The October 15, 1997 Punitaqui earthquake (Mw=7.1): a destructive event within the subducting Nazca plate in central Chile

Abstract

The 1943 Illapel seismic gap, central Chile (30–32°S), was partially reactivated in 1997–1998 by two distinct seismic clusters. On July 1997, a swarm of offshore earthquakes occurred on the northern part of the gap, along the coupled zone between Nazca and South American plates. Most of the focal mechanisms computed for these earthquakes show thrust faulting solutions. The July 1997 swarm was followed on October 15, 1997 by the Punitaqui main event (Mw=7.1), which destroyed the majority of adobe constructions in Punitaqui village and its environs. The main event focal mechanism indicates normal faulting with the more vertical plane considered as the active fault. This event is located inland at 68-km depth and it is assumed to be within the oceanic subducted plate, as are most of the more destructive Chilean seismic events. Aftershocks occurred mainly to the north of the Punitaqui mainshock location, in the central-eastern part of the Illapel seismic gap, but at shallower depths, with the two largest showing thrust focal mechanisms. The seismicity since 1964 has been relocated with a master event technique and a Joint Hypocenter Determination (JHD) algorithm, using teleseismic and regional data, along with aftershock data recorded by a temporary local seismic network and strong motion stations. These data show that the 1997 seismic clusters occurred at zones within the Illapel gap where low seismicity was observed during the considered time period. The analysis of P and T axis directions along the subduction zone, using the Harvard Centroid Moment Tensor solutions since 1977, shows that the oceanic slab is in a downdip extensional regime. In contrast, the Punitaqui mainshock is related to compression resulting from the flexure of the oceanic plate, which becomes subhorizontal at depths of about 100 km. Analog strong motion data of the Punitaqui main event show that the greatest accelerations are on the horizontal components. The highest amplitude spectra of the acceleration is in the frequency band 2.5–10 Hz, in agreement with the energy band responsible for the collapsed adobe constructions. The isoseismal map derived from the distribution of observed damage show that a high percentage of destruction is due to the proximity of the mainshock, the poor quality of adobe houses and probably local site amplification effects.