Geology topics

John O. Langbein

Publications and source records attributed to John O. Langbein.

At least 19 recordsLinked to original sources

USGS2021

Earthquake magnitudes from dynamic strain

Dynamic strains have never played a role in determining local earthquake magnitudes, which are routinely set by displacement waveforms from seismic instrumentation (e.g., M L ⁠ ). We present a magnitude scale for local earthquakes based on broadband dynamic strain waveforms. This scale is derived from the peak root‐mean‐squared strains ( ⁠ A ⁠ ) in 4589 records of dynamic strain associated with 365 crustal earthquakes and 77 borehole strainmeters along the Pacific‐North American plate boundary on the west coast of the United States and Canada. In this data set, catalog moment magnitudes range from 3.5≤ M w ≤ 7.2 ⁠ , and hypocentral distances range from 6≤ R ≤500 km . The 1D representation of geometrical spreading and attenuation of A common to all strain data is log A 0 ( R )=−0.00072R−1.45log(R) . After correcting for instrument gain, site terms, and event terms, the magnitude scale, M DS =log A −log A 0 ( R )−log(3×10−9) ⁠ , scales as ≈0.92 M w with a residual standard deviation of 0.19. This close association with M w holds for events east of the −124° meridian; west of this boundary, however, a constant correction of 0.41 is needed to adjust for additional along‐path attenuation effects. As a check on the accuracy of this magnitude scale, we apply it to dynamic strain records from three strainmeters located in the near field of the 2019 M 6.4 and 7.1 Ridgecrest earthquakes. Results from these six records are in agreement to within 0.5 magnitude units, and five out of six records are in agreement to within 0.34 units.

California, Oregon, Washington↗

USGS2018

Development of a geodetic component for the U.S. West Coast Earthquake Early Warning System

An earthquake early warning (EEW) system, ShakeAlert, is under development for the West Coast of the United States. This system currently uses the first few seconds of waveforms recorded by seismic instrumentation to rapidly characterize earthquake magnitude, location, and origin time; ShakeAlert recently added a seismic line source algorithm. For large to great earthquakes, magnitudes estimated from the earliest seismic data alone generally saturate. Real‐time Global Navigation Satellite System (GNSS) data can directly measure large displacements, enabling accurate magnitude estimates for M w 7 + "> M w 7 + events, possibly before rupture termination. GNSS‐measured displacements also track evolving slip and, alone or in combination with seismic data, constrain finite‐fault models. Particularly for large‐magnitude, long‐rupture events, GNSS‐based magnitude and rupture extent estimates can improve updates to predicted shaking and thus alert accuracy. GNSS data processing centers at ShakeAlert partner institutions provide real‐time streams to the EEW system, and three geodetic EEW algorithms have been developed through the ShakeAlert collaboration. These algorithms will undergo initial testing within ShakeAlert’s computational architecture using a suite of input data that includes simulated real‐time displacements from synthetic earthquakes and GNSS recordings from recent earthquakes worldwide. Performance will be evaluated using metrics and standards consistent with those adopted for ShakeAlert overall. This initial assessment will guide method refinement and synthesis of the most successful features into a candidate geodetic algorithm for the ShakeAlert production system. In parallel, improvements to geodetic networks and streamlining approaches to data processing and exchange will ensure robust geodetic data availability in the event of an earthquake.

