JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. B5, 2248, doi: /2001jb001665, 2003

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1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. B5, 2248, doi: /2001jb001665, 2003 V p and V p /V s images in the M w 6.0 Colfiorito fault region (central Italy): A contribution to the understanding of seismotectonic and seismogenic processes C. Chiarabba and A. Amato Istituto Nazionale di Geofisica e Vulcanologia, Centro Nazionale Terremoti, Rome, Italy Received 22 November 2001; revised 28 October 2002; accepted 12 December 2002; published 14 May [1] In 1997, the Umbria-Marche Apennines (central Italy) was struck by a series of moderate magnitude normal faulting earthquakes, accompanied by a prolonged sequence of aftershocks. Seismological analysis revealed a 40-km-long NW trending structure composed by several low-angle normal fault segments. In this paper we present the threedimensional (3-D) V p and V p /V s structure of the fault zone determined by the inversion of P and S wave arrival times from 1982 aftershocks. The high-resolution tomographic images reveal strong heterogeneity along and across the fault zone and allow us, along with the 3-D location of earthquakes, to define the fault geometry at depth and to improve our understanding on the seismotectonics of this part of the northern Apennines thrustand-fold belt. The relocated aftershocks suggest that the main ruptures occurred on planes slightly steeper than previously hypothesized and at depths shallower than those evidenced by 1-D locations. We find that the preexisting structure, which developed during the Neogene compressional tectonics, plays an important role in the earthquake generation and rupture evolution of the normal faulting events. We propose that the main shocks, whose hypocenters are all located at 6 km depth, originated along a shallow decollement of the Meso-Cenozoic cover, thrust over the metamorphic basement. Geometrical discontinuities and lateral steps along the inherited structures control the lateral fault extent and determine fault segmentation. This study offers a good example of how lithological heterogeneities and the complexity of the upper crust structure strongly affect the faulting process. INDEX TERMS: 8180 Tectonophysics: Tomography; 8015 Structural Geology: Local crustal structure; 7230 Seismology: Seismicity and seismotectonics; 8109 Tectonophysics: Continental tectonics extensional (0905); KEYWORDS: normal fault geometry, normal fault structure Citation: Chiarabba, C., and A. Amato, V p and V p /V s images in the M w 6.0 Colfiorito fault region (central Italy): A contribution to the understanding of seismotectonic and seismogenic processes, J. Geophys. Res., 108(B5), 2248, doi: /2001jb001665, Introduction [2] On 26 September 1997, two moderate magnitude earthquakes (M w 5.8 and M w 6.0) struck the Umbria-Marche region (central Italy, Figure 1), separated by 9 hours and heralding a sequence of aftershocks lasted for several months. During the sequence a third main shock on 14 October (M w 5.6) and three other earthquakes with M w larger than 5.2 occurred, renewing the aftershock release. All the large shocks originated on adjacent parallel NW trending normal faults whose extent varies between 5 and 10 km at hypocentral depth of 5 6 km [Amato et al., 1998]. Information on faulting mechanisms and source propagation for the largest shocks have been obtained from modeling of teleseismic and regional waveforms [Ekstrom et al., 1998; Olivieri and Ekstrom, 1999; Pino et al., 1999], Global Positioning System (GPS) and synthetic aperture radar Copyright 2003 by the American Geophysical Union /03/2001JB001665$09.00 (SAR) data [Hunstad et al., 1999; Stramondo et al., 1999; Salvi et al., 2000], strong motion data [Hernandez et al., 1999; Capuano et al., 2000], geologic data [Cello et al., 1998; Cinti et al., 1999], and references therein], and distribution of main shocks and aftershocks [Amato et al., 1998; Cattaneo et al., 2000]; Deschamps et al., 2000]. Seismological studies revealed several peculiarities which include a clear trend of earthquake migration along the fault system, a low-angle dip of the normal faults (40 ), and the rupturing of adjacent small segments in separate moderate magnitude earthquakes over a broad, 40-km-long, NW trending structure. Seismological and geodetic data suggest that the large shocks did not ruptured up to the surface, the upward tips of the faults being confined between 0 and 2 km below sea level. Although there are different views about the interpretation of the effects on the ground [see Basili et al., 1998; Cinti et al., 1999], no clear expression of primary faulting was observed. [3] Primary objective of seismology in our country is the definition of fault geometry and seismogenesis for regions affected by moderate magnitude earthquakes, that occur ESE 7-1

