The first month of the 2016 central Italy seismic sequence: fast determination of time domain moment tensors and finite fault model analysis of the ML 5.4 aftershock ANNALS OF GEOPHYSICS, 59, FAST TRACK 5, 2016; DOI: 10.4401/ag-7246 The first month of the 2016 central Italy seismic sequence: fast determination of time domain moment tensors and finite fault model analysis of the ML 5.4 aftershock LAURA SCOGNAMIGLIO*, ELISA TINTI, MATTEO QUINTILIANI Istituto Nazionale di Geofisica e Vulcanologia *laura.scognamiglio@ingv.it Abstract We present the revised Time Domain Moment Tensor (TDMT) catalogue for earthquakes with ML larger than 3.6 of the first month of the ongoing Amatrice seismic sequence (August 24th- September 25th). Most of the retrieved focal mechanisms show NNW–SSE striking normal faults in agreement with the main NE-SW extensional deformation of Central Apennines. We also report a preliminary finite fault model analysis performed on the larger aftershock of this period of the sequence (Mw 5.4) and discuss the obtained results in the framework of aftershocks distribution. I. INTRODUCTION he ML 6.0 Amatrice earthquake, which occurred at 01:36:32 UTC August 24th 2016 is the first mainshock of the Ama- trice seismic sequence. It is the largest earth- quake to strike this portion of central Apenni- nes since the M 6.2, 1639 October 7th Monti della Laga Earthquake archived as an I=IX-X (MCS) and M=6.2, [CPTI15, Rovida et al., 2016]. The earthquake caused 299 fatalities and partially destroyed the towns of Amatrice, Ac- cumoli and several surrounding small towns. The hypocenter is located at 42.70° N, 13.23° E and at depth of 8km, only 1 km far from the Accumoli village and just 9 km from the Ama- trice town [Marchetti et al., this issue]. Almost one hour after the mainshock, an aftershock of ML 5.4 occurred; it is located 12 km NW of the mainshock close to the Norcia town. No sig- nificant foreshocks were recorded. Soon after the mainshock evidence for 5.2 km of surface ruptures, preliminarily interpreted as a direct expression of coseismic fault rupture, was observed along the northwest striking Mt. Vet- tore fault trace by Emergeo Working Group ge- ologists [EMERGEO W. G., this issue] (Figure 1). The average surface offsets found is in the order of 15-20 cm for both heave and throw compo- nents. The kinematic finite fault model, retrieved starting from the TDMT mainshock solution, shows that the Amatrice earthquake ruptured a fault dimension 26 km long and 16 km width [Tinti et al., 2016]. The most relevant features of this model are (i) clear bilateral rupture, (ii) rela- T ANNALS OF GEOPHYSICS, 59, FAST TRACK 5, 2016; DOI: 10.4401/ag-7246 tively fast rupture velocity, (iii) heterogeneity of the slip distribution characterized by two main slip patches and iv) quite different value of the rake on the two patches [Tinti et al., 2016]. The rupture from this normal fault earthquake propagates bilaterally to SE and to N-NW direct- ing the strongest shaking toward the city of Ama- trice, where peak ground accelerations (PGAs) between 19%g and 43%g is recorded, and toward the town of Norcia, where PGAs ranges between 22%g and 37%g [Faenza et al., this issue]. The preliminary aftershocks distribution de- fines a main fault plane SW-dipping with an average dip angle of ~50° in agreement with the computed moment tensor solution. The main fault plane is simpler to the south while it be- comes more complex to the northernmost por- tion where it is evident the activation of a shal- lower and antithetic splay. The aftershocks dis- tribution suggests an along strike fault length of about 35 km [Michele et al., this issue]. Here, we report on source geometries of the, still active, Amatrice seismic