Export 66 results:
Sort by: Author Title Type [ Year  (Asc)]
and Aster, R.C., S*JS.  1993.  Comprehensive Characterization of Waveform Similarity in Large Microearthquake Data Sets. Bull. Seis. Soc. Am. Note. 83:1307-1314.
and Hartse, H., AFS* VRMJF.  1994.  Small-scale stress heterogeneity in the Anza seismic gap, southern California. J. Geophys. Res.. 99:6801-6818.
Haase, JS, Shearer PM, Aster RC.  1995.  Constraints on temporal variations in velocity near Anza, California from analysis of similar event pairs. Bulletin of the Seismological Society of America. 85:194-206. AbstractWebsite

Similar earthquake pairs recorded by the Anza Seismic Network in southern California are used as repeatable sources to place an upper limit on temporal changes in seismic velocity which occurred in the vicinity of the Anza seismic gap in the last 9 yr. Relative arrival times for each pair of events are found using a cross-correlation method and relative locations are calculated to verify that the pairs have nearly identical hypocenters. The time separation between events in these pairs varies from less than a day to almost 7 yr. The longterm changes in seismic travel times, as measured from the pairs with the longest time separation, are not significantly greater than the noise level estimated from the short-time-separation event pairs. Almost all P-wave paths show less than 0.06% (0.007 sec) change in travel time and all S-wave paths have less than 0.03% (0.004 sec) change. Sensitivity tests place an upper bound on traveltime changes that could be compensated by hypocenter mislocation at 0.2%. There is no evidence that localized stress accumulation causes measurable changes in seismic velocity in the Anza region.

Haase, JS, Hauksson E, Kanamori H, Mori J.  1995.  Global Positioning System re-survey of Southern California Seismic Network Stations. Bulletin of the Seismological Society of America. 85:361-374. AbstractWebsite

Systematic errors in travel-time data from local earthquakes can sometimes be traced to inaccuracies in the published seismic station coordinates. This prompted a resurvey of the stations of the Caltech/USGS Southern California Seismic Network (SCSN) using the Global Positioning System (GPS). We surveyed 241 stations of the SCSN using Trimble and Ashtech dual-frequency GPS receivers and calculated positions accurate to 3 m using differential positioning from carrier phase measurements, Twelve percent of the stations that were surveyed were found to be mislocated by more than 500 m. Stations of the TERRAscope and USC networks were also surveyed, as well as a network of portable seismic stations deployed shortly after the 1992 Joshua Tree and Landers earthquakes. The new coordinates and the offsets from the old coordinates are given below. The new coordinates are being used in SCSN locations as of 1 January 1994.

Haase, JS, Hauksson E, Vernon F, Edelman A.  1996.  Modeling of ground motion from a 1994 Northridge aftershock using a tomographic velocity model of the Los Angeles Basin. Bulletin of the Seismological Society of America. 86:S156-S167. AbstractWebsite

The 1994 Northridge mainshock and its aftershocks show a complex pattern of peak accelerations at stations located in the Los Angeles Basin. The waveforms contain multiples of body-wave phases and extensive surface waves at frequencies mostly below 1 Hz. In particular, for stations at distances greater than 18 km, secondary arrivals show larger accelerations than the direct S-wave arrivals. The mainshock waveforms are further complicated by irregularities of the source rupture. We use 2D finite difference to evaluate the effect of lateral variations in seismic velocity on the amplitude of shear-wave energy and to distinguish the effects of source and propagation path. We model waveforms from one aftershock recorded at nine stations deployed along a 60-km-long profile extending into the Los Angeles Basin. We use a two-dimensional slice through the 3D tomography model of the Los Angeles Basin in the 2D finite-difference calculations. These synthetic waveforms fit the aftershock waveforms significantly better than corresponding waveforms determined from simple 1D velocity models. With the addition of a thin low-velocity surface layer above the tomography model, the finite-difference synthetics reproduce most of the important features of the recorded data, in particular, the large-amplitude arrivals 7 to 10 sec following the direct S arrival. These arrivals correspond to the SS arrival, which is sharply refracted at the basin edge, and the S-wave with multiple legs trapped by the dipping near surface gradient. For large earthquakes located either inside or outside the basin, these phases can be the cause of the largest and hence potentially most hazardous shaking in the Los Angeles Basin.

