Repeating caldera collapse events constrain fault friction at the kilometer scale

Fault friction is central to understanding earthquakes, yet laboratory rock mechanics experiments are restricted to, at most, meter scale. Questions thus remain as to the applicability of measured frictional properties to faulting in situ. In particular, the slip-weakening distance dc strongly influences precursory slip during earthquake nucleation, but scales with fault roughness and is challenging to extrapolate to nature. The 2018 eruption of K̄ılauea volcano, Hawaii, caused 62 repeatable collapse events in which the summit caldera dropped several meters, accompanied by MW 4.7 to 5.4 very long period (VLP) earthquakes. Collapses were exceptionally well recorded by global positioning system (GPS) and tilt instruments and represent unique natural kilometer-scale friction experiments. We model a piston collapsing into a magma reservoir. Pressure at the piston base and shear stress on its margin, governed by rate and state friction, balance its weight. Downward motion of the piston compresses the underlying magma, driving flow to the eruption. Monte Carlo estimation of unknowns validates laboratory friction parameters at the kilometer scale, including the magnitude of steady-state velocity weakening. The absence of accelerating precollapse deformation constrains dc to be ≤10 mm, potentially much less. These results support the use of laboratory friction laws and parameters for modeling earthquakes. We identify initial conditions and material and magma-system parameters that lead to episodic caldera collapse, revealing that small differences in eruptive vent elevation can lead to major differences in eruption volume and duration. Most historical basaltic caldera collapses were, at least partly, episodic, implying that the conditions for stick–slip derived here are commonly met in nature.

Estimating plume heights of explosive eruptions using high-frequency seismic amplitudes

SUMMARY Seismic signals during explosive eruptions have been correlated to eruption size or eruption volume flux for individual eruptive episodes. However, the universality of these correlations has yet to be confirmed. We quantified the sources of high-frequency seismic signals associated with sub-Plinian and Vulcanian eruptions at Kirishima (Japan), Tungurahua (Ecuador) and other volcanoes in Japan using a simple approach based on highly scattered seismic waveform characteristics. We found that eruption plume heights scale to seismic source amplitudes and are described by two relations depending on the value of source amplitudes: power-law and exponential relations for plume height >6 km and <6 km, respectively. Though conceptually similar, our scaling relations differ from the previously proposed relation based on reduced displacement. By comparing seismic and geodetic data during sub-Plinian eruptions at Kirishima, we found that the source amplitude is proportional to eruption volume flux. Combining these relations, we show that our scaling relation for Plinian eruptions is consistent with predictions from plume dynamics models. We present a source model to explain the proportionality between the source amplitude and eruption volume flux assuming a vertical crack or a cylindrical conduit as the source. The source amplitude can be estimated in seconds without any complicated data processing, whereas eruption plumes take minutes to reach their maximum heights. Our results suggest that high-frequency seismic source amplitudes are useful for estimating plume heights in real time.

Volcanic architecture, eruption mechanism and landform evolution of a Plio/Pleistocene intracontinental basaltic polycyclic monogenetic volcano from the Bakony-Balaton Highland Volcanic Field, Hungary

Bondoró Volcanic Complex (shortly Bondoró) is one of the most complex eruption centre of Bakony-Balaton Highland Volcanic Field, which made up from basaltic pyroclastics sequences, a capping confined lava field (~4 km2) and an additional scoria cone. Here we document and describe the main evolutional phases of the Bondoró on the basis of facies analysis, drill core descriptions and geomorphic studies and provide a general model for this complex monogenetic volcano. Based on the distinguished 13 individual volcanic facies, we infer that the eruption history of Bondoró contained several stages including initial phreatomagmatic eruptions, Strombolian-type scoria cones forming as well as effusive phases. The existing and newly obtained K-Ar radiometric data have confirmed that the entire formation of the Bondoró volcano finished at about 2.3 Ma ago, and the time of its onset cannot be older than 3.8 Ma. Still K-Ar ages on neighbouring formations (e.g. Kab-hegy, Agár-teto) do not exclude a long-lasting eruptive period with multiple eruptions and potential rejuvenation of volcanic activity in the same place indicating stable melt production beneath this location. The prolonged volcanic activity and the complex volcanic facies architecture of Bondoró suggest that this volcano is a polycyclic volcano, composed of at least two monogenetic volcanoes formed more or less in the same place, each erupted through distinct, but short lived eruption episodes. The total estimated eruption volume, the volcanic facies characteristics and geomorphology also suggests that Bondoró is rather a small-volume polycyclic basaltic volcano than a polygenetic one and can be interpreted as a nested monogenetic volcanic complex with multiple eruption episodes. It seems that Bondoró is rather a “rule” than an “exception” in regard of its polycyclic nature not only among the volcanoes of the Bakony-Balaton Highland Volcanic Field but also in the Neogene basaltic volcanoes of the Pannonian Basin.

