Umbria-Marche Seismic Sequence Research Articles

The 2012 Emilia, Italy, Quasi-Consecutive Triggered Mainshocks: Implications for Seismic Hazard

The 2012 Emilia seismic sequence in central Italy represents an illustrative example of quasi‐consecutive triggering with several mainshocks occurring within a few hours or a few days. The sequence was characterized by seven earthquakes of moment magnitude M w>5, rupturing adjacent fault segments of the buried fold arc of the northern Apennines, for a total length of about 50 km (Scognamiglio et al. , 2012; Fig. 1). The occurrence of so many large earthquakes, in such a short time window, suggests a possible interpretation in terms of mutual, static (Ganas et al. , 2012), or dynamic (Convertito et al. , 2013) triggering. In this article, we discuss the consequences of a consecutive succession of earthquakes, that is, a succession of close earthquakes that occur almost simultaneously. Figure 1. Geographic representation of the area interested by the Emilia sequence. The figure shows the epicenters and magnitudes of the seven major events analyzed in this study (black stars). Gray stars represent the aftershocks of the whole sequence, whereas the black triangles identify the stations used to compute the response spectra. Multiple triggering—with variable time delays—has been observed in several damaging earthquake sequences of the Italian Apennine belt. In the central Apennines, the 2009 L’Aquila M w 6.3 earthquake was followed, the day after, by an M w 5.6 event occurred at the southeast edge of the main fault. This seismic sequence has been the deadliest one in Italy, with almost 300 victims, since 1980. The seismicity of the whole sequence spread over a 40 km long fault system (Chiarabba et al. , 2009). The 2002 Molise seismic sequence is remembered for two major shocks M w 5.8 and 5.7 that occurred within a few hours causing severe damage and destroying a school populated by children (Vallee and Di Luccio, 2005). The 1997–1998 Umbria–Marche seismic sequence was characterized by …

Laboratory measurements of the physical properties of Triassic Evaporites from Central Italy and correlation with geophysical data

The Triassic Evaporites (TE) of the Umbria–Marche Apennines, a sedimentary succession made up of a sequence of alternating sulphates (anhydrites and gypsum) and dolostones, represent a key lithology in terms of sealing properties and earthquake triggering. Here we: (1) report laboratory measurements of density, porosity, Vp, Vs, seismic anisotropy and permeability at effective confining pressures from 0 to 100 MPa, conducted on samples of TE collected from both outcrops and boreholes; and (2) attempt to upscale the laboratory results to larger scale geophysical investigation, such as Vp sonic logs, seismic tomography and in situ measurements of pore-fluid pressure. The average laboratory P-wave velocity is 6.0 km/s for dolostones, 4.6 km/s for gypsum–dolostones and 5.8 km/s for anhydrites, at ambient pressure. As effective confining pressure is increased up to 100 MPa, the average P-wave velocity increases to 7.0 km/s for dolostones, 5.3 km/s for gypsum and 6.4 km/s for anhydrites. Vp/Vs ratios appear to be independent of confining pressure, with average values of ∼ 1.8 to ∼ 2.2 for sulphates and ∼ 1.9 to ∼ 2.2 for dolostones, respectively, for dry and saturated conditions. All samples are characterized by very low permeability (10 − 18 m 2 to 10 − 21 m 2), with the higher values for gypsum–dolostones and fractured dolostones samples. The Vp profiles obtained from ultrasonic laboratory measurements match well the in situ Vp profiles measured using sonic logs. In the laboratory, the Vp/Vs ratio increases when pressurized pore fluids are present, in agreement with 4D seismic tomography that relates the increase in Vp/Vs ratio to the migration of fluids during the 1997 Umbria–Marche seismic sequence ( M max ∼ 6.0) that nucleates within the TE. Our low-permeability values are consistent with deep borehole measurements of high pore-fluid pressure trapped within the Triassic Evaporites.

