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Pacific Coast: Introduction
Because the Nazca plate is being subducted under the South American plate, the Pacific Coast of Colombia is an active seismic zone with a well-documented record of high-magnitude earthquakes. Seismic observations for the past century record at least three M 7.9 seismic events (1906, 1979, 1992), accompanied by co-seismic subsidence, soil liquefaction, and extensive landsliding that caused abundant river damming. The 1906 and 1979 events caused littoral co-seismic subsidence up to 1.5 m along the southern Pacific Coast between Buenaventura and the Mira delta. Both events triggered tsunami waves 2.5 m high that caused more than 3,500 deaths of inhabitants of the low coastal areas (Ramirez 1970; Ramírez and Goberna 1980; Herd et al. 1981; Meyer and Velásquez 1992).
Between Punta Ardita (at the Panamá border) and Cabo Corrientes (Fig. 2), the coastal zone corresponds to the western flanks of the Baudó Range (Serranía del Baudó), a 600-m-high region composed predominantly of oceanic basalts, diabases, and associated cherts and radiolarites (Duque-Caro 1990, Cossio 1994). The Baudó Range is a structurally controlled relief, densely fractured and faulted in the preferential directions NNE-SSE and N60ºE-N30ºW that control the general orientations of valleys and ridges.
South of the Baudó Range, the relief of the Pacific Coast is dominated by 20 to 100-m-high hills cut into Tertiary sedimentary sequences and Plio-Quaternary deposits at the piedmont of the Cordillera Occidental (González et al. 1988). Conspicuous (paleo-marine?) terraces between 20 and 30 m above present sea level are found along the hills between Cabo Corrientes and Buenaventura Bay (Fig. 2).
Coastal relief south of the Baudó Range abruptly limits the Holocene prisms of accretionary deposits that characterize the Pacific Coast of Colombia (Fig. 2). They include the three major Plio-Quaternary deltaic prisms of the San Juan, Patía, and Mira Rivers, and two narrow (<10 km wide) fringes of coalesced deltas between the Cabo Corrientes-Togoromá and Buenaventura-Guapi stretches of coast (West 1957, Martinez et al. 1995, Correa 1996). Holocene deposits along the narrow littoral fringe of the Pacific Coast form distinctive sets of contemporaneous barrier islands, extensive mangrove swamps and, from the San Juan delta to the Ecuador border, ancient barrier systems cut by "funnel-shaped" estuaries interconnected by a dense network of tidal channels. Extensive, highly unstable, barren sandy and muddy tidal flats are common at all the main tidal inlets of the Pacific Coast.
Climate of the Pacific Coast is humid tropical, dominated by the annual migration of the Intertropical Convergence Zone (ITCZ) and by high relief associated with the Cordillera Occidental. Mean daily temperatures are about 26ºC with minimum values of about 14ºC (West 1957, Eslava 1992). Annual rainfall reaches a maximum of about 10 m at the Buenaventura-San Juan River delta area, and decreases to the north and south to about 3 m/yr at the Panamá border and at the Tumaco-Mira River delta area (Eslava 1992). El Niño events regularly cause noticeable anomalies in rainfall, ocean-surface temperature, and sea level along the Pacific coast of Colombia. The 1997-1998 event was the most intense of the 20th century (IDEAM 2003).
Tides along the Pacific Coast are mixed semidiurnal, with mean amplitudes between 2 m at Tumaco and 4 m at Buenaventura. Maximum spring-tide amplitudes increase slightly from south to north, and vary between 3.5 m at Tumaco to 4.5 m at Buenaventura and Juradó (HIMAT 1992). Predominant winds along the Pacific Coast come from the south at the latitudes of the Baudó Range, and from the west to southwest along the central and southern sectors of the Pacific coast. Swell waves along the Pacific Coast are about 0.5 to 1.5 m high during calm periods, but can be as high as 2.5 to 3.5 m during strong wind periods. High waves are focused on the extensive subtidal and intertidal bars that front the major tidal inlets.
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