Localized megathrust slip controlled by metasomatic reactions in subduction mélanges

2021 ◽  
Author(s):  
Kohtaro Ujiie ◽  
Kazuya Noro ◽  
Norio Shigematsu ◽  
Åke Fagereng ◽  
Naoki Nishiyama ◽  
...  
Keyword(s):  
Mantle Wedge ◽  
Boundary Diffusion ◽  
Seismogenic Zone ◽  
Diffusion Creep ◽  
Slow Slip ◽  
Slow Slip Events ◽  
Fine Grained ◽  
Geological Conditions ◽  
Viscous Shear ◽  
Fluid Release

<p>Aseismic megathrust slip down-dip of the seismogenic zone is accommodated by either steady creep or episodic slow slip events (SSEs). However, the geological conditions defining the rheology of megathrust slip remain elusive. Here, we show that subduction mélanges deformed at ~370–500 °C in warm-slab environments record fluid release and viscous shear localization associated with metasomatic reactions between juxtaposed metapelitic and metabasaltic rocks. Metasomatic reactions induced albitization of metapelite, resulting in depth-dependent rheological behavior. In a mélange deformed at ~370 °C, near the down-dip limit of the seismogenic zone, very fine-grained metasomatic albite facilitated grain boundary diffusion creep at stresses less than those in the surrounding metapelite and metabasalt, contributing to an overall decreased megathrust strength. In a mélange deformed at ~500 °C, near the mantle wedge corner, metasomatic reactions led to brittle fracturing, albite grain growth, and incorporation of strengthened albitized metapelite blocks into a chlorite-actinolite matrix deforming at locally elevated strain rate of ~10<sup>-10</sup> s<sup>-1</sup>. We suggest that metasomatic reactions facilitate localized changes in megathrust slip mode with depth, potentially providing a mechanism for change from viscous creep to SSEs with tremor.</p>

Modeling the Interaction between Slow Slip Events and Earthquake Ruptures in the Nicoya Peninsula, Costa Rica. 

2021 ◽  
Author(s):  
Lise Alalouf ◽  
Yajing Liu
Keyword(s):  
Costa Rica ◽  
Subduction Zones ◽  
Computation Time ◽  
Seismogenic Zone ◽  
Model Parameters ◽  
Slow Slip ◽  
Coseismic Slip ◽  
Slow Slip Events ◽  
Megathrust Earthquakes ◽  
Earthquake Ruptures

<p>Subduction zones are where the largest earthquakes occur. In the past few decades, scientists have also discovered the presence of episodic aseismic slip, including slow slip events (SSEs), along most of the subduction zones. However, it is still unclear how these SSEs can influence megathrust earthquake ruptures. The Costa Rica subduction zone is a particularly interesting area because a SSE was recorded 6 months before the 2012 Mw7.6 earthquake in the Nicoya Peninsula, suggesting a potential stress transfer from the SSE to the earthquake slip zone. SSEs beneath the Nicoya Peninsula were also recorded both updip and downdip the seismogenic zone, making it a unique area to study the complex interaction between SSEs and earthquakes.</p><p>As most of the shallow SSEs were recorded around the Nicoya Peninsula, we chose to start using a 1D planar fault embedded in a homogeneous elastic half-space, with different dipping angles following several geometric profiles of the subduction fault beneath the Nicoya Peninsula section of the Costa Rica margin. This 1D modelling study allows us to better investigate the interaction between shallow and deep SSEs and megathrust earthquakes with high numerical resolution and relatively short computation time. The model provides information on the long-term seismic history by reproducing the different stages of the seismic cycle (interseismic slip, shallow and deep episodic slow slip, and coseismic slip).</p><p>We study the influence of the variation of numerical parameters and frictional properties on the recurrence interval, maximum slip velocity and cumulative slip of SSEs (both shallow and deep) and earthquakes and their interaction with each other. We then compare our results with GPS and seismic observations (i.e. cumulative slip, characteristic duration, moment rate, depth and size of the rupture, equivalent magnitude) to identify an optimal set of model parameters to understand the interaction between various modes of subduction fault deformation.</p>

Record by quartz veins of earthquakes and slow slip events

2020 ◽  
Author(s):  
Hugues Raimbourg ◽  
Vincent Famin ◽  
Kristijan Rajic ◽  
Saskia Erdmann ◽  
Benjamin Moris-Muttoni ◽  
...  
Keyword(s):  
Trace Element ◽  
Fluid Pressure ◽  
Trace Element Content ◽  
Element Content ◽  
Seismogenic Zone ◽  
Slow Slip ◽  
Slow Slip Events ◽  
Quartz Veins ◽  
Temperature Conditions ◽  
Montagne Noire

