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Silverii, F, D'Agostino N, Borsa AA, Calcaterra S, Gambino P, Giuliani R, Mattone M.  2019.  Transient crustal deformation from karst aquifers hydrology in the Apennines (Italy). Earth and Planetary Science Letters. 506:23-37.   10.1016/j.epsl.2018.10.019   AbstractWebsite

The increasing accuracy and spatiotemporal resolution of space geodetic techniques have positively impacted the study of shallow crustal deformation in response to the redistribution of water masses. Measurable deformations have been documented in areas where snow and water variability is large and persists over sufficiently long periods. Here we analyze GPS time series and hydrological data from the Central-Southern Apennines, a tectonically-active region hosting large karst aquifers. We document the occurrence of regional-scale horizontal and vertical transient deformation that is clearly correlated to seasonal and multiyear hydrological variability. These transient signals, which are most strongly observed at GPS sites surrounding the main karst aquifers, modulate long term tectonic deformation. Our results suggest that the karst aquifers in this region experience alternating periods of expansion and contraction in response to increasing/decreasing precipitation and, consequently, higher/lower hydraulic head in the aquifers. Thanks to the availability of a dense continuous GPS network and complementary hydrological datasets, we are able to verify the processes causing the observed deformation. We model the shallow crust in the region as a continuous anelastic solid and use Green's functions for finite strain cuboid sources to estimate the strain rate distribution associated with the GPS observations. We use the M-W 6.1 L'Aquila earthquake, which struck the Central Apennines in 2009 and whose effects are evident in geodetic data, to document the potential effects of moderate earthquakes on karst aquifers and to demonstrate the importance of correctly discerning tectonic from nontectonic signals in geodetic time series. Enhanced understanding of the karst aquifers behavior is of primary interest for improved management of this vital water resource and for a better understanding of the possible interactions between groundwater content and pore pressure variations in the crust and seismicity. (C) 2018 Elsevier B.V. All rights reserved.

Kraner, ML, Holt WE, Borsa AA.  2018.  Seasonal nontectonic loading inferred from cGPS as a potential trigger for the M6.0 South Napa earthquake. Journal of Geophysical Research-Solid Earth. 123:5300-5322.   10.1029/2017jb015420   AbstractWebsite

We analyze crustal strain corresponding to transient continuous Global Positioning System (cGPS) horizontal displacements in Northern California, detecting a seasonal positive dilatational strain and Coulomb stress transient in the South Napa region peaking just before the 24 August 2014 M6.0 South Napa earthquake. Using data from 2007 to 2014, we show that average dilatational strain within a 500-km(2) region encompassing South Napa and northern San Pablo Bay peaks in late summer at 7617x10(-9), accompanied by a Coulomb stress change of 1.90.8kPa. The situation reverses in winter, with an average dilatational strain of -5117x10(-9) and Coulomb stress change of -1.40.8kPa. Within a smaller 100-km(2) area centered on the South Napa rupture, peak values are considerably higher, including a summer Coulomb stress peak of 5.11.6kPa. We examine regional seismicity but see no statistically significant correlation with seasonal Coulomb stressing in the declustered earthquake catalog. Using western U.S. vertical cGPS displacements, we estimate that strain from hydrologic loading explains 10% of the observed long-wavelength strain and only 2-3% of peak strains around the South Napa rupture. Thermoelastic crustal strain estimated from temperature gradients between the San Francisco Bay and Sacramento Valley reaches values as high as 15% of the observed strain, but the strain patterns are not spatially consistent. Vertical deformation within the Sonoma and Napa Valley subbasins inferred from interferometric synthetic aperture radar explains large horizontal motions at nearby cGPS stations and suggests that seasonal changes in groundwater may contribute to observed strain and stress transients.

