UC Santa Barbara

Seismic noise is the continuous vibration of the ground due to various non-earthquake causes and is generally regarded as an unwanted component of the signal recorded by a seismograph. Its primary sources include human activity, ocean waves, wind, and atmospheric phenomena. It has long been discarded in seismic analysis but it contains valuable information about its excitation sources and Earth structure. This thesis aims at exploring two main components of seismic noise: anthropogenic noise (human activity) and microseisms (ocean waves). It attempts to clarify noise source characteristics, excitation mechanisms, and propagation processes. By using seismic records from seismometers and Distributed Acoustic Sensing (DAS), and applying several processing techniques, we show discoveries on patterns of human behavior, SH-wave microseisms excitation mechanisms, and precise microseism source locations.First, in Chapter 2, in regards to the anthropogenic noise, we find seismic noise is positively correlated with human activity and economic development over 20 years. We choose an iconic event: the COVID-19 pandemic to study human response recorded in seismic noise records, on the ground that cities in mainland China and Italy imposed restrictions on travel and daily activities in response to COVID-19. It gives us an unprecedented opportunity to study the relationship between human behavior and seismic noise. In this study, we are primarily concerned with seismic noise with frequencies above 1 Hz, known as "cultural noise", mainly generated by local transportation systems. We demonstrate that seismic noise can provide an absolute real-time, anonymous characterization of human activity. In Chapter 3, with respect to the microseisms in the frequency band 0.05-0.5 Hz, we present body-wave microseisms caused by two remote low-pressure systems off the coast of southeastern Australia and southeastern Greenland, detected by a large, dense array (~350 stations) in China. We then use two years of data to study the noise sources of body wave microseisms around the globe. The study points out that SH-wave microseisms, which theoretically should not be excited by the wave-to-wave interaction of ocean waves, can be clearly observed. We also demonstrate that SH waves can only be observed when the source region is close to an area of thick ocean-bottom sediments. Finally, in Chapter 4, we locate the precise sources of high-frequency microseisms in the frequency band 0.5-2 Hz using the new seismic measurement technique DAS. Although microseisms have been observed for more than 100 years, precise locations of their excitation sources in the oceans are still elusive. DAS data offer opportunities for deciphering the locations of excitation sources near the coast that were not possible at all by regular seismographs, including ocean-bottom seismographs. Using DAS data off the coast of Valencia, Spain, and applying a cross-correlation approach, we show that the sources of high-frequency microseisms (0.5-2 Hz) are confined between 7 and 27 km offshore, where the water depth varies from 25 to 100 m. Over time, we observe that these sources move quickly along narrow areas, often confined within an area of a few kilometers. Our method with DAS data allows us to characterize microseisms with high spatiotemporal resolutions, opening a new chapter in understanding the global and complex seismic phenomena that occur in the oceans.