Development of a volcanic hazard model for New Zealand

We commence development of a volcanic hazard model for New Zealand by applying the well- established methods of probabilistic seismic hazard analysis to volcanoes. As part of this work we use seismologically-based methods to develop eruption volume - frequency distributions for the Okataina and Taupo volcanoes of the central Taupo Volcanic Zone, New Zealand. Our procedure is to use the geologic and historical record of large eruptions (erupted magma volumes ≥ 0.01 cubic km for Taupo and ≥ 0.5 cubic km for Okataina) to construct eruption volume-frequency distributions for the two volcanoes. The two volcanoes show log-log distributions of decreasing frequency as a function of eruption volume, analogous to the shape of earthquake magnitude-frequency distributions constructed from seismicity catalogues. On the basis of these eruption volume-frequency distributions we estimate the maximum eruption volumes that Taupo and Okataina are capable of producing at probability levels of relevance to engineers and planners. We find that a maximum eruption volume of 0.1 cubic km is expected from Taupo with a 10% probability in 50 years, while Okataina may not produce a large eruption at this probability level. However, at the more conservative 2% probability in 50 years, both volcanoes are expected to produce large eruptions (0.5 cubic km for Okataina and 1 cubic km for Taupo). Our study therefore shows significant differences in eruption probabilities for volcanoes in the same physiographic region, and therefore highlights the importance of establishing unique eruption databases for all volcanoes in a hazard analysis.

The Taupo eruption, New Zealand I. General aspects

The ca . a.d. 186 Taupo eruption was the latest eruption at the Taupo Volcanic Centre, occurring from a vent, at Horomatangi Reefs, now submerged beneath Lake Taupo in the central North Island of New Zealand. Minor initial phreatomagmatic activity was followed by the dry vent 6 km 3 Hatepe plinian outburst. Large amounts of water then entered the vent during the 2.5 km 3 Hatepe phreatoplinian ash phase, eventually stopping the eruption, though large amounts of water continued to be ejected from the vent area, causing gullying of the ash deposits. After a break of several hours to weeks, phreatoplinian activity resumed, generating the 1.3 km 3 Rotongaio ash, notable for its fine grainsize and for containing significant quantities of non- or poorly-vesicular juvenile material. The vent area then became dry again, and eruption rates and power markedly increased into the 23 km 3 Taupo ‘ ultraplinian ’ phase, which is the most powerful plinian outburst yet documented. Synchronous with this ultraplinian activity, lesser volumes of non- to partially-welded ignimbrite were generated by diversion of ejecta from, or partial collapse of, the eruption column. The rapid rate of magma withdraw al during this phase removed support from the vent area, to trigger local vent collapse and initiate the catastrophic eruption of the ca. 30 km 3 Taupo ignimbrite. Finally, after some years, lava was extruded on to the floor of the reformed Lake Taupo, and floating fragments derived from the lava carapace were driven ashore. The known eruption volume is more than 65 km 3 , while additional volumes are represented by primary material now beneath Lake Taupo and layer 3 to the ignim brite phases; a total volume of more than 105 km 3 is likely, equivalent to more than 35 km 3 of magma plus more than 3 km 3 of lithic debris. Airfall deposits more than 10 cm thick blanketed 30000 km 2 of land east of the vent, while ignimbrite covers a near-circular area of 20000 km 2 . Widespread and locally severe ground shaking occurred during, but mostly after the eruption, associated with subsidence in the Lake Taupo basin. Secondary deposits are abundant above and extending beyond the Taupo ignimbrite, consisting of the products of surface water interacting with the still-hot ignimbrite and subsequent water reworking of the light, pumiceous materials. The complexity and size of this eruption preclude accurate forecasting of the size, nature and return period of the inevitable next eruption from the Taupo Volcanic Centre.