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Wang, MH, Wang JX, Bock Y, Liang H, Dong DA, Fang P.  2019.  Dynamic mapping of the movement of landfalling atmospheric rivers over Southern California with GPS data. Geophysical Research Letters. 46:3551-3559.   10.1029/2018gl081318   AbstractWebsite

Atmospheric rivers (ARs) are long, narrow, and transient corridors of strong horizontal water vapor transport that can result in heavy precipitation. Measuring the movement of these concentrated water vapor bands is important in gaining better insight into AR characteristics and forecasts of AR-caused precipitation. We describe a method to dynamically map the movement of landfalling ARs. The method utilizes high-rate GPS observations from a dense network to derive isochrones that represent the AR arrival time over specific locations. The generated isochrones show that the three ARs, during landfall over Southern California in January 2017, moved southeastward and took about 10 hr to pass over the study area. Overlaying the topography with isochrones reveals that the Peninsular Ranges slow the movement of the landfalling ARs. The large spacing between two adjacent isochrones, reflecting fast AR movement, is closely related to the increased hourly rain rate. Plain Language Summary Atmospheric rivers (ARs), "rivers in the sky," are "rivers" of water vapor rather than liquid water. The landfall of ARs can cause extreme rainfall that in turn induces disasters. We present a method with a dense high-rate GPS network to capture the movement of the landfalling ARs over Southern California. For the three landfalling AR cases in January 2017, results show that the ARs moved southeastward and the durations of AR passing over the study area were about 10 hr. The results also reveal that the landfalling AR movement is affected by local terrain and the fast AR movement is closely related to the large hourly rain rate. The use of the method provides a way to study ARs with high spatial-temporal resolution, which is important in gaining better insight into the forecasts of AR-caused rainfall.

Ruhl, CJ, Melgar D, Geng JH, Goldberg DE, Crowell BW, Allen RM, Bock Y, Barrientos S, Riquelme S, Baez JC, Cabral-Cano E, Perez-Campos X, Hill EM, Protti M, Ganas A, Ruiz M, Mothes P, Jarrin P, Nocquet JM, Avouac JP, D'Anastasio E.  2019.  A global database of strong-motion displacement GNSS recordings and an example application to PGD scaling. Seismological Research Letters. 90:271-279.   10.1785/0220180177   AbstractWebsite

Displacement waveforms derived from Global Navigation Satellite System (GNSS) data have become more commonly used by seismologists in the past 15 yrs. Unlike strong-motion accelerometer recordings that are affected by baseline offsets during very strong shaking, GNSS data record displacement with fidelity down to 0 Hz. Unfortunately, fully processed GNSS waveform data are still scarce because of limited public availability and the highly technical nature of GNSS processing. In an effort to further the use and adoption of high-rate (HR) GNSS for earthquake seismology, ground-motion studies, and structural monitoring applications, we describe and make available a database of fully curated HR-GNSS displacement waveforms for significant earthquakes. We include data from HR-GNSS networks at near-source to regional distances (1-1000 km) for 29 earthquakes between M-w 6.0 and 9.0 worldwide. As a demonstration of the utility of this dataset, we model the magnitude scaling properties of peak ground displacements (PGDs) for these events. In addition to tripling the number of earthquakes used in previous PGD scaling studies, the number of data points over a range of distances and magnitudes is dramatically increased. The data are made available as a compressed archive with the article.

