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Prawirodirdjo, L, McCaffrey R, Chadwell CD, Bock Y, Subarya C.  2010.  Geodetic observations of an earthquake cycle at the Sumatra subduction zone: Role of interseismic strain segmentation. Journal of Geophysical Research-Solid Earth. 115   10.1029/2008jb006139   AbstractWebsite

We use survey mode and continuous GPS data from 1991 to 2007 to examine fault segmentation in the earthquake cycle at the Sumatra megathrust, site of the 26 December 2004 M(w) 9.1 Sumatra-Andaman, the 28 March 2005 M(w) 8.7 Nias-Simeulue, and the 12 September 2007 M(w) 8.4 Mentawai earthquakes. These data, including new observations from 2006 and 2007, allow us to observe the final few years of one earthquake cycle and the beginning of the next. Our analysis reveals that the megathrust is segmented, a characteristic that may persist through multiple earthquake cycles. The Nias-Simeulue earthquake ruptured approximately the same region that broke in 1861, a 300 km long segment abutting the Sumatra-Andaman rupture zone. Farther southeast, the Mentawai segment of the megathrust (0.5 degrees S-5 degrees S), which produced M > 8 earthquakes in 1797 and 1833, is fully locked in the interseismic period but is flanked by two freely slipping regions, the Batu Islands in the NW and Enggano in the SE. The 12 September 2007 Mentawai earthquake sequence ruptured only the southern one third of the 1833 rupture zone. We model postseismic deformation from the Sumatra-Andaman and Nias-Simeulue earthquakes and find that afterslip was concentrated updip and downdip, respectively, from the main shocks. Comparing the velocity fields before and after 2001, we find the subduction zone underneath the Batu Islands and Enggano, which, prior to the earthquakes, was partially to fully coupled, appears now to be slipping freely. Thus, while the segmentation of the subduction zone is preserved, interseismic coupling on the subduction fault may vary with time.

Gagnon, K, Chadwell CD, Norabuena E.  2005.  Measuring the onset of locking in the Peru-Chile trench with GPS and acoustic measurements. Nature. 434:205-208.   10.1038/nature03412   AbstractWebsite

The subduction zone off the west coast of South America marks the convergence of the oceanic Nazca plate and the continental South America plate. Nazca - South America convergence over the past 23 million years has created the 6-km-deep Peru - Chile trench, 150 km offshore. High pressure between the plates creates a locked zone, leading to deformation of the overriding plate. The surface area of this locked zone is thought to control the magnitude of co-seismic release and is limited by pressure, temperature, sediment type and fluid content(1). Here we present seafloor deformation data from the submerged South America plate obtained from a combination of Global Positioning System (GPS) receivers and acoustic transponders. We estimate that the measured horizontal surface motion perpendicular to the trench is consistent with a model having no slip along the thrust fault between 2 and 40 km depth. A tsunami in 1996, 200 km north of our site, was interpreted as being the result of an anomalously shallow interplate earthquake(2). Seismic coupling at shallow depths, such as we observe, may explain why co-seismic events in the Peruvian subduction zone create large tsunamis.

Chadwell, CD, Bock Y.  2001.  Direct estimation of absolute precipitable water in oceanic regions by GPS tracking of a coastal buoy. Geophysical Research Letters. 28:3701-3704.   10.1029/2001gl013280   AbstractWebsite

A buoy-based CPS receiver and meteorological sensor are used to estimate directly the absolute precipitable water (PW) overlying a coastal ocean site 8 km from shore. During an 11-day experiment, one-second CPS data collected at the buoy and at a shore station are combined with 30-second data from four distant CPS stations to estimate the buoy position, zenith wet delay, phase biases, and receiver and satellite clocks using double-differenced phase processing with ambiguity resolution. GPS-derived PW at the buoy compared to radiosonde measurements (20) and to half-hourly GPS-PW values (384) from the nearby shore station show an rms agreement of +/-1.5 mm and +/-1.8 mm, respectively. Hourly means (170) of the GPS-measured vertical motion of the buoy show a +/- 24 mm rms agreement with a NOAA tide gauge, equivalent to about 4 mm of PW. GPS-derived PW from buoys may have the potential to improve weather forecasting, calibration of satellite-based sensors, and climate studies.

Chadwell, D, Spiess F, Hildebrand J, Young L, Purcell, George J, Dragert H.  1998.  Deep-sea geodesy; monitoring the ocean floor. GPS World. 9:44-50,52-55., Eugene, OR, United States (USA): Aster Pub. Corp., Eugene, OR AbstractWebsite
Chadwell, CD, Spiess FN, Hildebrand JA, Young LE, Purcell, George J, Dragert H, Segawa J, Fujimoto H, Okubo S.  1997.  Sea floor strain measurement using GPS and acoustics. International Association of Geodesy Symposia. 117:682-689., New York, NY, International (III): Springer-Verlag, New York, NY AbstractWebsite