Quantification Of Geodetic Strain Rate Uncertainties And Implications For Seismic Hazard Estimates

Geodetic velocity data provide first-order constraints on crustal surface strain rates, which in turn are linked to seismic hazard. Estimating the 2-D surface strain tensor everywhere requires knowledge of the surface velocity field everywhere, while geodetic data such as Global Navigation Satellite System (GNSS) only have spatially scattered measurements on the surface of the Earth. To use these data to estimate strain rates, some type of interpolation is required. In this study, we review methodologies for strain rate estimation and compare a suite of methods, including a new implementation based on the geostatistical method of kriging, to compare variation between methods with uncertainty based on one method. We estimate the velocity field and calculate strain rates in southern California using a GNSS velocity field and five different interpolation methods to understand the sources of variability in inferred strain rates. Uncertainty related to data noise and station spacing (aleatoric uncertainty) is minimal where station spacing is dense and maximum far from observations. Differences between methods, related to epistemic uncertainty, are usually highest in areas of high strain rate due to differences in how gradients in the velocity field are handled by different interpolation methods. Parameter choices, unsurprisingly, have a strong influence on strain rate field, and we propose the traditional L-curve approach as one method for quantifying the inherent trade-off between fit to the data and models that are reflective of tectonic strain rates. Doing so, we find total variability between five representative strain rate models to be roughly 40 per cent, a much lower value than roughly 100 per cent that was found in previous studies (Hearn et al.). Using multiple methods to tune parameters and calculate strain rates provides a better understanding of the range of acceptable models for a given velocity field. Finally, we present an open-source Python package (Materna et al.) for calculating strain rates, Strain 2D, which allows for the same data and model grid to be used in multiple strain rate methods, can be extended with other methods from the community, and provides an interface for comparing strain rate models, calculating statistics and estimating strain rate uncertainty for a given GNSS data set

Geodetic and Seismological Investigations of Earthquake Cycle Deformation and Fault Zone Properties

