![]() ![]() In contrast, in the Fiordland region in the southwest area of the South Island the Australian Plate subducts obliquely beneath the Pacific Plate at the Puysegur Trench. Beneath the North Island and northern-most South Island the Pacific Plate subducts obliquely beneath the Australian Plate at the Hikurangi Trough. New Zealand straddles the boundary of the Pacific and the Australian plates and its active tectonics are dominated by three main features (Fig. Tectonic setting and historical seismicity Aftershock relocations using double-difference tomography and RMT-derived focal mechanisms are used to examine how the aftershocks compare with previous aftershocks in the Canterbury sequence and with mapped faults in the region. Ground motions derived from strong-motion data for the two largest events are compared with the largest earthquakes in the Canterbury aftershock sequence. Coulomb stress analysis and analysis of apparent stresses shows how previous large events in the Canterbury aftershock sequence influenced the Pegasus Bay sequence. Focal mechanisms derived from regional moment tensor (RMT) analysis and kinematic source models derived from strong-motion data are used to characterize the source processes for the largest events. The vast majority of the events occurred in the offshore region several kilometres east of Christchurch, which helped minimize their impact on populated areas.Ī summary of the aftershock sequence is presented here with analysis using a variety of geophysical methods. The Pegasus Bay aftershock sequence began on 2011 December 23 UTC with three events of M w 5.4–5.9 in Pegasus Bay east of Christchurch, New Zealand's second largest city (population ca. The Pegasus Bay aftershock sequence is the most recent aftershock sequence resulting from the 2010 September 3 UTC moment magnitude ( M w) 7.1 Darfield earthquake in the Canterbury region of New Zealand (Gledhill et al. The trend of the aftershocks is similar to mapped reverse faults north of Christchurch, and the high number of reverse-faulting mechanisms suggests that similar reverse-faulting structures are present in the offshore region east of Christchurch.Įarthquake dynamics, Earthquake ground motions, Earthquake source observations, Seismicity and tectonics, New Zealand INTRODUCTION At greater depths, Coulomb stress decreases are predicted at the locations of the two events. Coulomb stress analysis indicates that previous large earthquakes in the Canterbury sequence generate Coulomb stress increases for the two events only at relatively shallow depths (3–5 km). Apparent stress estimates for the two largest events are 1.1 MPa ( M w 5.9) and 0.2 MPa ( M w 5.8), which are compatible with global averages, although lower than other large events in the Canterbury aftershock sequence. Peak ground accelerations ranging up to 0.98 g on the vertical component were recorded during the sequence, and the largest event produced horizontal accelerations of 0.2–0.4 g in the Christchurch central business district. Strong-motion data inversion favours a SE-dipping fault plane for the largest event ( M w 5.9) with a slip patch of 18 km × 15 km and a maximum slip of 0.8 m at 3.5 km depth. Regional moment tensor solutions calculated for the Pegasus Bay sequence indicate that the vast majority (45 of 53 events) are reverse-faulting events with an average P-axis azimuth of 125°. Relocations carried out using double-difference tomography show a well-defined NNE–SSW to NE–SW series of aftershocks with most of the activity occurring at depths >5 km and an average depth of ∼10 km. We present a summary of key aspects of the sequence derived using various geophysical methods. ![]() The Pegasus Bay aftershock sequence began on 2011 December 23 UTC with three events of M w 5.4–5.9 located in the offshore region of Pegasus Bay, east of Christchurch city. ![]() The Pegasus Bay aftershock sequence is the most recent aftershock sequence of the 2010 September 3 UTC moment magnitude ( M w) 7.1 Darfield earthquake in the Canterbury region of New Zealand. ![]()
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