Episodic tremor and accompanying slow slip, together called ETS, is most often observed in subduction zones of young and warm subducting slabs. ETS should help us to understand the mechanics of ...subduction megathrusts, but its mechanism is still unclear. It is commonly assumed that ETS represents a transition from seismic to aseismic behaviour of the megathrust with increasing depth, but this assumption is in contradiction with an observed spatial separation between the seismogenic zone and the ETS zone. Here we propose a unifying model for the necessary geological condition of ETS that explains the relationship between the two zones. By developing numerical thermal models, we examine the governing role of thermo-petrologically controlled fault zone rheology (frictional versus viscous shear). High temperatures in the warm-slab environment cause the megathrust seismogenic zone to terminate before reaching the depth of the intersection of the continental Mohorovičić discontinuity (Moho) and the subduction interface, called the mantle wedge corner. High pore-fluid pressures around the mantle wedge corner give rise to an isolated friction zone responsible for ETS. Separating the two zones is a segment of semi-frictional or viscous behaviour. The new model reconciles a wide range of seemingly disparate observations and defines a conceptual framework for the study of slip behaviour and the seismogenesis of major faults.
Subduction faults, called megathrusts, can generate large and hazardous earthquakes. The mode of slip and seismicity of a megathrust is controlled by the structural complexity of the fault zone. ...However, the relative strength of a megathrust based on the mode of slip is far from clear. The fault strength affects surface heat flow by frictional heating during slip. We model heat-flow data for a number of subduction zones to determine the fault strength. We find that smooth megathrusts that produce great earthquakes tend to be weaker and therefore dissipate less heat than geometrically rough megathrusts that slip mainly by creeping.
Geophysical observations including surface heat flow data indicate the subducting slab becomes fully coupled to the overlying mantle wedge at ∼70–80 km depth. This maximum depth of decoupling (MDD) ...separates cool, stagnant forearc mantle from warmer, convecting mantle capable of generating arc magmas. Thermodynamic calculations demonstrate that talc is stable in H2O‐undersaturated parts of the mantle wedge where its stability is controlled by the pressure‐dependent, fluid‐absent reaction: talc + forsterite = antigorite + enstatite, which occurs at pressures ∼2–2.5 GPa (∼70–80 km depth) and temperatures <650°C. At shallower depths, H2O‐undersaturated portions of the basal mantle wedge contain talc, which experimental studies show dramatically weakens rocks. At greater depths, talc is restricted to silica‐rich portions of the mantle wedge. The common MDD in subduction zones may reflect the downdip transition from a talc‐present decoupled shear zone to a talc‐absent fully coupled interface along the base of the mantle wedge.
Plain Language Summary
In many subduction zones, the relative strength of the plate interface compared to the overlying mantle changes dramatically at 70–80 km depth. At shallower levels, the interface acts as a weak shear zone decoupling the subducting plate from the overlying cool, stagnant mantle. At deeper levels, the strength of the shear zone increases such that subducting plate drives convection in the overlying mantle creating the high temperatures needed for arc magmatism. We propose the change in interface strength may be related to the stability of talc, a very weak mineral, in the forearc mantle. At shallower levels, talc is stable in a wide range of H2O‐undersaturated mantle compositions. At deeper levels, the stability field of talc is dramatically reduced and restricted to silica‐rich regions of the mantle.
Key Points
Talc, a very weak mineral, is stable in H2O‐undersaturated ultramafic (mantle) rocks at P < ∼2–2.5 GPa (70–80 km depth) and T < ∼650°C
At pressures greater than ∼2–2.5 GPa, talc breaks down via the P‐dependent reaction: talc + forsterite = antigorite + enstatite
The breakdown of talc may explain the change in subduction‐interface rheology at 70–80 km depth where full slab‐mantle coupling begins
Processes in subduction zones such as slab and mantle‐wedge metamorphism, intraslab earthquakes, and arc volcanism vary systematically with the age‐dependent thermal state of the subducting slab. In ...contrast, the configuration of subduction zones is rather uniform in that the arc is typically situated where the slab is ∼100 km deep. Toward reconciling the diversity and uniformity, we developed numerical thermal models with a nonlinear mantle rheology for seventeen subduction zones, spanning a large range of slab age, descent rate, and geometry. Where there are adequate observations, such as in Cascadia, northeast Japan, and Kamchatka, we find that surface heat flows can be explained if the interface between the slab and the mantle wedge is decoupled to a depth of 70–80 km. Models with this common decoupling depth predict that the region of high mantle temperatures and optimal fluid supply from the dehydrating slab, both required for melt generation for arc volcanism, occurs where the slab is ∼100 km deep. These models also reproduce the variations of the metamorphic, seismic, and volcanic processes with the thermal state of the slab. The shallow decoupling results in a stagnant fore arc whose thermal regime is controlled mainly by the subducting slab. The deeper coupling leads to a sudden onset of mantle wedge flow that brings heat from greater depths and the back arc, and its thermal effect overshadows that of the slab in the arc region. Our results serve to recast the research of subduction zone geodynamics into a quest for understanding what controls the common depth of decoupling.
