Rupture history of the Great Bolivian Earthquake: Slab interaction with the 660‐km discontinuity?

Abstract

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.