Aleutian Subduction Zone Research Articles

The 2023 Alaska National Seismic Hazard Model

US Geological Survey (USGS) National Seismic Hazard Models (NSHMs) are used extensively for seismic design regulations in the United States and earthquake scenario development, as well as risk assessment and mitigation for both buildings and infrastructure. This 2023 update of the long-term, time-independent Alaska NSHM includes substantial changes to both the earthquake rupture forecast (ERF) and ground motion models (GMMs). The ERF includes numerous additions to the finite-fault model, considers two deformation models, and introduces updated declustering and smoothing algorithms in the gridded background seismicity model. For the Alaska–Aleutian subduction zone, megathrust earthquakes occur on an updated structural and segmentation model, and the moment magnitude (M) 8+ rupture and rate model include a logic tree branch that considers slip rates derived from geodetic models of interface coupling. The megathrust model considers multiple models of down-dip width, and magnitudes are computed using newly developed scaling relations. For subduction intraslab events and subduction interface events with M < 7, the 2023 update uses a smoothed seismicity model with rupture depths derived from Slab2. The 2023 model updates GMMs in all tectonic settings using the recently published Next Generation Attenuation Subduction (NGA-Sub) GMMs for subduction interface and intraslab events, and the NGA-West2 GMMs for active crustal settings. Collectively, additions and updates to the Alaska NSHM result in hazard increases across most of south-central Alaska relative to the previous model, published in 2007. These changes are primarily due to the adoption of updated rate models for the large-magnitude interface events and the NGA-Sub GMMs that have much higher aleatory variability (sigma), consistent with global observations, and that include models of epistemic uncertainty.

Extensional forearc structures at the transition from Alaska to Aleutian Subduction Zone: slip partitioning, terranes and large earthquakes

Extensional forearc structures at the transition from Alaska to Aleutian Subduction Zone: slip partitioning, terranes and large earthquakes

Mechanical Essence of Double-Arc Formation Along the Eastern Aleutian Subduction Zone

The free-air gravity anomaly along the Aleutian subduction zone exhibits a single set of negative and positive trench-parallel belts in the western region, whereas it exhibits doubled negative–positive trench-parallel belts in the eastern region. The eastern inner–western positive gravitational belt corresponds to the topographic chain of the Alaska Peninsula and the Aleutian Islands. However, the eastern outer positive gravitational belt does not coincide with the chain of the topographic outer-arc highs. In this study, we determined the across-trench profiles of the plate interface geometry for the western and eastern Aleutian subduction zones on the basis of the hypocentre distribution. The surface uplift rates computed from the dislocation-based two-dimensional subduction model for the Aleutian plate interface profiles adequately reproduced the western single-arc and eastern double-arc characteristics. The essential factors of the double-arc formation are a low subduction dip angle and a bimodal plate interface curvature distribution within the elastic lithosphere. The double-arc highs of the computed uplift rates more closely coincided with the gravitational highs than the current topographic highs. This implies that tectonic events in the past caused the topographic activity shift towards the continental shelf edge and the subsequent topographic readjustment under the current tectonic state.

Local-S shear wave splitting along the length of the Alaska–Aleutian subduction zone

SUMMARY The Alaska–Aleutian subduction zone represents an ideal location to study dynamics within a mantle wedge. The subduction system spans several thousand kilometres, is characterized by a slab edge, and has ample seismicity. Additionally, the majority of islands along the arc house broad-band seismic instruments. We examine shear wave splitting of local-S phases originating along the length of the subduction zone. We have dense measurement spacing in two regions, the central Aleutians and beneath Alaska. Beneath Alaska, we observe a rotation in fast splitting directions near the edge of the subducting slab. Fast directions change from roughly trench perpendicular away from the slab edge to trench parallel near the boundary. This is indicative of toroidal flow around the edge of the subducting Alaska slab. In the central Aleutians, local-S splitting is primarily oriented parallel to, or oblique to, the strike of the trench. The local-S measurements, however, exhibit a depth dependence where deeper events show more consistently trench-parallel directions indicating prevalent trench-parallel mantle flow. Our local-S shear wave splitting results suggest trench-parallel orientation are likely present along much of the subduction zone excited by the slab edge, but that additional complexities exist along strike.

