Annals n.6/2003 ok 23/04 1315 ANNALS OF GEOPHYSICS, VOL. 46, N. 6, December 2003 Key words microseismicity – low-angle normal fault – seismic gap – seismic hazard 1. Introduction The Northern Apennines (Italy) undergo ac- tive crustal extension of about 0.5 × 10−7 yr–1 (Hunstad et al., 2003) accommodated by mod- erate to large normal faulting earthquakes along a NW-elongated seismic belt (fig. 1). A microseismic study in a low seismicity area of Italy: the Città di Castello 2000-2001 experiment Davide Piccinini (1), Marco Cattaneo (1), Claudio Chiarabba (1), Lauro Chiaraluce (1), Martina De Martin (1), Massimo Di Bona (1), Milena Moretti (1), Giulio Selvaggi (1), Paolo Augliera (2), Daniele Spallarossa (3), Gabriele Ferretti (3), Alberto Michelini (4), Aladino Govoni (4), Paolo Di Bartolomeo (4), Marco Romanelli (4) and Julius Fabbri (4) (1) Istituto Nazionale di Geofisica e Vulcanologia, Roma, Italy (2) Istituto Nazionale di Geofisica e Vulcanologia, Milano, Italy (3) Dipartimento per lo Studio del Territorio e le sue Risorse (Dip.Te.Ris.), Università di Genova, Italy (4) Istituto Nazionale di Oceanografia e di Geofisica Sperimentale – Centro di Ricerche Sismologiche (INOGS-CRS), Udine, Italy Abstract Recent seismological studies contribute to better understand the first order characteristics of earthquake oc- currence in Italy, identifying the potential sites for moderate to large size earthquakes. Ad hoc passive seis- mic experiments performed in these areas provide information to focus on the location and geometry of the active faults more closely. This information is relevant for assessing seismic hazard and for accurately con- straining possible ground shaking scenarios. The area around the Città di Castello Basin, in the Northern Apennines (Central Italy), is characterized by the absence of instrumental seismicity (M > 2.5), it is adjacent to faults ruptured by recent and historical earthquakes. To better understand the tectonics of the area, we in- stalled a dense network of seismic stations equipped with broadband and short period seismometers collect- ing data continuously for 8 months (October 2000-May 2001). The processing of ∼ 900 Gbyte of data re- vealed a consistent background seismicity consisting of very low magnitude earthquakes (ML < 3.2). Pre- liminary locations of about 2200 local earthquakes show that the area can be divided into two regions with different seismic behaviour: an area to the NW, in between Sansepolcro and Città di Castello, where seis- micity is not present. An area toward the SE, in between Città di Castello, Umbertide and Gubbio, where we detected a high microseismicity activity. These findings suggest a probable different mechanical behaviour of the two regions. In the latter area, the seismicity is confined between 0 and 8 km of depth revealing a rather well defined east-dipping, low angle fault 35 km wide that cuts through the entire upper crust down to 12-15 km depth. Beside an apparent structural complexity, fault plane solutions of background seismici- ty reveal a homogeneous pattern of deformation with a clear NE-SW extension. Mailing address: Dr. Lauro Chiaraluce, Istituto Nazio- nale di Geofisica e Vulcanologia, Via di Vigna Murata 605, 00143 Roma, Italy; e-mail: chiaraluce@ingv.it 1316 Davide Piccinini et al. In the past 30 years, three seismic sequences following Mw 5.0-6.0 mainshocks, struck a 70 km long section of the Northern Apennines from Nor- cia (1979, MS = 5.8) to Gubbio (1984, MS = 5.3). For all these sequences, seismological data show that earthquakes occur on NW-trending normal faults the lengths of which are usually less than 10 km and dipping towards the southwest (De- schamps et al., 1984; Haessler et al., 1988; West- away et al., 1989). During the most recent se- quence that struck the region in 1997 and 1998, seven mainshocks (5.0 < MW < 6.0, Ekström et al., 1998) ruptured adjacent SW-dipping fault seg- ments composing a 50 km long portion of the seis- mic belt (Chiaraluce et al., 2003). At the north- western end of this «seismically active» belt lies a «silent» region located near the town of Città di Castello (hereafter CDC). The southern portion of