Friday, May 16, 2025

Why oceanic subduction zones show contrasting seismicity

Thermal structure, fluid activity and earthquake mechanisms of oceanic subduction zones

Science China Press

Schematic diagram of structure and earthquake distribution in oceanic subduction zones — image:
(a) Geological units and earthquake distribution of an oceanic subduction zone. The orange shadow beneath the volcanic arc represents partially molten areas and magma channels. (b) Thermal structure of an oceanic subduction zone, modified from modeling results of Peacock and Wang (2021) for NE Japan. Black dashed lines are isotherms in ºC. Abbreviations: SSE, slow slip events; ETS, episodic tremor and slip.
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Credit: ©Science China Press

How the heat exchange between the subducted plate and the mantle wedge affects the fluid/melt migration and earthquakes in subduction zones is critical for our understanding of material and energy cycles during plate convergence. Earthquakes in the overriding plate often occur along thrust faults in the upper crust under compression. Interplate earthquakes not only include destructive megathrust earthquakes at the subduction plate boundary, but also slow earthquakes with longer source durations and lower frequencies. Intraslab earthquakes take place from the outer rise to the top of the lower mantle, showing variable focal mechanisms and distribution patterns from place to place.

“Because temperature is a controlling factor of mineral dehydration reactions and the brittle-ductile transition in rocks, the key to the contrasting seismicity in different oceanic subduction zones is thermal structure” explains Qin WANG, professor at School of Earth Sciences and Engineering at Nanjing University and corresponding author of the study.

Thermal structure of oceanic subduction zones

Geodynamic modeling results indicate the plate convergence rate is the primary factor controlling the thermal structure of subduction zones. Younger slabs exhibit higher slab surface temperature, and slower plate convergence velocity leads to higher slab surface temperature at depths less than 70 km. The maximum depth of decoupling (MDD) between the slab and the mantle wedge is 70–80 km. Updip of the MDD, the cooling effect of the subducting slab results in a "cold corner" in the forearc mantle wedge. Downdip of the MDD, the subducting slab drags the overlying mantle to move together (fully coupled) and induces the corner flow in the mantle wedge, which heats the slab surface rapidly.

It is interesting to notice that 70–80 km is also the peak exhumation depth of metamorphic rocks along the oceanic subduction plate interface. Mechanisms of the MDD remain unclear. Comparison of rock P-T records with thermal models of subduction zones reveal that exhumation of metamorphic rocks mainly occurs at high geothermal gradients at the beginning or ending subduction stages. Therefore, it is important to consider the effects of “survivorship bias” in geological records. Due to the widespread occurrence of oblique subduction and the thermal structure evolution of subduction zones, it is necessary to integrate global and regional scale observations, 3D thermodynamical modeling of subduction zones and global plate reconstructions.

Structure and deformation of the subduction plate interface

The subduction plate interface, also referred to as the subduction channel, comprises the roof décollement, the basal décollement, and a deformation zone between them. In partial locking areas, strain localization in weak rocks or lithologic contact could cause downward or upward migration of the basal and roof décollements. Consequently, rocks exhumed from the oceanic subduction channel are very complex. They could originate from the subducted slab, the forearc crust, and peridotites and pyroxenites from the mantle wedge.

The mechanical coupling degree and deformation of the subduction plate interface are controlled by temperature and lithology. From the trench to depths of 40–50 km, decoupling between the subducting plate and the relatively fixed upper plate is characterized by megathrust earthquakes along the subduction plate interface. Below depths of 70–80 km, the comparable rheological strength between the subducting plate interface and the mantle wedge causes viscous coupling. As a result, earthquakes distribute within the slab. Between the decoupling and viscous coupling domains, the subducting slab is partially coupled with the mantle wedge. Topography variations, material mixing and heterogeneous fluid activity in the subduction plate interface will affect the fault locking and earthquake nucleation process, resulting in coexistence of short-term brittle deformation (such as earthquakes and interseismic locking) and long-term ductile deformation.

Fluid activity and seismicity in subduction zones

Statistics on earthquakes in global subduction zones show that number of earthquakes decreases with depth and reaches the minimum at depth of ~300 km. In cold subduction zones, most hydrous minerals experience dehydration at subarc depths of 80–200 km, while water in lawsonite and phengite will be totally released until ~300 km. In warm subduction zones, complete dehydration is achieved in most hydrous minerals at subarc depths of <160 km. Earthquake distribution in the present-day cold and warm subduction zones is consistent with dehydration depths of hydrous minerals, demonstrating that dehydration embrittlement of hydrous minerals is the major mechanism of intermediate-depth earthquakes in oceanic subduction zones. Other mechanisms such as thermal runaway instability, eclogitization-related embrittlement, and metamorphism-facilitated instability in orthopyroxene may also contribute to intermediate-depth earthquakes.