New Data on the Rock and Mineral Composition of Kharchinsky and Zarechny Volcanoes (Central Kamchatka depression): Heterogeneity of the Mantle Source and Peculiarities of Magma Evolution in Crust

The Kharchinsky and Zarechny volcanoes and the Kharchinsky Lake zone of monogenetic cones are unique eruptive centers of magnesian lavas located above the northern margin of the Pacific Plate subducting beneath Kamchatka. This work presents new geochemical data on the composition of rocks (55 samples) and minerals (over 900 analyses of olivine, pyroxenes, amphibole, and plagioclase) of these centers analyzed by XRF and LA-ICP-MS (rocks) and electron microprobe (minerals). Most of the studied rocks are represented by magnesian (Mg# = 60–75 mol. %), medium-K basalts and basaltic andesites. Moderate-magnesian (Mg# = = 52–59 mol. %) basaltic andesites are present among the monogenic cones of the Kharchinsky Lake. The rare rock varieties include high-K basalts-basaltic andesites of dikes in the center of the Kharchinsky volcano and magnesian andesites (Mg# = 58–61 mol. %) of the extrusions of Zarechnу volcano. The distribution of trace element contents in these samples demonstrates the enrichment of large-ion lithophile elements, light REEs and depletion of high field strength elements and heavy REEs typical of arc rocks. High-K basalts and basaltic andesites show anomalous enrichment in Ba (1000 ppm), Th (3.8 ppm), U (1.8 ppm), Sr (800 ppm, Sr/Y 50) and light REE (La 20 ppm); their compositions are close to those of low-Si adakites. Basalts and basaltic andesites contain high-Mg olivine phenocrysts (up to Fo92.6) and clinopyroxene (Mg# up to 91 mol. %). The rocks show petrographic and geochemical signs of fractional crystallization along with the processes of mineral accumulation and magma mixing. Some of the olivine phenocrysts show high NiO contents (up to 5000 ppm) and elevated Fe/Mn ratio (up to 80), interpreted as evidence of participation of the pyroxenite source in the magma generation processes. The use of Ca/Fe and Ni/Mg ratios allowed us to distinguish the composition fields and evolution trends of olivines associated with different sources – peridotite and pyroxenite, formed by the reaction of mantle wedge peridotites and high-Si melts of the subducted oceanic crust. The new data are consistent with other evidence of melting of the subducted Pacific plate edge beneath the northern part of the Central Kamchatka depression at the Kurile-Kamchatka and Aleutian subduction zone junction and testify to significant heterogeneity of the mantle in this area.

Surface Wave Isotropic and Azimuthally Anisotropic Dispersion Across Alaska and the Alaska‐Aleutian Subduction Zone

AbstractComprehensive observations of surface wave anisotropy across Alaska and the Aleutian subduction zone would help to improve understanding of its tectonics, mantle dynamics, and earthquake risk. We produce such observations, using stations from the USArray Transportable Array, regional networks across Alaska, and the Alaska Amphibious Community Seismic Experiment in the Alaska‐Aleutian subduction zone both onshore and offshore. Our data include Rayleigh and Love wave phase dispersion from earthquakes (28–85 s) and ambient noise two‐ and three‐station interferometry (8–50 s). Compared with using two‐station interferometry alone, three‐station interferometry significantly improves the signal‐to‐noise ratio and approximately doubles the number of measurements retained. Average differences between both isotropic and anisotropic tomographic maps constructed from different methods lie within their uncertainties, which is justification for combining the measurements. The composite tomographic maps include Rayleigh wave isotropy and azimuthal anisotropy from 8 to 85 s both onshore and offshore, and onshore Love wave isotropy from 8 to 80 s. In the Alaska‐Aleutian subduction zone, Rayleigh wave fast directions vary from trench parallel to perpendicular and back to parallel with increasing periods, apparently reflecting the effect of the subducted Pacific Plate. The tomographic maps provide a basis for inferring the 3‐D anisotropic crustal and uppermost mantle structure across Alaska and the Aleutian subduction zone.

