abstract

A 15-station geodetic control network extending 10 km north and 40 km south of the Motagua fault in Guatemala was reobserved 2 yr after the 4 February 1976 MS = 7.5 earthquake. The initial first-order triangulation was observed in two parts: the Guatemala-Honduras border arc completed in 1935 and the Motagua valley arc completed in 1953. Both angles and distances were observed in the 1978 resurvey. The angle changes between the combined 1935 to 1953 survey and the 1978 resurvey were used to calculate earthquake-related fault slip on two dislocation models of the rupture surface. Left-lateral slip of 1.3 ± 0.1 m is calculated for a model fault with constant displacement from 0 to 15 km. For a model fault divided into three successively deeper sections, the calculated left-lateral slip is 0.8 ± 0.1 m near the surface, increasing to 2.1 ± 0.5 m in the intermediate 5- to 15-km section, and is undetermined at greater depths. Both models are consistent with the 0.9 to 1.1 m of accumulated slip and afterslip observed near the network by Bucknam et al. (1978). The second model is in better agreement with the 2 m of slip derived by Kanamori and Stewart (1978) from a long-period seismic surface wave estimate of the seismic moment.

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