RV Sonne Cruise 198-1, 03 May-14 Jun 2008. Singapore - Merak, Indonesia

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Description/Abstract

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 26th 2004 (MW=9.3) and March 28th 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 SO198-1, from Singapore to Merak between 3rd May and 14th June 2008 is the first 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. Arrays of 50 ocean-bottom seismometers (OBS) were deployed at each
of two locations – between the 2004 and 2005 ruptures, and at the southern end of the 2005
rupture - to record shots from a large-capacity airgun array. Approximately 7 days of
continuous airgun shooting at 60s interval was completed at each location. 10 OBS were
reconfigured for earthquake recording and deployed with a planned retrieval in early 2009.
Gravity, Parasound, and swath bathymetry data were recorded continuously while in the
permitted area, with magnetic field data recorded throughout the airgun shooting, and 101 XBT
casts taken at the OBS deployment locations.