FOCAL MECHANISMS OF EARTHQUAKES IN THE SUBDUCTION ZONE OF THE WESTERN PACIFIC PLATE

Deformation features of the subducting Pacific lithospheric plate are considered according to the data on earthquake focal mechanisms. The territory includes the convergent boundaries between the Pacific Plate and the North American (in the Aleutian arc region), the Okhotsk, the Eurasian and the Philippine plates.It has been shown that the angle of subducting Pacific Plate in the Aleutian subduction zone affects the focal mechanisms of earthquakes that occurred in the upper, 35 km part of the oceanic plate in the zone of its bending. There occur normal-fault earthquakes at a steep-angle subduction and rare thrust earthquakes at a shallow-angle subduction. The azimuthal orientation of P-axes of the focal mechanism solutions in the upper (1–70 km) contact zone corresponds to the Pacific Plate displacement vector when the plate fragments are subducting west-northwestwards. There occurs a change in azimuthal orientation of the compression axes in the subducting plate at a depth of more than 70 km: the axes occupy different azimuthal sectors showing difference in the orientation of their slope, with the orientations of the T-axes become multidirectional.The calculation of seismotectonic deformations was carried out based on the data on focal mechanisms of 7768 earthquakes. It was revealed that the Exx and Ezz deformation fields are the most homogeneous at depths of 1–70 km. The pattern of seismotectonic deformations changes abruptly for deep parts of the subducting plate (105–200, 200–400, and 400–700 km), there are observed heterogeneous deformation fields Exx, Eyy and Еzz with alternating episodes of extension and shortening.There has been proposed the author’s scheme of the influence of the upper mantle convection structure on the geometry of the subducting plate (slab) as a potential catalyst for the processes responsible for the separation of seismic activity zones and the change of earthquake types with depth and in different parts of the extended subduction zone.

Aseismic slip and recent ruptures of persistent asperities along the Alaska-Aleutian subduction zone

The frictional properties and slip behaviors of subduction thrusts play a key role in seismic and tsunami hazard assessment, especially in weakly coupled “seismic gaps”. Here, we rely on GPS observations in the Shumagin Gap of the Aleutian subduction zone to derive the slip distribution of the 2020 Mw 7.8 Simeonof Island, Alaska earthquake and of the subsequent afterslip during the first 87-day period. Our modeling results show that the mainshock ruptured at depths of ∼30–40 km beneath Simeonof Island. Kinematic and stress-driven models indicate that the afterslip occurred both updip and downdip of the mainshock rupture. Physically plausible locking models derived from interseismic GPS velocities suggest that the 2020 Simeonof and 2021 Mw 8.2 Chignik earthquakes ruptured persistent asperities on the subduction thrust. We infer that there are several additional persistent asperities at depths of 20–50 km west ∼157°W. However, it is still uncertain whether there are additional locked asperities at shallow depths because of the current lack of geodetic observations close to the trench.

Observation of stagnant slab material in the lower transition zone beneath the Aleutian Trench using teleseismic underside reflections

SUMMARY The goal of this project is to investigate the mantle transition zone (TZ) across the Aleutian subduction zone using SdS underside reflections. The Aleutian subduction zone is chosen as our study area because of its significant tectonic activity, coupled with a high-density data set of SdS midpoints, largely obtained from the U.S. Transportable Array. Seismic images were made using the wavefield iterative deconvolution stacking method. Our results are corroborated by comparing them with velocity anomalies observed in the 3-D GyPSuM Earth velocity model. The results of our investigation show that where the subducting Pacific Plate passes through the TZ, the 410 discontinuity is elevated by up to 20 km, and the 660 discontinuity is depressed by up to 40 km. We interpret the variations in the depth to the boundaries of the TZ in terms of Clapeyron slope of the olivine phase changes hypothesized to be responsible for these discontinuities. In this model, the 410 discontinuity is caused by a phase change of olivine to wadsleyite and has a positive Clapeyron slope, while the 660 discontinuity (phase change) represents a phase change of ringwoodite to perovskite and ferropericlase, and has a negative Clapeyron slope. Also, in the TZ, the 520 discontinuity (a phase change from wadsleyite to ringwoodite) occurs over a 30 km interval, resulting in a boundary that is too gradational to be observed globally in seismic imaging. However, in this study, the 520 is observed in regions close to the cold subducting slab in the Aleutian trench. We suggest this observation is a result of mantle chilling of the lower half of the TZ where the cold subducting Pacific slab does not penetrate the 660 km discontinuity, thereby cooling the mantle beneath the 520. This chilling of the mantle appears to sharpen the velocity contrast at the 520 depth. Finally, we infer that the Pacific slab pools atop the 660 discontinuity and undergoes dehydration that contributes to the observed deepening of the 660.