sequence relatively to the period of time spanning from August 24thto September 25th. We present the revised moment tensor solutions of all the events with ML > 3.3. Due to the complex aftershocks dis- tribution on the northern part of the seismic sequence, we decided to try to understand the larger aftershock of the sequence with the aim of identifying which is the activated fault plane and the main characteristic of the rup- ture. We believe that understanding the main features of this first period of the Amatrice seismic sequence could contribute to explain the complexity of the seismogenic processes active in the Central Apennines that has been marked by three mainshocks in two months of moment magnitude 6.0, 5.9 and 6.5 respec- tively (http://cnt.rm.ingv.it/en/tdmt). II. SEISMIC SEQUENCE MOMENT TENSOR SOLUTIONS Moment tensor solution presented in this study are computed by following the full-waveform TDMT technique originally proposed by Dre- ger & Helmberger (1993) and implemented in an automatic-way at INGV by Scognamiglio et al. (2009). Starting from a given hypocentral loca- tion, the algorithm inverts local to regional three component broad-band velocity waveforms to estimate moment tensor in a point-source ap- proximation. We adopt the pre-calculated and stored Green’s functions obtained using the CIA (Central Italian Apennines) velocity model [Herrmann et al., 2011] that has been inferred for the central Apennines during the 2009 L’Aquila sequence. Quality and reliability of moment ten- sors are based on the goodness of fit between synthetic and observed waveforms, which is quantified through the variance reduction (VR) parameter, that represents an L2-like norm [Scognamiglio et al., 2010]. For events in the magnitude ML 3.8 and larger range we have found that an appropriate filter is low-pass at 0.05 Hz followed by a high-pass filter of 0.02 Hz. Lower magnitude earthquakes were in- verted in the frequency band of 0.02-0.1Hz. We have revised TDMT solutions for 64 events of the first month of the Amatrice seismic se- quence including all the events with ML larger than 3.2 for which we were able to obtain a well-constrained solution (Figure 1 and Table 1). Comparing definitive to automatic solu- tions, computed immediately after the earth- quakes, we found really smaller adjustments to quick determinations. This highlights the robustness of the methodology adopted and the appropriate choice of the velocity model, that allows us to reach for this sequence a magnitude lower threshold equal to Mw 3.2. Most of these solutions have ‘Aa’ quality flag, that means very good fit between data and synthetics and high double-couple value (http://cnt.rm.ingv.it/en/help#TDMT). Cen- troid depth of all solutions is between 1 and 8 km, in most cases shallower than the INGV re- leased locations [Marchetti et al., this issue]. The 01:36 mainshock moment tensor solution, obtained inverting 50 stations in a distance range is of ~65-260 km, shows normal faulting with ANNALS OF GEOPHYSICS, 59, FAST TRACK 5, 2016; DOI: 10.4401/ag-7246 Figure 1. Map view of TDMT solutions of the first month of the Amatrice 2016 seismic sequence. Beach Balls are col- ored as a function of magnitude that is also reported on top of each mechanism. White stars are mainshock and biggest aftershock location. Red stars are the focal mechanism location. Black dots are the Michele et al. [this issue] relocated af- tershocks. Green triangles are the strong motion stations used for the finite fault analysis. nodal planes striking along the Apenninic di- rection equal to strike 155°/331°, dip 49°/41° and rake −85°/-93°. The estimated scalar seis- mic moment is 1.07 10+18 N·m for a preferred centroid depth of 5 km, corresponding to a moment