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 (Mw 7.2) was followed about 10 s later by a large moment release pulse of about 40 s 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 arc 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 lower mantle.