Особенности сейсмического процесса в западной части Алеутской субдукционной зоны и их возможная связь с климатическими изменениями в Арктике

In the second half of the XX century, there was a pronounced increase in the average annual temperature in the troposphere surface layer of the Russian Arctic region, which led to a scaling-up in the risks of natural disasters associated with the cryolithosphere degradation. Modern integrated studies of the Arctic have revealed a significant influence of geodynamic processes on the permafrost state. Previously, a seismogenic-trigger mechanism of the occurrence of climate warming phases in the Arctic proved to be possible due to strong mechanical disturbances of the marginal region of the Arctic lithosphere, caused by major earthquakes in the Aleutian subduction zone. The authors discuss the features of the seismic cycle in the Aleutian subduction zone, characterized the presence of a pronounced tangential component of the convergence vector of lithospheric plates. The study shows that the orientation of the plate convergence vector relative to subduction zone axis can have a significant impact on the preparation and occurrence of major earthquakes in subduction zones. In particular, the analysis of the seismic activity occurring in the western part of the Aleutian island arc showed that the seismic cycles here are shorter than in the eastern part of the arc. The authors revealed that major earthquakes, repeated in the same areas of the western part of the Aleutian subduction zone, differ both in magnitude and foci length. Taking into account the oblique subduction setting, the authors propose a keyboard model for the generation of major subduction-associated earthquakes as a mechanism potentially capable to explain the reduction in the seismic cycle duration and noticeable differences in the spatial extent and the foci localization of seismic events with similar magnitudes observed in the same segment of the western half of the Aleutian subduction zone.

Features of the Largest Earthquake Seismic Cycles in the Western Part of the Aleutian Subduction Zone

We discussed the peculiarities of the seismic cycle in Aleutian subduction zone, characterized by an oblique subduction setting. It was shown that the orientation of the plate convergence vector relative to the subduction zone axis can have a significant impact on the preparation and occurrence of the largest earthquakes in subduction zones. In particular, from the analysis of the seismic activity occurring in the western part of the Aleutian island arc, it was found that the seismic cycles here are shorter than in the eastern part of the arc. It was revealed that the strongest earthquakes, repeating in the same areas of the western part of the Aleutian subduction zone, differ both in magnitude and length of the fault zone. Taking into account the oblique subduction setting, we proposed the keyboard model of the largest megathrust earthquakes generation as a mechanism potentially capable of explaining the reduction in the seismic cycle duration and noticeable differences in the spatial extent and localization of the fault zones of events with similar magnitudes occurring in the same segment of the western half of the Aleutian subduction zone.

Effects of Depth of Fault Slip and Continental Shelf Geometry on the Generation of Anomalously Long‐Period Tsunami by the July 2020 Mw 7.8 Shumagin (Alaska) Earthquake

AbstractThe July 2020 Mw 7.8 Shumagin earthquake occurred in the seismic gap region along the Aleutian subduction zone. This interplate earthquake generated a small tsunami, but with unusual long‐period waves ranging between 40 and 90 min. We examined the cause of such an anomalous ocean wave through a source modeling inverted from tsunami and geodetic data. Our model indicates that the plate‐boundary rupture area was confined at depths of 20–40 km, although the slip resolvability decreases with depth. The coseismic seafloor displacement predominantly took place on the shallow continental shelf. Therefore, the initial water surface displacement at a mean water depth of ∼200 m is responsible for the long‐period waves, because tsunami period is inversely proportional to the square root of water depth. Furthermore, tsunami modeling implies that slip shallower than 20 km depth in the Aleutians would displace the seafloor beyond the continental shelf and generate shorter tsunami periods.

Stress Interactions between an Interplate Thrust Earthquake and an Intraplate Strike-Slip Event: A Case Study of 2018 Mw 7.9 Gulf of Alaska Earthquake

ABSTRACT Most major earthquakes that have occurred in Alaska are related to rupture of the megathrust along the Alaska–Aleutian subduction zone. Large intraplate earthquakes in the oceanic lithosphere are rare. Recently, one large intraplate earthquake occurred near the Alaska subduction zone: the Mw 7.9 Gulf of Alaska earthquake (23 January 2018). The stress interaction between interplate megathrust and intraplate earthquake has been of interest because it helps to understand stress transfer and seismic hazard estimation. To explore the phenomenon of stress transfer, a viscoelastic spherical finite-element-method model accounting for topographic relief and lateral viscosity variations was used to calculate time-dependent stress transfer associated with the 1964 Mw 9.2 Alaska earthquake. We used postseismic Global Positioning System measurements to probe the differences in rheological properties between the continental mantle and oceanic mantle. The best-fitting model determined the viscosities of the continental mantle and oceanic mantle to be 3×1019 and 1×1020 Pa·s, respectively. Then, we calculated the viscoelastic stress transfers based on this viscosity structure. Because of viscoelastic stress loading associated with the 1964 Alaska earthquake, the ΔCFS values on the 2018 Alaska earthquake rupture fault exceeded the threshold (0.01 MPa) for triggering earthquakes.

