UC Santa Cruz

Earthquake stress drop, strong ground shaking, recurrence interval and the rupture characteristics of large and great earthquakes are controlled by fault strength and the frictional properties of the plate interface. Monitoring the fault strength and its variability during the earthquake cycle is critical for earthquake hazard assessment. Subduction zones in particular, generate the largest and potentially more catastrophic earthquakes around the world. The rapid convergence (85-90 mm/yr) of the Cocos plate underneath the Caribbean plate along the Nicoya Peninsula, the northwestern margin of the Costa Rica subduction zone has generated M7+ earthquakes every 50 years (1853, 1900, 1950), with the last one of these events, a moment magnitude (Mw) 7.6 earthquake, occurring on 5 September 2012. Because of the advantageous location of the Nicoya Peninsula, extending seaward, over the seismogenic zone, a very dense seismic and geodetic network has recorded with incredible detail the megathrust activity throughout the last decade. Geodetic observations prior to the occurrence of the M7.6, 2012 earthquake, identified regions of strain accumulation that delineated the rupture area of this event. Regions of the plate interface up-dip and down-dip of this patch accommodate relative plate motions aseismically, hosting numerous Slow Slip events (SSEs) that have been well recorded since 2007, suggesting a partition of the plate interface into regions of frictionally stable and unstable or conditionally stable sliding behavior. This dissertation focuses on the study of fault zone heterogeneity and the frictional strength variability along the plate interface and upper plate of the Nicoya Peninsula subduction zone margin and its plausible relationship with the previously documented seismic and aseismic frictional slip behaviors.

The use of long-term cross-correlation of ambient seismic noise has become widely adopted at several seismological observatories for a) studying the susceptibility to crustal velocity variations along volcanic areas and the Earth’s subsurface due to transient events like volcanic eruptions, shallow earthquakes, landslides, etc., and b) seismic imaging. The use of random wave fields (coda waves or ambient seismic noise) for seismic imaging has proved to overcome certain limitations so far presented by the traditional tomography methods. This dissertation extends the use of the long-term cross-correlation of ambient noise vibrations to monitor variations in elastic properties and fault heterogeneity located along the overriding plate; before, during and after the 5 September, 2012 M7.6, Nicoya Peninsula megathrust earthquake. This large event was followed by thousands of aftershocks spatially localized in four clusters distributed along strike, as well as some geodetically detected post-seismic slip.

Earthquake focal mechanisms for nearly 300 of the Nicoya earthquake aftershocks were estimated using regional full waveform moment tensor inversions and P wave first motion polarities. The finite fault model of this large event generated by Liu et al., [2015] was used to compute the Coulomb static stress change imposed by the main shock on the nodal planes of the aftershock sequence in order to make a first evaluation of the static versus dynamic earthquake triggering hypotheses.

In addition, a total of 5 repeating earthquake families, earthquakes with nearly identical waveforms and thought to represent the rupture of the same fault asperity at a relatively constant repeat interval due to continuous loading by surrounding aseismic slip, were recognized using waveform matching techniques from 2007 through 2016. These earthquake families locate along the up-dip margin of the fault that ruptured during 5 September 2012, Mw 7.6 Nicoya megathrust. Due to the repeating nature of these events they do sense the temporal changes in the physical properties of the medium, including variations in the frictional strength of the plate interface. This dissertation extends the use of repeating earthquakes to derive stress drop or fault yield strength before and after the occurrence of the Nicoya peninsula earthquake.