Peter McGinty

Plate coupling in the northern South Island and southernmost North Island, New Zealand, as illuminated by earthquake focal mechanisms

Subduction of the Pacific plate in the northern South Island and southernmost North Island 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° 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.

The enigma of the Arthur's Pass, New Zealand, earthquake: 2. The aftershock distribution and its relation to regional and induced stress fields

The aftershock distribution of the 1994 Arthurs 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 σ1 and σ3 principal axes striking at 298° and 28°. Using dislocation theory, we calculate the stress induced by the Arthurs Pass earthquake and its largest aftershock (a strike-slip event) and add this to the regional field. There is a fair correspondence between the hypocenters of aftershocks away from the mainshock fault plane and regions of high induced Coulomb Failure Stress (CFS) on optimally oriented fault planes. However, there are regions of high induced CFS that are devoid of aftershocks. It appears that earthquake slip in this region of oblique (19°) plate convergence is, as observed elsewhere, partitioned into components parallel and perpendicular to the plate margin. Most of the slip is parallel, as occurs on the nearby dextral Alpine fault, the boundary between the Pacific and Australian plates. However, occasional reverse events, such as the Arthurs Pass earthquake, account for at least some of the perpendicular component of slip and the uplift that produced the Southern Alps.

Shallow subduction tectonics in the Raukumara Peninsula, New Zealand, as illuminated by earthquake focal mechanisms

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 117 earthquakes of M L 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 Taxes 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.

The 2000 Thompson Sound earthquake, New Zealand

Abstract The Mw 6.1 Thompson Sound earthquake occurred on 1 November 2000, with an epicentre near the Fiordland, New Zealand, coastline (–45.112°, 166.952°). Aftershocks, recorded on temporary seismographs as well as the National Seismograph Network, define a 12.5 × 12.5 km planar zone, taken as the mainshock fault, with a strike of 175° and dip of 65°W, ranging in depth from c. 12 to 24 km. This is in accord with the mainshock focal mechanism determined by a body‐wave inversion that indicates a rake of c. 59°, that is, mainly thrust motion with a component of left‐lateral strike‐slip. This event follows a series of moderate to large earthquakes in the Doubtful Sound region: Te Anau, 1988, Mw 6.7, depth 60 km; Doubtful Sound, 1989, Mw 6.4, depth 24 km; Secretary Island, 1993, Mw 6.8, depth 22 km. The Secretary Island event was a thrust event near, or on, the subduction interface, with a dip of 30–40°SE, and our interpretation is that both the Doubtful and Thompson Sound events were oblique thrusts (with a left‐lateral component) above the interface and shoreward of the Secretary Island earthquake. The Coulomb failure stress induced by all three large events prior to the Thompson Sound event would have loaded closer to failure faults such as that at Thompson Sound. Together, the four events induced a pattern of Coulomb failure stress on the nearby Alpine Fault that varies in sign with depth. Overall, little can be said about potential triggering or bringing forward/retarding of a large Alpine Fault event. Strong motion recordings for the Thompson Sound event are few, but peak accelerations are in accord with existing attenuation relationships.

Earthquake triggering in the Hawke's Bay, New Zealand, region from 1931 to 1934 as inferred from elastic dislocation and static stress modeling

During the 1930s the Hawkes Bay region of New Zealand experienced four large earthquakes, Napier (MW 7.6) and Hawke Bay (MW 7.3) in 1931, Wairoa (MW 6.9) in 1932, and Pahiatua (MW 7.4) in 1934. We address the question of whether these comprise a triggered sequence of events. There are significant difficulties in dealing with earthquakes that were recorded 70 years ago as fault parameters are difficult to obtain. With the exception of the Pahiatua earthquake, no primary surface fault ruptures were identified, and locations for the other three events may be in error by tens of kilometers. However, geodetic data were collected before and after the Napier and Wairoa earthquakes, and regions of uplift and subsidence from the former have been mapped from low-order leveling data. This information helps to constrain the fault parameters for the first of these events through elastostatic modeling. Results from recent teleseismic body wave modeling have been used to determine fault parameters for the Hawke Bay event. Our analysis of the induced static stresses with the Coulomb failure criterion shows that the Napier earthquake triggered both the Hawke Bay and Wairoa earthquakes but that the Hawke Bay earthquake probably delayed the Wairoa earthquake. We also conclude that these three events did not trigger the Pahiatua earthquake.

