Downdip Limit Research Articles

Subducted carbon weakens the forearc mantle wedge in a warm subduction zone

Subducting oceanic plates carry large amounts of carbon into the Earth’s interior. The subducted carbon is mobilized by fluid and encounters ultramafic rocks in the mantle wedge, resulting in changes to the mineral assemblage and mechanical properties of the mantle. Here, we use thermodynamic modeling of interactions between carbon-bearing multi-component fluids and mantle rocks to investigate the down-dip variation in mineral assemblage in the forearc mantle along subduction megathrusts. We found that fluids rich in aqueous carbon are preferentially generated in a warm subduction zone (e.g., Nankai, SW Japan), causing a change in mineral assemblage from serpentine-rich at the mantle wedge corner to talc + carbonate-rich at greater depths. The transition caused by the infiltration of aqueous carbon may influence the depth of the boundary between the seismogenic and aseismic zones, and the down-dip limit of episodic tremor and slip.

A pressure solution flow law for the seismogenic zone: Application to Cascadia.

We develop a linear viscous constitutive relationship for pressure solution constrained by models of deformed metasedimentary rocks and observations of exposed rocks from ancient subduction zones. We include pressure and temperature dependence on the solubility of silica in fluid by parameterizing a practical van't Hoff relationship. This general flow law is well suited for making predictions about interseismic behavior of subduction zones. We apply the flow law to Cascadia, where thermal structure, geometry, relative plate velocity, and Global Positioning System velocity field are well constrained. Results are consistent with the temperature conditions at which resolvable ductile strain is recorded in subducted mudstones (at depths near the updip limit of the seismogenic zone) and with relative plate motion accommodated completely by viscous deformation (at depths near the downdip limit of the seismogenic zone). The flow law also predicts the observed forearc tapering of slip rate deficit with depth.

Along-strike seismotectonic segmentation reflecting megathrust seismogenic behavior

Abstract Understanding the along-strike seismogenic behavior of megathrusts is crucial to anticipating seismic hazards in subduction zones. However, if and how spatiotemporal frictional heterogeneity (high and low kinematic coupling) at depth feeds back into the upper-plate deformation pattern and how the upper-plate elastic signals and permanent records may correlate have yet to be fully understood. Hence, we mimic subduction megathrust seismic cycles using an analog seismotectonic model of an elastoplastic wedge overlying a frictionally heterogeneous megathrust. Coseismically, the zone above the down-dip limit of the aseismic and seismogenic patches undergoes extension and contraction, respectively, while the strain state shows a switch in polarity from coseismic to interseismic. The down-dip limit of the creeping zone produces permanent along-strike extension or contraction, depending on the frictional barrier strength. Our experiments show that the frictional locking heterogeneity generates more segmented along-strike strain patterns elastically (short term) than permanently (long term). Moreover, our results suggest that along-strike upper-plate strain patterns could serve as a proxy for interpreting persistent lateral variations of seismogenic behavior in subduction megathrusts.

Imaging the 2007 Mw 7.7 Tocopilla earthquake from short-period back-projection

The 14 November 2007 Tocopilla (MW 7.7) earthquake was the first large rupture (M > 7.5) in the 1877 Northern Chile seismic gap. The event only ruptured the lower seismogenic part of the megathrust, raising questions about the mechanism that prevented the trenchward rupture propagation. Here, we present the short-period rupture process of the Tocopilla earthquake from local strong-motion (2.0–8.0 Hz) and teleseismic (0.5–2.0 Hz) back-projections. P and pP teleseismic back-projections were combined from two arrays formed on broadband networks in North America and Africa. The kinematics of the earthquake rupture was also characterized by back-propagating S-wave envelopes from local accelerometers (≤ 200 km from the mainshock). The results show a complex rupture process, including a sub-event, from the distribution of short-period rupture emissions and the main slip asperities. The sub-event location agrees with published estimates based on S-wave arrivals and kinematic inversion. We also observed rupture emissions around the slip patches and systemically classified them relative to the asperities, i.e., down-dip, up-dip, and inside. The short-period emissions are balanced around the asperities, highlighting up-dip rupture emissions. We interpreted the up-dip emissions as a proxy of the high-stress gradient caused by a kink in the slab interface, proposed in previous literature to be the mechanism that arrested the trenchward propagation of the 2007 rupture. The down-dip limit of the Tocopilla event coincided approximately where the slab interface intersects the continental Moho. Thus, it is a region expected to be extensively serpentinized, i.e., the most seaward part of the mantle wedge, defining a velocity-strengthening margin and a source of short-period radiation. The high-stress gradient around the asperities is proposed to define a third source of short-period radiation that may contribute to rupture encircling emissions.

