Seismic-aseismic Transition Research Articles

Heat flow and thermal structure in the Koyna seismic zone, western India: Implications for recurrent reservoir triggered seismicity

Scientific core drilling to a depth of 1522 m in the Koyna seismic zone, western India, provides a rare opportunity to characterize the geothermal regime and study its implications on seismogenesis. The borehole KBH-1 penetrated the 932.5 m thick Deccan Trap and continued for 589.5 m in the granite-gneiss basement. The equilibrium temperature profile was obtained from measurements made 10 months after drilling. Thermal conductivity was measured on core samples of Deccan basalt and basement granitoids. Radiogenic heat production of the basement granitoids was estimated from U, Th and K analyses. Salient results are as follows. (i) The temperature gradients in Deccan basalt and granite-gneiss basement, 26.5 mKm−1 and 15.7 mKm−1, respectively, combined with thermal conductivity measurements for the two rock types, yield a mean heat flow of 43 mW m−2. (ii) The low heat flow in Koyna is similar to values characteristic of Archaean cratons. (iii) Mean Th concentration of 6.8 ppm, U 0.9 ppm, and K 1.8% yielded a mean radiogenic heat production of 0.9 μWm−3 for the basement granitoids. (iv) New geological and geothermal constraints lead to improved estimates of temperature in the crust. Although geothermal considerations support a ∼ 25 km deep seismic-aseismic transition, the seismic activity is limited to the uppermost 10 km. (v) The infrequent occurrence of earthquakes below ∼10 km could be attributed to the low permeability of the basement granitoids, which reduces the influence of pore fluids in triggering failures along faults at deeper levels.

Revisiting evidence for widespread seismicity in the upper mantle under Los Angeles.

We revisit the finding of widespread deep seismicity in the upper mantle imaged with a dense, temporary nodal seismic array in Long Beach, California using back-projection to detect candidate events and trace randomization to develop a reliable imaging threshold for candidate detections. We find that nearly all detections of small events at depths greater than 20 kilometers in the upper mantle fall below the reliability threshold. We find a modest number of small, shallower events in the crust that appear to align with the active Newport-Inglewood Fault. These events occur primarily at 15- to 20-kilometer depth near the base of the seismogenic zone. Localized seismicity under fault zones suggests that the deep extensions of active faults are localized and deforming, with stress concentration leading to a concentration of small events, near the seismic-aseismic transition.

Magnitude-Dependent Transient Increase of Seismogenic Depth

AbstractThe thickness of the seismogenic zone in the Earth’s crust plays an important role in seismotectonics, affecting fault-system architecture and relative fault activity, earthquake size and distribution within a fault system, as well as long-term accumulation of tectonic deformation. Within the last two decades, several studies have revealed that aftershocks of large continental earthquakes may occur below the background depth of the seismogenic zone, that is, below the seismic–aseismic transition zone. This observation may be explained with a strain- and strain-rate-induced shift in rheological behavior that follows large mainshocks, transiently changing the deformation style below the seismogenic zone from incipient ductile to seismically brittle failure. As large earthquakes transiently deepen the seismic–aseismic transition zone, it is plausible to assume that larger mainshocks may cause stronger deepening than smaller mainshocks. Corresponding observations, however, have not yet been reported. Here, we use well-located seismic catalogs from Alaska, California, Japan, and Turkey to analyze if mainshock size positively correlates with the amount of transient deepening of the seismic–aseismic transition zone. We compare the depths of background seismicity with aftershock depths of 16 continental strike-slip earthquakes (6≤M≤7.8) and find that large mainshocks do cause stronger transient deepening than moderate-size mainshocks. We further suggest that this deepening effect also applies to the mainshocks themselves, with larger mainshock coseismic ruptures being capable of extending deeper into the normally aseismic zone. This understanding may help address fundamental questions of earthquake-source physics such as the assumed scale invariance of earthquake stress drop and whether fault-slip scales with rupture length or rupture width.

Hydrous oceanic crust hosts megathrust creep at low shear stresses.

