Imaging the lower-crustal structures and precisely determining earthquake depth via advanced analysis of seismic later phases

• Main Author: Li, Tianjue
• Other Authors: Tong Ping
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2024
• Subjects: Earth and Environmental Sciences; Mathematical Sciences
• Online Access: https://hdl.handle.net/10356/173515
• Institution: Nanyang Technological University

Within the earth especially in the crust lie key minerals and energy resources, which are necessities to maintain modern industries/technologies. In addition, the earth’s crust preserves the oldest records of our planet’s evolution, holding the key to unlock insights into the early earth. The crust also imposes social challenges in the form of various natural hazards, e.g., earthquakes, tsunamis, and volcano eruptions. Thus, studies regarding the crust have long been the focus of geoscience society. There are three crucial aspects of the earth crust. The first aspect pertains to the basal geometry of crust, Moho discontinuity, which is a petrological boundary separating the earth’s silicic crust from the ultramafic mantle. It demonstrates the earth differentiation at the global scale and plays a pivotal role in determining the lithosphere’s strength. Receiver function (RF) analysis of the Moho-converted signals such as the Ps phase which originates from teleseismic earthquakes is routinely used to constrain the Moho depth, but its success relies on the densely instrumented seismic stations thus the cost is high. For regions with active seismicity, it is preferred and costly less to jointly use the Moho-reflected PmP and Ps waves to delineate the Moho topography. Challenges remain regarding the PmP waves identification in an efficient way. To address such difficulty, I have developed a semiautomatic workflow to identify the PmP waves efficiently. Applying such new workflow to Southern California, I successfully construct the first PmP database there, and update the Moho geometry using the PmP data together with the Ps signals. The new Moho geometry demonstrates a high level of agreement with the community Moho model, and reveals unprecedented features, notably the presence of a deep buried Moho beneath the northern end of the central and western Transverse Ranges. This discovery aligns with the observed deep seismicity in the area naturally, indicating the presence of a thick seismogenic zone in the crust. The second aspect delves into the intracrustal P-wave velocity structure. By analyzing anomalies in comparison to the typical reference velocity profile, researchers can infer various properties of the earth crust like the rock type, temperature, rheology, etc. Such knowledge plays a key role in exploring underground resources and gaining insights into the physical mechanisms of earthquake and volcano eruption. Currently, inverting either first P-wave traveltimes or the full waveform of regional seismic records only provides good P-wave velocity structure in the middle-upper crust, as most natural quake sources (earthquakes) on the continents terminate at the depth of 20 km. To overcome such limitation, I build the tomographic image of the entire crust column through jointly inverting first arrivals and later phase PmP. By applying such imaging strategy to the young Clear Lake volcanic field (CLVF) in Northern California whose latest eruption occurred 10 ka ago, I find a multilevel trans-crustal magmatic system beneath the Geysers-Clear Lake area. The specific location, volume, and melt fraction of the deep magma reservoir in the lower crust are deciphered for the first time at the scale of tens of kilometers. This multilevel magmatic architecture not only offers a reasonable explanation for the diverse range of erupted rock compositions, observed over the past 2 Ma at the CLVF, but also provides an attractive model and mechanism for understanding the intermittent eruptions with wide spectrum seen at other continental volcanoes. The final aspect, though not the least important, is the study of earthquake locations within the crust. By carefully examining the seismicity pattern, researchers can delineate the fault geometry at depth, which is vital in understanding the seismogenic process and assessing the potential earthquake risks. The accurate earthquake location also serves as prerequisite for other studies like the seismic tomography. Currently, there are various algorithms available regarding earthquake location, e.g., HypoDD, Growclust and NonLL. However, challenges related to the estimation of earthquake location uncertainty and usually large location ambiguity in depth direction remain. The depth phase sP, a kind of signals that experience surface mode conversion on the source side, has long been recognized to have tight constraint on earthquake depth. But its applications are often limited to teleseismic earthquakes. Here I have developed a practical procedure for reliably and efficiently identifying the depth phase in local and regional seismic records, and I also propose a new earthquake location scheme within the Bayesian framework, which can efficiently incorporate the phase data of first P, S waves and depth phase sP wave to sequentially determine the earthquake hypocenter and origin time. The new earthquake location scheme not only offers an accurate estimate of the source position but also provides a more complete assessment of location uncertainty. By incorporating depth phase into the earthquake location algorithm, the ambiguity in earthquake depth estimation would be greatly reduced. As a result, the accuracy and reliability of seismicity pattern and body wave traveltime seismic tomography will be enhanced. Joint inversion of first arrivals and later phase PmP can image the entire crust column leading to a more comprehensive understanding of the seismogenic process and magmatic architecture in the lower crust.