D. Piccinini

The 1997 Umbria‐Marche, Italy, Earthquake Sequence: A first look at the main shocks and aftershocks

A long sequence of earthquakes, six with magnitudes between 5 and 6, struck Central Italy starting on September 26, 1997, causing severe damages and loss of human lives. The seismogenic structure consists of a NW-SE elongated fault zone extending for about 40 km. The focal mechanisms of the largest shocks reveal normal faulting with NE-SW extension perpendicular to the trend of the Apennines, consistently with the Quaternary tectonic setting of the internal sector of the belt and with previous earthquakes in adjacent regions. Preliminary data on the main shocks and aftershocks show that extension in this region of the Apennines is accomplished by normal faults dipping at low angle (∼40°) to the southwest, and confined in the upper ∼8 km of the crust. These normal faults might have reactivated thrust planes of the Pliocene compressional tectonics. The aftershock distribution and the damage patterns also suggest that the three main shocks ruptured distinct 5 to 15 km-long fault segments, adjacent and slightly offset from one another.

Temporal variation of seismic velocity and anisotropy before the 2009 MW 6.3 L'Aquila earthquake, Italy

We describe a set of seismological observations of the foreshock sequence preceding the 6 April 2009 M w 6.3 L9Aquila earthquake. The dense configuration of the seismic network in the epicenter area and the occurrence of a long foreshock sequence provide the opportunity for a detailed reconstruction of the preparatory phase of the main shock. Approaching the earthquake, we observed clear variations in the seismic wave propagation properties. The elastic properties of rocks in the fault region underwent a sharp change about a week before the earthquake. From our observations, we infer that a complex sequence of dilatancy-diffusion processes takes place and that fluids play a key role in the fault failure process.

Complex Normal Faulting in the Apennines Thrust-and-Fold Belt: The 1997 Seismic sequence in Central Italy

A long sequence of moderate-magnitude earthquakes (5 M 6) struck central Italy in September and October 1997. At the end of the sequence a year later, the seismogenic area extends for about 60 km along the Apennines. The analysis of historical seismicity suggests that this seismic sequence filled a 700-year gap in this portion of the chain. Other historical sequences in the same area are characterized by prolonged seismic release on adjacent fault segments, probably due to the in- volvement of shallow and complex structures inherited by the compressive tectonics. The distribution of seismicity and the fault-plane solutions show that the extension in this region is accomplished by normal faults dipping at relatively low angles (40) to the southwest. The focal mechanisms of the largest shocks reveal normal faulting with extension perpendicular to the Apenninic chain (northeast-southwest), consistently with the Quaternary tectonics of the internal sector of the northern Apen- nine belt and with previous earthquakes in adjacent regions. Three mainshocks oc- curred on distinct 5- to 10-km-long fault segments, adjacent and slightly offset be- tween each other. High-quality aftershock locations show that seismicity is confined within the sedimentary Mesozoic cover in the upper 8 km of the crust and that most of the aftershocks are shallower than the largest shocks, which nucleated at 6-km depth. Faults evidenced by aftershock locations have a planar geometry and show increased complexity toward the surface. Most of the aftershock focal mechanisms are dominated by normal faulting. Several strike-slip events occurred at shallow depths, reactivating portions of pre-existing thrust planes that segment the normal fault system. The spatiotemporal evolution of seismicity shows a peculiar migration of hypocenters along the strike of the main faults with multiple ruptures and the activation of fault segments before the occurrence of the main rupture episodes.

Spatio-temporal distribution of seismic activity during the Umbria-Marche crisis, 1997

We present the spatio-temporal distribution of more than 2000 earthquakesthat occurred during the Umbria-Marche seismic crisis, between September 26and November 3, 1997. This distribution was obtained from recordings of atemporary network that was installed after the occurrence of the first two largest shocks (Mw =, 5.7, Mw = 6.0) of September 26. This network wascomposed of 27 digital 3-components stations densely distributed in theepicentral area. The aftershock distribution covers a region of about 40 km long and about2 km wide along the NW-SE central Apennines chain. The activity is shallow,mostly located at less than 9 km depth. We distinguished three main zonesof different seismic activity from NW to SE. The central zone, that containsthe hypocenter of four earthquakes of magnitude larger than 5, was the moreactive and the more complex one. Sections at depth identify 40–50°dipping structures that agree well with the moment tensor focalmechanisms results. The clustering and the migration of seismicity from NW to SE and the generalfeatures are imaged by aftershock distribution both horizontally and at depth.

Quaternary faults and seismicity in the Umbro-Marchean Apennines (Central Italy): evidence from the 1997 Colfiorito earthquake

Abstract Analyses of structural and geomorphological data combined with remote sensing interpretation confirm previous knowledge on the existence of an extensional Quaternary tectonic regime in the Colfiorito area (Umbro-Marchean Central Apennines). This is characterized by a maximum principal axis of finite strain oriented approx. NE–SW, which is the result of a progressive deformation process due to pure and radial extension. Surface geological data, the crustal tectonic setting (reconstructed using a CROP 03 seismic reflection profile), and seismological data relative to the autumn 1997 Colfiorito earthquake sequence constrain the following seismotectonic model. We interpret the seismogenic SW-dipping low-angle normal fault pictured by seismic data as an inverted thrust ramp located in the basement at depth between 5 and 10 km. The surface projection of this seismogenic structure defines a crustal box within which high-angle normal faults are responsible for the deformation of the uppermost crust. The regional patterns of pre-existing basement thrusts therefore control the seismotectonic zoning of the area that cannot be directly related to the high-angle normal fault systems which cut through different crustal boxes; the latter system records, in fact, re-shear along pre-existing normal faults. Moreover, Quaternary slip-rates relative to high-angle normal faults in the Central Apennines are closely related to seismic hazard within each crustal box.

