Afterslip on the L’Aquila earthquake (M6.3) surface rupture captured in 4D using a Terrestrial laser scanner (TLS)

Normal faulting earthquakes produce coseismic vertical motions that are expected to amplify during the days and weeks after the mainshock. The amplitude, wavelength and timescales associated with such postseismic deformation can help constrain the seismic cycle and reveal whether the isostatic response to an earthquake is driven by fluid and poro-elastic effects, visco-elastic creep in the mantle or afterslip within a velocity strengthening zone in the shallow crust, or a combination of the above.
Here, we report the results of an innovative survey of the surface rupture formed during the 6th April LAquila earthquake (M6.3) using precise 3D terrestrial laser scan (TLS) technology. Using surface modelling techniques, we have produced a 4D afterslip survey across a 3 x 65 m area that has detected millimetre-scale movements on and adjacent to the rupture at exceptionally high horizontal spatial resolution (4cm). We identify and present surface motion observations of two distinct styles. On the surface rupture, we recorded surface motions totalling 14.6 mm slip and this was accompanied by 26.2 mm of continuous vertical subsidence in the form of a 30m wide hangingwall syncline. We model the surface motions from both zones using power law and Nason & Weertman approximations. Both surface rupture afterslip and hangingwall synclines on normal faults have been observed in previous studies but this is the first time the incremental growth of both features has been observed in 4D.
We conclude that surface motions measured from subsidence in the syncline to be indicative of the general postseismic deformation created by initially rapid local afterslip within the shallow portion of the fault zone. The surface motions measured from offsets across the surface rupture is a stick-slip low initial rate response that is retarded with respect to the syncline and thus does not represent the surface motion of greatest magnitude for this event. This is the first time such a discrepancy has been directly recorded and its recognition highlights that traditional survey techniques collecting sparsely distributed surface motion data are insufficient to characterise the true magnitude of surface motions related to afterslip. Empirical datasets of earthquake magnitude, rupture length and surface offset, such as those published in Wells & Coppersmith will have underestimated maximum surface offset, particularly for low magnitude events.
To capture the diversity in styles of afterslip on the hangingwall surface, pertaining to heterogeneous mechanical properties and slip styles within shallow fault zones, as well as identifying the maximum magnitude of afterslip we highlight the need for high-precision and high density monitoring over extended survey areas.