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2017
Goldberg, DE, Bock Y.  2017.  Self-contained local broadband seismogeodetic early warning system: Detection and location. Journal of Geophysical Research-Solid Earth. 122:3197-3220.   10.1002/2016jb013766   AbstractWebsite

Earthquake and local tsunami early warning is critical to mitigating adverse impacts of large-magnitude earthquakes. An optimal system must rely on near-source data to maximize warning time. To this end, we have developed a self-contained seismogeodetic early warning system employing an optimal combination of high-frequency information from strong-motion accelerometers and low-frequency information from collocated Global Navigation Satellite Systems (GNSS) instruments to estimate real-time displacements and velocities. Like GNSS, and unlike broadband seismometers, seismogeodetic stations record the full waveform, including static offset, without clipping in the near-field or saturating for large magnitude earthquakes. However, GNSS alone cannot provide a self-contained system and requires an external seismic trigger. Seismogeodetic stations detect Pwave arrivals with the same sensitivity as strong-motion accelerometers and thus provide a stand-alone system. We demonstrate the utility of near-source seismogeodesy for event detection and location with analysis of the 2010 M(w)7.2 El Mayor-Cucapah, Baja, California and 2014 M(w)6.0 Napa, California strike-slip events, and the 2014 M(w)8.2 Iquique, Chile subduction zone earthquake using observatory-grade accelerometers and GPS data. We present lessons from the 2014 M(w)4.0 Piedmont, California and 2016 M(w)5.2 Borrego Springs, California earthquakes, recorded by our seismogeodetic system with Micro-Electro Mechanical System (MEMS) accelerometers and GPS data and reanalyzed retrospectively. We conclude that our self-contained seismogeodetic system is suitable for early warning for earthquakes of significance (>M5) using either observatory-grade or MEMS accelerometers. Finally, we discuss the effect of network design on hypocenter location and suggest the deployment of additional seismogeodetic stations for the western U.S.

2016
Saunders, JK, Goldberg DE, Haase JS, Bock Y, Offield DG, Melgar D, Restrepo J, Fleischman RB, Nema A, Geng JH, Walls C, Mann D, Mattioli GS.  2016.  Seismogeodesy using GPS and low-cost MEMS accelerometers: Perspectives for earthquake early warning and rapid response. Bulletin of the Seismological Society of America. 106:2469-2489.   10.1785/0120160062   AbstractWebsite

The seismogeodetic method computes accurate displacement and velocity waveforms by optimally extracting high-frequency information from strong-motion accelerometers and low-frequency information from collocated Global Positioning System (GPS) instruments. These broadband observations retain the permanent (static) displacement, are immune to clipping and magnitude saturation for large earthquakes, and are sensitive enough to record P-wave arrivals. These characteristics make seismogeodesy suitable for real-time applications such as earthquake early warning. The Scripps Institution of Oceanography (SIO) has developed an inexpensive microelectromechanical systems (MEMS) accelerometer package to upgrade established GPS stations. We compare the performance of our MEMS accelerometer with an observatory-grade accelerometer using an experiment at the University of California San Diego Large High-Performance Outdoor Shake Table. We show that the two types of accelerometers agree in frequency ranges of seismological and engineering interest and produce equivalent seismogeodetic estimates of displacement and velocity. To date, 27 SIO MEMS packages have been installed at GPS monitoring stations in southern California and the San Francisco Bay area and have recorded four earthquakes (M4.2, M4.1, and two of M4.0). The P-wave arrivals are distinguishable in the seismogeodetic observations at distances of up to similar to 25 km away but not in the GPS-only displacements. There is no significant permanent deformation for these small events. This study demonstrates the lower limit of detectability and that seismogeodetic waveforms can also be a reliable early confirmation that an event is not large or hazardous. It also raises the possibility of rapid magnitude estimation through scaling relationships.

Crowell, BW, Bock Y, Liu Z.  2016.  Single-station automated detection of transient deformation in GPS time series with the relative strength index: A case study of Cascadian slow slip. Journal of Geophysical Research-Solid Earth. 121:9077-9094.   10.1002/2016jb013542   AbstractWebsite