magnitude of Mw 5.96. The ML 5.4 biggest aftershock, which occurred at 02:33:29 UTC 24 August 2016, also features a normal fault mechanism, the obtained focal parame- ters are strike 135°/327°, dip 47°/43°, and rake –98°/-81°. The solution is calculated using 62 stations in the distance range of 51-130 km. The preferred centroid depth is 5 km, while the seismic moment is 1.33 10+17 N·m resulting in a Mw 5.4. With the exception of the mainshock, this is the only earthquake of the first month of the sequence with Mw larger than 5. The remaining 62 moment tensor solutions shows that NNW–SSE striking normal faults dominate, which is in overall agree- ment with the trends of structures of this sector of Central Apennines. Only in a few cases, a strike-slip kinematic characterizes the source geometry of the obtained MT, as for the August 31, 11:52 and 18:12 (UTC) af- tershocks, with magnitude Mw 3.4 and 3.5 respectively, and for the September 3, 10:19 (UTC) Mw 4.3. These events are all located in ANNALS OF GEOPHYSICS, 59, FAST TRACK 5, 2016; DOI: 10.4401/ag-7246 the northern portion of the area activated by this first month of the sequence, the same area that would have been affected by the other two mainshocks of 26th October, Mw 5.9, and 30th October, Mw 6.5. The complete information and the fit to recorded wave- forms are available on the dedicated web- page (http://cnt.rm.ingv.it/en/tdmt), where automatic good quality solutions of smaller events are also available. Date Origin Time Longitude Latitude Depth Strike 1 Dip 1 Rake 1 Strike 2 Dip 2 Rake 2 Mw 2016-08-24 01:36:32 13.23 42.7 5 155 49 -87 331 41 -93 5.96 2016-08-24 01:56:00 13.28 42.6 2 120 60 -123 353 43 -47 4.34 2016-08-24 02:33:28 13.15 42.79 5 135 47 -98 327 43 -81 5.35 2016-08-24 02:59:35 13.13 42.8 5 332 58 -105 179 35 -67 3.89 2016-08-24 03:08:10 13.25 42.62 5 331 64 -85 139 26 -101 3.69 2016-08-24 03:17:59 13.14 42.76 5 351 62 -104 198 31 -66 3.64 2016-08-24 03:40:10 13.24 42.61 6 329 59 -90 150 31 -89 4.12 2016-08-24 04:06:50 13.12 42.77 3 149 47 -97 340 44 -82 4.40 2016-08-24 04:25:58 13.24 42.64 6 345 59 -87 159 31 -96 3.44 2016-08-24 04:33:09 13.21 42.62 5 325 49 -93 149 41 -87 3.42 2016-08-24 04:38:09 13.22 42.63 6 334 67 -96 169 23 -76 3.39 2016-08-24 04:44:38 13.18 42.73 6 155 54 -71 305 40 -114 3.46 2016-08-24 04:57:37 13.04 42.85 2 105 48 -109 311 45 -71 3.48 2016-08-24 05:02:24 13.29 42.46 7 139 47 -107 343 45 -72 3.35 2016-08-24 05:31:32 13.19 42.66 6 175 59 -69 318 37 -122 3.33 2016-08-24 05:36:19 13.14 42.8 7 338 81 -117 231 29 -19 3.27 2016-08-24 06:54:54 13.19 42.8 6 320 58 -134 201 52 -42 3.21 2016-08-24 07:10:55 13.16 42.78 5 340 70 -71 116 27 -131 3.2 2016-08-24 09:31:43 13.19 42.81 12 53 89 24 323 66 179 3.38 2016-08-24 11:50:30 13.16 42.82 6 321 59 -88 137 31 -94 4.53 2016-08-24 14:02:20 13.24 42.8 3 207 56 -55 336 47 -130 3.77 2016-08-24 17:46:09 13.21 42.66 7 340 57 -89 158 33 -91 4.24 2016-08-24 20:21:36 13.15 42.78 5 317 56 -117 180 43 -56 3.32 2016-08-24 23:22:05 13.21 42.65 6 338 63 -89 156 27 -91 4.00 2016-08-25 03:17:16 13.19 42.75 6 345 61 -105 193 32 -65 4.34 2016-08-25 04:12:11 13.23 42.69 5 180 47 -84 350 44 -97 3.25 2016-08-25 04:51:40 13.33 42.63 2 143 60 -91 325 30 -88 3.76 2016-08-25 12:36:05 13.28 42.6 5 129 55 -115 347 42 -60 4.42 2016-08-25 19:40:44 13.29 42.59 5 119 59 -132 360 50 -41 3.43 2016-08-26 00:04:09 13.28 42.66 3 149 58 -95 338 33 -82 3.56 2016-08-26 04:28:25 13.29 42.6 5 128 53 -106 333 40 -70 4.76 2016-08-26 05:17:05 13.21 42.75 7 125 69 98 283 22 70 3.20 2016-08-26 05:32:52 13.15 42.77 6 342 71 -119 222 34 -35 3.30 2016-08-26 16:05:29 13.16 42.69 6 319 62 -92 143 28 -86 3.47 2016-08-27 01:26:39 13.24 42.84 2 162 54 -79 324 37 -105 3.73 2016-08-27 02:50:59 13.24 42.84 3 158 53 -83 