Great earthquakes and subduction along the Peru trench

Subduction along the Peru trench, between 9 and 15° S, involves both large interplate underthrusting earthquakes and intraplate normal-fault earthquakes. The four largest earthquakes along the Peru trench are, from north to south, the 1970 (M_w = 7.9) intraplate normal-fault earthquake, and the interplate underthrusting earthquakes in 1966 (M_w = 8.0), 1940 (M = 8) and 1974 (M_w = 8.0). We have studied the rupture process of these earthquakes and can locate spatial concentrations of moment release through directivity analysis of source-time functions deconvolved from long-period P-wave seismograms. The 1966 earthquake has a source duration of 45 s with most of the moment release concentrated near the epicenter. Two intraplate normal-fault events occurred in 1963 (M_s = 6.7 and 7.0), at the down-dip edge of the 1966 dominant asperity. The 1940 earthquake is an underthrusting event with a simple source time function of 30 s duration that represents the rupture of a single asperity near the epicenter. The 1974 earthquake has a source duration of 45-50 s and two pulses of moment release. This earthquake has a bilateral rupture with the first pulse of moment release located northwest of the epicenter and the second pulse of moment release located southeast of the epicenter. Both pulses of moment release occur on the northern half of the aftershock area. The 1970 earthquake is one of the largest intraplate normal-fault earthquakes to occur in a subduction zone and has a moment release comparable with many large underthrusting events. The aftershocks for the 1970 earthquake form two distinct clusters, the smaller cluster near the epicenter has focal mechanisms characterized by down-dip tension but the second aftershock cluster, located 80 km southeast of the epicenter, has focal mechanisms characterized by down-dip compression. The P-waves for the main shock can be modeled as two sources with different focal mechanisms and depths similar to the two clusters of aftershocks. The first event has a down-dip tensional focal mechanism and is followed 40 s later by a distinct second event located 80 km southeast of the epicenter with a down-dip compressional focal mechanism and a somewhat shallower depth than the first event. The observable directivity indicates that the second source is located at the second cluster of aftershocks that have down-dip compressional focal mechanisms. The occurrence of both down-dip tensional and compressional focal mechanisms may be explained by extreme 'unbending' stresses associated with the anomalous slab geometry. The unusually large size of the 1970 earthquake may also be related to the subduction of the Mendaña fracture zone. The historic earthquake record along the Peru trench indicates that the previous event in 1746 was much larger than any of the three underthrusting earthquakes this century. The 1746 earthquake may have ruptured the entire segment in a multiple asperity earthquake. Thus, the mode of rupture along the Peru coast has changed between successive earthquake cycles.