Tree-lined cliffs and hills rise from a coastal beach where ocean waves gently wash ashore.
Earthquakes, landslides, erosion, and storms all interact to shape and reshape subduction zone landscapes, including beaches and cliffs on the coast of Oregon. Credit: ArtTower/Pixabay, Pixabay license

In the complicated regions where tectonic plates converge and landmasses and oceans meet, chains of landscape-shaping events can play out over years or even centuries. Storms and earthquakes dislodge rocks, sediment, and soil. Flooding rivers and strong currents carry these materials downstream and disperse them offshore. Along the way, this debris can carve wider channels, shift the courses of the streams and currents, or dam flows altogether. The resulting geomorphic changes in landscapes and seascapes profoundly affect the safety and livelihoods of tens of millions of people.

Fig. 1. Subduction zone landscapes and seascapes, such as those along the Cascadia subduction zone (center diagram), are shaped by many processes belowground and aboveground that create hazards for vulnerable populations living in the vicinity of the subduction zones. Clockwise from top left: Earthquakes along megathrusts or upper plate faults can cause tsunamis, whose runup on land is controlled in part by coastal bathymetry; precipitation and earthquakes can cause mass wasting events like landslides; subduction zone volcanoes can reshape landscapes and cause many hazards, for example, from lava flows, lahars, and ashfalls; volcanic processes can also send large amounts of sediment and debris down rivers, posing threats from river aggradation and flooding; sediments transported down deep-sea trenches can contribute to slip (i.e., earthquakes) along megathrust faults; fault slip can lead to rapid uplift or subsidence of coastal areas; and earthquake shaking can cause submarine landslides, which may cause tsunamis and damage underwater infrastructure. Click image for larger version. Credit: U.S. Geological Survey

Understanding the various types of Earth surface disturbances in subduction zone landscapes and seascapes—and the cascading impacts of these disturbances—has enormous practical importance because they pose substantial hazards to the ecosystems, communities, and infrastructure in these areas (Figure 1). However, we still lack an adequate understanding of when catastrophic surface disturbances are initiated, where the detritus produced by these events goes, and how long and how far the cascading impacts can extend.

Likewise, uncertainties persist regarding controls on the catastrophic events associated with tectonic convergence of subducting and overriding plates. Unknowns include how and where rock and sediment deform because of this convergence; the amount of convergence that accumulates in subduction zone fore arcs (i.e., between the deep-sea trench and volcanic arc); and where, how, and why the associated strain becomes localized or not. These knowledge gaps are relevant because strain accumulation and release determine the locations and magnitudes of earthquakes, tsunamis, and seismically triggered mass wasting that may affect population centers (Figure 1).

Advances in scientific knowledge and technologies are allowing us to address fundamental questions about subduction zone geohazards.

Accelerating advances in scientific knowledge and technologies are allowing us to address fundamental questions about subduction zone geohazards, one pillar of the Subduction Zones in 4 Dimensions (SZ4D) initiative. SZ4D, which began in 2018, is capitalizing on these advances by engaging a broad group of scientists in a decadal-scale, interdisciplinary research effort. Below we highlight major questions motivating these scientists and highlight critical developing technologies and key research opportunities to advance our understanding of subduction zone geohazards through this initiative.

A Complex Web of Interacting Systems

After major geomorphic events, punctuated and sometimes catastrophic erosional pulses can occur and initiate complicated responses and continuous adjustments in landscapes and seascapes that persist for years to centuries [e.g., Bruni et al., 2021]. Slope failures caused by storms, volcanic sector collapses during eruptions, and earthquake-mobilized debris can dam river channels (Figure 1). These events can lead either to continuous adjustments in channel shape in response to changes in sediment supply or to outburst floods that rapidly alter river channel morphology [e.g., Cook et al., 2018], both of which can damage downstream communities. Large volumes of detritus (e.g., sediment, boulders, trees) delivered to river channels by such subduction zone disturbances, as well as by extreme weather events, can promote channel widening and sediment infilling of channels, modifying river networks for years to decades [e.g., Major et al., 2016]. These changes in channel morphology can threaten infrastructure built on the banks and floodplains and, by altering the channel’s capacity to convey river flows, result in more frequent flooding.

The hazards produced by earthquakes, volcanic eruptions, and geomorphic events reflect the long-term operation of subduction zone processes.

The hazards produced by earthquakes, volcanic eruptions, and geomorphic events reflect the long-term operation of subduction zone processes (Figure 1). For example, deformation of fore-arc crust can modify and be modified by sediment transport systems, build climate-altering topography, and produce ground failures [e.g., Bhattacharya et al., 2018]. Sediment transport can change the hazards posed by megathrust earthquakes, as delivery of sediment to the ocean trench might affect slip behavior and earthquake recurrence in a subduction zone [Lamb and Davis, 2003]. The redistribution of surface loading due to both long-term and event-driven erosion can affect the stress on upper plate faults, promoting more frequent earthquakes [Steer et al., 2020].