2 ESE 7-2 CHIARABBA AND AMATO: TOMOGRAPHY OF COLFIORITO FAULT ZONE Figure 1. Seismicity map of the Umbria-Marche region, in central Italy. Epicenters are obtained relocating earthquakes occurred in the period by using data from the ING bulletin. CMT focal solutions of the largest shocks that occurred since 1979 and epicenters for the historical seismicity from CPTI Working Group [1999] are shown. frequently and create damage, cultural loss and great concern in the population. The availability of a very good aftershock data set in the Umbria-Marche allows us to understand the relationship between the inherited structure and the fault extent and geometry, that can be used for predicting rupture characteristics for future earthquakes. Since normal faulting earthquakes are not very frequent, this study offers original results to understand rupture mechanisms in extensional tectonics environment. [4] In past years, V p and V p /V s three-dimensional (3-D) models have proved to be a very useful tool to investigate the internal structure of fault system, clarifying seismotectonics and seismogenic processes [Thurber et al., 1995; Chiarabba and Selvaggi, 1997; Chiarabba et al., 1997; Eberhart-Phillips and Michael, 1998; Zhao and Negishi, 1998; Hauksson, 2000]. Most of the studies concentrated on strike slip and thrust faults, while a few focused on normal faults. In this paper, we help to constrain the geometry of the Colfiorito normal fault system and the relation between earthquake occurrence and subsurface structure by using 3- D located earthquakes and variations of material properties defined by tomographic images. The use of a complete data set of aftershocks with magnitude larger than 2.5 spanning all the earthquake sequence and recorded by a very dense network installed over the epicentral area ensure us to compute high resolution images of the uppermost crust. We have inverted 47,016 P wave and S-P arrival times from 1982 earthquakes to solve for V p, V p /V s, and hypocentral parameters by using the SimulPS13q technique [Thurber, 1993; Eberhart-Phillips, 1993, Eberhart-Phillips and Reyners, 1997]. The reliability of velocity models has been carefully verified by using synthetic tests and computing the whole resolution matrix [Menke, 1989; Toomey and Foulger, 1989]. 2. Tectonic Setting of the Area [5] The study area is part of the east verging Neogene thrust and fold belt of the northern Apennines (central Italy), displaced by Quaternary normal faults and intermountain tectonic depressions [Bally et al., 1986; Calamita et al., 1994; Lavecchia et al., 1994]. The structural setting of the area is characterized by several east verging thrust units composed by Meso-Cenozoic rocks, generally elongated in the N-S and NW-SE directions (Figure 2). The main outer front of the structure is broadly NNW elongated and rotates toward NNE in the south of the area. Three small intramountain basins define the Colfiorito plateau, where the two main shocks originated. Such basins, located at around 800 m of elevation above sea level, are filled by a thin

3 CHIARABBA AND AMATO: TOMOGRAPHY OF COLFIORITO FAULT ZONE ESE Assisi A 600 Nocera Umbra : A' 43 00' Foligno Colfiorito 00:33 15:23 km Sellano ' A A ' Sept. 26, 1997 mainshock Figure 2. (top) Structural sketch of the area showing the main fault traces, modified from Calamita et al. [1994]. Bold solid lines are thrusts, regular lines are normal faults, and light lines are topography contours every 200 m. Shaded regions indicate the Colfiorito basins composing the Colfiorito structural high. Fault boxes and main shock hypocenters (stars) are plotted for the three main events. (bottom) An E W geological section of the area, modified from Mirabella and Pucci, layer of alluvium and sediments and are surrounded by topographic highs of Mesozoic limestones and marls. [6] While most of the existing structure is related to the late Cenozoic compression, the more recent extensional tectonics is responsible for the generation of the small basins. On the basis of earthquake focal mechanisms and borehole breakouts, Montone et al. [1999] showed that the central part of the Apenninic belt is characterized by an active NE trending extension. Focal mechanisms computed for the largest shocks of the Colfiorito sequence are fully in agreement with this direction of regional extension [Ekstrom et al., 1998]. [7] Historical seismicity indicates that the region is lacking a relevant earthquake at least for the past 0.7 kyr (Figure 1), while three Imax X degree (MCS) occurred in 1747 and km to the north and in km to the south [Boschi et al., 1997]. More recently, a M5.9 event and a M5.4 event occurred in 1979 and in 1984 to the south and to the north, respectively. The only historical event that

4 ESE 7-4 CHIARABBA AND AMATO: TOMOGRAPHY OF COLFIORITO FAULT ZONE Figure 3. (a) Starting and final (bold line) one-dimensional V p models obtained by Velest. Dashed lines represent the interpolated model used for the three-dimensional inversion. (b) A scheme of the Umbria- Marche geologic units with corresponding V p values [from Bally et al., 1986]. (c) Station corrections obtained by Velest. occurred near the Colfiorito fault region is a X MCS degree in 1279 (Figure 1). [8] Just a few hours after the main shocks, several teams of scientists started to map in the field the surficial expression of the earthquake rupture. Even if the interpretation of the surficial breaks in terms of primary faulting is still ambiguous, no slip at the surface exceeding a few centimeters was observed (see Cello et al. [1998] and Cinti et al. [2000] for a thorough discussion). 3. Data and Inversion Procedure [9] During the operation of the local seismic network, a huge set of earthquakes was recorded. P and S wave arrival times were accurately picked on digital waveforms and weights were assigned to data using the criteria: weight 100% for reading errors less than 0.02 s, 75% for errors between 0.02 and 0.05 s, 50% for errors between 0.05 and 0.1, and 25% for errors higher than 0.1 s. [10] We first selected a subset of the best located aftershocks to define a reference one-dimensional model and station corrections for the area, i.e., our best choice of the average regional one-dimensional velocity structure. Subsequently, we have relocated the whole data set using the optimized one-dimensional model and station corrections. P wave and S-P arrival times, from all earthquake locations which satisfy a root-mean-square (RMS) residual less than 0.2 s, hypocentral errors less than 1.0 km, and at least eight readings were selected for the three-dimensional inversion. In order to improve the definition of the structure in the southern part of the model, where the station coverage decreases, we have also used earthquakes with a gap higher than 180 but with the closest stations at distance less than 5 km, and hypocentral coordinates well constrained by the one-dimensional locations. Since the a posteriori resolution of the model (see section 3.2) is strongly improved in the southern part using all the data, we believe in the effectiveness of our choice.

