Variations in the mode of great earthquake rupture along the Central Peru Subduction Zone

The historic record for the central Peru subduction zone suggests significant variations in the earthquake size during the last 400 years. During this century there have been four great underthrusting earthquakes along the central Peru seismic zone. From north to south these are the 17 October 1966 (M_w=8.1), 24 May 1940 (M∼8), 3 October 1974 (M_w=8.1), and 24 August 1942 (M∼8.2) earthquakes. Modified Mercalli intensity data and tsunami observations for the earthquakes in this century are compared with the 29 October 1746 and 20 October 1687 earthquakes. The 1746 earthquake had maximum intensity values between 9° and 13°S while the 1687 event had maximum values between 12° and 14°S suggesting that the two events failed different segments of the subduction zone. We find that the 1746 event occurred along the segiment that includes both 1940 and 1966 earthquakes. The size of the 1746 event is estimated to be M_w ∼8.8 based on the ratio of near‐field tsunami heights for the 1746 and 1966 earthquakes. The 1687 earthquake probably raptured the 1974 segment as well as the adjacent segment to the south where there is at present a gap between the 1942 and 1974 rupture zones. The size of the 1687 event is estimated to be M_w∼8.7 based on both far‐field and near‐field tsunami height ratios of the 1687 and 1974 events. Both 1746 and 1687 earthquakes appear to be much larger than the events of this century. In contrast to the simple, single asperity nature of the 20^th century earthquakes, these older and larger events may represent multiple‐asperity ruptures along the Peru subduction zone. Hence, variations in the mode of earthquake rupture from cycle to cycle along the central Peru seismic zone may explain the significant difference in earthquake size during die last 400 years.