The unique Chilean earthquake of May 22, 1960 (Mw =9.5)

The 1960/05/22 (Mw =9.5) great Chile earthquake is almost unique in different aspects including the magnitude, foreshocks , rupture initiation, and rupture process. We analyzed the rupture area of the event using the newly developed measure of “trench parallel Bouguer anomaly, TPBA” and its relative, the “slab reduced trench parallel Bouguer anomaly, SR-TPBA.” The slip distribution of the event (Barrientos and Ward, 1990) includes two separate sets of trench-parallel patches mirrored along the north-south axis of the Longitudinal Valley, 73 degrees W, with negligible slip over the valley. There is a clear tendency for the eastern slip set to peak on the positive-SR-TPBA patches along the north-south Principal Cordillera zone. Major parts of the western set of asperities, to the south of 38.5 degrees S, experienced coseismic subsidence. This is in conflict with the well observed cases that asperity areas should show coseismic uplift. Another serious mismatch is the negative SR-TPBA of the region covering the western set of asperities, this area was expected to show positive-SR-TPBA. These two apparent inconsistencies are the keys to the unraveling of this very complicated and unique rupture process. Based on the SR-TPBA distribution, current seismicity, coseismic slip, vertical displacement measurements, and geophysical data on the elastic thickness of the Longitudinal Valley and the Principle Cordillera we suggest a new scenario for the rupture process of the earthquake. It is essential to separate the smaller shallow-slip area to the north of 38.5 degrees S from the rest of the shallow rupture (western slip set) area. The asperities in the much smaller northern part behave like other asperities in usual subduction zones, while to the south the situation is totally different and unique. The Longitudinal Valley has no role in the coupling, its elastic thickness is less than the depth of the subducting slab’s top. The thick root of the dense Principal Cordillera with positive SR-TPBA makes the initial stage of strong coupling, while neither the Longitudinal Valley nor the shallow forearc with negative SR-TPBA make enough coupling. Of the controlling factors on the bulging of the subducting slab are the location and strength of the coupling. The strong coupling under the Principal Cordillera causes bulging of the subducting slab. For majority of subduction zones the distance between the strong coupling and the outer-rise bulge is between 60 to 130 km. The distance between the asperities beneath the Principal Cordillera and the trench is about 250 km. No bulging is observable seaward of the trench corresponding to the rupture area of the 1960/05/22 earthquake. The bulging axis is created at the locations where the western set of asperities are observed, about 135 km from the eastern set of asperities along the Principal Cordillera. Several decades (or centuries ?) after each great rupture, the bulging is enough to perturb the delicate balance between the overriding and the subducting slab, start developing the strong coupling at location of western slip sets, and simultaneously impede transfer of shallow plate motion to the eastern (deeper) slip set. This scenario explains all observed phenomena.