Acceleration and evolution of faults: An example from the Hunter Mountain-Panamint Valley fault zone, Eastern California

We present new space geodetic data indicating that the present slip rate on the Hunter Mountain–Panamint Valley fault zone in Eastern California (5.0 ± 0.5 mm/yr) is significantly faster than geologic estimates based on fault total offset and inception time. We interpret this discrepancy as evidence for an accelerating fault and propose a new model for fault initiation and evolution. In this model, fault slip rate initially increases with time; hence geologic estimates averaged over the early stages of the fault\u27s activity will tend to underestimate the present-day rate. The model is based on geologic data (total offset and fault initiation time) and geodetic data (present day slip rate). The model assumes a monotonic increase in slip rate with time as the fault matures and straightens. The rate increase follows a simple Rayleigh cumulative distribution. Integrating the rate-time path from fault inception to present-day gives the total fault offset

The Earthquake Cycle of Strike-Slip Faults

An earthquake is a mechanism of stress release along plate boundaries due to relative motion between the Earth\u27s lithospheric blocks. The period in which stresses are accruing across the plate boundary is known as the interseismic portion of the earthquake cycle. This dissertation focuses on interseismic portion of the earthquake cycle to extract characteristics of fault, shear zone and rock properties. Global Positioning System (GPS) data are used to observe the pattern of deformation across two primarily strike-slip fault systems: the Carrizo Segment of the San Andreas Fault (SAF) and the Eastern California Shear Zone (ECSZ). Two sets of GPS data are processed, analyzed and applied to analytic and numerical models describing the interseismic behavior of the earthquake cycle. The Carrizo segment is mature (i.e., had many earthquakes) and has juxtaposed terrains with varying rock properties laterally across the fault system. Lateral variations in rock properties affect the pattern of deformation around strike-slip faults and affect how surrounding rock deforms and if not considered may bias the interpretation of the faulted system. The Carrizo segment separates Franciscan terrain northeast of the fault from Salinian block to the southwest. GPS data are well fit to a model with a 15-25 km weak zone northeast of the Carrizo segment. The long-term slip rate estimated on the SAF is 34-38 mm/yr, with 2-4 mm/yr accommodated on faults to the west. The viscosity for the combined lower crust/upper mantle is estimated at 2-5x10^19 Pa s. This model is consistent with the distribution of rock type and corresponding laboratory data on their material properties, paleoseismic, seismic and magnetotelluric data. The ECSZ is a young (Myr) system of strike-slip faults including the Owens Valley - Airport Lake, Panamint Valley - Ash Hill - Hunter Mountain and Death Valley - Furnace Creek fault systems. The ECSZ study concentrates on fault evolution by finding the current position of maximum shear across the shear zone and estimating fault rates. Geologic studies suggest that the Death Valley - Furnace Creek fault system on eastern end of the ECSZ was the primary accommodator of slip early in the ECSZ history. This study suggests that the current locus of shear has shifted westward, and resides in the center of the ECSZ under the Panamint Valley - Ash Hill -Hunter Mountain fault system. The model dependent estimated geodetic rate of the Ash Hill - Panamint Valley -Hunter Mountain fault system (4.91-6.11 mm/yr) is faster than geologic estimates (1.6 - 4 mm/yr). The result is interpreted as a simplification of the ECSZ with time, combined with progressive westward migration of deformation. The best estimate for a combined rate across the shear zone is 10 mm/yr (20% of total Pacific-North America motion). The summation of rates obtained by this study is 49 mm/yr, well within estimates obtained by previous studies using independent techniques

Central Cascadia Subduction Zone Creep

Central Cascadia between 43ºN and 46ºN has reduced interseismic uplift observed in geodetic data and coseismic subsidence seen in multiple thrust earthquakes, suggesting elevated persistent fault creep in this section of the subduction zone. We estimate subduction thrust decade-scale locking and crustal block rotations from three-component continuous Global Positioning System (GPS) time series from 1997 to 2013, as well as 80 year tide gauge and leveling-derived uplift rates. Geodetic observations indicatecoastal central Oregon is rising at a slower rate than coastal Washington, southern Oregon and northern California. Modeled locking distributions suggest a wide locking transition zone that extends inland undercentral Oregon. Paleoseismic records of multiple great earthquakes along Cascadia indicate less subsidence in central Oregon. The Cascade thrust under central Oregon may be partially creeping for at least 6500 years(the length of the paleoseismic record) reducing interseismic uplift and resulting in reduced coseismic subsidence. Large accretions of Eocene age basalt (Siletzia terrane) between 43ºN and 46ºN may be less perme-able compared to surrounding terranes, potentially increasing pore fluid pressures along the fault interface resulting in a wide zone of persistent fault creep. In a separate inversion, three-component GPS time series from 1 July 2005 to 1 January 2011 are used to estimate upper plate deformation, locking between slow-slip events (SSEs), slip from 16 SSEs and an earthquake mechanism. Cumulative SSEs and tectonic tremor are weakest between 43ºN and 46ºN where partial fault creep is increased and Siletzia terrane is thick, suggesting that surrounding rock properties may influence the mode of slip

Strain Accumulation Across the Carrizo Segment of the San Andreas Fault, California: Impact of Laterally Varying Crustal Properties

Major strike slip faults juxtapose geologically dissimilar terrain which may vary in mechanical properties, leading to an asymmetric pattern of strain accumulation. We present new GPS data on the Carrizo segment of the San Andreas Fault, separating the Salinian block southwest of the fault from Franciscan terrane northeast of the fault, to better quantify asymmetric strain accumulation. We also present a series of finite element models to investigate the possible role of variable elastic layer thickness and material properties of the upper crust. The geodetic data are well fit with a simple model comprising a weak upper crustal zone 10–25 km wide northeast of the fault. This model is also consistent with geologic data on the distribution of major rock types and corresponding laboratory data on their material properties, as well as paleoseismic, seismic and magnetotelluric data. Using this model, we estimate a “long-term” (average over several seismic cycles) slip rate for the San Andreas Fault of 36−1.5+2 mm/yr in agreement with the known Holocene rate within uncertainties, and a viscosity for the combined lower crust/upper mantle of 2–5 × 1019 Pa s

Reconciling Patterns of Interseismic Strain Accumulation with Thermal Observations Across the Carrizo Segment of the San Andreas Fault

The Carrizo segment of the San Andreas Fault separates rocks of the Salinian Block southwest of the fault characterized by high heat flow (~ 75–95 mW/m2) and shallow seismicity (2) and deeper seismicity

Comparisons of corrected daily integrated erythemal UVR from the U.S. EPA/UGA network of Brewer spectroradiometers with model and satellite data

A network of 21 Brewer spectroradiometers, owned by the U.S. Environmental Protection Agency and operated by the University of Georgia, is measuring UV spectral irradiances throughout the United States. Corrections to the raw data have now been implemented. These corrections include (1) stray light rejection, (2) the cosine errors associated with the full sky diffuser, (3) the temperature dependence of the response of the instruments and (4) the temporal variation in the instrument response due to optical changes in the characteristics of the instruments. While for many sites the total corrections amount to less than 10%, for certain sites they are much larger, in some cases amounting to more than 25%. Application of these corrections brings the errors of the absolute irradiance values to approximately ± 5 to 7% for all sources of error. Comparisons of corrected daily integrated erythemal UVR data (DUV) to model and TOMS-inferred values are performed for sites at Acadia National Park, Bigbend National Park, Everglades National Park and the Virgin Islands. All sites show very good agreement with the TUVSPEC model but comparison with TOMS-inferred DUV values indicate a 10-20% overestimate by TOMS for the four sites