Volcanic and magmatic processes likewise build topography (Figure 1) [e.g., Karlstrom et al., 2018], sometimes producing eruptions, lava and debris flows, and other hazards in the process. Furthermore, the ascent of magma through the crust can alter the crust’s thermal state, producing rheological changes that can redistribute deformation throughout subduction zone systems. Communities within these landscapes are especially vulnerable to these hazards, the effects of which can be compounded by longer, more intense storms resulting from human-driven climate change.

New Tools for Deeper Understanding

New and emerging technologies allow us to study the processes in and around subduction zones in ways not previously possible, which has enabled a more complete and systems-level understanding. Advances in repeat mapping of Earth’s surface using high-resolution remote sensors aboard satellites, aircraft, drones, and submarine craft have revealed details of the initiation and long-term impacts of these events and of the transports of material they cause. Various types of sensors, including multispectral imagers, interferometric synthetic aperture radar (InSAR), and lidar, provide diverse data, with each type suited to particular landscapes, seascapes, and conditions.

High-resolution, repeat spaceborne imaging methods (e.g., InSAR and lidar) now allow us to locate where and when mass failures are initiated and, in some cases, to characterize rates of motion of transported materials (Figure 2) [Elliott et al., 2016; DeLong et al., 2012]. Suborbital plane and drone-based platforms, coupled with computer vision developments, allow images collected by simple camera systems to be used to build ultrahigh-resolution topography. This capability can help rapidly and cheaply characterize downstream effects, such as changes in river location and widespread deposition of sediment within rivers, produced over large areas by disturbances. Low-cost environmental sensor networks—measuring, for example, rainfall and soil moisture—allow for near-real-time monitoring and measurement of environmental conditions that also enable a better understanding of the triggering of mass wasting events and sediment transport. New submarine drone technologies, acoustic Doppler current profilers, and seafloor geodetic monuments can capture seascape changes produced by degradation of submarine fault scarps [Hughes et al., 2021] and sediment density currents that may be initiated by earthquake-generated submarine landslides (Figure 1).

Fig. 2. These average line-of-sight (LOS) velocity maps for two uninhabited aerial vehicle synthetic aperture radar (UAVSAR) flight paths (T33805 and T24500) over part of Northern California each cover the period between April 2016 and February 2018, illustrating the usefulness of the technique for identifying where and when mass failures are initiated and characterizing rates of motion of transported materials. Red and blue shading denotes active deformation, and changes in the activity of the landslides are based on comparison with previously published data. Digital elevation models are from OpenTopography and TanDEM-X. Credit: Handwerger et al. [2019]

These technologies are enabling, for the first time, both detailed and synoptic views of the ways that fore arcs deform in four dimensions, allowing researchers to begin to constrain the total energy budget of a subduction zone, which underlies our understanding of how solid Earth and atmospheric processes work together to produce geohazards. Simultaneously, developments in numerical methods and in computer hardware, which now provide petaflop-scale computational resources to researchers (compared with the teraflop-scale computations available 20 years ago), allow accurate simulations of the complex physics of subduction zone disturbances and their cascading impacts.

This holistic approach is foundational to better understanding the risks that hazardous tectonic events pose to communities occupying subduction zones.

Combined, these advances allow us to link observations and state-of-the-art numerical models to study the energetics and dynamics of the processes that shape subduction zone landscapes and seascapes. This holistic approach is foundational to better understanding the risks that hazardous tectonic events pose to communities occupying subduction zones.

Two Questions About Subduction Zone Geohazards

Researchers can leverage the suite of new observational technologies, computational capabilities, and model developments available to help investigate two overarching questions about subduction zone surface hazards identified by the SZ4D initiative. First, how do events within Earth’s atmosphere, hydrosphere, and solid Earth generate and transport sediment across subduction zone landscapes and seascapes? Second, what fraction of a subduction zone’s energy budget goes into building and shaping subduction zone landscapes and seascapes? These questions provide a framework for developing testable hypotheses that are of practical importance for understanding the risks of geohazards to humans.

The first question leads us to posit that the frequency with which different sediment-mobilizing drivers occur determines the extent to which these drivers control sediment generation and transport. When storms, atmospheric rivers, or other high-precipitation events mobilize sediments more frequently than do solid Earth events (e.g., earthquake shaking and volcanic unrest), the former will dominate sediment generation and transport [e.g., Major et al., 2021]. Conversely, solid Earth events play a larger role in shaping landscapes when such large storms are infrequent in an area [e.g., Bruni et al., 2021]. Thus, the ratio of the recurrence times of landscape-impacting storms to landscape-impacting solid Earth disturbances determines the relative imprints that atmospheric versus solid Earth processes have on shaping various parts of a subduction zone landscape.

Such framing provides a means of assessing the causes, timings, and locations of hazards and how these hazards might affect humans and infrastructure. It also allows us to anticipate how human-induced climate change alters the distribution of the processes giving rise to these hazards.

Regarding the second question, we might hypothesize that the partitioning of deformation in the upper plate of a subduction zone resulting from stress accumulation is regulated in part by plate motions and by coupling with the lower plate along the subduction megathrust. This partitioning is also regulated by the balance of elastic and inelastic deformation processes in the upper plate, the arrangement of rocks in the upper plate, and the ways in which they deform and flow in response to the weight of the topography [e.g., Penserini et al., 2017].