Seismic strain and stress field studies in Italy before and after the Umbria-Marche seismic sequence: a review

The seismic sequence that hit Umbria and Marche during 1997 and 1998 was particularly significant because it dramatically marked the evolution of analysis tools and the development of our geophysical knowledge of the region. Since September 1997, we have had a growing, coherent catalogue of source moment tensors that provides reliable information on earthquake sources in Italy and the surrounding regions. Together with borehole and other tectonic data, this has conspicuously improved our knowledge of the regional strain and stress fields. The main impact of these new data that include local information consists of the possibility to change the scale of the regional geodynamic frame. The simple description of extension tectonics that dominate the Apennines belt has evolved into the present-day maps of the strain and stress fields, where the active tectonics involve compression in the eastern Alps, extension and compression fronts in the northern Apennines, extension and strike-slip structures in the southern Apennines, and a compressional front along the southern Tyrrhenian Sea. This recent geodynamic evolution and the present-day seismotectonic sketch of the Italian peninsula are here described on the basis of the recognition of these tectonic features.

Extended sources of the main events of the Umbria-Marche (1997) seismic sequence inverted from geophysical data

The three largest events of the 1997 Umbria-Marche (Italy) sequence occurred on September 26, 1997 at 00:33 GMT (Event 1, MW=5.7) and 09:40 GMT (Event 2, MW=6.0) in the Colfiorito area and on the October 14, 1997 at 15:23 (Event 3, MW=5.6) in the Sellano area. The availability of different sets of geodetic and seismological data allowed several studies to characterize the extended sources of events 1-3. In this work, I review some of the studies that obtain the properties of the seismic sources by inversion of available data. Generally these studies assume the seismic sources as dislocations or distributions of equivalent point sources in elastic half-spaces. Following their chronological order, they model increasing complexities of the sources by using an increasing number of data. Some of the differences between results obtained, such as the top edge depth estimates, are shown to be due to the different approaches used. Commonly a 1-D crustal model is used in inverting strongmotion data. Instead homogeneous elastic half-spaces are mainly assumed in inverting geodetic data to obtain the three main sources of the 1997 Umbria-Marche sequence. Assuming the same crustal structure is important to make comparable results obtained analyzing seismological data or geodetic data separately, as it has been done till now for this sequence.

Earthquake forecasting in Italy, before and after Umbria-Marche seismic sequence 1997. A review of the earthquake occurrence modeling at different spatio-temporal-magnitude scales.

The main goal of this work is to review the scientific researches carried out before and after the Umbria-Marche sequence related to the earthquake forecasting/prediction in Italy. In particular, I focus the attention on models that aim addressing three main practical questions: was (is) Umbria-Marche a region with high probability of occurrence of a destructive earthquake? Was a precursory activity recorded before the mainshock(s)? What was our capability to model the spatio-temporal-magnitude evolution of that seismic sequence? The models are reviewed pointing out what we have learned after the Umbria-Marche earthquakes, in terms of physical understanding of earthquake occurrence process, and of improving our capability to forecast earthquakes and to track in real-time seismic sequences.

Long-term (1997-2007) geochemical monitoring of gases from the Umbria-Marche region

The paper presents the results of ten-year long geochemical monitoring carried out in the area hit by the 1997–98 Umbria–Marche seismic sequence (Northern Apennines, Italy) showing the advancement in our understanding of the reciprocal relationships between fluids and tectonic structures in this area. Long-term geochemical monitoring has allowed us to define the geochemical features of the gas phase and to model its origins and interactions. All the released gases, including dissolved and venting gases, have predominantly crustal origin and have suffered from substantial modifications in their geochemical features. The whole seismogenic process is considered to be responsible for the modifications recorded over a wide time interval, including the production of CO 2 from mechanical stress on carbonate rocks. We interpret the modifications occurred to the gas phase during the 1997–98 period as related to faulting activity, while those recorded after the end of the seismic crisis are interpreted as being a consequence of the crustal relaxation. The data show that it is almost impossible to interpret the temporal variation without coupling long-term geochemical monitoring and knowledge of the genetic features of the fluids. Therefore the information coming from the circulating fluids is a powerful tool to better understand the physical processes governing the faulting activity.

