Focal mechanisms of large earthquakes in the North Island of New Zealand: slip partitioning at an oblique active margin

Abstract

We have used body-wave modelling to determine the source parameters of 22 moderate to large earthquakes that have occurred along the Hikurangi subduction margin and elsewhere in the North Island of New Zealand since 1964. We have also included events from the Harvard CMT catalogue since 1977 as that catalogue contains smaller events than can be modelled with body waves. We have found that shallow earthquakes occurring in the back-arc Taupo Volcanic Zone and its extension to the north show predominantly normal faulting with nodal planes parallel to the regional fabric and T -axes indicating extension in a direction in agreement with geodetic measurements. A normal faulting solution for the 1974 Opunake earthquake is compatible with the mapping of active faults in the offshore Taranaki region, and the initiation depth of 17 km is close to the expected brittle-ductile transition for that area.Earthquakes within the subducted plate at the southwestern end of the margin are dominated by down-dip tension that is rotated towards the deepest part of the subducting plate. Further north, the events in the upper part of the Wadati-Benioff zone (<200 km) show pure down-dip tension, while this association is less obvious for the deeper events, suggesting that the aseismic part of the subducting slab inferred from plate reconstructions has reached the 670 km discontinuity. Normal faulting earthquakes associated with plate bending below the region of interplate contact occur along the length of the margin, while shallow normal faulting events also occur in the trench, indicating that some slab pull propagates to that region. We infer that large interplate earthquakes are not imminent because in other similar areas they are preceded by outer-rise compressional events that have not been recorded in the region in historical times.Slip vectors of interplate thrust earthquakes indicate that the oblique plate motion across the margin is fully partitioned; that is, the plate interface accommodates very little transcurrent motion, which is instead accommodated by faults in the overlying Australian Plate. Extensional geodetic strain has been measured across the margin in the northeast, which has been regarded as unusual in view of the plate convergence. We suggest that the topography at this part of the margin is too steep to be supported by the current horizontal forces due to friction on the plate interface. This may have occurred due to a reduction in friction because of the subduction of seamounts and the subsequent entrapment of sediments, this also being consistent with the idea of underplating at this part of the margin, or due to the subduction of less buoyant crust. Other margins, such as Japan and Mid-America, where there is known to be subduction of sediments, also show extension in the forearc. Compressional strain observed across the margin in the southwest is more usual for a convergent margin, but it does not show the partitioning we see in focal mechanisms. These data might only agree after the long-term compressional st...

Tectonics of the Taranaki region, New Zealand: Earthquake focal mechanisms and stress axes

“…1) as a means of elucidating the current tectonics. It complements previous work on active faulting (Nodder 1993;Hull 1994;Nicol et al 2005), and geodetic modelling (Wallace et al 2004), and expands significantly on the previously limited use of focal mechanisms to study Taranaki tectonics (Reyners 1980;Cavill et al 1997;Webb & Anderson 1998). It forms part of a multi-faceted study of Taranaki (Sherburn & White 2005;Sherburn et al in press) with the overall aim of understanding the structure and seismicity of the region and using that information to understand better earthquake monitoring data from Mt Taranaki volcano.…”

“…Our focal mechanism results are similar to those of Reyners (1980) for his 25-42 km deep earthquakes in this region, although we do not see the change in focal mechanism with depth that he observed. The mechanism of the Opunake earthquake (Webb & Anderson 1998), although southwest of Mt Taranaki (Fig. 1), is almost identical to two dominantly normal faulting mechanisms in eastern Taranaki (mechanisms 29 and 35, Fig.…”

“…Using waveform modelling, Webb & Anderson (1998) found an oblique normal focal mechanism for the 1974 M L 6.1 (MW5.5) Opunake earthquake with a fault plane aligned parallel to nearby faults in the CEFZ. Cavill et al (1997) determined a composite focal mechanism from small earthquakes immediately west of Mt Taranaki that was oblique normal with left-lateral slip.…”

“…3 and 4, respectively. Also shown are the focal mechanisms of the 1974 Opunake earthquake (Webb & Anderson 1998) and composite mechanisms from western Taranaki (Cavill et al 1997), corrected for polarity reversals; a lower hemisphere projection is used. CE is Cape Egmont and NP is New Plymouth.…”

New predictive equations for Arias intensity from crustal earthquakes in New Zealand

“…For the majority of events in the dataset, focal mechanism solutions exist, and for some, The shaded columns indicate the datasets used to derive the final predictive equations The columns L, W, h t , and h b correspond to the fault rupture length, rupture width, and depths to the top and bottom of the rupture surface respectively. Numbers in the Refs column correspond to the following references: (1) Anderson et al (1993), (2) Webb and Anderson (1998), (3) Dowrick and Rhoades (1998), (4) additional constraint is available in the form of spatial aftershock patterns, geodetic modelling, elastic dislocation modelling, Coulomb stress change modelling and considerations of structural geology. Information from the many references cited in Table 3 was extracted in order to determine the finite fault rupture parameters for each event.…”