Body wave inversion of the 1970 and 1963 South American large deep-focus earthquakes

摘要

A comprehensive set of teleseismic waveforms from two South American deep-focus earthquakes of the predigital era, the 1970 Colombia (M_w = 8.1) and 1963 Peru-Bolivia (M_w = 7.7) events, are inverted for source mechanism, seismic moment, rupture history, and centroid depth. The P and SH wave inversion of the Colombia event confirms previous work, indicating that rupture occurred on a plane that dips steeply west. Rupture direction paralleled the trend of the Wadati-Benioff zone. We decompose the source into subevents, based on a source time function which shows two major moment release pulses separated by approx 20 s. The first subevent is located near the initiation point at a depth of approx 630 km. The main moment release was located approx 70 km to the southeast and approx 20 km shallower. Rupture subsequently propagated farther southeast. The source time function has an initial subevent accounting for approx 30% of the moment release of the entire event, whereas the long-period centroid moment tensor (CMT) analysis [Russakoff et al., 1997] has the initial subevent yielding approx 50%. The high-angle nodal plane rotated approx 15 deg clockwise during the rupture, explaining the large compensated linear vector dipole (CLVD) component inferred from CMT solutions. Individual subevents have large CLVD and compressive isotropic components. A full moment tensor inversion of the Colombia and 1994 Bolivia events suggests that the initial subevents might contain a large non-double-couple (NDC) component. For the 1963 Peru-Bolivia event, using P waves, rupture propagated NNW for a distance of approx 70 km, parallel to the high-angle nodal plane and the trend of the Wadati-Benioff zone. The focal mechanism changed dramatically after the second subevent, causing a very large NDC component. Both events, together with the 1994 Bolivia earthquake, have a precursor separated in space and time from the main rupture and show rupture velocities varying between 3 and 4 km/s between subevents, with <2.0 km/s on average for the entire event. Low seismic efficiencies and rupture velocities support a highly dissipative, temperature-dependent rupture mechanism for large deep-focus South American earthquakes, compared with events in cold subducting slabs.