Chen, MC, Astroza R, Restrepo JI, Conte JP, Hutchinson T, Bock Y.  2017.  Predominant period and equivalent viscous damping ratio identification for a full-scale building shake table test. Earthquake Engineering & Structural Dynamics. 46:2459-2477.   10.1002/eqe.2913   AbstractWebsite

The predominant period and corresponding equivalent viscous damping ratio, also known in various loading codes as effective period and effective damping coefficient, are two important parameters employed in the seismic design of base-isolated and conventional building structures. Accurate determination of these two parameters can reduce the uncertainty in the computation of lateral displacement demands and interstory drifts for a given seismic design spectrum. This paper estimates these two parameters from data sets recorded from a full-scale five-story reinforced concrete building subjected to seismic base excitations of various intensities in base-isolated and fixed-base configurations on the outdoor shake table at the University of California, San Diego. The scope of this paper includes all test motions in which the yielding of the reinforcement has not occurred and the response can still be considered 'elastic'. The data sets are used with three system identification methods to determine the predominant period of response for each of the test configurations. One of the methods also determines the equivalent viscous damping ratio corresponding to the predominant period. It was found that the predominant period of the fixed-base building lengthened from 0.52 to 1.30 s. This corresponded to a significant reduction in effective system stiffness to about 16% of the original stiffness. The paper then establishes a correlation between predominant period and peak ground velocity. Finally, the predominant periods and equivalent viscous damping ratios recommended by the ASCE 7-10 loading standard are compared with those determined from the test building. Copyright (C) 2017 John Wiley & Sons, Ltd.

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.

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.

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.

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.

Bock, Y, Wdowinski S, Ferretti A, Novali F, Fumagalli A.  2012.  Recent subsidence of the Venice Lagoon from continuous GPS and interferometric synthetic aperture radar. Geochemistry Geophysics Geosystems. 13:Q03023.: AGU   10.1029/2011gc003976   AbstractWebsite

Coastal regions are increasingly affected by larger storms and rising sea level predicted by global warming models, aggravating the situation in the city of Venice where tidal-induced seasonal flooding coupled with natural and anthropogenic subsidence have been perennial problems. In light of accelerated efforts to protect Venice from the rise in sea level we assess land subsidence in the Venice Lagoon over the last decade. Through a combined analysis of GPS position time series from 2001.55 to 2011.00 for four stations installed by the Magistrato alle Acque di Venezia and thousands of observations of InSAR permanent scatterers using RADARSAT-1 images from 2003.3 to 2007.85, we determine that the northern lagoon subsides at a rate of 2–3 mm/yr, whereas the southern lagoon subsides at 3–4 mm/yr. The city of Venice continues to subside, at a rate of 1–2 mm/yr, in contrast to geodetic studies in the last decade of the 20th Century suggesting that subsidence has been stabilized. The GPS results indicate a general eastward tilt in subsidence and that the natural subsidence rate related to the retreat of the Adriatic plate subducting beneath the Apennines is at least 0.4–0.6 mm/yr. Our combined GPS and InSAR analysis demonstrates high spatial resolution in the vertical direction with a precision of 0.1–0.2 mm/yr with respect to a global reference frame. Continued efforts to secure the city of Venice from flooding must also take into account the significant local and regional subsidence rates as well as the expected rise in sea level.

Nof, RN, Ziv A, Doin MP, Baer G, Fialko Y, Wdowinski S, Eyal Y, Bock Y.  2012.  Rising of the lowest place on Earth due to Dead Sea water-level drop: Evidence from SAR interferometry and GPS. J. Geophys. Res.. 117:B05412.: AGU   10.1029/2011jb008961   AbstractWebsite

