The Taupō Volcanic Zone (TVZ) is the southern extent of the Tonga‐Kermadec volcanic arc and encompasses the Taupō Rift, with the two structures accommodating extension through the central North ...Island of New Zealand via magmatic and tectonic processes, respectively. Interplay between components of the TVZ and Taupō Rift are evident at Taupō volcano, which exhibits periods of increased seismicity on a decadal scale. One period of increased seismicity occurred in 2001 outside the caldera in the adjacent Taupō Fault Belt. We use seismological analysis to detect and characterize the Taupō Fault Belt seismicity, which aligns in clusters close to active faults, and was preceded by a large cluster of earthquakes beneath Lake Taupō's Western Bay. This cluster and a temporal change in the local stress field imply that a magmatic intrusion beneath the Western Bay initiated the unrest in the fault belt. Our analysis suggests this intrusion may have occurred outside the silicic reservoir at Taupō and that it represents an example of interaction between the regional tectonic and deep mafic magmatic systems at Taupō.
Plain Language Summary
The Taupō Volcanic Zone (TVZ) is a large area of volcanism that spans the central North Island of New Zealand. Active volcanism coincides with continental extension, named the Taupō Rift and both are the result of the interaction between the Australian and Pacific plates occurring offshore. At the center of the TVZ is Taupō volcano, a large caldera which undergoes regular phases of seismic unrest involving large numbers of earthquakes and ground shaking. A period of unrest in 2001 contrasts with the other unrest episodes in that most of the seismicity was concentrated in the Taupō Fault Belt rather than within the adjacent caldera, and we hypothesize that its occurrence may provide insight into the interaction between magmatic and tectonic processes around Taupō volcano. Our analysis suggests that the earthquakes in 2001 first occurred beneath Taupō volcano, initiating subsequent seismic activity in the Taupō Fault Belt and temporarily altering the surrounding stress state. We infer that the initial group of earthquakes beneath Taupō volcano coincided with and were triggered by a magmatic intrusion. Therefore, the seismicity in 2001 highlights an example of interaction between the tectonic and deep magmatic systems at Taupō volcano.
Key Points
Matched‐filter detection identified 3,782 earthquakes of ML 0.0–2.6 beneath and north of Lake Taupō between January and June 2001
Seismicity beneath Lake Taupō suggests an intrusion, possibly mafic, occurred prior to the seismicity within the Taupō Fault Belt
This implies direct interaction between the regional fault network and deep mafic feeder system at Taupō, which may occur elsewhere too
The Afar region in East Africa represents a key location to study continental breakup. We present an integrated structural analysis of the Western Afar Margin (WAM) aiming to better understand rifted ...margin development and the role of plate rotation during rifting. New structural information from remote sensing, fieldwork, and earthquake data sets reveals that the N‐S striking WAM is still actively deforming and is characterized by NNW‐SSE normal faulting as well as a series of marginal grabens. Seismicity distribution analysis and the first‐ever borehole‐calibrated sections of this developing passive margin show recent slip concentrated along antithetic faults. Tectonic stress parameters derived from earthquake focal mechanisms reveal different extension directions along the WAM (82°N), in Afar (66°N) and in the Main Ethiopian Rift (108°N). Fault slip analysis along the WAM yields the same extension direction. Combined with GPS data, this shows that current tectonics in Afar is dominated by the local rotation of the Danakil Block, considered to have occurred since 11 Ma. Earlier stages of Afar development (since 31–25 Ma) were most likely related to the large‐scale rotation of the Arabian plate. Various authors have proposed scenarios for the evolution of the WAM. Any complete model should consider, among other factors, the multiphase tectonic history and antithetic fault activity of the margin. The findings of this study are not only relevant for a better understanding of the WAM but also provide insights into the role of multiphase rotational extension during rifting and passive margin formation in general.