Seismic evidence on the complex rupture system of the 23 January 2018 MW 7.9 offshore Kodiak event

In this study, we probe the formation mechanisms of the 23 January 2018 Mw7.9 Kodiak earthquake in the Gulf of Alaska region from detailed structural analysis, combined with the regional crustal stress status evaluation via seismology methods measured by the b‐value imaging. The large earthquake not only aroused heated debate due to its intraplate deformation and complex rupture system approaching to the Aleutian subduction zone, but also the complex rupture system. Aftershocks of this event propagated along the ENE‐striking and NNW‐striking fractures in planar view, whereas it shows double‐layer seismicity zones at depth. The b‐value image indicates the highly fractured crustal stress status above 12 km and above 25 km, respectively, while the faulting was locked at a depth between 12 and 20 km. We suggest that mantle peridotite serpentinization induced the aseismic zone gap. Besides, the bathymetric map, the multichannel profiles, and the sub‐bottom seismic evidence across the study area have shown the sinistral seismogenic strike‐slip faults may have been reactivated from the deep‐seated oceanic fracture zones involving the subduction front. In addition, all these faults conjunction has formed the topographic uplift or fault blocks and contribute to the complex earthquake ruptures.

Ultra-long period and small-amplitude tsunami generated following the July 2020 Alaska Mw7.8 tsunamigenic earthquake

The July 2020 Mw7.8 Alaska tsunamigenic earthquake was a fresh call for potential large tsunamis associated with the Aleutian subduction zone. The second largest ever-recorded earthquake worldwide (Mw 9.2) occurred in this zone in 1964 indicating its massive earthquake and tsunami risk. Our analysis of the July 2020 tsunami revealed that it involves very long period waves (51–64 min) which is unusual for an Mw7.8 earthquake. The tsunami coastal amplitude was small (~0.5 m) which is much smaller than that usually expected from a tsunamigenic earthquake of this size. Here, through numerical simulations and spectral analyses, we explain the ultra-long period and small amplitude waves of the tsunami. Our analysis using an analytical equation showed that the ultra-long period of the tsunami (51–64 min) can be reproduced using the shallow water depth around the source region (100–200 m) and the length of the coseismic deformation area (~100 km). By comparing the coastal amplitude of this event with four other similar-size and similar-mechanism earthquakes (thrust Mw7.8), we attribute the relatively small coastal amplitude of this tsunami to the deep focal depth of the earthquake (28 km) and the extremely shallow water around the source region.

Tsunami Resonance Characterization in Japan Due to Trans‐Pacific Sources: Response on the Bay and Continental Shelf

AbstractThe tsunami resonance leads to the amplification of wave heights and extends the duration of wave activity. For tsunami warning and coastal planning, it is important to understand resonance behavior. We characterized the tsunami resonance of trans‐Pacific events in the bays and continental shelves of Japan, focusing on the Hokkaido and Sanriku regions. We considered five tsunami events with sources in the Peru–Chile subduction zone and the Alaska–Aleutian subduction zone. Spectral analysis was applied to observed and synthetic waveforms at coastal tide gauges and offshore GPS gauges to study the frequency content of tsunami energy, and modal analysis was conducted to investigate the eigen‐modes of the natural oscillation affected by regional and local bathymetry. It was found that the signatures of trans‐Pacific tsunami sources are lost in the spectra at most coastal gauges, but they partly remain in the spectra at offshore gauges, which are affected less significantly by topography or bathymetry. The predominant periods in the spectra at most coastal gauges match the eigen‐periods of natural oscillation calculated by modal analysis. Hence, we confirmed that regional or local bathymetry is the dominant factor in the resonance behaviors of trans‐Pacific tsunami events, rather than the source location or geometry. These findings are based on the systematic assessment of trans‐Pacific sources and have implications for tsunami warning and coastal planning along the Pacific coast of Japan.

