Steven Sherburn

Shallow seismicity of the central Taupo Volcanic Zone, New Zealand: Its distribution and nature

Abstract A deployment of 87 seismometers, including 23 broadband instruments, for a 5‐month period in 1995 yielded a detailed view of the distribution and nature of the shallow seismicity (depth <20 km) within the central part of the Taupo Volcanic Zone (TVZ), New Zealand. On a broad scale, the pattern of shallow seismicity observed during this study was similar to that recorded by the permanent National Seismograph Network between 1987 and 1994. The distribution of seismicity was not uniform in either time period. Rather, it was scattered throughout the currently active portion of the Taupo Fault Belt, with a number of distinct clusters of events near the northern end of the fault belt. Specifically, in 1995, there did not appear to be any correlation between the seismicity and individual faults. With the exception of a cluster of events near Rotorua, little seismicity occurred on the western side of the TVZ. Similarly, on the southeastern margin of the TVZ, the Taupo‐Reporoa Depression was characterised...

Seismic velocity structure of the central Taupo Volcanic Zone, New Zealand, from local earthquake tomography

Abstract The 3-D distribution of P-wave velocity (Vp) and the P-wave/S-wave velocity ratio (Vp/Vs) are derived for the crust in the central Taupo Volcanic Zone (TVZ), New Zealand, by tomographic inversion of P- and S-wave arrival time data from local earthquakes. Resolution in the seismogenic mid-crust (4–6 km) is good, but poorer above and below these depths. The 3-D velocity model has several Vp anomalies as large as ±5% in the mid–lower crust (4–10 km) and more than ±10% in the upper crust (0–4 km). The model achieves a 55% reduction in data variance from an initial 1-D model. Young caldera structures, Okataina, Rotorua, and Reporoa, are characterised by low Vp anomalies at a depth of about 4 km and these coincide with large negative residual gravity anomalies. We attribute these anomalies to large volumes of low Vp, low-density, volcaniclastic sediments that have filled these caldera collapse structures. Although there are no Vp anomalies which suggest the presence of molten or semi-molten magma beneath the TVZ, a large, high Vp anomaly of more than +15% and a high Vp/Vs anomaly are observed coincident with a diorite pluton beneath the Ngatamariki geothermal field. However, Vp anomalies cannot be seen beneath the largest geothermal fields, Waimangu, Waiotapu, and Reporoa, and, consequently, if such anomalies exist, they must be below the resolution of our data. A prominent Vp contrast of 5–10% occurs at a depth of about 6 km beneath the boundary between the Taupo–Reporoa Depression and the Taupo Fault Belt (TFB), coincident with the eastern limit of the seismic activity beneath the TFB. We interpret this velocity contrast as being caused by the presence of extensive, non-molten, intrusives beneath the Taupo–Reporoa Depression. We suggest that the high-velocity material beneath the Taupo–Reporoa Depression is isolated from regional extension in the TVZ, and from the resulting faulting and seismicity, which occurs preferentially within the weaker material of the TFB. We are unable to determine whether greywacke, which forms the basement beneath the eastern most part of the TVZ continues further west, or is replaced by a volcanic rock such as andesite which has similar Vp and density. Vp/Vs anomalies are much smaller than Vp anomalies and generally have little spatial relationship to the Vp pattern. There is a widespread decrease in Vp/Vs, from >1.76 to 1.70–1.73, between 4 and 6 km depth over much of the study area and Vp/Vs is high southwest of the Okataina caldera, where Vp is low. Hypocentres calculated using the 3-D velocity model differ little from those obtained using a 1-D model with station terms, however, some groups of earthquakes are more tightly clustered. Following relocation, there is a slight decrease in the estimated thickness of the seismogenic zone, with 73% of hypocentres between 4 and 7 km depth and a slight increase in the depth of the brittle–ductile transition from 6 to 6.5 km.

Seismicity of Ruapehu volcano, New Zealand, 1971–1996: a review

Abstract From 1971 until 1995, the style of seismicity at Ruapehu changed little, reflecting a period of relatively low eruptive activity and consequent long-term stability within the vent system. Volcanic earthquakes and volcanic tremor were both dominated by a frequency of about 2 Hz. Volcanic earthquakes accompanied all phreatic and phreatomagmatic eruptions, but not small hydrothermal eruptions that originated within Crater Lake. Furthermore, more than half of the ML>3 volcanic earthquakes and changes in the reduced displacement of 2 Hz volcanic tremor by as much as a factor of 20 occurred without any accompanying eruptive activity. Three and 7 Hz volcanic tremor were also recorded, although never at lower-elevation seismometers. At times, this tremor was stronger at the summit seismometer than the 2 Hz tremor. Their source regions were independent of the 2 Hz source, and located at shallower depths. Volcano-tectonic earthquakes were generally unrelated to eruptive activity. The seismicity accompanying the 1995–1996 eruptive activity was significantly different from that of the period 1971 to 1995, and included volcanic tremor with a frequency of less than 1 Hz, simultaneous changes in the amplitude of the previously independent 2 Hz and 7 Hz volcanic tremor, and finally a change in the frequency content of volcanic earthquakes and volcanic tremor from 2 Hz to wideband. Path transmission effects play an important role in determining the characteristics of seismograms at Ruapehu. The presence of Crater Lake affects both the style of eruptions and the accompanying seismicity.