The discovery of slow-slip events over the past decades has changed our understanding of tectonic hazards and the earthquake cycle. Proper geodetic characterization of slow-slip events is necessary for studies of regional interseismic, coseismic and postseismic deformation, and miscalculations can affect our understanding of the regional stress field and tectonic hazard. Because of the proliferation of GPS data over the last two decades, an automated algorithm is required to analyze the signals and model the deformation on a station by station basis. Using the relative strength index (RSI), a financial momentum oscillator, we test the ability to detect events of various sizes and durations. We first determine the statistics of the RSI under different noise conditions and then use this information as the basis for the automated transient detection algorithm by testing different synthetic signals. We then apply the technique to daily GPS displacement time series from 213 stations along the Cascadia subduction zone to form a record of transient deformation between 2005 and 2016. Our estimates of the spatial extent, duration, and propagation of major episodic tremor and slip (ETS) events are consistent with previous studies. We use the automated detections to remodel the displacement time series and obtain transient deformation rates over the past decade and discuss the tectonic implications. Finally, we analyze the correlation between transient detections and tremor showing good agreement between the two at slab depths commonly associated with ETS events.

Bock, Y, Melgar D.  2016.  Physical applications of GPS geodesy: a review. Reports on Progress in Physics. 79   10.1088/0034-4885/79/10/106801   AbstractWebsite

Geodesy, the oldest science, has become an important discipline in the geosciences, in large part by enhancing Global Positioning System (GPS) capabilities over the last 35 years well beyond the satellite constellation's original design. The ability of GPS geodesy to estimate 3D positions with millimeter-level precision with respect to a global terrestrial reference frame has contributed to significant advances in geophysics, seismology, atmospheric science, hydrology, and natural hazard science. Monitoring the changes in the positions or trajectories of GPS instruments on the Earth's land and water surfaces, in the atmosphere, or in space, is important for both theory and applications, from an improved understanding of tectonic and magmatic processes to developing systems for mitigating the impact of natural hazards on society and the environment. Besides accurate positioning, all disturbances in the propagation of the transmitted GPS radio signals from satellite to receiver are mined for information, from troposphere and ionosphere delays for weather, climate, and natural hazard applications, to disturbances in the signals due to multipath reflections from the solid ground, water, and ice for environmental applications. We review the relevant concepts of geodetic theory, data analysis, and physical modeling for a myriad of processes at multiple spatial and temporal scales, and discuss the extensive global infrastructure that has been built to support GPS geodesy consisting of thousands of continuously operating stations. We also discuss the integration of heterogeneous and complementary data sets from geodesy, seismology, and geology, focusing on crustal deformation applications and early warning systems for natural hazards.

Geng, JH, Bock Y.  2016.  GLONASS fractional-cycle bias estimation across inhomogeneous receivers for PPP ambiguity resolution. Journal of Geodesy. 90:379-396.   10.1007/s00190-015-0879-0   AbstractWebsite

The key issue to enable precise point positioning with ambiguity resolution (PPP-AR) is to estimate fractional-cycle biases (FCBs), which mainly relate to receiver and satellite hardware biases, over a network of reference stations. While this has been well achieved for GPS, FCB estimation for GLONASS is difficult because (1) satellites do not share the same frequencies as a result of Frequency Division Multiple Access (FDMA) signals; (2) and even worse, pseudorange hardware biases of receivers vary in an irregular manner with manufacturers, antennas, domes, firmware, etc., which especially complicates GLONASS PPP-AR over inhomogeneous receivers. We propose a general approach where external ionosphere products are introduced into GLONASS PPP to estimate precise FCBs that are less impaired by pseudorange hardware biases of diverse receivers to enable PPP-AR. One month of GLONASS data at about 550 European stations were processed. From an exemplary network of 51 inhomogeneous receivers, including four receiver types with various antennas and spanning about 800 km in both longitudinal and latitudinal directions, we found that 92.4 % of all fractional parts of GLONASS wide-lane ambiguities agree well within 0.15 cycles with a standard deviation of 0.09 cycles if global ionosphere maps (GIMs) are introduced, compared to only 51.7 % within 0.15 cycles and a larger standard deviation of 0.22 cycles otherwise. Hourly static GLONASS PPP-AR at 40 test stations can reach position estimates of about 1 and 2 cm in RMS from ground truth for the horizontal and vertical components, respectively, which is comparable to hourly GPS PPP-AR. Integrated GLONASS and GPS PPP-AR can further achieve an RMS of about 0.5 cm in horizontal and 1-2 cm in vertical components. We stress that the performance of GLONASS PPP-AR across inhomogeneous receivers depends on the accuracy of ionosphere products. GIMs have a modest accuracy of only 2-8 TECU (Total Electron Content Unit) in vertical which confines PPP-AR to an approximately km area in Europe. We expect that a regional ionosphere map with a better than 1 TECU accuracy is likely to improve the GLONASS PPP-AR efficiency.

