California Institute of Technology The Sept, 24, 2013, Mw7.7 Balochistan Earthquake J.P. Avouac, F. Ayoub, S. Wei, J.P. Ampuero, L. Meng, S. Leprince, R. Jolivet, Z. Duputel and D. Helmberger California Institute of Technology

Seismotectonic Context Mww moment tensor P T

Unusual Moment Tensor P T Characteristics of focal mechanisms of shallow (0-30km) earthquakes (Celerier, 2008)

Landsat-8 images: USGS website Pre-earthquake images September, 10, 2013 (14 day before) Post-earthquake images September, 26, 2013 (2 days after)

Image Processing (COSI-Corr) (Leprince et al, 2007)

Measured Surface Displacement Field Window size : 64x64 pixels (960x960 m) Step (GSD of displacement maps): 16 pixels (240 m) 1-s uncertainly: 30 cm - Amplitude of NS component - Horizontal displacement vector field

Strike-parallel Strike-perpendicular

The rupture falls on the Hosbah fault along the front of the Kech Band (Lawrence, Kahn,Dejong, Farah and Yeats, 1981)

Rupture kinematics from backprojection of Teleseismic waveforms Data: Japanese Hi-net seismic network Multitaper-MUSIC array processing technique (Meng et al, 2011) Frequency band: 0.5-2Hz HF source duration: 50s

Rupture kinematics from backprojection of Teleseismic waveforms Backprojection of teleseismic waveforms (Hi-NET), 0.5-2Hz

Finite Source Model Forward modeling: (assumes a pulse-like source) NB: Green’s functions are calculated with a crustal layered structure in the source region, then use ray theory to propagate to the receiver Meyer–Yamada wavelet. The MYM is an orthonormal base. 完备正交基，小波原子 Inversion: A simulated annealing algorithm is used to simultaneously invert for the slip, rise time and rupture velocity (Ji, C., D. J. Wald and D. V. Helmberger, 2002a.b, BSSA)

Finite Source Model

Rupture depth <12km, No shallow slip deficit (Fialko et al, 2005)

GPS velocities relative to India Comparison with interseismic strain PANG PANG- BEDI BEDI GPS velocities relative to India (Szeliga et al, JGR, 2012)

Long Term Tectonics Strike slip motion along the Hosbah fault potentially enhanced by eastward extrusion of easternmost outer Makran prism

Effect on the Makran megathrust and Chaman Fault system 1945 1765 Coulomb Stress Change on Makran Megathrust Coulomb Stress Change on NS strike-slip faults

Conclusion Rupture initiated at the southern tip of the Chaman Fault and propagated along the Hosbah fault within the Makran accretionnary prism Large strike-slip motion (6m on average, along 200km long fault trace, 0-12km depth) on pre-existing thrust fault Strike-slip enhanced due to eastward squeezing of Makran prims No shallow slip deficit Large rupture velocity on misoriented fault Rupture failed to propagate along the Chaman fault (despite most aftershocks there). Effect of shallow dip angle? shallow creep? unloading due to 1935 EQ?

Seismotectonic Context Mww moment tensor P T (Szeliga et al, JGR, 2012)

(Szeliga et al, JGR, 2012)

(Szeliga et al, JGR, 2012)