Some challenges of monitoring a potentially active volcanic field in a large urban area: Auckland volcanic field, New Zealand

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The city of Auckland (population 1.3 million) is built on and around a potentially active basaltic intraplate volcanic system, the Auckland volcanic field. This monogenetic field of around 50 small volcanoes covers an area of 360 km2 and may have been active for ca. 250 ka. Volcano monitoring can be difficult in small distributed volcanic fields such as the AVF as the next vent location is not known, and traditional techniques such as geochemical and ground deformation monitoring may therefore not be feasible as there is no obvious target for measurements. In such cases, seismic monitoring may be the only technique suitable. High levels of background (cultural) seismic noise and extremely low levels of historical seismicity make volcano monitoring particularly challenging in the AVF. Here, we report on recent attempts to improve monitoring capability in the AVF, which include the installation of downhole seismometers and a seismic refraction study to refine the velocity model used to locate earthquakes. We conclude that downhole instrumentation in a volcano-monitoring network allows better detection of local earthquakes, but note that several downhole sensors would be required for a network to realise the full benefits of downhole instrumentation. Our refinement of the general velocity model for Auckland has some effect on calculated earthquake locations and depths, and we suggest that further work on refining the velocity model and then its incorporation into the GeoNet earthquake location system could help us be better prepared for a future AVF eruption.