Volcanic Generation of Tsunamis: Two New Zealand Palaeo-Events

Abstract

Rapid emplacement of a mass via pyroclastic flows, or edifice failure, generates volcanic tsunamis. Physical modelling demonstrates that the efficiency of tsunami generation is influenced by the angle the mass enters the ocean. Efficiency decreases with increasing slope angle from 20° to 60°, before increasing to a maximum at 90°, which corresponds to a mass falling directly into the ocean without interacting with the slope (impact tsunami). Further, in the case of surging pyroclastic flows or regressive failures, successive closely spaced events may generate larger tsunami waves than a single event of comparable volume.It is difficult to assess if physical model results are meaningful for real world tsunami events due to limited observational data. This paper compares numerical models developed from physical simulations with palaeotsunami deposits from two New Zealand palaeo-events -pyroclastic flows from Mt Tarawera and edifice failure at Whakaari (White Island) -which constrains numerical simulations of the source mechanisms. The Mt Tarawera event involved multiple pyroclastic flows entering a lake during the AD 1314±12 Kaharoa Eruption. The interaction of multiple closely spaced pyroclastic flows is necessary to generate the 6-7 m maximum wave height inferred from near source tsunami deposits. Tsunami deposits in the Bay of Plenty, dated to 2962±52 BP, are consistent with edifice failure at Whakaari. In this case a single event with a volume of 0.23 km 3 is sufficient to account for the tsunami deposits. Hence, if the failure was regressive, the successive stages were sufficiently close together to be indistinguishable from a large single event.

Freshwater (paleo)tsunamis – a review

“…In 1314 AD, multiple pyroclastic flows emitted from Mt Tarawera during the Kaharoa eruption entered Lake Tarawera. This eruption coincides with the earliest evidence for human settlement in New Zealand (Hogg et al, 2003) and thus, represents a key dating event (De Lange and Moon, 2016). A paleotsunami deposit is described composed of two fining upward sequences overlying with a sharp, erosional basal contact with lacustrine silts.…”

“…These fining upward units consist of cobbles and gravels originating from beach deposit. Based on the threshold velocities for the entrainment of the different clasts in these units, these layers were interpreted as being the result of two waves of 7 and 1 m high, respectively (Magill, 2001;De Lange and Moon, 2016). De Lange and…”

“…The paleotsunami studies found in the literature are from Lakes Owen, Crescent, Managua, Lucerne, Geneva, Zurich, Sils, Two Yurts, Tekapo, Tarawera and Tahoe (Fig. 3 and Table 2) (Smoot et al, 2000;Schnellmann et al, 2002;Freundt et al, 2006;Moore et al, 2006;Dirksen et al, 2011;Kremer et al, 2014;Kremer et al, 2015;Leithold et al, 2019;De Lange and Moon, 2016;Strupler et al, 2018;Mountjoy et al, 2019;Nigg et al, submitted). Six studies report tsunami deposits in the sediments on J o u r n a l P r e -p r o o f lake shores (Lakes Managua, Tarawera and Sils) or within the basin (Lakes Owens, Two-Yurts and Tahoe)…”

“…Kempf et al, 2017). A review of the literature indicates that there are few publications with a specific focus on paleotsunamis generated in lakes (De Lange and Moon, 2016;Dirksen et al, 2011;Freundt et al, 2006;Kremer et al, 2014;Kremer et al, 2015;Leithold et al, 2019Moore et al, 2006Mountjoy et al, 2019;Nigg et al, submitted;Schnellmann et al, 2002;Strupler et al, 2018). There appear to be no publications referring specifically to river paleotsunamis or to paleo-meteotsunamis.…”