Trapped seismic fault zone energy recorded by a high-rate GPS station

San Francisco, California (USA)

The ground motion of the April 6th 2009 Mw 6.3 L'Aquila (Central Italy) earthquake, occurred on a SW-dipping normal fault, was successfully recorded by one of the GPS stations installed near L'Aquila before the seismic event and acquiring with a high sampling rate (10 Hz). These data provide valuable ground displacement time histories in the near source. GPS and strong motion data provided reliable reconstruction of the main features around the source. However, a low frequency (about 1 Hz) nearly-harmonic large oscillation (about 40 cm peak-to-peak) detected at this GPS station 5 s after the beginning of the coseismic dynamic deformation, and spanning about 6.5 s, was not possible to be fit by reliable source modeling. 1 Hz nearly-harmonic oscillations are also observed by a co-located seismological station installed one year after the mainshock. Both the GPS and the seismic stations are located on the eastern slope of the Mt. Ocre ridge, along which runs the the Monticchio-Fossa fault, a shallow and steep NE-dipping antithetic fault bordering the Aterno Valley graben. We have checked that the late large-amplitude coseismic displacement can be generated by energy trapped in this fault. We have obtained satisfactory results by using a line dislocation in a structure consisting in two quarter-spaces separated by vertical fault layers. Our best fit corresponds to a model characterized by a vertical 200 m-wide fault zone with impedance contrasts with respect to the adjacent left and right quarter-spaces of about 17% and 40%, respectively.