A low magnitude seismic sequence near Isernia (Molise,Central Italy) in January 1986

Following the increase in seismic activity which occurred near Isernia (Molise, Central Italy) in January 1986, a digital seismic network of four stations with three-component, short-period seismometers, was installed in the area by the Osservatorio Vesuviano. The temporary network had an average station spacing of about 8–10 km and, in combination with permanent local seismic stations, allowed the accurate determination of earthquake locations during an operating period of about one month. Among the 1517 detected earthquakes, 170 events were selected with standard errors on epicentre and depth not greater than respectively 0.5 and 1.5 km. The most frequent focal depths ranged between 4 and 8 km, while the epicentres distribution covered a small area NE of Isernia of about 10 km². A main rupture zone could not be clearly identified from the spatial distribution of the earthquakes, suggesting a rupture mechanism involving heterogeneous materials. The activity was characterized by low energy levels, the largest earthquake, on January 18, 1986, havingM _D =4.0. The time sequence of events and pattern of seismic energy release revealed a strong temporal clustering of events, similar to the behaviour commonly associated with seismic swarms.     

Scale invariance for a heterogeneous fault model

A very simple model of fault fracturing is presented. The fault is envisaged as a series of faultlets whose area is randomly distributed and whose strength have a Gaussian distribution. Faultlet slip occurs when the loading stress reaches its strength. Large earthquakes are generated by coalescence of smaller cracks. Theb value of the Gutenberg-Richter frequency magnitude relation is then calculated. Ab equal to 1 (scale invariance) is obtained only for events with magnitudeM greater than 3. At lower magnitude the number of events decreases with decreasing magnitude. A similar result can be explained by the experimental observation of an anomalous scaling law of real earthquakes. Here the suggestion is advanced that a similar observation could be interpreted by a multifractal approach.     

Frequency dependence of site amplification factor in the Messina Strait area

The spectral ratio method is used here for evaluating the frequency dependence of a site amplification factor in the Messina Strait area. All stations exhibit the same features of spectral ratios evidencing two peaks at about 10 Hz and 16 Hz. We relate this observation to the same geological structures. In fact, all the stations (except the referenced one) are situated over pleistocenic sediments with a similar grain size. This causes the disappearing of any site effect when the average spectrum is used as a reference. MES station, situated in the city of Messina, presents a clear site effect at a different frequency (6 Hz) which cannot be related to any geological structure because no substantial difference is observed between the sediments on which this station is situated and the sediments on which the other stations are situated. MT1 station exhibits a spectral peak at about 2 Hz which can be explained with oscillations of a little sedimentary basin.     

Assessment of focal mechanisms of microseismic events computed from two three‐component receivers: application to the Arkema‐Vauvert field (France)

We have developed a method that enables computing double‐couple focal mechanisms with only a few sensors. This method is based on a non‐linear inversion of the P, Sv and Sh amplitudes of microseismic events recorded on a set of sensors. The information brought by the focal mechanism enables determining the geometry of the rupture on the associated geological structure. It also provides a better estimate of the conventional source parameters. Full analysis has been performed on a data set of 15 microseismic events recorded in the brine production field of Vauvert. The microseismic monitoring network consisted of two permanent tools and one temporary borehole string. The majority of the focal mechanisms computed from both permanent tools are similar to those computed from the whole network. This result indicates that the double‐couple focal mechanism determination is reliable for both permanent 3C receivers in this field.     

Scaling properties of the spatio–temporal distribution of earthquakes: a multifractal approach applied to a Californian catalogue

We study the spatio–temporal scaling properties of the NCSN California catalogue for the period 1975–1995 (about 137 000 events) using a multifractal approach. The study is based on a new method of declustering earthquake catalogues based on looking for independent events. This technique makes use of the properties of the multifractal distribution of the inter-event times. A comparison test between our method and the Gardner and Knopoff (GK hereafter) approach reveals that our method produces a catalogue of independent events (monofractal Poissonian catalogue), while the GK declustered catalogue remains multifractal. The main difference lies in the assumption that the events can be considered to be correlated at large distances. The spatial properties of the declustered catalogue reveal a distribution of epicentres that is more complex than the distribution for the complete catalogue. We suggest that such results are very similar to the scaling properties of fully developed turbulence.