All plate boundaries are divided into segments - pieces of fault that are distinct from one another, either separated by gaps or with different orientations. The maximum size of an earthquake on a fault system is controlled by the degree to which the propagating rupture can cross the boundaries between such segments. A large earthquake may rupture a whole segment of plate boundary, but a great earthquake usually ruptures more than one segment at once.Earthquakes offshore of Sumatra on December 26 2004 (MW=9.3) and March 28 2005 (MW=8.7) ruptured, respectively, 1200-1300 km and 300-400 km of the subduction boundary between the Indian-Australian plate and the Burman and Sumatra blocks. Rupture in the 2004 event started at the southern end of the fault segment, and propagated northwards. The observation that the slip did not propagate significantly southwards in December 2004, even though the magnitude of slip was high at the southern end of the rupture strongly suggests a barrier at that place. Maximum slip in the March 2005 earthquake occurred within ~100 km of the barrier between the 2004 and 2005 ruptures, confirming both the physical importance of the barrier, and the loading of the March 2005 rupture zone by the December 2004 earthquake. Cruise SO198-2, from Merak to Merak between 18 June and 01 August 2008 is the second of three cruises, funded by the Natural Environment Research Council (NERC), which will form a coherent set of geophysical observations in the source regions of the 2004 and 2005 great Sumatra earthquakes. This cruise collected seismic reflection (MCS) profiles at SB1 and SB2 with the following objectives:1. To image the geometry and nature of the downgoing slab from the trench to 30-40 km depth within the forearc2. To image faults within the over-riding plate responsible for the development of the accretionary wedge3. To provide a set of shots that will calibrate the array of ocean-bottom seismometers deployed on cruise SO198-1, and be recorded by the land seismometer array established by a different part of the consortium.Cruise SO198-2 also included nine days of ship time funded by the United States National Science Foundation to investigators from the University of Texas Institute for Geophysics (UTIG). This allied study targeted the subject of rupture pathways, with a focus was on how the earthquake rupture propagates updip through the accretionary prism to ultimately move the seafloor and create the tsunami.Approximately 5000km of multichannel seismic reflection data were collected during the cruise, as well as continuous recording of gravity, magnetics, Parasound and swath bathymetry data while in the permitted area
