O OVGA é um Centro de Ciência que desenvolve atividade de investigação aplicada e de divulgação científica na área da Vulcanologia, da Sismologia e da Geotermia.
Implications of a Supervolcano Seismicity - Notícia OVGA 10-03-2020
Last year’s rumblings beneath New Zealand’s Taupō supervolcano, the site of Earth’s most recent supereruption, lend new urgency to research and outreach efforts in the region.
This aerial view shows Lake Taupō amid the whenua (land) of Ngāti Tūwharetoa on the North Island of New Zealand. This lake fills the caldera of a volcano that continues to alter the surrounding seismic and geothermal landscape. A new seismic network will provide a more detailed picture of the magmatic system and seismic activity in this culturally and economically important region.
Credit: Dougal Townsend/GNS Science
The deadly 9 December 2019 eruption at Whakaari (White Island) in New Zealand’s Bay of Plenty reminded the world of the variety of volcanoes in New Zealand capable of causing major impacts on modern society. However, the magnitude of this event was very small in comparison with other eruptions in the area’s geological past. Twenty-five millennia ago, another volcano in what is now New Zealand blasted a volume of ash and pumice into the air large enough to have covered all of California with 2.8 meters of debris. Since then, no other volcano on Earth has ejected more material.
Today, that volcano is known as Taupō, and it sits at one of the geographic and cultural focal points of New Zealand’s North Island (Figure 1). The odds of another such supereruption (an eruption that ejects at least 1,000 cubic kilometers of material) occurring at Taupō in the near future are effectively zero. But lower-magnitude eruptions have occurred there since, and the rumbling and shifting that continue under Taupō today are a source of ongoing concern and uncertainty that a hazardous blast could be on the horizon. Last year, Taupō was unusually seismically active, but scientists don’t know if that activity represents “business as usual” (volcanoes often show signs of stirring without any eruption) or not.
Any eruption today would have serious effects for people in the region, spreading ash and debris across the landscape [Barker et al., 2019]. An eruption would also affect Lake Taupō, which fills the volcano’s caldera and is the biggest freshwater lake in Australasia by volume. Lake Taupō and the surrounding volcanic landscape hold great cultural significance, especially to the Indigenous Māori people in the area. The region also provides important resources such as geothermal energy, agriculture, and forestry, and it sees more than a million tourists visit each year.
Serendipitously, the 2019 Taupō seismic unrest occurred during a research project called Eruption or Catastrophe: Learning to Implement Preparedness for future Supervolcano Eruptions (ECLIPSE), funded by the New Zealand government. ECLIPSE is a 5-year multi-institutional program designed to improve understanding of the physical state of New Zealand’s caldera volcanoes (including Taupō), identify the causes of unrest and tipping points that lead to eruptions, and develop decision support tools and mitigation strategies for the island nation.
Fig. 1. Lake Taupō and its geological features. The Oruanui caldera (light green) formed 25,500 years ago, and the Taupō caldera (dark green) formed in 232 CE. Red triangles show the locations of eruptions in the past 2,150 years. TVZ = Taupō volcanic zone.
A Turbulent Past, an Uncertain Future
The modern outline of Lake Taupō formed during Earth’s most recent known supereruption, the Oruanui event, which ejected more than 1,100 cubic kilometers of pumice and ash approximately 25,500 years ago [Wilson, 2001; Vandergoes et al., 2013]. Taupō has erupted at least 28 times since then, with the largest and most recent of these events occurring in 232 CE [Hogg et al., 2012], prior to the arrival of humans in New Zealand. This explosive eruption produced a high-speed flow of pumice, ash, and hot gases that devastated 20,000 square kilometers of the North Island [Wilson and Walker, 1985].
Variations in the composition of magmas erupted from Taupō over the past 12,000 years suggest that the mushy magma system below the volcano today has rebuilt to a substantial size (hundreds of cubic kilometers) and is capable of generating volumes of eruptible magma within only a few years to decades—a geologically rapid timescale [Barker et al., 2015, 2016].
Although future eruptions are probable in the long term, in the short term it is far more likely that volcanic unrest at Taupō will not lead to an eruption. However, such unrest could still cause significant social and economic disruption, given the volcano’s history and its reputation, which rests on the unusually large and exceptionally violent 232 CE eruption.
Signs of unrest, such as seismicity, deformation, and geothermal system changes, occur every few decades at Taupō [Potter et al., 2015]. However, because the volcano is located in an active tectonic rift, where portions of crust to the west and east of the volcano are pulling apart at about 1 centimeter per year, seismicity often occurs without any direct relationship to the magmatic system beneath Taupō. The presence of the rift makes it difficult to distinguish earthquakes that are related to volcanic unrest from those that are part of the background dynamic environment.
The 2019 Seismic Swarms
In 2019, Taupō experienced its most substantial seismic activity in several decades. New Zealand’s national monitoring network, GeoNet, reported more than 1,100 earthquakes within Taupō caldera during 2019, compared to the annual average of several hundred. The seismic activity occurred in discrete swarms starting in late April and culminated in a magnitude 5.2 earthquake in early September, the largest earthquake beneath the volcano since 1952.