West coast↗

USGS2018

Leveraging geodetic data to reduce losses from earthquakes

Seismic hazard assessments that are based on a variety of data and the best available science, coupled with rapid synthesis of real-time information from continuous monitoring networks to guide post-earthquake response, form a solid foundation for effective earthquake loss reduction. With this in mind, the Earthquake Hazards Program (EHP) of the U.S. Geological Survey (USGS) Natural Hazards Mission Area (NHMA) engages in a variety of undertakings, both established and emergent, in order to provide high quality products that enable stakeholders to take action in advance of and in response to earthquakes. Examples include the National Seismic Hazard Model (NSHM), development of tools for improved situational awareness such as earthquake early warning (EEW) and operational earthquake forecasting (OEF), research about induced seismicity, and new efforts to advance comprehensive subduction zone science and monitoring. Geodetic observations provide unique and complementary information directly relevant to advancing many aspects of these efforts (fig. 1). EHP scientists have long leveraged geodetic data for a range of influential studies, and they continue to develop innovative observation and analysis methods that push the boundaries of the field of geodesy as applied to natural hazards research. Given the ongoing, rapid improvement in availability, variety, and precision of geodetic measurements, considering ways to fully utilize this observational resource for earthquake loss reduction is timely and essential. This report presents strategies, and the underlying scientific rationale, by which the EHP could achieve the following outcomes: The EHP is an authoritative source for the interpretation of geodetic data and its use for earthquake loss reduction throughout the United States and its territories. The USGS consistently provides timely, high quality geodetic data to stakeholders. Significant earthquakes are better characterized by incorporating geodetic data into USGS event response products and by expanded use of geodetic imaging data to assess fault rupture and source parameters. Uncertainties in the NSHM, and in regional earthquake models, are reduced by fully incorporating geodetic data into earthquake probability calculations. Geodetic networks and data are integrated into the operations and earthquake information products of the Advanced National Seismic System (ANSS). Earthquake early warnings are improved by more rapidly assessing ground displacement and the dynamic faulting process for the largest earthquakes using real-time geodetic data. Methodology for probabilistic earthquake forecasting is refined by including geodetic data when calculating evolving moment release during aftershock sequences and by better understanding the implications of transient deformation for earthquake likelihood. A geodesy program that encompasses a balanced mix of activities to sustain missioncritical capabilities, grows new competencies through the continuum of fundamental to applied research, and ensures sufficient resources for these endeavors provides a foundation by which the EHP can be a leader in the application of geodesy to earthquake science. With this in mind the following objectives provide a framework to guide EHP efforts: Fully utilize geodetic information to improve key products, such as the NSHM and EEW, and to address new ventures like the USGS Subduction Zone Science Plan. Expand the variety, accuracy, and timeliness of post-earthquake information products, such as PAGER (Prompt Assessment of Global Earthquakes for Response), through incorporation of geodetic observations. Determine if geodetic measurements of transient deformation can significantly improve estimates of earthquake probability. Maintain an observational strategy aligned with the target outcomes of this document that includes continuous monitoring, recording of ephemeral observations, focused data collection for use in research, and application-driven data processing and analysis systems. Collaborate on research, development, and operation of affordable, high-precision seafloor geodetic methods that improve earthquake forecasting and event response. Advance computational techniques and instrumentation to enable use of strategies like repeat-pass imagery and low-cost geodetic sensors for earthquake response, monitoring, and research. Engage stakeholders and collaborate with partner institutions to foster operational and research objectives and to safeguard the continued health of geodetic infrastructure upon which we mutually depend. Maintaining a vibrant internal research program provides the foundation by which the EHP can remain an effective and trusted source for earthquake science. Exploiting abundant new data sources, evaluating and assimilating the latest science, and pursuing novel avenues of investigation are means to fulfilling the EHP’s core responsibilities and realizing the important scientific advances envisioned by its scientists. Central to the success of such a research program is engaging personnel with a breadth of competencies and a willingness and ability to adapt these to the program’s evolving priorities, enabling current staff to expand their skills and responsibilities, and planning holistically to meet shared workforce needs. In parallel, collaboration with external partners to support scientific investigations that complement ongoing internal research enables the EHP to strengthen earthquake information products by incorporating alternative perspectives and approaches and to study topics and geographic regions that cannot be adequately covered internally. With commensurate support from technical staff who possess diverse skills, including engineering, information technology, and proficiency in quantitative analysis combined with basic geophysical knowledge, the EHP can achieve the geodetic outcomes identified in this document.