Hauksson, E, Haase JS.  1997.  Three-dimensional V-P and V-P/V-S velocity models of the Los Angeles basin and central Transverse Ranges, California. Journal of Geophysical Research-Solid Earth. 102:5423-5453. AbstractWebsite

We use P and S arrival times from 5225 earthquakes and 53 explosions, recorded by the Southern California Seismographic Network, to invert for the three-dimensional P- velocity (V-p) and the P and S-velocity ratio (V-p/V-S) in the central Transverse Ranges and the Los Angeles basin. To model long-wavelength features of the velocity structure, we invert for the models by interpolating models determined successively from a sparse, medium, to a dense grid, with 40, 20, and 10 km spacing of horizontal grid nodes. Layers of grid nodes are placed at depths of 1, 4, 6, 8, 12, 16, and 20 km. The data variance decreased about 80% in the gradational inversion. Ample data from the 1994 Northridge and other earthquake sequences, the rich background seismicity, and the dense station distribution along with controlled sources made the model well resolved, except along the edges, to the southwest in the offshore region, and at depths greater than 20 km. The Vp model images the shape of the Los Angeles and east Ventura basins down to depths of 8 and 12 km. Three low-velocity areas at 1 km depth in the Los Angeles basin that coincide with recent sediment depositional areas are also imaged. The north edge of the Peninsular Ranges, the Santa Monica, and the San Gabriel Mountains, form discontinuous high-velocity ridges extending to depths df 20 km. The high V-P/V-S ratios in the near surface are consistent with high pore fluid pressures in the basin sediments. At depth beneath the east Ventura basin the high V-p/V-S and high. Vp suggest the presence of Ophiolitic assemblages or mid-Miocene volcanics. In contrast, a body with normal Vp and low V-P/V-S that is identified in the upper crust beneath Santa Monica Bay may be a fragment of quartz-rich continental crust. The Vp model of both the Ventura and the Los Angeles basins shows features that suggest deformation of the hanging wall or basin closure. At seismogenic depths of 16 km the hypocenters of moderate-sized and large earthquakes are located within or adjacent to high- Vp-velocity bodies. In most cases these high-velocity bodies form the upper block, consistent with shortening of the lower crust as described in thick-skinned tectonic interpretations.

Nicolas, J, Exertier P, Bonnefond P, Pierron F, Boudon Y, Mangin JF, Barlier F, Kasser M, Haase J.  1999.  Stability control of range biases on the French laser ranging stations. Laser Radar Ranging and Atmospheric Lidar Techniques II. 3865( Schreiber U, Werner C, Eds.).:27-32., Florence, Italy AbstractWebsite

Space geodetic techniques of artificial; satellites tracking have been greatly improved since this last decade. Particularly, the successful development of radio tracking systems like GPS, DORIS, and PRARE has to be emphasized, concerning precise positioning, orbit determination, and gravity field determination. All these techniques are able to operate under all weather conditions and are of very easy use. In fact, all the complexity of systems is to be found in space on board the satellites. To some extent, for laser satellite tracking, the situation is the opposite. It is weather dependent and comparatively complex and costly at the ground level. However, this technique is the most straightforward in concept for accurate measuring of the Earth-satellite distance. But, it is true, provided that instrumental biases can be well understood and precisely estimated. As a result, the role of laser tracking has to be reassessed and the control of range biases is nowadays of the utmost importance. Therefore, the international organization for laser cooperation (ILRS) has been reconsidered and the purpose of this paper is to present the effort made in this field at the Grasse, France observatory.

Nicolas, J, Pierron F, Kasser M, Exertier P, Bonnefond P, Barlier F, Haase J.  2000.  French Transportable Laser Ranging Station: Scientific objectives, technical features, and performance. Applied Optics: Lasers, Photonics, and Environmental Optics. 39:402-410.
Ge, MR, Calais E, Haase J.  2000.  Reducing satellite orbit error effects in near real-time GPS zenith tropospheric delay estimation for meteorology. Geophysical Research Letters. 27:1915-1918. AbstractWebsite

We investigate the influence of using IGS predicted orbits for near real-time zenith tropospheric delay determination from GPS and implement a new processing strategy that allows the use of predicted orbits with minimal degradation of the ZTD estimates. Our strategy is based on the estimation of the three Keplerian parameters that represent the main error sources in predicted orbits (semi-major axis, inclination, and argument of perigee), and their associated variance directly from the real-time GPS data set. A comparison with the use of IGS final orbits shows a negligible bias and an rms less than 6 mm in the ZTD estimates at all the stations. The improvement compared to a strategy based only on the quality index provided with the predicted orbits is 20%. The level of accuracy and turn-around time shown here meet the current requirements for operational meteorology.