Characteristics of earthquake sequences in the Central Volcanic Region, New Zealand

Abstract Earthquake sequences in the Central Volcanic Region (CVR) of New Zealand are part of a continuum of types of earthquake sequence that range from main shock ‐aftershock sequences to swarms. Most sequences, irrespective of the largest event in the sequence and the duration of the sequence, have swarm characteristics; that is, the difference in magnitude between the largest and second largest event is 0.3 magnitude units or less. Smaller sequences typically have a duration of a few hours to a day in length, whereas sequences with large magnitude events may continue for many weeks. The time distributions of earthquakes within the sequences range from simple foreshock or aftershock patterns to more complex patterns involving both foreshocks and aftershocks separated by periods of seismic quiescence. However, it does not follow that a sequence with an aftershock time distribution will have a magnitude distribution that will be representative of a main‐shock sequence. Routine location data indicate the ...

Seismicity at White Island volcano, New Zealand: a revised classification and inferences about source mechanism

The classification of earthquakes at White Island volcano, New Zealand, has been revised to address problems in existing classification schemes, to better reflect new data and to try to focus more on source processes. Seismicity generated by the direct involvement of magmatic or hydrothermal fluids are referred to as volcanic, and that generated by fault movement in response to stresses caused by those fluids, regional stresses, thermal effects and so on are referred to as volcano-tectonic. Spasmodic bursts form a separate category, as we have insufficient information to classify them as volcanic or volcano-tectonic. Volcanic seismicity is divided into short-duration, long-period volcanic earthquakes, long-duration volcanic earthquakes, and harmonic- and non-harmonic volcanic tremor, while volcano-tectonic seismicity is divided into shallow and deep volcano-tectonic earthquakes. Harmonic volcanic tremor is related to sub-surface intrusive processes, while non-harmonic volcanic tremor originates close to active craters at shallow depth, and usually occurs during eruptive activity. Short-duration, long-period volcanic earthquakes come from a single source close to the active craters, but originate deeper than non-harmonic volcanic tremor, and are not related to eruptive activity. Long-duration volcanic earthquakes often accompany larger discrete eruptions. The waveform of these events consists of an initial low-frequency part from a deep source, and a later cigar-shaped part of mixed frequencies from a shallow crater source.

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

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.

Volcanic tremor at Ruapehu: Characteristics and implications for the resonant source

Abstract Many volcanoes produce volcanic tremor with consistent sharp peaks, which suggests that one or more resonators have been excited by a volcanic process. Such “harmonic” tremor can be classified as single‐resonator or multiple‐resonator. Multiple‐resonator tremor has a number of peaks, not harmonically related, and the relative energy in the peaks varies rapidly with time. Single‐resonator tremor, as observed on Ruapehu, has a few sharp peaks in the spectrum, and the tremor signal shows some coherence. The main tremor observed on Ruapehu has a dominant frequency of c. 1.8–2.3 Hz, with very little second harmonic energy. Recordings within 1 km of Ruapehu Crater Lake sometimes show a strong third harmonic, but this is rapidly attenuated at greater distances. “Gliding” frequency changes are rarely seen. Tremor of c. 3 Hz also occurs, and this appears to come from another source, the intensity of which is independent of the main source. “2 Hz” Ruapehu tremor recorded near the source shows a complex par...

High-frequency earthquakes at White Island volcano, New Zealand: insights into the shallow structure of a volcano-hydrothermal system

Abstract Volcano-tectonic earthquakes at White Island are concentrated in a single seismically active zone, southeast of the active vents and at depths of less than 1 km. A few deeper earthquakes also occur beneath the active vents. A composite focal mechanism indicates that the stress regime in the shallow seismic zone is N-S extensional. Shallow seismicity occurs within the main volume of the volcano-hydrothermal system that underlies the Main Crater floor, and we interpret this as a region where the rocks have been weakened by past magmatic intrusions, elevated pore fluid pressure and physico-chemical effects of acid volcanic fluids, thereby allowing preferential seismic failure. Brittle seismic failure within this region requires a temperature less than about 400 °C, and implies high horizontal temperature gradients close to the active craters and fumaroles. Spasmodic bursts events are also a result of brittle failure, but occur close to zones of significant permeability in response to changes in local fluid pressure.

Seismic signals of snow-slurry lahars in motion: 25 September 2007, Mt Ruapehu, New Zealand

[1] Detection of ground shaking forms the basis of many lahar-warning systems. Seismic records of two lahar types at Ruapehu, New Zealand, in 2007 are used to examine their nature and internal dynamics. Upstream detection of a flow depends upon flow type and coupling with the ground. 3-D characteristics of seismic signals can be used to distinguish the dominant rheology and gross physical composition. Water-rich hyperconcentrated flows are turbulent; common inter-particle and particle-substrate collisions engender higher energy in cross-channel vibrations relative to channel-parallel. Plug-like snow-slurry lahars show greater energy in channel-parallel signals, due to lateral deposition insulating channel margins, and low turbulence. Direct comparison of flow size must account for flow rheology; a water-rich lahar will generate signals of greater amplitude than a similar-sized snow-slurry flow.

Seismicity of the Lake Taupo region, New Zealand, 1985–90

Abstract Earthquake activity in the Lake Taupo region during 1985–90 was dominated by the repeated occurrence of sequences of small earthquakes, and statistical analysis confirms that clustering was present. Earthquakes were concentrated beneath the central, eastern, and southern parts of the lake, with the Tokaanu, Motuoapa, and Te Hapua Bay areas being the main sites of repeated sequences. Western Bay, the western Taupo Fault Belt, and an area near the southwestern shore of the lake were almost aseismic. Almost all earthquakes occurred within the 15 km thick crust, with a concentration at 4 km depth. The distribution of earthquakes during 1985–90 contrasts with previous studies which snowed a concentration of seismicity in Western Bay and near the southwestern shore of the lake. The tendency for earthquakes to be concentrated in certain areas, together with large changes in the rate of activity, cause significant problems for the identification of earthquake sequences that may be precursors to volcanic ...