The rheology of the metamorphosed oceanic crust may be a critical control on megathrust strength and deformation style. However, little is known about the strength and deformation style of metamorphosed basalt. Exhumed megathrust shear zones exposed on Kyushu, SW Japan, contain hydrous metabasalts deformed at temperatures between ~300° and ~500°C, spanning the inferred temperature-controlled seismic-aseismic transition. Field and microstructural observations of these shear zones, combined with quartz grain-size piezometry, indicate that metabasalts creep at shear stresses <100 MPa at ~370°C and at shear stresses <30 MPa at ~500°C. These values are much lower than those suggested by viscous flow laws for basalt. The implication is that relatively weak, hydrous, metamorphosed oceanic crust can creep at low viscosities over a wide shear zone and have a critical influence on plate interface strength and deformation style around the seismic-aseismic transition.

Water hammers tremors during plate convergence

Research Article| October 23, 2018 Water hammers tremors during plate convergence An Yin An Yin * 1Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, California 90095-156702, USA *E-mail: ayin54@gmail.com Search for other works by this author on: GSW Google Scholar Geology (2018) 46 (12): 1031–1034. https://doi.org/10.1130/G45261.1 Article history received: 12 Jun 2018 rev-recd: 19 Sep 2018 accepted: 25 Sep 2018 first online: 23 Oct 2018 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Tools Icon Tools Get Permissions Search Site Citation An Yin; Water hammers tremors during plate convergence. Geology 2018;; 46 (12): 1031–1034. doi: https://doi.org/10.1130/G45261.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search nav search search input Search input auto suggest search filter All ContentBy SocietyGeology Search Advanced Search Abstract In this study, I show that rapid (∼100 km/h) dip-parallel tremor migration at seismic-aseismic transition depths (15–55 km) of a convergent margin can result from pressure-wave propagation in an anisotropic viscoplastic shear zone. The anisotropy is characterized by slip-parallel conduits in the dip direction composed of high-permeability brittle mafic rocks embedded in low-permeability ductile felsic materials. At the onset of a slow-slip event, the initial compaction of a mafic conduit causes permeability reduction that in turn can lead to partial or complete blockage of flow path. Due to the water-hammer effect, the sudden blockage of flow paths can trigger a propagating pressure wave with an elevated pore-fluid pressure, which is capable of initiating a progressive shear failure expressed as migrating tectonic tremors along the conduit. For a mafic conduit with a pre-slip static permeability of ∼10−13 m2 and a syn-slip dynamic permeability of 10−15 m2, the water-hammer model generates a pressure jump of up to 50 kPa, sufficient to initiate tremors along subduction zones and shear-zone deformation during the same slow-slip event. You do not currently have access to this article.

Inferring crustal viscosity from seismic velocity: Application to the lower crust of Southern California

We investigate the role of composition on the viscosity of the lower crust through a joint inversion of seismic P-wave (Vp) and S-wave (Vs) velocities. We determine the efficacy of using seismic velocity to constrain viscosity, extending previous research demonstrating robust relationships between seismic velocity and crustal composition, as well as crustal composition and viscosity. First, we calculate equilibrium mineral assemblages and seismic velocities for a global compilation of crustal rocks at relevant pressures and temperatures. Second, we use a rheological mixing model that incorporates single-phase flow laws for major crust-forming minerals to calculate aggregate viscosity from predicted mineral assemblages. We find a robust correlation between crustal viscosity and Vp together with Vs in the α-quartz regime. Using seismic data, geodetic surface strain rates, and heat flow measurements from Southern California, our method predicts that lower crustal viscosity varies regionally by four orders of magnitude, and lower crustal stress varies by three orders of magnitude at 25 km depth. At least half of the total variability in stress can be attributed to composition, implying that regional lithology has a significant effect on lower crustal geodynamics. Finally, we use our method to predict the depth of the brittle–ductile transition and compare this to regional variations of the seismic–aseismic transition. The variations in the seismic–aseismic transition are not explained by the variations in our model rheology inferred from the geophysical observations. Thus, we conclude that fabric development, in conjunction with compositional variations (i.e., quartz and mica content), is required to explain the regional changes in the seismic–aseismic transition.