5 CHIARABBA AND AMATO: TOMOGRAPHY OF COLFIORITO FAULT ZONE ESE 7-5 [11] A total of 24,172 P wave and 22,444 S-P arrival times from 1982 earthquakes has been inverted for V p, V p /V s and hypocentral parameters with the Simulps technique developed originally by Thurber [1983] and Um and Thurber [1987] and modified by Eberhart-Phillips [1993] and Eberhart-Phillips and Michael [1998]. The inversion of V p /V s parameters instead of V s has been recommended by Eberhart-Phillips and Michael [1998] in order to obtain V p /V s variations more easily interpretable in terms of rock properties and rheology. The velocity of the model is parameterized by velocity values at grid nodes of a three dimensional grid and is linearly interpolated through the medium. An approximate 3-D ray tracer producing circular ray paths with a pseudobending algorithm perturbing the circular ray path is used. The solution of model parameters is performed by iterative damped least squares inversions, and the damping parameters of V p, V p /V s and station corrections are selected empirically by optimizing data variance reduction and model complexity. In our final inversion, we used a damping equal to 100 for V p, 180 for V p /V s, and 100 for station corrections One-Dimensional Velocity Model Inversion [12] Since any linearized 3-D inversion strongly depends on the starting velocity model, we have used the procedure suggested by Kissling et al. [1994] and the Velest code [Kradolfer, 1989] to find a reference 1-D velocity model that only relies on our seismic data. We used as a priori information the velocity model computed by Deschamps et al. [1984] for an adjacent region of the Umbria-Marche Apennines by using P and S wave arrival times from local earthquakes occurred in The initial V p /V s ratio is set to 1.89 as computed with the Wadati diagram. In the 1-D model inversion, we achieve a final unweighted RMS equal to 0.14 s, after 4 iteration steps, the resolution of velocity parameters is higher than 0.9. The computed V p model (Figure 3a) is consistent with P wave velocities found for rocks composing the Apennines, as defined by velocity logs and laboratory measurements [Bally et al., 1986] (see Figure 3b). The V p values obtained in the upper layers (varying between 3.8 km/s at 0 km and 5.45 km/s at 4 km depth) indicate that the uppermost crust is composed of deep basinal marls deposits, marly limestones, and limestones of the Ceno-Mesozoic Umbrian sequence. Station corrections are very small, less than 0.1 s (see Figure 3c) Three-Dimensional Inversion: Geometry and Resolution [13] The three-dimensional nodal distribution has been chosen optimizing the fidelity of the parameterization and the parameter resolution, after trying different configurations of nodes and spacing. It is well known that the use of a coarse grid yields formally well resolved velocity parameters at the expense of a spatial aliasing of anomalies into the parameter space [see Toomey and Foulger, 1989]. The geometry and location of anomalies may be significantly biased by a poorly parameterized model while the resolution of parameters is high. Thus we have tried two different grid sets varying the grid spacing from 4 to 2 km and from 2 to 1 km in horizontal and vertical directions, respectively. By the analysis of the resolution matrix [see Menke, 1989], we found a degradation of the parameter resolution for the fine Figure 4. Vertical sections (section F in Figure 11) for (bottom) the coarse and (top) the fine V p models. inversion, while the trace of the resolution matrix of the fine inversion is significantly higher than that of the coarse inversion (752 versus 127). A comparison between results obtained by the coarse and fine parameterization on a vertical section of the V p model crossing the fault system is shown in Figure 4. The fine model allows to enhance the details of the buried structure. Since the structure of the upper crust beneath the study region is expected to be very complex, with wavelength of anomalies less than a few kilometers, we favored the use of the fine grid, that still presents a reasonably good resolution, as shown below. [14] The reliability of the V p and V p /V s models has been verified by a complete formal analysis of the resolution matrix [see Menke, 1989; Toomey and Foulger, 1989; Michelini and McEvilly, 1991] and performing synthetic tests using the observed ray geometry. The diagonal elements for V p and V p /V s parameters is shown in Figures 5a and 5b, respectively. The central part of the model has values higher than 0.45, and parameters present compact averaging vectors. The use of (a few) earthquakes with a gap higher than 180 but very close to the network increases the resolution at the border of the model. Visually inspecting all the averaging vectors, we are able to define the spatial properties of the velocity estimation, recognizing the amount and the preferential directions of smearing. The averaging vectors shown in Figure 6 are related to two nodes laying in the well resolved region, with the diagonal elements equal to 0.75 and 0.56, respectively. In both cases, the smearing of anomalies on adjacent nodes is small. The coupling between V p and V p /V s parameters is very small or absent. Conversely, peripherals nodes have higher smearing of anomalies, and their diagonal elements are usually lower than [15] To gain more confidence with the retrieved model, we have performed different synthetic tests by using various patterns of positive and negative perturbations. Figure 7

