Slip distributions on active normal faults measured from terrestrial laser scanning and field mapping of geomorphic offsets: an example from L’Aquila, Italy.

Surface slip distributions for an active normal fault in central Italy have been measured using terrestrial laser scanning (TLS), in order to assess the impact of changes in fault orientation and kinematics on subsurface slip distributions that control seismic moment release. The southeastern segment of the surface trace of the Campo Felice active normal fault near the city of LAquila was surveyed using TLS to define the vertical and horizontal offsets of geomorphic slopes that formed during the last glacial maximum. Field measurements of fault geometry and kinematics from 43 sites and throw/heave measurements from 250 scarp profiles were analysed using a modification of the Kostrov equations to calculate the magnitude and directions of the horizontal principal strain-rates. The studied segment of the fault is mostly linear, with a prominent bend where the fault has linked across a former left-stepping relay-zone.

Throw-rates defined by offsets across the post-glacial bedrock fault scarp decrease linearly from the fault centre to the tip, except in the location of the prominent bend where throw rates increase over a distance of ~1 km (Figs. 1a & 1d). Vertical coseismic offsets averaged between two palaeoearthquake ruptures that manifest themselves as fresh strips of rock at the base of the bedrock scarp also increase across the prominent bend. The dip of the fault and slip direction is constant across the bend (Figs. 1b & 1c), they combine to produce a principal strain-rate that decreases linearly from the fault centre towards the tip (Fig. 1e); the strain-rate does not increase across the prominent fault bend.

The above shows that changes in fault strike, whilst having no effect on the principal horizontal strain-rate, can produce local maxima in throw-rates and these throw-rate maxima can also be seen in slip distributions for palaeoearthquakes. Detailed geomorphological and structural investigation of active faults is therefore a critical input in order to properly define fault activity for the purpose of accurate seismic hazard assessment.
lity of ductile deformation.