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Physical and Statistical Models in Deformation Geodesy
Get PDFGeodetic techniques involving spaceborne measurement have revolutionized scientific understanding of Earth surface deformation, and this thesis presents several independent methodological developments concerning such data. The primary results are: a more general method for the optimal design of geodetic networks; an algorithm for detecting transient deformation events in large geodetic datasets; the identification of a previously unmapped fault in the San Francisco Bay Area, California; a new approach to quantifying the permeability structure of shallow, near-surface, strike slip faults; and a regional evaluation of correlated seasonal deformation in the Bay Area. These results are not strictly related, however, several themes are present throughout. Particular emphasis is placed on the development of statistical and numerical methodology. Although the results presented in this thesis are not strictly related, they represent a significant contribution to our scientific understanding of Earth surface deformation phenomena
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Investigating Seismic Hazard Through Source, Path, and Site Effects Using Various Instrumentation
Get PDFSocietal and economic losses in earthquakes are strongly dependent on the strength of shaking of the ground. Such shaking, or ‘ground motion’ can vary widely from place to place for a given earthquake based on characteristics of the earthquake source, crustal properties affecting seismic wave behavior, and differences in the strength of near-surface geologic material. We investigate the variability in ground motion through a range of methods and techniques and address several of these parameters (e.g., source, path, or site effects) that influence seismic hazard in tectonically active regions, namely: King County in Washington State, U.S. near the Cascadia subduction zone, Bío Bío in Chile near the Atacama Trench, and Christchurch, New Zealand near the Pacific-Australian plate boundary. To study these variations in ground motion, we utilize computational simulations of earthquake hazard and economic loss using the FEMA HAZUS (Federal Emergency Management Agency HAZards U.S) model and seismic data acquired through high-density deployments of small, low-cost micro-electro-mechanical-systems (MEMS) Quake-Catcher Network (QCN) accelerometers. While these sensors are lower-resolution compared to traditional, more expensive seismometers, our results suggest that, with appropriate quality control, QCN sensors provide good-quality data that can be integrated with local networks. We propose that the more economical MEMS sensor technology can drive future studies of seismic hazard and risk at higher spatial resolution than previously available and, with ample amounts of seismic data collected (i.e., “big data”), is posed to revolutionize modern seismology
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Searching for the Geodetic Signature of Wastewater Injection at The Geysers Geothermal Field
Get PDFThe Geysers geothermal field in northern California has seen subsidence, attributed to net volume loss during power production, since at least the 1960s. Over the last three decades this has been accompanied by reductions in reservoir steam pressure and power generation. To combat these effects, wastewater has been injected in the field since 1997. In order to better understand the effects of variations in production and wastewater injection on geothermal reservoir volume and surface subsidence over time, two continuously-recording GPS stations (TG01 and TG02) in the northern Geysers in 2012 and one in the southern Geysers (TG03) in 2013 were installed in the field.In this thesis, I will present our first analyses of the continuous GPS data and our first attempt at modeling the data. Both TG01 and TG02 show early periods of uplift and later subsidence while TG03 shows ongoing subsidence. Next, we downsample steam extraction and injection data onto a rectangular grid and calculate ‘observed’ monthly volume changes as a function of position. We then use these to drive a forward elastic dislocation model to predict surface deformation changes each month in The Geysers field. The models cannot reproduce the GPS data, specifically the uplift that occurs at TG01. We then compare the observed volume changes with inverse elastic dislocation models of the volume changes required to reproduce the GPS time series. Due to low spatial resolution, we compare the total volume change in the field, which is comparable amplitude, and shows periods where the peaks are in phase, and periods where they are out of phase. In these cases, the peaks in the inverse modeled volume changes lag those in the reported data by one month. These 'out-of-phase periods' correlate with periods of peak injection in the field, typically in the winter and early spring months, suggesting that the GPS data are detecting a delayed deformation response to injection in the field, possibly related to the finite permeability of the geothermal reservoir rocks.We also look at a case study of The Geysers Mw 5.0 earthquake. This earthquake is a rare small shallow strike-slip earthquake capable of producing surface displacements. Our three continuous GPS stations in the field recorded coseismic displacement of up to 2 cm. We also were able to see the earthquake displacement in a 6-day descending interferogram. We ran forward and inverse models of both nodal planes provided by earthquake catalogs and found that both nodal planes produce consistent deformation patterns although the inverse model closely matched the NNW-striking nodal plane
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A Renewed Look at the Coseismic Surface Deformation and Fault Slip of the 1994 Northridge Earthquake Using Space Geodesy