Open-File Report↗

USGS2017

Improved efficiency of maximum likelihood analysis of time series with temporally correlated errors

Most time series of geophysical phenomena have temporally correlated errors. From these measurements, various parameters are estimated. For instance, from geodetic measurements of positions, the rates and changes in rates are often estimated and are used to model tectonic processes. Along with the estimates of the size of the parameters, the error in these parameters needs to be assessed. If temporal correlations are not taken into account, or each observation is assumed to be independent, it is likely that any estimate of the error of these parameters will be too low and the estimated value of the parameter will be biased. Inclusion of better estimates of uncertainties is limited by several factors, including selection of the correct model for the background noise and the computational requirements to estimate the parameters of the selected noise model for cases where there are numerous observations. Here, I address the second problem of computational efficiency using maximum likelihood estimates (MLE). Most geophysical time series have background noise processes that can be represented as a combination of white and power-law noise, 1 / f α "> 1 / f α 1/fα with frequency, f . With missing data, standard spectral techniques involving FFTs are not appropriate. Instead, time domain techniques involving construction and inversion of large data covariance matrices are employed. Bos et al. (J Geod, 2013 . doi: 10.1007/s00190-012-0605-0 ) demonstrate one technique that substantially increases the efficiency of the MLE methods, yet is only an approximate solution for power-law indices >1.0 since they require the data covariance matrix to be Toeplitz. That restriction can be removed by simply forming a data filter that adds noise processes rather than combining them in quadrature. Consequently, the inversion of the data covariance matrix is simplified yet provides robust results for a wider range of power-law indices.

Journal of Geodesy↗

USGS2015

Borehole strainmeter measurements spanning the 2014, M w 6.0 South Napa Earthquake, California: The effect from instrument calibration

The 24 August 2014 M w 6.0 South Napa, California earthquake produced significant offsets on 12 borehole strainmeters in the San Francisco Bay area. These strainmeters are located between 24 and 80 km from the source and the observed offsets ranged up to 400 parts-per-billion (ppb), which exceeds their nominal precision by a factor of 100. However, the observed offsets of tidally calibrated strains differ by up to 130 ppb from predictions based on a moment tensor derived from seismic data. The large misfit can be attributed to a combination of poor instrument calibration and better modeling of the strain fit from the earthquake. Borehole strainmeters require in-situ calibration, which historically has been accomplished by comparing their measurements of Earth tides with the strain-tides predicted by a model. Although the borehole strainmeter accurately measure the deformation within the borehole, the long-wavelength strain signals from tides or other tectonic processes recorded in the borehole are modified by the presence of the borehole and the elastic properties of the grout and the instrument. Previous analyses of surface-mounted, strainmeter data and their relationship with the predicted tides suggest that tidal models could be in error by 30%. The poor fit of the borehole strainmeter data from this earthquake can be improved by simultaneously varying the components of the model tides up to 30% and making small adjustments to the point-source model of the earthquake, which reduces the RMS misfit from 130 ppb to 18 ppb. This suggests that relying on tidal models to calibrate borehole strainmeters significantly reduces their accuracy.

California↗

USGS2015

Renewed inflation of Long Valley Caldera, California (2011 to 2014)

Slow inflation began at Long Valley Caldera in late 2011, coinciding with renewed swarm seismicity. Ongoing deformation is concentrated within the caldera. We analyze this deformation using a combination of GPS and InSAR (TerraSAR-X) data processed with a persistent scatterer technique. The extension rate of the dome-crossing baseline during this episode (CA99 to KRAC) is 1 cm/yr, similar to past inflation episodes (1990–1995 and 2002–2003), and about a tenth of the peak rate observed during the 1997 unrest. The current deformation is well modeled by the inflation of a prolate spheroidal magma reservoir &sim;7 km beneath the resurgent dome, with a volume change of &sim;6 × 10 6  m 3 /yr from 2011.7 through the end of 2014. The current data cannot resolve a second source, which was required to model the 1997 episode. This source appears to be in the same region as previous inflation episodes, suggesting a persistent reservoir.

California↗

USGS2015

Crowdsourced earthquake early warning

Earthquake early warning (EEW) can reduce harm to people and infrastructure from earthquakes and tsunamis, but it has not been implemented in most high earthquake-risk regions because of prohibitive cost. Common consumer devices such as smartphones contain low-cost versions of the sensors used in EEW. Although less accurate than scientific-grade instruments, these sensors are globally ubiquitous. Through controlled tests of consumer devices, simulation of an M w (moment magnitude) 7 earthquake on California’s Hayward fault, and real data from the M w 9 Tohoku-oki earthquake, we demonstrate that EEW could be achieved via crowdsourcing.

