Slip distributions on active normal faults measured from Terrestrial Laser Scan (TLS) data and field mapping of geomorphic offsets: An example from L’Aquila, Italy, and implications for modeling seismic moment

Surface slip distributions for an active normal fault in Italy have been measured using terrestrial laser scanning (TLS), concentrating on offsets developed since 15 3 ka and for 2 palaeoearthquake ruptures, in order to assess the impact of spatial changes in fault orientation and kinematics on sub-surface slip distributions that control seismic moment release. The southeastern half of the surface trace of the Campo Felice active normal fault near the city of LAquila, central Italy, was scanned with TLS to define the vertical and horizontal offsets of geomorphic slopes that formed during the last glacial maximum (15 3 ka) from the center of the fault to its southeastern tip. Field measurements were made to define the strike and dip of the fault plane and plunge and plunge direction of the slip vector from striations on slickensides. Throw measurements from 250 TLS-derived scarp profiles were analyzed using the crossint cross section interpretation program developed by the authors specifically for this study. Field data of fault kinematics from 43 sites were combined with the TLS-derived throw measurements using a modification of the Kostrov equations to calculate the magnitude and directions of the horizontal principle strain-rates. The studied 5 km long portion of the fault has an overall strike of 140, but has a prominent bend where the strike is 100-140, where the fault has linked across a former left-stepping relay-zone which had an along strike length of ~600 m and across strike width of ~300 m. Throw-rates defined by TLS-derived profiles across a 15 3 ka bedrock fault scarp decrease linearly from 0.95 0.025 mm/yr at the fault center through 0.5 0.025 mm/yr to zero at the fault tip, except in the position of the prominent bend where throws rates increase by 0.15 0.025 mm/yr over a distance of ~1 km. The vertical co-seismic offsets averaged between two palaeoearthquake ruptures that manifest themselves as fresh stripes of rock at the base of the bedrock scarp, also increase across the prominent bend from 0.66 0.05 m to 1 0.05 m. Both the dip of the fault (~50), and slip-vector azimuth (205-218) are constant across the prominent bend. These combine to produce a principle strain-rate calculated in 250 x 250 m boxes centered on the fault trace that decreases linearly from ~3.5 ppm/yr to ~1 ppm/yr from the fault center towards its tip; 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 principle horizontal strain-rate, can produce local maxima in throw-rates of ~25%, and these throw-rate maxima can also be seen in slip distributions for palaeoearthquakes. The above has implications for modelling sub-surface slip distributions for earthquake ruptures through inversion of GPS, InSAR and strong motion data using planar fault approximations. For example, on the nearby Paganica fault that ruptured in the Mw 6.3 2009 LAquila Earthquake, where slip anomalies of 20-30% of the total slip are considered significant, yet small-scale changes in fault orientation are not modeled.