Enzminger, TL, Small EE, Borsa AA.  2018.  Accuracy of snow water equivalent estimated from GPS vertical displacements: A synthetic loading case study for western US mountains. Water Resources Research. 54:581-599.   10.1002/2017wr021521   AbstractWebsite

GPS monitoring of solid Earth deformation due to surface loading is an independent approach for estimating seasonal changes in terrestrial water storage (TWS). In western United States (WUSA) mountain ranges, snow water equivalent (SWE) is the dominant component of TWS and an essential water resource. While several studies have estimated SWE from GPS-measured vertical displacements, the error associated with this method remains poorly constrained. We examine the accuracy of SWE estimated from synthetic displacements at 1,395 continuous GPS station locations in the WUSA. Displacement at each station is calculated from the predicted elastic response to variations in SWE from SNODAS and soil moisture from the NLDAS-2 Noah model. We invert synthetic displacements for TWS, showing that both seasonal accumulation and melt as well as year-to-year fluctuations in peak SWE can be estimated from data recorded by the existing GPS network. Because we impose a smoothness constraint in the inversion, recovered TWS exhibits mass leakage from mountain ranges to surrounding areas. This leakage bias is removed via linear rescaling in which the magnitude of the gain factor depends on station distribution and TWS anomaly patterns. The synthetic GPS-derived estimates reproduce approximately half of the spatial variability (unbiased root mean square error similar to 50%) of TWS loading within mountain ranges, a considerable improvement over GRACE. The inclusion of additional simulated GPS stations improves representation of spatial variations. GPS data can be used to estimate mountain-range-scale SWE, but effects of soil moisture and other TWS components must first be subtracted from the GPS-derived load estimates.

Sun, XL, Abshire JB, Borsa AA, Fricker HA, Yi DH, DiMarzio JP, Paolo FS, Brunt KM, Harding DJ, Neumann GA.  2017.  ICESAT/GLAS Altimetry Measurements: Received Signal Dynamic Range and Saturation Correction. Ieee Transactions on Geoscience and Remote Sensing. 55:5440-5454.   10.1109/tgrs.2017.2702126   AbstractWebsite

NASA's Ice, Cloud, and land Elevation Satellite (ICESat), which operated between 2003 and 2009, made the first satellite-based global lidar measurement of earth's ice sheet elevations, sea-ice thickness, and vegetation canopy structure. The primary instrument on ICESat was the Geoscience Laser Altimeter System (GLAS), which measured the distance from the spacecraft to the earth's surface via the roundtrip travel time of individual laser pulses. GLAS utilized pulsed lasers and a direct detection receiver consisting of a silicon avalanche photodiode and a waveform digitizer. Early in the mission, the peak power of the received signal from snow and ice surfaces was found to span a wider dynamic range than anticipated, often exceeding the linear dynamic range of the GLAS 1064-nm detector assembly. The resulting saturation of the receiver distorted the recorded signal and resulted in range biases as large as similar to 50 cm for ice-and snow-covered surfaces. We developed a correction for this "saturation range bias" based on laboratory tests using a spare flight detector, and refined the correction by comparing GLAS elevation estimates with those derived from Global Positioning System surveys over the calibration site at the salar de Uyuni, Bolivia. Applying the saturation correction largely eliminated the range bias due to receiver saturation for affected ICESat measurements over Uyuni and significantly reduced the discrepancies at orbit crossovers located on flat regions of the Antarctic ice sheet.

Trugman, DT, Shearer PM, Borsa AA, Fialko Y.  2016.  A comparison of long-term changes in seismicity at The Geysers, Salton Sea, and Coso geothermal fields. Journal of Geophysical Research-Solid Earth. 121:225-247.   10.1002/2015jb012510   AbstractWebsite

Geothermal energy is an important source of renewable energy, yet its production is known to induce seismicity. Here we analyze seismicity at the three largest geothermal fields in California: The Geysers, Salton Sea, and Coso. We focus on resolving the temporal evolution of seismicity rates, which provides important observational constraints on how geothermal fields respond to natural and anthropogenic loading. We develop an iterative, regularized inversion procedure to partition the observed seismicity rate into two components: (1) the interaction rate due to earthquake-earthquake triggering and (2) the smoothly varying background rate controlled by other time-dependent stresses, including anthropogenic forcing. We apply our methodology to compare long-term changes in seismicity to monthly records of fluid injection and withdrawal. At The Geysers, we find that the background seismicity rate is highly correlated with fluid injection, with the mean rate increasing by approximately 50% and exhibiting strong seasonal fluctuations following construction of the Santa Rosa pipeline in 2003. In contrast, at both Salton Sea and Coso, the background seismicity rate has remained relatively stable since 1990, though both experience short-term rate fluctuations that are not obviously modulated by geothermal plant operation. We also observe significant temporal variations in Gutenberg-Richter b value, earthquake magnitude distribution, and earthquake depth distribution, providing further evidence for the dynamic evolution of stresses within these fields. The differing field-wide responses to fluid injection and withdrawal may reflect differences in in situ reservoir conditions and local tectonics, suggesting that a complex interplay of natural and anthropogenic stressing controls seismicity within California's geothermal fields.