<p>Veins that form contemporaneously with deformation are the best recorders of the fluids circulating in the depths of orogenic and subduction zones. We have analyzed syn-kinematic quartz veins from accretionary prisms (Shimanto Belt in Japan, Kodiak accretionary prism in Alaska) and tectonic nappes in collisional orogens (Flysch à Helminthoïdes in the Alps, the southern domain of the variscan Montagne Noire), which formed at temperature conditions between 250 and 350°C, i.e. spanning the downdip limit of large subduction earthquakes and the generation of slow slip events. In all geological domains, veins hosted in rocks with the lower temperature conditions (~250-300°C) show quartz grains with crystallographic facets and growth rims. Cathodoluminescence (CL) imaging of these growth rims shows two different colors, a short-lived blue color and a brown one, attesting to cyclic variations in precipitation conditions. In contrast, veins hosted in rocks with the higher temperature conditions (~350°C), show a homogeneous, CL-brown colored quartz, except for some very restricted domains of crack-seal structures of CL-blue quartz found in Japan, Kodiak and Montagne Noire. Based on laser ablation and electron microprobe mapping, the variations in CL colors appear correlated with the trace element content of quartz, the short-lived CL-blue being associated with the substitution of Si<sup>4+</sup> by Al<sup>3+</sup>+Li<sup>+</sup>/H<sup>+</sup>.</p><p>Due to their ubiquitous presence in various settings, the variations in CL colors in the lower T range reflect a common, general process. We interpret these cyclic growth structures as a reflection of deformation/fracturing events, which triggered transient changes in (1) the fluid pressure through fluid flow and (2) the chemistry of the fluid due to enhanced reactivity of the fractured material. The CL-blue growth rims delineate zones where quartz growth was rapid and crystals incorporated a large proportion of Al and Li. Crystal growth continued at a lower pace after fluid pressure and composition evolved to equilibrium conditions, leading to the formation of CL-brown quartz with few substitutions of tetrahedral Si. The variations in fluid pressure fluctuated at values close to lithostatic conditions, as indicated by growth in cavities that remained open.</p><p>The crack-seal microstructures have been interpreted as the result of slow-slip events near the base of the seismogenic zone (Fisher and Brantley, 2014; Ujiie et al., 2018). Our observations on quartz composition suggest that the quartz in crack-seal microstructures records episodic variation in fluid pressure and composition, similar to vein quartz at T<~300 °C. In contrast to the cooler and shallower domain, the variations are significantly smaller, as recorded by the very limited extent of the CL-blue domains, and most if not all of the quartz growth occurred under constant physico-chemical conditions, including a near lithostatic fluid pressure. </p><p>We conclude that quartz trace element content is a useful tool to track variations in fluid conditions. In particular, at seismogenic depths (i.e. near 250°C), fluid pressure varies significantly around a lithostatic value. In contrast, deeper, near the base of the seismogenic zone where slow slip events occur (i.e. near 350°C), the variations in fluid pressure conditions are smaller.</p>

scholarly journals Fluid pressure variations recorded by quartz vein geochemistry

2021 ◽  
Author(s):  
Hugues Raimbourg ◽  
Vincent Famin ◽  
Kristijan Rajic ◽  
Saskia Erdmann ◽  
Benjamin Moris-Muttoni ◽  
...  
Keyword(s):  
Trace Element ◽  
Fluid Pressure ◽  
Trace Element Content ◽  
Element Content ◽  
Seismogenic Zone ◽  
Slow Slip ◽  
Slow Slip Events ◽  
Temperature Conditions ◽  
Montagne Noire ◽  
Lower Temperature

