Shallow afterslip following the 2003 May 21, Mw= 6.9 Boumerdes earthquake, Algeria

SUMMARY We investigated post-seismic deformation following the 2003 May 21, Mw= 6.9 Boumerdes, Algeria, earthquake using surface displacements from six continuous Global Positioning System sites that operated in the epicentral area for 2.5 yr following the event. We find up to 4 cm of cumulative horizontal displacement during that time period, with a time-dependence well fit by a logarithmic decay. Post-seismic deformation appears to continue at all sites after the 2.5-yr observation period, with rates on the order of 1 cm yr−1 or less. The data is consistent with shallow afterslip (0–5 km) and shows no evidence for afterslip downdip of the coseismic rupture. The data is poorly explained by viscoelastic relaxation in the lower crust or upper mantle, or by poroelastic rebound. The concentration of afterslip adjacent to and updip of the coseismic rupture, at least in the western half of the fault, suggests that afterslip is driven by coseismic stresses. The correlation between the depth of afterslip and that of the sedimentary wedge along the Algerian margin, while coseismic slip occurs in deeper basement rocks, suggests (1) that post-seismic deformation may also involve folding and (2) that spatial variations in frictional properties along the fault correlate with the type of rocks involved.

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