A number of geologic processes on the surface and in the interior of the Earth result in ground deformation on human timescales. These processes include the steady motion of tectonic plates, the internal deformation of heterogeneous regions in the crust, the response of the Earth to surface loads, and deformation at plate boundaries, all acting to produce millimeter-scale displacements each year. These same processes are also responsible for many natural hazards that cause harm to human communities: earthquakes and tsunamis are the most directly related, but sea level rise and extreme flooding are also pertinent natural threats exacerbated by the deformation of the ground. In order to understand these natural hazards and to mitigate their impacts, it is important to quantify surface deformation and to understand the physical mechanisms underlying it. The chapters of this dissertation address several questions in the field of tectonic deformation that relate to these processes. In particular, I focus on understanding earthquake hazards and fault zone properties using a variety of tools at the intersection of space geodesy and seismology. Zones of weakness in the Earth’s crust at all scales are relevant for the study of natural hazards. They respond to external stresses and often host earthquake ruptures. Fault zones, as one small-scale example, are thought to be mechanically weaker than their surroundings because the accumulated near-fault damage results in lower mechanical strength, forming compliant fault zones. Importantly, the presence of a compliant fault zone around a fault affects the types of earthquake ruptures the fault can sustain. In order to study compliant fault zones around a major fault, I investigated the elastic properties of the crust near the San Andreas Fault Zone in northern California. I used GPS measurements to characterize compliant fault structures along strike and found that their distribution is heterogeneous even over short spatial scales. Larger zones of weakness also impact present-day deformation. On continental scales, inherited zones of weakness from previous tectonic episodes can affect the seismic activity of a region even millions of years later. In the craton of southern Africa, I studied the rupture process of a rare M6.5 earthquake in Botswana that ruptured the lower crust in a continental plate interior. Using a joint analysis of teleseismic waveforms, InSAR data, and relocated aftershocks, I identified the rupture plane out of the two possible focal planes. The modeled strike matches with the boundary of an ancient collisional mountain belt that has been reactivated in the present day as a set of normal faults. Intraplate earthquakes such as this one are challenging to forecast because they occur in regions of low interseismic strain, but they can be especially damaging because they occur in places that aren’t expecting earthquakes. Understanding the structure of pre-existing weak zones in the crust may be key in such situations. Clues to the internal structure of the crust may also come from the study of periodic hydrological loads on the Earth’s surface. Seasonal loads from snow, rivers, lakes, groundwater, the ocean, and the atmosphere cause surface deformation that depends on the structure of the underlying medium and the processes involved. In order to analyze these processes in a tectonically active region, I performed a study on hydrological loading in GPS data from South Asia and Southeast Asia, a region impacted by a strong yearly monsoon. The annual deformation varies across the region but is generally consistent with the elastic loading modeled from an independent gravity dataset. The appropriate modeling of the hydrological loading deformation in the future could help quantify the storage of aquifers, improve our understanding of earthquake cycle deformation, and aid in the accurate detection of transient fault behavior. The remaining two chapters of this dissertation relate to transient fault behavior of the Mendocino Triple Junction in northern California. This region lies at the intersection of three major plate-bounding faults, and it produces some of the largest earthquakes in California. Several of the plate-bounding faults at the Mendocino Triple Junction produce a mix of seismic and aseismic moment release; this results in fascinating time-dependent slip behavior and an interplay between aseismic and seismic slip modes. In one study of this behavior, I used characteristically repeating earthquakes to identify regions of the Mendocino Fault Zone that are creeping aseismically. Using a dataset from 2008 to 2018, I found several dozen families of small-magnitude repeating earthquakes that show a high degree of active creep on the plate boundary fault. The creep rate is calculated to be about 65% of the overall slip budget, a significant amount but in line with previous estimates of aseismic slip on oceanic transform faults. During this time interval, the slip appears to be relatively steady, but the repeating earthquake catalog allows us to study any time-dependent variations of the creep rate in the future. On the neighboring Cascadia Subduction Zone, I also find aseismic creep with time-dependent rate variations. Well-known slip transients called Episodic Tremor and Slip events occur on the southern Cascadia Subduction Zone margin every 7-10 months, resulting in several centimeters of slip at about 30 km depth. However, in the final chapter of this dissertation, I document changes in surface velocity that appear uncorrelated with the process of Episodic Tremor and Slip. I find that surface GPS velocities near the Mendocino Triple Junction show systematic variations in their east/west component that last for several years apiece. The timing of several velocity changes is coincident with the timing of large (M>6.5) offshore earthquakes. The spatial pattern and temporal pattern of the observations do not indicate more usual processes like afterslip, viscoelastic relaxation, or hydrological loading are sufficient to explain the observations. Instead, inversion of the velocity changes suggests that in a region slightly updip of the Episodic Tremor and Slip portion of the interface, interseismic coupling may both increase and decrease in connection with the offshore earthquakes. A speculative dynamic triggering mechanism is presented. Such observations suggest that understanding fault zone properties is of paramount importance in the study of earthquake hazards. The findings also show that in light of newer geodetic and seismological datasets, there is still much to be learned and many unanswered questions about the behavior of fault zones throughout the earthquake cycle

Dynamically Triggered Changes of Plate Interface Coupling in Southern Cascadia

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.National Science Foundation (NSF). Grant Number: EAR-1841371NSF Graduate Research Fellowship Program and NSF. Grant Number: OCE-1905098NSF Cooperative Agreement. Grant Number: EAR-073515

Tracking the weight of Hurricane Harvey’s stormwater using GPS data

On 26 August 2017, Hurricane Harvey struck the Gulf Coast as a category four cyclone depositing ~95 km3 of water, making it the wettest cyclone in U.S. history. Water left in Harvey’s wake should cause elastic loading and subsidence of Earth’s crust, and uplift as it drains into the ocean and evaporates. To track daily changes of transient water storage, we use Global Positioning System (GPS) measurements, finding a clear migration of subsidence (up to 21 mm) and horizontal motion (up to 4 mm) across the Gulf Coast, followed by gradual uplift over a 5-week period. Inversion of these data shows that a third of Harvey’s total stormwater was captured on land (25.7 ± 3.0 km3 ), indicating that the rest drained rapidly into the ocean at a rate of 8.2 km3 /day, with the remaining stored water gradually lost over the following 5 weeks at ~1 km3 /day, primarily by evapotranspiration. These results indicate that GPS networks can remotely track the spatial extent and daily evolution of terrestrial water storage following transient, extreme precipitation events, with implications for improving operational flood forecasts and understanding the response of drainage systems to large influxes of water

Geodetic and Seismological Investigations of Earthquake Cycle Deformation and Fault Zone Properties

