Abstract. The Raukumara Peninsula affords an excellent opportunity to study the subduction process, as subduction of the buoyant Hikurangi Plateau on the Pacific plate has resulted in exposure of the forearc above the shallow part of the subduction thrust. Here we report on the focal mechanisms of 1 17 earthquakes of ML 2.4-4.9 and shallower than 80 km, recorded during a 5-month deployment of 36 portable seismographs on the peninsula. Mechanisms have been constrained using both first motion polarity data and amplitudes of seismogram envelopes. Downdip tensional strain predominates in the subducted plate, with T axes of events in both the upper and lower planes of the dipping seismic zone generally paralleling the local dip of the zone. Trenchward extensional strain is seen in the uppermost part of the overlying Australian plate, in line with geodetic and geological results. This can be related to extension and gravity sliding of surficial rocks due to uplift of the Raukumara range resulting from underplating of subducted sediment. There is a marked change in earthquake mechanisms along strike in the lower part of the overlying plate and at the plate interface. A concentration of low-angle thrust events at the plate interface in the northeastern half of the peninsula suggests that the plate interface is less coupled there than to the southwest. This along-strike change in plate coupling corresponds closely to a change in the crustal structure of the overlying plate and also to a change in tectonic rotation domain determined paleomagnetically.
Abstract. The aftershock distribution of the 1994 Arthur's Pass earthquake, Mw6.7, is unusual for a reverse faulting event in that it extends 12 km NNW and 30 km SSE of the actual fault plane, which strikes NE-SW. We have used several methods to infer the regional stress field in the region, including geodetic results, earthquake mechanisms, and inversion of P wave polarity data for the stress tensor orientation. The inversion method is new and does not require the focal mechanisms of the events used. It also incorporates the Coulomb failure criterion. All results point to a stress field favoring strike-slip faulting, not thrusting, with near-horizontal cvl and cv3 principal axes striking at 298 ø and 28 ø . Using dislocation theory, we calculate the stress induced
Abstract. Subduction of the Pacific plate in the northern South Island and southernmost NorthIsland of New Zealand is transitional, insofar as the crustal thickness of the Pacific plate increases significantly along strike in the northern South Island. Focal mechanisms of 145 events shallower than 100 km in this region have been determined using both first motion polarity data and amplitudes of seismogram envelopes. The stress regime in the subducted plate appears to be dominated by slab pull. T axes in both the upper and lower planes of the dipping seismic zone generally parallel the local dip of the zone, and the average azimuth of these T axes is rotated some 25 ø clockwise out of the direction of dip of the subducted plate. This can be related to the asymmetrical shape of the subducted slab. In contrast, the stress regime in the overlying plate appears to be dominated by subhorizontal compression. Low-angle thrust events near the plate interface in Cook Strait and the southernmost North Island concentrate in two areas which may mark the updip and downdip edges of a locked region identified from Global Positioning System (GPS) observations. An absence of low-angle thrust events near the plate interface in the northern South Island and the tendency of P axes of events in the subducted plate to become more horizontal suggest that plate coupling there is stronger than in the southernmost North Island. Differential coupling at the plate interface provides a viable mechanism for producing the large tectonic rotations seen in the northern South Island.
Summary In the Fiordland region of the South Island of New Zealand, the subducted Australian Plate steepens from south to north, becoming near‐vertical below 75 km depth between Doubtful and Milford Sounds. This steepening is accompanied by a sharp change in strike of the subduction zone of about 17°. Here we investigate stress and strain in this twisted subduction zone using earthquakes recorded during a three‐month deployment of 24 portable digital seismographs. We determine focal mechanisms of 115 earthquakes of ML 2.5–4.5 using both first motion polarity data and amplitudes of seismogram envelopes. We also invert for the stress tensor orientation for regions within both the subducted and overlying plates, using a method which invokes the Coulomb failure criterion and considers all P wave first motions together, regardless of whether or not they are sufficient to define single event focal mechanisms. Focal mechanisms and stress inversions for earthquakes in the overlying plate indicate normal faulting shallower than 16 km and thrust faulting at greater depths. This stress and strain regime is related to the 17° change in strike of the subduction zone, which leads to an arching up of the subducted plate at the bend. Such arching is consistent with the exposure of lower crustal granulite facies rocks near the bend. Earthquakes in the subducted plate show predominantly normal faulting relative to the plate. Arrival time inversions for 3‐D seismic velocity structure indicate that a zone of high Vp (>8.5 km s−1) in the uppermost mantle of the overlying plate abuts the vertical portion of the subduction zone. Stresses and strains in the subducted plate are consistent with a model in which the sharp change in strike of the subduction zone is a consequence of the obliquely converging subducted plate having to bend around this zone. The steep dip of the subducted plate to the north appears to be a natural consequence of the minimization of membrane stress in the plate arising from the sharp bend along strike. This mechanism for steepening the plate is quite different to that of trench retreat usually used to explain steep subduction.
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