- The dissertation concentrates on seismicity of the Blanco Transform Fault (BTF), a 350-km-long oceanic transform fault (OTF) in the northeast Pacific Ocean. I analyze data collected during the Blanco Transform Fault OBS experiment, which comprised of a 1-year deployment of 55 three-component seismometers co-located with differential pressure gauges on and around the fault. The Blanco Transform Fault OBS experiment represents the second – and up to this day the largest – high- resolution seismic investigation of an oceanic transform fault and provides a unique dataset for studying seismicity and tectonics of the BTF.
The dissertation has six chapters. The first chapter provides a general introduction into OTF slip and tectonics and summarizes rationale for the Blanco Transform Fault study.
Chapter II presents a detailed analysis of seismicity and tectonics of the BTF. Using the OBS dataset, I determined precise hypocentral parameters of almost 8,000 seismic events occurring at the BTF during the observation period. The spatial distribution of seismicity reveals a complex structure of the BTF, with multiple segments of transform and extensional tectonics. The younger, western part of the Blanco fault (including West Blanco, East Blanco and Surveyor depressions) shows higher tectonic complexity than the eastern BTF. Seismicity occurs predominantly in the lower crust and suggests that the lithospheric thermal structure is relatively unaffected by hydrothermal circulation. In contrast, seismicity in the central and eastern Blanco (including Cascadia depression, Blanco Ridge fault and Gorda depression) is deeper, reaching the uppermost mantle and suggests significant hydrothermal cooling. The frequency-magnitude distribution of earthquakes together with earthquake stress-drops reflect the variable rheology along the fault. A slip deficit is indicated along the most of the BTF, which suggests that a large portion of the slip is released aseismically.
Chapter III reports on interactions between the seismic and aseismic slip at the Blanco Ridge fault. The Blanco Ridge fault represents the longest continuous transform fault segment of the BTF and hosts the strongest BTF earthquakes. Using the seismicity catalogue enhanced with small earthquakes by the cross-correlation technique, I analyzed the spatial and temporal distribution of earthquakes at the BR. Based on the local seismicity and existing regional and teleseismic observations, I formulated a new, detailed model of the mode of slip at oceanic transform faults. In the model, the mantle part of the fault, shallower than the depth of the 600°C isotherm, creeps partially and episodically, while loading the brittle and fully seismically coupled crustal part of the fault. The mantle creep activates small asperities in the mantle, producing seismic swarms. Large-magnitude earthquakes, slipping both the crustal and the mantle part of the fault, are preceded by an episode of the shallow mantle creep that is accompanied by seismic swarms. The seismic swarms then appear as foreshocks of large earthquakes and explain observations of foreshock sequences at OTFs.
Chapter IV describes analysis of the correlation integral for identification of characteristic patterns in the spatial and temporal distribution of earthquakes, which was used for identification of earthquake swarms at the BR fault (Chapter III). The correlation integral displays seismicity distribution over a wide range of inter-event times and distances and helps to recognize patterns in spatial and temporal distribution of earthquakes. I provide a MATLAB script C_d for the correlation integral computation and demonstrate application of the analysis on synthetic earthquake catalogues and the Blanco Ridge fault seismicity catalogue.
Chapter V does not focus on seismicity of the BTF, but also utilizes signals recorded at the Blanco OBS network. In Chapter V, I introduce a new approach for seismic imaging of the upper oceanic crust using recordings of fin whale songs. Fin whale songs are abundant, globally available signals that are among the strongest animal vocalizations detectable over great distances in the oceans. I show that OBS recordings of the fin whale calls contain – apart from waterborne wave arrivals – seismic waves reflected and refracted from interfaces within the shallow oceanic crust. The fin whale calls can be used similarly as conventional air-gun sources for seismic imaging of the upper oceanic crust and constrain the thickness and the compressional- and transverse-wave velocity of the oceanic sediments as well as the seismic velocity of the basaltic basement beneath and around the ocean bottom seismic station.
Overall, this dissertation shows the importance of collecting high-resolution OBS data at OTFs and aims to improve our knowledge of the Blanco Transform Fault as well as understanding of the process of earthquake generation at oceanic transform faults.