Melgar, D, Allen RM, Riquelme S, Geng JH, Bravo F, Baez JC, Parra H, Barrientos S, Fang P, Bock Y, Bevis M, Caccamise DJ, Vigny C, Moreno M, Smalley R.  2016.  Local tsunami warnings: Perspectives from recent large events. Geophysical Research Letters. 43:1109-1117.   10.1002/2015gl067100   AbstractWebsite

We demonstrate a flexible strategy for local tsunami warning that relies on regional geodetic and seismic stations. Through retrospective analysis of four recent tsunamigenic events in Japan and Chile, we show that rapid earthquake source information, provided by methodologies developed for earthquake early warning, can be used to generate timely estimates of maximum expected tsunami amplitude with enough accuracy for tsunami warning. We validate the technique by comparing to detailed models of earthquake source and tsunami propagation as well as field surveys of tsunami inundation. Our approach does not require deployment of new geodetic and seismic instrumentation in many subduction zones and could be implemented rapidly by national monitoring and warning agencies. We illustrate the potential impact of our method with a detailed comparison to the actual timeline of events during the recent 2015 M(w)8.3 Illapel, Chile, earthquake and tsunami that prompted the evacuation of 1 million people.

2015
Moore, AW, Small IJ, Gutman SI, Bock Y, Dumas JL, Fang P, Haase JS, Jackson ME, Laber JL.  2015.  National Weather Service forecasters use GPS precipitable water vapor for enhanced situational awareness during the Southern California summer monsoon. Bulletin of the American Meteorological Society. 96:1867-1877.   10.1175/bams-d-14-00095.1   AbstractWebsite

During the North American Monsoon, low-to-midlevel moisture is transported in surges from the Gulf of California and Eastern Pacific Ocean into Mexico and the American Southwest. As rising levels of precipitable water interact with the mountainous terrain, severe thunderstorms can develop, resulting in flash floods that threaten life and property. The rapid evolution of these storms, coupled with the relative lack of upper-air and surface weather observations in the region, make them difficult to predict and monitor, and guidance from numerical weather prediction models can vary greatly under these conditions. Precipitable water vapor (PW) estimates derived from continuously operating ground-based GPS receivers have been available for some time from NOAA's GPS-Met program, but these observations have been of limited utility to operational forecasters in part due to poor spatial resolution. Under a NASA Advanced Information Systems Technology project, 37 real-time stations were added to NOAA's GPS-Met analysis providing 30-min PW estimates, reducing station spacing from approximately 150 km to 30 km in Southern California. An 18-22 July 2013 North American Monsoon event provided an opportunity to evaluate the utility of the additional upper-air moisture observations to enhance National Weather Service (NWS) forecaster situational awareness during the rapidly developing event. NWS forecasters used these additional data to detect rapid moisture increases at intervals between the available 1-6-h model updates and approximately twice-daily radiosonde observations, and these contributed tangibly to the issuance of timely flood watches and warnings in advance of flash floods, debris flows, and related road closures.

Reuveni, Y, Bock Y, Tong XP, Moore AW.  2015.  Calibrating interferometric synthetic aperture radar (InSAR) images with regional GPS network atmosphere models. Geophysical Journal International. 202:2106-2119.   10.1093/gji/ggv253   AbstractWebsite

Interferometric synthetic aperture radar (InSAR) technology provides a valuable tool for obtaining Earth surface deformation and topography at high spatial resolution for crustal deformation studies. Similar to global positioning system (GPS), InSAR measurements are affected by the Earth's ionospheric and tropospheric layers as the electromagnetic signals significantly refract while propagating through the different layers. While GPS signals propagating through the neutral atmosphere are affected primarily by the distribution, pressure and temperature of atmospheric gases, including water vapour, the propagation through the ionosphere is mainly affected by the number of free electrons along the signal path. Here, we present the use of dense regional GPS networks for extracting tropospheric zenith delays and ionospheric total electron content (TEC) maps in order to reduce the noise levels in InSAR images. The results show significant reduction in the root mean square (RMS) values when simultaneously combining the two corrections, both at short time periods where no surface deformation is expected, and at longer periods, where imaging of localized subsidence and fault creep is enhanced.

