Russell Robinson

The 1968 May 23 Inangahua, New Zealand, earthquake: An integrated geological, geodetic, and seismological source model

Abstract The 1968 Inangahua, New Zealand, earthquake occurred in the West Coast Basin and Range Province, northwest of the main plate boundary zone in northern South Island. At MS 7.4, it is not the largest known earthquake in the province, but it has been the subject of thorough seismological, geological, and geodetic documentation. Re interpretation of past observations and more recent data, in the light of new structural and tectonic theories, has produced a new source model for the earthquake. The data suggest that at least 4 m of reverse slip occurred on a fault plane dipping 45° to the northwest beneath the northern part of the Grey‐Inangahua Depression, an area previously inferred to be on the footwall of major reverse faults bounding the ranges on either side of the depression. The seismogenic fault may have propagated north and south across older geological structures in recent times. Faulting within basement is occurring on pre‐existing faults and is accommodating some of the compressional compo...

Relative teleseismic travel-time residuals, North Island, New Zealand, and their relation to upper-mantle structure

Abstract Relative travel-time residuals computed from clear P-wave arrivals at fourteen seismograph stations in the North Island, New Zealand, from five deep-focus events in the Banda Sea region, show large spatial variations of up to 3 sec. The variations can be explained by higher than normal velocities in the oceanic lithosphere which is underthrust to depths of 350 km beneath the North Island. After correction for crustal structure, the residuals imply an average P-wave velocity about 11% higher than in the surrounding mantle. The lack of suitable source events at azimuths other than northwest prevents a more detailed investigation by this means.

Delineation of a low‐velocity body under the Roosevelt Hot Springs geothermal area, Utah, using teleseismic P‐wave data

To assess the nature of the heat source associated with the Roosevelt Hot Springs geothermal area, we have investigated the P‐wave velocity structure of the crust and uppermost mantle in the vicinity of the Mineral Mountains, southwest Utah, a region of late Cenozoic rhyolitic and basaltic volcanic activity. A roughly square (30 × 30 km) array of 15 seismographs, centered on the mountains, was operated for a period of 46 days, during which 72 teleseismic events were recorded with sufficient quality for calculation of P‐wave traveltime residuals. Relative residuals, using the array average for each event as reference, show a clear pattern of azimuthal variation of up to 0.3 sec. This pattern implies the existence of a localized region of relatively low‐velocity material extending up from the upper mantle to depths of about 5 km under the Mineral Mountains. A three‐dimensional (3-D) inversion of the data confirms this conclusion and yields a model featuring a region of low velocity (5 to 7 percent less than...

The Opunake, New Zealand, earthquake of 5 November 1974

Abtract The Opunake earthquake (ML = 6.1) was centred at 39°.54S, 173°.45E, very near the Maui gas field, at a shallow depth. This is about 35 km off the SW Taranaki coast in a region where the Cape Egmont fault zone forms the western margin of the Taranaki graben. Damage due to the event was minor. The focal mechanism solution implies an equal amount of thrust and strike-slip motion, and a nearly horizontal compression-axis that trends NE-SW. The aftershock sequence (128 events with ML ⩾ 3.5 up to the end of November) was of long duration and had a high b-value of 1.35. The aftershock epicentres occupied an area of about 15 X 10 km. The main event is somewhat unusual in that it was, for its magnitude, a very compact event (radius of equivalent circular dislocation 6 km) with a large displacement (0.17 m) and high stressdrop (1.2 X 106 N/m2) .

Aftershocks of the 1987 Edgecumbe earthquake, New Zealand: Seismological and structural studies using portable seismographs in the epicentral region

Abstract The Edgecumbe earthquake (1987 Mar 2d01h42m34s UT; M L 6. 3, M S 6. 6) was a normal faulting event accompanied by surface fauft breaks. The earthquake occurred in an onshore area of active back-arc extension characterised by recent volcanic activity. Over 100 aftershocks have been located using 11 portable, and 2 permanent, seismographs in the epicentral region. The events occurred during the period from 3 to 10 days after the main shock, and most have magnitudes of 3. 0 or greater. They have been located using a velocity model derived from the arrival-time data itself. Station terms, which form a part of that model, reflect the lateral variation in near-surface geology. The epicentres define a region 65 km long, in the northeast-southwest direction, and 10 km broad. This is much longer than the observed surface faulting or what would be expected from the main-shock magnitude. There is a gap in the aftershock distribution near the Kawerau Geothermal Field and the recently active andesite volcano,...

Variation of energy release, rate of occurrence and b-value of earthquakes in the Main Seismic Region, New Zealand

Abstract Time variations in the rate of occurrence, b -value and cumulative energy release have been calculated for earthquakes in the Main Seismic Region (MSR) of New Zealand from 1950 to 1977, and energy release also estimated over the longer period from 1840 to 1977. Although there were no clear premonitory changes in these parameters prior to the magnitude 7.1 earthquake at Inangahua in May, 1968, this event was followed by a marked increase in both the rate of occurrence and b -value of shallow earthquakes, other than aftershocks, throughout the MSR. There was also an increase in the rate of deep shocks. Despite the increase in numbers of events the rate of energy release since the Inangahua earthquake is much less than in the preceding 16 years. These observations seem to require more extensive stress relief than can be accounted for by Inangahua event in itself. The overall rate of energy release for shallow shocks, 1950–1977, is only about 12% of the average rate for the longer period 1840–1977, thus continuing a period of relative quiescence that has existed since the very active period 1929–1934. A similar long quiescent period followed another active period from 1843 to 1855. The energy release during the two active periods is approximately the same. If this long-term pattern of energy release is extrapolated the years from about 2000 could mark a change to another active period.

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...

A microearthquake survey at the Ngawha geothermal field, New Zealand

A twenty day microearthquake survey of the Ngawha geothermal field, New Zealand, was undertaken in order to establish the level of preproduction seismicity and to test the usefulness of such surveys in geothermal exploration. The Ngawha geothermal field, in the far northwest of the North Island (Northland) is associated with a region of Quaternary basaltic volcanism. It is not a part of the much more extensive Taupo volcanic zone in the central North Island, site of the well‐known Wairakei geothermal field, among others. Although surface thermal activity at Ngawha is limited to a few relatively small hot springs, resistivity surveys have outlined a 25-50km2 area of hot water at the 1-km depth level (Macdonald et al. , 1977). Test bores to that depth have encountered temperatures of up to 250u2009°C within Mesozoic graywacke. Overlying the graywacke is about 500 m of Cenozoic claystone and siltstone which forms an impermeable cap.

An anomaly in borehole radon content at Wellington, New Zealand, in 1993

Abstract The radon content of the air in a 60 m deep well at Wellington has been monitored since 1978. The purpose is to test the proposal that anomalous radon content is a precursor to large local earthquakes. The data from 13 January 1990 to 31 July 1994 are homogeneous, free from data gaps, and have been corrected for air pressure and waterlevel changes. During this period there were eight large (magnitude 6.0 or more), shallow (depth <40 km), regional (distance of 800 km or less) earthquakes, but none closer than 172 km. The radon data show only a single anomalous period of 147 days (9% of the observation period) from 17 May to 10 October 1993 when the content was significantly higher than normal. Although two out of eight (25%) of the large, shallow, regional earthquakes occurred during this time, the evidence for a physical connection is weak, since this could happen with probability 0.15 for random earthquake occurrence times.