Plain Language Summary
The Earth's continents are in gradual but perpetual motion, driven by large plate tectonic forces in the Earth's deep interior. A crucial process is the stretching and breaking up of continents, initially forming localized rift or graben depressions, followed by the opening of a new ocean flanked by the margins of the newly divided continents (passive margins). Rifts and passive margins are important because of their vast resource potential (e.g., oil, gas, geothermal energy), but pose also severe risks (volcanism, earthquakes, and landslides) since they are often home to large populations.
We focus on the Western Afar Margin in Ethiopia, where the African continent is currently splitting apart, providing a unique research opportunity. In the framework of an international collaborative effort, we combined satellite imagery, topography data, field observations, GPS, and earthquake measurements in order to map out the present‐day geology and tectonic activity along the margin.
Our results show the complexity of tectonic plate movements: We find evidence for two phases of continental stretching in Ethiopia that both involve the rotation of tectonic plates. We also observe active faulting, which may help to assess seismic risks in the area. Our study helps to better understand passive margins worldwide.
Key Points
Structural, borehole, and seismicity data from the Western Afar Margin serve to study continental breakup processes and rotational rifting
Antithetic faults bounding marginal grabens are currently accommodating significant deformation along the margin
Current extension is due to rotation of the Danakil Block; a previous oblique extension phase was likely related to Arabian plate motion
Abstract
The role of lithospheric heterogeneities, presence or absence of melt, local and regional stresses, and gravitational potential energy in strain localization in continental rifts remains ...debated. We use new seismic and geodetic data to identify the location and orientation of the modern Nubia‐Somalia plate boundary in the 300‐km‐wide zone between the southern Main Ethiopian Rift (MER) and Eastern Rift (ER) across the Mesozoic Anza rift in the Turkana Depression. This region exhibits lithospheric heterogeneity, 45 Ma‐Recent magmatism, and more than 1,500 m of base‐level elevation change, enabling the assessment of strain localization mechanisms. We relocate 1716 earthquakes using a new 1‐D velocity model. Using a new local magnitude scaling with station corrections, we find 1 ≤
M
L
≤ 4.5, and a
b
‐value of 1.22 ± 0.06. We present 59 first motion and 3 full moment tensor inversions, and invert for opening directions. We use complementary geodetic displacement vectors and strain rates to describe the geodetic strain field. Our seismic and geodetic strain zones demonstrate that only a small part of the 300 km‐wide region is currently active; low elevation and high‐elevation regions are active, as are areas with and without Holocene magmatism. Variations in the active plate boundary's location, orientation and strain rate appear to correspond to lithospheric heterogeneities. In the MER‐ER linkage zone, a belt of seismically fast mantle lithosphere generally lacking Recent magmatism is coincident with diffuse crustal deformation, whereas seismically slow mantle lithosphere and Recent magmatism are characterized by localized crustal strain; lithospheric heterogeneity drives strain localization.
Plain Language Summary
Continental rifting involves the stretching, thinning, and heating of plates. Where present, magma adds heat, fluid, and new rock material into the plate, altering its physical and chemical properties. Pre‐existing lithospheric thickness variations and crustal shear zones may also influence when and where rifting occurs. Rifting processes can be monitored by tracking the vertical and horizontal movement of plates, and by analyzing earthquake patterns. To address debates concerning the role of plate thinning, magmatism, and pre‐rift lithospheric structures, we analyze new seismic and geodetic data from the Turkana Depression, a 300‐km‐wide region linking the Main Ethiopian Rift to the north and the Eastern Rift to the south. Additionally, the region has a failed Mesozoic rift with pre‐existing crustal shear zones. We identify regions that are actively deforming, defining the active plate boundary. The breadth of the actively rifting zone decreases above mantle low‐velocity zones, thinned mantle lithosphere, and/or hotter melt‐infiltrated zones. We also find little to no evidence of reactivation of faults within the thin crust of a failed Mesozoic rift that is circumnavigated by the active rifting in the region. Our findings suggest that differences in the plate‐scale physical and chemical properties dictate where and how faulting and magmatism occur.
