Despite recent important contributions to the completeness of earthquake catalogs, there are still significant gaps in our knowledge about the preinstrumental seismicity of the Iberian Peninsula. This is true even for the second half of the nineteenth century, in spite of the vast increase in the number of scientific institutions and newspapers in these decades. In the last extensive revision of the Iberian earthquake catalog (Rodríguez de la Torre, 1990a,b), the number of known earthquakes occurring between 1851 and 1900 increased from 620 to 2066 as a result of the systematic search conducted by Rodríguez de la Torre mainly using Spanish sources.

Relative to earlier earthquake catalogs, the new data enabled (Martínez Solares and Mezcua, 2002; Mezcua et al., 2004) to present a very much improved catalog of the earthquakes that occurred in the Iberian Peninsula and its vicinity, with a new assessment of epicenters and magnitudes of the major events, through application of the Bakun and Wentworth (B&W) method (Bakun and Wentworth, 1997, 1999; Bakun et al., 2003).

As Martínez Solares and Mezcua (2002) and Mezcua et al. (2004) pointed out, the B&W procedure was only applied to events for which there were enough intensity values. Although registered in this catalog, one major Iberian earthquake that is still waiting for epicenter and magnitude determinations is the 21 October 1880 (6:41 a.m.) earthquake.

The event was unknown until 1990, when it was discovered by Fernando Rodríguez de la Torre (1990a) in Spanish and Portuguese nineteenth century newspapers. With this new information revealed, the 1880 earthquake was included in the catalog of Martínez Solares and Mezcua (2002), which was subsequently transferred into the Spanish Catálogo Sísmico Nacional (Instituto Geográfico Nacional [IGN]) and the SHEEC (SHARE European Earthquake Catalogue, 1000–1899) (Stucchi et al., 2013) with an entry based on Rodríguez de la Torre’s compilation and comprising data from 27 localities. The search was later expanded (Rodríguez de la Torre, 1998) to eventually include macroseismic information from 44 Portuguese and Spanish cities, towns, and villages, although not all are related to the 21 October 1880 event.

Based on the distribution of the intensities, Rodríguez de la Torre (1990a) suggested an Atlantic location “far removed from the coast” and a “high magnitude,” a hypothesis he later reaffirmed (Rodríguez de la Torre, 1998). Martínez Solares and Mezcua (2002) also located the epicenter in the Atlantic Ocean at the approximate coordinates 41.0° N and 10.0° W (i.e., at the latitude of Porto), a location that was also adopted by the SHEEC. In both cases, however, no specific magnitude values were reported, and no calculations were presented to support these hypotheses.

We assume that the number and the spatial distribution of intensity values were not enough to allow the assignment of a reliable epicenter and magnitude to this earthquake through the application of any of the several methods for macroseismic locations now available. Thus, further advances in the knowledge of the macroseismic effects of that event are necessary to determine these source parameters. Finding such additional data and using it to determine the epicenter and the magnitude of the earthquake was the main purpose of our present work.