Becker, TW, Lowry AR, Faccenna C, Schmandt B, Borsa A, Yu CQ.  2015.  Western US intermountain seismicity caused by changes in upper mantle flow. Nature. 524:458-+.   10.1038/nature14867   AbstractWebsite

Understanding the causes of intraplate earthquakes is challenging, as it requires extending plate tectonic theory to the dynamics of continental deformation. Seismicity in the western United States away from the plate boundary is clustered along a meandering, north-south trending 'intermountain' belt(1). This zone coincides with a transition from thin, actively deforming to thicker, less tectonically active crust and lithosphere. Although such structural gradients have been invoked to explain seismicity localization(2,3), the underlying cause of seismicity remains unclear. Here we show results fromimproved mantle flowmodels that reveal a relationship between seismicity and the rate change of 'dynamic topography' (that is, vertical normal stress from mantle flow). The associated predictive skill is greater than that of any of the other forcings we examined. Wesuggest that active mantle flow is amajor contributor to seismogenic intraplate deformation, while gravitational potential energy variations have a minor role. Seismicity localization should occur where convective changes in vertical normal stress are modulated by lithospheric strength heterogeneities. Our results on deformation processes appear consistent with findings from other mobile belts(4), and imply that mantle flow plays a significant and quantifiable part in shaping topography, tectonics, and seismic hazard within intraplate settings.

Trugman, DT, Borsa AA, Sandwell DT.  2014.  Did stresses from the Cerro Prieto Geothermal Field influence the El Mayor-Cucapah rupture sequence? Geophysical Research Letters. 41:8767-8774.   10.1002/2014gl061959   AbstractWebsite

The M-w 7.2 El Mayor-Cucapah (EMC) earthquake ruptured a complex fault system in northern Baja California that was previously considered inactive. The Cerro Prieto Geothermal Field (CPGF), site of the world's second largest geothermal power plant, is located approximately 15km to the northeast of the EMC hypocenter. We investigate whether anthropogenic fluid extraction at the CPGF caused a significant perturbation to the stress field in the EMC rupture zone. We use Advanced Land Observing Satellite interferometric synthetic aperture radar data to develop a laterally heterogeneous model of fluid extraction at the CPGF and estimate that this extraction generates positive Coulomb stressing rates of order 15 kPa/yr near the EMC hypocenter, a value which exceeds the local tectonic stressing rate. Although we cannot definitively conclude that production at the CPGF triggered the EMC earthquake, its influence on the local stress field is substantial and should not be neglected in local seismic hazard assessments.

Borsa, AA, Agnew DC, Cayan DR.  2014.  Ongoing drought-induced uplift in the western United States. Science.   10.1126/science.1260279   AbstractWebsite

The western United States has been experiencing severe drought since 2013. The solid earth response to the accompanying loss of surface and near-surface water mass should be a broad region of uplift. We use seasonally-adjusted time series from continuously operating GPS stations to measure this uplift, which we invert to estimate mass loss. The median uplift is 4 mm, with values up to 15 mm in California’s mountains. The associated pattern of mass loss, which ranges up to 50 cm of water equivalent, is consistent with observed decreases in precipitation and streamflow. We estimate the total deficit to be about 240 Gt, equivalent to a 10 cm layer of water over the entire region, or the annual mass loss from the Greenland Ice Sheet.