Galetzka, J, Melgar D, Genrich JF, Geng J, Owen S, Lindsey EO, Xu X, Bock Y, Avouac JP, Adhikari LB, Upreti BN, Pratt-Sitaula B, Bhattarai TN, Sitaula BP, Moore A, Hudnut KW, Szeliga W, Normandeau J, Fend M, Flouzat M, Bollinger L, Shrestha P, Koirala B, Gautam U, Bhatterai M, Gupta R, Kandel T, Timsina C, Sapkota SN, Rajaure S, Maharjan N.  2015.  Slip pulse and resonance of the Kathmandu basin during the 2015 Gorkha earthquake, Nepal. Science. 349:1091-1095.   10.1126/science.aac6383   AbstractWebsite

Detailed geodetic imaging of earthquake ruptures enhances our understanding of earthquake physics and associated ground shaking. The 25 April 2015 moment magnitude 7.8 earthquake in Gorkha, Nepal was the first large continental megathrust rupture to have occurred beneath a high-rate (5-hertz) Global Positioning System (GPS) network. We used GPS and interferometric synthetic aperture radar data to model the earthquake rupture as a slip pulse similar to 20 kilometers in width, similar to 6 seconds in duration, and with a peak sliding velocity of 1.1 meters per second, which propagated toward the Kathmandu basin at similar to 3.3 kilometers per second over similar to 140 kilometers. The smooth slip onset, indicating a large (similar to 5-meter) slip-weakening distance, caused moderate ground shaking at high frequencies (>1 hertz; peak ground acceleration, similar to 16% of Earth's gravity) and minimized damage to vernacular dwellings. Whole-basin resonance at a period of 4 to 5 seconds caused the collapse of tall structures, including cultural artifacts.

Melgar, D, Crowell BW, Geng JH, Allen RM, Bock Y, Riquelme S, Hill EM, Protti M, Ganas A.  2015.  Earthquake magnitude calculation without saturation from the scaling of peak ground displacement. Geophysical Research Letters. 42:5197-5205.   10.1002/2015gl064278   AbstractWebsite

GPS instruments are noninertial and directly measure displacements with respect to a global reference frame, while inertial sensors are affected by systematic offsetsprimarily tiltingthat adversely impact integration to displacement. We study the magnitude scaling properties of peak ground displacement (PGD) from high-rate GPS networks at near-source to regional distances (similar to 10-1000 km), from earthquakes between M(w)6 and 9. We conclude that real-time GPS seismic waveforms can be used to rapidly determine magnitude, typically within the first minute of rupture initiation and in many cases before the rupture is complete. While slower than earthquake early warning methods that rely on the first few seconds of P wave arrival, our approach does not suffer from the saturation effects experienced with seismic sensors at large magnitudes. Rapid magnitude estimation is useful for generating rapid earthquake source models, tsunami prediction, and ground motion studies that require accurate information on long-period displacements.

Melgar, D, Geng JH, Crowell BW, Haase JS, Bock Y, Hammond WC, Allen RM.  2015.  Seismogeodesy of the 2014 M(w)6.1 Napa earthquake, California: Rapid response and modeling of fast rupture on a dipping strike-slip fault. Journal of Geophysical Research-Solid Earth. 120:5013-5033.   10.1002/2015jb011921   AbstractWebsite

Real-time high-rate geodetic data have been shown to be useful for rapid earthquake response systems during medium to large events. The 2014 M(w)6.1 Napa, California earthquake is important because it provides an opportunity to study an event at the lower threshold of what can be detected with GPS. We show the results of GPS-only earthquake source products such as peak ground displacement magnitude scaling, centroid moment tensor (CMT) solution, and static slip inversion. We also highlight the retrospective real-time combination of GPS and strong motion data to produce seismogeodetic waveforms that have higher precision and longer period information than GPS-only or seismic-only measurements of ground motion. We show their utility for rapid kinematic slip inversion and conclude that it would have been possible, with current real-time infrastructure, to determine the basic features of the earthquake source. We supplement the analysis with strong motion data collected close to the source to obtain an improved postevent image of the source process. The model reveals unilateral fast propagation of slip to the north of the hypocenter with a delayed onset of shallow slip. The source model suggests that the multiple strands of observed surface rupture are controlled by the shallow soft sediments of Napa Valley and do not necessarily represent the intersection of the main faulting surface and the free surface. We conclude that the main dislocation plane is westward dipping and should intersect the surface to the east, either where the easternmost strand of surface rupture is observed or at the location where the West Napa fault has been mapped in the past.