<p>Veins that form contemporaneously with deformation are the best recorders of the fluids circulating in the depths of orogenic and subduction zones. We have analyzed syn-kinematic quartz veins from accretionary prisms (Shimanto Belt in Japan, Kodiak accretionary Complex in Alaska) and tectonic nappes in collisional orogens (Flysch à Helminthoïdes in the Alps, southern nappes of the variscan Montagne Noire), which formed at temperature conditions between 250 and 350°C, i.e. spanning the downdip limit of large subduction earthquakes and the generation of slow slip events. In all geological domains, veins hosted in rocks that have experienced the lower temperature conditions (~250-300°C) show quartz grains with crystallographic facets and growth rims. Cathodoluminescence (CL) imaging of these growth rims shows two different colors, a short-lived blue color and a brown one, attesting to cyclic variations in precipitation conditions. In contrast, veins hosted in rocks that have experienced the higher temperature conditions (~350°C), show a homogeneous, CL-brown colored quartz, except for some very restricted domains of crack-seal structures of CL-blue quartz found in Japan, Kodiak and Montagne Noire.</p><p>Based on laser ablation analysis and electron microprobe mapping, variations in CL colors appear correlated with the trace element content of quartz. The highly luminescent quartz contains high concentrations of aluminum (Al) and lithium (Li), up to 3000 and 400 ppm, respectively. Variations in Al and Li correlate well, so that Li appears as the main charge‐compensating cation for SiàAl substitution.</p><p>Due to their ubiquitous presence in various settings, the variations in CL colors in the lower temperature range reflect a common, general process. We interpret these cyclic growth structures as a result of deformation/fracturing events, which triggered transient changes in fluid pressure. The CL-blue growth rims delineate zones where quartz growth was rapid and crystals incorporated a large proportion of Al and Li. Crystal growth continued at lower pace after fluid pressure evolved to equilibrium conditions, leading to the formation of CL-brown quartz with fewer substitutions of tetrahedral Si. The variations in fluid pressure fluctuated at values close to lithostatic conditions, as indicated by growth in cavities that remained open.</p><p>The crack-seal microstructures have been interpreted as the result of slow-slip events near the base of the seismogenic zone (Fisher and Brantley, 2014; Ujiie et al., 2018). Our observations on quartz composition suggest that the quartz in crack-seal microstructures records episodic variation in fluid pressure, similar to vein quartz at T<~300 °C. In contrast to the cooler and shallower domain, the variations are significantly smaller, as recorded by the very limited extent of the CL-blue domains, and most if not all of the quartz growth occurred under constant physico-chemical conditions, including a near lithostatic fluid pressure. </p><p>We conclude that quartz trace element content might be a useful tool to track variations in fluid conditions. In particular, at seismogenic depths (i.e. near 250°C), fluid pressure varies significantly around a lithostatic value. In contrast, deeper, near the base of the seismogenic zone where slow slip events occur (i.e. near 350°C), the variations in fluid pressure are smaller.</p>

scholarly journals Shallow slow earthquakes to decipher future catastrophic earthquakes in the Guerrero seismic gap.

2020 ◽  
Author(s):  
Raymundo Plata-Martínez ◽  
Satoshi Ide ◽  
Masanao Shinohara ◽  
Emmanuel Soliman Garcia Mortel ◽  
Naoto Mizuno ◽  
...  
Keyword(s):  
Seismogenic Zone ◽  
Seismic Gap ◽  
Slow Slip ◽  
Dynamic Rupture ◽  
Plate Interface ◽  
Slow Slip Events ◽  
Repeating Earthquakes ◽  
Slow Earthquakes ◽  
Slip Area ◽  
Large Earthquakes

Abstract The Guerrero seismic gap is presumed to be a major source of seismic and tsunami hazard along the Mexican subduction zone. Until recently, there were limited observations to describe the shallow portion of the plate interface in Guerrero. For this reason, we deployed offshore instrumentation to gain new seismic data and identify the extent of the seismogenic zone inside the Guerrero gap. We discovered episodic shallow tremors and potential slow slip events which, together with repeating earthquakes, seismicity, residual gravity and residual bathymetry suggest that a portion of the shallow plate interface in the Guerrero seismic gap undergoes stable slip. This mechanical condition may not only explain the long return period of large earthquakes with origins inside the Guerrero seismic gap, but also reveal why the rupture from past M<8 earthquakes on adjacent megathrust fault segments did not propagate into the gap to encompass a larger slip area. Nevertheless, a large enough earthquake initiating nearby could rupture through the entire Guerrero seismic gap if driven by dynamic rupture effects.

scholarly journals Excitation of San Andreas tremors by thermal instabilities below the seismogenic zone

2020 ◽  
Vol 6 (36) ◽  
pp. eabb2057 ◽  
Author(s):  
Lifeng Wang ◽  
Sylvain Barbot
Keyword(s):  
Low Frequency ◽  
Frictional Resistance ◽  
Seismogenic Zone ◽  
Slow Slip ◽  
Slow Slip Events ◽  
Tectonic Plates ◽  
San Andreas ◽  
Wide Range ◽  
Shear Heating ◽  
Increasing Temperature