Opal‐CT in chert beneath the toe of the Tohoku margin and its influence on the seismic aseismic transition in subduction zones

AbstractThick accumulation of chert is a ubiquitous feature of old oceanic plates at convergent margins. In this study, we investigate chert fragments recovered by the Integrated Ocean Drilling Program expedition 343 at the Japan Trench where the 2011 Tohoku‐Oki earthquake (Mw 9.0) occurred. This sample provides a unique opportunity to investigate in situ chert diagenesis at an active subduction margin and its influence on the kinematics of megathrust faulting. Our mineralogical analyses revealed that the chert is characterized by hydrous opal‐CT and may therefore be highly deformable via pressure solution creep and readily accommodate shear strain between the converging plates at driving stresses of kilopascal order. As chert diagenesis advances, any further deformation requires stresses of &gt;100 MPa, given the increasing transport distances for solutes as represented in cherts on land. The chert diagenesis is thus related to the mechanical transition from a weakly to strongly coupled plate interface at this margin.

Aftershock seismicity and tectonic setting of the 2015 September 16 Mw 8.3 Illapel earthquake, Central Chile

Abstract Powerful subduction zone earthquakes rupture thousands of square kilometres along continental margins but at certain locations earthquake rupture terminates. To date, detailed knowledge of the parameters that govern seismic rupture and aftershocks is still incomplete. On 2015 September 16, the Mw 8.3 Illapel earthquake ruptured a 200 km long stretch of the Central Chilean subduction zone, triggering a tsunami and causing significant damage. Here, we analyse the temporal and spatial pattern of the coseismic rupture and aftershocks in relation to the tectonic setting in the earthquake area. Aftershocks cluster around the area of maximum coseismic slip, in particular in lateral and downdip direction. During the first 24 hr after the main shock, aftershocks migrated in both lateral directions with velocities of approximately 2.5 and 5 km hr−1. At the southern rupture boundary, aftershocks cluster around individual subducted seamounts that are related to the downthrusting Juan Fernández Ridge. In the northern part of the rupture area, aftershocks separate into an upper cluster (above 25 km depth) and a lower cluster (below 35 km depth). This dual seismic–aseismic transition in downdip direction is also observed in the interseismic period suggesting that it may represent a persistent feature for the Central Chilean subduction zone.

Low-grade metamorphism around the down-dip limit of seismogenic subduction zones: Example from an ancient accretionary complex in the Shimanto Belt, Japan

Reactions involving clay minerals during low-grade metamorphism at the depth of an ancient accretionary complex in the Shimanto Belt, Kyushu, Japan, were studied by integrated transmission electron microscopy–energy dispersive X-ray spectroscopy and X-ray diffraction analyses of the bulk rock and clay fraction. The analyzed metasediment (the Kitagawa unit) contain an incipient sub-horizontal slaty cleavage. Illite crystallinity data and mica b dimensions indicate that the conditions of metamorphic deformation were anchizone–epizone grade and intermediate pressure. Cleavage formation was linked to two reactions involving clay minerals: (1) the recrystallization of 1 M-dominant matrix mica, inherited from the original sedimentary fabric, into thick, defect-free 2 M 1 packets along cleavage planes; and (2) the formation of chlorite from 7 Å berthierine. Balanced equations among the clay phases, based on compositional data and their relative abundance, suggest that the decomposition of matrix mica resulted in the formation of paragenetic mica and chlorite along the cleavage planes, without significant elemental outflux. Although a modal increase in phyllosilicates is not indicated by the data, the growth of chlorite and mica along cleavage planes may have a large influence on the rheological properties of a décollement and may be related to the occurrence of the seismic–aseismic transition at ~ 350 °C.

Learning from dynamic triggering of low-frequency tremor in subduction zones

Abstract Remote triggering of small low-frequency seismic tremor (non-volcanic tremor) near the seismic-aseismic transition zone of subduction zones, by surface waves from large distant earthquakes, has been reported in southwest Japan and the Cascadia region. Recent observed triggering in southwest Japan from three large earthquakes (2003 Tokachi-oki, 2007 Solomon, and 2008 Wenchuan) covering wide azimuthal information provides strong evidence for the influence of fluids in the source area. The Coulomb failure stress analyses suggest the effective friction coefficient is large for southwest Japan and small for the Cascadia region, which could be related to the amount of fluid in the source regions of the low-frequency tremor.

