RUPTURE HISTORY OF THE GREAT BOLIVIAN EARTHQUAKE - SLAB INTERACTION WITH THE 660-KM DISCONTINUITY

Teleseismic body waves of the great Bolivian earthquake of June 9, 1994 are analyzed to determine the focal parameters and rupture history. Broadband seismograms reveal a complex rupture process: A small initial event (M(w) 7.2) was followed about 10 s later by a large moment release pulse of about 40 a duration. Focal mechanisms determined for the mainshock indicate normal faulting with one very shallow NE dipping plane. Azimuthal variation in body-wave displacement pulse widths suggest northward rupture. From master event and body-wave inversion, the main moment release is located 25-50 km NE Of the initiation point at about 650-km depth with only small depth variations between the initial and main event. This suggests that rupture was N directed on the near-horizontal plane. Because the slab along other parts of the Andean are at about 600-km depth dips steeply, a sub-horizontal plane may imply shearing perpendicular to slab dip. Downdip compression on a sub-horizontal plane would then imply that the slab does not penetrate the 660-km seismic discontinuity, but rather, is being sheared out to the NE, This interpretation is not unique as other scenarios are also possible. Such an event immediately above the 660-km discontinuity suggests massive deformation above the discontinuity with no smooth slab penetration into the fewer mantle.