Деформационные тектонические волны как возможный триггерный механизм активизации эмиссии метана в Арктике

The authors consider a model of tectonic waves in the lithosphere, caused by the strongest earthquakes in the Aleutian subduction zone and propagating from it into the Arctic shelf and adjacent land areas. They demonstrate that changes in thermodynamic conditions at the boundary between the rigid lithosphere and viscous asthenosphere can generate traveling wave disturbances. The mechanism of this phenomenon is associated with the fact that the rate of movement of the base of the lithosphere, which is the solidus line, does not coincide with the rate of movement of its surface, and the viscous reaction of the asthenosphere becomes a function of the kinetics of the melting process. The emerging traveling tectonic wave generates a decrease in normal stresses in the near-surface layer of the earth’s crust, which, according to the L. I. Lobkovsky hypothesis, leads to the activation of methane emission from the accumulation zones of metastable relict gas hydrates and the onset of sharp warming phases of the actual climate in the Arctic.

Seismogenic-Triggering Mechanism of Gas Emission Activizations on the Arctic Shelf and Associated Phases of Abrupt Warming

A seismogenic trigger mechanism is proposed to explain the abrupt climate warming phases in the Arctic as a result of strong mechanical disturbances in the marginal region of the Arctic lithosphere. Those disturbances might have been caused by great earthquakes in the Aleutian subduction zone, and slowly propagated across the Arctic shelf and adjacent regions, triggering the methane release from permafrost and metastable gas hydrates, followed by greenhouse gas emissions into the atmosphere. The proposed mechanism is based on the identified correlation between the series of the great earthquakes in the Aleutian island arc, which occurred in the early and middle of the 20th century, and the two phases of sharp climate warming, which began in 1920 and 1980. There is a 20-year time lag between these events, which is explained by the time of arrival of deformation waves in the lithosphere (propagating with a velocity of about 100 km per year) at the Arctic shelf and adjacent land from the Aleutian subduction zone, the region of their generation. The trigger mechanism causing the methane release from permafrost and metastable gas hydrates is related to the destruction of micro-sized ice films covering gas hydrate particles, the elements highly important for hydrate self-preservation, as well as destruction of gas-saturated micropores in permafrost rocks due to the slight additional stresses associated with deformation waves, and thus emergence of conditions favorable for gas filtration and its subsequent emission.

Aleutian island arc magma production rates and primary controlling factors

Island arc magmatism is considered a major mechanism of continental crustal growth; hence, the variation in island arc magma production rates and its influencing factors are of great importance. The along-arc strike variation in the island arc magma production rate is important for elucidating whether the subduction zone temperature is a controlling factor on arc magmatism. No back-arc spreading or ridge subduction is occurring in the intraoceanic subduction region of the Aleutian subduction zone, and the along-arc temperatures vary due to differences in the subducting plate age, convergence rate, and slab dip. Therefore, this is an ideal region to study the correlation between magma production rates and subduction zone temperatures. However, previous studies estimated the magma production rates at only a few locations due to the insufficient coverage of seismic data. Here, we employ the gravity inversion method based on density modelling and seismic data constraints to calculate the present-day arc crustal thickness and then obtain the magmatic thickness by removing the pre-existing crustal thickness from the present-day arc crustal thickness. Finally, the magma production rates along the main intraoceanic subduction region of the Aleutian Arc are mapped by estimating the island arc magmatic thickness and the most recent subduction inception dating result. In addition, the subduction zone temperatures and mantle melt amounts along five profiles across the Aleutian Arc are calculated. The magma production rate variation is highly correlated with the mantle melt amount. The slab temperature controls the extent of the slab dehydration, which is essential for mantle wedge melting, and the mantle wedge temperature controls the size of the hydrated mantle melting region and the melting fraction therein. Therefore, as the subduction zone temperature controls the mantle melt amount, we suggest that the subduction zone temperature is the first-order controlling factor on the magma production rate.

Great Pending Himalaya Earthquakes

Abstract Geological, historical, and geophysical observations show that the entire Himalayan arc is poised to produce a sequence of great earthquakes, possibly similar to that which occurred in the twentieth century along the Aleutian subduction zone. The human catastrophe in the densely populated countries astride the arc is likely to be unprecedented when these earthquakes occur.