Melgar, D, Bock Y.  2015.  Kinematic earthquake source inversion and tsunami runup prediction with regional geophysical data. Journal of Geophysical Research-Solid Earth. 120:3324-3349.   10.1002/2014jb011832   AbstractWebsite

Rapid near-source earthquake source modeling relying only on strong motion data is limited by instrumental offsets and magnitude saturation, adversely affecting subsequent tsunami prediction. Seismogeodetic displacement and velocity waveforms estimated from an optimal combination of high-rate GPS and strong motion data overcome these limitations. Supplementing land-based data with offshore wave measurements by seafloor pressure sensors and GPS-equipped buoys can further improve the image of the earthquake source and prediction of tsunami extent, inundation, and runup. We present a kinematic source model obtained from a retrospective real-time analysis of a heterogeneous data set for the 2011 M(w)9.0 Tohoku-Oki, Japan, earthquake. Our model is consistent with conceptual models of subduction zones, exhibiting depth dependent behavior that is quantified through frequency domain analysis of slip rate functions. The stress drop distribution is found to be significantly more correlated with aftershock locations and mechanism types when off-shore data are included. The kinematic model parameters are then used as initial conditions in a fully nonlinear tsunami propagation analysis. Notably, we include the horizontal advection of steeply sloping bathymetric features. Comparison with post-event on-land survey measurements demonstrates that the tsunami's inundation and runup are predicted with considerable accuracy, only limited in scale by the resolution of available topography and bathymetry. We conclude that it is possible to produce credible and rapid, kinematic source models and tsunami predictions within minutes of earthquake onset time for near-source coastal regions most susceptible to loss of life and damage to critical infrastructure, regardless of earthquake magnitude.

2014
Lindsey, EO, Sahakian VJ, Fialko Y, Bock Y, Barbot S, Rockwell TK.  2014.  Interseismic strain localization in the San Jacinto Fault Zone. Pure and Applied Geophysics. 171:2937-2954.   10.1007/s00024-013-0753-z   AbstractWebsite

We investigate interseismic deformation across the San Jacinto fault at Anza, California where previous geodetic observations have indicated an anomalously high shear strain rate. We present an updated set of secular velocities from GPS and InSAR observations that reveal a 2-3 km wide shear zone deforming at a rate that exceeds the background strain rate by more than a factor of two. GPS occupations of an alignment array installed in 1990 across the fault trace at Anza allow us to rule out shallow creep as a possible contributor to the observed strain rate. Using a dislocation model in a heterogeneous elastic half space, we show that a reduction in shear modulus within the fault zone by a factor of 1.2-1.6 as imaged tomographically by Allam and Ben-Zion (Geophys J Int 190:1181-1196, 2012) can explain about 50 % of the observed anomalous strain rate. However, the best-fitting locking depth in this case (10.4 +/- A 1.3 km) is significantly less than the local depth extent of seismicity (14-18 km). We show that a deep fault zone with a shear modulus reduction of at least a factor of 2.4 would be required to explain fully the geodetic strain rate, assuming the locking depth is 15 km. Two alternative possibilities include fault creep at a substantial fraction of the long-term slip rate within the region of deep microseismicity, or a reduced yield strength within the upper fault zone leading to distributed plastic failure during the interseismic period.

Lindsey, EO, Fialko Y, Bock Y, Sandwell DT, Bilham R.  2014.  Localized and distributed creep along the southern San Andreas Fault. Journal of Geophysical Research-Solid Earth. 119:7909-7922.   10.1002/2014jb011275   AbstractWebsite

We investigate the spatial pattern of surface creep and off-fault deformation along the southern segment of the San Andreas Fault using a combination of multiple interferometric synthetic aperture radar viewing geometries and survey-mode GPS occupations of a dense array crossing the fault. Radar observations from Envisat during the period 2003-2010 were used to separate the pattern of horizontal and vertical motion, providing a high-resolution image of uplift and shallow creep along the fault trace. The data reveal pervasive shallow creep along the southernmost 50 km of the fault. Creep is localized on a well-defined fault trace only in the Mecca Hills and Durmid Hill areas, while elsewhere creep appears to be distributed over a 1-2 km wide zone surrounding the fault. The degree of strain localization is correlated with variations in the local fault strike. Using a two-dimensional boundary element model, we show that stresses resulting from slip on a curved fault can promote or inhibit inelastic failure within the fault zone in a pattern matching the observations. The occurrence of shallow, localized interseismic fault creep within mature fault zones may thus be partly controlled by the local fault geometry and normal stress, with implications for models of fault zone evolution, shallow coseismic slip deficit, and geologic estimates of long-term slip rates. Key PointsShallow creep is pervasive along the southernmost 50 km of the San Andreas FaultCreep is localized only along transpressional fault segmentsIn transtensional areas, creep is distributed over a 1-2 km wide fault zone

2013
Geng, JH, Bock Y.  2013.  Triple-frequency GPS precise point positioning with rapid ambiguity resolution. Journal of Geodesy. 87:449-460.   10.1007/s00190-013-0619-2   AbstractWebsite