B-value variations in the Central Chile seismic gap assessed by a Bayesian transdimensional approach

The b-value can be used to characterize the seismic activity for a given earthquake catalog and provide information on the stress level accumulated at active faults. Here we develop an algorithm to objectively estimate variations of b-value along one arbitrary dimension. To this end, we employ a Bayesian transdimensional approach where the seismic domains will be self-defined according to information in the seismic catalog. This makes it unnecessary to prescribe the location and extent of domains, as it is commonly done. We first show the algorithm’s robustness by performing regressions from synthetic catalogs, recovering the target models with great accuracy. We also apply the algorithm to a microseismicity catalog for the Central Chile region. This segment is considered a seismic gap where the last major earthquake with shallow slip was in 1730. Our results illuminate the downdip limit of the seismogenic zone and the transition to intraslab seismicity. In the along-strike direction, low b-value coincides with the extent of locked asperities, suggesting a high-stress loading at the Central Chile seismic gap. Our results indicate the reliability of the Bayesian transdimensional method for capturing robust b-value variations, allowing us to characterize the mechanical behavior on the plate interface of subduction zones.

Numerical Simulation of the Effects of Wedge Subduction on the Lithospheric Thermal Structure and the Seismogenic Zone South of Chile Triple Junction

In contrast to common subduction, the young and thin part of the Antarctic Plate subducts first to the south of the Chile Triple Junction (CTJ), followed by the old and thick part, corresponding to wedge subduction. A finite element model was used to simulate the wedge subduction of the Antarctic Plate and to compare it with the slab subduction of the Nazca Plate. The results show that the CTJ is not only a wedge subduction boundary but also an important factor controlling the lithospheric thermal structure of the overriding plate. The computed heat flow curves are consistent with the data observed near the trench of the two selected profiles. The different slab dips to the north and south of the CTJ are considered to be caused by wedge subduction. When the slabs are young and at the same age, the deep dip of the Antarctic slab is 22° smaller than the Nazca slab. Southward from the CTJ, the slab age of the wedge subduction increases, which leads to a larger slab dip, a colder slab, and a wider seismogenic zone. The effect of the slab age of wedge subduction on the focal depth is smaller than that of the convergence rate. A 4.8-cm/year difference in convergence rate of the wedge subduction results in an 11-km difference in the width of the seismogenic zone and a 10-km difference in the depth of the downdip limit. Among these controlling factors, the convergence rate plays a major role in the different focal depths south and north of the CTJ.

Estimation of size of megathrust zone in the Makran subduction system by thermal modelling

SUMMARYTo estimate the maximum possible size of megathrust earthquakes, we calculate the thermal structure along two profiles in west and east Makran subduction zone by solving the steady-state 2-D energy equation. For the western profile, we derive the slab geometry from a recent receiver function study along IASBS (Institute for Advanced Studies in Basic Sciences) seismic profile in the onshore part of the Iranian Makran. For the eastern profile, the slab geometry is derived from a recent relocation of seismicity of Makran. Using the improved slab geometry and a force balance establishment in the accretionary wedge, the effective coefficient of friction, $\mu ^{\prime}$, is assumed to be equal to 0.03. We estimate the updip and downdip of the megathrust zone by simultaneously considering the seismicity related to the events with thrust and normal mechanisms and intersection between 100–150 and 350–450 °C isotherms and the subducting slab interface. Along the western profile, the megathrust updip locates ∼95 km north of the deformation front (DF) at the depth of ∼20 km and the downdip locates ∼300 km north of the DF at the depth of ∼35 km. Presence of normal mechanism events at deeper depths indicates that the downdip limit of the megathrust zone is consistent with the 350 °C isotherm. The megathrust width is ∼205 km along the western profile. Along the eastern profile, the megathrust updip locates ∼60 km north of the DF at the depth of ∼15 km and the downdip locates ∼280 km north of the DF at the depth of ∼35 km. The downdip limit of the megathrust zone is closely related to the 350 °C isotherm. The megathrust width is ∼220 km along the eastern profile. Assuming a segmentation of the thrust zone into the western and eastern parts, the areal size of the megathrust zones in west and east Makran is ∼82 000 and 88 000 km2, respectively. We estimate the magnitude of the largest possible megathrust earthquakes in the west and east Makran to be 8.65 ± 0.26 and 8.75 ± 0.26 Mw, respectively.

