Geologic setting, field survey and modeling of the Chimbote, northern Peru, tsunami of 21 February 1996

Whereas the coast of Peru south of 10 degrees S is historically accustomed to tsunamigenic earthquakes, the subduction zone north of 10 degrees S has been relatively quiet. On 21 February 1996 at 21:51 GMT (07:51 local time) a large, tsunamigenic earthquake (Harvard estimate M-w = 7.5) struck at 9.6 degrees S, 79.6 degrees W, approximately 130 km off the northern coast of Peru, north of the intersection of the Mendana fracture zone with the Peru- Chile trench. The likely mechanism inferred from seismic data is a low-angl e thrust consistent with subduction of the Nazca Plate beneath the South Am erican plate, with relatively slow rupture characteristics. Approximately o ne hour after the main shock, a damaging tsunami reached the Peruvian coast , resulting in twelve deaths. We report survey measurements, from 7.7 degre es S to 11 degrees S, on maximum runup (2-5 m, between 8 and 10 degrees S), maximum inundation distances, which exceeded 500 m, and tsunami sediment d eposition patterns. Observations and numerical simulations show that the hy drodynamic characteristics of this event resemble those of the 1992 Nicarag ua tsunami. Differences in climate, vegetation and population make these tw o tsunamis seem more different than they were. This 1996 Chimbote event was the first large (M-w > 7) subduction-zone (int erplate) earthquake between about 8 and 10 degrees S, in Peru, since the 17 th century, and bears resemblance to the 1960 (M-w 7.6) event at 6.8 degree s S. Together these two events are apparently the only large subduction-zon e earthquakes in northern Peru since 1619 (est. latitude 8 degrees S, est. M-w 7.8); these two tsunamis also each produced more fatalities than any ot her tsunami in Peru since the 18th century. We concur with PELAYO and WIENS (1990, 1992) that this subduction zone, in northern Peru, resembles others where the subduction zone is only weakly coupled, and convergence is large ly aseismic. Subduction-zone earthquakes, when they occur, are slow, common ly shallow, and originate far from shore (near the tip of the wedge). Thus they are weakly felt, and the ensuing tsunamis are unanticipated by local p opulations. Although perhaps a borderline case, the Chimbote tsunami clearl y is another wake-up example of a "tsunami earthquake."