UC Santa Cruz

Earthquakes occur suddenly and often are followed with many serious consequences. Interpreting what causes earthquakes and how they respond to outside perturbations is a crucial goal for many geophysical scientists. Our approach to understanding the earthquake nucleation process depends on the amount and quality of data available. Given that the basic physical processes of earthquakes are now fairly well understood and that high-quality geophysical data are being collected, it should be possible to improve our general understanding of how earthquakes are triggered. Seismology is a key geophysical tool that can help us study the Earth’s interior and gain insights into processes that are difficult to observe directly. It also allows us to measure the internal disturbances in the crust where earthquakes nucleate. In addition, hydrological measurements, such as liquefaction, stream discharge, temperature, and turbidity, fluctuations in well water levels, and the eruption of mud volcanoes, have been observed to respond to earthquakes. By analyzing changes in water levels, we can estimate the hydraulic properties of fault damage zones, understand how fluid interacts with faults and quantify poroelastic responses to static stress changes or the dynamic stress associated with seismic waves. Besides, geological measurements also provide a comprehensive view of the history and properties of faults, adding constraints to geophysical data and improving our understanding on a tectonic scale. We use these geophysical tools to study the earthquake-triggering processes, which are challenging to observe directly due to the lack of in situ measurements at subsurface. This dissertation encompasses two seemingly distinct areas: fault maturity and earthquake triggering. These areas of study can ultimately be instrumentally and scientifically connected in understanding faults through various types of observation techniques for dual-purpose research goals as suggested by the title of this dissertation. Chapter 1 explores the relationship between the structure of faults and the seismological aspects of an earthquake rupturing. Earthquakes occur on faults, which are often complex structures. Variations in fault zone maturity have intermittently been invoked to explain variations in some seismological observations for large earthquakes. However, the lack of a unified geological definition of fault maturity makes quantitative assessment of its importance difficult. Here we empirically compare geological and geometrical aspects of strike-slip fault zones and surface ruptures of major events with seismological attributes of the events. We consider factors such as the total offset on the fault that has accumulated over geological time and the number of segments in maps of earthquake rupture and investigate how they correlate with aspects of the resulting earthquake such as the number of aftershocks or the rupture speed. Several of these factors co-evolve as a fault accumulates slip and matures, thus the trends can be interpreted as indicative of the type of earthquakes observed on mature versus immature faults. We find that less mature faults tend to generate more aftershocks and have lower rupture velocity. Energy estimated from seismograms is relatively low for very immature faults, increases with fault evolution and then decreases as maturity further increases. These relationships help elucidate seismic hazard for fault systems of different maturity and delineate the important fault zone factors that have bearing on earthquake rupture and aftershock generation process. In Chapters 2 and 3, this dissertation investigates various types of earthquake triggering, the process through which stress changes associated with an earthquake can either induce or inhibit seismic activity in the surrounding region or trigger other earthquakes over considerable distances. Chapter 2 studies the hydraulic parameters of a fault zone which help understand the triggering mechanisms of a fluid-driven active fault, including either pore pressure changes or poroelastic stresses perturbations to the surrounding rock. Chapter 3 explores dynamic triggering, a rapid form of triggering over large distances explained by the passage of transient seismic waves, which may immediately induce Coulomb-type failure or initiate a secondary mechanism that leads to delayed triggering. Chapter 2 focuses on determining the hydraulic properties (e.g., diffusivity, permeability) which are critical factors determining how fast the fluid can flow and are necessary and useful for studying induced seismicity. Pore pressure diffusion along faults influences induced seismicity and rupture mechanics. However, in situ hydraulic diffusivity measurements along faults are rare and generally lower than inferred from seismicity migration. Here we use the tidal response of deep geothermal boreholes to measure fault diffusivity and permeability. Initial interpretations of the observation with a homogeneous confined aquifer model result in diffusivities of 10-3-10-1 m2/s. However, this model mixed signals from both the conduit and the host rock. We develop a model for tidal response with a fault passing through the aquifer based on the fault-guided fracture network and solve for hydraulic properties in both the fault and the host rock. The resulting fault permeability is 2×10-14-7×10-14 m2 (90% CI) and fault diffusivity is 0.08-0.33 m2/s (90% CI), which is 2 orders of magnitude higher than the host rock diffusivity in some wells, thus highlighting the role of faults as fluid conduits. In Chapter 3, we work on quantifying dynamic triggering which presents a unique opportunity to understand earthquake interactions and associated hazard implications. The extent and timing of dynamic triggering at given specific stress changes still remain inadequately predicted due to limited studies and datasets. In particular, the requirement for complete, well-characterized catalogs to detect triggering systematically seriously limits the types of studies possible. To address this, we utilized 7-year continuous waveform data from 239 stations in southern California and used PhaseNet for phase picking to identify local earthquakes and measure triggering without constructing any earthquake catalog. Our analysis reveals a similar power-law relationship between triggering intensity and peak stress changes as prior works. Spatial mapping of triggering intensity can be affected by the Ridgecrest earthquake and shows variations across the region. We further observe a slow decay rate of dynamic triggering and conclude that low-frequency waves (0.04-0.1 Hz) may be more effective in dynamic triggering than high-frequency waves (1-3 Hz) which is consistent with a rate-state assisted aseismic creep or hydrological triggering mechanism.