Zonas sísmicas de Oaxaca, México: sismos máximos y tiempos de recurrencia para el periodo 1542-1988

Abstract

References on historical and recent earthquakes in Oaxaca City, or that have caused damage in other localities o"f the Oaxaca state from 1542 untill988, are reviewed. Macroseismic data are used to defme sets of similar earthquakes. On the basis of present knowledge about seismotectonics of the region, wesuggest the defmition of eight seismogenic zones. Data references and our interpretation are given in a catalogue. The 68 larger events (m>6.5, Ms>7.0) are assigned to each one of the eight seismographic zones. Main results are: - The occurrence of great earthquakes (Ms ~ 8.5) at the subduction zone, similar to that on March 29, 1787, is probable.- During the period 1928-1988 eleven earthquakes (Ms>7.0) occurred at the subduction zone, four of them in thc eastern flank and seven in the western flank. The period of activity was preceded by 56 years of quiescence (Ms>7.0). The focal region of the 1787 earthquake was followed by a seismic gap (Ms>7.0) of 141 years, untill928. - From February to October 1928, four subduction earthquakes (Ms>7.4) occurred, as well as two intraplate intermediate depth earthquakes (mb= 7.7) with epicenters at the north of the state. In January 1931 a shallow intraplate earthquake (Ms = 8.0) shaked the southern part of the state. This pattern of seismic energy released in a short period of time supports the fact that great earthquakes in Oaxaca are due to simultaneousrupture of more than one or two seismogenic units. - The "Huajuapan" and "Mixe" zones are affected by intermediate depth earthquakes with normal fault mechanism (60 ~ 60 ~ 120 km, mb ~ 7.0 - 7.7); their recurrence times appear very regular (95 ± 11 yr, and 80 ± 16 yr, respectively). - At the "Valle" zone both shallow and normal fault earthquakes (Ms ~ 8.0) occurred; their recurrence time is also very regular (82 ± 16 yr). At the "Mixteca Media" zone, located to the west of the "Valle" zone, earthquakes of the same nature probably also occur. If our hypothesis is correct, then the "Mixteca Media" would present today the highest seismic potential (recurrence time 156 ± 22 yr, last earthquake, 1854). - At the "Istmo" zone, the intermediate depth seismic activity reaches moderate magnitudes (mb ~ 6.5). The December 29, 1917 (Ms= 7.1) shallow earthquake presents a particular interest. Our relocation put its epicenter in the Gulf of Tehuantepec, at a seismic gap whose seismic potential is difficult to evaluate. - The analysis of seismic activity (mb>6.5, Ms>7.0) reported for the last 480 years, suggests that studies based in catalogues of the last 150 years may lead to erroneous conclusions on seismotectonics of the region, underestimates of the maximum earthquakes, and to defming of recurrence periods with insufficient information.