Walpersdorf, A, Calais E, Haase J, Eymard L, Desbois M, Vedel H.  2001.  Atmospheric gradients estimated by GPS compared to a high resolution numerical weather prediction (NWP) model. Physics and Chemistry of the Earth Part a-Solid Earth and Geodesy. 26:147-152. AbstractWebsite

The estimation of horizontal atmospheric gradients, in addition to zenith delays, is a strategy now commonly used in geodetic Global Positioning System (GPS) positioning. This strategy compensates for inhomogeneities in the atmospheric water vapor distribution above GPS sites, and has shown to increase the positioning precision, e.g. in geodynamic networks. While the zenith delay has been successfully related to the pressure at the GPS site and the water vapor above the site, the relation of the GPS estimated horizontal gradients to atmospheric quantities remains unclear. To get a better understanding of the nature of these gradients inferred by GPS, this study compares GPS tropospheric observations from the MAGIC permanent network on the NW side of the Mediterranean Sea with simulations based on the high resolution NWP model ALADIN (Meteo France). To verify the model performance, we use meteorological measurements from the FETCH ship campaign in the Gulf of Lyon in March-April 1998. For this study, five stations of the MAGIC network close to the Golf of Lyon have been selected. Results from two periods, representing two different weather situations occuring within the FETCH observation campaign, are presented. (C) 2001 Elsevier Science Ltd. All rights reserved.

Ge, M, Calais E, Haase J.  2001.  Automatic orbit quality control for near real-time GPS zenith tropospheric delay estimation. Physics and Chemistry of the Earth Part a-Solid Earth and Geodesy. 26:177-181. AbstractWebsite

We implement an automatic orbit quality control procedure in order to reduce the effect of satellite orbit error for near real-time estimating of zenith total delay (ZTD). We estimate the three Keplerian parameters of the GPS orbits that represent the main error sources (semimajor axis, inclination, and argument of perigee). We start with an a priori constraint corresponding to the typical orbit accuracy for each parameter and adjust it iteratively according to its estimated value and variance. This data analysis procedure shows an improvement of 20% in ZTD rms compared to a strategy based only on the quality index provided with the IGS predicted orbits. A real-time test with IGS ultra-rapid orbits also shows significant improvement compared to fixing IGS ultra-rapid orbits. This strategy allows for a totally automated estimation of orbital parameters directly from the real-time GPS data, without altering the accuracy of the ZTD estimates. (C) 2001 Elsevier Science Ltd. All rights reserved.

Haase, J, Calais E, Talaya J, Rius A, Vespe F, Santangelo R, Huang XY, Davila JM, Ge M, Cucurull L, Flores A, Sciarretta C, Pacione R, Boccolari M, Pugnaghi S, Vedel H, Mogensen K, Yang X, Garate J.  2001.  The contributions of the MAGIC project to the COST 716 objectives of assessing the operational potential of ground-based GPS meteorology on an international scale. Physics and Chemistry of the Earth Part a-Solid Earth and Geodesy. 26:433-437. AbstractWebsite

MAGIC (Meteorological Applications of GPS Integrated Column Water Vapor Measurements in the Western Mediterranean) is a 3 year project financed in part by the European Commission for research on deriving and validating robust GPS integrated water vapor (IWV) and zenith tropospheric delay (ZTD) data sets and developing methods to assimilate the data into numerical weather prediction models (NWP) and test their impact. It was conceived independently from the COST 716 action, which seeks to coordinate research in the domain at an international scale, but addresses some of the same objectives. This has led to a productive cooperation between the two initiatives and their participants, and motivated the decision of MAGIC participants to provide research results as part of the COST demonstration system. Currently a database of 1.5 years of ZTD data are available on the MAGIC web site which has been validated through comparisons with radiosondes which gives differences with a standard deviation of 10 mm ZTD or the equivalent error in IWV of 1.6 kg/m(2). NWP assimilation tests will be carried out in the final year of the project. (C) 2001 Elsevier Science Ltd. All rights reserved.

