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, 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.

Towards real-time eruption forecasting in the Auckland Volcanic Field: application of BET_EF during the New Zealand National Disaster Exercise ‘Ruaumoko’

“…Comparison with life spans of analogue volcanic fields (e.g. Sherburn et al 2007) and the presence of a mantle anomaly at depths of about 70-90 km beneath Auckland that has been interpreted as a zone of partial melting (Horspool et al 2006) suggest the field will erupt again. Thus, having a procedure for determining volcanic risk in a more rigorous and quantitative manner is important for long-term planning and volcanic hazard mitigation in the Auckland region.…”

Post-Miocene faults in Auckland: insights from borehole and topographic analysis

“…The most recent earthquakes have been located in the Hauraki Gulf north of Auckland (Sherburn et al 2007) with the largest three tremors all recorded on 21 February 2007, 30 km east of Ō rewa, at depths of 7Á15 km and Richter magnitudes of 4.5, 3.7 and 3.8. Further south, the 5.5Á5.9 magnitude Waikato Heads earthquake of 1891 caused minor damage in Auckland (Hull et al 1995).…”

“…Although the seismic hazard in Auckland is low (Stirling et al 2002;Sherburn et al 2007), the presence of low-level seismicity together with numerous faults of unknown age surrounding the central Auckland area indicates that there could be concealed faults in central Auckland. Unknown concealed faults could have implications for seismic hazard in the region and also for volcanism, as pre-existing faults may provide future magma ascent pathways.…”

“…Despite this, the area does experience low-level seismicity; since 1983, the Auckland seismic network has recorded c. 80 earthquakes above c. M L 2.5 (Sherburn et al 2007). The Kerepēhi Fault within the Hauraki Rift to the east and south of Auckland is still considered to be active (De Lange & Lowe 1990).…”

Evacuation planning in the Auckland Volcanic Field, New Zealand: a spatio-temporal approach for emergency management and transportation network decisions

“…This is based on the expected fast magma ascent rates (1-10 cm s -1 ) that basaltic volcanic fields are known to exhibit (Blake et al 2006, Sherburn et al 2007). Early detection of precursory activity is therefore critical.…”