Capable faulting, environmental effects and seismic landscape in the area affected by the 1997 Umbria–Marche (Central Italy) seismic sequence

The September–October 1997 seismic sequence in the Umbria–Marche regions of Central Italy has been one of the best studied from the seismological, macroseismic and geological point of view. Numerous papers have been published in the period immediately after the seismic sequence, providing a significant database of effects triggered by the earthquake on natural environment. In the following years, further studies have provided additional pieces of evidence that allow to better relate the seismic sequence with its geological background. Moreover, recent developments in the characterization of coseismic environmental effects provide new horizons in seismic hazard assessment (SHA) procedures, which should take into account even long term geomorphological and geological features resulting from repeated characteristic earthquakes (concept of “seismic landscape”). This paper reviews the current state of knowledge on the 1997 Umbria–Marche seismic sequence, with particular regard to coseismic environmental effects (primary and secondary), that have been used for ESI seismic intensity assessment, in order to verify if i) they are consistent with geological, seismological and macroseismic data in the location and characterization of the seismogenic structure, and ii) if they fit the “seismic landscape” features that mark the epicentral area of Colfiorito.

Directivity effects for moderate-magnitude earthquakes (Mw 5.6–6.0) during the 1997 Umbria–Marche sequence, central Italy

The Colfiorito Mw 6.0 (September 26, 1997, 09:40 UTC) and the Sellano Mw 5.6 (October 14, 1997) normal-faulting earthquakes are the two strongest events of the 1997–1998 Umbria–Marche seismic sequence (central Italy). They provided accelerograms recorded at epicentral distances within nearly two fault lengths. Previous studies on these events revealed clear evidence of unilateral rupture propagation and directivity effects on the source time functions. We developed an approximate procedure to correct the observed peak ground motion for the site effects, and we found a systematic increase of peak ground acceleration (PGA) and peak ground velocity (PGV) at sites located in the direction of the rupture propagation respect to the stations located to the backward. In particular we showed that, for the Colfiorito earthquake, the average PGV at the north-west sites is 2.8 times larger than the PGV at south-east sites (2.1 for PGA); an opposite behaviour is found during the Sellano earthquake, with an amplification factor of about 1.9 for PGV (1.5 for PGA) recorded at south-east sites respect to the north-west stations. We explained these behaviours in terms of directivity effect, by comparing recorded ground accelerations with synthetics simulated by a deterministic–stochastic technique. This method computes high-frequency ground motions ( f > 0.5 Hz) from extended faults, generalizing the point-source stochastic method to capture the essence of near-source effects. The synthetic PGAs decrease with distance in agreement with the ground-motion empirical predictive model available for the area and describe the azimuthal variation of recorded data in terms of source effects.

Insights on the geometry and mechanics of the Umbria–Marche earthquakes (Central Italy) from the integration of field and laboratory data

The integration of seismic reflection profiles with well-located earthquakes shows that the mainshocks of the 1997–1998 Umbria–Marche seismic sequence nucleated at a depth of ∼ 6 km on normal faults within the Triassic Evaporites. In the same lithology, made of interbeds of dolomites and anhydrites, two deep boreholes (∼ 4–5 km), drilled northwest of the epicentral area, encountered CO 2 at near lithostatic pressure. In order to investigate the deformation processes operating at depth in the source region, we have combined field studies on a major evaporite-bearing normal fault (the Roccastrada fault) with mechanical and permeability data from rock deformation experiments on anhydrite rocks. The Roccastrada fault is a mature normal fault (inferred displacement > 100 m and exhumation from depths > 1 km) that dips in the range 40°–45°. The fault zone structure is characterized by a 5–6 m thick fault core, which appears to be zoned. The inner fault core is made of fine-grained fault rocks (∼ 1 m thick), with deformation localized along continuous and straight slip surfaces associated with a dolomite-rich cataclasite (brittle deformation). The outer fault core is mainly characterized by distributed deformation accommodated by a fault parallel fabric consisting of interbeds of cataclastic dolostones and foliated Ca-sulphate rocks. The damage zone consists of foliated Ca-sulphate rocks (foliation almost perpendicular to the fault zone) and heavily fractured and boudinaged dolostones. Mechanical data obtained from triaxial loading tests on borehole-recovered anhydrites with different grain size and mesoscopic fabric, show that the transition from localized to distributed deformation occurs at effective pressures of about 20 MPa. The permeability measured under hydrostatic stress conditions, before loading, is generally low and ranges between 10 − 21 ≤ k ≤ 10 − 19 m 2. During sample loading, the permeability increases up to 3 (prior to brittle localized failure) and 2 (prior to distributed ductile failure) orders of magnitude, with measured k values of 10 − 17 m 2 and 10 − 18 m 2, respectively. The integration of field observations with mechanical data allows us to propose a fault evolution model where fault initiation occurs as distributed deformation within the anhydrites, along fault zones dipping at ∼ 45°. With increasing displacement, dolostones are concentrated into the inner fault core. The ultimate result is a fault zone structure dipping at ∼ 45° , made of an inner fault core affected by brittle processes surrounded by a macroscopically ductile outer fault core. This fault zone evolution can explain the low rupture dip (38°–48°) of the Umbria–Marche 1997–1998 mainshocks. The combination of field observations and permeability measurements suggests a fault zone permeability low enough for fluid overpressures to develop. Therefore in the active area of the Umbria–Marche Apennines deep-seated CO 2-rich fluids can be trapped at seismogenic depths within evaporite-bearing faults and can potentially promote earthquake nucleation.

