generated by an increase of the differential stress acting at a regional scale in the north–south direction shortly before the M 3.6 event. The strain change associated to this earthquake was estimated to be of the order of 10 -9 using data from the straingram recorded at a Sacks–Evertson dilatometer located about 3 km from the epicenter. The given information allows us to estimate the sensitivity of the the measured parameters to the strain change induced by the M 3.6 earthquake. The sensitivity is of the order of 1.4 x 10 9 (QC-1/strain units) for QC-1 and is of the order of 2 x 10 10(msec/strain units) for Td.