Glennie, CL, Hinojosa-Corona A, Nissen E, Kusari A, Oskin ME, Arrowsmith JR, Borsa A.  2014.  Optimization of legacy lidar data sets for measuring near-field earthquake displacements. Geophysical Research Letters. 41:3494-3501.   10.1002/2014gl059919   AbstractWebsite

Airborne lidar (light detection and ranging) topography, acquired before and after an earthquake, can provide an estimate of the coseismic surface displacement field by differencing the preevent and postevent lidar point clouds. However, estimated displacements can be contaminated by the presence of large systematic errors in either of the point clouds. We present three-dimensional displacements obtained by differencing airborne lidar point clouds collected before and after the El Mayor-Cucapah earthquake, a M-w 7.2 earthquake that occurred in 2010. The original surface displacement estimates contained large, periodic artifacts caused by systematic errors in the preevent lidar data. Reprocessing the preevent data, detailed herein, removed a majority of these systematic errors that were largely due to misalignment between the scanning mirror and the outgoing laser beam. The methodology presented can be applied to other legacy airborne laser scanning data sets in order to improve change estimates from temporally spaced lidar acquisitions.

Borsa, AA, Moholdt G, Fricker HA, Brunt KM.  2014.  A range correction for ICESat and its potential impact on ice-sheet mass balance studies. The Cryosphere. 8:345-357.: Copernicus Publications   10.5194/tc-8-345-2014   AbstractWebsite

We report on a previously undocumented range error in NASA's Ice, Cloud and land Elevation Satellite (ICESat) that degrades elevation precision and introduces a small but significant elevation trend over the ICESat mission period. This range error (the Gaussian-Centroid or "G-C" offset) varies on a shot-to-shot basis and exhibits increasing scatter when laser transmit energies fall below 20 mJ. Although the G-C offset is uncorrelated over periods ≤ 1 day, it evolves over the life of each of ICESat's three lasers in a series of ramps and jumps that give rise to spurious elevation trends of −0.92 to −1.90 cm yr−1, depending on the time period considered. Using ICESat data over the Ross and Filchner–Ronne ice shelves we show that (1) the G-C offset introduces significant biases in ice-shelf mass balance estimates, and (2) the mass balance bias can vary between regions because of different temporal samplings of ICESat. We can reproduce the effect of the G-C offset over these two ice shelves by fitting trends to sample-weighted mean G-C offsets for each campaign, suggesting that it may not be necessary to fully repeat earlier ICESat studies to determine the impact of the G-C offset on ice-sheet mass balance estimates.

Hodgkinson, K, Langbein J, Henderson B, Mencin D, Borsa A.  2013.  Tidal calibration of plate boundary observatory borehole strainmeters. Journal of Geophysical Research-Solid Earth. 118:447-458.   10.1029/2012jb009651   AbstractWebsite

The Plate Boundary Observatory, the geodetic component of the EarthScope program, includes 74 borehole strainmeters installed in the western United States and on Vancouver Island, Canada. In this study, we calibrate 45 of the instruments by comparing the observed M-2 and O-1 Earth tides with those predicted using Earth tide models. For each strainmeter, we invert for a coupling matrix that relates the gauge measurements to the regional strain field assuming only that the measured strains are linear combinations of the regional areal and shear strains. We compare these matrices to those found when constraints are imposed which require the coupling coefficients to lie within expected ranges for this strainmeter design. Similar unconstrained and constrained coupling matrices suggest the instrument is functioning as expected as no other coupling matrix can be found that better reduces the misfit between observed and predicted tides when the inversion is unconstrained. Differences imply a coupling matrix with coefficients outside typical ranges gives a better fit between the observed and predicted tides. We find that 22 of the strainmeters examined have coupling matrices for which there is little difference between the constrained and unconstrained inversions. If we allow a greater divergence in the shear coupling coefficients and consider the possibility that one gauge may not function as expected, the discrepancies between the unconstrained and constrained coupling matrices are resolved for a subset of the remaining strainmeters. Our results also indicate that most of the strainmeters are less sensitive to areal strain than expected from theory. Citation: Hodgkinson, K., J. Langbein, B. Henderson, D. Mencin, and A. Borsa (2013), Tidal calibration of plate boundary observatory borehole strainmeters, J. Geophys. Res. Solid Earth, 118, 447-458, doi:10.1029/2012JB009651.

