Geological and seismological studies in the epicentral region of the major 1887 Sonoran Basin and Range Province earthquake: A project review

Abstract

A collaborative research project between Universidad Nacional Autónoma de México (UNAM) and Centro de Investigación Científica y de Educación Superior de Ensenada (CICESE) about the major 1887 earthquake in the Basin and Range Province of northeastern Sonora and the persisting background seismicity in its epicentral region yielded numerous results that are summarized in this paper. Three first-order range-bounding north-south normal faults (from south to north: Otates, Teras, and Pitáycachi) ruptured in this earthquake as evidenced by a multi-segment surface rupture with the maximum vertical displacement of 5.2 m and a length of 102 km, which is the longest extensional dip-slip surface rupture in the worldwide historical record. Empirical scaling relations between surface rupture length and moment magnitude for normal faults estimate a magnitude M_w of 7.5 ± 0.3 for this earthquake. The rupture was arrested in the north as well as in the south at major cross faults, and the rupture displacement profile tapers rapidly toward these faults. The 1887 rupture is characterized at the surface and at focal depth in the mid-to-lower crust by extensional dip slip on 55° to 72°W dipping faults. The rupture kinematics and epicenter of the 1887 earthquake can be inferred from the bilateral branching pattern observed along the trace of the Pitáycachi segment. The focal mechanisms and the inversion of slickenlines exposed on the 1887 surface rupture indicate a normal-fault-type tectonic strain field with an ENE-WSW oriented regional extension. The long-term geological slip rates of the faults that ruptured in 1887 is estimated at 0.02 to 0.08 mm/yr, and the average repeat time of 1887 size ruptures at 15 to 42 kyr. A regional seismic network, Red Sísmica del Noreste de Sonora (RESNES), was installed within this project in the epicentral region of the 1887 earthquake. The network consisted of nine digital autonomous seismological stations and operated from 2002 to 2011. Its recordings indicate ongoing microseismicity not only along the faults that ruptured in 1887, but also along two major Basin and Range Province normal faults located farther south, in the Granados-Huásabas region. In addition to the parametric data of the seismicity distribution, the high quality of the RESNES digital recordings permitted the characterization of the regional seismic attenuation and a spatial analysis of the seismic energy radiated by the recorded earthquakes. A model of the change in static Coulomb stress throughout this region resulting from the slip on the three individual segments of the 1887 rupture can explain the long-term seismicity pattern in the area. The model suggests that stress increases caused by singularities at the tips of the three rupture segments triggered seismicity in the step-over between the northern and central segments of the surface rupture, as well as in the Granados-Huásabas region, to the south of the documented surface rupture. Furthermore, the model explains the moderate earthquakes that occurred after the 1887 event in the region of the neighboring Fronteras fault. According to the model, these were the result of bending of the hanging-wall blocks of the Pitáycachi and Teras segments.