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PKU Bachelor Theses Studying New Zealand Recent Earthquakes
26/03/2019 – The past few years have seen a growing interest in New Zealand among Peking University undergraduates. Some of them have chosen New Zealand Studies as the theme of their bachelor theses. The new interest adds to what graduate and doctoral students and faculty have been doing in the field of New Zealand Studies.
In the past few years some of Peking University undergraduates became interested in choosing New Zealand Studies as the theme of their bachelor theses. The new interest among PKU undergraduates adds to what graduate and doctoral students and faculty have been doing in the field of New Zealand Studies.
From 2017 to 2018 three bachelor theses in the School of Earth and Space Sciences were completed as a series of research of the recent earthquakes that took place in New Zealand. In 2018, Wang Jing completed her bachelor thesis entitled “Estimating Lithospheric Anisotropy Parameters in the North Island of New Zealand Using the Time-delays of Shear Wave” (《利用横波分裂到时差确定新西兰北岛地区各向异性分层参数 》).
The theoretical study of the shear-wave splitting shows that the polarization of the fast shear-wave is usually parallel to the direction of the maximum principal compressive stress in the region. The time delay between fast and slow shear-waves has a strong dependence on the degree of anisotropy of the medium in the region. The observational data show that there are significant differences in the directions of polarization and time delays between fast and slow shear-waves in different observation areas and observation stations. These differences indicate that the seismic anisotropy may be related to both local geological conditions and tectonic environments. At the same time, the shear-wave splitting characteristics should be associated with more detailed stratification and physical information.
In this study, the author of the article uses the near-earthquake data from January 2011 to December 2013 at the BKZ station in the central part of the North Island of New Zealand, where sixty-one pieces of seismic data located in the transverse wave window are selected and visualized to measure the direction of polarization of the fast shear-wave and the time delay between fast and slow shear-waves. According to the seismic structure of the sub-regions, the multi-path “transverse wave pair” is compared with the observed time and it is inferred that the result of the of the time delay between fast and slow shear-waves at the BKZ station is related to the anisotropic structure of the medium. The thickness of the first and second layer of anisotropic medium below the fault area south of the Taupo volcanic area and near the NIDFB fault area are obtained, and the thickness of the first layer is basically consistent with the thickness of the crust. The normalized time delay between fast and slow shear-waves in the first, second and third layer of anisotropic medium in the two regions are calculated which reflect the anisotropic intensity.
In 2017 both Tan Fengzhou and Chen Chenxu of the School of Earth and Space Sciences completed their bachelor theses on New Zealand earthquakes. Tan Fengzhou’s thesis is “The Application of Phase Weighted Relative Back Projection Method on the 2016 Mw 7.8 Kaikoura, New Zealand, Earthquake”(《相位加权的相对反投影法在2016新西兰Mw7.8地震中的应用 》). Based on Relative Back Projection Method, the article develops Phase Weighted Relative Back Projection Method with higher resolution. Then it uses it in studying the Mw 7.8 earthquake in Kaikoula, New Zealand, on November 13th, 2016. Phase and magnitude are the two parts of the signal. Phase changes faster and greater than magnitude. Phase and magnitude of correlated signals change synchronously, respectively. Thus, combining information from phase and magnitude enhances the ability to pick correlated signals. Therefore, Phase Weighted Relative Back Projection has higher resolution. The complex rupture process of the New Zealand earthquake contains a lot of faults. Our results show coincidence with field observations and Peak Ground Acceleration (PGA) distribution. Compared to the results derived from other related methods, our results are more reliable. The rupture process consists of two periods. Its total duration time is 76s with the average rupture speed of 1.5-1.7 km/s.
Chen Chenxu also completed her bachelor thesis in 2017 as well，which was entitled “A Study of the Seismic Mechanism of the 2016 Mww7.8 Earthquake in New Zealand” (《对2016年新西兰Mww7.8级地震震源机制的探索》). The article is a study of the the Mww 7.8 earthquake that took place on Nov. 13th 2016 in a central South Island in New Zealand. New Zealand is an earthquake-prone country, which is in a compressed area of Plate and Indian Ocean Plate, so the seismic faults here are very complicated. The earthquake speeded northeast and then went through more than ten faults. As a result, the earthquake had a very complicated and rare seismic mechanism. The earthquake was a two-stage process, in which the rupture in the previous stage triggered larger rupture in the second stage. Also, earthquake mechanism existed differences in depth daring the rupture in the second stage. The rupture in deep part is a thrust one, in superficial part is a strike-slip one. |