the study area had no significant earthquakes for the past 1.0 kyr (see the historical catalogue of Boschi et al., 1997) and an impressive lack of in- strumentally recorded seismicity ML > 2.5 earth- quakes, which represent the lower threshold mag- nitude for the located earthquakes detected by the Italian network (RSNC). To investigate the CDC seismically silent region, we acquired digital, mostly broadband, data with a seismic array consisting of 30 12 00′ 12 24′ 12 48′ 42 48′ 43 12′ 43 36′ 0 5 10 km 1192 1352 1458 1768 1789 1799 1832 1917 1918 1919 1741 1751 1781 1279 1328 1353 Sansepolcro Città di CastelloArezzo Gubbio Perugia Umbertide 2001/11/25 1998/03/04 1984/29/04 1997/09/26 Tiber Valley Fig. 1. Historical earthquakes (empty squares, from CFTI-CPTI) and instrumental seismicity (ML > 2.5) of the area (from earthquake bulletin 1990-1999): in black the focal mechanisms of Gubbio (1984), Gualdo Tadino (1998) and Sansepolcro (2001); in red the mainshock of Colfiorito seismic sequence (1997). The dashed box en- close the studied area. 1317 A microseismic study in a low seismicity area of Italy: the Città di Castello 2000-2001 experiment three-component stations. The experiment last- ed for 8 months. The experiment aims at defining possible fault location, geometry and kinematics by us- ing the recorded microearthquakes. In this study, we present the recorded data with pre- liminary hypocentral determinations and focal mechanisms of the recorded seismicity. 2. The CDC experiments Between October 2000 and May 2001, a dense temporary local network of 30 digital three-component stations was installed in an area extending 70 × 40 = 2800 km2, from Sansepolcro (in the north) to 20 km south of Gubbio (fig. 2). We installed Reftek, Nanometrics and Lennartz digital stations equipped with three-component sensors (Lennartz LE3d-5s, Guralp CMG40T, Lennartz LE3d-1s, Mark L22). The geometry of the network varied during the experiment in order to fulfil the aims of the project. 2.1. The test In the first week of October 2000, a data acqui- sition test was performed to provide a first look at the ongoing local seismicity. This initial part of the 12 00′ 12 15′ 12 30′ 12 45′ 43 00′ 43 15′ 43 30′ 43 45′ INGV OGS-Trieste DIPTERIS-Genova Other permanent Network RSNC Perugia Gubbio Città di Castello Sansepolcro Arezzo 20 km T iber V alley Fig. 2. Map of the temporary seismic network operating in the area. 1318 Davide Piccinini et al. experiment was also helpful to optimize the net- work geometry. The test network consists of 10 sta- tions positioned around the nodes of a regular grid spaced at about 10 km. The stations continuously telemetered the data to a seismic acquisition center located in a mobile van. After optimizing the trig- ger and the coincidence parameters of the acquisi- tion system, it was possible to monitor the seismic- ity of ML ≤ 1 in real-time. A total of 181 events (143 both local and regional, and 38 teleseisms) were recorded during the month-long experiment. 2.2. The main experiment As the test confirmed the presence of a signif- icant level of background seismicity, we distrib- uted the remaining 20 stations around the 10 sta- tions of the test network (see fig. 2). From the be- ginning of November 2000, the network was com- posed of «stand alone stations» only recording continuously. The radio transmitters were re- placed by systems recording on hard disks (Lennartz Digital Uher). At a sampling rate of 125 sps per channel, the memory capacity of the sta- tions lasted for more than 1 month. The instruments were powered by high capaci- ty batteries (200 Ah) and solar panels. Stations were located along secondary country roads, mainly in woodland, far away from any anthropic noise. Sites characterized by solid rock outcrop, good insula- tion for solar panels and visible sky for GPS were favoured. The seismometers were buried, when possible, and covered with a heavy cap to avoid wind and rain disturbances. The stations were visit- ed once a month for data collection and servicing. After processing the data of the first 4 months, a cluster of shallow (from 2 to 4 km depth) hypocenters was identified roughly 10 km to the north of Gubbio. Thus, we decided to move some of the stations from the peripheries of the network to the Gubbio area to improve the network geom- etry around the hypocentral cluster. 