The Mw6.3Mw6.3 Ölfus earthquake at 15:45 UTC on 29 May 2008 in South Iceland: ICEARRAY strong-motion recordings

At 15:45 UTC on 29 May 2008 a Mw6.3Mw6.3 earthquake occurred in the western part of the South Iceland Seismic Zone (SISZ). Preliminary results indicate that the first motion originated approximately 6.5 km east–southeast of the town of Hveragerdi on what aftershocks appear to identify as an almost 10 km long north–south trending fault. However, most aftershocks outlined another almost 20 km long north–south trending fault less than 2 km from the town of Hveragerdi, which happens to be the location of the recently deployed ICEARRAY, the first small-aperture strong-motion array in Iceland. The ICEARRAY produced high-quality recordings on 11 stations over an aperture of ∼1.9km and a ∼50m minimum interstation distance. The recordings are characterized by strong motion of short duration and high intensity, manifested by the geometric mean of horizontal peak ground acceleration ranging between 0.44 and 0.87 g. Moreover, a prominent long-period near-fault velocity pulse is observed both along the strike-normal and strike-parallel directions. Its apparently repetitive behavior may reflect the complex source effects of the two-fault system. The linear response spectra of the ICEARRAY data indicate that the long-period energy of the velocity pulse seen along the strike-normal direction is not present in the strike-parallel direction. However, the long-period spectral energy seen along the strike-parallel direction is most likely caused by permanent tectonic displacement. Along the strike-normal direction, the Eurocode 8 “Type 2” design spectrum combined with a design spectrum for near-fault motion with distinct pulses appears to capture well the overall spectral composition of the ICEARRAY response spectra. We believe that these globally unique ICEARRAY recordings are of importance to the fields of earthquake engineering and engineering seismology.