Subcrustal earthquakes in the plate boundary zone of New Zealand's South Island
Abstract
Sporadic, intermediate-depth earthquakes have been observed for ~40 years in the vicinity of the Alpine Fault, a 460 km-long transpressive fault forming the western boundary of the Southern Alps. The Alpine Fault represents the plate boundary between the Australian and Pacific Plates in New Zealand and links two subduction zones of opposite polarity in the North and South. Several earthquakes at depths of 59-85 km have been recorded by the Southern Alps Microearthquake Borehole Array (SAMBA) since its deployment in November 2008. Due to large numbers of impulsive phase arrivals, focal mechanisms were obtained for these events during routine processing. In 2009 and early 2010, several additional temporary seismometer networks were operating in the central Southern Alps (Alpine Fault Array ALFA, Deep Fault Drilling Project 2010 DFDP10) and the offshore region west of the South Island (Marine Observations of Anisotropy Near Aotearoa MOANA). To gain more insight about the cause and mechanism of these deep events, a comprehensive analysis has been performed incorporating data from all available instruments. Accurate hypocentres of 22 earthquakes (ML<4) and focal mechanisms of at least 14 events have been obtained. The focal mechanisms reveal that reverse faulting predominates at depth in the continental collision zone between the Pacific and Australian Plates. The intermediate-depth events occur below the Moho discontinuity, which has been mapped in detail using wide-angle reflection/refraction data obtained during the South Island Geophysical Transect (SIGHT) project in 1995/96. Although the cause for these subcrustal earthquakes is not yet clear, they have previously been interpreted to result from intra-continental subduction (Reyners 1987), high shear-strain gradients due to depressed geotherms and viscous deformation of mantle lithosphere (Kohler and Eberhart-Phillips 2003). On the basis of the locations and mechanisms obtained using SAMBA, we have argued that these events are caused due to high strain rates resulting from plate bending, but this is the subject of further investigations. Reyners, M., Subcrustal earthquakes in the central South Island, New Zealand, and the root of the Southern Alps, Geology, 15, 1168-1171, 1987. Kohler, M., and D. Eberhart-Phillips, Intermediate-depth earthquakes in a region of continental convergence: South Island, New Zealand, Bulletin of the Seismological Society of America, 93 , 85, 2003.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.T33F2721B
- Keywords:
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- 7230 SEISMOLOGY / Seismicity and tectonics;
- 8108 TECTONOPHYSICS / Continental tectonics: compressional