Lithospheric Structure of the Ecuadorian Orogenic System and Event Location using the Seismoacoustic Wave Field

Abstract

Seismologists use the seismic wavefield to image the Earth’s structure at a wide range of scales, from a few meters to 1000s of km. Sources (earthquakes, explosions, etc.) of seismic waves can also be located and distinguished using the seismic wavefield. In this dissertation, I utilize both of these aspects of seismology. The major part of this dissertation focuses on the use of naturally occurring seismic sources (earthquakes) to elucidate the structure of the crust and upper mantle beneath the Ecuadorian orogenic system (Appendices A-C). In the final section, I explore the seismic location problem by combining seismic and infrasound phenomena in a Bayesian framework (Appendix D). Ecuador, the focus of the first three studies, is a complex tectonic region spanning several tectonic provinces. Offshore, the Nazca plate subducts beneath the South American plate creating major stresses that build up and result in megathrust earthquakes along the boundary between the two plates. Following a magnitude 7.8 earthquake offshore Pedernales, Ecuador in 2016, seismic instruments were deployed to study the seismicity and tectonics of the region. This collaboration between US institutions (University of Arizona and Lehigh University) and the Instituto Geofísico at the Escuela Politécnica Nacional in Ecuador also opened up a wealth of data from the Ecuadorian permanent seismic network which enabled a higher resolution study of the arc region. Appendix A presents a detailed study of the tectonics of the forearc region and the relationship with the megathrust behavior. The results indicate that the complex accretionary history of Ecuador resulted in a forearc that exhibits significant variations in the seismic velocities along the strike of the trench. These variations appear to align with the style and behavior of the seismicity in the region, suggesting that the structure of the upper plate may play an important role in controlling megathrust behavior. Appendix B shifts the focus towards the Andean region and the active volcanic arc. The Ecuadorian Andes contain a broad (~150 km wide), active, arc that extends from the Western Cordillera into the Subandean zone. Here, a map of crustal thickness beneath the Ecuadorian Andes is presented, which shows that it is largely in isostatic equilibrium at the Moho. Observed low-velocity regions are beneath several active volcanoes are interpreted as regions of long-term magma storage, consistent with crystal mush zones. To connect the arc and forearc, earthquake-generated surface waves and the Automated Surface Wave Phase Velocity Measuring System are used to measure phase velocities in Ecuador. Appendix C reports on the results of this method. Periods between 25-50 seconds show good coverage across the array and image a faster forearc region and a slower arc region, likely reflecting a thicker crust in the arc region. At periods ≥ 60 seconds coverage is limited to the arc region where a longer period of data was available. These results serve to extend the phase velocity measurements from ambient noise deeper and begin to offer constraints on the upper mantle beneath Ecuador. As more data and more stations are deployed in Ecuador it may be beneficial to revisit this analysis at a later time. In the final Appendix, the focus shifts from lithospheric structure to explore the event location problem. Here, we combine seismic and infrasound observations for locating a seismoacoustic event. A Bayesian framework is developed to better estimate the uncertainty associated with the location. This new method is tested on data from a surface explosion from the Bingham mine in Utah and shows that combining the two phenomena can improve the location beyond what either method can obtain individually.