Comment on ‘Evidence for a large strike-slip component during the 1960 Chilean earthquake’ by H. Kanamori, L. Rivera, and S. Lambotte

Summary Based on numerous studies of the relevant geodetic data, a low-angle thrusting mechanism has been assigned to the 1960 Chile earthquake. Kanamori, Rivera, and Lambotte recently suggested that a component of dextral slip comparable to the thrusting be included in the mechanism to satisfy long-period, teleseismic observations. The absence of geodetic evidence for that huge strike-slip component is the subject of this comment. The geodetic data are largely measurements of coseismic uplift associated with the earthquake but include 8 measurements of the coseismic change in shear strain. Because strike slip produces relatively little uplift except near the end points of the rupture, identification of that strike-slip component in the geodetic data depends upon the measured, shear-strain change. I consider elastic, half-space models of oblique slip on the plate interface possibly supplemented by simultaneous dextral slip on the nearby, intra-arc Liquiñe-Ofqui Fault Zone. Slip is assumed to be uniform along strike. The best fits to the geodetic data for these models furnish little evidence for strike slip on those structures. To satisfy the long-period, teleseismic data, Kanamori et al. proposed 6 examples, each of which requires a large amount of dextral slip. Because the long-period, teleseismic data do not define the slip distributions, I have used the best fits of those examples to the geodetic data to define those distributions. The large thrusting near the deformation front required by those slip distributions implies large uplift there, contrary to the uplift inferred from the inversion of tsunami data. However, an acceptable fit to the geodetic data and the tsunami data for the 6 examples suggested by Kanamori et al. can be obtained if the seismic moments specified by them are reduced by a factor ∼1.8, a factor within the uncertainties in estimating seismic moments of the 1960 Chile earthquake. The presence of strike slip in those reduced-moment examples despite the lack of geodetic evidence for strike slip is due to a remarkable coincidence that requires careful balancing of contributions from the shallower (depths < 70 km) coseismic sources against those from the deeper coseismic sources to nullify the geodetic evidence for strike slip. Such balancing is possible, but it is remarkable that the balancing is so nearly perfect that it nullifies the geodetic evidence for strike slip and thereby confounds the interpretation of the geodetic data.

Large earthquakes, mean sea level, and tsunamis along the Pacific Coast of Mexico and Central America

abstract Along the Pacific Coast of Mexico and Central America, 26 local tsunamis have been reported during the period 1732 to 1973. Nine of these were caused by earthquakes with teleseismic hypocenters, all of which were located well inland. If these epicenters were correct, these earthquakes could not have generated tsunamis. Under the assumption that the true epicenters must have been located at the coast or off shore, it was estimated that teleseismic hypocenters in this area are mislocated by about 75 km toward the northeast, and 20 km toward greater depth. We propose that most teleseismic locations in this area are afflicted by this same error. The most likely cause for the mislocations are shorter than expected travel times for rays in the down-dip direction of the subducted lithospheric slab. These rays travel to North American stations which contribute strongly to hypocenter locations in Middle America. The annual mean sea level of 13 tide gauge stations along the Pacific coast of Mexico and Central America were examined for evidence of vertical crustal deformation changes that could have been associated with earthquakes along this coast. Only one coseismic change could be identified in the annual mean sea level data. It occurred at Acapulco, Mexico, during the 11 May (Ms = 7.0) and 19 May (Ms = 7.2) 1962 earthquakes. The crustal uplift was about 22 cm, estimated from the difference of the 10-yr sea level means before and after the events. By comparing annual mean with daily mean sea level data, it appears that about 23 per cent of the permanent uplift observed at Acapulco was due to aseismic slip or aftershocks in this area. If tide gauge data in this area are kept current, long-term precursory crustal movements might be detectable if they exceed several centimeters.