6 ESE 7-6 CHIARABBA AND AMATO: TOMOGRAPHY OF COLFIORITO FAULT ZONE Figure 5a. Diagonal elements of V p parameters. The well-resolved regions have values higher than 0.45, while regions with smaller values suffer for a slight horizontal and vertical smearing of anomalies. shows the results of the checker board test for the V p in all the inverted layers. In this test, synthetic rays are traced in a heterogeneous model with perturbation of ±0.30 km/s alternated in the three directions. A random noise equal to the final variance of the real inversion is added to synthetic data. We observe that the recovering of synthetic features is very good in most of the inverted model, beneath the central Colfiorito basin and at both its northwest and southeast borders. Furthermore, we explored the ability of resolving larger anomalies, by using the model obtained by the inversion of the real data as the synthetic model (restore test). Data have been inverted, after adding random noise. The recovering of location, shape, and amplitude of the synthetic anomalies is very good for almost all the volume with diagonal elements higher than 0.45 (Figure 7). On the basis of the resolution matrix and the synthetic tests (Figures 5, 6, and 7), we are confident with the retrieved model in the area that will be discussed in sections 3.3 and Three-Dimensional Inversion: Results [16] A total of 1896 V p and V p /V s parameters and 33 station corrections have been estimated, achieving 69% of variance reduction, and a final RMS of 0.08 s, slightly above the estimated reading errors, after four iteration steps. Relocated earthquakes present high RMS location improvements on average, with location errors less than 0.5 km.

7 CHIARABBA AND AMATO: TOMOGRAPHY OF COLFIORITO FAULT ZONE ESE 7-7 Figure 5b. Diagonal elements of V p /V s parameters. The well-resolved regions have values higher than 0.45, while regions with smaller values suffer for a slight horizontal and vertical smearing of anomalies. [17] Figure 8 shows V p, V p /V s and earthquake hypocenters for each inverted layer. V p values range between 3.6 and 6.1 km/s, while V p /V s range between 1.84 and 2.0, variations consistent with those found in other active faults [Michelini and McEvilly, 1991; Thurber et al., 1995; Zhao and Kanamori, 1995; Zhao and Negishi, 1998]. [18] In the upper kilometers of the crust, layers 1 and 2, low-velocity anomalies (V p lower than 4.0 and 4.4 km/s at 0 and 1 km depth, respectively) are found in the central area, beneath the Colfiorito basin, and extend both northward and southward close to the 14 October main shock. Regions with V p higher than 4.6 km/s are those where limestone rocks outcrop at the surface, corresponding to the topographic reliefs which surround the basins. The very few earthquakes are mostly aligned on a small NNE trending linear structure bordering the Colfiorito basin to the west. The heterogeneity of the V p /V s model is high, with a predominance of local high-v p /V s zones within the Colfiorito basin (Figure 8). A high-v p /V s region is present between the two main shocks, at 1 km depth, also bordered by shallow seismicity to the east. [19] At 2 km depth, regions with high V p (up to 5.4 km/s) are found around and beneath the Colfiorito basin. An about 10-km-long, NW trending high-velocity anomaly is present in the northeastern part of the epicentral region and rotates to the south outside this area. Two separate high-velocity

8 ESE 7-8 CHIARABBA AND AMATO: TOMOGRAPHY OF COLFIORITO FAULT ZONE Figure 6. (a) Averaging vectors for two well resolved parameters for NODE 0074 located in the uppermost layer with a diagonal value of 0.75, the shape of the averaging vector is very compact. (b) Averaging vectors for two well resolved parameters for NODE 0645 located in the fourth layer with the diagonal value of 0.57, the averaging vector is picked on the diagonal and the smearing on adjacent nodes is small. features are found to the south of the Colfiorito basin. Earthquakes are clustered on two linear N-S trending structures located in the region between the two main shocks and mostly surround positive V p and V p /V s anomalies. Only two events with magnitude larger than 4.0 occur at this depth, one along the linear structure (the M w 4.6, 16 October, strike slip earthquake) and the other one to the southwest of the Colfiorito basin. A main NNW trending region of high V p /V s is observed to the east of the Colfiorito basin. [20] At 3 km depth, a broad positive velocity anomaly is found in the central area with local spots of high V p (as high as 6.1 km/s), bordered both to the south and to the north by low-velocity anomalies (V p less than 5.2 km/s). Earthquakes are mostly concentrated around the bulk of the central high-velocity features and, in the southern part of the model, at its western border. The V p /V s pattern of anomalies is similar to that in the layer above, but with less pronounced variations. Also at this depth, most of the high-v p regions have high V p /V s values. [21] At 4 km depth, the main fault region is marked by positive anomaly, with four local highs of V p higher than 6.0 km/s. Aftershocks occur within the whole high-v p region, mostly clustering around the local maxima. A diffuse region of low-v p /V s is found beneath the central part of the model, interrupted by two local highs. A well pronounced N-S elongated, high-v p /V s anomaly is found slightly to the north of the 14 October main shock. [22] At 5 km depth, the central area of Colfiorito is characterized by P wave velocities lower than those retrieved at both the northern and southern borders of the model (V p less than 5.8 km/s in the center and up to 6.1 km/s at the borders). In the central part of the model V p are generally smaller than those found at 4 km depth. In the footwall of the 14 October main shock a prominent high V p, high-v p /V s region is found. A significant low-v p /V s anomaly (V p /V s less than 1.86) is located close to the nucleation region of the two 26 September main shocks and of the relevant aftershocks. [23] At 6 km depth, the central area is characterized by a NW elongated low-velocity anomaly with values generally lower than 6.0 km/s. A parallel, thinner low-v p /V s region is evident, slightly offset to the west, where earthquakes appear to cluster. Even if the resolution of the model