A number of geologic processes on the surface and in the interior of the Earth result in ground deformation on human timescales. These processes include the steady motion of tectonic plates, the internal deformation of heterogeneous regions in the crust, the response of the Earth to surface loads, and deformation at plate boundaries, all acting to produce millimeter-scale displacements each year. These same processes are also responsible for many natural hazards that cause harm to human communities: earthquakes and tsunamis are the most directly related, but sea level rise and extreme flooding are also pertinent natural threats exacerbated by the deformation of the ground. In order to understand these natural hazards and to mitigate their impacts, it is important to quantify surface deformation and to understand the physical mechanisms underlying it. The chapters of this dissertation address several questions in the field of tectonic deformation that relate to these processes. In particular, I focus on understanding earthquake hazards and fault zone properties using a variety of tools at the intersection of space geodesy and seismology. Zones of weakness in the Earth’s crust at all scales are relevant for the study of natural hazards. They respond to external stresses and often host earthquake ruptures. Fault zones, as one small-scale example, are thought to be mechanically weaker than their surroundings because the accumulated near-fault damage results in lower mechanical strength, forming compliant fault zones. Importantly, the presence of a compliant fault zone around a fault affects the types of earthquake ruptures the fault can sustain. In order to study compliant fault zones around a major fault, I investigated the elastic properties of the crust near the San Andreas Fault Zone in northern California. I used GPS measurements to characterize compliant fault structures along strike and found that their distribution is heterogeneous even over short spatial scales. Larger zones of weakness also impact present-day deformation. On continental scales, inherited zones of weakness from previous tectonic episodes can affect the seismic activity of a region even millions of years later. In the craton of southern Africa, I studied the rupture process of a rare M6.5 earthquake in Botswana that ruptured the lower crust in a continental plate interior. Using a joint analysis of teleseismic waveforms, InSAR data, and relocated aftershocks, I identified the rupture plane out of the two possible focal planes. The modeled strike matches with the boundary of an ancient collisional mountain belt that has been reactivated in the present day as a set of normal faults. Intraplate earthquakes such as this one are challenging to forecast because they occur in regions of low interseismic strain, but they can be especially damaging because they occur in places that aren’t expecting earthquakes. Understanding the structure of pre-existing weak zones in the crust may be key in such situations. Clues to the internal structure of the crust may also come from the study of periodic hydrological loads on the Earth’s surface. Seasonal loads from snow, rivers, lakes, groundwater, the ocean, and the atmosphere cause surface deformation that depends on the structure of the underlying medium and the processes involved. In order to analyze these processes in a tectonically active region, I performed a study on hydrological loading in GPS data from South Asia and Southeast Asia, a region impacted by a strong yearly monsoon. The annual deformation varies across the region but is generally consistent with the elastic loading modeled from an independent gravity dataset. The appropriate modeling of the hydrological loading deformation in the future could help quantify the storage of aquifers, improve our understanding of earthquake cycle deformation, and aid in the accurate detection of transient fault behavior. The remaining two chapters of this dissertation relate to transient fault behavior of the Mendocino Triple Junction in northern California. This region lies at the intersection of three major plate-bounding faults, and it produces some of the largest earthquakes in California. Several of the plate-bounding faults at the Mendocino Triple Junction produce a mix of seismic and aseismic moment release; this results in fascinating time-dependent slip behavior and an interplay between aseismic and seismic slip modes. In one study of this behavior, I used characteristically repeating earthquakes to identify regions of the Mendocino Fault Zone that are creeping aseismically. Using a dataset from 2008 to 2018, I found several dozen families of small-magnitude repeating earthquakes that show a high degree of active creep on the plate boundary fault. The creep rate is calculated to be about 65% of the overall slip budget, a significant amount but in line with previous estimates of aseismic slip on oceanic transform faults. During this time interval, the slip appears to be relatively steady, but the repeating earthquake catalog allows us to study any time-dependent variations of the creep rate in the future. On the neighboring Cascadia Subduction Zone, I also find aseismic creep with time-dependent rate variations. Well-known slip transients called Episodic Tremor and Slip events occur on the southern Cascadia Subduction Zone margin every 7-10 months, resulting in several centimeters of slip at about 30 km depth. However, in the final chapter of this dissertation, I document changes in surface velocity that appear uncorrelated with the process of Episodic Tremor and Slip. I find that surface GPS velocities near the Mendocino Triple Junction show systematic variations in their east/west component that last for several years apiece. The timing of several velocity changes is coincident with the timing of large (M>6.5) offshore earthquakes. The spatial pattern and temporal pattern of the observations do not indicate more usual processes like afterslip, viscoelastic relaxation, or hydrological loading are sufficient to explain the observations. Instead, inversion of the velocity changes suggests that in a region slightly updip of the Episodic Tremor and Slip portion of the interface, interseismic coupling may both increase and decrease in connection with the offshore earthquakes. A speculative dynamic triggering mechanism is presented. Such observations suggest that understanding fault zone properties is of paramount importance in the study of earthquake hazards. The findings also show that in light of newer geodetic and seismological datasets, there is still much to be learned and many unanswered questions about the behavior of fault zones throughout the earthquake cycle