Science Advances↗

USGS2014

Real-time inversions for finite fault slip models and rupture geometry based on high-rate GPS data

We present an inversion strategy capable of using real-time high-rate GPS data to simultaneously solve for a distributed slip model and fault geometry in real time as a rupture unfolds. We employ Bayesian inference to find the optimal fault geometry and the distribution of possible slip models for that geometry using a simple analytical solution. By adopting an analytical Bayesian approach, we can solve this complex inversion problem (including calculating the uncertainties on our results) in real time. Furthermore, since the joint inversion for distributed slip and fault geometry can be computed in real time, the time required to obtain a source model of the earthquake does not depend on the computational cost. Instead, the time required is controlled by the duration of the rupture and the time required for information to propagate from the source to the receivers. We apply our modeling approach, called Bayesian Evidence-based Fault Orientation and Real-time Earthquake Slip, to the 2011 Tohoku-oki earthquake, 2003 Tokachi-oki earthquake, and a simulated Hayward fault earthquake. In all three cases, the inversion recovers the magnitude, spatial distribution of slip, and fault geometry in real time. Since our inversion relies on static offsets estimated from real-time high-rate GPS data, we also present performance tests of various approaches to estimating quasi-static offsets in real time. We find that the raw high-rate time series are the best data to use for determining the moment magnitude of the event, but slightly smoothing the raw time series helps stabilize the inversion for fault geometry.

Journal of Geophysical Research↗

USGS2014

Response of Global Navigation Satellite System receivers to known shaking between 0.2 and 20 Hertz