The Dead Sea water-level has been dropping at an exceedingly increasing rate since 1960, and between 1993 and 2001, the interval of the InSAR data examined in this study, it has dropped at an average rate of 0.88 m per year. Such a water-level change could potentially give rise to a resolvable lithospheric rebound and regional uplift, with spatial extent and amplitude that are controlled by the effective mechanical properties of the crust and upper mantle combined. We measure that deformation for the years 1993 to 2001, using 149 short baseline interferograms made of 31 ERS-1 and ERS-2 Synthetic Aperture Radar (SAR) images and continuous GPS data from the Survey of Israel recorded between 1997 and 2011. The uplift rate at the Dead Sea is small (up to 4 mm/year), and the basin topography is almost a mirror of the displacement, introducing a strong trade-off between uplift and stratified atmosphere noise. To overcome this complication, we impose a linearity constraint on the satellite to ground Line Of Sight (LOS) phase changes based on the steady uplift observed by a continuous GPS station in the area of interest, and simultaneously solve for the LOS change rate, Digital Elevation Model (DEM) errors and the elevation-phase correlation. While the LOS rate and DEM errors are solved for each pixel independently, the elevation-phase correlation is solved for each SAR acquisition independently. Using this approach we separated the stratified atmospheric delay from the ground displacement. We observed a regional uplift around the Dead Sea northern basin, with maximum uplift close to the shorelines, and diminishing to zero by the Mediterranean coast. We modeled the effect of water load changes using a homogeneous elastic half-space, and found a good agreement between modeled and observed ground displacements using elastic properties that are compatible with seismic and gravity data down to a depth of 15 km below the Dead Sea basin, suggesting that the response of the crust to the sea level drop is controlled mainly by the elastic properties of the upper-crust immediately below the Dead Sea basin.

Noll, C, Bock Y, Habrich H, Moore A.  2009.  Development of data infrastructure to support scientific analysis for the International GNSS Service. Journal of Geodesy. 83:309-325.   10.1007/s00190-008-0245-6   AbstractWebsite

The International GNSS Service provides data and products to support a wide range of global, multidisciplinary scientific research. The service has established a hierarchy of components to facilitate its mission: a globally distributed network of Tracking Stations, Data Centers, Analysis Centers, a Central Bureau, and a Governing Board. The Data Centers, in conjunction with the Central Bureau, serve as the primary means of distributing GNSS data, products, and general information to the user community through ftp and Web servers and email services. The requirements of analysis centers and the scientific community have evolved over the lifetime of the IGS, requiring enhancement and extension of the supporting data center infrastructure. The diversity of IGS data and products extends today from the realm of the real-time and near real-time to the long-term archive and thus forms a basis for multidisciplinary research spanning decades. Reliability of all components is a key requirement within the IGS and is accomplished through the geographic distribution of data centers and the creation of independent, redundant, parallel channels for the transmission of data and products. We discuss the development of the IGS data infrastructure, current status, and plans for future enhancements. Descriptions of IGS data and products and associated metadata are also included.

King, NE, Argus D, Langbein J, Agnew DC, Bawden G, Dollar RS, Liu Z, Galloway D, Reichard E, Yong A, Webb FH, Bock Y, Stark K, Barseghian D.  2007.  Space geodetic observation of expansion of the San Gabriel Valley, California, aquifer system, during heavy rainfall in winter 2004-2005. Journal of Geophysical Research-Solid Earth. 112   10.1029/2006jb004448   AbstractWebsite

[1] Starting early in 2005, the positions of GPS stations in the San Gabriel valley region of southern California showed statistically significant departures from their previous behavior. Station LONG moved up by about 47 mm, and nearby stations moved away from LONG by about 10 mm. These changes began during an extremely rainy season in southern California and coincided with a 16-m increase in water level at a nearby well in Baldwin Park and a regional uplift detected by interferometric synthetic aperture radar. No equivalent signals were seen in GPS station position time series elsewhere in southern California. Our preferred explanation, supported by the timing and by a hydrologic simulation, is deformation due to recharging of aquifers after near-record rainfall in 2004 - 2005. We cannot rule out an aseismic slip event, but we consider such an event unlikely because it requires slip on multiple faults and predicts other signals that are not observed.

