Aquila destructive earthquake was successfully recorded by closely spaced 10 Hz and 1 Hz recording GPS receivers and strong motion accelerometers located above or close to the 50°dipping activated fault. We retrieved both static and dynamic displacements from very high rate GPS (VHRGPS) recordings by using Precise Point Positioning kinematic analysis. We compared the GPS positions' time series with the closest displacement time series obtained by doubly integrating strong motion data, first, to assess the GPS capability to detect the first seismic arrivals (P waves) and, second, to evaluate the accelerometers' capability to detect coseismic offsets up to ∼45 s after the earthquake occurrence. By comparing seismic and VHRGPS frequency contents, we inferred that GPS sampling rates greater than 2.5 Hz (i.e., 5 or 10 Hz) are required in the near field of moderate-magnitude events to provide "alias-free" solutions of coseismic dynamic displacements. Finally, we assessed the consistency of the dynamic VHRGPS results as a constraint on the kinematic rupture history of the main shock. These results suggested that the high-rate sampling GPS sites in the near field can be as useful as strong motion stations for earthquake source studies.

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S U M M A R YWe investigate shear wave polarization in the Hayward fault zone near Niles Canyon, Fremont, CA. Waveforms of 12 earthquakes recorded by a seven-accelerometer seismic array around the fault are analysed to clarify directional site effects in the fault damage zone. The analysis is performed in the frequency domain through H/V spectral ratios with horizontal components rotated from 0 • to 180 • , and in the time domain using the eigenvectors and eigenvalues of the covariance matrix method employing three component records. The near-fault ground motion tends to be polarized in the horizontal plane. At two on-fault stations where the local strike is N160 • , ground motion polarization is oriented N88 ± 19 • and N83 ± 32 • , respectively. At a third on-fault station, the motion is more complex with horizontal polarization varying in different frequency bands. However, a polarization of N86 ± 7 • , similar to the results at the other two on-fault stations, is found in the frequency band 6-8 Hz. The predominantly high-angle polarization from the fault strike at the Hayward Fault is consistent with similar results at the Parkfield section of the San Andreas Fault and the Val d'Agri area (a Quaternary extensional basin) in Italy. In all these cases, comparisons of the observed polarization directions with models of fracture orientation based on the fault movement indicate that the dominant horizontal polarization is near-orthogonal to the orientation of the expected predominant cracking direction. The results help to develop improved connections between fault mechanics and near-fault ground motion.