The relative motion of tectonic plates is accommodated at boundary faults through slow and fast ruptures that encompass a wide range of source properties. Near the Parkfield segment of the San Andreas fault, low-frequency earthquakes and slow-slip events take place deeper than most seismicity, at temperature conditions typically associated with stable sliding. However, laboratory experiments indicate that the strength of granitic gouge decreases with increasing temperature above 350°C, providing a possible mechanism for weakening if temperature is to vary dynamically. Here, we argue that recurring low-frequency earthquakes and slow-slip transients at these depths may arise because of shear heating and the temperature dependence of frictional resistance. Recurring thermal instabilities can explain the recurrence pattern of the mid-crustal low-frequency earthquakes and their correlative slip distribution. Shear heating associated with slow slip is sufficient to generate pseudotachylyte veins in host rocks even when fault slip is dominantly aseismic.

scholarly journals Shallow slow earthquakes to decipher future catastrophic earthquakes in the Guerrero seismic gap

2021 ◽  
Vol 12 (1) ◽  
Author(s):  
R. Plata-Martinez ◽  
S. Ide ◽  
M. Shinohara ◽  
E. S. Garcia ◽  
N. Mizuno ◽  
...  
Keyword(s):  
Seismogenic Zone ◽  
Seismic Gap ◽  
Slow Slip ◽  
Dynamic Rupture ◽  
Plate Interface ◽  
Slow Slip Events ◽  
Repeating Earthquakes ◽  
Residual Gravity ◽  
Slow Earthquakes ◽  
Large Earthquakes

AbstractThe Guerrero seismic gap is presumed to be a major source of seismic and tsunami hazard along the Mexican subduction zone. Until recently, there were limited observations at the shallow portion of the plate interface offshore Guerrero, so we deployed instruments there to better characterize the extent of the seismogenic zone. Here we report the discovery of episodic shallow tremors and potential slow slip events in Guerrero offshore. Their distribution, together with that of repeating earthquakes, seismicity, residual gravity and bathymetry, suggest that a portion of the shallow plate interface in the gap undergoes stable slip. This mechanical condition may not only explain the long return period of large earthquakes inside the gap, but also reveals why the rupture from past M < 8 earthquakes on adjacent megathrust segments did not propagate into the gap to result in much larger events. However, dynamic rupture effects could drive one of these nearby earthquakes to break through the entire Guerrero seismic gap.

scholarly journals What’s down there? The structures, materials and environment of deep-seated slow slip and tremor

2021 ◽  
Vol 379 (2193) ◽  
pp. 20200218 ◽  
Author(s):  
Whitney M. Behr ◽  
Roland Bürgmann
Keyword(s):  
Subduction Zones ◽  
Source Region ◽  
Fluid Pressure ◽  
Plate Boundary ◽  
Seismogenic Zone ◽  
Slow Slip ◽  
Earthquake Dynamics ◽  
Plate Interface ◽  
Viscous Shear ◽  
Geophysical Imaging

Deep-seated slow slip and tremor (SST), including slow slip events, episodic tremor and slip, and low-frequency earthquakes, occur downdip of the seismogenic zone of numerous subduction megathrusts and plate boundary strike-slip faults. These events represent a fascinating and perplexing mode of fault failure that has greatly broadened our view of earthquake dynamics. In this contribution, we review constraints on SST deformation processes from both geophysical observations of active subduction zones and geological observations of exhumed field analogues. We first provide an overview of what has been learned about the environment, kinematics and dynamics of SST from geodetic and seismologic data. We then describe the materials, deformation mechanisms, and metamorphic and fluid pressure conditions that characterize exhumed rocks from SST source depths. Both the geophysical and geological records strongly suggest the importance of a fluid-rich and high fluid pressure habitat for the SST source region. Additionally, transient deformation features preserved in the rock record, involving combined frictional-viscous shear in regions of mixed lithology and near-lithostatic fluid pressures, may scale with the tremor component of SST. While several open questions remain, it is clear that improved constraints on the materials, environment, structure, and conditions of the plate interface from geophysical imaging and geologic observations will enhance model representations of the boundary conditions and geometry of the SST deformation process. This article is part of a discussion meeting issue ‘Understanding earthquakes using the geological record’.