Williams, SDP, Bock Y, Fang P, Jamason P, Nikolaidis RM, Prawirodirdjo L, Miller M, Johnson DJ.  2004.  Error analysis of continuous GPS position time series. Journal of Geophysical Research-Solid Earth. 109   10.1029/2003jb002741   AbstractWebsite

[1] A total of 954 continuous GPS position time series from 414 individual sites in nine different GPS solutions were analyzed for noise content using maximum likelihood estimation (MLE). The lengths of the series varied from around 16 months to over 10 years. MLE was used to analyze the data in two ways. In the first analysis the noise was assumed to be white noise only, a combination of white noise plus flicker noise, or a combination of white noise plus random walk noise. For the second analysis the spectral index and amplitude of the power law noise were estimated simultaneously with the white noise. In solutions where the sites were globally distributed, the noise can be best described by a combination of white noise plus flicker noise. Both noise components show latitude dependence in their amplitudes ( higher at equatorial sites) together with a bias to larger values in the Southern Hemisphere. In the regional solutions, where a spatially correlated ( common mode) signal has been removed, the noise is significantly lower. The spectral index of the power law in regional solutions is more varied than in the global solutions and probably reflects a mixture of local effects. A significant reduction in noise can be seen since the first continuous GPS networks began recording in the early 1990s. A comparison of the noise amplitudes to the different monument types in the Southern California Integrated GPS Network suggests that the deep drill braced monument is preferred for maximum stability.

Bock, Y, Prawirodirdjo L, Genrich JF, Stevens CW, McCaffrey R, Subarya C, Puntodewo SSO, Calais E.  2003.  Crustal motion in Indonesia from Global Positioning System measurements. Journal of Geophysical Research-Solid Earth. 108   10.1029/2001jb000324   AbstractWebsite

[1] We present the crustal motion velocity field for the Indonesian archipelago based on Global Positioning System (GPS) field surveys conducted from 1991 to 1997, and 2001, totaling more than 150 sites, as well as on a reanalysis of global tracking data in the Scripps Orbit and Permanent Array Center archive from 1991 to 2001 in International Terrestrial Reference Frame 2000. We compute poles of rotation for the Australia, Eurasia, and Pacific plates based on our analysis of the global GPS data. We find that regional tectonics is dominated by the interaction of four discrete, rotating blocks spanning significant areas of the Sunda Shelf, the South Banda arc, the Bird's Head region of New Guinea, and East Sulawesi. The largest, the Sunda Shelf block (SSH), is estimated to be moving 6 +/- 3 mm/yr SE relative to Eurasia. The South Banda block (SBB) rotates clockwise relative to both the SSH and Australia plate, resulting in 15 +/- 8 mm/yr of motion across the Timor trough and 60 +/- 3 mm/yr of shortening across the Flores Sea. Southern New Guinea forms part of the Australia plate from which the Bird's Head block (BHB) moves rapidly WSW, subducting beneath the Seram trough. The East Sulawesi block rotates clockwise about a nearby axis with respect to the Sunda Shelf, thereby transferring east-west shortening between the Pacific and Eurasia plates into north-south shortening across the North Sulawesi trench. Except for the Sunda Shelf, the crustal blocks are all experiencing significant internal deformation. In this respect, crustal motion in those regions does not fit the microplate tectonics model.

Lyons, SN, Bock Y, Sandwell DT.  2002.  Creep along the imperial fault, southern California, from GPS measurements. Journal of Geophysical Research-Solid Earth. 107   10.1029/2001jb000763   AbstractWebsite

[1] In May of 1999 and 2000, we surveyed with Global Positioning System (GPS) 46 geodetic monuments established by Imperial College, London, in a dense grid (half-mile spacing) along the Imperial Fault, with three additional National Geodetic Survey sites serving as base stations. These stations were previously surveyed in 1991 and 1993. The Imperial College sites were surveyed in rapid-static mode (15-20 min occupations), while the NGS sites continuously received data for 10 h d(-1). Site locations were calculated using the method of instantaneous positioning, and velocities were determined relative to one of the NGS base stations. Combining our results with far-field velocities from the Southern California Earthquake Center (SCEC), we fit the data to a simple elastic dislocation model with 35 mm yr(-1) of right-lateral slip below 10 km and 9 mm yr(-1) of creep from the surface down to 3 km. The velocity field is asymmetrical across the fault and could indicate a dipping fault plane to the northeast or a viscosity contrast across the fault.