The source regions of slow earthquakes in subduction zones are characterized by low effective stress and high pore fluid pressure, which may be caused by dehydration reactions of multiple hydrous minerals. Nominally anhydrous minerals and dense hydrous magnesium silicates in cold slabs can transport water to depths below 300 km, resulting in localized water enrichment in the mantle transition zone. Transformational faulting of metastable olivine is considered as the main mechanism for deep-focus earthquakes, while the role of water in deep-focus earthquakes remain controversial.

“There are still many puzzles in the interplay of metamorphism, seismicity, and fluid/melt activity during plate subduction,” says Qin WANG. Experiments and theoretical calculations of phase stability in an open system, high-precision earthquake locations, as well as integrated studies of fossil earthquakes in ancient subduction zones will provide new insights into dynamic evolution of subduction zones and improve seismic risk assessment in subduction zones.

(a) Stress state and focal mechanisms of a subducting plate (modified from Goes et al., 2017; Zhan, 2020). Stress state of the slab is dominated by plate-parallel extensional stress in the upper mantle, and plate-parallel compressive stress in the mantle transition zone. (b) Histogram of focal depth of global earthquakes. (c) Anticrack in metastable olivine (modified from Zhan, 2020). Mineral abbreviations: Brg: bridgmanite; Mws: magnesiowüstite; Rdw: ringwoodite; Wds: wadsleyite.

Hydrous minerals in the water-saturated MORB and water-saturated peridotites are shown in blue and red, respectively. Hydrous minerals at the bottom of the mantle wedge is shown in black

Credit

©Science China Press

Journal

Science China Earth Sciences

DOI

10.1007/s11430-024-1514-4

Method of Research

Literature review

Tiny gas bubbles reveal secrets of Hawaiian volcanoes

Cornell UniversiTY

ITHACA, N.Y. – Using advanced technology that analyzes tiny gas bubbles trapped in crystal, a team of scientists led by Cornell University has precisely mapped how magma storage evolves as Hawaiian volcanoes age.

Geologists have long proposed that, as the Hawaiian Islands slowly drift northwest with the Pacific Plate, they move away from a deep, heat-rich plume rising from near Earth’s core. Young volcanoes like Kilauea – positioned directly above the hotspot on Hawaii’s main island – receive a steady flow of magma. Far less is known about older volcanoes like Haleakala – located northwest on the island of Maui – where magma flow has significantly diminished.

The new research, under embargo until 2pm EST on May 14 in Science Advances, finds that as volcanoes move off the hotspot, their magma flow not only shrinks, but shifts deeper underground, reshaping assumptions about how Hawaii’s volcanic “pluming system” has evolved.

“This challenges the old idea that eruptions are fueled by magma stored in the Earth’s crust and suggests a new possibility,” said lead author Esteban Gazel, “that magma is stored and matures in the Earth’s mantle, and eruptions are fueled from this deep mantle reservoir.”

By analyzing fluid inclusions – tiny gas bubbles trapped inside crystals formed in magma – the researchers calculated the pressure, and therefore the depth, at which the inclusions were trapped before an explosive eruption ejects them to the surface.

“The technology allows us to measure pressure from depths with an uncertainty as small as just hundreds of meters, which is very, very precise for depths that are tens of kilometers below the surface,” Gazel said. “Before this, measuring magma storage was much more difficult, with uncertainties that could span kilometers.”

To achieve such level of precision, researchers optimized a custom gas chamber that fits under a laser-based Raman spectrometer.

“Our contribution to significantly increase accuracy was to get the thermocouple inside the chamber and precisely control and measure temperature and pressure,” Gazel said. By analyzing carbon dioxide behavior, researchers can determine its density and calculate the original depth of magma storage, he added.

The method was applied to samples from three Hawaiian volcanoes representing different evolutionary stages:

Kilauea, an active “shield” volcano, showed magma storage at shallow depths of 1–2 kilometers, consistent with previous findings;
Haleakala, in the post-shield stage, revealed dual storage zones: one shallow at approximately 2 kilometers and one deep at 20–27 kilometers in the Earth’s mantle; and
Diamond Head, a rejuvenation-stage volcanic vent on the island of O’ahu, showed magma stored around 22–30 kilometers deep, all within the Earth’s mantle.

“Knowing these depths precisely matters, because to understand the drivers of eruptions, one of the most important constraints is where magma is stored,” Gazel said. “That is fundamental for physical models that will explain eruptive processes and is required for volcanic risk assessment.”

