Taupō is a large caldera volcano located beneath a lake in the centre of the North Island of New Zealand and most recently erupted ~1800 years ago. The volcano has experienced at least 16 periods of unrest since 1872, each of which were characterised by increased seismic activity. Here we detail seismic activity during the most recent period of unrest from May 2022 to May 2023. The unrest was notable for the highest number of earthquakes detected during instrumented unrest episodes, and for one of the largest magnitude earthquakes detected beneath the lake for at least 50 years (ML 5.7). Relocated earthquakes indicate seismic activity was focused around an area hosting overlapping caldera structures and a hydrothermal system. Moment tensor inversion for the largest earthquake includes a non-negligible inflationary isotropic component. We suggest the seismic unrest was caused by the reactivation of faults due to an intrusion of magma at depth.
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.
The purpose of this short note is to correct the misidentification of diurnal (24 hr) and semi-diurnal (12 hr) periodicities in volcanic seismicity reported for Ruapehu Volcano, New Zealand.
We obtain estimates of the seismic velocity and attenuation for White Island volcano by use of high‐impact sand‐bag drops from helicopter. Three drops were attempted, two at either end of a 6‐station linear array within the crater floor, and the third in the volcano's crater lake. The bags were dropped from ∼310–380 m height and contained ∼700 kg of sand. The impact velocity was estimated at ∼60–70 m/s yielding a kinetic energy of about 10 6 Nm, giving P ‐wave onsets to a distance of ∼1 km. We obtained a seismic velocity estimate of Vp = 1.2 km/s for the unconsolidated crater floor and Vp = 2.2 km/s for rays traversing through consolidated rock outside the crater. Attenuation was very strong ( Q < 10) for both consolidated and unconsolidated parts of the volcano. This trial shows that low cost helicopter mass drops can be successfully applied to safely determine sub‐surface properties at hazardous volcanoes.
Abstract Volcanic lakes often capture a significant amount of volcanic heat emission and thus provide a unique opportunity to monitor changes inside the volcano. We present a Bayesian inversion method to automatically infer changes in volcanic heat emission over time at the base of a volcanic lake from lake monitoring data using a non-linear Kalman Smoother. Our method accounts for the, sometimes large, uncertainties in observations and the underlying physics-based model to generate probabilistic estimates of heat emission. We verify our results using a synthetic test case and then estimate the daily heat input rate into Mt. Ruapehu’s Crater Lake, New Zealand, between 2016 and 2022. Time-frequency analysis of the heat input rate shows dominant periods of heating cycles ranged between 100 - 250 days. The period between 2017 and 2020 was dominated by shorter cycles and greater-than-average heat input rate which points to changes in the magmatic heat supply and the hydrothermal system during this time.
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