Au-quartz mineralization near the base of the continental seismogenic zone

Abstract The base of the continental seismogenic zone is defined within individual fault zones by the transition with depth from pressure-sensitive factional (FR) faulting to temperature-sensitive quasi-plastic (QP) ductile shearing. The depth of this FR-QP transition fluctuates principally as a consequence of variations in geothermal gradient and crustal lithology but other factors (e.g. fluid pressure level, strain rate) also play a role. For quartz-dominant and feldspar-dominant lithologies, respectively, it corresponds approximately to isotherms at 300–350°C and c. 450 °C., defining an undulating transition zone in the mid-crust with a relief of the order of 5–10 km. This transition zone correlates with the greenschist-facies metamorphic environment where the bulk of mesozonal Au-quartz lodes form in mixed continuous-discontinuous shear zones. In areas of crustal convergence and thickening, where fluid release results from prograde metamorphic dehydration, especially at the greenschistamphibolite-facies transition in the middle to deep crust, the seismogenic carapace acts as an upper crustal stress guide and low-permeability lid to overpressured metamorphic fluids migrating through shear zone conduits. Under appropriate combinations of stress and fault architecture in the brittle carapace, substantial fluid volumes may be trapped beneath this elastic lid at sufficient overpressure to generate dilatant fault-fracture meshes discharging episodically by fault-valve action following fault rupture, with permeability locally enhanced by aftershock activity distributed about the rupture zones. Topographic irregularities in the seismic-aseismic transition determine rupture nucleation sites and probably play a critical role in focusing the discharge of overpressured metamorphic fluids into the seismogenic layer. This helps to account for the observed spacing of mesozonal lode systems along transcrustal shear zones.

Comparison of smectite- and illite-rich gouge frictional properties: application to the updip limit of the seismogenic zone along subduction megathrusts

Along plate boundary subduction thrusts, the transformation of smectite to illite within fault gouge at temperatures of ∼150°C is one of the key mineralogical changes thought to control the updip limit of seismicity. If correct, this hypothesis requires illite-rich gouges to exhibit frictionally unstable (velocity-weakening) behavior. Here, we report on laboratory experiments designed to investigate the frictional behavior of natural and synthetic clay-rich gouges. We sheared 5-mm-thick layers of commercially obtained pure Ca-smectite, a suite of smectite–quartz mixtures, and natural illite shale (grain size ranging from 2 to 500 μm) in the double-direct shear geometry to shear strains of ∼7–30 at room humidity and temperature. XRD analyses show that the illite shale contains dominantly clay minerals and quartz; within the clay-sized fraction (<2 μm), the dominant mineral is illite. Thus, we consider this shale as an appropriate analog for fine-grained sediments incoming to subduction zones, within which smectite has been transformed to illite. We observe a coefficient of friction (μ) of 0.42–0.68 for the illite shale, consistent with previous work. Over a range of normal stresses from 5 to 150 MPa and sliding velocities from 0.1 to 200 μm/s, this material exhibits only velocity-strengthening behavior, opposite to the widely expected, potentially unstable velocity-weakening behavior of illite. Smectite sheared under identical conditions exhibits low friction (μ=0.15–0.32) and a transition from velocity weakening at low normal stress to velocity strengthening at higher normal stress (>40 MPa). Our data, specifically the velocity-strengthening behavior of illite shale under a wide range of conditions, do not support the hypothesis that the smectite–illite transition is responsible for the seismic–aseismic transition in subduction zones. We suggest that other depth- and temperature-dependent processes, such as cementation, consolidation, and slip localization with increased shearing, may play an important role in changing the frictional properties of subduction zone faults, and that these processes, in addition to clay mineralogy, should be the focus of future investigation.

