and Navigation Group at DLR’s German Space Operations

member of the Executive and Governing Board of th

Practical formulas for the refraction coefficient

Knowledge of the actual refraction coefficient is essential in leveling surveys and precise electromagnetic distance measurement reduction. The most common method followed by the surveyor for its determination is based on the use of simultaneous reciprocal zenith observations. The commonly used formula is only an approximation valid for approximately horizontal sightings, whereas the exact geometric solution turns out to be very complicated so that an iterative computation procedure is suggested instead. In the present paper, the goal is to derive a compact formula from the complete solution that is easy to implement and retains the necessary accuracy for horizontal and slanted sightings. In addition, the paper will also focus on the common situation for the surveyor where isolated observations have to be done and no partially compensating procedures—e.g., leap-frog or middle point—are possible. If temperature vertical profiles are unknown then the refraction coefficient cannot be reliably determined. Some surveyors may customarily use then an average value, e.g., k 5 0:13, perhaps being unaware of the risks involved in such simplistic assumption. In the present paper, it is also a goal to present a useful and simple formula for approximately estimating the refraction coefficient in terms of easily accessible parameters to correct the bulk of the refraction effect in single observations, always bearing in mind that determination of the refraction coefficient by means of a model may turn out to be somewhat inaccurate, but still better than the blind use of a universal k.The authors are grateful to the editor and the anonymous reviewers for their valuable suggestions, corrections, and comments that helped improve the original manuscript. This research is funded by the Spanish Ministry of Science and Innovation (Grant No. AYA2011-23232).Baselga Moreno, S.; García-Asenjo Villamayor, L.; Garrigues Talens, P. (2014). Practical formulas for the refraction coefficient. Journal of Surveying Engineering. 140(2):1-5. https://doi.org/10.1061/(ASCE)SU.1943-5428.0000124S15140

The potential of high-rate GPS for strong ground motion assessment

We show that high-rate GPS can have a vital role to play in near real-time monitoring of potentially destructive earthquakes. We do this by investigating the potential of GPS in recording strong ground motions from earthquakes in Switzerland and Japan. The study uses finite-fault stochastic ground motion simulation based on Fourier amplitude spectra and duration models previously developed for both countries, allowing comparisons in terms of both Fourier and time domain characteristics (here the Peak Ground Velocity, PGV). We find that earthquakes of magnitude Mw>5.8 can be expected to be recorded by GPS in real-time at 10 km distance, i.e. their Fourier spectrum exceeds the noise of the instruments enough to be used in strong motion seismology. Post-processing of GPS time series lowers the noise and can improve the minimum observable magnitude by 0.1-0.2. As GPS receivers can record at higher rates (> 10 sps), we investigate which sampling rate is sufficient to optimally record earthquake signals and conclude that a minimum sampling rate of 5 sps is recommended. This is driven by recording events at short distances (below 10 km for magnitude 6 events and below 30 km for magnitude 7 events). Furthermore, the Maximum Ground Velocity derived from GPS is compared to the actual PGV for synthetic signals from the stochastic simulations and the 2008 Mw=6.9 Iwate earthquake. The proposed model, confirmed by synthetic and empirical data, shows that a reliable estimate of PGV for events of about magnitude 7 and greater can be basically retrieved by GPS in real-time and could be included for instance in ShakeMaps for aiding post-event disaster management

Crustal Evolution of the Mid-Atlantic Ridge near the Fifteen-Twenty Fracture Zone in the last 5 Ma

Author Posting. © American Geophysical Union, 2003. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 4 (2003): 1024, doi:10.1029/2002GC000364.The Mid-Atlantic Ridge around the Fifteen-Twenty Fracture Zone is unique in that outcrops of lower crust and mantle rocks are extensive on both flanks of the axial valley walls over an unusually long distance along-axis, indicating a high ratio of tectonic to magmatic extension. On the basis of newly collected multibeam bathymetry, magnetic, and gravity data, we investigate crustal evolution of this unique section of the Mid-Atlantic Ridge over the last 5 Ma. The northern and southern edges of the study area, away from the fracture zone, contain long abyssal hills with small spacing and fault throw, well lineated and high-amplitude magnetic signals, and residual mantle Bouguer anomaly (RMBA) lows, all of which suggest relatively robust magmatic extension. In contrast, crust in two ridge segments immediately north of the fracture zone and two immediately to the south is characterized by rugged and blocky topography, by low-amplitude and discontinuous magnetization stripes, and by RMBA highs that imply thin crust throughout the last 5 Ma. Over these segments, morphology is typically asymmetric across the spreading axis, indicating significant tectonic thinning of crust caused by faults that have persistently dipped in only one direction. North of the fracture zone, however, megamullions are that thought to have formed by slip on long-lived normal faults are found on both ridge flanks at different ages and within the same spreading segment. This unusual partitioning of megamullions can be explained either by a ridge jump or by polarity reversal of the detachment fault following formation of the first megamullion.This work was completed while T. Fujiwara was a Guest Investigator at Woods Hole Oceanographic Institution with funding from Japan Marine Science and Technology Center (JAMSTEC), National Science Foundation, and the JAMSTEC Research Overseas Program. J. Lin’s contributions to this research were supported by NSF Grant OCE-9811924. B. E. Tucholke’s contributions were supported by NSF Grant OCE-9503561 and by the Andrew W. Mellon Endowment Fund for Innovative Research and the Henry Bryant Bigelow Chair at Woods Hole Oceanographic Institution