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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.