In Search of the Source of the 1865-1866 Nicaraguan Earthquakes: Paleoseismic Data from the Cofradia Fault, Managua Graben (Nicaragua)

Abstract

In 1865-1866, a series of catastrophic earthquakes struck Managua, Nicaragua. We present new data supporting the view that these earthquakes were caused by the seismogenic Cofradia fault. The data were collected at three paleoseismological sites: La Vaqueria (central-northern area), El Cocal (central area), and Piedra Menuda (located at an antithetic strand of the southern fault segment). Coseismic evidence includes liquefaction features, offset layers, and colluvial wedges dated using radiocarbon ages and pottery fragments, providing relative cultural ages. The minimum event displacement observed at the central site (1 m) and the total length of the mapped geomorphological trace (39 km) are consistent with a maximum expected magnitude around 7. A minimum slip rate between 1.1 and 1.4 mm/yr is obtained from the new data, reinforcing earlier estimates. In line with paleoseismic chronology and compared with a Bayesian OxCal model of the stratigraphic sequence, at least three seismic events have occurred since A.D. 300, the last one occurring after A.D. 790 and the twentieth century. Thus, the earthquakes of 1865-1866 that resulted in surface alterations in the Tipitapa River probably correspond to the last paleoseismic event on the Cofradia fault. This assumption yields a range between 1511 and 1245 yr for the best estimated maximum recurrence interval between the ultimate and penultimate event.