327 38 -99 4.00 2016-08-27 06:20:30 13.31 42.55 3 334 50 -65 118 46 -117 3.23 2016-08-27 10:40:14 13.24 42.85 2 173 65 -60 299 39 -137 3.52 2016-08-28 06:37:19 13.2 42.72 7 326 56 -102 166 35 -73 3.33 ANNALS OF GEOPHYSICS, 59, FAST TRACK 5, 2016; DOI: 10.4401/ag-7246 2016-08-28 13:07:32 13.29 42.6 6 156 61 -100 356 30 -72 3.52 2016-08-28 15:37:38 13.12 42.77 6 342 52 -83 150 39 -99 3.45 2016-08-28 15:55:35 13.23 42.82 5 206 55 -87 22 35 -94 4.19 2016-08-28 16:42:01 13.14 42.82 5 340 51 -86 154 39 -95 3.73 2016-08-29 01:44:25 13.19 42.76 7 129 50 -113 342 45 -66 3.36 2016-08-30 00:35:55 13.14 42.8 6 347 60 -51 108 48 -137 3.27 2016-08-31 11:26:01 13.13 42.83 5 345 56 -80 147 35 -105 3.91 2016-08-31 11:52:31 13.22 42.85 4 99 89 177 189 87 1 3.41 2016-08-31 13:23:04 13.23 42.75 6 161 71 -67 288 29 -140 3.32 2016-08-31 18:12:52 13.26 42.82 7 318 83 -164 225 74 -8 3.48 2016-09-01 03:53:04 13.31 42.62 5 139 57 -88 315 33 -93 3.58 2016-09-01 11:35:57 13.3 42.56 5 290 81 -124 186 35 -17 3.27 2016-09-03 01:34:12 13.13 42.77 4 345 51 -92 169 39 -87 4.22 2016-09-03 10:18:51 13.22 42.86 8 199 88 -7 289 83 -178 4.30 2016-09-07 18:13:26 13.24 42.8 1 134 49 -91 315 41 -89 3.27 2016-09-15 14:40:52 13.13 42.77 2 356 54 -102 196 37 -74 3.67 2016-09-19 23:34:25 13.28 42.67 4 322 61 -52 83 46 -138 3.65 2016-09-20 01:20:53 13.29 42.68 6 323 87 -68 59 23 -173 3.23 2016-09-22 20:03:55 13.19 42.76 5 337 58 -71 123 37 -118 3.42 2016-09-30 19:22:28 13.25 42.9 5 177 57 -67 318 39 -122 3.34 2016-09-30 19:38:37 13.25 42.89 5 182 59 -61 315 41 -129 3.44 2016-10-02 23:47:07 13.23 42.79 2 133 53 -88 309 37 -93 3.21 2016-10-04 12:41:35 13.12 42.85 5 103 57 -136 345 54 -42 3.37 2016-10-08 12:19:03 13.17 42.74 3 334 47 -86 148 43 -94 3.53 2016-10-08 18:11:09 13.19 42.74 2 156 51 -91 338 39 -88 3.91 Table 1. Moment tensor solutions obtained using the TDMT technique. The complete information and the fit to re- corded waveforms are available on the dedicated web-page (http://cnt.rm.ingv.it/en/tdmt). III. ANALYSIS OF THE MW 5.4 AFTERSHOCK FAULT PLANE One hour after the 24th August mainshock, a Mw 5.4 event occurred almost 12 km NW from the mainshock. TDMT procedure reveals a nor- mal faulting moment tensor solution having strike directions diverging from the mainshock fault rupture of ~20° for the west-dipping plane and ~4° for the east-dipping plane. This is the biggest aftershock of the sequence, the only one with magnitude larger than 5. It is located in the northern portion of the region activated by the Amatrice seismic sequence, ~5 km far from Norcia town. To unravel the ambiguity on what fault plane actually ruptured, we perform a series of in- versions using waveform data recorded by 15 strong-motion INGV [Michelini et al., 2016] and RAN (http://ran.protezionecivile.it/ET/index.php) stations, and adopting the TDMT source geome- try (strike and dip) on an overly-large-dimension fault plane centered on the NLL relocated hypocenter: latitude 42.793° N, longitude 13.162° E and depth 6.835 km [Michele et al., this issue]. The epicentral distances of the selected record- ing sites are less than 45 km We use the inver- sion code based on the method of Hartzell and Heaton [1983], and implemented by Dreger et al. [2005] consisting in a non-negative, least- squares inversion method with simultaneous smoothing and damping, the same used to in- fer the source parameters of the mainshock. This approach assumes a constant rupture ve- locity and rise time and the best fitting values have been selected iteratively by performing ANNALS OF GEOPHYSICS, 59, FAST TRACK 5, 2016; DOI: 10.4401/ag-7246 inversions with different values of these pa- rameters and quantitatively measuring the fit based on a variance reduction, as defined above for TDMT