Over the past decade, several technological advances have allowed Global Navigation Satellite Systems (GNSS) receivers to have the capability to record displacements at high frequencies, with sampling rates approaching 100 samples per second (sps). In addition, communication and computer hardware and software have allowed various institutions, including the U.S. Geological Survey (USGS), to retrieve, process, and display position changes recorded by a network of GNSS sites with small, less than 1-s delays between the time that the GNSS receiver records signals from a constellation of satellites and the time that the position is estimated (a method known as “real-time”). These improvements in hardware and software have allowed the USGS to process GNSS (or a subset of the GNSS, the Global Positioning System, GPS) data in real-time at 1 sps with the goal of determining displacements from earthquakes and volcanoes in real-time. However, the current set of GNSS equipment can record at rates of 100 sps, which allows the possibility of using this equipment to record earthquake displacements over the full range of frequencies that typically are recorded by acceleration and velocity transducers. The advantage of using GNSS to record earthquakes is that the displacement, rather than acceleration or velocity, is recorded, and for large earthquakes, the GNSS sensor stays on scale and will not distort the observations due to clipping of the signal at its highest amplitude. The direct observation of displacement is advantageous in estimating the size and spatial extent of the earthquake rupture. Otherwise, when using velocity or acceleration sensors, the displacements are determined by numerical integration of the observations, which can introduce significant uncertainty in the estimated displacements. However, GNSS technology can, at best, resolve displacements of a few millimeters, and for most earthquakes, their displacements are less than 1 mm. Consequently, to be useful, GNSS data are only relevant for the large earthquakes with magnitudes (M) exceeding M5.5 at best. With the capability to record GNSS data at high-rate, at sampling rates typical for seismological applications, experiments are needed to quantify the response of GNSS to shaking from earthquakes. There have been a few studies that examine the response of GNSS to strong shaking. One of the first was Elosegui and others (2006), where they simulated surface waves from a distant earthquake and mechanically applied the shaking to a GPS antenna. They processed the 1 sps observations and compared the estimated displacements with the simulated displacements. They determined that the GPS could accurately track the simulated surface wave whose primary frequency spans from 0.01 to 0.1 Hertz (Hz), which spanned the frequency band of the simulation. To test GNSS equipment due to shaking from a large earthquake in the near-field, Wang and others (2012) used a mechanical simulator or shake table with 6 degrees of freedom and studied two different inputs to the simulator—(1) the accelerometer record from one station that was located near the 2010 M8.8 Maule, Chile earthquake, and (2) a 2-Hz sinusoid. Wang and others (2012) analyzed the 2-Hz data with spectral analysis and determined that the displacements observed by the GPS included higher harmonics along with the 2-Hz signal. In addition, the background spectral amplitude was greater during periods of 2-Hz shaking than when at rest. With the simulated M 8.8 earthquake, Wang and others (2012) observed decreased signal to noise for L1 and L2 carrier frequencies of the GPS signal, at times corresponding to high acceleration and jerk (first derivative of acceleration). One of the principal limitations of these experiments was that the displacements of the shake table itself could not be measured independently. Although with the 2-Hz sinusoidal measurements, the input displacements were purely translational, Wang and others (2012) analysis of the data showed that the shake table also included rotational motions which affect horizontal inertial sensors like accelerometers and seismometers at first order. More recently, Ebinuma and Kato (2012) used a GPS simulator to electronically test several GNSS receivers and obtain the receiver characteristics at three frequencies: 1, 2, and 5 Hz. The results showed that the amplitude of 5-Hz displacements recorded by the GPS was, depending on the receiver model, between 30 and 125 percent more than the displacement input to the simulator. At low frequencies, the GPS displacement was nearly equal to the input displacement. In addition, Ebinuma and Kato (2012) examined how each receiver model amplified an earthquake displacement record in the 2–8 Hz band. The simulated earthquake was the 2008 moment magnitude (Mw) 6.8 Iwate-Miyagi earthquake where, for the simulated record, acceleration peaked at 1 G. The study discussed here builds on the tests by Ebinuma and Kato (2012), but rather than using electronic simulation, the tests are setup outdoors and closer to actual field installations of GNSS equipment. We used a one-dimensional shake table capable of 400 mm of displacement and high acceleration; the shake table also is constrained by a precision linear slider to have very low tilt that would affect inertial sensors. In addition, the stage position can be accurately monitored independent of the GNSS hardware and, importantly, provides a reference to compare with the estimated displacements from the GNSS data. Our tests spanned a greater frequency range from 0.2 to 20 Hz and we used equipment from three different manufacturers covering five different combinations of receivers and antennas. In addition, we have been able to simulate the frequency response of the GNSS equipment using a simple, causal filter. The quality of the filter was tested using additional test data where a step function in displacement was applied to the shake table. The observed displacements from the GNSS data show an overshoot in displacement at the time of the step or transition of the stage. That overshoot was accurately predicted using the filter design derived from our sinusoidal displacement tests. Similar to Wang and others (2012), we also examined the GPS displacement records using standard spectral techniques. However, we extended their work by evaluating several models of GNSS receivers using a variety of input frequencies. Because our shake table was limited on acceleration and displacement, we did not attempt to duplicate the high shaking associated with high magnitude earthquakes. However, because our shake table could measure the table displacement, we could directly compare the measured GPS displacements with the true displacements.

Open-File Report↗

USGS2014

Long Valley Caldera 2003 through 2014: Overview of low level unrest in the past decade

Long Valley Caldera is located in California along the eastern escarpment of the Sierra Nevada Range. The caldera formed about 760,000 years ago as the eruption of 600 km 3 of rhyolite magma (Bishop Tuff) resulted in collapse of the partially evacuated magma chamber. Resurgent doming in the central part of the caldera occurred shortly afterwards, and the most recent eruptions inside the caldera occurred about 50,000 years ago. The caldera remains thermally active, with many hot springs and fumaroles, and has had significant deformation and seismicity since at least 1978. Periods of intense unrest in the 1980s to early 2000s are well documented in the literature (Hill and others, 2002; Ewert and others, 2010). In this poster, we extend the timeline forward, documenting seismicity and deformation over the past decade.