Geodetic Coupling Models as Constraints on Stochastic Earthquake Ruptures: An Example Application to PTHA in Cascadia

AbstractCurrent stochastic rupture modeling techniques do not consider the potential influence of inter‐seismic coupling, a first‐order property of a megathrust, which can show correlation between areas of high coupling and areas of greater slip as seen in recent large ruptures globally. Therefore, it is reasonable to assume that it should be considered as prior information in rupture modeling. Here, we first present a mathematical formalism to introduce coupling models as prior information into stochastic rupture modeling. We then focus on how introducing slip deficit information into the stochastic rupture models influences slip distributions for the Cascadia subduction zone (CSZ). We compare rupture models created with two end member models of coupling, one with a shallow coupling and another with coupling deeper downdip. We also discuss the comparison to models created without assuming knowledge of the coupling distribution except for variation in the downdip limit of slip. Variations occur and correlate well with areas with the largest differences in slip deficit rates. The ruptures are then used for regional probabilistic tsunami hazard assessment. Overall, the tsunami amplitudes generated are much more hazardous in the northern extent of the CSZ where differences in coupling distribution are more prevalent. Models obtained from assuming a shallower downdip limit have tsunami amplitudes more similar to those from the geodetic coupling models. Although uncertainties are present in the accuracy of coupling, imposing either constraint created different hazard estimations when compared to those where no prior coupling information was used.

New Megathrust Locking Model for the Southern Kurile Subduction Zone Incorporating Viscoelastic Relaxation and Non‐Uniform Compliance of Upper Plate

AbstractDense Global Navigation Satellite System (GNSS) observations enable the development of megathrust interseismic locking models for the southern Kurile subduction zone where many great earthquakes have occurred. Inversion of these data assuming uniform elastic Earth has yielded slip deficit rates that are unreasonably high and/or full locking depth that is unreasonably large. Using the finite element method, here we construct a new Kurile locking model that includes interseismic viscoelastic stress relaxation and non‐uniform compliance of the elastic upper plate. Inverting the same geodetic data using the new subduction zone model alleviates the previously seen unreasonable features in inferred megathrust locking state. In the new model, full locking extends to shallower depths than the downdip limit of some large megathrust earthquakes including the 2003 Mw 8.0 Tokachi‐oki earthquake, supporting the notion of the shrinking of the locked area before the earthquakes and/or propagation of seismic rupture into creeping areas as previously predicted by friction or dynamic rupture models. By modeling the effects of a few recent M 8 earthquakes, we show that postseismic transients of recent earthquakes, although second‐order, should be addressed in deriving megathrust locking models. The locking state near the trench cannot be resolved by the land‐based GNSS data regardless of the improved model rheology and structure, although independent observations, such as slow earthquakes, may be used to speculate on the near‐trench locking state in various part of the margin in the absence of seafloor geodetic observations.

