Earthquake Analysis Suggests Dyke Intrusion in 2019 Near Tarawera Volcano, New Zealand

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.

Storage, evolution, and mixing in basaltic eruptions from around the Okataina Volcanic Centre, Taupō Volcanic Zone, Aotearoa New Zealand

“…This suggests that basaltic magmas, in addition to rhyolitic magmas, are stored and evolve polybarically within the crust. This agrees with current geochemical and geophysical constraints from previous Tarawera clinopyroxene barometry (100-300 MPa, with some >700 MPa, reported in Sable et al, 2009) and the presence of partial melt bodies at similar depths around the OVC, such as at 6-16 km using receiver functions (Bannister et al, 2004), 10-20 km (as shallow as 8 km beneath Waimungu) using magnetotelluric and electrical resistivity inversions (Heise et al, 2016(Heise et al, , 2010, and 8-10 km from earthquake swarms attributed to a basaltic dike intrusion (Benson et al, 2021). Additionally, conceptual models based on petrological modelling have mafic sheets residing at 11-15 km, with some isolated pods found at 8-6 km depths (Cole et al, 2014;Deering et al, 2010).…”

“…Some rhyolitic eruptions were preceded by basaltic eruptions, with either no (e.g., Matahi prior to Rotoiti) or direct (e.g., mixed basaltic-rhyolitic clasts in Okareka) evidence for magma mixing prior to eruption, whereas others (e.g., Rerewhakaaitu and Kaharoa) host basaltic blebs and enclaves (e.g., Burt et al, 1998;Cole, 1973a;Cole et al, 2014;Leonard et al, 2002;Nairn, 1992;Pullar and Nairn, 1972;Schmitz and Smith, 2004;Shane et al, 2007Shane et al, , 2008a. The OVC is passively degassing CO2 and heat today and inferred basaltic dike events also occur (e.g., Benson et al, 2021;Hughes et al, 2019b;Mazot et al, 2014). (n.d.), but are likely very small due to their limited occurrence (Nairn, 2002).…”

“…As the upper section runs parallel to the Tarawera 1886 fissure only one vent is indicated but the orange horizontal bar represents the extent of the vents. The location of the 2019 earthquake swarm attributed to a basaltic dyke (Benson et al, 2021) is shown by a pentagon. The white transparent areas are low-resistivity regions images using magneto-telluric and electric resistivity surveys by Heise et al (2016Heise et al ( , 2010 the whole region has not been surveyed so other low-resistivity regions likely exist.…”

Stretching, Shaking, Inflating: Volcanic-Tectonic Interactions at a Rifting Silicic Caldera

“…Although mafic magmas are inferred to maintain the main silicic reservoir at Taupō, mafic magmas also provide the high heat flow to the other parts of the central TVZ (Bibby et al, 1995), and likely accommodate a significant proportion of crustal extension at depth (i.e., Rowland et al, 2010;Gómez-Vasconcelos et al, 2017). Thus mafic intrusions are likely to be a common occurrence throughout the TVZ, almost all of which stall in the crust with minimal consequence and may escape notice, even in the instrumented age (Outcome 3: Benson et al, 2021;Ellis et al, in review;Figure 9). However, dike-induced stress changes can encourage fault slip (Rubin and Pollard, 1988), and are inferred to have triggered past eruptions from Taupō and other silicic systems globally (e.g., 2005 Dabbahu rifting episode (Ethiopia), 1350 CE Mono-Inyo eruption (United States), 25.5 ka Oruanui eruption (New Zealand); Bursik et al, 2003;Rowland et al, 2007;Allan et al, 2012;Wright et al, 2012).…”

Volcanic Unrest at Taupō Volcano in 2019: Causes, Mechanisms and Implications

“…Using the vent spacing and inferred crystallization depths, Barker et al (2015) estimated the volume of the modern silicic magmatic system to be between 250 and 1000 km 3 . However, this estimate is only a first-order calculation and does not take into account the lateral movement of magmas, which is thought to be potentially important in this rifted setting (Allan et al, 2012;Benson et al, 2021;Wilson et al, 2009). The minimum geographic areas of the magmatic system is determined by the collapse structure that formed during the 232 CE Taupō eruption, which is also the area of most intense modern heat flow on the lake floor (Davy & Caldwell, 1998;de Ronde et al, 2002;Whiteford, 1996).…”