9 CHIARABBA AND AMATO: TOMOGRAPHY OF COLFIORITO FAULT ZONE ESE 7-9 Figure 7. Synthetic tests for the V p model in each inverted layer. In the checkerboard test V p variations between ±0.3 km/s and in the restore test perturbations of ±8% are used. Note the very high resolution in the central part of the model and the good resolution in the southeastern border. Fault boxes and main shock hypocenters (stars) are plotted. decreases at this depth, the central low-velocity anomaly is still a reliable feature Three-Dimensional Earthquake Locations [24] Three-dimensional earthquake locations are strongly improved with respect to the 1-D locations, with an average RMS decrease of about 60% and an average final RMS close to 0.05 s (with almost 90% of events with final RMS smaller than 0.1 s). Hypocentral variations are less than 1 km in x and y and less than 2 km in z, with average displacements of about 0.4 km in x and y and 0.6 in z. Figure 9 shows the 1-D and 3-D earthquake epicenters. It is evident that all the 3-D located earthquakes are more clustered with respect to the 1-D locations. Epicentral variations are higher at the northern and southern peripheries of the area, while are much smaller in the central region where the station distribution is denser. [25] In the epicentral map, the two N10 E striking linear alignments of epicenters are evident: their geometry being strongly enhanced by the three-dimensional earthquake locations. These two linear structures are located between the two main shocks of 26 September. A clear gap of seismicity is present at the southern border of the Colfiorito basin, separating the northern faults (0940, 0033 UT) from the southern, 1523 UT fault. Three other regions with a small density of aftershocks are visible on the three fault planes, suggesting the location of the main slipped patches (see Figure 9). [26] The improvement of 3-D locations is impressive looking at the cross sections perpendicular to the faults of the three main shocks (Figure 10). The 0940 UT main shock (upper panel) ruptured toward NW on a fault plane that is very clear for about 6 km from the hypocenter, with a dip of and an apparent increase of dip to 50 northward. In the central part of Colfiorito (Figure 10, middle), two subparallel faults are clearly visible. The SW fault plane appears as a ramp and flat structure and was activated by a M w 5.4 earthquakes on 6 October. The 0033 UT main shock ruptured the eastern fault and propagated southward. To the south, the separation between these two faults increases. The lateral extent of the 0033 UT fault is clear for almost 5 km. More to the south (Figure 10, bottom), only one fault plane is present along which the 1523 UT (14 October) earthquake originated. This fault is the southern continuation of the western fault of the Colfiorito basin, separated by a region with few earthquakes (see Figure 9). 4. Interpretation [27] Among the several factors influencing P wave velocities in the upper crust, lithological contrasts are the

10 ESE 7-10 CHIARABBA AND AMATO: TOMOGRAPHY OF COLFIORITO FAULT ZONE Pleistocene. A more recent extensional tectonics replaced the former compression but the associated extensional structures are often hidden by the preexisting structure. As a result, the subsurface structure enlightened by tomography mostly consists of high-v p Mesozoic limestone uplifted units (T1, T2,T3, and T4 in Figures 8 and 11) thrust generally eastward over low-v p flysch and sedimentary basins. In addition, Triassic evaporitic rocks (with i up to 6.1 km/s) are present at the base of the Mesozoic cover, with thickness of about 1 3 km. Evaporites can be found at very shallow depth if they are involved in thrust structures, as found in deep wells drilled in the region. [28] The deep structure beneath the Colfiorito basin, in the central part of the model, is complex. At 1 km depth, the basin is characterized by low V p (less than 4 km/s), indicating the presence of subsided Cenozoic sedimentary rocks. The depth of the basin is less than 1 km and beneath 2 km depth Mesozoic limestone rocks are found (V p higher 4.6 km/s). At 3 km depth, a pronounced high velocity anomaly is found in the central part of the area, closed both to the north and to the south by low velocities (Figure 8). V p is as high as 6.1 km/s in the basin, suggesting a structural high of the Mesozoic cover with the bulk composed by Triassic evaporites. This interpretation is consistent with information from deep wells where the Triassic evaporites (with V p as high as 6.1 km/s) are found at shallow depth. At 4 km depth, the Triassic rocks are present also to the south, close to the 1523 UT fault. Figure 8. (left) Three-dimensional V p and V p /V s (right) models, main shocks (large stars), M L 4 aftershocks (small stars), and smaller magnitude aftershocks (circles) in the inverted layers. The boxes indicate the location and geometry of the fault planes ruptured by the three main shocks [from Stramondo et al., 1997]. The white lines are the diagonal of resolution 0.45 and 0.25, the latter indicating the limit of the modeled region. most relevant cause. In peninsular Italy, tomographic studies of the crust revealed that high-v p anomalies in the upper kilometers (V p 4.6 km/s) define the geometry of the Mesozoic limestone units, while low velocities are found for orogenic flysch units (V p between 3.0 and 4.0 km/s) or postorogenic Plio-Pleistocene sedimentary basin (V p lower than 3.0 km/s) [Amato et al., 1992; [Chiarabba et al., 1995]. These values are consistent with those measured on rocks extracted by deep drilling studies [Bally et al., 1986, see Figure 3b]. The area is part of the northern Apennines thrust belt originated by the compressional tectonics during Plio- Figure 8. (continued)

