POSTSEISMIC DEFORMATION on THE 2009 L'Aquila EARTHQUAKE (M6.3, CENTRAL ITALY) SURFACE RUPTURE CAPTURED IN 4D USING TERRESTRIAL LASER SCANNING

We report the results of an innovative survey of the normal surface rupture formed during the 6th April 2009 L’Aquila earthquake (M6.3, Central Italy) using precise 3D terrestrial laser scanning (TLS) technology. Our measurements began 8 days after the main shock, with repeated measurements at 11, 35, 39, 43, 48 and 124 days. Using surface modeling 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 (4 cm). We identify and present surface motion observations of two distinct styles. After 124 days, we recorded a total of 13.4 mm vertical slip across the rupture, accompanied by a further 14.3 mm of continuous vertical subsidence in the form of a 30 m wide hangingwall syncline developing 7 m from the surface rupture. The localised proximity of these observations to the surface rupture suggests that shallow afterslip dominates the deformation we have measured. The total vertical offset recorded between 8 and 124 days after the earthquake is 27.7 mm (rupture slip and syncline subsidence combined). However, 52 % of the measured postseismic deformation within tens of metres of the surface rupture occurred via syncline growth, which would have been difficult to identify without the precision and spatial resolution provided by TLS. In an attempt to calculate the magnitude of afterslip that occurred prior to initiation of our measurements, on days 0 - 8 after the earthquake, and how this was partitioned between the rupture and the syncline, we back extrapolated the surface motions measured between days 8 - 124 using a variety of published creep functions. We conclude that centimetric to decimetric scale postseismic deformation occurred on days 0 - 8, an appreciable portion of which we estimate occurred via syncline growth that would be difficult to observe without the use of TLS. This raises the question as to how much slip has gone unobserved for other historical earthquakes that were not measured using this new technology. Both surface rupture afterslip and hangingwall synclines on normal faults have been observed in palaeoseismic studies of other earthquakes, but this is the first time the incremental growth of both features has been observed in 4D.