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Antonella Cirella, Alessio Piatanesi, Elisa Tinti, Massimo Cocco

INGV. Ground Motion and Source Process of the 6 th April 2009 L’Aquila, central Italy, Earthquake. Antonella Cirella, Alessio Piatanesi, Elisa Tinti, Massimo Cocco. NERA Project- JRA3 (WP13) : INGV, Roma, 17-18 May 2011. Goals.

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Antonella Cirella, Alessio Piatanesi, Elisa Tinti, Massimo Cocco

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  1. INGV Ground Motion and Source Process of the 6th April 2009 L’Aquila, central Italy, Earthquake Antonella Cirella, Alessio Piatanesi, Elisa Tinti, Massimo Cocco NERA Project- JRA3 (WP13) : INGV, Roma, 17-18 May 2011

  2. Goals Global search kinematic inversion technique of seismological & geodetic data; 2. We retrieve the rupture process of the 2009 April 6th L’Aquila, central Italy, mainshock (Mw 6.1), by using a nonlinear separate and joint inversions of strong motion, GPS, DInSAR data; 3. In order to capture the heterogeneity of the rupture history, we give particular attention to the variability of model parameters and we attempt to constrain the local rupture velocity on the fault plane; 4. The goal is to constrain the mechanics of the causative fault as well as the observed ground motion. NERA Project- JRA3 (WP13) : INGV, Roma, 17-18 May 2011

  3. Kinematic Inversion TechniqueData & Fault Parameterization • finite fault is divided into sub-faults; • Inverted Parameters: • Peak Slip Velocity; • Rise Time; • Rupture Time; • Rake. • joint and separate inversion of strong motion, GPS and DInSAR data; • kinematic parameters are allowed to vary within a sub-fault; • models having a local rupture velocity larger than P-wave velocity are discarded (not acausal rupture propagation); • several analytical slip velocity source time functions (STFs) are implemented; • different crustal models can be adopted • to compute Green's functions at different receivers. NERA Project- JRA3 (WP13) : INGV, Roma, 17-18 May 2011

  4. Kinematic Inversion TechniqueOutput • Average Model: • Standard Deviation: Output of kinematic inversion: Ω Rupture Models m & Cost Function C(m) Model EnsembleΩ = • Best Model NERA Project- JRA3 (WP13) : INGV, Roma, 17-18 May 2011

  5. 2009 L’Aquila (central Italy) Earthquake, Mw=6.1 The 2009 L’Aquila earthquake (Mw 6.1) 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. NERA Project- JRA3 (WP13) : INGV, Roma, 17-18 May 2011

  6. Input 2009 L’Aquila (central Italy) Earthquake, Mw=6.1 Datasets: 2009 April 6th 1:32 UTC • 14 accelerograms (strong motion records from the RAN and the MedNet station AQU); • 36 GPS stations (INGV-Ring, GNSSA, ISPRA, ITALPOS and ASI network, Cheloni & al, 2010); •   70 km; • frequency-band: (0.02÷0.5) Hz; • 60 sec (body & surface waves); NERA Project- JRA3 (WP13) : INGV, Roma, 17-18 May 2011

  7. Input DInSAR • Satellite: Envisat descending. SAR Sensor: C-band, wavelength = 5.6 cm, look angle: 23°; • Each fringe represents a deformation of 2.8 cm in Line of Sight (LOS). • The images have been acquired on April 27, 2008 and April 12, 2009, respectively. • In green the 2625 resampled pixels (size=300 m) selected for the inversion. NERA Project- JRA3 (WP13) : INGV, Roma, 17-18 May 2011

  8. Crustal Structure Input • Receiver function: 1D velocity model resulting from the analysis of receiver functions at AQU & AQG sites (Bianchi & al., 2010). Used to compute synthetics at AQU and AQG. • nnCIA.mod: 1D velocity model resulting from the surface wave dispersion analysis (Herrmann & Malagnini, 2009). Used to compute synthetics at all other stations. NERA Project- JRA3 (WP13) : INGV, Roma, 17-18 May 2011

  9. Fault Geometry • The proposed fault geometry agrees with the DInSAR data and the aftershock pattern. It is also consistent with both the hypocenter location and the induced surface breakages. Fault Parametrization • W=17.5km; L= 28km; =3.5km; • all kinematic parameters are inverted simultaneously • (0-3.5) m/s psv; (0.75-3)s ; (1.4-4.0)km/s vr; (230-310)° rake angle. Input • hypocenter: 42.35°N, 13.38°E, 9.5km depth (Chiarabba et al., 2009); • strike: N133°E; • dip: 54° to SW; NERA Project- JRA3 (WP13) : INGV, Roma, 17-18 May 2011

