The rupture history of the 2009 L’Aquila earthquake by non-linear joint inversion of strong motion and GPS data Antonella Cirella, Alessio Piatanesi, Massimo Cocco, Elisa Tinti, Laura Scognamiglio, Alberto Michelini, Anthony Lomax Precaria assegno di ricerca- scadenza 31/12/2009 Precaria art.23 - scadenza 31/07/2010 Precaria art.23 - scadenza 30/11/2009 INGV Convegno Annuale dei Progetti Sismologici, Roma, 19-21 Ottobre 2009

Goals: The 2009 L’Aquila earthquake (Mw 6.3) occurred in the Central Apennines (Italy) on April 6th at the 01:32 UTC and caused nearly 300 casualties and heavy damages in the L’Aquila town and in several villages nearby. The main shock ruptured a normal fault striking along the Apennine axis and dipping at nearly 50° to the SW. Most of the aftershocks are also associated with normal faulting, which is consistent with the present-day tectonic setting of this sector of the Apennines. The 2009 L’Aquila earthquake provided the collection of an excellent data set of seismograms and geodetic data for a normal faulting event. In this study, we investigate the rupture process of the L’Aquila main shock by using a nonlinear joint inversion of strong motion and GPS data. The goal is to constrain the rupture history to better understand the mechanics of the causative fault as well as the observed ground shaking. Convegno Annuale dei Progetti Sismologici, Roma, 19-21 Ottobre 2009

Kinematic Inversion Technique: Data & Fault Parameterization 1) joint inversion of strong motion and GPS data; 2) finite fault is divided into sub-faults; Inverted Parameters: Peak Slip Velocity; Rise Time; Rupture Velocity; Rake. 3) kinematic parameters are allowed to vary within a sub-fault; 4) several analytical slip velocity source time functions (STFs) are implemented. 5) different crustal models can be adopted to compute Green's functions at different receivers. Convegno Annuale dei Progetti Sismologici, Roma, 19-21 Ottobre 2009

C(m) + = Strong motion L1+L2 norm GPS L2 norm Kinematic Inversion Technique : Stage I: Building Model Ensemble–HB Simulated Annealing random model m0 START loop over parameters N (Vr,rise time,…) Loop over model values M Forward Modeling: DWFE Method - Compsyn (complete response 1D vertically varying Earth Structure) + = Strong motion L1+L2 norm To quantify the misfit… GPS L2 norm C(m) To quantify the misfit we compute the cost function that give us the goodness of the explored model! Convegno Annuale dei Progetti Sismologici, Roma, 19-21 Ottobre 2009

Kinematic Inversion Technique : Stage II: Appraisal of the Ensemble Output of kinematic inversion: Cost Function iterations Ω Rupture Models m & Cost Function C(m) Model Ensemble Ω = Average Model: Standard Deviation: Best Model Convegno Annuale dei Progetti Sismologici, Roma, 19-21 Ottobre 2009

2009 L’Aquila (Central Italy) Earthquake, Mw=6.3 Kinematic Inversion: 2009 L’Aquila (Central Italy) Earthquake, Mw=6.3 Data: 2009 April 6th 1:32 UTC 14 accelerograms (strong motion records from the RAN and the MedNet station AQU); 17 GPS stations (INGV-Ring & ASI network) ;   70 km; frequency-band: (0.02÷0.5) Hz; 60 sec (body & surface waves); Convegno Annuale dei Progetti Sismologici, Roma, 19-21 Ottobre 2009

2009 L’Aquila (Central Italy) Earthquake, Mw=6.3 Kinematic Inversion: 2009 L’Aquila (Central Italy) Earthquake, Mw=6.3 Crustal Structure: 1D velocity model resulting from the analysis of receiver functions at AQU & AQG sites (I. Bianchi, pers comm, 2009); a regional 1D velocity model obtained by Bagh et al. (2007) inverting P-wave arrival times of digital waveforms (INGV networks); shallow low velocity layer (vp  4km/s) consistent with surface wave dispersion analysis (Malagnini & Hermann, pers comm, 2009). Convegno Annuale dei Progetti Sismologici, Roma, 19-21 Ottobre 2009