Seismic and infrasonic evidences for an impulsive source of the shallow volcanic tremor at Mt. Etna, Italy

During a seismo-acoustic experiment we recorded volcanic tremor around the summit craters of Mt. Etna volcano. Tremor shows amplitude modulation, which disappear ≈ 900 m from the crater area. The infrasonic wavefield is coherent even at distances of ≈ 750 m. Time delay between infrasonic transient is stable around 1.3 s and is consistent with the position of the source in the Voragine crater. Amplitude modulation of tremor is well correlated (0.72) with infrasound amplitude with a time lag of 0.37 s. coherent with a shallow position of the source. Amplitude of volcanic tremor decays over increasing distances according to geometrical spreading of body waves. Tremor wavefield shows a linear polarization following the same time occourrence as the infrasonic pulses. Polarization azimuth indicates that wavefield rectilinearity is mostly due to P-waves. We infer that most of the volcanic tremor we recorded at Mt. Etna is generated by superimposition of small impulsive sources acting at 1–2 s rate caused by pressure instability during magma degassing.

Foreshock sequence of September 26th, 1997 Umbria-Marche earthquakes

On September 3rd (22:07 UTC), 1997 a small earthquake with Mw=4.54 started the foreshocks sequence (≈1500 events with ML <3.1) of the September 26th seismic sequence. Two days after, three seismic stations of the University of Camerino were installed around the macroseismic epicenter of the foreshock. We present in this paper the location of foreshocks (with 2.1<ML<3.3) which occurred between September 3rd and 26th. Foreshocks location, with horizontal (ERH) and vertical (ERZ) error less than 1.5 km, define an area ≈4 km large. Foreshocks have been localized between the epicenters of the two major events of September 26th, which occurred at 00:33 UTC with Mw=5.6 and at 09:40 UTC with Mw=6.0 (Amato et al., 1998; Ekström et al., 1998). In a vertical cross-section, hypocenters show a low angle (≈30°) structure with SW dip-direction. Focal mechanisms for three of the major events show dip-slip fault solutions with strike direction of about N130, in agreement with the CMT solutions of September 3rd and September 26th earthquakes (Ekström et al., 1998). Data recorded at two stations Popola (POP) e Capodacqua (CPQ) located on the rupture area of the September 26th faults, allowed us to calculate a mean Vp/Vs ratio of 1.84±0.03 for the foreshock. This value is lower than the Vp/Vs ratio of 1.89±0.02 calculated for the aftershock sequence occurred in the same area. Besides, the Vp/Vs ratio during the foreshocks sequence is not stable in time but it seems to increase approaching September 26th. After September 26th mainshocks, this value tends to stabilize around a higher value of 1.89. Following the dilatancy model, we suggest that the relative low Vp/Vs ratio before the main shocks could indicate the presence of fluid in the focal volume. The presence of fluids could have increased the effective stress on the fault plane and could be responsible for the long foreshock activity before the two main earthquakes of September 26th. Therefore, we suggest that this foreshock activity could have also contributed to reduce the friction along the September 26th fault plane, breaking the active structure in two smaller segments. In this hypothesis, foreshock activity could have drastically contributed to mitigate the seismic potential of the Colfioritos active structure.

ANISOMAT+: An automatic tool to retrieve seismic anisotropy from local earthquakes

Abstract An automatic analysis code called ANISOMAT+ has been developed and improved to automatically retrieve the crustal anisotropic parameters fast polarization direction ( ϕ ) and delay time (δ t ) related to the shear wave splitting phenomena affecting seismic S-wave. The code is composed of a set of MatLab scripts and functions able to evaluate the anisotropic parameters from the three-component seismic recordings of local earthquakes using the cross-correlation method. Because the aim of the code is to achieve a fully automatic evaluation of anisotropic parameters, during the development of the code we focus our attention to devise several automatic checks intended to guarantee the quality and the stability of the results obtained. The basic idea behind the development of this automatic code is to build a tool able to work on a huge amount of data in a short time, obtaining stable results and minimizing the errors due to the subjectivity. These behaviors, coupled to a three component digital seismic network and a monitoring system that performs automatic pickings and locations, are required to develop a real-time monitoring of the anisotropic parameters.

Discovering geothermal supercritical fluids: a new frontier for seismic exploration

Exploiting supercritical geothermal resources represents a frontier for the next generation of geothermal electrical power plant, as the heat capacity of supercritical fluids (SCF),which directly impacts on energy production, is much higher than that of fluids at subcritical conditions. Reconnaissance and location of intensively permeable and productive horizons at depth is the present limit for the development of SCF geothermal plants. We use, for the first time, teleseismic converted waves (i.e. receiver function) for discovering those horizons in the crust. Thanks to the capability of receiver function to map buried anisotropic materials, the SCF-bearing horizon is seen as the 4km-depth abrupt termination of a shallow, thick, ultra-high (>30%) anisotropic rock volume, in the center of the Larderello geothermal field. The SCF-bearing horizon develops within the granites of the geothermal field, bounding at depth the vapor-filled heavily-fractured rock matrix that hosts the shallow steam-dominated geothermal reservoirs. The sharp termination at depth of the anisotropic behavior of granites, coinciding with a 2 km-thick stripe of seismicity and diffuse fracturing, points out the sudden change in compressibility of the fluid filling the fractures and is a key-evidence of deep fluids that locally traversed the supercritical conditions. The presence of SCF and fracture permeability in nominally ductile granitic rocks open new scenarios for the understanding of magmatic systems and for geothermal exploitation.