The 1960 Chile earthquake: inversion for slip distribution from surface deformation

Abstract

SUMMARY A total of 166 observations of sea-level change, 130 measurements of elevation difference, and 16 determinations of horizontal strain provide an excellent view of the (quasi-)static source process of the great 1960 Chilean earthquake. These surface deformation data were employed in classical uniform slip fault models as well as more recently developed models that allow spatial variability of slip. The best uniform slip planar (USP) model is 850km long, 130km wide, and dips 20°. Seventeen metres of fault displacement contributed to a USP moment of 9.4 times 1022 N m. The variable slip planar (VSP) model concentrates slip on a 900 km long, 150 km wide band parallel to the coast. Several peaks of slip with dimensions of 50–100 km appear in this band and are thought to represent major subduction zone asperities. Important fractures of the oceanic lithosphere bound the 1960 rupture and are offered as a potential source of fault segmentation within the Chilean subduction zone. The VSP moment for 1960 earthquake totals 9.5 times 1022 N m, about one fifth of the value estimated for the foreshock-mainshock sequence from seismic methods. Except for areas out to sea, geodetic resolution on the fault is fairly uniform. Thus, it is unlikely that slip missed by the network could increase the VSP moment much beyond 1.8 times 1023 N m. Several patches of moment, isolated from the main body at 80–110 km depth, are found down dip in the VSP model and are presumably indicative of aseismic slip. One patch at the northern end of the rupture is probably associated with the initiation phase of the mainshock, although the time sequence of the relationship is unknown. Tide gauge records suggest that another patch between 40° and 43° S, responsible for the observed strain and uplifts inland at those latitudes, is not of coseismic origin, but derives from in-place, post-seismic creep over several years. Apparently, great 1960-type events are not typical members of the ∼ 128 yr earthquake cycle in south-central Chile. The Nazca-South America boundary here is characterized by a variable rupture mode in which major asperities are completely broken by great earthquakes only once in four or five earthquake cycles. The more frequent large earthquakes, that geographically overlap the great events, fill in between the locked zones.