2009 L’Aquila (Central Italy) Earthquake, Mw=6.3 Kinematic Inversion: 2009 L’Aquila (Central Italy) Earthquake, Mw=6.3 Fault Geometry: hypocenter: 42.35°N, 13.38°E, 9.5km depth (Chiarabba et al., 2009); strike: N133°E; dip: 54° to SW; The proposed fault geometry agrees with the InSAR data and the aftershock pattern. It is also consistent with both the hypocenter location and the induced surface breakages. all kinematic parameters are inverted simultaneously (0-2.5) m/s psv; (1-2)s ; (1.8-2.8)km/s vr; (250-290)° rake angle. Fault Parametrization: W=17.5km; L= 28km; =3.5km; Convegno Annuale dei Progetti Sismologici, Roma, 19-21 Ottobre 2009

2009 L’Aquila (Central Italy) Earthquake, Mw=6.3 Kinematic Inversion: 2009 L’Aquila (Central Italy) Earthquake, Mw=6.3 Rupture Process - Inversion Results Mo = 3.5  1018 Nm Convegno Annuale dei Progetti Sismologici, Roma, 19-21 Ottobre 2009

Data Fit - Inversion Results

2009 L’Aquila (Central Italy) Earthquake, Mw=6.3 Local Rupture Velocity Vr  2.8 - 3.0 km/s Vr  1.8 - 2.2 km/s Convegno Annuale dei Progetti Sismologici, Roma, 19-21 Ottobre 2009

2009 L’Aquila (Central Italy) Earthquake, Mw=6.3 Rupture Velocity & Crustal Structure I. Bianchi, personal comm., 2009 1.6 km/s 2.7 km/s 4 km/s 3.2 km/s Convegno Annuale dei Progetti Sismologici, Roma, 19-21 Ottobre 2009

2009 L’Aquila (Central Italy) Earthquake, Mw=6.3 Rupture Process & on-fault Seismicity Pattern Convegno Annuale dei Progetti Sismologici, Roma, 19-21 Ottobre 2009

Conclusions We image the rupture history of the 2009 L’Aquila (Central Italy) earthquake using a nonlinear joint inversion of strong motion and GPS data. The inferred slip distribution is heterogeneous and characterized by a small, shallow slip patch located up-dip from the hypocenter (9.5 km depth) and a large, deeper patch located southeastward. The rupture velocity is larger in the up-dip than in the along-strike direction. This difference can be partially accounted by the crustal structure, which is characterized by a high velocity layer above the hypocenter and a lower velocity below. The imaged slip distribution correlates well with the on-fault aftershock pattern as well as with mapped surface breakages. Cirella, A., A.Piatanesi, M.Cocco, E. Tinti, L. Scognamiglio, A. Michelini, A. Lomax and E.Boschi (2009), Rupture history of the 2009 L'Aquila (Italy) earthquake from non-linear joint inversion of strong motion and GPS data, Geophys. Res. Lett., 36, L19304, doi:10.1029/2009GL039795

Un ringraziamento speciale ai Vigili del Fuoco, ai volontari della Protezione Civile ed ai lavoratori precari dell’INGV che per tutta la durata dello stato di emergenza hanno continuato e continuano a garantire con il massimo impegno le attività tecnico/scientifiche, specie quelle di monitoraggio. Agli aquilani, per tutto quello che ancora c’è da fare…