-30-

Journal

Science Advances

DOI

10.1126/sciadv.adu9332

Article Title

Crustal to Mantle Melt Storage During the Evolution of Hawaiian Volcanoes

Article Publication Date

14-May-2025

Making automation more human through innovative fabrication tools

UC Santa Barbara’s Jennifer Jacobs wins NSF CAREER Award to develop adaptive, interactive fabrication systems

University of California - Santa Barbara

Jennifer Jacobs in her Lab — image:
“You can think of this as the next step in personal computing,” said UCSB professor Jennifer Jacobs, whose research explores more intuitive, responsive digital fabrication systems.
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Credit: Matt Perko

The boundary between automation and human creative production is shifting as new possibilities emerge in digital fabrication.

Among those leading this transformation is Jennifer Jacobs, assistant professor in the Media Arts and Technology Program at UC Santa Barbara, who has received a prestigious CAREER Award from the National Science Foundation to pioneer fabrication systems that respond dynamically to human input, material behavior and domain expertise.

The $680,000 award supports a five-year project to redefine how manufacturers, researchers and designers interact with machines like 3D printers and CNC mills. Instead of locking users into rigid, pre-programmed workflows, Jacobs envisions tools that foster improvisation, expertise and adaptation.

“Digital fabrication has enormous potential, but the way we interact with these machines hasn’t evolved much beyond industrial manufacturing,” Jacobs said. “We want to rethink how people work with machines, not by simplifying what the user can do but by designing tools that adapt to how people actually make things.”

Today, most digital fabrication workflows require users to model designs in specialized software and convert them into instructions that a machine executes automatically. While precise, this process leaves little room for the kind of real-time feedback, learning and hands-on knowledge that skilled makers bring to their work, especially when working with complex or sensitive materials.

Jacobs’ research focuses on changing that. By designing systems that blend manual and automated operations, her lab enables creators to intervene during fabrication, adjusting variables mid-print, responding to material behavior and even drawing or demonstrating toolpaths in physical space.

For example, in Jacobs’ Expressive Computation Lab, her team works in clay 3D printing. While the hardware and software of a clay printer closely resemble that of a plastic printer, the material behaves very differently. Clay shifts with humidity, reacts to changes in speed and requires constant attention. Jacobs’ lab, in collaboration with Ilan Moyer at MIT, developed a control system that allows users to fine-tune the flow of clay or adjust the print geometry in real time, creating a more flexible and intuitive fabrication process. In one project, they reimagined a clay printer to resemble a pottery wheel, merging traditional craft practices with digital controls.

“We’re not trying to replace hands-on expertise,” Jacobs said. “We’re trying to create systems that honor and incorporate it.”

This vision guided her NSF proposal, which calls for the development of new control methods, machine architectures and design tools for domain-specific digital fabrication. Rather than treating machines as one-size-fits-all, Jacobs wants to support workflows tailored to specific industries, including occupational therapy, product design and construction.

“We’re especially interested in supporting people who already know something about making,” she said. “They’re not novices, but the tools available to them don’t reflect how they work.”

The CAREER Award also supports an educational mission. Jacobs plans to engage students from UCSB and local public schools in computer science and entrepreneurship through hands-on fabrication projects. During the pandemic, her lab distributed low-cost 3D printers to UCSB students, prompting a wave of experimentation with materials, machine settings and new applications.

Her work also contributes to understanding who uses digital fabrication and how. A previous grant from the National Science Foundation supported her research into the real-world use of clay 3D printers. Her team found that while the technology is gaining traction in construction and design, its adoption in traditional ceramics studios remains limited — partly because of the complexity of the tools and the difference between digital and analog practices.

By making digital fabrication more responsive to the ways people already work, Jacobs hopes to make it more powerful for domain professionals.

“You can think of this as the next step in personal computing,” she said. “Just like software environments became customizable based on whether you were a musician or a writer or a scientist, we want to make fabrication tools that people can adapt to their specific professions or workflows. That’s when we’ll start to see the full potential of digital fabrication.”