At present, reliable ambiguity resolution in real-time GPS precise point positioning (PPP) can only be achieved after an initial observation period of a few tens of minutes. In this study, we propose a method where the incoming triple-frequency GPS signals are exploited to enable rapid convergences to ambiguity-fixed solutions in real-time PPP. Specifically, extra-wide-lane ambiguity resolution can be first achieved almost instantaneously with the Melbourne-Wubbena combination observable on L2 and L5. Then the resultant unambiguous extra-wide-lane carrier-phase is combined with the wide-lane carrier-phase on L1 and L2 to form an ionosphere-free observable with a wavelength of about 3.4 m. Although the noise of this observable is around 100 times the raw carrier-phase noise, its wide-lane ambiguity can still be resolved very efficiently, and the resultant ambiguity-fixed observable can assist much better than pseudorange in speeding up succeeding narrow-lane ambiguity resolution. To validate this method, we use an advanced hardware simulator to generate triple-frequency signals and a high-grade receiver to collect 1-Hz data. When the carrier-phase precisions on L1, L2 and L5 are as poor as 1.5, 6.3 and 1.5 mm, respectively, wide-lane ambiguity resolution can still reach a correctness rate of over 99 % within 20 s. As a result, the correctness rate of narrow-lane ambiguity resolution achieves 99 % within 65 s, in contrast to only 64 % within 150 s in dual-frequency PPP. In addition, we also simulate a multipath-contaminated data set and introduce new ambiguities for all satellites every 120 s. We find that when multipath effects are strong, ambiguity-fixed solutions are achieved at 78 % of all epochs in triple-frequency PPP whilst almost no ambiguities are resolved in dual-frequency PPP. Therefore, we demonstrate that triple-frequency PPP has the potential to achieve ambiguity-fixed solutions within a few minutes, or even shorter if raw carrier-phase precisions are around 1 mm. In either case, we conclude that the efficiency of ambiguity resolution in triple-frequency PPP is much higher than that in dual-frequency PPP.

Geng, JH, Bock Y, Melgar D, Crowell BW, Haase JS.  2013.  A new seismogeodetic approach applied to GPS and accelerometer observations of the 2012 Brawley seismic swarm: Implications for earthquake early warning. Geochemistry Geophysics Geosystems. 14:2124-2142.   10.1002/ggge.20144   AbstractWebsite

The 26 August 2012 Brawley seismic swarm of hundreds of events ranging from M1.4 to M5.5 in the Salton Trough, California provides a unique data set to investigate a new seismogeodetic approach that combines Global Positioning System (GPS) and accelerometer observations to estimate displacement and velocity waveforms. First in simulated real-time mode, we analyzed 1-5 Hz GPS data collected by 17 stations fully encircling the swarm zone at near-source distances up to about 40km using precise point positioning with ambiguity resolution (PPP-AR). We used a reference network of North American GPS stations well outside the region of deformation to estimate fractional-cycle biases and satellite clock parameters, which were then combined with ultrarapid orbits from the International GNSS Service to estimate positions during the Brawley seismic swarm. Next, we estimated seismogeodetic displacements and velocities from GPS phase and pseudorange observations and 100-200 Hz accelerations collected at three pairs of GPS and seismic stations in close proximity using a new tightly coupled Kalman filter approach as an extension of the PPP-AR process. We can clearly discern body waves in the velocity waveforms, including P-wave arrivals not detectable with the GPS-only approach for earthquake magnitudes as low as M-w 4.6 and significant static offsets for magnitudes as low as M-w 5.4. Our study shows that GPS networks upgraded with strong motion accelerometers can provide new information for improved understanding of the earthquake rupture process and be of critical value in creating a robust early warning system for any earthquake of societal significance.

Crowell, BW, Bock Y, Sandwell DT, Fialko Y.  2013.  Geodetic investigation into the deformation of the Salton Trough. Journal of Geophysical Research-Solid Earth. 118:5030-5039.   10.1002/jgrb.50347   AbstractWebsite

The Salton Trough represents a complex transition between the spreading center in Baja California and the strike-slip San Andreas fault system and is one of the most active zones of deformation and seismicity in California. We present a high-resolution interseismic velocity field for the Salton Trough derived from 74 continuous GPS sites and 109 benchmarks surveyed in three GPS campaigns during 2008-2009 and previous surveys between 2000 and 2005. We also investigate small-scale deformation by removing the regional velocity field predicted by an elastic block model for Southern California from the observed velocities. We find a total extension rate of 11mm/yr from the Mesquite Basin to the southern edge of the San Andreas Fault, coupled with 15mm/yr of left-lateral shear, the majority of which is concentrated in the southern Salton Sea and Obsidian Buttes and is equivalent to 17mm/yr oriented in the direction of the San Andreas Fault. Differential shear strain is exclusively localized in the Brawley Seismic Zone, and dilatation rate indicates widespread extension throughout the zone. In addition, we infer clockwise rotation of 10 degrees/Ma, consistent with northwestward propagation of the Brawley Seismic Zone over geologic time.