11 CHIARABBA AND AMATO: TOMOGRAPHY OF COLFIORITO FAULT ZONE ESE ' km ' A B C D E F G H I L km ' Figure 9. Earthquake epicenters obtained by (top) the 1-D location and (bottom) the 3-D location. Triangles are the used seismic stations, the crosses in Figure 9 (top) are the grid nodes used in the inversion. Fault boxes, main shock hypocenters (stars), and vertical section traces are shown and labeled. Note the strong clustering of earthquakes in the 3-D locations, and the two NNE trending linear features located between the two 26 September main shocks. Lateral discontinuities of the NW trending high V p structure are observed. At 5 km depth, lateral velocity contrasts are small in the central part of the model, while a high V p is found to the south. This suggests a lateral discontinuity between a structural high located beneath the Colfiorito basin and a structural low to the south. The area of transition between these two structures corresponds with a seismicity gap (see Figure 9) and an evident geometrical right lateral offset of the fault system. At depth of 6 km, a broad low V p anomaly is evident in the central region, with V p less than 6.0 km/s indicating an inversion of velocity with respect to the layer above. Independent information on the regional structure indicate that the top of the metamorphic basement is composed by Permo-Triassic conglomerates and phyllades with V p much less than 6.0 km/s. Thus we interpret the low V p found at 6 km depth as due to the presence of the top of the metamorphic basement. [29] V p /V s variations are related to difference in rock properties and rheology [O Connell and Budiansky, 1977]. Within a fault zone, V p /V s is mostly controlled by a different level of cracking in the subsurface rocks, and fluid pressure at depth [Zhao and Negishi, 1998, and references therein]. [30] The presence of fluids within the Mesozoic cover of the Apenninic belt derives from large regional aquifers [Boni et al., 1991] and a flux of CO 2 of deep origin [Chiodini et al., 1999, 2000]. In our interpretation, V p /V s anomalies in the upper Mesozoic cover indicate a different level of saturation and fluid pressure, with high V p /V s suggesting abundance of fluids and high pore pressure. The relatively low-v p /V s anomalies found at 5 and 6 km depth, in correspondence of reduced V p, could be explained by the presence of two phase fluids, by a lower saturation in fluids or by a quartz-enriched lithology [Kern, 1982;

12 ESE 7-12 CHIARABBA AND AMATO: TOMOGRAPHY OF COLFIORITO FAULT ZONE 1 D Locations 3 D Locations SW NE 0-5 SW NE 0 A B C D -5 09:40 Fault SW NE 0-5 SW NE 0 E F G -5 00:33 Fault SW NE SW NE 0 H I L -5 15:23 Fault distance (km) depth (km) Tatham, 1982; Koch, 1992]. Considering that the V p /V s is on average high for the whole fault system (values higher than 1.84), we interpret the low-v p, low-v p /V s anomalies observed beneath 5 km depth as due to a less developed system of cracks within the metamorphic basement with respect to the overlaying sedimentary cover. 5. Seismotectonic Inference [31] In this section, we discuss the mechanism of normal faulting and fault segmentation in the central Apennines. Recently, Collettini et al. [2000] and Boncio and Lavecchia [2000] proposed structural models in which the Colfiorito earthquakes ruptured SW dipping normal faults antithetic to a east dipping low angle normal fault (called Alto Tiberina fault, ATF ). The ATF is visible as a gently east dipping reflector on seismic profiles crossing the Apennines at 20 to 60 km to the north of Colfiorito [see Barchi et al., 1998]. In both reconstructions, the main tectonic feature of this portion of the Apenninic belt is the east dipping ATF where large normal faulting earthquakes are hypothesized, though never observed, while secondary normal faulting occur on antithetic southwest dipping segments, like those activated during the 1997 sequence. The depth at which the SW dipping normal faults of the Colfiorito system should intersect the ATF is presumed to be at 5 to 7 km, extrapolating results obtained further to the north. [32] In our study, aftershocks and main shocks are consistent with rupturing on SW dipping normal faults confined above 6 km depth. However, no earthquakes with magnitude larger than 2.5 are associated to the ATF, suggesting that no significant seismic deformation occurred on this fault during the Colfiorito seismic sequence. Our tomographic images do not show evidence of a east verging ATF in the epicentral area of Colfiorito. Seismicity distribution and velocity structure suggest that the metamorphic basement is scarcely affected by the normal faulting tectonics, and the ruptured fault segments are confined within the upper Mesozoic cover. We speculate that the mechanism of normal faulting tectonics is controlled by the presence of the shallow decollement at the base of the Triassic evaporites above the metamorphic basement. [33] On the basis of surface geology, the tectonic structure shows a rotation of the frontal thrusts from NNW to N-S [see Calamita and Deiana, 1987, Figure 2]. In contrast, seismological data clearly show that the normal faults are NW trending. Also, tomographic images show a NW trending pattern of anomalies at 3 km depth and below. The two N-S trending, subvertical faults that ruptured during the 1997 sequence, displacing the main ruptures in the center of the Colfiorito basin (Figure 9), are very Figure 10. (opposite) NE trending vertical sections of seismicity across the three main faults (section traces in Figure 7). Each trace is separated by 2 km, and the width for plotting earthquakes is ±1 km. Stars indicate the three main shocks (large stars) and all the aftershocks with magnitude larger than 4 (small stars). We note an improvement of the fault definition obtained by the 3-D locations; faults are steeper and reach shallower depth than those defined by 1-D locations.

