SUMMARY We have analysed the seismic sequence that occurred in October 1996 near the town of Reggio Emilia on the southern edge of the Po Plain. The onset of the sequence was marked by a 5.4 moment magnitude main shock, located at 15 km depth. The main‐shock focal mechanism is a reverse solution with a strike‐slip component and the scalar moment is 1.46 × 1017 N m. We used broad‐band digital recordings from a borehole station, located at about 70 km from the epicentre, for a spectral analysis in order to estimate attenuation and source parameters for the main shock. In addition, the empirical Green's function method has been applied to evaluate the source time function in terms of both moment rate and stress rate. We infer an asperity‐like rupture process for the main shock, as suggested by the short duration of the stress release with respect to the overall duration of the moment rate function. This analysis also allows us to estimate the average dynamic stress drop of the main shock (600 bar). We analysed the digital recordings of the temporary local seismic network deployed after the main shock and of a permanent local network maintained by the Italian Petroleum Agency (AGIP). During 15 days of field experiments, we recorded more than 800 aftershocks, which delineate a 9 km long, NE‐elongated distribution, confined between 12 and 15 km depth, suggesting that the basement is involved in the deformation processes. 102 focal mechanism of aftershocks have been computed from P‐wave polarities, showing mainly pure reverse solutions. We calculate the principal stress axes from a selected population of earthquakes providing a constraint on the stress regime of this part of the Po Plain. The focal mechanisms are consistent with a N–S subhorizontal σ1. All the seismological data we have analysed confirm that this region is undergoing active compressional tectonics, as already inferred from recent earthquakes, geomorphological data and other stress indicators. Moreover, the elongation of the Reggio Emilia aftershock sequence is consistent with the regional direction of the thrust fronts cropping out in the area, suggesting that they are still active.

S U M M A R YA joint study of microearthquake source and medium parameters was carried out by analysing a seismic sequence that occurred in the Tuscan Emilian Apenninic region. Signal processing and graphic techniques were applied to study the amplitude and polarization of wave motion and thus identify the main and secondary body-wave arrivals on the seismic records.The evidence for similar waveforms (denoted as 'similar events') in a microearthquake subset allowed application of a non-linear technique, which provided the accurate relative location of the events. Microearthquakes occurred within the shallow sediments (average depth of 3.5 km), and all appeared to be concentrated in a small volume of about 1 km3. The composite fault mechanism of the 'similar events' was computed by non-linear inversion of P-polarity readings. The maximumlikelihood solution appeared to be well constrained, and indicated a normal-faulting mechanism with the Taxis approximately oriented in a northerly direction.Interpretation of several secondary arrivals was performed by direct modelling of traveltimes and wave-motion amplitudes using a double-couple point-source model. Green's functions in a layered medium were computed using two different methods, based on ray theory (Farra 1990) and discrete wavenumber representation of the wavefield (Bouchon 1981). The study of secondary arrivals indicates a depth of 11-12 km for the top of the crystalline pre-Tertiary basement. This estimate concerns a region located at around 3-6km from the epicentres of the 'similar events', along the N290"E direction. The modelling of arrival times for an S-to-P converted phase at the basement discontinuity suggests an increase of about 10 per cent in the basement seismic-wave velocity (V, = 6.9 km SKI), or, equivalently, an increase of the V, to V, ratio in the upper sediments. The increments were defined with respect to an existing reference model by AGIP (Italian Petroleum Agency). Recordings at the Minerbio station (MIN) (located about 40 km SE of the epicentral area) show converted/reflected phases at shallow crustal discontinuities and waveresonance phenomena. These path effects appear repeatedly on seismic records independently of the size of earthquakes within the magnitude range considered. Waveform and traveltime modelling of these secondary arrivals provide further constraints on the depth and velocity of main-structure discontinuities.

The model developed by Aki and Chouet for the coda wave generation and propagation has been used to calculate the quality factor Q for the zone of the Aeolian Islands, southern Italy, in the frequency range of 1 to 12 Hz, and the scaling properties of the seismic spectrum in the magnitude range of 0.4 to 4.7. The Q found for the Aeolian area has a frequency dependence of the form Q = qfv. The absolute values of Q seem to be dependent on the station and location of the seismic events, confirming the strong lateral heterogeneities in the geological structure beneath the Aeolian Arc. A temporal variation has been noted in the Q calculated at Vulcano station (VPL) in a period of 3 weeks soon after the occurrence of a main shock of ML = 5.5 located near the station. The scaling behavior of this sequence is similar to that obtained in two areas of California and one portion of Japan, with a corner frequency that remains constant with an increasing seismic moment between magnitudes 1 and 4. It differs substantially from the scaling properties of the Hawaian earthquakes that show a linear pattern, without an increase of the stress drop with magnitude. The fact that Vulcano is an active volcano seems not to influence the scaling properties of the seismic sequence localized very near it. It probably indicates that the aftershocks used for calculating the scaling law are generated out of the volcanic complex Lipari-Vulcano, in a zone with a good capability of accumulating the stress.

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Developing a model of hydraulic fracture development to characterize the effective fracture volume and identify potential means for re-stimulation of previously treated wells by utilizing seismic moment tensor inversion techniques.

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