Get PDFThe January 17, 1994 M6.7 Northridge earthquake occurred in the densely populated suburbs northwest of Los Angeles, California, causing 33 deaths and ~$20 billion in damage. To quantify the influence, in terms of stress changes, of the Northridge event on surrounding faults, detailed knowledge of the location, orientation and amount of fault slip is important. Existing InSAR models of this earthquake typically were developed by fitting the pattern of displacements by trial and error, and were therefore somewhat subjective. In the 15 years since the original studies were published a number of new modeling tools and community data products have been developed that should enable us to produce more detailed, objective and robust results. We measure the coseismic deformation of this earthquake using InSAR data from the ERS-1 and JERS-1 satellites, combined with GPS measurements (Hudnut et al., 1996) that together show uplift of ~42 cm. Using these data, we first employ a nonlinear inversion to determine the parameters of a best-fitting model using rectangular, uniform slip dislocations. Our best-fitting fault solution contains two faults, a main fault with 2.3 m of slip and a secondary fault to the northwest with 0.8 m.In detail, however, the deformation pattern of the Northridge event is more complex than can be described by rectangular dislocations. To investigate this, we solve for a detailed slip distribution for the event using a non-planar triangular element fault mesh modified from the SCEC Community Fault Model (Plesch et al.,2007). This model shows a main asperity on a protrusion on the fault, with peak slip of ~2.7m, bounded at its western edge by a geometrical barrier, a steep down-dip parallel lateral ramp in the fault. Secondary slip of about 0.6m to the northwest of this feature is also present. These two slip patches together shows that the geometry of the fault strongly influences the slip pattern of the event
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Combining Seismology and Geodesy to Better Constrain Earthquake Source Parameters and Shallow Fault Behavior
Get PDFOur current understanding of the Earth’s interior structure and processes is limited to observations made at the surface that are mapped to the subsurface using inverse methods. The complexity of geophysical inverse problems mainly arises from the existence of many free parameters that sometimes have traded off with each other. This can cause inaccuracies, low resolution and non-uniqueness problems in geophysical models. The main focus of my dissertation is on how we can use two independent geophysical data types – geodesy and seismology – to increase knowledge, resolution and accuracy of Earth’s structure, and of interseismic and coseismic processes in the earthquake cycle. For example, in my first project (Chapter 2) I search for repeating earthquakes (REs) using similarity search on recorded seismic waveforms from the northern San Francisco Bay Area. Evidence from the San Andreas fault and elsewhere indicates that REs are correlated with, and likely driven by, aseismic slip (creep) at depth. This is complementary knowledge to the geodetic observation of creep at the surface. The source information of REs can also be used to constrain the interseismic slip models inverted from geodetic data such as GPS and InSAR. By using a new fast similarity search algorithm, that I developed specifically for probing big seismic data sets (described in Chapter 3), we found 198 RE groups, including periodic and nonperiodic repeating earthquake 'families', and repeating event pairs. Our results can not only help us to map the depth and extent of creep on several major faults but also reveal previously unknown structural complexity – e.g. that subparallel strands of the Maacama fault are active and creep simultaneously. Source parameters and locations of these REs can be used to update seismic hazard models, by better constraining the creeping areas of faults in the region, and to improve community models of fault geometry.In a second major project (Chapter 4), I aim to reconcile earthquake source parameters and locations determined by long-period teleseismic source inversions with those obtained from InSAR data. The latter observes earthquakes in situ and thus, we presume, accurately locates them. Previous studies suggest that the discrepancies between these two catalogs arise from the existing inaccuracy in Earth models and are caused by the historic (and circular) problem that earthquake locations estimated using inaccurate velocity models are themselves inaccurate, and vice-versa.In several case studies of various locations (e.g. California, Iceland, central Italy) we observe and quantify the biases of the S40RTS Earth velocity model that cause a delay or early arrival of the predicted seismic waves to the seismic stations at certain azimuths. We gather these misestimations of predicted seismic wave arrivals as corrections that can be applied to teleseismic source inversions in order to improve location accuracy. The similarity of corrections that we observe for events in the same region suggests they could be used as regional corrections. We also show that these corrections not only can be used to accurately locate global events but also can help us to accurately obtain the source mechanisms of these events. In future, by gathering these corrections for all the events with existing InSAR source models (i.e. more than 120 global events so far) we might be able to increase accuracy of velocity models of the upper mantle, e.g. by using finite-frequency tomography
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Rapid, Robust Characterization of Subduction Zone Earthquakes
Get PDFRapid, Robust Characterization of Subduction Zone Earthquake
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Complex Faulting in the Yuha Desert: Implications for Fault Interaction
Get PDFWe have determined precise hypocentral locations for over 3,600 aftershocks that occurred in the Yuha Desert region following the 4 April 2010 Mw 7.2 El Mayor-Cucapah (EMC) earthquake until 14 June 2010 through a series of absolute and relative relocation procedures with algorithms including hpoinverse, velest and hypoDD. Location errors were reduced to ~20 m horizontally and ~80 m vertically. The locations reveal a complex pattern of faulting with en echelon fault segments trending in toward the northwest, approximately parallel to the North American-Pacific plate boundary and en echelon, conjugate features trending to the northeast. The relocated seismicity is highly correlated with the mapped faults showing triggered surface slip in response to the EMC mainshock. Aftershocks are located between depths of 2 km and 11 km, consistent with previous studies of seismogenic thickness in the region. Three-dimensional analysis reveals individual and