References Anzidei, M., Boschi, E., Cannelli, V., Devoti, R., Esposito, A., Galvani, A., Melini, D., Pietrantonio, G., Riguzzi, F., Sepe, V., and E. Serpelloni (2009), Coseismic deformation of the destructive April 6, 2009 L'Aquila earthquake (central Italy) from GPS data, Geophys. Res. Lett., doi: 10.1029/2009GL039145. Atzori, S., Hunstad, I., Chini, M., Salvi, S., Tolomei, C., Bignami, C., Stramondo, S., Trasatti, E. Antonioli, A. and E. Boschi (2009), Finite fault inversion of DInSAR coseismic displacement of the 2009 L’Aquila earthquake (Central Italy), Geophys. Res. Lett., doi: 10.1029/2009GL039293. Bagh, S., L. Chiaraluce, P. De Gori, M. Moretti, A. Govoni, C. Chiarabba, P. Di Bartolomeo, M. Romanelli (2007), Background seismicity in the Central Apennines of Italy: The Abruzzo region case study, Tectonophysics, 444, 80-92. Chiarabba, C., A. Amato, M. Anselmi, P. Baccheschi, I. Bianchi, M. Cattaneo, G. Cecere, L. Chiaraluce, M. G. Ciaccio, P. De Gori, G. De Luca, M. Di Bona, R. Di Stefano, L. Faenza, A. Govoni, L. Improta, F. P. Lucente, A. Marchetti, L. Margheriti, F. Mele, A. Michelini, G. Monachesi, M. Moretti, M. Pastori, N. Piana Agostinetti, D. Piccinini, P. Roselli, D. Seccia, and L. Valoroso (2009), The 2009 L'Aquila (central Italy) MW6.3 earthquake: Main shock and aftershocks, Geophys. Res. Lett., 36, L18308, doi:10.1029/2009GL039627 Cirella, A., A.Piatanesi, M.Cocco, E. Tinti, L. Scognamiglio, A. Michelini, A. Lomax and E.Boschi (2009), Rupture history of the 2009 L'Aquila (Italy) earthquake from non-linear joint inversion of strong motion and GPS data, Geophys. Res. Lett., 36, L19304, doi:10.1029/2009GL039795 EMERGEO WORKING GROUP (2009), Evidence for surface rupture associated with the Mw 6.3 L'Aquila earthquake sequence of April 2009 (central Italy), submitted to Terranova. Piatanesi, A., A. Cirella, P. Spudich, and M. Cocco (2007), A global search inversion for earthquake kinematic rupture history: Application to the 2000 western Tottori, Japan earthquake, J. Geophys. Res., 112, B07314, doi:10.1029/2006JB004821.

The strike direction agrees with the InSAR data; The selected dip angle is consistent with both the hypocenter location and the surface breakages observed near Paganica. The proposed fault geometry: - agrees with the relocated aftershocks, availed of manually picked INGV bulletin arrival time data; - lie within the range of values inferred from moment tensor solutions. Convegno Annuale dei Progetti Sismologici, Roma, 19-21 Ottobre 2009

2009 L’Aquila (Central Italy) Earthquake, Mw=6.3 Rupture Directivy & Observed PGV 2.5491 cm/s 11.3967cm/s 2.7685 cm/s 6.6854 cm/s Convegno Annuale dei Progetti Sismologici, Roma, 19-21 Ottobre 2009

http://earthquake. rm. ingv. it/shakemap/shake/2206496920/intensity http://earthquake.rm.ingv.it/shakemap/shake/2206496920/intensity.html Seismic Stations ANT: Agency: RAN Lat: 42.4182 Lon: 13.0786 Distance: 21.7 km Station Comp Max Vel (cm/s) Max Acc (%g) HNE 1.7646 2.0102 HNZ 1.1741 1.1721 HNN 2.5491 2.6341 AQU: Agency: IV Lat: 42.3539 Lon: 13.4019 Distance: 0.0 km HNE 25.6389 32.8408 HNZ 32.2634 41.7600 HNN 38.5778 39.5897 CLN: Agency: RAN Lat: 42.0852 Lon: 13.5207 Distance: 22.3 km HNE 4.7703 8.0816 HNZ 7.0866 4.5887 HNN 6.6854 8.9818 FMG: Agency: RAN Lat: 42.2680 Lon: 13.1172 Distance: 19.0 km HNE 2.7685 2.3882 HNZ 1.2667 1.9756 HNN 1.6637 2.6862 GSA: Agency: RAN Lat: 42.4207 Lon: 13.5194 Distance: 11.2 km HNE 11.3967 14.8838 HNZ 4.2237 10.9188 HNN 7.9026 14.5186

2009 L’Aquila (Central Italy) Earthquake, Mw=6.3 Total Slip & Crustal Structure Convegno Annuale dei Progetti Sismologici, Roma, 19-21 Ottobre 2009

Auxiliary Material 1 Strong motion L1+L2 norm GPS L2 norm Spudich & Miller, 1990 GPS L2 norm Hudnut et al., 1996

