The Historical Seismicity of the Loreto Region, Baja California Peninsula, Mexico (1684–1878

Loreto, the earliest permanent Novohispanic settlement in the Californias, is located on the east coast of the Baja California peninsula between the oblique-divergent plate boundary in the Gulf of California and onshore crustal normal-fault segments with Quaternary activity belonging to the main gulf escarpment. This article documents earthquake intensity observations in the Loreto region during the Novohispanic and early Mexican periods (1684–1878) from primary historical sources and additionally includes (as Ⓔ Electronic Supplement S1) a descriptive earthquake catalog (1684–1908) covering the entire central and southern parts of the Baja California peninsula. A major earthquake in A.D. 1810 is described in detail in a previously unpublished document preserved at Archivo General de la Nación in Mexico City. The earthquake was destructive in Loreto and was followed by a tsunami. The damage area likely included San Ignacio, 200 km northwest of Loreto, and rockfalls were described from Loreto all along the main gulf escarpment to La Paz, 230 km to the south. The earthquake source was most likely the Farallon transform fault in the Gulf of California. Loreto was again razed in A.D. 1878 by an earthquake doublet that is documented by letters from eye witnesses and the report of a governmental rescue team from La Paz, published in contemporary Mexican and American newspapers. Because the reported damage is limited to Loreto, this was possibly a local crustal earthquake caused by a nearby fault of the main gulf escarpment such as the Loreto or Escondido fault. Based on the observed effects of the A.D. 1810 and 1878 earthquakes, Loreto has a significant seismic groundshaking hazard and is highly vulnerable to amplification of seismic ground shaking and related fissuring and soil liquefaction, which can be explained by its location on unconsolidated alluvium of the coastal plain and the associated high groundwater level.