California↗

USGS2014

Key recovery factors for the August 24, 2014, South Napa Earthquake

Through discussions between the Federal Emergency Management Agency (FEMA) and the U.S. Geological Survey (USGS) following the South Napa earthquake, it was determined that several key decision points would be faced by FEMA for which additional information should be sought and provided by USGS and its partners. This report addresses the four tasks that were agreed to. These tasks are (1) assessment of ongoing fault movement (called afterslip) especially in the Browns Valley residential neighborhood, (2) assessment of the shaking pattern in the downtown area of the City of Napa, (3) improvement of information on the fault hazards posed by the West Napa Fault System (record of past earthquakes and slip rate, for example), and (4) imagery acquisition and data processing to provide overall geospatial information support to FEMA.

California↗

USGS2013

Workshop on New Madrid geodesy and the challenges of understanding intraplate earthquakes

On March 4, 2011, 26 researchers gathered in Norwood, Massachusetts, for a workshop sponsored by the U.S. Geological Survey and FM Global to discuss geodesy in and around the New Madrid seismic zone (NMSZ) and its relation to earthquake hazard. The group addressed the challenge of reconciling current geodetic measurements, which show low present-day surface strain rates, with paleoseismic evidence of recent, relatively frequent, major earthquakes in the region. Several researchers were invited by the organizing committee to give overview presentations while all participants were encouraged to present their most recent ideas. The overview presentations appear in this report along with a set of recommendations.

Open-File Report↗

USGS2012

Improved stability of a deeply anchored geodetic monument for deformation monitoring

A test of two different monument designs used in geodetic networks shows that monuments installed to depths of 5 to 10 meters, and laterally braced, exhibit less environmentally caused displacement than those installed to 2 meters depth. At Parkfield, California, we have been monitoring the lengths of 17 baselines over the past decade with a 2 to 3 day interval between measurements using a two-color geodimeter with a nominal precision of 0.5 mm over 5 km long baselines. Significant variations are observed on many of these baselines which appear to be related to the seasonally occurring rainfall, with the larger variations approaching 10 mm over the past decade. To test whether we could improve upon the measurements on some of the more susceptible lines, at two sites we installed new monuments within about 30 meters of the original monuments. After 1.5 years of measurements it is evident that the new monuments significantly attenuate the seasonal displacements to less than 1 mm. The use of deeply anchored monuments should improve the ability of fault-scale geodetic monitoring arrays to detect small tectonic displacements.

California↗

USGS2012

Estimating rate uncertainty with maximum likelihood: differences between power-law and flicker–random-walk models

Recent studies have documented that global positioning system (GPS) time series of position estimates have temporal correlations which have been modeled as a combination of power-law and white noise processes. When estimating quantities such as a constant rate from GPS time series data, the estimated uncertainties on these quantities are more realistic when using a noise model that includes temporal correlations than simply assuming temporally uncorrelated noise. However, the choice of the specific representation of correlated noise can affect the estimate of uncertainty. For many GPS time series, the background noise can be represented by either: (1) a sum of flicker and random-walk noise or, (2) as a power-law noise model that represents an average of the flicker and random-walk noise. For instance, if the underlying noise model is a combination of flicker and random-walk noise, then incorrectly choosing the power-law model could underestimate the rate uncertainty by a factor of two. Distinguishing between the two alternate noise models is difficult since the flicker component can dominate the assessment of the noise properties because it is spread over a significant portion of the measurable frequency band. But, although not necessarily detectable, the random-walk component can be a major constituent of the estimated rate uncertainty. None the less, it is possible to determine the upper bound on the random-walk noise.

Journal of Geodesy↗

USGS2011

Earthquake prediction lessons from Parkfield Experiment

In 1985, the National Earthquake Prediction Evaluation Council (NEPEC), and later its California counterpart (CEPEC), approved an earthquake prediction for the first time. This was the Bakun and Lindh [1985] forecast, with 95% confidence, that an M = 5.5 to 6 earthquake would rupture along the San Andreas fault near the town of Parkfield before 1993. This forecast was an important factor in selecting the Parkfield area for a major geophysical and geological monitoring study. The experiments installed there are aimed at producing a better understanding of the earthquake generation process and earthquake effects, gaining experience with real-time monitoring of a variety of geophysical data, and, if possible, making a short-term prediction of the future earthquake. Twenty-eight different observational networks now exist in the Parkfield area. The forecast also generated interest that led researchers to reexamine past Parkfield earthquakes with current knowledge and to develop new models of the earthquake cycle.