Granat, R, Parker J, Kedar S, Dong DA, Tang BY, Bock Y.  2013.  Statistical Approaches to Detecting Transient Signals in GPS: Results from the 2009-2011 Transient Detection Exercise. Seismological Research Letters. 84:444-454.   10.1785/0220130039   AbstractWebsite
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Melgar, D, Bock Y, Sanchez D, Crowell BW.  2013.  On robust and reliable automated baseline corrections for strong motion seismology. Journal of Geophysical Research-Solid Earth. 118:1177-1187.   10.1002/jgrb.50135   AbstractWebsite

Computation of displacements from strong motion inertial sensors is to date an open problem. Two distinct methodologies have been proposed to solve it. One involves baseline corrections determined from the inertial data themselves and the other a combination with other geophysical sensors such as GPS. Here we analyze a proposed automated baseline correction algorithm using only accelerometer data and compare it to the results from the real-time combination of strong motion and GPS data. The analysis is performed on 48 collocated GPS and accelerometers in Japan that recorded the 2011 Mw 9.0 Tohoku-oki earthquake. We study the time and frequency domain behavior of both methodologies. We find that the error incurred from automated baseline corrections that rely on seismic data alone is complex and can be large in both the time and frequency domains of interest in seismological and engineering applications. The GPS/accelerometer combination has no such problems and can adequately recover broadband strong motion displacements for this event. The problems and ambiguities with baseline corrections and the success of the GPS/accelerometer combination lead us to advocate for instrument collocations as opposed to automated baseline correction algorithms for accelerometers. Citation: Melgar, D., Y. Bock, D. Sanchez, and B. W. Crowell (2013), On robust and reliable automated baseline corrections for strong motion seismology, J. Geophys. Res. Solid Earth, 118, 1177-1187, doi: 10.1002/jgrb.50135.

Crowell, BW, Melgar D, Bock Y, Haase JS, Geng JH.  2013.  Earthquake magnitude scaling using seismogeodetic data. Geophysical Research Letters. 40:6089-6094.   10.1002/2013gl058391   AbstractWebsite

The combination of GPS and strong-motion data to estimate seismogeodetic waveforms creates a data set that is sensitive to the entire spectrum of ground displacement and the full extent of coseismic slip. In this study we derive earthquake magnitude scaling relationships using seismogeodetic observations of either P wave amplitude or peak ground displacements from five earthquakes in Japan and California ranging in magnitude from 5.3 to 9.0. The addition of the low-frequency component allows rapid distinction of earthquake size for large magnitude events with high precision, unlike accelerometer data that saturate for earthquakes greater than M 7 to 8, and is available well before the coseismic displacements are emplaced. These results, though based on a limited seismogeodetic data set, support earlier studies that propose it may be possible to estimate the final magnitude of an earthquake well before the rupture is complete.

Lindsey, E, Sahakian V, Fialko Y, Bock Y, Barbot S, Rockwell T.  2013.  Interseismic strain localization in the San Jacinto Fault Zone. Pure and Applied Geophysics. :1-18.: Springer Basel   10.1007/s00024-013-0753-z   AbstractWebsite

We investigate interseismic deformation across the San Jacinto fault at Anza, California where previous geodetic observations have indicated an anomalously high shear strain rate. We present an updated set of secular velocities from GPS and InSAR observations that reveal a 2–3 km wide shear zone deforming at a rate that exceeds the background strain rate by more than a factor of two. GPS occupations of an alignment array installed in 1990 across the fault trace at Anza allow us to rule out shallow creep as a possible contributor to the observed strain rate. Using a dislocation model in a heterogeneous elastic half space, we show that a reduction in shear modulus within the fault zone by a factor of 1.2–1.6 as imaged tomographically by Allam and Ben-Zion (Geophys J Int 190:1181–1196, 2012) can explain about 50 % of the observed anomalous strain rate. However, the best-fitting locking depth in this case (10.4 ± 1.3 km) is significantly less than the local depth extent of seismicity (14–18 km). We show that a deep fault zone with a shear modulus reduction of at least a factor of 2.4 would be required to explain fully the geodetic strain rate, assuming the locking depth is 15 km. Two alternative possibilities include fault creep at a substantial fraction of the long-term slip rate within the region of deep microseismicity, or a reduced yield strength within the upper fault zone leading to distributed plastic failure during the interseismic period.