The earthquakes occurred in locales where there is evidence of past young eruptive activity, like the 7,000-year-old lava dome called Motutaiko Island (Figure 1), and at depths of more than 5 kilometers in the crust, where Taupō’s magmatic system is inferred to reside [Barker et al., 2015]. Accurate earthquake detection and source location are crucial in allowing scientists to search for migrating earthquake swarms and source mechanisms suggestive of fluid interaction, both of which are telltale signs of magmatic unrest. However, the national seismic sensor network is sparse near Taupō (only five sensors are within 10 kilometers of the caldera), making it difficult to detect and locate small-magnitude earthquakes accurately, especially their depths.
As part of the ECLIPSE program and in response to the 2019 earthquake swarms, our team of researchers has installed eight additional seismometers—on loan for 2 years from the Incorporated Research Institutions for Seismology (IRIS)—around Taupō, and we are in the process of deploying another five. This will improve our understanding of and, ultimately, our monitoring capability for the volcano (Figure 2). Crucially, the deployment effort is being undertaken with the full involvement of local communities and landowners as well as emergency managers and the volcano observatory—we call this a coproduction approach to research.
Fig. 2. Gray circles show the locations of earthquakes in the 2019 swarms, from GeoNet. Blue triangles show the GeoNet seismic network; red triangles show the ECLIPSE seismic network. Asterisks denote planned sites.
Working with the People of the Land
Land ownership has been a contentious issue in New Zealand since European colonization, stemming largely from the differing views and interpretations of land ownership between the Indigenous Māori peoples and colonizers. The Ngāti Tūwharetoa iwi (tribe) are the kaitiaki (guardians) of the Taupō region, and they have a deep sense of belonging and inherent responsibility to the land. This relationship extends beyond notions of land ownership conventional in Western cultures and is more closely described as the people belonging to the maunga (mountains), moana (lake), and whenua (land).
Because of this, our research is intrinsically linked to understanding Ngāti Tūwharetoa’s whenua and taonga (cultural treasures). Installing seismometers can be viewed as intrusive, so we have sought to ensure that the Indigenous community is aware of, and approves of, our research techniques. For example, all of the new seismometer locations have been planned alongside the hapū (family groups) that have dominion over each site. This approach has engaged the hapū in our work and has allowed us to avoid impinging on waahi tapu (sacred sites), including burial sites, former settlements, and sacred landscapes. This consultation is vital because the locations of the numerous culturally significant sites around the lake are generally not known to people outside the iwi.
Our coproduced research represents a modern approach to volcano science in New Zealand, and it opens the opportunity for a level of outreach and engagement in the local community that has not previously been achieved. Conveying the concept of volcanic hazards is, however, a challenge in the context of the Māori worldview (Te Ao Māori), which posits the people as direct descendants of their whenua and maunga. Māori communities might not be receptive to the idea that these landscapes could pose hazards to the people, so hazard communication methods familiar to most Westerners are not effective.
Western concepts of science are often presented to Indigenous communities without much thought about cultural differences in how scientific knowledge is received and digested. The ECLIPSE project, however, includes Māori and other researchers who are investigating the effectiveness of science dissemination in communities in New Zealand as well as how we can integrate the Māori worldview into our scientific communication. With the findings from these efforts, we will be able to engage and inform the population living around the volcano more effectively and meaningfully. Our coproduction approach is also relevant for research in other countries, and we hope it contributes to helpful collaborations between Indigenous peoples and scientists elsewhere.
The ECLIPSE project is investigating the effectiveness of science dissemination in communities in New Zealand as well as how to integrate the Māori worldview into scientific communication. Credit: Dougal Townsend/GNS Science
Improved Understanding of Taupō
The new seismic network will dramatically increase the station coverage around Taupō. We have two main ambitions here: improved detection and location of earthquakes and direct imaging of the magma system beneath the lake. With the current GeoNet seismic network at Taupō, the earthquake catalog is complete only above magnitude 2.2, meaning that many smaller earthquakes are missed and accurately locating earthquake sources is problematic. Therefore, trying to understand geodynamic activity and define unrest events at Taupō has been challenging.
The addition of the new ECLIPSE seismic network will significantly improve the detection threshold and increase the size of the earthquake catalog for events in the Taupō area, allowing us to track seismicity and volcanotectonic behavior in much greater detail. The network will also allow researchers to directly image the magma system beneath Taupō using geophysical techniques. Because the system is concealed beneath Lake Taupō, this imaging has not been achieved before, resulting in uncertainties regarding the volume and current physical state of the magmatic system [Barker et al., 2015].
We will use the expanded earthquake catalog, alongside measurements of the ambient seismic noise field, to perform a joint inversion analysis of seismic velocities and create the first 3-D image of the crust beneath Taupō. This work should help reveal the extent and depth of the magmatic system and, potentially, details about its internal structure. It will also enable us to interpret earthquake activity, such as the 2019 seismic swarms, in the context of the magma system location and physical state. Such information is vital for considering the processes responsible for unrest at Taupō, making plausible estimates of future eruption sizes and locations, and providing appropriate advice through GeoNet to Ngāti Tūwharetoa, civil authorities, emergency managers, and the public.
Fonte: eos.org (05-03-2020)
1 - Localização do vulcão Taupō (Nova Zelândia)
2– Na coleção OVGA existe uma amostra de pedra-pomes de Taupō.