Analysis of atmospheric delays and asymmetric positioning errors in the global positioning system

Thesis: S.B., Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences, 2014.15Cataloged from PDF version of thesis.Includes bibliographical references (pages 50-51).Abstract Errors in modeling atmospheric delays are one of the limiting factors in the accuracy of GPS position determination. In regions with uneven topography, atmospheric delay phenomena can be especially complicated. Current delay models used in analyzing GPS data from the Plate Boundary Observatory (PBO) are successful in achieving millimeter-level accuracy at most locations; however, at a subset of stations, the time series for position estimates contain an unusually large number of outliers. In many cases these outliers are oriented in the same direction. The stations which exhibit asymmetric outliers occur in various places across the PBO network, but they are especially numerous in California's Mammoth Lakes region, which served as a case study for this project. The phenomenon of skewed residuals was analyzed by removing secular trends and variations with periods longer than 75 days from the signal using a median filter. The skewness of the station position residuals was subsequently calculated in the north, east and up directions. In the cases examined, typical position outliers are 5-15 mm. In extreme cases, skewed position residuals, not related to snow on antennas, can be as large as 20 mm. I examined the causes of the skewness through site-by-site comparisons with topographic data and various forms of weather data such as numerical weather models, radiosondes, and satellite images. Analysis suggests that the direction of the skewness is generally parallel to the local topographic gradient at a scale of several kilometers. Comparison with weather data suggests that outlier data points in the Mammoth Lakes region occur when lee waves are likely to form downstream of the Sierra Nevada Mountains. The results imply that coupling between the atmosphere and local topography, e.g. lee waves, is responsible for the phenomenon of skewed residuals.by Kathryn Materna.S.B

Timeseries Data from Superstition Hills and Imperial Fault creep events in 2023

<p>GNSS and creepmeter time series associated with the 2023 creep events on the Superstition Hills and Imperial faults.  Supplementary data for Materna, Burgmann, Lindsey, Bilham, Crowell, Herring, and Szeliga, "Shallow slow slip events in the Imperial Valley with pulse-like propagation".</p&gt

Shallow Slow Slip Events in the Imperial Valley With Along‐Strike Propagation

Abstract Shallow creep events provide opportunities to understand the mechanical properties and behavior of faults. However, due to physical limitations observing creep events, the precise spatio‐temporal evolution of slip during creep events is not well understood. In 2023, the Superstition Hills and Imperial faults in California each experienced centimeter‐scale slip events that were captured in unprecedented detail by satellite radar, sub‐daily Global Navigation Satellite Systems, and creepmeters. In both cases, the slip propagated along the fault over 2–3 weeks. The Superstition Hills event propagated bilaterally away from its initiation point at average velocities of ∼9 km/day, but propagation velocities were locally much higher. The ruptures were consistent with slip from tens of meters to ∼2 km depths. These slowly propagating events reveal that the shallow crust of the Imperial Valley does not obey purely velocity‐strengthening or velocity‐weakening rate‐and‐state friction, but instead requires the consideration of fault heterogeneity or fault‐frictional behaviors such as dilatant strengthening

Relatively stable pressure effects and time-increasing thermal contraction control Heber geothermal field deformation

Abstract Due to geological complexities and observational gaps, it is challenging to identify the governing physical processes of geothermal field deformation including ground subsidence and earthquakes. In the west and east regions of the Heber Geothermal Field (HGF), decade-long subsidence was occurring despite injection of heat-depleted brines, along with transient reversals between uplift and subsidence. These observed phenomena contradict current knowledge that injection leads to surface uplift. Here we show that high-yield production wells at the HGF center siphon fluid from surrounding regions, which can cause subsidence at low-rate injection locations. Moreover, the thermal contraction effect by cooling increases with time and eventually overwhelms the pressure effects of pressure fluctuation and poroelastic responses, which keep relatively stable during geothermal operations. The observed subsidence anomalies result from the siphoning effect and thermal contraction. We further demonstrate that thermal contraction dominates long-term trends of surface displacement and seismicity growth, while pressure effects drive near-instantaneous changes