THE MECHANISM OF THE DEEP BOLIVIA EARTHQUAKE OF JUNE 9, 1994

We analyzed the IRIS broad-band seismograms for the June 9, 1994, Bolivia earthquake, the largest deep-focus earthquake ever recorded. We inverted the record sections with a duration of 330 sec which include P, pP and sP phases, and obtained a dip-slip mechanism: (dip(delta), rake(lambda), strike(phi)) = (13-degrees, -4-degrees, 1-degree) or (89-degrees, -103-degrees, 95-degrees), with a scalar moment of Mo = 2.9 x 10(21) Nm (Mw = 8.2). This mechanism is similar to that obtained from long-period (175 to 250 sec) Rayleigh and Love waves: (delta, lambda, phi) = (83-degrees, -100-degrees, 90-degrees); Mo = 3.0 x 10(21) Nm (Mw = 8.3). The main rupture was preceded by about 10 sec by a cluster of smaller events with a total moment of 1.2 x 10(20) Nm (Mw = 7.3). The main rupture starts at about 20 km east or northeast of the initial break, and propagates horizontally in the east to northeast direction. The source duration is 40 sec, and the rupture dimension is about 40km. If a fault area of 40x40km2 is assumed, the stress drop is 110 MPa. An inversion of body waves (P, pP, and sP) with 6 basis moment tensors yielded a best-fit solution with a 10% implosive component. This result is of marginal significance and its confirmation would require more detailed information on the structures near the source region, pP bounce points and stations. The observed amplitude of the radial mode, 0S0, (1226 sec) is consistent with a double couple model with less than 2 % isotropic component.