Fault plane orientations of small earthquakes of the 1997 Umbria-Marche (Italy) seismic sequence fromP-wave polarities and rise times

SUMMARY A two-step technique has been developed with the aim of retrieving the fault plane orientation of a small-magnitude earthquake. The technique uses a set of non-linear equations, which relate the rise times of first P waves to source parameters (source dimension L, dip δ and strike φ of the fault) and intrinsic Qp. At the first step of the technique, the inversion of P polarities provides two fault plane orientations for each focal mechanism solution. At the second step, the inversion of rise times is carried out to retrieve L and Qp by fixing δ and φ to the values of the fault plane orientation inferred at the first step. A robust analysis, based on the random deviates technique, allows us to evaluate if one fault plane exists which systematically better fits data and can be considered the ‘true’ fault plane. A parameter is introduced to quantify the level of resolution of the ‘true’ fault plane. A probabilistic approach, based on the assumption that errors on data are Gaussian distributed, allows us to a posteriori validate or reject the ‘true’ fault plane. The technique has been applied to 47 small earthquakes (1.3 < ML < 3.9) occurred below the town of Sellano, during the 1997 Umbria-Marche (Central Italy) seismic crisis. The strike of the inferred fault planes is generally along the Apennine direction. Fault plane solutions can be mainly subdivided into two groups: a first group of SW-dipping faults and a second group of NE-dipping faults. These results agree with the present day knowledge of the fault systems in the area. An average Qp equal to 354 ± 63 has been estimated, in agreement with previous attenuation studies.

Spatial structures in earthquakes and faults: quantifying similarity in simulated stress fields and natural data sets

This paper presents an adaptation for fault analysis of the coherence method of Kagan and Knopoff [Kagan, Y.Y., Knopoff, L., 1985b. The two-point correlation function of the seismic moment tensor. Geophysical Journal of the Royal Astronomical Society 83, 637–656], understanding coherence as a measure of the similarity between fault-slip data for two faults, which can be analytically computed from earthquake or fault moment tensors. Through the use of a customized GIS routine, the degree and distribution of similarity in synthetic and natural fault populations can be calculated as a function of distance between fault/earthquake pairs and their relative orientation. Stress simulations using synthetic data sets reveal that fault populations, which developed under homogeneous stress fields, have a high degree of similarity that increases with the slip tendency of faults. Stress ratio variations, and especially stress axis rotations, lower the coherence values. Spatial analysis of natural earthquake and fault data sets from the Umbria–Marche region (Northern Apennines, Italy) reveals a common pattern of coherence spatial cyclicities in faults/earthquakes controlled by two consecutive tectonic phases, Miocene compression and Plio-Pleistocene extension. The proposed interpretation attributes the spatial cyclicities of Miocene data to syn- to post-faulting folding, in agreement with wavelengths derived from field data. The observed periodicities in earthquakes (1997 Colfiorito–Sellano Umbria–Marche seismic sequence) could, in turn, have been controlled by the Miocene thrust structures, with possible spatial stress field axis rotations and/or stress ratio variations. The spatial characteristics of younger fault/earthquake populations are thus possibly controlled by the characteristics of older tectonic structures, such as fold wavelength or fault spacing.

