Shallow subsurface structure of the 2009 April 6Mw 6.3 L’Aquila earthquake surface rupture at Paganica, investigated with ground-penetrating radar

Abstract

The shallow subsurface structure of the 2009 April 6 Mw 6.3 L’Aquila earthquake surface rupture at Paganica has been investigated with ground penetrating radar to study how the surface rupture relates spatially to previous surface displacements during the Holocene and Pleistocene. The discontinuous surface rupture stepped between en-echelon/parallel faults within the overall fault zone that show clear Holocene/Pleistocene offsets in the top 10 m of the subsurface. Some portions of the fault zone that show clear Holocene offsets were not ruptured in 2009, having been bypassed as the rupture stepped across a relay zone onto a fault across strike. The slip vectors, defined by opening directions across surface cracks, indicate dip-slip normal movement, whose azimuth remained constant between 210◦ and 228◦ across the zone where the rupture stepped between faults. We interpret maximum vertical offsets of the base of the Holocene summed across strike to be 4.5 m, which if averaged over 15 kyr, gives a maximum throw-rate of 0.23–0.30 mm yr–1, consistent with throw-rates implied by vertical offsets of a layer whose age we assume to be ∼33 ka. This compares with published values of 0.4 mm yr–1 for a minimum slip rate implied by offsets of Middle Pleistocene tephras, and 0.24 mm yr–1 since 24.8 kyr from palaeoseismology. The Paganica Fault, although clearly an important active structure, is not slipping fast enough to accommodate all of the 3–5 mm yr–1 of extension across this sector of the Apennines; other neighbouring range-bounding active normal faults also have a role to play in the seismic hazard.