Different ways of ‘getting a grip’

Researchers uncover new evidence of how ancient human relatives in South Africa used their hands, revealing varying levels of dexterity and climbing ability

Max Planck Institute for Evolutionary Anthropology

Fossil hands of Australopithecus sediba and Homo naledi — image:
The fossil hands of *Australopithecus sediba* (around two million years old) and *Homo naledi* (around 250,000 years old) show that these South African hominins may have had different levels of dexterity, as well as different climbing abilities.
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Credit: © Tracy Kivell

To the point

Different hand use: Two ancient human relatives, Australopithecus sediba and Homo naledi, had different finger bone morphologies that indicate they used different types of hand grips, both when using tools and when climbing
Internal structure of the finger bones: A. sediba had a mix of ape-like and human-like features, while H. naledi had a unique pattern of bone thickness, suggesting different loading patterns and possible grip types.
Human Evolution: Ancient human relatives adapted to their environments in diverse ways, balancing tool use, food processing, and locomotion, challenging the traditional view of a single, linear transition from upright walking to advanced tool use.

Scientists have found new evidence for how our fossil human relatives in South Africa may have used their hands. Research led by Samar Syeda, postdoctoral researcher at the American Museum of Natural History, together with scientists at the Max Planck Institute for Evolutionary Anthropology, the University of the Witwatersrand, University of Kent, Duke University and the National Geographic Society, investigated variation in finger bone morphology to determine that South African hominins not only may have had different levels of dexterity, but also different climbing abilities.

This new research focuses on two, almost complete fossil hand skeletons found in South Africa. One is the hand of Australopithecus sediba, first discovered in 2010 at the site of Malapa and dated to approximately two million years old. The other hand skeleton is from the more recent, but perhaps more enigmatic, Homo naledi, first found in 2015 deep within the Rising Star Cave system and dated to around 250,000 years ago.

Neither hominin has yet been found in direct association with stone tools, but several aspects of their hand and wrist morphology suggest that they had a degree of hand dexterity much more similar to that of humans than to living chimpanzees or gorillas. “Since stone tools are found in South Africa by at least 2.2 million years ago (and in East Africa by as early as 3.3 million years ago), and many primates are all excellent stone tool users, it is not surprising that A. sediba and H. naledi would be dexterous tool users as well. However, how exactly they used tools and if they manipulated their tools in similar ways is unclear,” says senior author, Tracy Kivell.

Moreover, both A. sediba and H. naledi are also found with many other bones of their skeleton that preserve ape-like features, particularly bones of their upper limbs, that would be advantageous for climbing. If these features reflect actual climbing in these individuals or are simply evolutionary hold-overs from an ancestor that climbed, is a longstanding debate in palaeoanthropology.

Fossil hands reveal ancient human behavior

To help address these questions, Syeda and her colleagues investigated variation in the internal structure—the cortical bone—of the fingers in A. sediba and H. naledi. Bone is a living tissue that can adapt its structure in response to how we use and load our skeleton during life, getting thicker where loads are higher and thinner where loads are lower. Therefore, variation in the internal cortical thickness can provide new insights in how these two hominin fossils may have actually used their hands during their lifetimes.

“We found that A. sediba and H. naledi show different functional signals in the cortical bone structure of their fingers,” says Samar Syeda, lead author of the study. In A. sediba, the distribution of the cortical bone within the proximal and intermediate phalanges of most of its fingers is like that of apes. However, bones of its thumb and pinky finger are more like those of humans. Syeda concludes that “these two digits are more likely to reflect potential signals of manipulation because they are less often used or experience less load during climbing or suspensory locomotion. When we combine these results with the remarkably long, human-like thumb of A. sediba, it suggests that A. sediba used its hand for both tool use and other dexterous behaviours, as well as climbing.”

Homo naledi's hands show unique grip pattern

H. naledi, in contrast, is unusual in showing a human-like signal in its proximal phalanges (the bones that articulate with the palm) but an ape-like signal in its intermediate phalanges (the bones within the middle of the fingers). “This distinct pattern was unexpected and indicates that H. naledi likely used and loaded different regions of its fingers in different ways,” says Syeda. This kind of loading pattern is typical of only certain grip types used today, like crimp grips, used often by rock climbers, where the surface is grasped primarily by just the tips of the fingers. H. naledi also has unusually highly curved finger bones, particularly for a hominin that lived at the same time as the earliest members of our species, Homo sapiens, which is another indication that it used its hands for locomotion.

While more research is needed to further test if H. naledi may have used crimp-like grips or climbed rocks, it is clear that throughout human evolution there were different ways of combining enhanced dexterity for tool use and food processing with the continued need to use the hands to climb, be it trees or rocks, within the South African palaeolandscape. “This work offers yet more evidence that human evolution is not a single, linear transition from upright walking to increasingly better tool use, but is rather characterized by different ‘experiments’ that balanced the need to both manipulate and to move within these past environments,” says Kivell.

The fossil hands of Australopithecus sediba (around two million years old) and Homo naledi (around 250,000 years old) show that these South African hominins may have had different levels of dexterity, as well as different climbing abilities.