Terry Webb

The enigma of the Arthur's Pass, New Zealand, earthquake: 1. Reconciling a variety of data for an unusual earthquake sequence

The 1994 Arthurs Pass earthquake (MW6.7) is the largest in a recent sequence of earthquakes in the central South Island, New Zealand. No surface rupture was observed, the aftershock distribution was complex, and routine methods of obtaining the faulting orientation of this earthquake proved contradictory. We use a range of data and techniques to obtain our preferred solution, which has a centroid depth of 5 km, M0=1.3 × 1019 N m, and a strike, dip, and rake of 221°, 47°, 112°, respectively. Discrepancies between this solution and the Harvard centroid moment tensor, together with the Global Positioning System (GPS) observations and unusual aftershock distribution, suggest that the rupture may not have occurred on a planar fault. A second, strike slip, subevent on a more northerly striking plane is suggested by these data but neither the body wave modeling nor regional broadband recordings show any complexity or late subevents. We relocate the aftershocks using both one-dimensional and three-dimensional velocity inversions. The depth range of the aftershocks (1–10 km) agrees well with the preferred mainshock centroid depth. The aftershocks near the hypocenter suggest a structure dipping toward the NW, which we interpret to be the mainshock fault plane. This structure and the Harper fault, ∼15 km to the south, appear to have acted as boundaries to the extensive aftershock zone trending NNW-SSE. Most of the ML ≥ 5 aftershocks, including the two largest (ML6.1 and ML5.7), clustered near the Harper fault and have strike slip mechanisms consistent with motion on this fault and its conjugates. Forward modeling of the GPS data suggests that a reverse slip mainshock, combined with strike slip aftershock faulting in the south, is able to match the observed displacements. The occurrence of this earthquake sequence implies that the level of seismic hazard in the central South Island is greater than previous estimates.

The MW 6.2 Cass, New Zealand, earthquake of 24 November 1995: Reverse faulting in a strike‐slip region

Abstract On 24 November 1995 an earthquake of moment magnitude MW 6.2 struck near the small settlement of Cass in the Southern Alps, South Island, New Zealand. Body‐wave modelling using teleseismic arrivals gives an oblique reverse focal mechanism for the mainshock, with the fault plane striking approximately north‐south, and a shallow centroid depth of 3–6 km. Aftershock recordings at the station SNZO near Wellington were used as empirical Greens functions to estimate a source time function duration of 7 s. A joint inversion for velocity and location of 169 selected events was used to derive a one‐dimensional velocity model with station terms, and this velocity model was then used to relocate all recorded aftershocks. A subset of the best 803 events was then selected for further analysis. The apparent trend of the aftershock zone is NNW‐SSE, with the mainshock near the centre. However, projections of the aftershocks on north‐south and east‐west cross‐sections show a band of activity shallowing to the south and dipping to the west. The north‐striking, west‐dipping nodal plane of the mainshock focal mechanism is therefore most likely to be the fault plane. Early aftershocks occurred mainly to the south of the mainshock location, suggesting rupture to the south, a feature supported by the mainshock modelling. The aftershock focal mechanisms are mixed but reflect the regional stress field (NW‐SE compression).

Large earthquakes near doubtful sound, New Zealand, 1989–93

Abstract Aftershock data recorded on portable seismographs deployed immediately after the MW 6.4 Doubtful Sound earthquake of 1989 May 31 and the MW 6.8 Secretary Island earthquake of 1993 August 10 have been used to constrain the rupture zones of these events. Both earthquakes involved slip at the interface between the subducted Australian plate and overlying Pacific plate. Their rupture zones abut rather than overlap, and the region where they meet lies vertically beneath the surface trace of the East Branch of the Alpine Fault. Slip during the deeper 1989 event was approximately in the plate convergence direction, whereas slip during the shallower 1993 event was approximately down the dip of the subducted plate. This requires slip partitioning in the shallow part of the subduction zone, and suggests that the East Branch of the Alpine Fault is active in this part of Fiordland. The 1989 earthquake produced very few aftershocks, whereas the 1993 earthquake had a rich aftershock sequence. This difference, and the mismatch in slip direction between the two events, can be attributed to changes in the frictional regime at the plate interface with depth. Static stress changes produced by slip in the 1989 earthquake promoted down‐dip thrusting on the rupture zone of the 1993 event. Thus, the 1989 event appears to have triggered the 1993 event, in the sense that it moved it closer to failure.

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.

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).