The partitioning of deformation is important because it affects the locations and scales of potentially hazardous events. For example, absorption of subduction zone convergence between the arc and the trench—and resulting upper plate deformation (Figure 1)—may manifest as particularly damaging earthquakes because these events often occur closer to human populations than larger events that occur along the subduction megathrust itself.

From Hypothesis to Experiment

Broad working hypotheses and conceptual models can be tested using experiments to explore driving factors in different parts, or segments, of subduction zone systems, holding most conditions constant while systematically varying a single factor. However, such experiments are impractical at the scale and complexity of entire subduction zones.

Nature has provided us with opportunities to compare subduction zone segments.

Fortunately, nature has provided us with opportunities to compare subduction zone segments where many of the relevant factors are known, thereby yielding insights into the effects of different individual factors. Similar to strictly controlled experiments, this approach can also be used to test quantitative models of geomorphic transport and fore-arc deformation within subduction zones.

For example, we might test hypotheses related to our first guiding research question on sediment generation and transport by selecting a pair of subduction zone segments that either have similar solid Earth characteristics or experience similar atmospheric conditions. We can explore the influence of atmospheric events by selecting subduction zone segments that share similar convergence rates, subduction angles (i.e., the angle at which the subducting plate dives below the surface), and frequency-magnitude relationships for large earthquakes and magmatic events but whose climatic parameters (e.g., precipitation, storminess, and temperature) differ. Differences in climatic conditions would presumably manifest within variations in the frequencies, magnitudes, and spatial extents of storms affecting the subduction zone landscapes and seascapes.

In this case, a successful experiment would require field measurements and data capturing the effects of single events and the integrated effects of multiple events. Furthermore, long-term data would be necessary to elucidate how these events combined to produce and transport mass across the subduction zone system over long timescales.

Comparisons of natural landscapes can be augmented by numerical modeling to help constrain the energy available for hazardous earthquakes, volcanic eruptions, and landslides.

The same data that characterize mass transport can be integrated with data on tectonic convergence and uplift, seismicity, and heat flux to constrain components of the energy budget that are similar or different between subduction zones, thus addressing our second research question. For example, we might identify and compare subduction zone segments that have similar convergence rates, degrees of seismogenic coupling between plates, magmatic fluxes, fore-arc structural complexity, and climate but have different topographies. In this way, we can isolate the interplay between topographic loading and upper plate deformation and magmatism. Such comparisons of natural landscapes may not allow perfect isolation of controlling factors, but they can be augmented by numerical modeling to help constrain the energy available for hazardous earthquakes, volcanic eruptions, and landslides.

The hypothetical experiments described above might be carried out by contrasting, for example, the Cascadia and southern Chilean subduction zone systems. Both have experienced large megathrust ruptures and are situated in temperate climates, and their arc physiographies, their degrees of subduction megathrust coupling, and the presence of a young downgoing oceanic slab in some areas are similar.

However, the rate of convergence along the southern Chilean subduction zone is far more rapid than along the Cascadia subduction zone. Comparing these zones offers a way, albeit an imperfect one, to investigate systematic effects of variations in the recurrence time of landscape-impacting solid Earth versus atmospheric processes. With a higher convergence rate, the rate of energy addition to the upper plate due to frictional coupling along the megathrust is also plausibly higher in the southern Chilean subduction zone system, indicating another informative comparison between it and the Cascadia system. This example comparison is one of many in which researchers could exploit natural geographic variations in subduction zone properties to advance understanding of subduction zone surface process geohazards.

Putting New Science into Action

Subduction zone landscapes host many communities that are vulnerable to the potent impacts of Earth surface processes operating in these areas. New measurement and modeling capabilities promise to substantially increase our understanding of the phenomena that produce these hazards.

A concerted effort organized under the SZ4D initiative provides a unique opportunity to take advantage of new capabilities to advance the science of subduction zone systems. The decadal-scale focus of the SZ4D Landscapes and Seascapes group promises to provide a synoptic view of the operation of surface processes and the hazards they create at the scale of an entire subduction zone system—a scale that has never been achieved as part of an integrated scientific effort. Strong stakeholder involvement will also accelerate the pace at which this science can be put into action to reduce the risks of these hazards to vulnerable subduction zone communities by developing partnerships between local stakeholder organizations and community groups.

References

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Author Information

Mong-Han Huang, Department of Geology, University of Maryland, College Park; Kristin Morell, Department of Earth Science, University of California, Santa Barbara; Alison Duvall, Department of Earth and Space Sciences, University of Washington, Seattle; Sean F. Gallen, Department of Geosciences, Colorado State University, Fort Collins; and George E. Hilley (hilley@stanford.edu), Department of Geological Sciences, Stanford University, Stanford, Calif.

Citation: Huang, M.-H., K. Morell, A. Duvall, S. F. Gallen, and G. E. Hilley (2022), Exploring subduction zone geohazards on land and at sea, Eos, 103, https://doi.org/10.1029/2022EO220159. Published on 25 March 2022.
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