Abstract
Tarawera volcano (New Zealand) is volumetrically dominated by rhyolitic lavas and pyroclastic deposits, but the most recent event in AD 1886 was a basaltic Plinian fissure eruption. In March 2019 a swarm of at least 64 earthquakes occurred to the NE of Tarawera volcano, as recorded by the New Zealand Geohazard Monitoring Network (GeoNet). We use seismological analysis to show that this swarm was most likely caused by a dyke that intruded into the brittle crust between depths of 8–10 km and propagated toward Tarawera volcano for 2 km at a rate of 0.3–0.6 m s−1. We infer that this was a dyke of basaltic composition that was stress-guided toward Tarawera volcano by the topographic load of the volcanic edifice. Dyke intrusions of this nature are most likely a common occurrence but a similar process may have occurred during the 1886 eruption with a dyke sourced from some lateral distance away from the volcano. The 2019 intrusion was not detected by InSAR geodesy and we use synthetic models to show that geodetic monitoring could only detect a ≥6 m wide dyke at these depths. Improvements to geodetic monitoring, combined with detailed seismological analysis, could better detect future magmatic intrusions in the region and serve to help assess ongoing changes in the magmatic system and the associated possibilities of a volcanic event.
Highlights
Forecasting volcanic eruptions is inherently challenging due to the wide range of unrest signals that can occur at variable rates and over variable timescales prior to eruption (e.g., Sparks et al, 2012)
The earthquakes migrated from NE to SW at an apparent rate of 0.3–0.6 m s−1, and their range of depths increased at the same time (Figure 4)
We have shown that dyke intrusions sourced from outside the Okataina Volcanic Center (OVC) caldera can propagate toward Tarawera, and we speculate that similar “external” dyke intrusions may have influenced past eruptive activity
Summary
Forecasting volcanic eruptions is inherently challenging due to the wide range of unrest signals (e.g., ground deformation, elevated seismicity, gas emissions) that can occur at variable rates and over variable timescales prior to eruption (e.g., Sparks et al, 2012). Many of these unrest signals can occur without leading to eruption, highlighting the complex nature of the subsurface plumbing systems and the numerous processes that occur beneath dormant volcanoes (Moran et al, 2011). Disentangling the signals of magmatic unrest (e.g., variations in the pressure of the system due to magma recharge or ascent) vs. non-magmatic unrest (e.g., changes in the hydrothermal system or tectonic earthquakes) can be difficult, with the potential for many different mechanisms that can produce similar unrest symptoms (Acocella et al, 2015)
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