Borsa, A, Minster JB.  2012.  Rapid Determination of Near-Fault Earthquake Deformation Using Differential LiDAR. Bulletin of the Seismological Society of America. 102:1335-1347.   10.1785/0120110159   AbstractWebsite

Improved near-field measurements of earthquake slip and deformation patterns have the potential for expanding our understanding of fault behavior and the relationship of active faulting to topography. Current techniques for obtaining these measurements-including field observation, Global Navigation Satellite Systems displacement estimation, and optical or radar remote sensing-have limitations that can be mitigated by the inclusion of results from differential airborne Light Detection and Ranging (LiDAR) analysis of the rupture zone. The 2005 airborne LiDAR survey of the southern San Andreas, San Jacinto, and Banning faults (the B4 survey) mapped 1100 km of the most seismically active fault systems in southern California for the purpose of providing a baseline for determining slip from a future earthquake. We used the B4 survey to develop a processing algorithm that yields rapid estimates of near-fault ground deformation using simultaneous cross correlation of both topography and backscatter intensity from pre-earthquake and simulated postearthquake LiDAR datasets. We show robust recovery of the direction and magnitude of an applied synthetic slip of 5 m in the horizontal and 0.5 m in the vertical within our area of study, with clear discrimination between areas with and without applied slip. We also successfully recovered more complex deformation from a modeled fault stepover in the same study area. Our results indicate that we should be able to recover slip to accuracies of better than 20 cm in the horizontal and 1 cm in the vertical, at a spatial resolution of <= 15 m for LiDAR datasets with sample densities as low as 0: 5 points/m(2).

Borsa, AA, Bills BG, Minster JB.  2008.  Modeling the topography of the salar de Uyuni, Bolivia, as an equipotential surface of Earth's gravity field. Journal of Geophysical Research-Solid Earth. 113   10.1029/2007jb005445   AbstractWebsite

The salar de Uyuni is a massive dry salt lake that lies at the lowest point of an internal drainage basin in the Bolivain Altiplano. A kinematic GPS survey of the salar in September 2002 found a topographic range of only 80 cm over a 54 x 45 km area and subtle surface features that appeared to correlate with mapped gravity. In order to confirm the correlation between topography and gravity/geopotential, we use local gravity measurements and the EGM96 global geopotential model to construct a centimeter-level equipotential surface corresponding to the elevation of the salar. Our comparison of GPS survey elevations with equipotential surface estimate shows that 63% of the variance of the GPS elevations can be explained by equipotential surface undulations (and long-wavelength error) in the EGM96 model alone, with an additional 30% explained by the shorter-wavelength equipotential surface derived from local gravity. In order to establish a physical connection between topography and the geopotential, we also develop and test a simple surface process model that redistributes salt via the dissolution, transport, and redeposition of salt by precipitated water. Forcing within the model pushes the system to evolve toward constant water depth, with the salt surface approximating the shape of the local equipotential surface. Since the model removes almost all topographic relief with respect to the equipotential surface within a matter of decades, it appears that observed (similar to 5 cm amplitude, similar to 5 km wavelength) residual topography is actively maintained by a process independent of gravity-driven fluid flow.

Bills, BG, Borsa AA, Comstock RL.  2007.  MISR-based passive optical bathymetry from orbit with few-cm level of accuracy on the Salar de Uyuni, Bolivia. Remote Sensing of Environment. 107:240-255.   10.1016/j.rse.2006.11.006   AbstractWebsite

We demonstrate that, under ideal circumstances, passive optical measurements can yield surface water depth estimates with an accuracy of a few centimeters. Our target area is the Salar de Uyuni, in Bolivia. It is a large, active salt flat or playa, which is maintained as an almost perfectly le vel and highly reflective surface by annual flooding, to a mean depth of 30-50 cm. We use MISR data to estimate spatial and temporal variations in water depth during the waning portion of the 2001 flooding cycle. We use a single ICESat laser altimetry profile to calibrate our water depth model.-Though the salt surface is probably the smoothest surface of its size on Earth, with less that 30 cm RMS height variations over an area of nearly 10(4) km(2), it is not completely featureless. Topography there includes a peripheral depression, or moat, around the edge of the salt, and several sets of prominent parallel ridges, with 5 km wavelength and 30 cm amplitude. The process by which these features form is still not well characterized. (c) 2006 Elsevier Inc. All rights reserved.