Quartz Vein Geochemistry Records Deformation Processes in Convergent Zones

AbstractIn several examples of subduction zones, we compared pairs of quartz veins formed either at the lower temperatures of the seismogenic zone (260°C or below), or at the higher temperatures of its downdip limit (∼330°C). All the veins analyzed here are mode I cracks that formed contemporaneously with the host‐rock main stage of deformation at peak burial conditions. Lower‐temperature veins show examples of quartz crystals with euhedral shapes and growth rims, while higher‐temperature veins contain crack‐seal microstructures. In the lower‐temperature realm, quartz growth rims have alternatingly either: (1) high cathodoluminescence (CL), CL‐blue color and high concentration in trace elements and fluid inclusions, or (2) low luminescence, CL‐brown color and low concentration in trace elements and fluid inclusions. In contrast, the quartz from higher‐temperature samples is homogeneously low luminescent and CL‐brown, except for very restricted domains of the crack‐seal microstructures where patches of CL‐blue quartz are present. The highly luminescent quartz contains high concentrations of aluminum and lithium, up to 3,000 and 400 ppm, respectively. Variations in Al and Li correlate well, so that Li appears as the main charge‐compensating cation for Al. We propose that the incorporation of Al and Li reflects the amplitude of the fluid pressure variations, which control crystal growth rates. Quartz geochemistry might therefore unravel the contrast between the seismogenic zone, where large fluid pressure variations are present, and its downdip limit, where fluid pressure variations are much more limited in amplitude.

Cascadia megathrust earthquake rupture model constrained by geodetic fault locking.

Paleo-earthquakes along the Cascadia subduction zone inferred from offshore sediments and Japan coastal tsunami deposits approximated to M9+ and ruptured the entire margin. However, due to the lack of modern megathrust earthquake records and general quiescence of subduction fault seismicity, the potential megathrust rupture scenario and influence of downdip limit of the seismogenic zone are still obscure. In this study, we present a numerical simulation of Cascadia subduction zone earthquake sequences in the laboratory-derived rate-and-state friction framework to investigate the potential influence of the geodetic fault locking on the megathrust sequences. We consider the rate-state friction stability parameter constrained by geodetic fault locking models derived from decadal GPS records, tidal gauge and levelling-derived uplift rate data along the Cascadia margin. We incorporate historical coseismic subsidence inferred from coastal marine sediments to validate our coseismic rupture scenarios. Earthquake rupture pattern is strongly controlled by the downdip width of the seismogenic, velocity-weakening zone and by the earthquake nucleation zone size. In our model, along-strike heterogeneous characteristic slip distance is required to generate margin-wide ruptures that result in reasonable agreement between the synthetic and observed coastal subsidence for the AD 1700 Cascadia Mw∼9.0 megathrust rupture. Our results suggest the geodetically inferred fault locking model can provide a useful constraint on earthquake rupture scenarios in subduction zones. This article is part of the theme issue 'Fracture dynamics of solid materials: from particles to the globe'.

In situ monitoring of an inclined cover made with mine waste materials to control water infiltration on a reactive waste rock dyke

The development of a cover for inclined acid-generating areas, such as the external face of dykes and the slope of waste rock piles, is undoubtedly one of the biggest technical reclamation challenges at several mine sites. The LaRonde mine site, owned and operated by Agnico Eagle Mines (Quebec, Canada) is currently engaged to identify an optimal reclamation scenario for the Dyke 1 of its acid-generating tailings storage facilities. One of the promising reclamation options for controlling water infiltration in the acid-generating waste rock on the Dyke 1 is the use of an inclined cover built with available mine waste materials. An instrumented inclined cell with an inclination angle of 18.3 degrees was built on a slope of this dyke to validate if low sulfide tailings and non potentially acid-generating waste rock can be used as cover material to reclaim the Dyke 1. The instrumented inclined cell was monitored for 3 years (2017 to 2019) using volumetric lysimeters, suction sensors, and volumetric water content sensors. The monitoring was done under natural climatic conditions and artificial wetting events. Under natural conditions, less than 1% (5 mm) of incident rainfall percolated in the volumetric lysimeters installed along the slope of the inclined cell. Under controlled conditions associated with artificial wetting events of 6.4 mm/h over a period of 12 h, net percolation values between 1 and 9% (4 to 60 mm) of the sum of incident precipitation were measured. The distance between the top of the cell and the Down Dip Limit (DDL) point was greater than the slope length of the cover under natural conditions and the DDL point moved from the bottom toward the top to reach values between 12 and 20 m from the top of the slope when the wetting events were applied on the cover. These results confirmed the suitability of mining materials as an inclined cover material to control water infiltration in reactive mine waste rocks.