Auxiliary Material 2 - Earthquake locations In our study we have adopted the INGV revised main shock hypocenter location (Chiarabba et al, 2009). We note, however, that the horizontal and vertical errors (0.1 and 0.2 km, respectively) published on the web page and provided by the Hypoellipse programme likely underestimate the true location uncertainties. To appraise the solution, we have applied the global search, non-linear location algorithm NonLinLoc (Lomax, 2005; Lomax et al. 2001; Lomax et al. 2000) to 30 manually picked phases. The resulting axes of the dispersion ellipsoid feature lengths of 0.4, 0.45 and 0.83 km and a root mean square of the arrival time residuals of 0.071 s. The hypocenter location used in this study was found to lie within the probability density function scatter values. In addition, in this study we have used the same non-linear inversion algorithm to locate early aftershocks and to select those situated near the main shock fault plane. References Lomax, A., J. Virieux, P. Volant and C. Berge, 2000. Probabilistic earthquake location in 3D and layered models: Introduction of a Metropolis-Gibbs method and comparison with linear locations, in Advances in Seismic Event Location Thurber, C.H., and N. Rabinowitz (eds.), Kluwer, Amsterdam, 101-134.. Lomax, A., A. Zollo, P. Capuano, and J. Virieux, 2001. Precise, absoute earthquake location under Somma-Vesuvius volcano using a new 3D velocity model, Gephys. J. Int., 146, 313-331. Lomax, A. (2005). A reanalysis of the hypocentral location and related observations for the great 1906 California earthquake, Bull. Seismol. Soc. Am., 95, 861–877.

Auxiliary Material 3 - Synthetic Test Synthetic data are generated using a target rupture model obtained by assumig a regularized Yoffe function with Tacc (time to peak slip velocity) equal to 0.225 sec. Slip is concentrated only on one main asperity, characterized by a peak slip velocity of 1.5 m/s and a rise time of 2.5 s. The rake angle is fixed equal to 270°. The rupture front propagates at 2.2 km/s, except in the portion of the fault located between 6 km SE from the nucleation and the right edge of the fault plane, where it accelerates to nearly 2.8 km/s. We invert simultaneously the kinematic parameters (peak slip velocity, rise time and rupture time) at nodal points equally spaced along strike and dip every 3.5 km. We compute synthetic ground velocities in the frequency band 0.02 and 0.5 Hz and horizontal and vertical components of static displacement and we use these as our target dataset. During the inversion, the peak slip velocity is allowed to vary between 0 and 2.5 m/s with 0.25 m/s interval; the rise time between 1.0 and 3.0 sec at 0.25 sec step increment and the rupture time of each grid node is bounded by a rupture velocity ranging between 2 and 3 km/s. The rake angle is kept fixed. We apply a two stages nonlinear global inversion technique [see Piatanesi et al., 2007]. The synthetic test proves that the azimuthal coverage of the selected stations is good enough to obtain reliable results.

Auxiliary Material 3: Synthetic Test

Auxiliary Material 3: Synthetic Test

Regularized Yoffe Tacc=0.2sec Tacc must be much shorter than the rise time. In this inversion attempt we have chosen a value of the Tacc parameter (0.225 sec) that is consistent with the relatively short rise time (ranging between 1 and 2 sec) expected for a moderate magnitude earthquake and it is close to previous applications of the Yoffe function (Cirella et al., 2008). We have verified that, in the frequency band used in this study (0.02-0.5 Hz), changing the adopted value (between 0.225 and 0.4 sec) does not affect the spatial distribution of slip and rupture times and the effect on the rise time is very modest. The choice of Tacc influences the inferred peak slip velocity value (see equation 7 in that paper). For the L’Aquila earthquake the inversion of available data with Tacc equal to 0.225 or 0.4 sec yields a decrease of maximum peak slip velocity of nearly 12 %. Box-car Cosine Regularized Yoffe Tacc=0.2sec Regularized Yoffe Tacc=0.4sec Tacc

2009 L’Aquila (Central Italy) Earthquake, Mw=6.3 Rupture Process & on-fault Seismicity Pattern Convegno Annuale dei Progetti Sismologici, Roma, 19-21 Ottobre 2009