13 CHIARABBA AND AMATO: TOMOGRAPHY OF COLFIORITO FAULT ZONE ESE 7-13 Figure 11. Vertical cross section of V p and V p /V s models across the fault system (for traces see Figure 7). From top to bottom: sections C, D, E, F, G, I, and L. The same as Figure 8. shallow and confined within the upper 2 km depth. The present-day extensional tectonics cut the preexisting thrust structures, while fault segmentation appears to be related to discontinuities of the uppermost crust. The competitive role played by the N-S and the NW-SE trending structures in controlling the complexity of tectonics and the depth extent of the N-S faults still need to be further explored, integrating evidence from seismology and geology. [34] The relationship between earthquake occurrence and crustal structure is clarified by vertical cross section across the fault system. Figure 11 shows seven parallel vertical sections of V p, V p /V s and relocated earthquakes from the northwest to southeast of the area. The most impressive feature is the presence of several distinct high-v p, high-v p /V s anomalies that we interpret as high-velocity material constituting the major folds, thrust eastward during the building of the belt. These folds are composed by Meso-Cenozoic limestones and basinal sequence materials and their inner bulk is composed by the Triassic evaporites. The high-v p /V s anomalies suggest that high pore pressure is present within the folds (see Figure 11) The 26 September, 0940 UT Fault [35] In the northern portion of the fault system, a welldefined east verging high-v p fault-related fold is revealed (T1 in Figure 11, sections C and D). Seismicity indicates that the SW dipping normal fault along which the 0940 UT earthquake occurred coincides with the preexisting thrust.

14 ESE 7-14 CHIARABBA AND AMATO: TOMOGRAPHY OF COLFIORITO FAULT ZONE The geometry of the high-v p fold is well defined both at depth (4 5 km) and close to the surface (1 2 km), while velocity contrasts are feeble in the middle. Most of the coseismic slip is concentrated in a 6-km northwestward elongated, high V p, patch at depth between 6 and 3 km in agreement with the source model by Hernandez et al. [1999], coincident with the high-v p zone between 3 and 5 km depth (see also Figure 8). The fault-related fold is characterized by high V p /V s down to 4 km depth, interpreted as a high pore pressure region. Further to the north, the structural high terminates in a region of low V p (Figure 8) The 26 September, 0033 UT Fault [36] In the central part of the fault system, two main faults splay from the northern segment and are sub parallel for 6 8 km southward, spaced each other up to 6 km (sections D, E, F, and G in Figure 11). The two faults end abruptly southward at the closure of the Colfiorito basin. The main shock originated along the easternmost fault and broke in the back limb of a high-v p fault-related fold (T2 on Figure 11, sections E, F, and G), a few kilometers west of the thrust plane, in agreement with there construction by Mirabella and Pucci [2002]. In the hanging wall of the 0033 UT fault, a very small low-velocity basin is imaged down to 1 km depth (X = 3 5 in sections F and G) possibly suggesting previous earthquakes occurred on the same normal fault. The repeated activity of this fault might have created the small depression of the Colfiorito basin. [37] The western fault located in the hanging wall of the 0033 UT fault was ruptured by a M w 5.4 shock on 6 October and by several M w 4 earthquakes. This fault coincides with a second thrust (T3 in Figure 11, section G). At shallow depth, the high-v p folds (T2-T3) present very high-v p /V s anomalies, that we interpret as regions of pronounced pore pressure The 14 October, 1523 UT Fault [38] In the southern part of the fault system, a main northwest trending high-v p anomaly is present and interpreted as a Mesozoic thrust unit (T4 on Figure 11, sections I and L). The fault segment ruptured by 1523 UT main shock is located in a sharp velocity contrast in the back limb of the thrust unit. In the hanging wall, a well defined local velocity down warp is evident down to 3 km depth. Also this down warp is consistent with repeated normal events occurred along the same fault segment. High pore pressure is inferred within the thrust structure in coincidence with the inner part of the fold, probably constituted by Triassic evaporites(v p larger than 6.0 km/s). V p and V p /V s anomalies across this fault are consistent with those found by Michelini et al. [2000] in the southern part of the Colfiorito region Rupture Evolution [39] In complex fault system, an important issue is to understand which factors control rupture evolution and segmentation. More specifically, which is the competitive role played by the lateral variation and geometrical discontinuities. Tomographic images across the fault system (see Figure 11) show large lateral complexity in the structure from north to south. The Colfiorito structural high is a Figure 12. (opposite) Sketch of the Colfiorito normal faulting system. Cross sections are the same of Figure 11. Solid lines are V p contours every 0.5 km/s (5.0, 5.5, and 6.0 km/s). Circles are magnitude less than 4.0 aftershocks, small stars are M w larger than 4.0 earthquakes, and large stars are the three main shocks.

