Source areas of the Acapulco-San Marcos, Mexico earthquakes of 1962 (M 7.1; 7.0) and 1957 (M 7.7), as constrained by tsunami and uplift records

Abstract

A reliable location of the rupture areas for the earthquake doublet of 11 May 1962 (M 7.1) and 19 May, 1962 (M 7.0), and the earthquake of 1957 (M 7.8) in the Acapulco-San Marcos region is essential for the assessment of the width and location of the strongly-coupled patch along the plate interface and for the evaluation of possible rupture scenarios of future earthquakes in the Guerrero seismic gap. We analyze tsunamis recorded by tide gauges at Acapulco and Salina Cruz. A re-examination of the Acapulco record shows permanent uplifts of 15±3 cm and 7±3 cm during the 11 May and 19 May 1962 earthquakes, respectively. No detectable permanent uplift was found for the 28 July 1957 event. To model the tsunami records we select appropriate focal mechanisms, initial rupture dimensions (L and W), and average dislocations on the fault. By trial and error, we optimize the dimension and location of the rupture area so that the synthetic tsunami fits the residual tsunami waveform. In the case of the 1962 events, the predicted uplift is fitted to the observed uplift. The surface projection of the rupture areas of 11 and 19 May 1962 earthquakes mostly lies onshore, NW and SE from Acapulco, respectively. The estimated uncertainty in the rupture length and its offshore extension is about 5 km. For the 1957 earthquake we model the tsunami at Acapulco as well as at Salina Cruz. The results suggest that NW limit of the 1957 rupture area was 30(±5) km SE of Acapulco and L = 90(±20) km. The offshore extension of the rupture was less than ~10 km.