An official website of the United States government
Abstract Earthquake clustering can be promoted by local, regional, and remote triggering. The interaction between faults by static and dynamic stress transfer has received much attention. However, the role of quasi‐static stress interaction mediated by viscoelastic flow is still poorly understood. Here, we investigate whether the tight synchronization of moment‐magnitude 6 earthquakes every about 6 years on distant asperities in the Gofar‐Discovery fault system of the East Pacific Rise may be caused by mechanical coupling within the lithosphere‐asthenosphere system. We build a three‐dimensional numerical model of seismic cycles in the framework of rate‐ and state‐dependent friction with a brittle layer overlaying a viscoelastic mantle with nonlinear rheology to simulate earthquake cycles on separate asperities. The brittle section of the West Gofar fault consists of two frictionally unstable 20 km‐long by 5 km‐wide asperities separated by a velocity‐strengthening barrier, consistent with seismic observations, allowing stress transfer by afterslip and viscoelastic relaxation. We find that viscoelastic stress transfer can promote the synchronization of earthquakes. Even if the asperities are separated by as far as 30 km, synchronization is still possible for a viscosity of the underlying mantle of 1017 Pa s, which can be attained by dislocation creep or transient creep during the postseismic period. Considering the similarities in tectonic and structural settings, viscoelastic stress transfer and earthquake synchronization may also occur at 15’20 (Mid‐Atlantic Ridge), George V (Southeast Indian Ridge), Menard and Heezen transform fault (Pacific‐Antarctic Ridge).
more »
« less
Heterogeneous Power‐Law Flow With Transient Creep in Southern California Following the 2010 El Mayor‐Cucapah Earthquake
Abstract The rheology of the crust and mantle and the interaction of viscoelastic flow with seismic/aseismic slip on faults control the state of stress in the lithosphere over multiple seismic cycles. The rheological behavior of rocks is well constrained in a laboratory setting, but thein situproperties of the lithosphere and its lateral variations remain poorly known. Here, we access the lower‐crustal rheology in Southern California by exploiting 8 years of geodetic postseismic deformation following the 2010 El Mayor‐Cucapah earthquake. The data illuminate viscoelastic flow in the lower crust with lateral variations of effective viscosity correlated with the geological province. We show that a Burgers assembly with dashpots following a nonlinear constitutive law can approximate the temporal evolution of stress and strain rate, indicating the activation of nonlinear transient creep before steady‐state dislocation creep. The transient and background viscosities in the lower crust of the Salton Trough are on the order of ~1018and ~1019 Pa s, respectively, about an order of magnitude lower than those in the surrounding regions. We highlight the importance of transient creep, nonlinear flow laws, and lateral variations of rheological properties to capture the entire history of postseismic relaxation following the El Mayor‐Cucapah earthquake.
more »
« less
- Award ID(s):
- 1848192
- PAR ID:
- 10380383
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Solid Earth
- Volume:
- 125
- Issue:
- 9
- ISSN:
- 2169-9313
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract Flexure occurs on intermediate geologic timescales (∼1 Myr) due to volcanic‐island building at the Island of Hawaii, and the deformational response of the lithosphere is simultaneously elastic, plastic, and ductile. At shallow depths and low temperatures, elastic deformation transitions to frictional failure on faults where stresses exceed a threshold value, and this complex rheology controls the rate of deformation manifested by earthquakes. In this study, we estimate the seismic strain rate based on earthquakes recorded between 1960 and 2019 at Hawaii, and the estimated strain rate with 10−18–10−15s−1in magnitude exhibits a local minimum or neutral bending plane at 15 km depth within the lithosphere. In comparison, flexure and internal deformation of the lithosphere are modeled in 3D viscoelastic loading models where deformation at shallow depths is accommodated by frictional sliding on faults and limited by the frictional coefficient (μf), and at larger depths by low‐temperature plasticity and high‐temperature creep. Observations of flexure and the seismic strain rate are best‐reproduced by models withμf = 0.3 ± 0.1 and modified laboratory‐derived low‐temperature plasticity. Results also suggest strong lateral variations in the frictional strength of faults beneath Hawaii. Our models predict a radial pattern of compressive stress axes relative to central Hawaii consistent with observations of earthquake pressure (P) axes. We demonstrate that the dip angle of this radial axis is essential to discerning a change in the curvature of flexure, and therefore has implications for constraining lateral variations in lithospheric strength.more » « less
-