Key Points
New 1‐D velocity model relocates 1716 earthquakes with local magnitude ranging from 1 to 4.5 and
b
‐value of 1.22 ± 0.06 in Turkana Depression
Brittle strain localizes in narrow areas overlying uppermost mantle low velocity zones and avoids previously thinned crust of Mesozoic rift
We redefine the modern Nubia‐Somalia plate boundary across the Turkana Depression; only a part of the 300‐km‐wide region is active
Although magmatism may occur during the earliest stages of continental rifting, its role in strain accommodation remains weakly constrained by largely 2‐D studies. We analyze seismicity data from a ...13 month, 39‐station broadband seismic array to determine the role of magma intrusion on state‐of‐stress and strain localization, and their along‐strike variations. Precise earthquake locations using cluster analyses and a new 3‐D velocity model reveal lower crustal earthquakes beneath the central basins and along projections of steep border faults that degas CO2. Seismicity forms several disks interpreted as sills at 6–10 km below a monogenetic cone field. The sills overlie a lower crustal magma chamber that may feed eruptions at Oldoinyo Lengai volcano. After determining a new ML scaling relation, we determine a b‐value of 0.87 ± 0.03. Focal mechanisms for 65 earthquakes, and 13 from a catalogue prior to our array reveal an along‐axis stress rotation of ∼60° in the magmatically active zone. New and prior mechanisms show predominantly normal slip along steep nodal planes, with extension directions ∼N90°E north and south of an active volcanic chain consistent with geodetic data, and ∼N150°E in the volcanic chain. The stress rotation facilitates strain transfer from border fault systems, the locus of early‐stage deformation, to the zone of magma intrusion in the central rift. Our seismic, structural, and geochemistry results indicate that frequent lower crustal earthquakes are promoted by elevated pore pressures from volatile degassing along border faults, and hydraulic fracture around the margins of magma bodies. Results indicate that earthquakes are largely driven by stress state around inflating magma bodies.
Key Points
Lower crustal earthquakes along projections of steep border faults
Local stress field rotation facilitates strain transfer from border fault to central zone of magma intrusion
Volatile release from magma intrusion localizes strain in early‐stage rift systems
Transform faults are a fundamental tenet of plate tectonics, connecting offset extensional segments of mid‐ocean ridges in ocean basins worldwide. The current consensus is that oceanic transform ...faults initiate after the onset of seafloor spreading. However, this inference has been difficult to test given the lack of direct observations of transform fault formation. Here we integrate evidence from surface faults, geodetic measurements, local seismicity, and numerical modeling of the subaerial Afar continental rift and show that a proto‐transform fault is initiating during the final stages of continental breakup. This is the first direct observation of proto‐transform fault initiation in a continental rift and sheds unprecedented light on their formation mechanisms. We demonstrate that they can initiate during late‐stage continental rifting, earlier in the rifting cycle than previously thought. Future studies of volcanic rifted margins cannot assume that oceanic transform faults initiated after the onset of seafloor spreading.
