Abstract The Auckland Volcanic Field (AVF) in New Zealand is monitored by a network of five telemetered, vertical‐component, 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 ML 1.6 to 3.3, and five earthquakes of ML ≥ 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 ML 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.
Abstract A short, intense sequence of volcano‐tectonic earthquakes preceded a period of strong volcanic tremor at White Island volcano, New Zealand, in July—September 1991. The tremor was initially harmonic with clear higher harmonics, but after 3 days was gradually replaced by broadband non‐harmonic tremor. Good examples of both harmonic and non‐harmonic tremor were recorded. Shock waves were observed in the eruption column of May 91 vent from early August, coinciding with the period of non‐harmonic tremor. The harmonic tremor is interpreted to have been due to a standing wave vibration in vesicular magma in the conduit beneath May 91 Vent, and the non‐harmonic tremor to open‐vent degassing activity near the top of the vent.
A large, dense network of three-component, broad-band seismographs was used to determine accurate hypocentres for earthquakes in Taranaki, New Zealand. They allow us to characterize seismicity around Mt Taranaki, a large, dormant, andesite, cone volcano, and to map precisely two major lineations of crustal seismicity. A minimum 1-D velocity model was used to locate 389 local earthquakes using the probabilistic, non-linear earthquake location program nonlinloc. There are few earthquakes beneath Mt Taranaki itself, and all are relatively small and shallow (≤10 km deep). The shallow seismogenic zone can be explained by the crust being unusually hot, thus causing the base of the brittle—ductile transition to be shallower than normal beneath Mt Taranaki. This is supported by a high heat flow anomaly in this area. The absence of any volcanic earthquakes beneath Mt Taranaki suggests that active volcanic processes are currently unlikely, and the shallow brittle—ductile transition depth means that precursory volcano—tectonic seismicity from any future magmatic intrusion is unlikely to occur below 10-km depth. The permanent seismic network can locate earthquakes in Taranaki reasonably accurately and can reproduce most of the details seen by the temporary seismograph deployment provided that only the best hypocentres are considered. However, beneath Mt Taranaki, which is the most important area for volcano monitoring, hypocentres determined by the permanent network are too deep by 4–12 km. The active Cape Egmont fault zone (CEFZ), west of Mt Taranaki, is the most seismically active area, with earthquakes in the upper crust to about 22-km depth. Spatial and temporal clustering, earthquakes with similar waveforms, and an absence of obvious main shocks imply that earthquake swarms make up a significant proportion of the seismicity in this area. Earthquakes in eastern Taranaki occur primarily along the Taranaki—Ruapehu Line (TRL), thought to be a major boundary across which the crustal thickness changes by about 10 km. These earthquakes are less clustered, have a b value typical of tectonic earthquakes, and occur in the lower crust to a depth of 35 km, with the upper crust almost aseismic. The abrupt cessation of seismicity at 35-km depth is consistent with this boundary marking the Moho, with no earthquakes in the mantle. The concentration of earthquakes in the lower crust requires it to be drier and more mafic than the wet, quartzo-feldspathic composition often used to model crustal rheology. There is no change in maximum earthquake depth across the currently accepted location of the TRL, but there is a 10-km decrease in maximum earthquake depth some 25 km to the north of the currently accepted location. This suggests that the true position of the TRL is 25 km north of the hitherto accepted position.
Abstract 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. Lower crustal earthquakes in eastern Taranaki display both strike‐slip and normal faulting mechanisms; σ3 is subhorizontal and aligned northwest‐southeast, while the best fit σ1is aligned northeast‐southwest with a dip of 27–38° to the horizontal. The principal stress directions in eastern Taranaki are similar to those near the southern Taupo Volcanic Zone, suggesting that the back‐arc extension that characterises the Taupo Volcanic Zone continues into eastern Taranaki. Swarm earthquakes from the Cape Egmont Fault Zone, west of Mt Taranaki, have dominantly strike‐slip focal mechanisms. The maximum (σ1) and minimum (σ3) compressive stresses west of Mt Taranaki are both subhorizontal, with σ1 aligned east‐west and σ3 north‐south. The focal mechanisms and principal stress directions do not agree with the geologically inferred northwest‐southeast extension direction. We suggest that western Taranaki may be affected by stresses induced by magmatism beneath Mt Taranaki and that the normal faulting seen at the surface mainly occurs associated with significant eruptions from the volcano. The failure angle on faults in western Taranaki exceeds that expected for Byerlee friction and hydrostatic fluid pressure and this suggests that these faults have a relatively low coefficient of friction or relatively high pore fluid pressure.
Abstract Tilde is a new in-house developed solution that the GNS Science’s GeoNet program has recently developed to provide storage and access to low sample rate datasets used to monitor tsunami, landslides, and volcanoes in Aotearoa New Zealand. It includes datasets covering sample rates of 15 s or longer. Time series data are stored and disseminated in JSON and CSV formats, and users can access these through an application programming interface (API) and through graphical user interfaces (GUIs). Tilde’s GUIs were created to allow technical and non-technical users easy access to the available data. The introduction of the Tilde system as one of the GeoNet program delivery channels has represented a big step forward for GeoNet’s volcano data holdings by providing a single point to access all low to medium-sample rate volcano-specific monitoring data. We designed the system and developed a domain model, an API, a graphical data discovery interface, and associated data tutorials. This work leverages the open-by-default data policy for data generated through the GeoNet program. This paper is intended to highlight how we made many of the key decisions that shaped the Tilde system, how they were impacted by our multi-hazard monitoring requirements, and how they have improved access to volcano monitoring data. We conclude with some open questions about the need to develop common standards to share analysis-ready time series data within different disciplines in volcanology and geophysics.
Abstract Vertical deformation and shallow seismicity around Lake Taupo, which occupies much of the Taupo Volcanic Centre, displayed two regionally distinct patterns during the 1985–90 study period. In the Taupo Fault Belt, north of the lake, there was aseismic sag. A high rate of relative subsidence of 10 ± 1 mm/yr resulted in inward tilt exceeding 1.0 ± 0.1 |μrad/yr, but was accompanied by almost no seismic activity. In contrast, the lakeshore and islands within the central and southern part of the lake displayed repeated small oscillations in height, superimposed on a pattern of slow tilt to the southwest, while the seismicity of this area was characterised by frequent small earthquake sequences. Four periods were identified when rates of deformation and seismicity within the southern area appeared to increase above previous levels, but there was no evidence of a one‐to‐one correspondence between deformation and individual seismic events. Both phenomena are thought more likely to reflect responses, at different time scales and depths, to the underlying strain release.
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