Abstract The rheology of the upper mantle impacts a variety of geodynamic processes, including postseismic deformation following great earthquakes and post-glacial rebound. The deformation of upper mantle rocks is controlled by the rheology of olivine, the most abundant upper mantle mineral. The mechanical properties of olivine at steady state are well constrained. However, the physical mechanism underlying transient creep, an evolutionary, hardening phase converging to steady state asymptotically, is still poorly understood. Here, we constrain a constitutive framework that captures transient creep and steady state creep consistently using the mechanical data from laboratory experiments on natural dunites containing at least 94% olivine under both hydrous and anhydrous conditions. The constitutive framework represents a Burgers assembly with a thermally activated nonlinear stress-versus-strain-rate relationship for the dashpots. Work hardening is obtained by the evolution of a state variable that represents internal stress. We determine the flow law parameters for dunites using a Markov chain Monte Carlo method. We find the activation energy $$430\pm 20$$ 430 ± 20 and $$250\pm 10$$ 250 ± 10 kJ/mol for dry and wet conditions, respectively, and the stress exponent $$2.0\pm 0.1$$ 2.0 ± 0.1 for both the dry and wet cases for transient creep, consistently lower than those of steady-state creep, suggesting a separate physical mechanism. For wet dunites in the grain-boundary sliding regime, the grain-size dependence is similar for transient creep and steady-state creep. The lower activation energy of transient creep could be due to a higher jog density of the corresponding soft-slip system. More experimental data are required to estimate the activation volume and water content exponent of transient creep. The constitutive relation used and its associated flow law parameters provide useful constraints for geodynamics applications. Graphical Abstractmore » « less
-
Abstract Localized frictional sliding on faults in the continental crust transitions at depth to distributed deformation in viscous shear zones. This brittle‐ductile transition (BDT), and/or the transition from velocity‐weakening (VW) to velocity‐strengthening (VS) friction, are controlled by the lithospheric thermal structure and composition. Here, we investigate these transitions, and their effect on the depth extent of earthquakes, using 2D antiplane shear simulations of a strike‐slip fault with rate‐and‐state friction. The off‐fault material is viscoelastic, with temperature‐dependent dislocation creep. We solve the heat equation for temperature, accounting for frictional and viscous shear heating that creates a thermal anomaly relative to the ambient geotherm which reduces viscosity and facilitates viscous flow. We explore several geotherms and effective normal stress distributions (by changing pore pressure), quantifying the thermal anomaly, seismic and aseismic slip, and the transition from frictional sliding to viscous flow. The thermal anomaly can reach several hundred degrees below the seismogenic zone in models with hydrostatic pressure but is smaller for higher pressure (and these high‐pressure models are most consistent with San Andreas Fault heat flow constraints). Shear heating raises the BDT, sometimes to where it limits rupture depth rather than the frictional VW‐to‐VS transition. Our thermomechanical modeling framework can be used to evaluate lithospheric rheology and thermal models through predictions of earthquake ruptures, postseismic and interseismic crustal deformation, heat flow, and the geological structures that reflect the complex deformation beneath faults.more » « less
-
Abstract Previous studies of flexure in continental settings assert that the elastic thickness of a multilayer lithosphere is controlled by the mechanically competent layer thicknesses only, following a cubic rule. More specifically, the cubic rule statesTe = (Te13+Te23+ … Ten3)1/3, whereTeis the total elastic thickness, andTeiis the elastic thickness of each competent layer. However, it is not necessarily clear thatTeshould be insensitive to the properties of intermediate weak layers (e.g., a weak lower crust) which may act to decouple the surface load from lower competent layers. To test this idea, we formulate 2D viscoelastic loading models with layered viscosity to compute the fully dynamic, time‐dependent response of a multilayer lithosphere with a weak lower crust. Results show that the flexural response of a multilayer lithosphere to a surface load is initially consistent with the cubic rule. However, this solution is transient because the stress associated with the load cannot be transmitted through the weak lower crust on long timescales. Stress in the mantle lithosphere relaxes with time and eventually does not support the load at all due to the decoupling effect of flow in the weak lower crust. The steady state flexure of a multilayer lithosphere is controlled solely by the mechanically competent upper crust such thatTe = Te1, and this new rule is the major finding of this study. Our new findings now explain small estimates ofTein continental settings with thick mantle lithosphere such as the Northern Tien Shan, which previously, were poorly explained by the cubic rule.more » « less