solutions [Scognamiglio et al., 2010]. Rake parameter can be heterogeneous on the fault plane or assumed constant all over the fault and chosen iteratively as the rupture velocity. The slip velocity is modeled by imposing a sim- ple box-car source-time function. The Green’s functions are computed on a regular grid sam- pling the focal volume every 1 km horizontally and 1 km vertically and filtered between 0.02 and 0.5 Hz, the same as for the recorded data. The velocity structure is CIA model [Herrmann et al., 2011], the same adopted for TDMT solu- tions. We are aware that the maximum se- lected frequency could prevent us to identify the details of the rupture history for a M 5 earthquake, but it allows us to reduce the im- pact of site effects like those reported for Nor- cia station (NRC) for higher frequencies [Bindi et al., 2011]. We run more than 1000 inversions for both planes by setting rise time and rupture velocity ranging between 0.3 ÷ 1 s and 2.0 ÷ 4 km/s respectively, while rake is allowed to vary in a 30°-long range respect to the TDMT values. The faults are parameterized using sub- faults having a 2 x 2 km2 area. The overly-large modeled fault planes are 10 km x 10km. Figure 2(a, b) shows the resulting preferred slip mod- els for both the inverted planes and the corre- sponding waveforms fit. The east-dipping plane displays a main slip concentration located ~4 km southeastward from the hypocenter at similar depth. This patch accounts for a maximum slip of about 20 cm. The picked rise time is 0.3 s, the rupture velocity is 2.7 km/s, and the rake is -91°. The total inferred seismic moment is 9.78 1016 Nm corresponding to Mw = 5.29. The west-dipping plane features two slip patches both located 2 km deeper than the hypocenter. The first as- perity is located just below the nucleation and has average slip equal to 16 cm, the second one is located ~ 4 km SE and reports a maximum slip of 22 cm. The retrieved rise time is 0.3 s, while the rupture velocity is 3.7 km/s and the rake -115°. The total inferred seismic moment is 9.53 1016 Nm corresponding to Mw = 5.29. Both the inverted rupture planes prefer the shortest rise time that, for the inverted fre- quencies, means to consider a delta-like slip velocity function. Otherwise, while the preferred rupture velocity for the N327° plane finds a clear maximum within the explored rupture veloci- ties, the high value found for the N135° plane cannot be considered well-constrained due to a very similar variance reduction (~30%) we obtain for rupture velocities between 3.0 and 3.9 km/s (Figure 3a). Although we adopt an overly- large fault plane to image the model parame- ters, significant slip occurs only on an area of 8 x 5 km2. The comparison between recorded and synthetic data retrieved from the two rup- ture models is presented in Figure 2c. The syn- thetic ground velocities generated by the N327° slip model match fairly well the main body wave pulses of the majority of the re- corded seismograms. On the contrary, the rup- ture model obtained for the west-dipping plane poorly fits the main features of real data except for the three farthest stations and NRC. Both models show discrepancies at some sites most likely resulting from 3D variations of ve- locity structure in this area not included in our adopted 1D velocity model [Casarotti et al., this issue]. As an example, stations located in the northern side show a poor fit (e.g. FEMA) and would require a faster velocity structure to allow a better alignment between real and syn- thetic phases. Figure 3b shows the evolution of the VR values for the performed inversions with rise time equal to 0.3s. Among all the performed inver- sions, we have found more than 60% of the so- lutions adopting the