Source characteristics of historic earthquakes along the central Chile subduction zone

We have analyzed four large to great historic earthquakes that occurred along the central Chile subduction zone from north to south on November 11, 1922 (M-s = 8.3), April, 1943 (M-s = 7.9), December 1, 1928 (M-s = 8.0) and January 25, 1939 (M-s = 7.8). Waveform modeling and P-wave first motions indicate that the 1922, 1928 and 1943 earthquakes are shallow and consistent with underthrusting of the Nazca Plate beneath the South American plate. In contrast, the 1939 earthquake is not an underthrusting event but rather a normal fault event within the down-going slab. The 1922 earthquake is by far the largest event with a complex source time function showing three pulses of moment release and a duration of 75 a. The 1943 earthquake has a simple source time function with one pulse of moment release and a duration of 24 s. This event had a local tsunami of 4 m and a far-field tsunami height in Japan of 10-30 cm. The 1928 earthquake also has a simple source time function with a duration of 28 s. The aftershocks and highest intensities are south of the epicenter indicating a southward rupture with most of the seismic moment release occurring 50-80 km south of the 1928 epicenter but still north of the adjacent 1939 earthquake region. The 1939 Chillan earthquake was not an underthrusting but rather a complex normal fault earthquake. Our preferred model is a normal fault mechanism at a depth of 80 to 100 km with two pulses of moment release and a total duration of approximately 60 s. The high intensities, lack of a tsunami, and inland location associated with the 1939 event are all consistent with an intraplate event within the down-going slab. The 1939 earthquake was clearly more destructive than the other similar size or larger events. This may in part be due to the intraplate nature of the event but also due to high amplification of the sites in the Central Valley of south central Chile. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.