The Role of Fluid‐Related Heterogeneous Structures in Controlling the Fault Slip Behavior in the Slow‐Earthquake Source Region Along the Nankai Subduction Zone, Southwest Japan

AbstractA seismic array observation across the slow‐earthquake source region in the eastern Kii Peninsula, southwest Japan, reveal the detailed structure and seismicity along the upper boundary of the subducted Philippine Sea Plate. Tomography analysis and earthquake reflection imaging clarify the geometry of the upper boundary of the Philippine Sea Plate with a subduction angle of ∼15° at a depth range of 22–30 km. We observe intraslab low‐frequency earthquakes (LFEs) in and around the low‐Vp and high‐Vp/Vs zones, which are located above the intraslab earthquakes that are controlled by the dehydration embrittlement of serpentine. The intraslab LFE activity may be related to fluid movement. Fluids, which are derived from both dehydration of the serpentinized oceanic mantle and crustal eclogitization, may control the fault slip behavior. Therefore, the fluid behavior around the subducting plate boundary has an important role in controlling the downdip limit of the seismogenic zone.

Fluid overpressure in subduction plate boundary caused by mantle-derived fluids

Subduction plate boundaries are commonly weak owing to fluid overpressure. The generation of fluid overpressure has been invoked for compaction disequilibrium and mineral dehydration in subducting sediments and altered oceanic crust. The subduction mélange in the Shimanto accretionary complex, southwest Japan, preserves the quartz-calcite-filled shear veins and sigmoidal extension veins formed under fluid overpressure near the downdip limit of the thermally-controlled seismogenic zone. The shear veins record the repeated low-angle brittle thrusting under fluid overpressure, while the extension veins constitute the Riedel shear zones developed during subduction under vertical maximum principle stress. To examine the origin of overpressured fluids, we conducted fluid inclusion and noble gas analyses on the shear and extension veins. The result of fluid inclusion analysis shows various fluid pressures between lithostatic to hydrostatic pressures during the vein formation, which were associated with warm fluid infiltration. 3He/4He values of shear veins and extension veins range 1.6–2.5 Ra and 1.6–2.3 Ra, respectively, indicating the presence of mantle helium. 40Ar/36Ar values and 84Kr/36Ar and 130Xe/36Ar ratios of the veins are similar to those of serpentinized mantle, suggesting that mantle helium originated from serpentinized mantle. Because the mantle-derived rocks such as peridotite and serpentinite are absent in the mélange, the results of noble gas analyses represent the infiltration of fluids from the serpentinized mantle into the mélange shear zone. Our results show that an infiltration of fluids derived from serpentinized mantle contributed the fluid overpressure in the mélange shear zone near the downdip limit of the seismogenic zone.

Modeling Megathrust Earthquakes Across Scales: One‐way Coupling From Geodynamics and Seismic Cycles to Dynamic Rupture

AbstractTaking the full complexity of subduction zones into account is important for realistic modeling and hazard assessment of subduction zone seismicity and associated tsunamis. Studying seismicity requires numerical methods that span a large range of spatial and temporal scales. We present the first coupled framework that resolves subduction dynamics over millions of years and earthquake dynamics down to fractions of a second. Using a two‐dimensional geodynamic seismic cycle (SC) model, we model 4 million years of subduction followed by cycles of spontaneous megathrust events. At the initiation of one such SC event, we export the self‐consistent fault and surface geometry, fault stress and strength, and heterogeneous material properties to a dynamic rupture (DR) model. Coupling leads to spontaneous dynamic rupture nucleation, propagation, and arrest with the same spatial characteristics as in the SC model. It also results in a similar material‐dependent stress drop, although dynamic slip is significantly larger. The DR event shows a high degree of complexity, featuring various rupture styles and speeds, precursory phases, and fault reactivation. Compared to a coupled model with homogeneous material properties, accounting for realistic lithological contrasts doubles the amount of maximum slip, introduces local pulse‐like rupture episodes, and relocates the peak slip from near the downdip limit of the seismogenic zone to the updip limit. When an SC splay fault is included in the DR model, the rupture prefers the splay over the shallow megathrust, although wave reflections do activate the megathrust afterward.

