Elizabeth Harding Hearn

Dynamics of İzmit Earthquake Postseismic Deformation and Loading of the Düzce Earthquake Hypocenter

We have developed dynamic finite-element models of Izmit earthquake postseismic deformation to evaluate whether this deformation is better explained by afterslip (via either velocity-strengthening frictional slip or linear viscous creep) or by distributed linear viscoelastic relaxation of the lower crust. We find that velocity-strengthening frictional afterslip driven by coseismic shear stress loading can reproduce time-dependent Global Positioning System data better than either linear viscous creep on a vertical shear zone below the rupture or lower crustal viscoelastic relaxation. Our best frictional afterslip model fits the main features of postseismic slip inversions, in particular, high slip patches at (and below) the hypocenter and on the western Karadere segment, and limited afterslip west of the Hersek Delta (Burgmann et al., 2002). The model requires a weakly velocity-strengthening fault, that is, either low effective normal stress in the slipping regions or a smaller value for the parameter describing rate-dependence of friction ( a - b ) than is indicated by laboratory experiments. Our best afterslip model suggests that the Coulomb stress at the Duzce hypocenter increased by 0.14 MPa (1.4 bars) during the Izmit earthquake (assuming right-lateral slip on a surface dipping 50° to the north), and by another 0.1 MPa during the 87 days between the Izmit and Duzce earthquakes. In the Marmara Sea region (within about 160 km of the Izmit earthquake rupture), this model indicates that the Coulomb stresses increased by 15%-25% of the coseismic amount during the first 300 days after the earthquake. Three hundred days after the earthquake, postseismic contributions to Coulomb stressing rate on the Maramara region faults had fallen to values equal to or less than the inferred secular stress accumulation rate. Our estimates of postseismic Coulomb stress are highly model dependent: in the Marmara region, the linear viscous shear zone and viscoelastic lower crust models predict greater postseismic Coulomb stresses than the frictional afterslip model. Near-field stress and fault-zone rheology estimates are sensitive to the Earth9s elastic structure. When a layered elastic structure is incorporated in our model, it yields a Coulomb stress of 0.24 MPa at the Duzce hypocenter, significantly more than the 0.14 MPa estimated from the uniform elastic model. Because of the higher near-field coseismic stresses, the layered elastic model requires a higher value of velocity-strengthening parameter ( A - B ) ([ a - b ] times effective normal stress r ′) to produce comparable postseismic slip. ( A - B ) is estimated at 0.4 and 0.2 MPa, respectively, for the layered and uniform elastic models. These results highlight the importance of understanding the Earth9s elastic structure and the mechanism for postseismic deformation if we wish to accurately model coseismic and postseismic crustal stresses.

Time-Dependent Distributed Afterslip on and Deep below the İzmit Earthquake Rupture

Surface deformation transients measured with the Global Positioning System during the 87 days between the 17 August 1999 Izmit earthquake and the 12 November 1999 Duzce earthquake indicate rapidly decaying aseismic fault slip on and well below the coseismic rupture. Elastic model inversions for time-dependent distributed fault slip, using a network inversion filter approach, show that afterslip was highest between and below the regions of maximum coseismic slip and propa- gated downward to, or even below, the base of the crust. Maximum afterslip rates decayed from greater than 2 m/yr, immediately after the I zmit earthquake to about 1.2 m/yr just prior to the Duzce earthquake. Maximum afterslip occurred below the eastern Karadere rupture segment and near the I zmit hypocenter. Afterslip in the upper 16 km decayed more rapidly than that below the seismogenic zone. These observations are consistent with a phase of rapid aseismic fault slip concentrated near the base of the seismogenic zone. Continued loading from the rapid deep afterslip along the eastern rupture zone is a plausible mechanism that helped trigger the nearby, Mw 7.2, 12 November Duzce earthquake.