Nucleation of large earthquakes determined by the seismic–aseismic boundary: agreement between models and observations

Using a slip-dependent constitutive law, we investigate how the seismic–aseismic boundary determines the nucleation process of large earthquakes. We model the depth variation of the breakdown strength drop and the critical weakening displacement on the basis of several studies by Ohnaka and co-workers. In this model, the breakdown strength drop increases linearly with depth in an unstable seismogenic zone and decreases sharply in a seismic–aseismic transition at the bottom of the seismogenic zone. On the other hand, the critical weakening displacement is almost constant in the seismogenic zone, and increases sharply with depth in the seismic–aseismic transition at the bottom of the seismogenic region as the fracture energy remains constant. It has been reported that large intra-plate earthquakes appear to initiate at the place where the cut-off depth of seismicity changes sharply. We examine a case in which there is a steep depth change in the seismic–aseismic transition at the bottom of the seismogenic region. The results of the simulation show two stages of the accelerating slip: slip velocity enhancement widely along the seismic–aseismic transition at the bottom of the seismogenic region, and subsequent localized accelerated slip in the region of a steep depth change in the seismic–aseismic transition at the bottom of the seismogenic region. The first stage of the accelerating slip will be important for the intermediate-term earthquake prediction. We also consider a broad aseismic weak zone in the seismogenic zone. In this case, first, slip velocity is enhanced in a broad weak zone, then nucleation starts at lower corner of the surrounding strong region. Our numerical results can explain the location of rupture initiation of some recent large earthquakes.

Fluid flow in mid-to deep crustal shear systems: Experimental constraints, observations on exhumed high fluid flux shear systems, and implications for seismogenic processes

The aseismic parts of shear systems at mid-to deep crustal levels can localise the supply of deeply-sourced, high pressure fluids into the shallower level parts of these systems in the seismogenic regime. Even during deformation at elevated temperatures in mid-to deep crustal shear zones, high pore fluid factors promote grain-scale to macroscopic fracture growth and permeability enhancement. The evolution of permeability is governed by dynamic competition between crack growth and crack sealing/healing processes. Steady state creep below the seismic-aseismic transition leads to steady state permeability and continuous fluid flow. In contrast, within and near the base of the seismogenic regime, large cyclic changes in permeability can lead to episodic fluid flow and fluctuations in fluid pressure. At mid-crustal depths, temporal and spatial variations in pore fluid pressure and shear stress within shear networks influence rupture nucleation via cyclic changes in shear strength. Fluid pressure and shear stress cycling can also drive repeated transitions between interseismic creep and rapid, co-seismic slip. Reaction-weakening and reaction-strengthening, during hydrothermal alteration in fluid-active shear systems, can also drive transitions between seismic and aseismic behaviour on longer time-scales.

Deformational controls on the dynamics of fluid flow in mesothermal gold systems

Abstract Coupling between deformation processes and rock permeability is a major factor influencing fluid migration and fluid-rock interaction during the formation of mesothermal lode gold systems in mid- to upper crustal regimes. The evolution of permeability during deformation is controlled by a dynamic competition between deformation-induced porosity creation processes and porosity destruction processes. Localization of deformation in faults and shear zones leads to flow localization, with large scale flow systems forming when active faults and shear zones link to create percolation networks. Broad regions of fluid focusing develop around the upstream segments of active shear networks and fluid discharge regions develop in the downstream parts of these systems. The architecture of flow within shear networks is influenced by the relative proportions of backbone, dangling and isolated structures within the network. Connectivity in the network may increase with growth of the shear network, but is expected to continually change as the locus and intensity of deformation changes. The typical distribution of mesothermal lode deposits within crustal scale shear networks indicates that mesothermal systems might develop most efficiently in networks that are close to the percolation threshold, i.e. with most flow occurring along a flow backbone forming only a small part of the total shear network. Dangling elements adjacent to the backbone, particularly in the downstream (discharge) parts of the system, provide some of the best potential for gold deposition by fluid-rock interaction processes. Contrasting styles of fluid flow are expected between the seismogenic and aseismic regimes of the shear- or fault-hosted hydrothermal systems associated with mesothermal lode gold formation. Below the seismic-aseismic transition, creep processes can lead to near-steady state permeabilities and produce continuous fluid flow regimes. In the seismogenic regime, large cyclic changes in fault permeability, fluid pressures and fluid flux occur during the seismic cycle, and lead to fault-valve behaviour and episodic fluid flow. The seismogenic regime allows for a richer variety of gold deposition processes than is generally available in the aseismic regions of hydrothermal systems associated with lode gold formation.