2.3. Event detection In order to extract seismic events from the continuous raw data, we developed a detection al- gorithm, denominated DETECTOR hereafter. DE- TECTOR is based on the same principles as the au- tomatic detection systems used by centralized networks. We applied an sta/lta (short time aver- age/long time average) trigger algorithm to the continuous data. The values for sta, lta, ratio threshold and other trigger parameters were opti- mized by performing numerous test runs: sta = 2 s, lta = 40 s, threshold = 3, duration = 2 s, coinci- dence interval = 40 s, coincidence level = 5. This resulted in an earthquake detection level less than ML ≈ 1 and minimum number of false events. The detection procedure was composed as follows: firstly we selected intervals with sta/lta (i.e. short versus long time averages) greater than a chosen threshold on each sta- tion data. Then a trigger coincidence of five stations was chosen to declare an event. Event time windows were extracted from the contin- uous data of all 30 stations based upon the se- lected triggers. We also find the best combi- nation for the time length of sta/lta required to be greater of the threshold, the time inter- val during which triggers have to be consid- ered coincident and finally the number of co- incident triggers necessary to define a possi- ble seismic event. 3. Data analysis Despite the low magnitude of the events, a high signal/noise ratio allowed us to read P- wave and S-wave onset on digital waveforms with the accuracy of few samples for the P on- set and less than 0.1 s for S-wave readings. The phase readings has been weighted according to reading accuracy and the following «01234»- HYPO71 (Lee and Lahr, 1975) scheme was adopted: 0 (≤ 0.03 s), 1 (0.03-0.06 s), 2 (0.06- 0.1 s), 3 (≥ 0.1 s), and 4 (ignore). A total of 2200 earthquakes were located by using HYPOELLIPSE (Lahr, 1989) and a gradient 1D velocity model obtained by inverting 1994 earthquakes for a total of 14245 P-phases and 14105 S-phases by using VELEST (Kradolfer, 1989; Kissling et al., 1994) code. The starting velocity model was derived from studies performed on adjacent region (De- schamps et al., 1984; Chiaraluce et al., 2003). The best 1D velocity model was reached after 4 1319 A microseismic study in a low seismicity area of Italy: the Città di Castello 2000-2001 experiment iterations, with a final unweighted RMS of 0.04. The P-wave velocity values found are consistent with those derived by logs and labo- ratory experiment (Bally et al., 1986). Inversion results (fig. 3) show the presence of a well constrained, low velocity layer (5.7 km/s) at a depth of 20 km. Although the distri- bution of seismicity is mainly located in the first 15 km in depth, the crustal volume affect- ed by the low velocity layer is sampled by a good number of seismic rays (> 500 rays and about 20 hypocenters). The deepest earth- quakes recorded were localized around 60 km at depth. The Vp / Vs ratio computed by VELEST is also well constrained around the value of 1.84. Similar values were found in adjacent re- gions (Haessler et al., 1988; Ripepe et al., 2000). Finally, a total of 2215 events were lo- cated by using HYPOELLIPSE (Lahr, 1989). To determine ML, synthetic Wood-Anderson seismograms were calculated by removing the instruments response of each recording and convolving the displacement ground motion with the standard Wood-Anderson torsion seis- mograph response (Anderson and Wood 1925; Richter, 1935). Observed NS and EW ampli- tude values were averaged to constitute a single measurement (i.e. sqrt (ns2 + ew2)). Following the method originally used by Richter (1935,1958), ML is given by M A A SLog LogL 0= - - (3.1) where A is the maximum trace amplitude in millimetres measured from a Wood-Anderson seismogram, A0 is a distance dependent atten- uation curve, and S is a station dependent ML adjustment. We assume a parametric form of Log A0 proposed by Alsaker et al. (1991) and derived by Bakun and Joyner (1984) as A R n R R R R R Log Log ref ref ref 0 = + -k V- - ^ ` ^ ^ h j h h (3.2) where R is the hypocentral distance (in km), n and k are parameters related to geometrical spreading and attenuation of S-waves. Rref is the reference distance and V (Rref) is the reference magnitude value corresponding to Rref. For instance, Rref = 100 km corresponds to V (Rref) = 3 whereas Rref = 17 km corresponds to V (Rref) ~ 2. To establish the new magnitude scale, we combined eqs. (3.1) and (3.2) to give an over determined sparse system of equations. Under the constraints that station corrections sum to zero, the inversion of the linear system was performed by a least-squares procedure based on LSQR algorithm (Paige and Saunders, 1982). Bootstrap resampling techniques (e.g., Efrom, 1979) were used to estimate the inver- sion stability; station corrections, attenuation parameters and magnitude values are computed by averaging the results of 5000 repeated inver- sions on randomly resampled data sets. Boot- strap results are also used to estimate the stan- dard deviation of the inverted parameters. Inversion was performed normalizing the lo- cal magnitude scale to motions at Ref = 17 km (Vref ∼ 2.0). We obtain n = (1.420 ± 0.03), k = = (0.009 ± 0.0009) and station corrections rang- ing between 0.46 and − 0.30. 0 1 2 3 4 5 6 7 8 9 100 90 80 70 60 50 40 30 20 10 0 Velocity (km/s) D e p th ( km ) P - wave (km/s) S - wave (km/s) Fig. 3. Best 1D velocity model for the study area. 1320 Davide Piccinini et al. Fig. 4a-d. Statistical plots for the localized events. a b c e d 1321 A microseismic study in a low seismicity area of Italy: the Città di Castello 2000-2001 experiment 11 48′ 12 00′ 12 12′ 12 24′ 12 36′ 12 48′ 13 00′ 13 12′ 42 48′ 43 00′ 43 12′ 43 24′ 43 36′ 43 48′ 44 00′ 0 5 10 km Arezzo Perugia Umbertide 0 10 20 30 40 50 60 -30 -20 -10 0 10 20 30 0 10 20 30 40 50 60 -30 -20 -10 0 10 20 30 Gubbio Città di Castello Sansepolcro Depth(km) 0 60 A B T iber V alley BA Fig. 5. Map and two vertical sections of located earthquakes. The dashed box enclose the hypothesized seis- mic gap discussed in the text. White dots represent quarry blasts localized at very shallow depth. 1322 Davide Piccinini et al. Finally some statistical analyses were per- formed on the temporal distribution of seis- micity. The main results can be summarized as follows: i) The distribution of the local magnitude for the recorded events is complete down to M = = 1.2, while ranging between 0 and 3 (fig. 4a). ii) The number of phases per events spans from a minimum of 8 phases (about 30 events) to 60 phases per event (including P- and S-wave readings from the INGV-National Seismic Net- work), indicating the good design of the acquisi- tion experiment, the choice of low-noise sites and minimum failure of the instrumentation (fig. 4b). iii) The RMS mean value around 0.08 s indi- cates an overall high quality of phase data and of the velocity model adopted (fig. 4c). This is also shown by the final locations depicted on the plan view map and along the vertical sections. The number of events versus hour of the day is almost constant. The cumulative number of events (fig. 4d) shows a constant increase between October 2000 and February 2001 while between March and April 2001 this trend shows an evident tilt-up (fig. 4d,e), corresponding to the period when the network was densified around the main clusters of seismicity. We also used blasts produced by some quarries to test the goodness of earthquake location (A in fig. 5). For the best located events, we computed focal mechanisms by using those events which has at least 12 P-wave polarities (fig. 6). Here we present the solutions for 150 events comput- ed by using FPFIT code (Reasemberg and Op- penheimer, 1985) which have quality factors equal to A. 12 30′ 43 30′ 0 5 10 km Città di Castello Perugia Umbertide Gubbio Fig. 6. 150 focal solutions for the best located earthquakes. 