Subduction zone segmentation and controls on earthquake rupture: The 2004 and 2005 Sumatra earthquakes - Seismicity and deep structure

Abstract

The first objective of this project was the installation of a dense, amphibious, temporary seismic along the central part of the Sumatran margin between April 2008 and February 2009 as part of the NERC funded consortium “Sumatra Segmentation”. Here we summarise the station distribution and performance of the network. Furthermore, local seismicity results are presented. For hypocentre locations, the data from this network were combined with OBS from an ocean bottom network (SONNE cruise SO198, University of Southampton) and with landstations from temporary networks adjacent north (“GFZ“) and south (“Aftershock study of the 2007 Sumatran earthquakes“, University Liverpool, University Cambridge) of the working area. Additionally, data from 8 permanent stations (BMG, GEOFON) were incorporated. In total we located 1,783 local events in a 11 month period. For 1,220 events in the central part of the area under investigation we were able to calculate local magnitudes with magnitudes Ml between 0.8 and 5.6, of which 860 events occurred within the crust (depths <20 km) along the Great Sumatran Fault (GSF). The high number of events on the mainland reflects the denser station distribution along the GSF where the station spacing was ~15 km. In the working area the Investigator Fracture Zone (IFZ) is subducted below the Sumatra. This topography appears to influence seismicity at all depth intervals. A well-defined linear streak of seismicity extending from 80 to 200 km depth is lying along the prolongation of closely spaced IFZ sub-ridges. More intermediate depth seismicity is located to the southeast this string of seismicity and is presumably related to subducted rough oceanic seafloor. The plate interface beneath Siberut Island which ruptured last in 1797 is characterised by almost complete absence of seismicity. The configuration of the network was designed such that the data captured by the seismic array will allows a detailed mapping of the velocity, attenuation and anisotropy structure through recordings of both earthquakes and airgun shots. The data from this network are archived at SeisUK and the IRIS-DMC with network code ZB.