April 17 @ 9:00 am - 12:00 pm CDT
Chenhao Yang will hold his PhD Oral Defense on Tuesday, April 17, 2018
Who: Chenhao Yang, Doctoral Candidate
When: 9:00 AM, Tuesday, April 17
Where: Keith Wiess Geological Laboratories, Room 123
What: “Seismic Investigation on Temporal Structure Changes and Joint Inversion of Sedimentary and Crustal Structure Beneath North China”
Abstract: In recent years, time-lapse seismic imaging (4D) has received wide attentions as accurate imaging of the evolving subsurface structure has significant applications in disaster reduction, resource exploration, and environmental monitoring. One particularly interesting area to seismologists is monitoring the time-varying stress field associated with major earthquakes and magmatic eruptions through the detection of temporal changes in the crustal velocity at seismogenic and volcanogenic depths. In the first part of my study, we continue to explore the feasibility to measure and monitor stress transient in the crust by examining the stress sensitivity of seismic velocity. To do so, we conduct an active source cross-well experiment at Parkfield SAFOD drill site to verify the possibility of detecting stress-induced seismic velocity changes at the top part of seismogenic zone. Our result shows a ~0.04% S-wave variation that are strongly correlated with the fluctuation of barometric pressure. The stress sensitivity we calculated is 2.0×10-7 Pa-1, which is consistent with the results from previous studies.
Using the ambient seismic noise to monitor temporal changes has become a significant technique with many applications. While ambient noise provides a natural source to monitor subsurface structural changes, it is still debated whether the observed temporal changes from ambient noise data are caused by structural damage in the top a few hundreds meters or stress-induced structural changes at great depth (a few kilometers to tens of kilometers. To clarify the differences between effects associated with shallow and deep structural changes, we conduct numerical experiments associated with simple 1D models that include low velocities and attenuation coefficients characterizing the very shallow crust. We find significant phase-velocity drops in the period range of 5-20s when large structural changes are limited to the top three kilometers. Our results highlight the importance of accounting for effects of the low velocities and attenuation structure in the top few hundred meters to three kilometers in studies aiming to determine the source region of temporal changes.
North China is one of earthquake prone regions in China. Knowing sedimentary structure within a basin is also of great importance to predict strong ground motions caused by earthquakes. We employ two different methods to image the shallow sedimentary structure for the Bohai Bay Basin and other basins located within North China Plain. A joint inversion of Rayleigh wave phase velocity and ellipticity are applied to obtain a high-resolution S-wave velocity model for the North China region. The results show a good correlation between sedimentary structure and sub-terranes geology, indicating the Rayleigh wave ellipticity has a very good constrain for the shallow sedimentary structure. We also use the frequency-dependent P-wave splitting observed from teleseismic records to estimate the sedimentary thickness and velocity at the same study region. The sediment thickness maps obtained from the two methods have very similar variation patterns, although there are some differences in details.