Fluid and Melt Pathways in the Central Chilean Subduction Zone Near the 2010 Maule Earthquake (35–36°S) as Inferred From Magnetotelluric Data

AbstractThe subduction zone of central Chile (36°S) has produced some of the world's largest earthquakes and significant volcanic eruptions. Understanding the fluid fluxes and structure of the subducting slab and overriding plate can provide insight into the tectonic processes responsible for both seismicity and magmatism. Broadband and long‐period magnetotelluric data were collected along a 350‐km profile in central Chile and Argentina and show a regional geoelectric strike of 15 ± 19° east of north. The preferred two‐dimensional inversion model included the geometry of the subducting Nazca plate as a constraint. On the upper surface of the Nazca plate, conductors were interpreted as fluids expelled from the downgoing slab via compaction at shallow depth (C1) and metamorphic reactions at depths of 40–90 km (C2 and C3). At greater depths (130 km), a conductor (C7) is interpreted as a region of partial melt related to deserpentinization in the backarc. A resistor on the slab interface (R1) is coincident with a high‐velocity anomaly which was interpreted as a strong asperity which may affect the coseismic slip behavior of large megathrust earthquakes at this latitude. Correlations with seismicity suggest slab fluids alter the forearc mantle and define the downdip limit of the seismogenic zone. Beneath the volcanic arc, several upper crustal conductors (C4 and C5) represent partial melt beneath the Tatara‐San Pedro Volcano and the Laguna del Maule Volcanic Field. A deeper lower crustal conductor (C6) underlies both volcanoes and suggests a connected network of melt in a thermally mature lower crust.

Large scale lithospheric detachment of the downgoing Cocos plate: The 8 September 2017 earthquake (Mw 8.2)

On 8 September 2017, a great earthquake (Mw 8.2) took place in the Mexican subduction zone in the Tehuantepec gap, where no large subduction earthquakes have taken place since 1902. However, the 8 September earthquake did not occur on the contact between the Cocos and North American plates. The centroidal hypocentral depths reported by different agencies, including the National Seismological Service, range from 45 to 47.4 km, placing it immediately beneath the down-dip limit of the interplate locked zone. The source mechanism reflects down-dip tensional faulting. The inversion of the fault process shows a rupture that initiated at the bottom of the lithosphere and propagated upward, breaking through the entire subducted lithosphere. An unusually long, complex and copious aftershock sequence followed the main event. The relocated aftershocks, in the first 20 days following the main event, delineate a 160-km-long fault, sub-parallel to the oceanic trench, immediately beneath the interplate contact. The aftershocks also concentrate in secondary intraslab faults ∼50 km down-dip from the mainshock rupture, revealing that pervasive tensional stresses are present within the subducted plate. This deformation pattern suggests a large-scale tensional regime in the slab, apparently induced by its own gravitational weight that pulls it away from the strongly coupled interplate contact. Other large intraplate earthquakes in the vicinity, in 1931 (Mw 7.8) and 1999 (Mw 7.5), also reflect this detachment of the downgoing Cocos plate that sinks into the mantle under its own gravitational weight. The slab detachment suggests that the up-dip segment of the plate interface near the Isthmus of Tehuantepec is locked and possibly primed for a megathrust earthquake, such as the Mw 8.6 that took place in 1787, on an adjacent segment of the Mexican subduction zone.