Thermal regime of the San Andreas Fault near Parkfield, California

Knowledge of the temperature variation with depth near the San Andreas fault is vital to understanding the physical processes that occur within the fault zone during earthquakes and creep events. Parkfield is near the southern end of the Coast Ranges segment of the San Andreas fault. This segment has higher mean heat flow than the Cape Mendocino segment to the northwest or the Mojave segment to the southeast. Boreholes were drilled specifically for the U.S. Geological Survey's Parkfield earthquake prediction experiment or converted from other uses at 25 sites within a few kilometers of the fault near Parkfield. These holes, which range in depth from 150 to over 1500m, were intended mainly for the deployment of volumetric strain meters, water‐level recorders, and other downhole instruments. Temperature profiles were obtained from all the holes, and heat flow values were estimated from 17 of them. For a number of reasons, including a paucity of thermal conductivity data and rugged local topography, the accuracy of individual determinations was not sufficiently high to document local variations in heat flow. Values range from 54 to 92 mW m−2, with mean and 95% confidence limits of 74±4 mW m−2. This mean is slightly lower than the mean (83±3) of 39 previously published values from the central Coast Ranges, but it is consistent with the overall pattern of elevated heat flow in the Coast Ranges, and it is transitional to the mean of 68±2 mW m−2 that characterizes the Mojave segment of the San Andreas fault immediately to the south. The lack of a heat flow peak near the fault underscores the absence of a frictional thermal anomaly and provides additional support for a very small resolved shear stress parallel to the San Andreas fault and the nearly fault‐normal maximum compressive stress observed in this region. Estimates of subsurface thermal conditions indicate that the seismicaseismic transition for the Parkfield segment corresponds to temperatures in the range of 350°–400°C. Increasing heat flow to the northwest of Parkfield corresponds to a transition from locked to creeping sections and to a shallowing of the base of seismicity and confirms the importance of temperature in controlling the thickness of the seismogenic crust. Lateral variations in heat flow do not appear to have any major role in determining the regularity of M5.5–6 earthquakes at Parkfield.

The relationship between plate motions and seismic moment tensors, and the rates of active deformation in the Mediterranean and Middle East

This paper is concerned with the relationship between the overall motion across a zone of distributed continental deformation and the seismic moment tensors of earthquakes that occur within it. The overall deformation in the zone is described by the deformation gradient tensor L, which may be split into a symmetric part, S, and an antisymmetric part, A. S is the strain tensor, and can always be determined from the sum of the moment tensors, following the result of Kostrov (1974). A corresponds to a rigid body rotation, and is in principle unobservable seismically: the moment tensors contain no information about A, regardless of whether the ambiguity between fault and auxiliary planes is resolved. From S the integrated rates of motion normal and parallel to the zone boundary, as well as vertically, can be calculated. Of these, only the motion normal to the zone is specified by the motion across its boundaries. In general, S (and hence L) is not specified by the motion of the plates bounding the zone. Only if some a priori assumptions about L are made, can information about A be recovered from the seismic moment tensors. Otherwise A must be determined independently from paleomagnetic or geodetic measurements. These results are applied to the Mediterranean region to see whether the motion between the relatively rigid regions of central Iran, Turkey, Arabia, Africa, the Adriatic Sea and Eurasia is accommodated seismically within the upper crust of the wide deforming zones that bound these regions. In NE Iran, the North Anatolian Fault Zone and the Aegean Sea all or most of the deformation is probably taken up seismically. In the Zagros, Caucasus, Hellenic Trench and western Mediterranean probably 10 per cent or less of the upper-crustal deformation is seismic and the rest must be accommodated by creep. The Cyprus arc, the East Anatolian and Dead Sea Fault Zones have had insufficient seismicity this century for any conclusions to be drawn. The seismicity in central Italy and Yugoslavia accounts for velocities of about 2 mm yr−1 normal to the deforming zones, but there is no independent estimate of the velocities on the borders of the Adriatic Sea with which to compare these. The seismicity in the Aegean region indicates very high stretching velocities (c. 60 mm yr−1) and strain rates (c. 4 × 10−15 s−1). These in turn require correspondingly high subduction rates (c. 100 mm yr−1) in the Hellenic Trench. If these rates have been constant in time, it is unlikely that the tip of the sinking slab beneath the southern Aegean began to be subducted more than 5 Ma ago. These high-stretching rates in the Aegean, if extrapolated back to the Pliocene, are compatible with observed finite strains and paleomagnetic rotations. They are also likely to have raised to a shallower depth the isotherm corresponding to the seismic-aseismic transition in the crust, perhaps accounting for the relatively shallow focal depths of normal faulting earthquakes in the Aegean compared with those in other areas of continental extension where strain rates are lower. It is possible that the reason for the dominantly aseismic deformation in the Zagros and Hellenic Trench is related to the great thickness of sediments, partly decoupled from the basement by salt, in both places. This may lead to elevated basement temperatures and inhibit upward fault propagation, thus restricting the size of seismogenic fault planes (and hence seismic moment) and causing the sedimentary cover to deform independently from the basement, partly by folding. However, this explanation has serious drawbacks and is not easily applicable to other areas, notably the western Mediterranean.