Key Points
We document the initiation of a proto‐transform fault during late‐stage continental rifting in Afar, Ethiopia
Surface faulting, seismicity, and geodetic observations reveal extensional transfer between magmatic segments
Numerical modeling predicts that the region between the offset magmatic segments will evolve to a stable oceanic transform fault
Ahi Tupua, the central section of the Taupō Volcanic Zone in the central North Island of New Zealand, encompasses Taupō and Ōkataina calderas and has been the most frequently active and productive ...silicic magma system worldwide during the Quaternary. The entire Taupō Volcanic Zone is underlain by the Hikurangi Subduction Zone where a Large Igneous Province, the Hikurangi Plateau, is being subducted, but Ahi Tupua exhibits much higher rates of magma output than either the northern or the southern sections. Trace element signatures of Ahi Tupua eruptive products suggest that both decompression melting and flux melting play a role in this rifted arc setting, with fluid flux signatures being more prevalent in regions of active caldera volcanism. Intermediate‐depth (50–300 km) earthquakes provide a means of studying faulting and fluid flow processes within and around the subducting slab beneath Ahi Tupua. Using data from the national seismic network and a 13‐station temporary network (“ECLIPSE”), we repick and relocate 397 intermediate‐depth earthquakes of magnitudes M2.5+ that occurred in a 29‐month interval, and compute focal mechanisms for a subset of 47 earthquakes. We observe some seismicity that may be within the mantle wedge but most of the relocated earthquakes occur within the crust and uppermost mantle of the subducted slab and exhibit a weak transition from predominantly normal‐faulting in the slab crust to predominantly reverse‐faulting in the slab mantle. No double seismic zone is observed beneath Ahi Tupua. These observations are consistent with dehydration of the slab and flow of slab‐derived fluids along existing faults into the mantle wedge, that drives flux melting and accounts for the distinctive geochemical signatures and voluminous output of Ahi Tupua calderas.
Plain Language Summary
Ahi Tupua is a volcanic region in the North Island of New Zealand that is the most active silicic volcanic system on Earth. It includes the active caldera volcanoes Taupō and Ōkataina. This volcanic region lies above the Hikurangi Subduction Zone, and studies of past eruptions suggest that Ahi Tupua's high rate of activity is driven by fluids released deep in the subduction zone. In this study, we use seismic data from across the entire North Island to investigate earthquakes occurring at depths of 50–300 km within the Hikurangi Subduction Zone. We show that these earthquakes suggest that the subducting oceanic crust is releasing a substantial amount of water into the overlying mantle, causing a large amount of melting and, ultimately, the distinctive volcanism of Ahi Tupua.
Key Points
We relocate 397 intermediate‐depth earthquakes beneath Ahi Tupua, central Taupō Volcanic Zone, and compute 47 focal mechanisms
Most seismicity occurs within the slab crust and mantle, with subordinate mantle wedge and plate interface faulting
Hypocenters and focal mechanisms are consistent with slab‐derived fluid fluxing melting of the mantle wedge beneath Ahi Tupua
Abstract Several studies suggest that large earthquakes ( M > 7.0) can act as external triggers of volcanic unrest, and even eruption. This triggering is attributed to either ground shaking (dynamic ...stresses) or to permanent ground deformation (associated with static stress changes). However, large earthquakes are rare and testing triggering hypotheses has proven difficult. We use geodetic data to show that the 13 November 2016 Kaikōura earthquake ( M w 7.8) triggered local deformation of up to 11 mm at Taupō volcano, 500 km away, which lasted for approximately twelve days. Using elastic geodetic models, we infer that the observed deformation was caused by either aseismic fault slip or a dike intrusion. We then use strong motion data from the surrounding area to show that the Kaikōura earthquake caused maximum dynamic stress changes in the range of 0.15–0.9 MPa in the vicinity of Taupō volcano and conclude that these dynamic stress changes triggered local faulting or dike activity and the associated deformation at Taupō volcano.
Plain Language Summary It has often been proposed that large earthquakes can trigger volcanic eruptions. However, large earthquakes are rare and testing these triggering hypotheses is therefore difficult. We show that the 13 November 2016 Kaikōura earthquake triggered local ground movement at Taupō volcano, 500 km away, which lasted for approximately two weeks. We show that the observed ground movement was caused by either slow movement on a fault or by the movement of magma at depth. We then use seismology data from the surrounding area to show that the Kaikōura earthquake caused large pressure changes in the vicinity of Taupō volcano. We conclude that the ground shaking from the earthquake caused local faulting or magmatic activity and the associated ground movement at Taupō volcano.