•SOC, TN, and TP were remarkably higher in dolomite and limestone than in clasolite.•In natural succession, dynamics of SOC, TN, and TP differed between dolomite and limestone.•For clasolite, only ...SOC and TN in grassland differed significantly with arable land.•Human interference intensity affected the response of soil nutrients to land use change.
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Soil carbon (C), nitrogen (N), and phosphorus (P) are the main soil nutrients required for plant development and their stoichiometric ratios are important indicators of ecosystem functions. However, there have been few studies on the effects of land use and lithology on soil nutrients and stoichiometric ratios, especially in karst areas with extremely fragile geology and intensive human disturbance. To evaluate the synergistic effect of land use and lithology, soil samples at depth of 0–15cm were collected from five land-use types (arable land, plantation forest, grassland, shrubland, and secondary forest) over three lithologies (karst dolomite and limestone and non-karst clasolite) in a typical karst area in southwest China. For natural succession, grassland, shrubland, and secondary forest corresponded to the early, middle, and late successional stage after agricultural abandonment, while from arable land to plantation forest can be treated as a manual reversion after agricultural abandonment. The results showed that, in dolomite, soil organic C (SOC) and total N (TN) increased continuously with natural succession and increased in plantation forest compared to arable land. Total P (TP) continued to decrease from arable land to grassland and then to shrubland. In limestone, SOC and TN did not follow the same pattern because SOC and TN were slightly higher in grassland than shrubland, while TN was slightly lower in plantation forest compared to arable land. TP was remarkably higher in arable land than the other land-use types. For clasolite, SOC was highest in grassland, while TN was not significantly different among land-use types. Compared to arable land, TP was lower in other types of land use. These soil nutrient characteristics led to various stoichiometric ratios under the five land-use types over different lithologies. Therefore, ecological restoration projects based on land use conversion should consider differences in regional lithology and human disturbance.
Subduction zones produce the largest earthquakes. Over the past two decades, space geodesy has revolutionized our view of crustal deformation between consecutive earthquakes. The short time span of ...modern measurements necessitates comparative studies of subduction zones that are at different stages of the deformation cycle. Piecing together geodetic 'snapshots' from different subduction zones leads to a unifying picture in which the deformation is controlled by both the short-term (years) and long-term (decades and centuries) viscous behaviour of the mantle. Traditional views based on elastic models, such as coseismic deformation being a mirror image of interseismic deformation, are being thoroughly revised.
Soil micro‐organisms play a key role in soil biogeochemical cycles, but their growth and activities are often limited by resource availability. Understanding soil processes that are driven by ...micro‐organisms and resource limitation of microbes will help to elucidate controls on soil fertility and improve the ability to predict the responses of an ecosystem to global changes. As a widespread ecosystem type, karst ecosystem develops from limestone or dolomite with unique soil; however, karst ecosystems remain poorly understood regarding their soil microbial processes and microbial resource limitation.
Here, ecoenzymatic stoichiometry was used as an indicator of microbial resource limitation, and to model major microbial processes (i.e. decomposition of soil organic carbon and microbial respiration) in a karst and a non‐karst forest.
Results showed that the modelled decomposition and respiration rates were significantly higher in the karst forest than in the non‐karst forest. In addition, results of ecoenzymatic stoichiometry showed that the karst forest was more carbon‐limited than the non‐karst forest. In contrast, the karst forest was likely saturated with nitrogen, but the non‐karst forest was limited by nitrogen. Both the karst and non‐karst forests were limited by phosphorus, but phosphorus deficiency was more evident in the non‐karst forest than in the karst forest.
These findings highlight the specific profiles of karst ecosystems, and they suggest that the responses of karst ecosystems to global changes should be very different compared to other ecosystems.
A plain language summary is available for this article.
