Intermediate Depth Earthquakes: Investigations of the Predominance and Cause of Subhorizontal Faults

Saint Louis University, Saint Louis MO

Investigators

Abstract

Deep earthquakes were discovered nearly 100 years ago, but how they can occur outside the normal pressure-temperature conditions for brittle failure remains a mystery. Following up on recent observations that earthquakes from ~75-300 km depth often occur along subhorizontal faults, we have developed new hypotheses and tests to investigate the predominance and cause of these faults. Do outer rise faults remain weak as the slab bends and subducts? Alternatively, are the fossil outer rise faults strong because they have healed? Does the stress field in the bending slab result in downward pumping of water that initiates earthquakes? Do sustained fault zones exist? Is rupture propagation an isobaric process? Different seismic observations would support or refute each of these hypotheses, and we have selected two study areas, Japan and Colombia, with features and regional data sets ideal for answering these questions. In Japan, we are using the broadband F-Net records to analyze the directivity of ~500 earthquakes down to magnitude ~4.5. In Colombia, we are collaborating with Professor German Prieto of the Universidad de los Andes to analyze the Cauca cluster of ~2000 earthquakes through relocation and source characterization. These analyses will advance understanding of a long-unresolved question in the Earth sciences. In the process, the participation of a graduate student, an undergraduate student, and an early-career scientist will promote teaching and training, and the international collaboration will enhance the infrastructure for research and education.