Crustal and upper mantle structure of the northwestern North Island, New Zealand, from seismic refraction data

The Auckland Volcanic Field (AVF) in New Zealand is monitored by a network of five telemetered, verticalcomponent, short-period seismographs. Between 1995 and 2005, 24 earthquakes were located in the Auckland region. Ten of these were located reasonably reliably (position and depth uncertainty 10 km) and all of these were <15 km deep. Only one of these earthquakes occurred within the AVF. Magnitudes ranged from M L 1.6 to 3.3, and five earthquakes of M L 2.4 were felt. There were few reliably located earthquakes because most were not recorded by the whole network owing to their relatively low magnitude and a high level of background noise. The Auckland earthquakes are believed to represent normal background seismicity and are not thought to be eruption precursors. All earthquakes were of high-frequency, tectonic type; no low-frequency, volcanic earthquakes were recorded. Based on seismic precursors to eruptions from historically active volcanic fields, we estimate that precursory earthquakes could occur as little as 2 weeks before an Auckland eruption and they could be as large as M L 4.5-5.5. Based on the depth of the background seismicity in Auckland, and previous estimates of the ascent rate and source depth of AVF magmas, we calculate a precursory period as short as a few days. Our best estimate of the length of preeruption seismicity is therefore a few days to a few weeks. The largest precursory earthquakes could be large enough to be felt by most of the population who live in Auckland City. During a magmatic intrusion, deep long-period earthquakes might occur at c. 30 km as magma ascends into the crust. Earthquakes would probably have to be a lot shallower, perhaps only 5 km, before their epicentres might be useful for estimating the location of any eruption. Geodetic monitoring methods (GPS and InSAR) might perform as well as seismic monitoring for identifying unrest, but they have significant limitations. To better monitor and interpret precursory seismicity from the AVF, an increase in the number of seismographs and an improvement in our understanding of the local crustal structure are needed.

Section: 2°h Auraki Gulfsupporting

confidence: 77%

Monitoring seismic precursors to an eruption from the Auckland Volcanic Field, New Zealand

Sherburn

Scott

Olsen

et al. 2007

“…Previous geophysical studies indicate that attenuated continental crust underlies the greater region. Long offset seismic refraction surveys record a depth to the mantle of c. 15 km under the New Caledonia Basin, 20 km under the Lord Howe Rise and Norfolk Ridge (Shor et al 1971), and c. 25 km beneath the Northland peninsula (Stern et al 1987). Marine gravity studies reveal similar crustal thicknesses under the other major ridges and reduced thicknesses of 15-20 km under the basins (Uruski & Wood 1991;Wood 1991;Zhu & Symonds 1994).…”

Section: Introductionmentioning

confidence: 97%

Seismic stratigraphy and structural history of the Reinga Basin and its margins, southern Norfolk Ridge system

Herzer

Chaproniere

Edwards

et al. 1997

102

The Reinga Basin northwest of the North Island of New Zealand was initially formed by crustal extension in Cretaceous time. Gravity models suggest up to 35-40% crustal thinning. The seismic stratigraphy of the basin is continuous with that of the offshore western North Island, where reflectors are well constrained by oil exploration data. In the Reinga Basin, there are two Cretaceous sequences above an older Mesozoic basement. The lower sequence is apparently terrestrial and may include both pre-rift and synrift subsequences; the upper is a rift-filling marine sequence. These are overlain by Paleocene and Eocene blanket sequences that were laid down during a period of relative tectonic quiescence consistent with cooling subsidence, continued submergence, a northeast-facing continental shelf, and absence of a significant active plate boundary. A strong regional reflector, caused by a combined unconformity and G97011Received 13 February 1997; accepted 14 August 1997 Oligocene condensed sequence, separates the Paleogene and Neogene sequences.The Neogene sequences record sedimentary infill from several source directions, not only from the New Zealand landmass, but from the north and west as well. Near the Northland coast, sediment accumulated in clastic wedges and ponded sub-basins from the Miocene to the present day. Along the flanking ridges to the northwest, similar deposition occurred in the Early and Middle Miocene but changed in the Late Miocene to sedimentation in drifts flanked by scours. This change reflects the end of tectonism, a diminishing clastic sediment supply, and the establishment of a throughgoing oceanic current regime as the marginal ridges submerged. This pattern of sedimentation persists today.Post-Cretaceous volcanism occurred in two parts of the basin. In the central southeastern part, volcanic bodies in the ?Oligocene to Early Miocene sequences could be a northwestern extension of the Northland volcanic arc. In the western part, small intrusive and extrusive bodies appear to be of Pliocene intraplate origin.Compression (or transpression) had an important role in developing the basin's present form. Miocene compressional structures-asymmetric anticlines, reverse faults, everted basins, and pop-ups-are present everywhere but at the southeastern end. The present marginal ridges have structurally complex origins. The Reinga Ridge which forms the northeastern margin is a transform boundary with the Norfolk backarc basin. Deformation thought to be caused by the action of the transform is recorded in folded and faulted Cretaceous-Paleogene sequences and syntectonic Early and Middle Miocene sequences along its length. The southwestern margin of the basin is a double ridge comprising the Wanganella Ridge, an early Middle to early Late Miocene, compressional uplift, and the older, eroded West Norfolk Ridge, which contains Cretaceous halfgrabens. The northern half of the Wanganella Ridge is an everted ?Oligocene to Early Miocene aulacogen in which slivers of basement rock were thrust up alo...

“…1) is thought to overlie the juxtaposition of a thin continental crust (25 km) to the north against a normal thickness crust (36 km) to the south (Stern et al 1987), which is marked by the Taranaki-Ruapehu Line (TRL). From the orientation of the regional gravity anomaly, Stern et al (1987) considered the TRL to be aligned east-west, although Reyners et al (2006), using 3D velocity tomography results, think it is aligned northwest-southeast, parallel to the dip of the subducting Pacific plate beneath the North Island. Although the area is seismically active (Sherburn & White 2005), there are no known active faults, possibly because the earthquakes are concentrated deeper than 25 km below the surface (Sherburn & White 2005).…”

Section: Introductionmentioning

confidence: 99%

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

Sherburn

White

2006

We present 39 well-determined focal mechanisms for crustal earthquakes from the Taranaki region in western North Island, New Zealand. Earthquake locations, azimuths, and take-off angles were calculated using a 3D velocity model and only those mechanisms with at least 15 clear first motions were considered. Principal stress axes were determined by inverting focal mechanisms and independently by inverting earthquake first motions. Based on misfit values and differences in seismicity and geology we interpret data east and west of Mt Taranaki separately.