California↗

USGS2010

The effect of error in theoretical Earth tide on calibration of borehole strainmeters

Since the installation of borehole strainmeters into the ground locally distorts the strain in the rock, these strainmeters require calibration from a known source which typically is the Earth tide. Consequently, the accuracy of the observed strain changes from borehole strainmeters depends upon the calibration derived from modeling the Earth tide. Previous work from the mid-1970s, which is replicated here, demonstrate that the theoretical tide can differ by 30% from the tide observed at surface-mounted, long-baseline strainmeters. In spite of possible inaccurate tidal models, many of the 74 borehole strainmeters installed since 2005 can be “calibrated”. However, inaccurate tidal models affect the amplitude and phase of observed transient strain changes which needs to be considered along with the precision of the data from the inherent drift of these borehole instruments. In particular, the error from inaccurate tidal model dominates the error budget in the observation of impulsive, sub-daily, strain-transients.

Geophysical Research Letters↗

USGS2010

Computer algorithm for analyzing and processing borehole strainmeter data

The newly installed Plate Boundary Observatory (PBO) strainmeters record signals from tectonic activity, Earth tides, and atmospheric pressure. Important information about tectonic processes may occur at amplitudes at and below tidal strains and pressure loading. If incorrect assumptions are made regarding the background noise in the strain data, then the estimates of tectonic signal amplitudes may be incorrect. Furthermore, the use of simplifying assumptions that data are uncorrelated can lead to incorrect results and pressure loading and tides may not be completely removed from the raw data. Instead, any algorithm used to process strainmeter data must incorporate the strong temporal correlations that are inherent with these data. The technique described here uses least squares but employs data covariance that describes the temporal correlation of strainmeter data. There are several advantages to this method since many parameters are estimated simultaneously. These parameters include: (1) functional terms that describe the underlying error model, (2) the tidal terms, (3) the pressure loading term(s), (4) amplitudes of offsets, either those from earthquakes or from the instrument, (5) rate and changes in rate, and (6) the amplitudes and time constants of either logarithmic or exponential curves that can characterize postseismic deformation or diffusion of fluids near the strainmeter. With the proper error model, realistic estimates of the standard errors of the various parameters are obtained; this is especially critical in determining the statistical significance of a suspected, tectonic strain signal. The program also provides a method of tracking the various adjustments required to process strainmeter data. In addition, the program provides several plots to assist with identifying either tectonic signals or other signals that may need to be removed before any geophysical signal can be identified.

Computers & Geosciences↗

USGS2008

Noise in GPS displacement measurements from southern California and southern Nevada

Time series of position changes estimated from data from 236 continuously recording GPS receivers operating in Southern California and Southern Nevada are evaluated for noise models that characterize their temporal correlations. The lengths of the time series range between 3.5 and 10 years. After adjusting these data for postseismic deformation, offsets, and annual periodicities, I find that about one-half of the time series have temporal correlations that are categorized as either flicker or random-walk noise. The remaining time series can be best categorized as either a combination of flicker and random-walk; power law noise; first-order Gauss-Markov plus random-walk noise; or power law plus broadband, seasonal noise. A variety of geodetic monuments are used in Southern California and Nevada, including deeply braced designs, cement piers, pins drilled in outcrop, and buildings. When I evaluate the noise for each time series in terms of an estimate of the standard error in velocity, I find that the sites with the smallest errors are those located in Nevada using deeply braced monuments. Sites that are installed within regions of active pumping, both for groundwater and oil, had the largest standard errors in velocity. Comparison of monument stability, as measured by standard error in rate, with average, annual rainfall nearby indicates a marginally significant correlation. In addition, even though regional filtering removed much of the common-mode signals in these time series, there still remains a common-mode seasonal signal which can and should be removed.

California, Nevada↗