Also, work by Prawirodirdjo et al. [GRL, in press, 1997] provided geodetic insights into the deformation and segmentation along the Sumatra subduction zone, suggesting an abrupt change in plate coupling coincident with the boundary between the rupture zones of the 1833 and 1861 thrust earthquakes. Their subsequent modeling assigned the geodetic coupling parameter (similar to seismic coupling parameter) of 1.0 to the 1833 rupture zone and 0.4 to the 1861 zone.
With over 10 more years of Harvard CMT solutions and improved understanding of the mechanics of oblique subduction [Dmowska et al., JGR, 1996, Taylor et al., JGR, submitted, 1997] we look at the Sumatra subduction segment in an attempt to assess new clues to its current seismic potential.
The additional 10 years of seismic data confirms the observation by Newcomb and McCann (1987) that the seismicity in the interplate-interface stays in place, suggesting possible permanence of seismotectonic heterogeneities. The well known barrier between the 1833 and 1861 rupture zones is seismically well visible and marked by some tensional/or more complex events. There is a seismically striking difference in event rate observed between the 1833 rupture zone and the area south of it, suggesting much less coupling in the south. However, by measures of current seismicity, there is no discernible difference between the 1833 and 1861 zones (as opposed to the strongly different strain accumulation in those zones as measured by GPS, Prawirodirdjo et al., 1997). The cumulative seismic moment release, as measured from 1964, in both rupture zones increases with time; such behavior is sometimes observed and interpreted as an intermediate-term precursor (e.g. Alaska-Aleutians, Bufe et al., 1994).
Possibly interesting seismic behavior is observed along the Sumatra
fault, a well-pronounced right-lateral strike-slip fault system that runs
in the upper plate parallel to the subduction zone and releases part of
the oblique plate motion in the area. Viewing the data from the last 30
years, it can be noted that seismicity along the Sumatra fault in 1964-1976
is not pronounced (typical mb is less than 5.4, with 1 event
with mb =5.8) but that it increases in the last 20 years (1977
to present) with events reaching Mw = 6.7 (in 1995). It seems
that the Sumatra fault is currently more active and the seismic energy
release is higher than before. We discuss questions concerning the possible
relation of the strike-slip seismicity in the upper plate to subduction
ruptures for this as well as other areas in the world (Andreanof Islands,
Aleutians; Biak, Indonesia; Kobe, Japan).