The Effect of Elastic Layering on Inversions of GPS Data for Coseismic Slip and Resulting Stress Changes: Strike-Slip Earthquakes

We investigate the effect of depth-dependent elasticity on slip inversions and coseismic stress change estimates for large strike-slip earthquakes, using a series of hypothetical models and the 1999 Izmit, Turkey earthquake as examples. Slip inversions are performed using both semianalytical and finite-element solutions for surface displacements due to a shear dislocation in layered and uniform elastic Earth models. We find that incorporating realistic increases to shear modulus ( μ ) with depth in our inversions increases recovered centroid depth and seismic potency relative to uniform elastic half-space models. Recovered seismic moment is up to 40% greater for models incorporating depth-dependent μ than it is for uniform elastic half-space models. Incorporating depth-dependent μ also increases our estimate of the maximum slip depth for the Izmit, Turkey, earthquake (to at least 20 km). Our estimates of coseismic stress change in the upper crust do not change significantly when we incorporate depth-dependent elasticity in our inversions, as long as slip at depth is tuned (increased) to match surface displacements. Coseismic differential stresses in the lower crust increase by up to a factor of 3 in the near field, but further from the fault, stresses from layered and uniform elastic models are approximately equal. With increasing depth in the mantle, the ratio of modeled differential stresses for the layered and uniform elastic models approaches the ratio of mantle μ values for these two models. We conclude that models of postseismic viscoelastic relaxation following large strike-slip earthquakes should incorporate depth-dependent elasticity, but that uniform elastic half-space models are adequate to calculate coseismic Coulomb stresses in the upper crust for most triggering studies.

Coupled variations in helium isotopes and fluid chemistry: Shoshone Geyser Basin, Yellowstone National Park

Early studies of 3He/4He variations in geothermal systems have generally attributed these fluctuations to either differences in the source of the magmatic 3He-rich helium or to local differences in the deep flux of magmatic 3He-rich helium. Kennedy et al. (1987), however, show that near-surface processes such as boiling and dilution may also drastically affect 3He4He ratios of geothermal vapors. Helium isotope ratios were determined for several hot springs at Shoshone Geyser Basin of Yellowstone National Park for this study, along with other noble gas data. Stable isotope data and water and gas chemistry data for each spring were also compiled. The water chemistry indicates that there is one deep, hot thermal water in the area which is mixing with dilute meteoric water that has entered the system at depth. Spring HCO3− concentrations correlate with 3He4He values, as in nearby Lower Geyser Basin. This correlation is attributed to variable amounts of deep dilution of thermal waters with a relatively cool water that inhibits boiling at depth, thus preventing the loss of CO2 (and therefore HCO3−) and magmatic He in the most diluted samples. Oxygen and hydrogen isotope data also support a boiling and dilution model, but to produce the observed fractionations, the boiling event would have to be extensive, with steam loss at the surface, whereas the boiling that affected the helium isotope ratios was probably a small scale event with steam loss at depth. It is possible that deep boiling occurred in the basin and that small amounts of steam escaped along fractures at about 500 m below the surface while all subsequently produced steam was lost near or at the surface.

Can Lateral Viscosity Contrasts Explain Asymmetric Interseismic Deformation around Strike‐Slip Faults?

Abstract Geodetic studies have shown that deformation rates around several major strike‐slip faults are asymmetric. This asymmetry is often explained in terms of a crustal‐scale contrast in elastic properties across the fault. Motivated by the fact that elasticity variations for different rock types under similar ambient conditions are generally modest, whereas effective viscosity may vary over orders of magnitude, we have developed earthquake‐cycle models to evaluate whether contrasts in viscosity structure and effective plate thickness can explain observed asymmetric surface deformation. We find that an increased plate‐thickness contrast results in a more asymmetric surface‐velocity profile. Furthermore, for the same plate‐thickness contrast, asymmetry of the surface deformation is most pronounced in models with a low‐viscosity substrate. Initially, velocities relative to a point on the fault are higher on the side with the thin plate; however, late in the interseismic interval this reverses, and these velocities are higher on the side with the thick plate. Models with a contrast in the substrate viscosity on either side of the fault and a uniform plate thickness show behavior similar to that of the variable‐thickness plate models. In both suites of asymmetric models, the surface velocity at the fault varies through the earthquake cycle. This is necessary to reconcile symmetric coseismic deformation, asymmetric interseismic deformation, and the requirement of zero strain in the blocks on either side of the fault over an earthquake cycle. Given the modest asymmetry in surface deformation for models capable of producing localized interseismic deformation around the fault (i.e., models with high substrate viscosities), we conclude that lateral contrasts in viscosity or effective plate thickness cannot produce dramatic asymmetries in Global Positioning System surface‐velocity profiles across major strike‐slip faults.