Haase, JS, Vedel H, Ge M, Calais E.  2001.  GPS zenith tropospheric delay (ZTD) variability in the Mediterranean. Physics and Chemistry of the Earth Part a-Solid Earth and Geodesy. 26:439-443. AbstractWebsite

In the context of the MAGIC project (Meteorological Applications of GPS Integrated Column Water Vapor Measurements in the Western Mediterranean), zenith tropospheric delays (ZTD) are computed from Global Positioning System (GPS) signals at permanent sites in the western Mediterranean area continuously since November 1998. These time series are compared to the equivalent values estimated by integrating the atmospheric column derived from radiosonde profiles and the HIRLAM numerical weather prediction model in order to determine the error statistics of the data. From a year and a half of data, we show that the difference between radiosonde and GPS ZTD has a standard deviation of 12 mm and a bias of less than 5 mm for most stations. The biases between the GPS ZTD and HIRLAM estimates are smaller, but the standard deviation is greater, usually on the order of 17 mm. The standard deviation of the residuals from both comparisons depends greatly on the humidity which produces an annual signal because of the much higher humidity variability in the summer months. (C) 2001 Elsevier Science Ltd. All rights reserved.

Healy, SB, Haase J, Lesne O.  2002.  Abel transform inversion of radio occultation measurements made with a receiver inside the Earth's atmosphere. Annales Geophysicae. 20:1253-1256. AbstractWebsite

Radio occultation measurements made with a receiver inside the Earth's atmosphere can be inverted, assuming local spherical symmetry, with an Abel transform to provide an estimate of the atmospheric refractive index profile. The measurement geometry is closely related to problems encountered when inverting seismic time-travel data and solar occultation measurements, where the Abel solution is well known. The method requires measuring both rays that originate from above and below the local horizon of the receiver. The Abel transform operates on a profile of "partial bending angles" found by subtracting the positive elevation measurement from the negative elevation value with the same impact parameter. In principle, the refractive index profile can be derived from measurements with a single frequency GPS receiver because the ionospheric bending is removed when the partial bending angle is evaluated.

Ge, MR, Calais E, Haase J.  2002.  Sensitivity of zenith total delay accuracy to GPS orbit errors and implications for near-real-time GPS meteorology. Journal of Geophysical Research-Atmospheres. 107   431510.1029/2001jd001095   AbstractWebsite

[1] Global Positioning System (GPS) measurements have been demonstrated to provide precipitable water vapor (PWV) estimates with a level of accuracy that is comparable to that of radiosondes and microwave radiometers. GPS measurements therefore have the potential to become a significant source of data for operational weather forecasting, provided that PWV (or the intermediate zenith total delay (ZTD)) can be made available in near real-time with a minimum accuracy degradation. Despite the recent decrease in the latency and increase in accuracy provided by the International GPS Service (IGS) ultrarapid predicted GPS orbit products, we show that the accuracy of these orbits continues to be a limiting factor for the accuracy of near real-time GPS-derived atmospheric estimates. In this work, a coefficient matrix is derived from the normal equations of the least squares adjustment model for the GPS observables that maps the orbital parameter errors into ZTD errors. This is used to analyze the sensitivity of GPS derived tropospheric errors to an extensive set of parameters, including their time dependence, in a computationally efficient manner. We show that ZTD errors are dominated by biases in the orbital semimajor axis, with minor contributions from the inclination and argument of perigee, and that this error increases significantly after the fourth to fifth hour of the prediction window. We implemented a GPS data processing strategy based on an iterative estimation of the three most critical orbital parameters (semimajor axis, inclination and argument of perigee) together with the ZTD parameters. We tested this strategy in a 3500 3500 km network of 15 GPS sites in western Europe providing hourly data files. We show that the standard deviation improvement compared to a strategy based only on the orbit quality index provided with the predicted orbit products is on the order of 20%. The analysis of one month of data in near-real-time shows a bias lower than 1 mm ZTD and a standard deviation lower than 6 mm ZTD compared to using the most precise IGS final orbits. We also show that this strategy is robust and capable of dealing with very large orbit errors appropriately. We demonstrate that the same quality is achievable with a 1500 1500 km network which has positive implications for decentralized processing strategies. The near real-time processing methodology described here meets the current timeliness requirements of operational meteorology (30 mn to 2 hours, depending on the application), while ensuring a level of accuracy similar to that provided in postprocessed mode with precise final IGS orbits (1 mm ZTD bias, 6 mm ZTD RMS). The method we propose can also be considered as an "on-the-fly'' orbit quality control for near real-time GPS applications.

