Seismic, gravity and magnetic studies, Broadlands geothermal field, New Zealand

The New Zealand Taupo Volcanic Zone (TVZ) is an area with considerable crustal seismicity, but the shallowness of the seismicity has made it difficult to determine good focal mechanisms from the permanent seismic stations. Within the central TVZ, the only focal mechanisms previously available were composite ones derived from earthquake swarms. These mechanisms were dominantly strike-slip, rather than the normal mechanisms that might be expected from the dominant southeastnorthwest extension seen from deformation measurements and geology. The 1987 M L 6.3 Edgecumbe earthquake near the northern end of the zone had a normal mechanism consistent with the deformation, as did some large earthquakes just offshore.We present focal mechanisms obtained from a 6 month (Jan-Jun 1995) seismometer deployment consisting of 64 short-period and 23 broadband seismometers together with the 15 permanent stations within the TVZ. This close-spaced network enabled us to get focal mechanisms for a considerable number of crustal earthquakes. We present here 23 TVZ mechanisms, mainly for events around the Rotorua lakes, and southwestwards towards Lake Taupo, together with one strike-slip mechanism for an earthquake east of the TVZ.The focal mechanisms are mainly normal, consistent with the observed extension, although most mechanisms have a strike-slip component. Three earthquakes appeared to have thrust mechanisms, but a non-double-couple solution is also a possibility for two of these events. Even within a single swarm, earthquakes with normal focal mechanisms had considerable variation in the strike direction. These variations suggest that the high pore pressure has produced a very low strength region in which slip can occur in a range of directions. These normal mechanisms make unnecessary the model of extension by conjugate strike-slip events proposed by Smith & Webb in 1986. We also carried out a direct stress inversion from the first-motions of a larger set of earthquakes. The principal stress directions obtained by this method agree with the average for the 19 normal earthquakes, and both support

Section: Introductionmentioning

confidence: 89%

Earthquake focal mechanisms in the central Taupo Volcanic Zone and their relation to faulting and deformation

Hurst¹,

Bibby²,

Robinson³

2002

“…1, apart from the few caused by topographic effects, reflect local demagnetisation of ancient geothermal reservoirs. Negative magnetic anomalies have been observed over many active geothermal fields in the central and eastern TVZ which are surrounded by unaltered, normally magnetised volcanic rocks (Hochstein & Hunt 1970;Ignacio 1985;Soengkono 1985;Salt 1986;van Dijck 1988).…”

Section: Low-altitude Aeromagnetic Surveys Over the Western Tvzmentioning

confidence: 99%

Geophysical evidence for widespread reversely magnetised pyroclastics in the western Taupo Volcanic Zone (New Zealand)

Soengkono

Hochstein

Smith

et al. 1992

Low-altitude aeromagnetic data show that negative residual anomalies are widespread over the western Taupo Volcanic Zone, New Zealand. Paleomagnetic study of eight rhyolitic ignimbrite units and two lava flows which are exposed in this area, together with new K-Ar dates of four of the ignimbrite units, indicate that the two lava units and seven of the ignimbrite units were erupted during the Matuyama geomagnetic epoch (>0.73 Ma B.P.) and suggest that rhyolitic volcanism in the western Taupo Volcanic Zone began as early as 1.6 Ma B.P. These results provide the basis for an interpretation of our aeromagnetic data which confirms the hypothesis that the magnetic anomalies observed in the western Taupo Volcanic Zone are caused by widespread, thick, reversely magnetised pyroclastic and lava flows. Magnetic modelling also allows thickness estimates of the younger, normally magnetised cover rocks which reach a maximum thickness of the order of 0.5 km in the Mangakino area. The magnetic structure of these volcanic rocks defines approximately the lateral extent of the Mangakino Volcanic Centre.

“…Seismic refraction surveys have shown that it contains up to 2 kID of low velocity volcanic material underlain by a high velocity refractor. (Hochstein & Hunt 1970;Robinson et aI. 1981).…”

Section: Introductionmentioning

confidence: 99%

A seismic refraction experiment southeast of Ruapehu volcano

Sissons

Dibble

1981

A seismic refraction survey parallel to the strike of the Taupo Volcanic Zone, North Island, New Zealand, and about 9 km southeast of Ruapehu volcano, shows 3 principal refractors with velocities of 2.0 km S-I, 4.9 km S-I, and 5.4 km S-I. The first refractor underlies shallow surface layers, while the last 2 occur at average depths of approximately 600 m and 1700 m respectively. Delay time analysis shows that the 4.9 kID s -I refractor is approximately horizontal, but is upthrown 150 m on the Ruapehu side of a previously mapped fault which curves around the volcano. Surface layers are tephras and the 2.0 km s -1 layer is composed of lahar deposits. Seismograms from the southern portion of the survey show second arrivals with velocities of about 3.0 kID S-I. These are attributed to Tertiary sediments which underlie and predate the volcanics. The velocities of 4.9 km S-1 and 5.4 km S-1 are within the range measured for Mesozoic greywackes which constitute the sedimentary basement of the surrounding region.