Key Points The M w 7.8 Kaikōura earthquake was followed by local deformation at Taupō volcano, c. 500 km away The deformation observed is consistent with either a dike intrusion or aseismic slip on a fault Dynamic stress changes in the range of 0.15–0.9 MPa caused by seismic waves from the Kaikōura earthquake triggered this response at Taupō
Several studies suggest that large earthquakes (M > 7.0) can act as external triggers of volcanic unrest, and even eruption. This triggering is attributed to either ground shaking (dynamic stresses) ...or to permanent ground deformation (associated with static stress changes). However, large earthquakes are rare and testing triggering hypotheses has proven difficult. We use geodetic data to show that the 13 November 2016 Kaikōura earthquake (Mw 7.8) triggered local deformation of up to 11 mm at Taupō volcano, 500 km away, which lasted for approximately twelve days. Using elastic geodetic models, we infer that the observed deformation was caused by either aseismic fault slip or a dike intrusion. We then use strong motion data from the surrounding area to show that the Kaikōura earthquake caused maximum dynamic stress changes in the range of 0.15–0.9 MPa in the vicinity of Taupō volcano and conclude that these dynamic stress changes triggered local faulting or dike activity and the associated deformation at Taupō volcano.
Plain Language Summary
It has often been proposed that large earthquakes can trigger volcanic eruptions. However, large earthquakes are rare and testing these triggering hypotheses is therefore difficult. We show that the 13 November 2016 Kaikōura earthquake triggered local ground movement at Taupō volcano, 500 km away, which lasted for approximately two weeks. We show that the observed ground movement was caused by either slow movement on a fault or by the movement of magma at depth. We then use seismology data from the surrounding area to show that the Kaikōura earthquake caused large pressure changes in the vicinity of Taupō volcano. We conclude that the ground shaking from the earthquake caused local faulting or magmatic activity and the associated ground movement at Taupō volcano.
Key Points
The Mw7.8 Kaikōura earthquake was followed by local deformation at Taupō volcano, c. 500 km away
The deformation observed is consistent with either a dike intrusion or aseismic slip on a fault
Dynamic stress changes in the range of 0.15–0.9 MPa caused by seismic waves from the Kaikōura earthquake triggered this response at Taupō
Abstract
Although magmatism may occur during the earliest stages of continental rifting, its role in strain accommodation remains weakly constrained by largely 2‐D studies. We analyze seismicity data ...from a 13 month, 39‐station broadband seismic array to determine the role of magma intrusion on state‐of‐stress and strain localization, and their along‐strike variations. Precise earthquake locations using cluster analyses and a new 3‐D velocity model reveal lower crustal earthquakes beneath the central basins and along projections of steep border faults that degas CO
2
. Seismicity forms several disks interpreted as sills at 6–10 km below a monogenetic cone field. The sills overlie a lower crustal magma chamber that may feed eruptions at Oldoinyo Lengai volcano. After determining a new
M
L
scaling relation, we determine a
b
‐value of 0.87 ± 0.03. Focal mechanisms for 65 earthquakes, and 13 from a catalogue prior to our array reveal an along‐axis stress rotation of ∼60° in the magmatically active zone. New and prior mechanisms show predominantly normal slip along steep nodal planes, with extension directions ∼N90°E north and south of an active volcanic chain consistent with geodetic data, and ∼N150°E in the volcanic chain. The stress rotation facilitates strain transfer from border fault systems, the locus of early‐stage deformation, to the zone of magma intrusion in the central rift. Our seismic, structural, and geochemistry results indicate that frequent lower crustal earthquakes are promoted by elevated pore pressures from volatile degassing along border faults, and hydraulic fracture around the margins of magma bodies. Results indicate that earthquakes are largely driven by stress state around inflating magma bodies.
Key Points
Lower crustal earthquakes along projections of steep border faults
Local stress field rotation facilitates strain transfer from border fault to central zone of magma intrusion
Volatile release from magma intrusion localizes strain in early‐stage rift systems
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