Geology Scholarly Works
http://hdl.handle.net/1808/88
2021-09-24T00:53:34ZDynamically Triggered Changes of Plate Interface Coupling in Southern Cascadia
http://hdl.handle.net/1808/31682
Dynamically Triggered Changes of Plate Interface Coupling in Southern Cascadia
Materna, Kathryn; Bartlow, Noel; Wech, Aaron; Williams, Charles; Bürgmann, Roland
In subduction zones, frictional locking on the subduction interface produces year-by-year surface deformation that is measurable with GPS. During the interseismic period of the earthquake cycle, lasting hundreds of years between major earthquakes, these ground motions are usually constant with time because the locking on the plate interface is relatively unchanging. However, at the Mendocino Triple Junction in Northern California, we find evidence for slight changes in GPS interseismic motion within the last decade that challenge the assumption of constant interseismic deformation. Our results suggest changes in interseismic coupling on the southernmost Cascadia Subduction Zone. Interestingly, these coupling changes appear to be related to large offshore earthquakes and are perhaps triggered by the seismic shaking during those events. These results have important implications for our understanding of seismic hazard in subduction zones.
2019-10-23T00:00:00ZAn Improved Analytical Solution for the Temperature Profile of Ice Sheets
http://hdl.handle.net/1808/31647
An Improved Analytical Solution for the Temperature Profile of Ice Sheets
Rezvanbehbahani, Soroush; van der Veen, C. J.; Stearns, Leigh A.
The one-dimensional steady state analytical solution of the energy conservation equation obtained by Robin (1955, https://doi.org/10.3189/002214355793702028) is frequently used in glaciology. This solution assumes a linear change in surface velocity from a minimum value equal to minus the mass balance at the surface to zero at the bed. Here we show that this assumption of a linear velocity profile leads to large errors in the calculated temperature profile and especially in basal temperature. By prescribing a nonlinear power function of elevation above the bed for the vertical velocity profile arising from use of the Shallow Ice Approximation, we derive a new analytical solution for temperature. We show that the solution produces temperature profiles identical to numerical temperature solutions with the Shallow Ice Approximation vertical velocity near ice divides. We quantify the importance of strain heating and demonstrate that integrating the strain heating and adding it to the geothermal heat flux at the bed is a reasonable approximation for the interior regions. Our analytical solution does not include horizontal advection components, so we compare our solution with numerical solutions of a two-dimensional advection-diffusion model and assess the applicability and errors of the analytical solution away from the ice divide. We show that several parameters and assumptions impact the spatial extent of applicability of the new solution including surface mass balance rate and surface temperature lapse rate. We delineate regions of Greenland and Antarctica within which the analytical solution at any depth is likely within 2 K of the actual temperatures with horizontal advection.
An edited version of this paper was published by AGU. Copyright 2019 American Geophysical Union.
2019-01-11T00:00:00ZSegmented strain accumulation in the High Himalaya expressed in river channel steepness
http://hdl.handle.net/1808/31593
Segmented strain accumulation in the High Himalaya expressed in river channel steepness
Cannon, J.M.; Murphy, M.A.; Taylor, Michael Halford
We investigate segmentation of High Himalayan strain by cross-orogen structures separating western and eastern obliquely convergent sectors from a central orthogonally convergent sector, and evaluate the relationship between the size of regions accumulating strain, their proximity to the toe of the thrust wedge, and recurrence of Mw >7 earthquakes. We present a map of river channel steepness (ksn)—a proxy for rock-uplift rate over 105 yr, for the Himalayan arc—and evaluate the strength of its correlation with Main Himalayan thrust (MHT) coupling (–0.6), earthquake density (0.6), topography (0.6), lithotectonic units (0.5), and precipitation (–0.3) along 40 profiles spanning the Himalaya from 78°E to 92°E. We interpret the ksn map to be foremost a function of recent strain accumulation. This reveals prominent offsets of hinterland strain accumulation collocated with cross-orogen strike-slip and extensional fault systems. Clusters of high-ksn rivers are located near the boundary between the strongly and weakly coupled portions of the MHT, where fault behavior changes from seismogenic to sliding at the rheologic brittle-to-plastic transition (BPT). We propose that the rate at which major MHT earthquakes repeat is related to four parameters: convergence rate (nearly uniform); spatial dimensions of the high-ksn cluster (proxy for volume of material accumulating strain); the high ksn clusters distance from the toe of thrust wedge (fault surface area over which static friction must be overcome); and the degree of obliquity between India-Asia convergence and the local trend of the orogen (proxy for the magnitude of strain partitioning).
2018-03-12T00:00:00ZPermeability Prediction in Rocks Experiencing Mineral Precipitation and Dissolution: A Numerical Study
http://hdl.handle.net/1808/31474
Permeability Prediction in Rocks Experiencing Mineral Precipitation and Dissolution: A Numerical Study
Niu, Qifei; Zhang, Chi
In this study, we focus on the electrical tortuosity‐based permeability model k = reff2/8F (reff is an effective pore size, and F is the formation factor) and analyze its applicability to rocks experiencing mineral precipitation and dissolution. Two limiting cases of advection‐dominated water‐rock reactions are simulated, that is, the reaction‐limited and transport‐limited cases. At the pore scale, the two precipitation/dissolution patterns are simulated with a geometrical model and a phenomenological model. The fluid and electric flows in the rocks are simulated by directly solving the linear Stokes equation and Laplace equation on the representative elementary volume of the samples. The numerical results show that evolutions of k and F differ significantly in the two limiting cases. In general, the reaction‐limited precipitation/dissolution would result in a smooth variation of k and F, which can be roughly modeled with a power function of porosity ϕ with a constant exponent. In contrast, the transport‐limited precipitation/dissolution mostly occurs near the pore throats where the fluid velocity is high. This induces a sharp change in k and F despite a minor variation in ϕ. The commonly used power laws with constant exponents are not able to describe such variations. The results also reveal that the electrical tortuosity‐based permeability prediction generally works well for rocks experiencing precipitation/dissolution if reff can be appropriately estimated, for example, with the electrical field normalized pore size Λ. The associated prediction errors are mainly due to the use of electrical tortuosity, which might be considerably larger than the true hydraulic tortuosity.
An edited version of this paper was published by AGU. Copyright 2019 American Geophysical Union.
2019-04-01T00:00:00Z