Geotechnical Site Characterisation in the Umbria-Marche Area and Evaluation of Earthquake Site-Response

In this research a site classification is proposed for the Umbria-Marche area (central Italy), based on geotechnical and geomorphological criteria. Two empirical techniques to evaluate site effects are used, namely the generalised spectral inversion, and the horizontal to vertical spectral ratio. The soil transfer functions obtained by the two techniques are compared in order to capture the features common to each class and verify the accuracy of the classification. We used strong ground motions recorded during the Umbria-Marche seismic sequence (September 1997-May 1998) and we selected 25 sites that were classified into four groups. The results of the empirical techniques have also been tested using theoretical 1-D or 2-D methods that are commonly used to simulate linear soil behaviour. The results, expressed in terms of amplitude transfer functions, are shown according to each soil class and confirm the consistency of the site characterisation. The empirical techniques are efficient in detecting the fundamental frequency of vibration but seldom the higher harmonics. Conversely, the peak amplitude of the transfer functions is not consistently determined, especially for sites located in deep sedimentary basins, where 2-D effects occur and simplified methods, such as the horizontal to vertical spectral ratios (HVSR) or 1-D theoretical models frequently fail. Significant amplifications are also found for 2-D rock structures such as rock crests or cliffs.

Site Effect Evaluation In Sellano (Italy) By1-D And 2-D Numerical Analyses

The 1997–1998 Umbria-Marche seismic sequence induced severe damage to the historical centre of Sellano (Perugia, Italy). During the long aftershock sequence two temporary accelerometric stations, about 300m apart, provided several ground motion records. These data allowed us to investigate the amplification of surface motion which can essentially be ascribed to local seismic effects. A site response study of the historical centre of the village was carried out by means of 1-D and 2-D numerical analyses to clarify the role played by the stratigraphic and topographic effects. The applicability of 1-D and 2-D models was examined by comparing the recorded and calculated acceleration response spectra. 1-D response analyses seem to indicate a significant stratigraphic effect on the ground response. Conversely, 2-D response analyses have shown only moderate amplification effects due to surface topography, which somewhat improved the predictions of site response spectra.

Hydrogeological Changes Related to the Umbria?Marche Earthquake of 26 September 1997 (Central Italy)

Evidences of variations of the hydrologic regime have been observed after the Umbria–Marche seismic sequence, started on September 26th 1997, at the boundary between Umbria and Marche regions, central Italy. In order to quantify the hydrological variations, several data regarding groundwater level, spring discharge and river flow rates, have been collected from local authorities and mineral water companies. They allowed to confine the area affected by anomalies and to quantify the hydrological variations by a comparison with the average yearly regime, estimated from the data referring to previous years. An attempt to identify precursor phenomena has also been carried out. The results of the analysis sustain the close connection of two water reservoirs, the Umbria–Marche ridge and the Valnerina unit, that have both been affected by changes of the hydrologic regime. Two of the river gauge stations registered anomalies several months before the crisis started, acting as earthquake precursors.

Evaluation of site effects by means of Joint Analysis of Sonogram and Standard Spectral Ratio (JASSSR)

In order to evaluate thefrequency dependence of site effects, thejoint analysis of the standard spectralratio technique with a sonogram approach isproposed (Joint Analysis of Sonogram andStandard Spectral Ratio). Two tools aresuggested: the enhanced sonograms andthe differential sonograms. Inparticular, the local geology effects areinvestigated by considering both theamplification of the seismic signals andtheir prolongation. Tests carried out onsynthetic signals show the capability ofthe sonogram approach to evaluate thefrequency-dependent lengthening of theseismic signal. The JASSSR is applied todata recorded by a dense local networkdeployed during the Umbria-Marche seismicsequence of 1997 and to data recorded inthe Volvi Test site (Euroseistest, Greece).The results show that using the JASSSR gives amore complete overview of the effects oflocal geology on seismic ground motion than maybe obtained by using the StandardSpectral Ratio alone. In particular, theeffect of secondary arrivals, both on theamplitude and on the lengthening of theseismic ground motion, can be pointed out.In addition, the characteristic frequencyof these arrivals can be evaluated.