Borsa, AA, Fricker HA, Bills BG, Minster JB, Carabajal CC, Quinn KJ.  2007.  Topography of the salar de Uyuni, Bolivia from kinematic GPS. Geophysical Journal International. 172:31-40.   10.1111/j.1365-246X.2007.03604.x   AbstractWebsite

The salar de Uyuni in the Bolivian Andes is the largest salt flat on Earth, exhibiting less than 1 m of vertical relief over an area of 9000 km(2). We report on a kinematic Global Positioning System (GPS) survey of a 45-by-54 km area in the eastern salar, conducted in September 2002 to provide ground truth for the Ice Cloud and land Elevation Satellite (ICESat) mission. GPS post-processing included corrections for long-period GPS noise that significantly improved survey accuracy. We fit corrected GPS trajectories with 2-D Fourier basis functions, from which we created a digital elevation model (DEM) of the surface whose absolute accuracy we estimate to be at least 2.2 cm RMSE. With over two magnitudes better vertical resolution than the Shuttle Radar Topography Mission data, this DEM reveals decimetre-level topography that is completely absent in other topographic data sets. Longer wavelengths in the DEM correlate well with mapped gravity, suggesting a connection between broad-scale salar topography and the geoid similar to that seen over the oceans.

Borsa, AA, Minster JB, Bills BG, Fricker HA.  2007.  Modeling long-period noise in kinematic GPS applications. Journal of Geodesy. 81:157-170.   10.1007/s00190-006-0097-x   AbstractWebsite

We develop and test an algorithm for modeling and removing elevation error in kinematic GPS trajectories in the context of a kinematic GPS survey of the salar de Uyuni, Bolivia. Noise in the kinematic trajectory ranges over 15 cm and is highly autocorrelated, resulting in significant contamination of the topographic signal. We solve for a noise model using crossover differences at trajectory intersections as constraints in a least-squares inversion. Validation of the model using multiple realizations of synthetic/simulated noise shows an average decrease in root-mean-square-error (RMSE) by a factor of four. Applying the model to data from the salar de Uyuni survey, we find that crossover differences drop by a factor of eight (from an RMSE of 5.6 to 0.7 cm), and previously obscured topographic features are revealed in a plan view of the corrected trajectory. We believe that this algorithm can be successfully adapted to other survey methods that employ kinematic GPS for positioning.

Fricker, HA, Borsa A, Minster B, Carabajal C, Quinn K, Bills B.  2005.  Assessment of ICESat performance at the Salar de Uyuni, Bolivia. Geophysical Research Letters. 32   10.1029/2005gl023423   AbstractWebsite

The primary goal of the Ice, Cloud and land Elevation Satellite (ICESat) mission is ice sheet elevation change detection. Confirmation that ICESat is achieving its stated scientific requirement of detecting spatially-averaged changes as small as 1.5 cm/year requires continual assessment of ICESat-derived elevations throughout the mission. We use a GPS-derived digital elevation model (DEM) of the salar de Uyuni, Bolivia for this purpose. Using all twelve ICESat passes over the salar survey area acquired to date, we show that the accuracy of ICESat-derived elevations is impacted by environmental effects (e.g., forward scattering and surface reflectance) and instrument effects (e.g., pointing biases, detector saturation, and variations in transmitted laser energy). We estimate that under optimal conditions at the salar de Uyuni, ICESat-derived elevations have an absolute accuracy of <2 cm and precision of <3 cm.

Hudnut, KW, Borsa A, Glennie C, Minster JB.  2002.  High-resolution topography along surface rupture of the 16 October 1999 Hector Mine, California, earthquake (M-w 7. 1) from airborne laser swath mapping. Bulletin of the Seismological Society of America. 92:1570-1576.   10.1785/0120000934   AbstractWebsite

In order to document surface rupture associated with the Hector Mine earthquake, in particular, the area of maximum slip and the deformed surface of Lavic Lake playa, we acquired high-resolution data using relatively new topographic-mapping methods. We performed a raster-laser scan of the main surface breaks along the entire rupture zone, as well as along an unruptured portion of the Bullion fault. The image of the ground surface produced by this method is highly detailed, comparable to that obtained when geologists make particularly detailed site maps for geomorphic or paleoseismic studies. In this case, however, for the first time after a surface-rupturing earthquake, the detailed mapping is along the entire fault zone rather than being confined to selected sites. These data are geodetically referenced, using the Global Positioning System, thus enabling more accurate mapping of the rupture traces. In addition, digital photographs taken along the same fight lines can be overlaid onto the precise topographic data, improving terrain visualization. We demonstrate the potential of these techniques for measuring fault-slip vectors.