east-dipping plane having VR larger than 30% with an overall VRMAX= 35%, while, for the alternative west-dipping plane, only 13% feature VR larger than 30% with an overall VRMAX= 31%. The finite fault inversion result opts for the activation of the strike 327° fault plane. However, the small dif- ANNALS OF GEOPHYSICS, 59, FAST TRACK 5, 2016; DOI: 10.4401/ag-7246 Figure 2. Rupture models imaged by inverting ground velocity time histories for the (a) east dipping plane (N327°) and (b) west dipping plane (N135°) respectively. The black arrows indicate the slip direction (rake angle) for slip larger than 15 cm. (c) Fit to the data: synthetic ground velocity filtered between 0.02 and 0.5 Hz (green and red lines are for the east and west dipping planes respectively) and recorded strong motions (black lines). Numbers in box represent the amplitude range in cm/s. Numbers in percentage represent the Variance Reduction for each station. ference between the resulting VRMAX values do not allow us to exclude a priori the activation of the strike 135° fault. To solve the ambiguity of the kinematic result we decide to analyze the aftershocks distribution and verify the existence of earthquakes align- NRC RQT CSC AMT FEMA MNF LSS SNO SPM GUMA SPO1 TRE TRL TERO ANT 4 km 13 km 14 km 21 km 21 km 30 km 30 km 30 km 34 km 34 km 35 km 35 km 41 km 41 km 42 km yl im [- 1. 2 1. 2] c m /s yl im [- 0. 5 0. 5] c m /s 50.3% 31.4% 43.4% 16.3% -16.1% 36.6% 40.0% 47.5% 15.3% 33.7% 17.8% -8.4% 26.9% 17.3% 14.0% 62.5% 7.9% 41.0% -39.1% 12.9% -44.4% 16.5% -22.6% -1.6% -1.4% -2.1% -12.6% 34.6% 32.3% 22.6% 30 seconds 30 seconds 0. 5 0 .5 1 1 1 1.5 1. 5 1.5 2 2 2 0. 5 0 .5 1 1 1 1.5 1. 5 1.5 2 2 2 0. 5 0 .5 1 1 1 1.5 1. 5 1.5 2 2 2 2.5 2 .5 2 .5 3 327° strike, EAST dipping plane Along strike (km) 0 1 2 3 4 5 6 7 8 9 10 A lo n g d ip ( km ) 0 1 2 3 4 5 6 7 8 9 10 0 5 10 15 20 25 30 0 .5 0 .5 1 1 1 1 1 1. 5 1.5 1.5 1. 5 0 .5 0 .5 1 1 1 1 1 1. 5 1.5 1.5 1. 5 135° strike, WEST dipping plane Along strike (km) 0 1 2 3 4 5 6 7 8 9 10 A lo n g d ip ( km ) 0 1 2 3 4 5 6 7 8 9 10 0 5 10 15 20 25 30 SE NW NW SE (cm) (cm) EAST NORTH VERTICAL EAST NORTH VERTICAL ANNALS OF GEOPHYSICS, 59, FAST TRACK 5, 2016; DOI: 10.4401/ag-7246 Figure 3 (above). Resulting variance reduction for a subset of 336 inversions performed with rise-time equal to 0.3s. Rupture velocity and rake angle vary within the explored range. Green dots are for the strik- ing N327° plane while red dots are for the striking N135°plane. (a) VR is shown as a function of rup- ture velocity; (b) VR is sorted for increasing values. The variability of VR for each rupture velocity (panel a) reflects the variability in rake. Figure 4 (right column). (a) Map view of the re- located aftershocks [Michele et al., this issue] with traces of the active mapped faults [EMERGEO W. G., this issue]. White stars represent the location of the largest events of the studied sequence. Earthquakes are colored as a function of date of occurrence. Yellow and green boxes are the map projection of the inverted fault while the yellow and green lines correspond to the vertical sections reported in panels (b) and (c) showing the earthquakes occurring within 8km from the vertical line. ment on the investigated rupture planes. We plot the first 8 days of the aftershocks sequence relocated by Michele et al. [this issue] along two vertical sections oriented N45°E and N57°E, per- pendicular to the strike of the TDMT planes (Figure 4). Due to the not completely 100% dou- ble couple component of the moment tensor solution, the profiles are rotated 12° from each other. Aftershocks are shown with dots colored as function of date of occurrence. Each section reports earthquakes occurring in the 8 x 15 km2 boxes mapped in