Lesne, O, Haase J, Kirchengast G, Ramsauer J, Poetzi W.  2002.  Sensitivity Analysis of GNSS radio occultation for airborne sounding of the troposphere. Phys. and Chem. of the Earth. 27:291-299. AbstractWebsite

The usual geometry for radio occultation sounding using GNSS (Global Navigation Satellite System) signals has the receiver placed on a LEO (Low Earth Orbit) satellite. We investigate a new geometric approach, assuming an airborne rather than a spaceborne receiver. Information on the refractivity structure and hence the pressure, temperature, humidity can be retrieved from accurate airborne measurements of amplitude and phase delay of the signals occulted by the troposphere. We present some advantages and disadvantages for the concept of making measurements from commercial aircraft equipped with proper GNSS receivers and antennae compared to the spaceborne case. We simulated realistic airborne occultation observations and assessed the characteristics of their geometry and sampling. We also compared the dynamic range of the signal with the magnitude of error sources that affect the measurements. Findings include that an airborne system has the potential to provide many more profiles below 10 km height than a single LEO or constellations of up to 25 satellites over the North Atlantic (though with inferior global coverage), and that the SNR (signal to noise ratio) should be better below 5 km than in the LEO case. Though the receiver velocity error is larger than for the LEO system, it is still small enough relative to the signal level to retrieve useful information. Because of the large horizontal drift of the tangent point of up to 450 km, the assumption of spherical symmetry in the existence of significant 3D variations in structure is expected to be a major error source, in addition to the airplane velocity uncertainty.

Vey, S, Calais E, Llubes M, Florsch N, Woppelmann G, Hinderer J, Amalviet M, Lalancette MF, Simon B, Duquenne F, Haase JS.  2002.  GPS measurements of ocean loading and its impact on zenith tropospheric delay estimates: a case study in Brittany, France. Journal of Geodesy. 76:419-427.   10.1007/s00190-002-0272-7   AbstractWebsite

The results from a global positioning system (GPS) experiment carried out in Brittany, France, in October 1999, aimed at measuring crustal displacements caused by ocean loading and quantifying their effects on GPS-derived tropospheric delay estimates, are presented. The loading effect in the vertical and horizontal position time series is identified, however with significant disagreement in amplitude compared to ocean loading model predictions. It is shown that these amplitude misfits result from spatial tropospheric heterogeneities not accounted for in the data processing. The effect of ocean loading on GPS-derived zenith total delay (ZTD) estimates is investigated and a scaling factor of 4.4 between ZTD and station height for a 10degrees elevation cutoff angle is found (i.e. a 4.4-cm station height error would map into a 1-cm ZTD error). Consequently. unmodeled ocean loading effects map into significant errors in ZTD estimates and ocean loading modeling must be properly implemented when estimating ZTD parameters from GRS data for meteorological applications. Ocean loading effects Must be known with an accuracy of better than 3 cm in order to meet the accuracy requirements of meteorological and climatological applications of GPS-derived precipitable water vapor.

Calais, E, Haase JS, Minster JB.  2003.  Detection of ionospheric perturbations using a dense GPS array in Southern California. Geophysical Research Letters. 30 AbstractWebsite

[1] We present observations of high-frequency ionospheric perturbations detected using Global Positioning System (GPS) data from the Southern California Integrated GPS Network (SCIGN), a densely spaced GPS array of about 250 stations centered on the Los Angeles area. We show examples of perturbations with 3 - 10 minute periods that may result from coupling between the ionosphere and neutral gravity waves generated in the lower atmosphere. Although the signal-to-noise ratio of the perturbations is relatively small, we show how multi-station array processing techniques can take advantage of the high spatial density of the measurements and the coherence of the signal over a region the size of the SCIGN to considerably improve the detection capability.

