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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.

Phillips, KA, Chadwell CD, Hildebrand JA.  2008.  Vertical deformation measurements on the submerged south flank of Kilauea volcano, Hawai'i reveal seafloor motion associated with volcanic collapse. Journal of Geophysical Research-Solid Earth. 113   10.1029/2007jb005124   AbstractWebsite

A four-year seafloor geodetic study was conducted to measure vertical deformation of the submerged south flank of Kilauea volcano on the active Hilina slump. The Hilina slump is a site of significant deformation, major earthquakes with ground cracking and associated tsunami. A new technique was developed to measure vertical deformation on the seafloor using pressure sensors in campaign-style surveys. The data revealed the midslope bench of the offshore slump is uplifting at 9.0 +/- 2.4 cm/a, but transitions to no significant deformation on the outer bench and slump toe. Elastic half-space inverse modeling of these data show that the observed deformation can be fit by seaward slip of 28.1 +/- 7.3 cm/a extending from the East rift zone 27.0 +/- 0.5 km on a gently island-dipping decollement fault plane at a depth of approximately 7 km. Modeling suggests that the outer bench is not currently deforming. Because the majority of Kilauea is submerged, these offshore measurements are necessary to constrain the seaward extent of fault slip and the motion of the outer bench.