Constraints on the shallow crustal model of the Northern Apennines (Italy) from the analysis of microearthquake seismic records

Abstract

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.

Slip‐generated patterns of swarm microearthquakes from West Bohemia/Vogtland (central Europe): Evidence of their triggering mechanism?

“…Only events with at least seven arrival time readings were relocated. As location method, a slightly modified master event procedure after Zollo et al [1995] with use of Málek et al 's [2005] one‐dimensional (1‐D) gradient velocity model was applied. Zollo et al's method is based on the minimization of the variance V of the difference Δ T 0 between the origin times of the master event and secondary events, where n is the number of arrival times.…”

The Mw 5.4 Reggio Emilia 1996 earthquake: active compressional tectonics in the Po Plain, Italy

“…Both main-shock and aftershock fault plane solutions indicate a subhorizontal s 1 (plunge 18u) in a N±S direction. The 1987 shallow seismic sequence analysed by Zollo et al (1995) involves earthquakes with rupture dimensions of a few hundred metres, and stress at this scale can be highly variable in space. The different results obtained by Zollo et al (1995) can be explained in terms of the high variability of stress in space.…”

Active tectonics of the outer northern Apennines: Adriatic vs. Po Plain seismicity and stress fields

“…There, focal mechanisms for events preceding the 2013 earthquakes were characterized by a wide range of orientation of P and T axes, and included thrust, strike-slip and extensional fault plane solutions (e.g. Gasparini et al, 1985;Zollo et al, 1995;Amato, 1997, 2000;Mariucci et al, 1999;Selvaggi et al, 2001;Santini et al, 2011;Macchiavelli et al, 2012). Furthermore, thrusts buried beneath the Adriatic Sea appear to be sealed by undeformed Middle-Upper Pleistocene deposits (e.g.…”