Haase, JS, Ge MR, Vedel H, Calais E.  2003.  Accuracy and variability of GPS tropospheric delay measurements of water vapor in the western Mediterranean. Journal of Applied Meteorology. 42:1547-1568. AbstractWebsite

As a preliminary step for assessing the impact of global positioning system (GPS) refractive delay data in numerical weather prediction (NWP) models, the GPS zenith tropospheric delays (ZTDs) are analyzed from 51 permanent GPS sites in the western Mediterranean. The objectives are to estimate the error statistics necessary for future assimilation of GPS ZTD data in numerical models and to investigate the variability of the data in this area. The time series, which were derived continuously from November 1998 to June 2001, are compared with independent equivalent values derived from radiosonde profiles and the High-Resolution Limited-Area Model (HIRLAM) NWP model. Based on over two years of data, the difference between radiosonde and GPS ZTD has a standard deviation of 12 mm of delay and a bias of 7 mm of delay. Some sites have biases as high as 14 mm of delay. The bimodal distribution of residuals, with a higher bias for daytime launches, indicates these biases may be due to radiosonde day-night measurement biases. The biases between the GPS ZTD and HIRLAM estimates are smaller, but the 18-mm ZTD standard deviation is significantly greater. The standard deviation of the residuals depends strongly on the amount of humidity, which produces an annual signal because of the much higher variability of water vapor in the summer months. The better agreement with radiosonde data than HIRLAM estimates indicates that the NWP models will benefit from the additional information provided by GPS. The long-term differences between the observational data sources require further study before GPS-derived data become useful for climate studies.

Vedel, H, Huang XY, Haase JS, Ge M, Calais E.  2004.  Impact of GPS Zenith Tropospheric Delay data on precipitation forecasts in Mediterranean France and Spain. Geophysical Research Letters. 31   L0210210.1029/2003gl017715   AbstractWebsite

[1] Forecasting precipitation in the western Mediterranean is difficult because of the interactions among dynamical forcing, orographic lifting and moisture advection from the warm Mediterranean Sea. Torrential rainfall events are not uncommon, especially during the autumn. This type of event motivated an effort to improve precipitation forecasting by incorporating additional information on the initial state of the humidity field from Global Positioning System measurements of refractive delay. In this study we process data from a network of sites in Western Europe and assimilate the data over a two week period into the HIRLAM numerical weather prediction model. The overall impact for the two week period is neutral, however, for a severe rain event taking place during that period, the forecasts show improved skill when including GPS data. The work implies that the GPS data have good potential for influencing numerical models in rapidly developing, high moisture flux situations.

Miyazaki, S, Larson KM, Choi KH, Hikima K, Koketsu K, Bodin P, Haase J, Emore G, Yamagiwa A.  2004.  Modeling the rupture process of the 2003 September 25 Tokachi-Oki (Hokkaido) earthquake using 1-Hz GPS data. Geophysical Research Letters. 31   L2160310.1029/2004gl021457   AbstractWebsite

[ 1] High-rate GPS has the potential to recover both dynamic and static displacements accurately. We analyze 1-Hz GPS data recorded during the 2003 Tokachi-Oki earthquake. The 1-Hz GPS displacement waveforms show good agreement with integrated accelerometer records except for low frequency noise that are inherently present in integrated seismic records. The GPS waveforms were inverted to model the spatio-temporal evolution of the fault slip during the rupture. The slip is found to propagate downdip in the subduction zone with largest moment release -50 km northwest of the hypocenter. The region of largest slip agrees in general with traditional seismic studies, indicating that 1-Hz GPS can be used for finite fault studies. The 1-Hz GPS slip model shows clearer contrast with afterslip distributions than those inferred from strong motion data, possibly because 1-Hz GPS is more sensitive to cumulative slip distribution.

Emore, GL, Haase JS, Choi K, Larson KA, Yamagiwa A.  2007.  Recovering seismic displacements through combined use of 1-Hz GPS and strong-motion accelerometers. Bulletin of the Seismological Society of America. 97:357-378.   10.1785/0120060153   AbstractWebsite

Retrieving displacement from seismic acceleration records is often difficult because unknown small baseline offsets in the acceleration time series will contaminate the doubly integrated record with large quadratic errors. One-hertz Global Positioning System (GPS) position estimates and collocated seismic data are available from the 2003 M-W 8 Tokachi-Oki (Hokkaido) earthquake. After a process of correcting for possible misorientation of the seismic sensors, an inversion method is used to simultaneously solve for ground displacement with both data sets as input constraints. This inversion method takes into account the presence of unknown offsets in the acceleration record, and the relatively large uncertainties in the estimated 1-Hz GPS positions. In this study, 117 channels of seismic data were analyzed. Only 5% of the time does the static displacement retrieved from traditional baseline correction processing without GPS information agree with the absolute displacement measured with 1-Hz GPS to within the errors of the GPS data. In solving simultaneously for constrained displacements that agree with both the seismic and GPS data sets, an optimal solution was found that included only one- or two-step functions in the acceleration records. Potential explanations for the offsets are analyzed in terms of tilt of the sensor or electronic noise. For nine stations, clear misorientations of the seismic sensors of more than 20 deg from the reported orientation were found. For this size event, the 30-sec sampled GPS solutions were also a sufficient constraint for establishing the offset errors and recovering reliable displacements. The results significantly extend the frequency band over which accelerometer data are reliable for source inversion studies.