Reconciling viscoelastic models of postseismic and interseismic deformation: Effects of viscous shear zones and finite length ruptures

We have developed a suite of earthquake cycle models for strike-slip faults to investigate how finite ruptures and lithosphere-scale viscous shear zones affect interseismic deformation. In particular, we assess whether localized and stationary interseismic deformation and large-scale, rapidly decaying postseismic transients may be explained with models incorporating either or both of these features. Models incorporating viscous shear zones give more stationary interseismic deformation than layered half-space, Maxwell viscoelastic models producing similar early postseismic deformation in the near field. This tendency is accentuated when the effective viscosity per unit width of the shear zone increases either with depth or with interseismic time. Models with finite (200 km long) ruptures produce time-dependent deformation similar to that from models with infinitely long ruptures, though with smaller-magnitude and somewhat more localized surface velocity fluctuations. Models incorporating a stiff lithosphere, a low-viscosity mantle asthenosphere, and a crust- to lithosphere-scale viscous shear zone can replicate postseismic and interseismic deformation typical of large, strike-slip earthquakes. Such models require depth-dependent, power law, or transient rheologies for the viscous shear zone material in the crust, as long as the effective viscosity per unit shear zone width is approximately 1015 Pa s/m at crustal depths during the postseismic interval. Below the Moho, the shear zone effective viscosity per unit width must be higher throughout the seismic cycle, at least over a short depth interval. For example, if shear zone viscosity per unit width is 5×1016 Pa s/m in the mantle lithosphere, a model with a 50 km thick lithosphere and asthenosphere effective viscosities of 1 to 5×1018 Pa s can reproduce reference velocity profiles for both postseismic and later interseismic deformation.

Space Geodetic Data Improve Seismic Hazard Assessment in California: Workshop on Incorporating Geodetic Surface Deformation Data Into UCERF3; Pomona, California, 1–2 April 2010

A workshop was held to begin scientific consideration of how to incorporate space geodetic constraints on strain rates and fault slip rates into the next generation Uniform California Earthquake Rupture Forecast, version 3 (UCERF3), due to be completed in mid-2012. Principal outcomes of the meeting were (1) an assessment of secure science ready for UCERF3 applications within the next year, and (2) an agenda of new research objectives for the Southern California Earthquake Center (SCEC), the U.S. Geological Survey (USGS), and others in support of UCERF3 and related probabilistic seismic hazard assessments (PSHA). A number of goals potentially achievable within a year were identified, including (1) slip rate and fault locking depth estimates, with uncertainties or ranges, for all major and some minor faults of the extended San Andreas system; (2) strain rate estimates or bounds on rates for selected regions lying off the major faults of the San Andreas system; and (3) corrections or bounds on perturbing effects of postseismic deformation and elastic modulus heterogeneities on the observed Global Positioning System (GPS) velocity field (needed as input to models for estimating fault slip and strain rates in goals 1 and 2 above).

Investigating Ductile Lithosphere Deformation in Southern California

A workshop to investigate properties and processes of ductile lithosphere deformation was held in Menlo Park, Calif. This workshop provided overviews of the newest results from lab, field, and geodetic modeling disciplines relevant to better understanding earthquake processes in Southern California. A principal goal of the workshop was to bring together niche experts in tectonic geodesy, laboratory rock mechanics, and field observations of ductile rocks to gain better interdisciplinary understanding of the stress transfer process between brittle-elastic upper crust and ductile lithosphere in Southern California.