Browsing by Autor "Kevin Ahlgren"

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A robust approach to terrestrial relative gravity measurements and adjustment of gravity networks
(Springer Science+Business Media, 2024) Franco S. Sobrero; Kevin Ahlgren; Michael Bevis; Demián D. Gómez; Jacob Heck; Arturo Echalar; Dana J. Caccamise; Eric Kendrick; Paola Montenegro; Ariele Batistti
Abstract Like many geophysical observations, relative gravity (RG) measurements are affected by random errors, systematic errors, and occasional blunders. When RG measurements are used to build large gravity networks in remote areas under adverse environmental or logistical conditions (such as extreme temperatures, heavy precipitation, rugged terrain, difficult or dangerous roads, and high altitudes), it is more likely for significant errors to occur and accumulate. Therefore, obtaining accurate gravity estimates at regional gravity networks largely depends on defensive data collection protocols and robust adjustment techniques. In this work, we present a measurement field protocol based on highly redundant observation patterns, and a two-step least squares adjustment scheme implemented as a MATLAB package. This software helps us identify blunders, mitigates the impact of random errors, and downweights or removes outlier observations. The methodology also guarantees that adjusted gravity values have well-constrained standard error estimates. We illustrate the capabilities of our approach through the case study of the Bolivian gravity network, where we determined the acceleration due to gravity at 2548 stations that spread over difficult and sometimes extreme environments, with a typical level of uncertainty of 0.10–0.15 mGal.
Isolating active orogenic wedge deformation in the southern Subandes of Bolivia
(Wiley, 2016) Jonathan Weiss; B. A. Brooks; James H. Foster; Michael Bevis; Arturo Echalar; Dana J. Caccamise; Jacob Heck; Eric Kendrick; Kevin Ahlgren; David Raleigh
Abstract A new GPS‐derived surface velocity field for the central Andean backarc permits an assessment of orogenic wedge deformation across the southern Subandes of Bolivia, where recent studies suggest that great earthquakes (> M w 8) are possible. We find that the backarc is not isolated from the main plate boundary seismic cycle. Rather, signals from subduction zone earthquakes contaminate the velocity field at distances greater than 800 km from the Chile trench. Two new wedge‐crossing velocity profiles, corrected for seasonal and earthquake affects, reveal distinct regions that reflect (1) locking of the main plate boundary across the high Andes, (2) the location of and loading rate at the back of orogenic wedge, and (3) an east flank velocity gradient indicative of décollement locking beneath the Subandes. Modeling of the Subandean portions of the profiles indicates along‐strike variations in the décollement locked width ( W L ) and wedge loading rate; the northern wedge décollement has a W L of ~100 km while accumulating slip at a rate of ~14 mm/yr, whereas the southern wedge has a W L of ~61 km and a slip rate of ~7 mm/yr. When compared to Quaternary estimates of geologic shortening and evidence for Holocene internal wedge deformation, the new GPS‐derived wedge loading rates may indicate that the southern wedge is experiencing a phase of thickening via reactivation of preexisting internal structures. In contrast, we suspect that the northern wedge is undergoing an accretion or widening phase primarily via slip on relatively young thrust‐front faults.