Detection of local site effects through the estimation of building damages

After the Umbria Marche seismic sequence of September–October 1997, an extensive microzoning study was conducted, from October 1997 to May 1998, to detect the zones where the ground motion was amplified because of local geological and geomorphological settings. The goal of the investigation was to return amplification coefficients to local authorities for the post-seismic reconstruction and repairing of pre-existing buildings, by examining the seismic behaviour of the most damaged 60 villages. The analysis consisted in the application of well known computer codes and the results were arranged as reference table, used to assign the amplification coefficients to the rest of damaged localities. The present study has the aim of evaluating the accuracy of that methodology, following a completely different approach. The actual damages, detected by an aerial photo interpretation, are compared to the theoretical damages estimated through empirical curves, obtained from the analysis of recent Italian earthquakes. This approach was reliable enough to calibrate the results of the microzoning study and appropriate when an immediate assessment of the damage is required, to assign funds and estimate the amplification zones, that need to be investigated in finer detail.

Can Seismic Waves Be Trapped inside an Inactive Fault Zone? The Case Study of Nocera Umbra, Central Italy

In March 1998 a small-aperture (∼350 m) linear array of five broadband stations was installed across a Pliocene strike-slip fault crossing the town of Nocera Umbra, central Italy. Three-component seismograms of local earthquakes recorded during the Umbria-Marche seismic sequence have shown that the degradation of mechanical properties in damaged rock causes a significant amplification of ground motion within the fault zone (Marra et al. , 2000). During the operation of the array, an M W 5.3 subcrustal earthquake occurred at a depth of 48 km and an epicentral distance of 10 km from Nocera Umbra. The near-vertical incidence of this event offers a unique opportunity for constraining the 2D geometry of the laterally heterogeneous low-velocity structure responsible for the large amplification across the fault. Based on the observed symmetry of the amplification pattern, a 2D wedge-shaped model of the fault zone is used for the computation of antiplane ( SH ) synthetic waveforms to be compared with observations along the array. Two parameters of the wedge model can be constrained by independent geophysical and geological data: (1) shear-wave velocity in pristine rock outside the fault zone ( v S = 2200 m/sec) and (2) the lateral extent of the damaged rock zone ( L = 160 m). The other two model parameters, namely, the wedge depth ( h ) and shear-wave velocity of the fault zone ( v F S ), are varied. Computer simulations show that trapped waves, which have been observed so far for earthquakes occurring inside the fault zone, can develop inside a 2D wedge model when struck by external, vertically incident seismic radiation. Results of numerical modeling indicate that, in spite of its extreme simplicity, such an idealized model is successful in reproducing many of the essential characters of ground motion within the fault zone of Nocera Umbra. Even though the best-fit solution is not unique and the model parameters trade off each other, a study of parameter sensitivity, comparing synthetics with recorded data, indicates that the variability of h and v F S can be restricted to a limited range of values: the quality of fit is optimal when the shear-velocity reduction lies in the range of 40%–50% and the depth of the trapping zone is comprised between 1 and 2 km.

Quaternary basin evolution and present tectonic regime in the area of the 1997–1998 Umbria–Marche seismic sequence (central Italy)