  10. Inversion Results - Rupture History Output NERA Project- JRA3 (WP13) : INGV, Roma, 17-18 May 2011

  11. Output Inversion Results - Slip Velocity History NERA Project- JRA3 (WP13) : INGV, Roma, 17-18 May 2011

  12. Output Local Rupture Velocity & Rupture Index Mode Pulido & Dalguer (2009) NERA Project- JRA3 (WP13) : INGV, Roma, 17-18 May 2011

  13. Conclusions • We investigate the rupture history of the 2009 L’Aquila (Central Italy) earthquake using a nonlinear inversion of strong motion, GPS and DInSAR data. • Both the separate and joint inversions reveal a complex rupture history and a heterogeneous slip distribution characterized by a shallow slip patch located up-dip from the hypocenter and a large, deeper patch located southeastward. • The rupture history is characterized by two distinct phases: a rupture initiation with a modest moment release lasting nearly 0.5 sec, followed by a sharp increase in slip velocity and rupture speed (4.0 km/s) located 2 km up-dip from the hypocenter and a second stage (starting 2.0 sec after the nucleation) characterized by a slower along strike rupture propagation and the failure of the deep larger slip patch. • The up-dip and along-strike rupture propagations are separated in time and associated with two distinct rupture modes: Mode II and Mode III, respectively. NERA Project- JRA3 (WP13) : INGV, Roma, 17-18 May 2011

  14. Conclusions Cirella, A., A.Piatanesi, E. Tinti, M.Chini and M.Cocco, Source Complexity of the 2009 L’Aquila, Italy, earthquake: Evidence for a Rheological Control on Rupture Process, submitted to Geophysical Journal International. 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 • Our rupture model confirms the evident along strike directivity (Pino&Di Luccio (2009), Akinci&al (2010)) and it also reveals an initial up-dip directivity that lasted for nearly 2sec and likely affected the ground motion observed in the L’Aquila town; • Our results show that the 2009 L’Aquila mainshock featured a very complex rupture history for a moderate Mw 6.1, with strong spatial and temporal heterogeneities suggesting a strong frictional properties’ control of the rupture process. ..for details see.. NERA Project- JRA3 (WP13) : INGV, Roma, 17-18 May 2011

  15. Slip Velocity History-Directivity

  16. Key Issues • To investigate the relationship between the observed ground motion variability & the kinematic source parameters taking into account the strong heterogeneity of the rupture process; • To study the effects of the observed directivity; • To better analyze the effect of frictional properties on the retrieved rupture history; • These aspects are crucial to generate & to interpret shaking scenarios in near source regions. This afternoon.. 13:45 – 14:00 Task 2 “Identification of ground motion dominated by the source” (A. Piatanesi - INGV) NERA Project- JRA3 (WP13) : INGV, Roma, 17-18 May 2011

  17. Thank You

  18. Kinematic Inversion TechniqueStage I: Building-up the Model Ensemble START random model m0 Loop over temperatures Loop over iterations Loop over parameters (Vr,…) Loop over model values Simulated annealing Heat-bath algorithm Forward Modeling: Compsyn Misfit computation end end end To quantify the misfit… end Strong motionL1+L2 norm DInSAR L2 norm GPS L2 norm = C(m) + + Aux NERA Project- JRA3 (WP13) : INGV, Roma, 17-18 May 2011

  19. Aux Inversion Results - DataFit NERA Project- JRA3 (WP13) : INGV, Roma, 17-18 May 2011

  20. Aux Inversion Results - DataFit only DInSAR & only GPS

  21. Aux Inversion Results - DataFit: only SM NERA Project- JRA3 (WP13) : INGV, Roma, 17-18 May 2011

  22. Aux Inversion Results - Snapshots NERA Project- JRA3 (WP13) : INGV, Roma, 17-18 May 2011

  23. 2009 L’Aquila (Central Italy) Earthquake, Mw=6.1 Rupture Process & on-fault Seismicity Pattern NERA Project- JRA3 (WP13) : INGV, Roma, 17-18 May 2011

  24. is the angle between the rupture velocity vector and the fault strike direction NERA Project- JRA3 (WP13) : INGV, Roma, 17-18 May 2011

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