BACKARC THRUST FAULTING AND TECTONIC UPLIFT ALONG THE CARIBBEAN SEA COAST DURING THE APRIL 22, 1991 COSTA-RICA EARTHQUAKE

Surface deformation and a tsunami accompanied the destructive April 22, 1991, Costa Rica-Panama earthquake (Ms=7.5). Along a 135 km stretch of Caribbean coast, coseismic uplift was measured between the lower and upper limits of sessile intertidal organisms stranded on coral reefs, the preearthquake and postearthquake high tide levels located from driftwood lines on beaches, and the preearthquake and postearthquake tide levels as pointed out by local residents, The nature and distribution of offshore vertical displacements were further constrained from analysis of measured run-up heights and reported arrival times of the tsunami. Uplift detected along the coast jumped, within 4 km, from zero to 157 cm near Limon and generally decreased over a distance of 70 km southward to the border with Panama. These data map an axis of uplift that intersects the coastal beach ridge just north of the port of Moin and runs offshore to the east and south roughly parallel to the coast. No surface faulting was found. The earthquake and tsunami were generated by backarc thrusting along faults that bound the north Panama deformed belt and dip from the Caribbean Sea beneath Costa Rica and northern Panama. Combined geodetic and seismological data indicate that the main rupture dips landward at an angle of about 30-degrees and is approximately 40 km wide and 80 km long. Dislocation models suggest 2.2 m of slip on a causative thrust fault striking between 105-degrees and 120-degrees. We estimate that the repeat time for this type of earthquake is 200 to 1100 years. The historical record and new isotopic data favor the middle of the range.