Interseismic Coupling and Slow Slip Events on the Cascadia Megathrust

In this study, we model geodetic strain accumulation along the Cascadia subduction zone between 2007.0 and 2017.632 using position time series from 352 continuous GPS stations. First, we use the secular linear motion to determine interseismic locking along the megathrust. We determine two end member models, assuming that the megathrust is either a priori locked or creeping, which differ essentially along the trench where the inversion is poorly constrained by the data. In either case, significant locking of the megathrust updip of the coastline is needed. The downdip limit of the locked portion lies ∼ 20–80 km updip from the coast assuming a locked a priori, but very close to the coast for a creeping a priori. Second, we use a variational Bayesian Independent Component Analysis (vbICA) decomposition to model geodetic strain time variations, an approach which is effective to separate the geodetic strain signal due to non-tectonic and tectonic sources. The Slow Slip Events (SSEs) kinematics is retrieved by linearly inverting for slip on the megathrust the Independent Components related to these transient phenomena. The procedure allows the detection and modelling of 64 SSEs which spatially and temporally match with the tremors activity. SEEs and tremors occur well inland from the coastline and follow closely the estimated location of the mantle wedge corner. The transition zone, between the locked portion of the megathrust and the zone of tremors, is creeping rather steadily at the long-term slip rate and probably buffers the effect of SSEs on the megathrust seismogenic portion.

Geodetic inversion for spatial distribution of slow earthquakes under sparsity constraints

In geodetic data inversion, insufficient observational data and smoothness constraints for model parameters make it difficult to clearly resolve small-scale heterogeneous structures with discontinuous boundaries. We therefore applied sparse modelling to geodetic data inversion. In this paper, we reported two examples; one is developed a novel regularization scheme for the inversion problem that uses discontinuity, sparsity, and smoothness constraints. In order to assess its usefulness and applicability, the proposed method was applied to synthetic displacements calculated by a ring-shaped and sharply varying afterslip distribution on the plate interface beneath the Hyuga-nada region in southwest Japan. The discontinuous boundary was sharper than that obtained by using smoothness constraint only. The other is used the fused regularization, a type of sparse modelling suitable for detecting discontinuous changes in the model parameters. We estimated spatial distribution of the long-term slow slip events beneath the Bungo channel in southwest Japan. We found that the largest slip abruptly becomes zero at the down-dip limit of the seismogenic zone, and is immediately reduced to half at the up-dip limit of the deep low-frequency tremors, and becomes zero near its down-dip limit. Such correspondences imply that some thresholds exist in the generation processes for both tremors and SSEs. These results suggest that geodetic data inversion with sparse modelling can detect such abrupt changes and discontinuous boundaries in the slip distribution of slow earthquakes.

Tearing, segmentation, and backstepping of subduction in the Aegean: New insights from seismicity

This study revisits subduction processes at the Hellenic Subduction Zone (HSZ) including tearing, segmentation, and backstepping, by refining the geometry of the Nubian slab down to 150–180 km depth using well-located hypocentres from global and local seismicity catalogues. At the western termination of the HSZ, the Kefalonia Transform Fault marks the transition between oceanic and continental lithosphere subducting to the south and to the north of it, respectively. A discontinuity is suggested to exist between the two slabs at shallow depths. The Kefalonia Transform Fault is interpreted as an active Subduction-Transform-Edge-Propagator-fault formed as consequence of faster trench retreat induced by the subduction of oceanic lithosphere to the south of it. A model reconstructing the evolution of the subduction system in the area of Peloponnese since 34 Ma, involving the backstepping of the subduction to the back-side of Adria, provides seismological evidence that supports the single-slab model for the HSZ and suggests the correlation between the downdip limit of the seismicity to the amount of subducted oceanic lithosphere. In the area of Rhodes, earthquake hypocentres indicate the presence of a NW dipping subducting slab that rules out the presence of a NE-SW striking Subduction-Transform-Edge-Propagator-fault in the Pliny-Strabo trenches region. Earthquake hypocentres also allow refining the slab tear beneath southwestern Anatolia down to 150–180 km depth. Furthermore, the distribution of microseismicity shows a first-order slab segmentation in the region between Crete and Karpathos, with a less steep and laterally wider slab segment to the west and a steeper and narrower slab segment to the east. Thermal models indicate the presence of a colder slab beneath the southeastern Aegean that leads to deepening of the intermediate-depth seismicity. Slab segmentation affects the upper plate deformation that is stronger above the eastern slab segment and the seismicity along the interplate seismogenic zone.