Seismic and transient slip characteristics of frictional-viscous shear zones in subduction environments

2021 ◽  
Author(s):  
Whitney Behr ◽  
Taras Gerya
Keyword(s):  
Shear Zone ◽  
Numerical Models ◽  
Low Frequency ◽  
Shear Zones ◽  
Finite Width ◽  
Slow Slip ◽  
Deep Roots ◽  
Slow Slip Events ◽  
Aspect Ratios ◽  
Viscous Shear

&lt;p&gt;The deep roots of subduction megathrusts exhibit aseismic slow slip events, commonly accompanied by tremor and low-frequency earthquakes. Observations from exhumed rocks suggest that the deep subduction interface is a shear zone in which frictional lenses are embedded in a weaker, distributed viscous matrix deformed under high fluid pressures and low stresses. Here we use numerical models to explore the transient slip characteristics of finite-width frictional-viscous shear zones. Our model formulation utilizes an invariant form of rate- and state-dependent friction (RSF) and simulates earthquakes along spontaneously evolving faults embedded in a 2D continuum. The setup includes two elastic plates bounding a viscoelastoplastic shear zone (subduction interface) with inclusions (clasts) of varying sizes, aspect ratios, distributions and viscosity contrasts with respect to the surrounding matrix. The entire shear zone exhibits the same velocity-weakening RSF parameters, but the low viscosity matrix in the shear zone has the capacity to switch between RSF and linear viscous creep as a function of its local viscosity and stress state. Results show that for a range of matrix viscosities near a threshold viscosity (representative of the frictional-viscous transition), viscous damping and stress heterogeneity in these shear zones both 1) sets the &amp;#8216;speed limit&amp;#8217; for earthquake ruptures that nucleate in clasts such that they propagate at velocities similar to observed slow slip events; and 2) simultaneously permits the transmission of slow slip from clast to clast, allowing slow ruptures to propagate substantial distances over the model domain. For reasonable input parameters, modeled events have moment-duration statistics, stress drops, and rupture propagation rates that match natural slow slip events. Events resembling very low-frequency earthquakes appear to be favored at high clast densities and low matrix viscosities, whereas longer duration, higher-magnitude slow slip events are favored at intermediate clast densities and near-threshold viscosities. These model results have potential to reconcile geophysical constraints on slow slip phenomena with the exhumed geological record of the slow slip environment.&lt;/p&gt;

scholarly journals Earthquake swarms and slow slip on a sliver fault in the Mexican subduction zone

2019 ◽  
Vol 116 (15) ◽  
pp. 7198-7206 ◽  
Author(s):  
Shannon L. Fasola ◽  
Michael R. Brudzinski ◽  
Stephen G. Holtkamp ◽  
Shannon E. Graham ◽  
Enrique Cabral-Cano
Keyword(s):  
Subduction Zone ◽  
Local Network ◽  
Seismogenic Zone ◽  
Slow Slip ◽  
Plate Interface ◽  
Slow Slip Events ◽  
Parallel Motion ◽  
Oblique Convergence ◽  
Seismicity Catalog ◽  
Mexican Subduction Zone

The Mexican subduction zone is an ideal location for studying subduction processes due to the short trench-to-coast distances that bring broad portions of the seismogenic and transition zones of the plate interface inland. Using a recently generated seismicity catalog from a local network in Oaxaca, we identified 20 swarms of earthquakes (M < 5) from 2006 to 2012. Swarms outline what appears to be a steeply dipping structure in the overriding plate, indicative of an origin other than the plate interface. This steeply dipping structure corresponds to the northern boundary of the Xolapa terrane. In addition, we observed an interesting characteristic of slow slip events (SSEs) where they showed a shift from trenchward motion toward an along-strike direction at coastal GPS sites. A majority of the swarms were found to correspond in time to the along-strike shift. We propose that swarms and SSEs are occurring on a sliver fault that allows the oblique convergence to be partitioned into trench-perpendicular motion on the subduction interface and trench-parallel motion on the sliver fault. The resistivity structure surrounding the sliver fault suggests that SSEs and swarms of earthquakes occur due to high fluid content in the fault zone. We propose that the sliver fault provides a natural pathway for buoyant fluids attempting to migrate upward after being released from the downgoing plate. Thus, sliver faults could be responsible for the downdip end of the seismogenic zone by creating drier conditions on the subduction interface trenchward of the sliver fault, promoting fast-slip seismogenic rupture behavior.

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