(Dumpster) Diving from the Land into the Sea with Seismology
Seismology is a geophysical tool that can probe the earth and provide insight into processes that are otherwise difficult to observe. I am primarily interested in using seismological methods to detect and study ubiquitous processes that are challenging to observe due to noise sources and other observational limitations. This dissertation encompasses two seemingly distinct areas: aseismic fault slip and oceanic internal waves. These areas of study can ultimately be instrumentally and scientifically connected in offshore regions using ocean-bottom seismometers (OBS) for dual-purpose research goals. In particular, as the title of this dissertation suggests, we make use of the often-discarded portion of seismic signals (i.e., “noise”) to study earth processes.
Slip on faults occurs as a spectrum, ranging from stably sliding (aseismic creep) to rapid ruptures (earthquakes). Within the fault slip spectrum are transient events of slow, aseismic slip known as slow slip events (SSEs). SSEs may precede and even trigger large megathrust earthquakes and are therefore critical to detect and understand. SSEs are too slow to generate seismic waves and are generally detected geodetically. However, the shallow portion (<15 km) of many subduction zones is typically located offshore, out of the range of land geodetic networks. This presents an observational limitation of shallow SSEs that may be improved using related secondary seismicity, such as tremor and microearthquakes, to infer the location of shallow offshore SSEs.
Chapter 1 and Chapter 2 study a specific form of microseismicity, burst-type or near-repeating earthquakes, that can be used to improve the detection of transient aseismic slip. Near-repeating or burst-type repeating earthquakes are families of closely spaced earthquakes (within 200 m) with highly-correlated waveforms, aperiodic repeat intervals and variable magnitudes. We test the hypothesis that sequences of burst-type or near-repeating earthquake families concentrated in time and space are driven to failure by transient aseismic slip on the surrounding fault and can therefore be used as a proxy for aseismic slip transients. Although not included in this dissertation, I am additionally a coauthor on a study that analyzed a cluster of highly similar earthquakes in the southern Cascadia subduction zone likely related to an aseismic slip transient or changes in plate interface coupling (Alongi et al., 2021).
Chapter 1 analyzes burst-type repeating earthquakes related to a well-recorded shallow SSE offshore of Gisborne, New Zealand in September/October 2014 using OBS data. (Shaddox et al., 2019). These burst-type repeating earthquakes are coincident with tectonic tremor and are located within an upper plate fracture network above a subducted seamount after the 2014 Gisborne SSE. I was additionally a coauthor on a study of the microseismicity during the entire OBS deployment (Yarce et al., 2019). We have also performed a brief follow-up study during a similarly located SSE in 2019, and once again find burst-type repeating earthquakes within the upper plate fracture network after the plate boundary SSE. We propose that the SSEs caused fluid migration from over-pressured sediments down-dip of the seamount into the upper plate fracture network, triggering further slow slip on preexisting faults. This is strong evidence that burst-type repeating earthquakes are a promising proxy for transient aseismic slip. These findings also demonstrate the need for offshore seismic observations to detect microseismicity related to shallow, offshore transient aseismic slip. However, although OBS instrumentation is necessary to detect small offshore earthquakes not detected by land seismometers, they generally have low signal-to-noise ratios below ~3 Hz due to oceanographic processes. This is a current limitation in OBS networks.
In Chapter 2 we further demonstrate the utility of near-repeating earthquakes as a proxy for transient aseismic slip. We investigate the occurrence of near-repeating earthquake families during aseismic transients independently detected by borehole strainmeter data but beneath the noise level of continuous Global Positioning System (cGPS) stations in the trifurcation area of the San Jacinto fault zone near Anza in southern California (Shaddox, Schwartz et al., 2021). We find that all moderate-sized earthquakes occurring in this region during the time studied have afterslip signals on borehole strainmeter data and are accompanied by near-repeating earthquakes and elevated seismicity rates. Afterslip geometries defined by the near-repeating earthquake families are consistent with strain change observations. We conclude that families of near-repeating earthquakes, similar to low-frequency earthquakes within tremor, can be useful indicators of aseismic slip transients and can reveal faulting complexities during aseismic slip.
Oceanic internal gravity waves propagate along density stratification within the water column and are ubiquitous. They can propagate thousands of kilometers before breaking in shoaling bathymetry and the ensuing turbulent mixing affects coastal processes and climate feedbacks. Despite their importance, internal waves are intrinsically difficult to detect as they result in only minor amplitude deflection of the sea surface; the need for global detection and long time series of internal waves motivates a search for geophysical detection methods. The pressure coupling of a propagating internal wave with the sloping seafloor provides a potential mechanism to generate seismically observable signals.
In Chapter 3 we use data from the South China Sea where exceptional oceanographic and satellite time series are available for comparison to identify internal wave signals in an onshore passive seismic dataset for the first time (Shaddox, Brodsky et al., 2021). We analyze potential seismic signals on broadband seismometers in the context of corroborating oceanographic and satellite data available near Dongsha Atoll in May-June 2019 and find a promising correlation between long-period transient seismic tilt signals and internal wave arrivals and collisions in oceanic and satellite data. It appears that we have successfully detected oceanic internal waves using a subaerial seismometer. This initial detection suggests that the onshore seismic detection and amplitude determination of oceanic internal waves is possible and can potentially be used to expand the historical record by capitalizing on existing island and coastal seismic stations.
After the completion of the terrestrial study, we acquired data from a broadband OBS station that was deployed offshore of Dongsha Atoll from November 2019 – December 2020, providing a great opportunity to detect and quantify the signal of internal waves on an OBS. We were able to successfully identify internal waves on this OBS, opening up the possibility of a year-long study of internal wave activity near Dongsha Atoll. Further, this may allow us to ultimately remove the long-period “noise” from internal waves on this OBS to improve the detection of microseismicity.
There is important information within the “noise” of geophysical data that can help identify and characterize processes that are difficult to otherwise observe. We have successfully identified offshore aseismic slip on minor faults using burst-type or near-repeating earthquakes on noisy OBS instruments, identified and modeled small aseismic slip transients beneath the noise threshold of cGPS stations with near-repeating earthquakes and borehole strainmeter data, and have performed the first subaerial seismic detection of oceanic internal waves as well as observed internal waves on an OBS at periods that are generally considered “noise” for microseismic investigations. The following dissertation provides in depth details of the methods and implications of these findings.