Geological and geomorphological surveys have been performed in the area affected by the 1997–1998 Umbria–Marche seismic sequence ( M max=6.0) aimed at defining the Quaternary tectonic history and the characteristics of the present tectonic regime. Data have been collected from: (1) the analysis of the remnant landsurfaces by means of aerial photos and field surveys; (2) geological surveys in the Cesi–San Martino basin and in the easternmost sector of the Colfiorito basin in order to identify deformative features affecting the Quaternary deposits; (3) the analysis of boreholes and geo-electrical data (derived from previous surveys performed in the 1960s) in order to reconstruct the top of the pre-Quaternary substratum in the Colfiorito basin. Two different successions of remnant landsurfaces have been identified along the faults bounding the basins to the east, in the hangingwall and the footwall, respectively. The difference accounts for a fault-controlled evolution of the landscape at least during the Upper Pliocene–Early Pleistocene. The deformation affecting the Quaternary deposits and landforms in the investigated basins indicates a decreasing tectonic activity along the master faults since the Middle Pleistocene. Surface deformation due to tectonics is faint and displayed by gentle warping of the landforms during the late Quaternary. As for the basin geometry, subsurface data show that two minor depressions formed in the Colfiorito Basin during the Quaternary, the oldest one close to the fault bounding the basin, while the youngest (and deepest) formed in the inner portion of the basin. Therefore, the present geometry is different from that of other fault-bounded Quaternary depressions of the central Apennines (typically half-graben basins), showing the maximum depth of the substratum in the area close to the master fault. Tectonic history may be summarised as follows: (1) origin of the Quaternary fault-bounded Colfiorito and Cesi–San Martino basins; (2) evolution of the basins with a half-graben style; (3) significant reduction of tectonic activity since the Middle Pleistocene. During the third phase, the evolution of the basins is no longer related to a half-graben style. In the case of the Colfiorito basin, a new depression is superimposed to on the previous half-graben whose evolution is related to the lowering of the inner portion of the basin through warping. Moreover, present activity does not result in fault-related surficial displacements but only in “continuous” deformation spread over the basins. These conclusions have fundamental implications for the seismotectonic framework of the 1997–1998 earthquake sequence. This deformation style is, indeed, in agreement with the coseismic deformation modelled by means of the SAR interferometry analyses carried out by other institutions during the seismic sequence, and with the lack of evident surface faulting related to the mentioned events in the investigated area. This evidence indicates that the evolution of the investigated area since the Middle Pleistocene resulted from the summation of deformative episodes similar to that which occurred during the recent seismic sequence. As a consequence, no earthquakes with magnitude larger than 6 are expected in the area.

Source Parameters Estimated from the Aftershocks of the 1997 Umbria-Marche (Italy) Seismic Sequence

We derived the source parameters of 563 aftershocks (1.4 ≤ M L ≤ 4.5) of the 1997 Umbria–Marche seismic sequence by analyzing three-component digital recordings of a temporary network and developed a local magnitude scale using the whole available data set (more than 25,000 waveforms) from synthetic Wood–Anderson seismograms. Then, considering a subnetwork composed of nine portable stations, more than 12,000 earthquake spectra were corrected for the near-surface attenuation parameter k and the path attenuation. The corrected spectra were used to compute the seismic moment, the source radius, and stress drop assuming an ω–2 Brune source model. We calculate the relationship log M = 17.46 + 1.12 M L between seismic moment and local magnitude for the studied area. The source dimension versus seismic moment relationship did not show any breakdown in the similarity of the rupture process, even when events of magnitude less than 2.2 (the threshold value for the completeness of the considered catalog.) were considered. The average stress drop obtained analyzing the S -wave spectra was 38 ± 10 bar, and the source dimension ranged between about 40 and 500 m.

Stochastic Simulation of Strong-Motion Records from the 26 September 1997 (Mw 6), Umbria-Marche (Central Italy) Earthquake

We simulated the strong-motion time histories recorded during the main shock ( M w 6.0) of the Umbria-Marche seismic sequence (Central Italy) of September–October 1997. Ground-motion waveforms were computed using the stochastic modeling technique proposed by Beresnev and Atkinson (1997, 1998) for finite faults. In this approach, the high-frequency amplitudes are simulated as a summation of stochastic point sources. We used the FINSIM code (Beresnev and Atkinson, 1998), which incorporates regional attenuation and frequency-dependent site-amplification factors. We divided the fault plane into 60 elements whose length and width are 1.2 km and 1.5 km, respectively. The resulting subfault corner frequency and rise time are 0.91 Hz and 0.5 sec. We found that the site-amplification functions play an important role in the simulation process, improving the fit to the observed time histories and spectra. The strong-motion waveforms recorded at the Nocera (NOC) station, located at the northern end of the causative fault, show an important directivity effect. Thus, to fit the observed ground motions, we used an inhomogeneous slip distribution, weighting the slip on the fault heavier toward the north. We tested two models: one that simulates a fault rupture with two main slip patches and a second model that ruptures northward with a less heterogeneous slip distribution where slip is mostly concentrated near the rupture nucleation at the southern edge of the fault plane. The simulated low-frequency amplitudes at NOC, however, require an additional frequency-dependent directivity correction (e.g., Bernard et al., 1996). In conclusion, we found that stochastic finite-fault simulations calculated using adequate site amplification functions and crustal attenuation reproduce reasonably well the ground motions from the M w = 6.0 Umbria-Marche earthquake.