1323 A microseismic study in a low seismicity area of Italy: the Città di Castello 2000-2001 experiment 4. Data interpretation Figure 5 shows the epicentral map of the lo- cated earthquakes with horizontal and vertical er- rors less than 1 km and an azimuthal gap less than 240° (80% of the seismicity has a gap less than 180°). Earthquakes concentrate on a NW-trend- ing structure elongated for 20-30 km to the north of Gubbio and toward the south. Background seismicity in the area close to CDC town is scarse, leaving the hypothesized seismic gap al- most silent (A in fig. 5). This behaviour is neither referable to the network geometry nor to the de- tection method which was applied to the data us- ing the same trigger parameters. This is granted by the presence of some microearthquakes in the north-western part of the network (see A in fig. 5) which were localized also using data from the RSNC network. From the vertical sections across the fault sys- tem (fig. 5), we observe an east-dipping cloud of hypocenters that, from around 2 km depth be- neath the western side of the Tiber Valley, reach about 12-15 km of depth to the east, cutting the upper crust beneath the Apenninic belt. We inter- pret this seismicity to occur on a gently east-dip- ping fault, possibly related to the Alto Tiberina Fault previously recognized comparing available seismological data with seismic reflection pro- files and geological investigation (Barchi et al., 1998; Boncio et al., 2002; Collettini et al., 2002). In the central part of the study area, the cluster of hypocenters located to the north of Gubbio is confined at depth by the east-dipping fault inter- secting the west-dipping seismicity at a depth of about 6 km. A few earthquakes occur at mid- crustal depth and down to 50 km in the upper- most mantle. The occurrence of this sub-crustal seismicity could be related to the flexured sub- ducting Adriatic lithosphere as previously ob- served by Selvaggi and Amato (1992). The focal solutions for a very high number of events (> 80%) show a general NE-trending direction of extension, with NW-trending both SW and NE dipping normal fault planes, con- sistent with the regional stress observed by large earthquakes and borehole breakouts (Mar- iucci et al., 1999; Chiaraluce et al., 2003). Some inverse solutions are found for the deep- est earthquakes. 5. Conclusions The microseismicity distribution, revealed by the high detection capability of the local ar- ray, let us define two different zones: one in the SE, where low magnitude background seismic- ity is present and well organized, and the other to the NW characterized by the lack of micro- seismicity. Consistently with larger magnitude events, background seismicity occurs on small NW-trending normal fault segments governed by the regional NE trending extension. In the southern and central portions (from Gubbio to CDC), the microseismicity defines a seismic volume elongated for 50 km and from 0 to 15 km in depth, confined on the hanging-wall of the low angle east-dipping normal fault (Al- to Tiberina Fault). This latter fault seems to play a dominant role in controlling the defor- mation style of the area, cutting the upper crust almost completely. In the northern part, background seismicity is nearly absent defining a ≈ 20 km long area north- ward limited by an area (Sansepolcro) where three small seismic sequences (ML < 4.5) oc- curred in the past 15 years (Braun et al., 2003). These sequences are referable to the northern segment of the ATF fault, or to the antithetic SW dipping normal fault. In the silent area, no earth- quakes occur on the east-dipping low angle nor- mal fault hypothesized by active seismic data. The presence of historical earthquakes (1458, MCS = IX; 1789, MCS = VIII-IX; 1917, MCS = IX) suggests that this portion of the fault could be presently locked, supporting the hypothesis of a seismic gap. Acknowledgements This experiment was partially founded by Gruppo Nazionale per la Difesa dai Terremoti within the project: «Development and Compar- ison among Methodologies for the Evaluation of Seismic Hazard in Seismogenic Areas: Ap- plication to the Central and Southern Apen- nines», coordinated by M. Cocco. We thank all the people who contributed to the field work and in particular the technical staff of the INGV Mobile Network Laboratory. 1324 Davide Piccinini et al. Thanks to M. 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