Plain Language Summary
Among the wide range of thermal, petrologic, hydrological, and structural factors that potentially affect subduction earthquakes, the roughness of the subducting seafloor is among the most important. ...By reviewing seismic and geodetic studies of megathrust locking/creeping state, we find that creeping is the predominant mode of subduction in areas of extremely rugged subducting seafloor such as the Kyushu margin, Manila Trench, northern Hikurangi, and southeastern Costa Rica. In Java and Mariana, megathrust creeping state is not yet constrained by geodetic observations, but the very rugged subducting seafloor and lack of large earthquakes also suggest aseismic creep. Large topographic features on otherwise relatively smooth subducting seafloor such as the Nazca Ridge off Peru, the Investigator Fracture Zone off Sumatra, and the Joban seamount chain in southern Japan Trench also cause creep and often stop the propagation of large ruptures. Similar to all other known giant earthquakes, the Tohoku earthquake of March 2011 occurred in an area of relatively smooth subducting seafloor. The Tohoku event also offers an example of subducting seamounts stopping rupture propagation. Very rugged subducting seafloor not only retards the process of shear localization, but also gives rise to heterogeneous stresses. In this situation, the fault zone creeps because of distributed deformation of fractured rocks, and the creep may take place as transient events of various spatial and temporal scales accompanied with small and medium-size earthquakes. This process cannot be described as stable or unstable friction along a single contact surface. The association of large earthquakes with relatively smooth subducting seafloor and creep with very rugged subducting seafloor calls for further investigation. Seafloor near-trench geodetic monitoring, high-resolution imaging of subduction fault structure, studies of exhumed ancient subduction zones, and laboratory studies of low-temperature creep will greatly improve our understanding of the seismogenic and creep processes and their hazard implications.
•Subsurface runoff dominates the runoff components in karst hillslope.•Thinner soils decrease surface runoff and increase subsurface runoff.•Bedrock topography controlled subsurface runoff generation ...in the thinner soils.•Thinner soils had a higher contribution of new water than that in the thick soils.
Hydrological processes in the critical zone are closely related to the soil–bedrock structures. However, the effect of soil thickness on the rainfall-runoff relationship on the hillslope with complex topography remains unclear. Surface runoff, lateral subsurface runoff from soil–epikarst interface, and epikarst runoff from the epikarst–bedrock interface were monitored on two adjacent plots with deep and shallow (66.0 vs. 35.4 cm) soil thicknesses from June 2019 to December 2020 in the karst region of southwest China. During the monitoring period, surface and subsurface runoff account for 20% and 37% of the total runoff in the deep-soil plot (DSP), and 3% and 43% in the shallow-soil plot (SSP). This demonstrates that runoff from the soil-epikarst system is predominant compared to the relatively small contribution of surface runoff. In the SSP, the surface topography wetness index (TWI) was highly coupled with bedrock TWI, and the bedrock TWI had a significant negative linear relationship (p < 0.01) with subsurface runoff. Moreover, isotope hydrogen-separation results showed that subsurface and epikarst runoff were dominated by pre-event water, but a higher contribution of event water was observed in the SSP than in the DSP. These findings supported the hypothesis that rainwater could infiltrate the epikarst more easily in shallow soil slopes. Rainfall and surface runoff exhibited a linear relationship in the dry season and a non-linear relationship in the rainy season, indicating the occurrence of threshold rainfall–runoff behavior. The rainfall amount threshold for surface runoff was higher in DSP (44.7 mm) than in SSP (39.5 mm), and the corresponding variation of rainfall intensity interpretation was greater (54% vs. 38%). For subsurface runoff, the rainfall amount threshold was higher in the DSP than in the SSP (91.0 vs 79.4 mm), and the corresponding variation of soil moisture interpretation was higher (56% vs. 20%). This demonstrated that runoff can be better predicted at deeper soil hillslopes by rainfall and antecedent soil moisture. Accordingly, this study emphasizes the importance of evaluating the spatial heterogeneity of soil thickness in hydrological process research.
•Portion of karst landform is the key factor for elasticity of actual E in SW China.•Actual E in karst catchments is more sensitive to P but less to E0 than in non-karst.•Karst catchments exhibited ...higher degradation stress brought by climate change.
Karst landform represents about 10% of the continental area and plays key roles in water supplies for almost a quarter of the global population. Knowledge of ecohydrological responses of karst landform to climate change is critical for both water resources management and ecological protection in these regions. This study investigated the effects of karst landform on the elasticity of actual evapotranspiration (derived by the Budyko equation), estimated the contribution of climate change and evaluated the implications, on the basis of 13 typical catchments that have different karst landform coverages in southwest China. Catchment properties, including the vegetation coverage, portion of karst landform (POK), drainage area, surface roughness, mean topographic wetness index, mean slope, and mean aspect, were selected to test the influencing factors for the elasticity of actual evapotranspiration. Results indicate that POK is the most influencing factor for the elasticity of actual evapotranspiration in this region. Moreover, the actual evapotranspiration in karst catchments is more sensitive to precipitation change and less sensitive to the potential evapotranspiration change than that in the non-karst catchments. On the other hand, the contribution of climate change to actual evapotranspiration was generally negative in this region. Furthermore, relatively large negative contributions mainly occurred in the karst-dominated catchments, suggesting that the karst catchments were exposed to higher degradation stress brought by the climate change than that in non-karst catchments.
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