Figure 4a and the trace of the inverted fault planes, as well as the mapped main normal fault systems [EMERGEO W. G., this issue]. At first glance, both profiles show a cross-like events distribution that reveals the activation in this area of both synthetic and antithetic ANNALS OF GEOPHYSICS, 59, FAST TRACK 5, 2016; DOI: 10.4401/ag-7246 structures. In the N45°E oriented A-A’ profile, the seismicity does not clearly image a con- tinuous single west-dipping plane, but it is quite diffuse and probably related to the NW termination of the fault activated during the mainshock [Tinti et al., 2016, Michele et al., this issue]. On the contrary, along the B-B’ section, the distribution of the aftershocks along the dip of the inverted N327° rupture plane is ir- regular but in some way more evident. For this plane we also find a quite good geometrical correspondence with the mapped Norcia anti- thetic fault. Both the vertical sections also image a sepa- rated hypocenters cluster below the Mt. Vet- tore fault system. IV. CONCLUSION In this paper we have presented the revised TDMT catalogue for all the events with ML larger than 3.2 belonging to the first month if the 2016 Amatrice (Central Italy) seismic se- quence. The moment tensor solutions immedi- ately pointed up a prevalent normal faulting mechanism for the occurring earthquakes, in agreement with the present-day active stress field in this sector of the Apennines. Only in a few cases, mainly located in the northern por- tion of the area activated by the seismic se- quence, a strike-slip kinematic characterizes the source geometry. Centroid depths are be- tween 1 and 8 km, typically shallower than the INGV released location [Marchetti et al., this issue]. We have found a really good agreement be- tween revised and automatic solutions, com- puted immediately after the earthquakes oc- currence, confirming once again the imple- mented method as a robust and practical tool for real-time determinations of the point-source focal parameters and magnitude. We have analyzed the largest aftershock of the first month of the sequence (Mw 5.4) in terms of finite rupture model with the aim of identi- fying the activated fault plane. We have per- formed more than 1000 inversions to model both the fault plane geometries identified by TDMT solution and we have found a quite sat- isfactory fit by using strong motion data at epicentral distance less than 45 km and fre- quencies up to 0.5 Hz. The finite fault inver- sion results opt for the activation of the strike N327° fault plane. The small difference of vari- ance reduction between the two best solutions does not allows us to exclude a priori the acti- vation of the strike 135° fault. In any case, both the models reveal, as a common feature, a main asperity located south-east from the hypocenter characterized by a maximum slip of ~ 20 cm. To give an additional constraint to the kine- matic result we have decided to analyze the first 8 days of aftershocks distribution and ver- ify the existence of earthquakes alignment on the investigated rupture planes. The preliminary analysis of finite fault model and aftershocks distribution suggests that, for the Mw5.4 aftershock, the most probably acti- vated fault plane is the one striking N327°and dipping 43° toward NE. For this plane we find a quite good geometrical correspondence with the mapped Norcia antithetic fault. REFERENCES [Bindi et al., 2011] Bindi, D., L. Luzi, S. Parolai, D. Di Giacomo, and G. Monachesi (2011). Site effects observed in alluvial basins: the case of Norcia (Central Italy), Bulletin of Earthquake Engineering, 9(6), 1941-1959, doi:10.1007/s1051 8-011-9273-3. [Dreger et al., 2005] Dreger, D. S., L. 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