intersecting fault planes between 5 km and 10 km in the along-strike and along-dip directions. These fault planes remain distinct structures at depth, indicative of conjugate faulting, and do not appear to coalesce onto a through-going fault segment. Measurements of surface displacement along the Laguna Salada-West and East (LS-W and LS-E, respectively) branches reveal no triggered surface creep on either branch following the EMC mainshock. Approximately 2 mm of triggered surface creep was observed on the LS-W in response to the 14 June 2010 M5.7 Ocotillo earthquake, however no creep was measured along the LS-E. Relocated seismicity shows that aftershock activity occurs along the LS-E, however not along the LS-W in the time period between these earthquakes. The area east of the LS-W was seismically active following the EMC mainshock, however; no systematic migration patterns were observed. Aftershock activity abruptly shuts off in this area following the Ocotillo earthquake. The majority of seismicity in the two-year period following the Ocotillo earthquake suggesting a complex spatial and temporal distribution of aftershock activity. The conjugate pattern of faulting, combination of seismicity and triggered surface creep and non-uniform aftershock migration patterns suggest that strain in the Yuha Desert is being accommodated in a complex m
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Understanding Fault Creep From the Macroscopic to the Microscopic Scale
Get PDFFault creep is a behavior of some faults where the two sides of a fault slowly slide past one another in the absence of large earthquakes. This is a form of aseismic slip and can be continuous or episodic. Understanding what drives fault creep is important as it can reduce the amount of strain that accumulates in the crust and can impede future earthquake ruptures. Both of these aspects play an important role in analyzing the seismic hazard of a region, therefore mapping the extent of where fault creep is occurring and identifying common lithology between creeping faults is necessary for accurate hazard assessment. In this dissertation, we focus on fault creep in northern California using three different methods -- satellite imagery, 3D models derived from photographs, and rock mechanics experiments. In the first project, we focus on mapping the extent of fault creep along on two faults in the North Bay (north of the San Francisco Bay Area), the Maacama and Rodgers Creek faults. Both faults have observations of fault creep at specific locations but the extent and variability of fault creep on each fault is not well mapped. We use Interferometric Synthetic Aperture Radar (InSAR) to map the extent of fault creep along both faults and estimate at least 55% of the Maacama fault and 40% of the Rodgers Creek faults are creeping at the surface. The second project focuses on offset sidewalk observations along the southern Hayward Fault, known to be creeping at rates up to ~8 mm/yr and how fault creep is being expressed in an urban setting. We construct 3D models from 2D photos taken from 2015 to 2018 to measure 3D displacements of each offset sidewalk. In this way we can monitor how the fault is being expressed in an urban environment due to the creeping fault. We find that on average, individual offset curbs sample < 40% of the overall creep rate measured from nearby alignment arrays (which span a fault-perpendicular distance of 100 m or more). In some locations, multiple adjacent curbs are actively deforming. These findings imply that there is significant off-fault deformation along the southern Hayward fault, and suggests that the `fault trace' can more correctly be considered a zone of deformation, narrower than an alignment array width but wider than one curb length.The third and fourth projects center on an exposure of the Bartlett Springs fault core near Lake Pillsbury, which is known to be creeping at 3.4 mm/yr from a nearby alignment array. We collected the fault gouge and ran two sets of experiments to investigate the frictional and mineralogical properties needed for a fault to creep. The first set of experiments investigated which of the minerals that we found in the fault gouge, are promoting or not promoting fault creep. We find that the presence of talc has the strongest influence on creep behavior. The second set of experiments explored the frictional properties of the natural fault gouge and compared their frictional properties to samples collected from the creeping section of the San Andreas fault. We found that both fault gouges have similar compositions and frictional properties. In order for fault creep to occur on the Bartlett Springs fault, we estimate that there needs to be at least 50% talc in the gouge, concentrated into layers in which the majority of shear is taking place. Through the various approaches to understanding fault creep used in this dissertation and the compilation of previous studies using various techniques (e.g. GPS-derived models, seismology, additional geodetic observations), we have a better understanding of fault creep mechanisms and distribution in the North Bay. We estimate a larger extent of the Maacama and Rodgers Creek faults to be creeping than previously observed, supplementing previous estimates from repeating earthquake families, GPS-based models, alignment arrays, and prior InSAR studies in the area. The measurement of offset sidewalks along the Hayward fault shows along-strike variations in the rates of movement but also allows us to quantify how much of the total creep occurs in a narrow zone near the mapped surface trace (3-10 meters wide) versus the wider zone measured by alignment arrays (~100 meters wide). The low frictional strength of talc within the Bartlett Springs fault gouge is the main driving factor of fault creep on the fault, and can promote creep at depths up to 9 km. The mineral assemblage is also similar in its elemental composition to the fault gouge collected within the creeping section of the San Andreas fault, suggesting that the creep in both places is controlled by a similar lithology sampled by each fault. This suggests that the Bartlett Springs fault may also be creeping at deeper depths, up to 9 km, consistent with the depths of repeating earthquakes located on the fault
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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