The Ormond, New Zealand, earthquake of 1993 August 10: Rupture in the mantle of the subducted Pacific plate

Abstract Data from temporary seismographs installed immediately after the Ml 6.3 Ormond earthquake of 1993 August 10 have been used to determine the nature of faulting which took place during the event. The rupture began at 37 km depth, within the mantle of the subducted Pacific plate, and aftershocks extended from near the base of the subducted crust to c. 20 km into the subducted mantle. Aftershocks in the mantle decayed exceptionally rapidly compared with those in the crust of the subducted plate. This may reflect a hotter, more ductile mantle and/or relatively homogeneous rupture within the mantle during the mainshock. Aftershocks within the mantle show a variety of thrusting mechanisms. Focal mechanisms of aftershocks within the subducted crust indicate that compression along strike dominates over slab pull, and that the down‐dip stress has a similar magnitude to the vertical stress. This suggests that, at least after the Ormond earthquake, the tectonic stress coupled across the plate interface is ra...

The 1990 Lake Tennyson earthquake sequence, Marlborough, New Zealand

Abstract Aftershocks from the 1990 Lake Tennyson earthquake (ML 5.8) recorded at nine temporary portable seismographs have been used to invert travel‐time data simultaneously for both hypocentre and velocity parameters, resulting in a 1‐D velocity model and station terms for the Lake Tennyson region. The distribution of the best relocated aftershocks outlines a main fault lineation in a ENE direction, and several off‐fault clusters. The main fault lineation is 8 km long, with a strike of about 60° and a dip that is nearly vertical. It is located between and subparallel to the Awatere and Fowler Faults, on a previously unknown fault. The mainshock has been relocated in the middle of this lineation zone, which suggests that the fault ruptured bilaterally. The distribution of aftershocks matches that expected from the Coulomb failure criterion, which identifies areas of increased and decreased stress levels due to the occurrence of the mainshock. Focal mechanisms for the mainshock and aftershocks that make u...

The 2001 ML 6.2 Jackson Bay earthquake sequence, South Island, New Zealand

Abstract Following the 2001 December 7 Jackson Bay earthquake (ML 6.2, MW 5.8), a temporary network of five portable seismographs was deployed in the region to record aftershock activity. Data recorded by the temporary network and nearby New Zealand National Seismological Network stations have been used to define a velocity model for the region and station corrections for each recording station. The locations of the best recorded aftershocks and the revised location of the mainshock indicate that the Jackson Bay earthquake sequence occurred 3–10 km to the east of the Alpine Fault, which is vertical in this region. A fault plane solution obtained from body‐wave modelling suggests the mainshock was primarily a reverse event (rake = 103°) centred at c. 4 km depth on a fault striking northeast‐southwest (48°) and probably dipping to the southeast (45°), which is roughly consistent with the Harvard CMT solution for this earthquake. However, on examination of the aftershock locations, such a fault plane is not clear, nor is any other. The aftershocks are located mainly in two clusters near each other at depths between 3 and 8 km and aligned approximately north‐south. Their positions are in accord with induced stress considerations and the mainshock fault plane lying between the clusters. Individual focal mechanisms for 33 aftershocks have a wide range of solutions. As a group, however, their P and T axes are reasonably well aligned and consistent with the background stress regime in the region as determined by direct inversion of P‐wave polarities. The Jackson Bay earthquake was the third thrust earthquake of magnitude >6 to occur just east of the Alpine Fault in a 7 yr period. Consideration of the mechanics of this earthquake, and the previous two, suggests that the regional stress is at a high level, in accord with the long elapsed time since a large Alpine Fault event. Although the area is small, the Jackson Bay mainshock induced a mainly positive change in Coulomb Failure Stress (CFS) on the closest section of the Alpine Fault, up to c. 0.7 MPa (7 bars).

Slip distribution of the Lake Tennyson earthquake, New Zealand, as inferred from static stress changes and off fault aftershocks

The fault plane of an earthquake can be estimated in many different ways. One is to examine the distribution of a well located aftershock sequence to see if a fault plane is evident. Here we present a new method where we take an earthquake aftershock sequence with distinguishable off fault clusters and vary the slip distribution to achieve a good correlation between regions of increased Coulomb failure stress (CFS) and aftershock occurrence. We find with the Lake Tennyson earthquake that simply taking an area outlined by aftershocks to estimate a fault plane may not be valid. If we do this, patterns of increased CFS have negative correlation with off-fault aftershock occurrence. However if we restrict slip to only deeper regions, and over a smaller area, then the correlation between regions of increased CFS and aftershock off-fault occurrence is good.