Migration of the seismic-aseismic transition during uniform and nonuniform extension of the continental lithosphere

Research Article| April 01, 1987 Migration of the seismic-aseismic transition during uniform and nonuniform extension of the continental lithosphere Philip England; Philip England 1Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138 Search for other works by this author on: GSW Google Scholar James Jackson James Jackson 2Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, England Search for other works by this author on: GSW Google Scholar Author and Article Information Philip England 1Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138 James Jackson 2Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, England Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1987) 15 (4): 291–294. https://doi.org/10.1130/0091-7613(1987)15<291:MOTSTD>2.0.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Philip England, James Jackson; Migration of the seismic-aseismic transition during uniform and nonuniform extension of the continental lithosphere. Geology 1987;; 15 (4): 291–294. doi: https://doi.org/10.1130/0091-7613(1987)15<291:MOTSTD>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract We investigate the thermal and seismic implications of two extreme configurations of lithospheric extension. One extreme treats the extension as being approximately uniform with depth, the upper crust extending by the rotation of fault-bounded blocks above a diffusely deforming lower crust; in the other extreme, crustal extension occurs only above a low-angle fault (dip less than about 10°). Very low-angle shear zones that initially move aseismically within a uniformly extending lithosphere may be raised through the depth range that is normally seismically active without cooling appreciably; the condition for this to occur is that extension should take place faster than about 10−14/s. In contrast, extension accommodated entirely above a very low-angle shear zone rapidly brings that zone to temperatures at which it should be seismically active, even if initial movement on it were aseismic. The different thermal histories implied by the two configurations of extension offer the possibility of distinguishing between modes of extension by observations of temperature-time and pressure-temperature paths on rocks close to low-angle faults. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

On the nomenclature of mode of failure transitions in rocks

Attention is drawn to the increasing identification in the geological literature of the concept of ductility with a particular mechanism of rock deformation, namely intracrystalline plasticity. It is argued here that “ductility” only describes the capacity of a material to deform to a substantial strain without the tendency to localize the flow into bands (faults), and should not have a mechanistic connotation. A simple diagrammatic visualization of the concepts of mode-of-failure transitions is proposed. By force of repetition, the depth in the Earth to the seismic—aseismic transition is being regarded as a brittle—ductile transition in the sense of a brittle—plastic transition. This tendency is criticized and it is suggested that the nomenclature of mode of failure of rocks be rationalized, in the face of growing prospects of imprecision and misunderstandings of the communication of data and ideas.

Fault zone models, heat flow, and the depth distribution of earthquakes in the continental crust of the United States

abstract Models of fault zones in continental crust, based on the analysis of rock deformation textures, suggest that the depth of seismic activity is controlled by the passage from a pressure-sensitive, dominantly frictional regime to strongly temperature-dependent, quasi-plastic mylonitization at greenschist and higher grades of metamorphism. Sufficient knowledge now exists concerning the frictional and rheological properties of quartz-bearing rocks to construct crude strength-depth curves for different geotherms. In such models, shear resistance peaks sharply at the inferred seismic-aseismic transition. The maximum depth of microseismic activity in various heat flow provinces of the conterminous United States generally correlates well with the frictional to quasi-plastic transition modeled for the different geotherms. Larger earthquakes ( M L > 5.5) also tend to nucleate near the base of the seismogenic zone. This region is postulated to have the highest concentration of distortional strain energy for stress levels at failure, and can be regarded as the prime asperity in crustal fault zones.