Garrison, JL, Lee SCG, Haase JS, Calais E.  2007.  A method for detecting ionospheric disturbances and estimating their propagation speed and direction using a large GPS network. Radio Science. 42   Rs601110.1029/2007rs003657   AbstractWebsite

A technique is developed for detecting short period (3-10 min) ionospheric disturbances and estimating their propagation speed and direction using data from a large GPS network (a hundred or more receivers). This method increases the signal-to-noise ratio of small signals and could be applied, autonomously, to process a large set of data for the study of the potential signal sources and statistical distributions of these disturbances. The integral electron content (IEC) for every satellite-station pair in the network is extracted from dual frequency phase data. These IEC time series are then band-pass-filtered and cross-correlated with each other. The resulting correlation power is an indication of the presence of a common disturbance recorded at the two stations, and the delay to the maximum correlation is a measurement of the propagation time between the ionospheric pierce points of the respective stations. A threshold on correlation power is used to select a subset of these delay measurements. The velocity of the detected perturbation is then estimated by fitting a two-dimensional plane wave model to this subset of measurements. A technique is developed to remove the effects of time-varying satellite motion and to reconstruct the waveform that would have been observed at a fixed point within the ionosphere. Consistency of the resulting velocity estimates is checked using a stacking-alignment method and a time-distance mapping that accounts for the motion of the GPS satellites. The sensitivity of the velocity estimate to both the assumed height of a thin-layer ionosphere and the detection threshold value is studied. A simulation is used to demonstrate the IEC waveform distortion due to satellite motion, and an example is shown in which this distortion is able to shift the dominant frequencies of an actual disturbance outside of the passband of the filter, thereby preventing detection. Four weeks of data, in different seasons, collected using the Southern California Integrated GPS Network (SCIGN), were processed. Over the total of 28 days, 127 significant disturbances were detected, most with horizontal propagation speeds between 50-1000 m/s and westward directions of propagation. A few cases with exceptionally high speed (>2000 m/s) were observed. It is hypothesized that these are manifestations of disturbances that occur simultaneously throughout the ionosphere, rather than traveling waves. The rate of occurrence of disturbances in the 3-10 min band was found to be larger than expected. Observational biases of this method are discussed.

Dautermann, T, Calais E, Haase J, Garrison J.  2007.  Investigation of ionospheric electron content variations before earthquakes in southern California, 2003-2004. Journal of Geophysical Research-Solid Earth. 112   B0210610.1029/2006jb004447   AbstractWebsite

[1] It has been proposed that earthquakes are preceded by electromagnetic signals detectable from ground- and space-based measurements. Ionospheric anomalies, such as variations in the electron density a few days before earthquakes, are one of the precursory signals proposed. Since Global Positioning System (GPS) data can be used to measure the ionospheric total electron content (TEC), the technique has received attention as a potential tool to detect ionospheric perturbations related to earthquakes. Here, we analyze 2 years ( 2003 - 2004) of data from the Southern California Integrated GPS Network (SCIGN), a dense network of 265 continuous GPS stations centered on the Los Angeles basin, for possible precursors. This time period encompasses the December 2003, M6.6, San Simeon and September 2004, M6.0, Parkfield earthquakes. We produce TEC time series at all SCIGN sites and apply three different statistical tests to detect anomalous TEC signals preceding earthquakes. We find anomalous TEC signals but no statistically significant correlation, in time or in space, between these TEC anomalies and the occurrence of earthquakes in southern California for the 2003 - 2004 period. This result does not disprove the possibility of precursory phenomena but show the signal-to-noise ratio of a hypothetical TEC precursor signature is too low to be detected by the analysis techniques employed here. Precursors may still be revealed for future large earthquakes in well instrumented areas such as California and Japan, if the tests can be developed into techniques that can better separate external influences from the actual TEC signal.