Using low-frequency earthquakes as a fault probe in Guerrero, Mexico

Low-frequency earthquakes (LFEs) are the smallest (in magnitude and duration) member of a newly discovered class of phenomena called slow earthquakes that also includes slow-slip events (SSEs) and non-volcanic or tectonic tremor, all of whom occur downdip of the seismogenic zone where classic earthquakes are observed. Small-scale frictional instabilities in the vicinity of the subduction interface generate LFEs. Their most distinctive feature is that each LFE source activates very frequently, generating bursts of seismicity made up of event multiplets. These multiplets can then be detected by using one of the events as a template and stacked to obtain waveforms with high signal-to-noise ratios. Clearly distinguishable impulsive arrivals of the P- and the S-waves can then be used to accurately locate and determine the focal mechanisms of LFEs. The subduction zone of Guerrero, Mexico, the study region of this thesis, is a seismic gap where the tectonic strain from plate convergence is not fully accommodated by seismicity within the seismogenic zone and is partially released by aseismic slip within the slow earthquake source region. I analyze the data from a dense linear seismic network operated that in Guerrero during 2.5 years to investigate the slip processes along the Mexican subduction interface using LFEs as a fault probe. I report the first observations of LFEs in Guerrero, Mexico and demonstrate that each LFE represents a release of tectonic stress along the subduction interface. After developing a new automatic template detection algorithm, the full analysis of the 2.5 year-long continuous records produces an unprecedented LFE catalog containing almost two million events spread over 1120 sources. I distinguish two distinct regions of LFE activity: (1) at the downdip extent of the SSE source region where LFEs primarily occur during strong SSEs; (2) further downdip in the "sweet spot", previously identified by near-continuous TT activity. I divide the LFE catalog into subcatalogs representing different portions of the subduction interface and investigate variations of their occurrence. I show that the subcatalog behavior during a strong SSE can be explained by an along-fault updip migration of fluids, which are released by the metamorphic slab dehydration in the vicinity of the sweet spot. I also identified anthropogenically-induced (AI) superficial repeating seismic events. The AI activity significantly increases during a strong SSE that I suggest represents a transient perturbation of the effective normal stress close to the surface due to volumetric extensional deformation that accompanies the slow slip on the subduction interface.