Vigny, C, Perfettini H, Walpersdorf A, Lemoine A, Simons W, van Loon D, Ambrosius B, Stevens C, McCaffrey R, Morgan P, Bock Y, Subarya C, Manurung P, Kahar J, Abidin HZ, Abu SH.  2002.  Migration of seismicity and earthquake interactions monitored by GPS in SE Asia triple junction: Sulawesi, Indonesia. Journal of Geophysical Research-Solid Earth. 107   10.1029/2001jb000377   AbstractWebsite

[1] Global Positioning System (GPS) measurements made in Sulawesi, Indonesia, from 1992 to 1999 detected coseismic and transient postseismic deformation related to the 1 January 1996, M-w = 7.9 earthquake on the North Sulawesi (Minahassa) trench. These motions are superimposed on the long-term secular motion (40 mm/yr) of the left-lateral Palu fault in central Sulawesi and continued for about 1.5-2 years. Following the earthquake, a string of earthquakes (of magnitude >6) migrated along the Minahassa trench, from west to east. Subsequently, two earthquakes of magnitude >6 occurred on or near the Palu fault migrating toward the south. Modeling the increase in Coulomb stress generated by the successive earthquakes agrees with the hypothesis of interacting events. An unclamping effect, possibly due to fluid migration in the Palu area, is also suggested by the stress computations and the detected (GPS) displacements.

Dong, D, Fang P, Bock Y, Cheng MK, Miyazaki S.  2002.  Anatomy of apparent seasonal variations from GPS-derived site position time series. Journal of Geophysical Research-Solid Earth. 107   10.1029/2001jb000573   AbstractWebsite

[1] Apparent seasonal site position variations are derived from 4.5 years of global continuous GPS time series and are explored through the "peering'' approach. Peering is a way to depict the contributions of the comparatively well-known seasonal sources to garner insight into the relatively poorly known contributors. Contributions from pole tide effects, ocean tide loading, atmospheric loading, nontidal oceanic mass, and groundwater loading are evaluated. Our results show that similar to40% of the power of the observed annual vertical variations in site positions can be explained by the joint contribution of these seasonal surface mass redistributions. After removing these seasonal effects from the observations the potential contributions from unmodeled wet troposphere effects, bedrock thermal expansion, errors in phase center variation models, and errors in orbital modeling are also investigated. A scaled sensitivity matrix analysis is proposed to assess the contributions from highly correlated parameters. The effects of employing different analysis strategies are investigated by comparing the solutions from different GPS data analysis centers. Comparison results indicate that current solutions of several analysis centers are able to detect the seasonal signals but that the differences among these solutions are the main cause for residual seasonal effects. Potential implications for modeling seasonal variations in global site positions are explored, in particular, as a way to improve the stability of the terrestrial reference frame on seasonal timescales.

Stevens, C, McCaffrey R, Bock Y, Genrich J, Endang, Subarya C, Puntodewo SSO, Fauzi, Vigny C.  1999.  Rapid rotations about a vertical axis in a collisional setting revealed by the Palu fault, Sulawesi, Indonesia. Geophysical Research Letters. 26:2677-2680.   10.1029/1999gl008344   AbstractWebsite

Global Positioning System (GPS) measurements from 1992 to 1995 indicate that the left-lateral Palu fault in central Sulawesi slips at a rate of 38+/-8 mm/a with a locking depth between 2 and 8 km. From the measured slip rate and the historic seismicity of the fault, we estimate that the Palu fault currently has stored enough strain to produce a M-w>7 earthquake. The Palu and other nearby faults accommodate rapid clockwise rotation of nearly 4 degrees/Ma of E Sulawesi relative to eastern Sunda. The rotation of east Sulawesi transfers E-W shortening between the Pacific and Eurasian plates to NS subduction of the Celebes Basin beneath Sulawesi.