15 CHIARABBA AND AMATO: TOMOGRAPHY OF COLFIORITO FAULT ZONE ESE 7-15 Figure 13. Distribution of hypocenters on a 45, SW dipping fault plane comprising the 0940 and the 0033 UT main shocks (stars). The hypothesized main asperities ruptured by the two shocks are shown. region of high V p, high V p /V s at 1 to 4 km depth and lower V p /V s below. At seismogenic depth, 4 to 6 km depth, local spots of high-v p materials are found for each of the three segments, but without a lateral continuity over all the structure. Such high-v p spots may represent the most competent patches on the fault planes where coseismic slip concentrates during the main shocks. Moreover, lateral steps between the three segments are clear both considering earthquake distribution and velocity anomalies (Figures 8, 9, and 11) and fault geometry modeled by SAR, GPS and strong motion [see Stramondo et al., 1999; Capuano et al., 2000]. Our results suggest that the geometrical discontinuities of the structure control fault segmentation and the delayed rupture of distinct segments during the 1997 sequence. Figure 12 shows a sketch of the Colfiorito fault system, where the major fault segments and the relation with the preexisting structure are defined. For the 0940 UT shock, the old thrust plane is activated with normal mechanisms. Conversely, for the 0033 and the 1523 UT shocks the activated fault lays at the back limb of the thrust unit (see Figures 11 and 12). This difference could be due to the orientation of the thrust planes relative to the direction of extension. In this area, the minimum stress is NE-SW oriented, approximately perpendicular to the structural trend north of Colfiorito. To the south, the rotation of these trends to N-S might have forced the NE extension to be accommodated on newly formed steeper normal fault planes, on the back limb of the ancient thrust. In our hypothesis, the largest shocks originated along the shallow decollement of the thrust units in zones of increased strength (low V p /V s ). The upward propagation of the faults in the Mesozoic cover is controlled by the heterogeneity of the shallow structure. [40] The coseismic slip of the main shocks concentrates in small regions of relatively high V p and low V p /V s (Figures 8a and 11). Aftershocks concentrated around the area of maximum slip, defining small patches of high strength material that we interpret as the main asperities of the fault system (Figure 13). Numerous tomographic applications evidenced a tendency of coseismic slip to concentrate on confined patches of high-v p and low-v p /V s along the fault, being the P wave and S wave velocities related to the elastic moduli [Lees, 1990; Michael and Eberhart-Phillips, 1991; Eberhart-Phillips and Michael, 1993; Foxall et al., 1993; Zhao and Kanamori, 1993; Chiarabba and Amato, 1994; Zhao and Kanamori, 1995; Eberhart-Phillips and Michael, 1998]. In the upper part of the fault, extremely high V p /V s values are observed. We interpret the increase of V p /V s as a sudden drop of shear at the upper tip of the fault, possibly causing the upward termination of rupture and the absence of significant slip at the surface. A similar mechanism has been proposed by Thurber et al. [1995] for faulting mechanisms at Loma Prieta. [41] The very shallow decollement and the strong complexity of the structure confines the lateral dimension of fault segments, thus limiting the maximum magnitude of earthquakes along these small fault planes. This structural style may be characteristic for the northern Apennines. 6. Conclusions [42] In the 1997 Colfiorito sequence, normal faulting earthquakes originate along SW dipping planes confined within the Mesozoic cover and possibly splaying from a decollement above the metamorphic basement. The faults appear to be slightly deeper than previously suggested. The comparison of our results with the local tectonics suggests that the segmentation of normal faults in the central northern Apennines, and hence the maximum magnitude expected, is controlled by both the shallow decollement and the intersection between NW-SE trending faults and N- S transverse structure (lateral thrust ramps, strike slip transfer faults, etc.). This observation could be a key to recognize fault segmentation for earthquakes that will strike the northern Apennines in the future. [43] A striking problem raised by the 1997 sequence is the reactivation of preexisting thrust planes as normal faults. Our results show that in some case the old planes are utilized, whereas in others they are not. We propose that slight changes of strike and dip of the preexisting faults, on the scale of a few kilometers, can determine the evolution of the faulting process, even under a uniform regional stress field. [44] Finally, we observed that lateral variations of material properties along the faults control the rupture evolution.

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