Melgar, D, Bock Y.  2013.  Near-field tsunami models with rapid earthquake source inversions from land- and ocean-based observations: The potential for forecast and warning. Journal of Geophysical Research-Solid Earth. 118:5939-5955.   10.1002/2013jb010506   AbstractWebsite

Computation of tsunami models in the region adjacent to large earthquakes immediately after rupture initiation remains a challenging problem. Limitations of traditional seismological instrumentation in the near field and concern by tsunami modelers regarding the nonuniqueness of source inversions and the use of indirect observations have in the past been hurdles for such efforts. Employing near-field data from the Mw 9.0 2011 Tohoku-oki data, we test source models obtained from newly developed algorithms and multisensor data. We demonstrate the ability of such source models determined from land-based coseismic data from the combination of GPS and strong-motion sensors to forecast near-source tsunamis. We also demonstrate that rapid ingestion of offshore shallow water (100-1000m) wave gauge data substantially improves the earthquake source and tsunami forecast. To assess the success of such tsunami models, we rely on detailed comparisons to 2000+ tsunami survey measurements collected after the event. We argue that deployments of shallow water wave gauges coupled with land-based geophysical sensors can, in the future, provide enough information to issue timely and accurate forecasts of tsunami intensity immediately or shortly after rupture initiation of large earthquakes.

Geng, JH, Melgar D, Bock Y, Pantoli E, Restrepo J.  2013.  Recovering coseismic point ground tilts from collocated high-rate GPS and accelerometers. Geophysical Research Letters. 40:5095-5100.   10.1002/grl.51001   AbstractWebsite

Rotational along with translational and strain measurements are essential for a complete description of the motion of a deformable body in a seismic event. We propose a new seismogeodetic approach where collocated high-rate GPS and accelerometer measurements are combined to estimate permanent and dynamic coseismic ground tilts at a point, whereas at present, only dynamic tilts are measured with either a dense seismic array or an expensive ring laser gyroscope. We estimate point tilts for a five-story structure on a shake table subjected to 13 earthquake strong motion records of increasing intensity. For the most intense record from the 2002 M7.9 Denali earthquake, we observe a peak-to-peak dynamic tilt of 0.12 degrees and a permanent tilt of 0.16 degrees for the structure's roof. Point tilts derived from networks of collocated GPS and accelerometers can be used to estimate the rotational component of the seismic wavefield for improved earthquake source characterization.

Melgar, D, Crowell BW, Bock Y, Haase JS.  2013.  Rapid modeling of the 2011 Mw 9.0 Tohoku-oki earthquake with seismogeodesy. Geophysics Research Letters. 40:1-6.   10.1002/grl.50590  
2012
Sadeh, M, Hamiel Y, Ziv A, Bock Y, Fang P, Wdowinski S.  2012.  Crustal deformation along the Dead Sea Transform and the Carmel Fault inferred from 12 years of GPS measurements. Journal of Geophysical Research-Solid Earth. 117   10.1029/2012jb009241   AbstractWebsite

Large-scale crustal deformation in the Levant is mainly related to the DST and the CFS. The former is an active left lateral transform, bounding the Arabian plate and the Sinai sub-plate, and the latter branches out of the former and separates the Sinai sub-plate into two tectonic domains. In this study we obtain the velocities of 33 permanent GPS stations and 145 survey stations that were surveyed in three campaigns between 1996 and 2008. We use a simple 1-D elastic dislocation model to infer the slip rate and locking depth along various segments of the DST. We infer a 3.1-4.5 mm/yr slip rate and a 7.8-16.5 km locking depth along the DST north of the CFS, and a slip rate of 4.6-5.9 mm/yr and locking depth of 11.8-24 km along the Jericho Valley, south of the CFS. Further south, along the Arava Valley we obtain a slip rate of 4.7-5.4 mm/yr and a locking depth of 12.1-23 km. We identify an oblique motion along the Carmel Fault with similar to 0.7 mm/yr left-lateral and similar to 0.6 mm/yr extension rates, resulting in N-S extension across the Carmel Fault. This result, together with the decrease in DST slip velocity from the Jericho Valley to the segment north of the CFS, confirms previous suggestions, according to which part of the slip between Arabia and Sinai is being transferred from the DST to the CFS.