小地震震源参数和破裂机理研究

RESEARCH ON SOURCE PAPAMETERS AND RUPTURE MECHANISM OF SMALL EARTHQUAKES

龚文正

11849204

硕士

力学

陈晓非

2020-05-30

2020-05-30

哈尔滨工业大学

深圳

地震破裂相图表明小地震（约小于 6 级）有两种不同的破裂类型，一种是 自停止破裂，其震源谱光滑、震源时间函数简单。一种是被迫停止破裂，其震 源谱在特定频率处存在凹陷、震源时间函数复杂。本文计算了实际小地震的震 源谱和震源时间函数并估计它们的拐角频率和应力降，以此来研究小地震的破 裂类型和破裂机理。本文使用多分量、多台站的 P 波数据，运用经验格林函数 （EGF）方法来获取目标地震的震源谱和震源时间函数，并使用 Boatwright 和 Brune 震源谱模型估计拐角频率和应力降。EGFs 的选择包括多个方面因素比如 波形互相关系数、EGFs 地震矩大小、数据信噪比以及震源谱拟合稳定性等等。 其中波形互相关系数是一个重要因素，它 反应了目标地震和 EGF 地震之间震源 位置和震源机制的相近性。本文使用了多窗口频谱分析方法来获取稳定的震源 谱曲线并使用 Nelder-Meade 方法拟合震源谱。本文使用了 Bucaramanga nest 地 区震级为 3-5.7 Ml 的中深源（125-165 km）地震和 Fukushima-Ibaraki 地区震级 为 3-6.7 Mw 的浅源（小于 20 km）地震作为目标地震，最终在这两个地区分别 获得 714 和 575 个地震的震源谱和震源时间函数。在 Bucaramanga nest 地区地 震中，516 个地震为自停止破裂地震，198 个地震为被迫停止破裂地震。在 Fukushima-Ibaraki 地区地震中，281 个地震为自停止破裂地震，292 个地震为被 迫停止破裂地震。对于被迫停止破裂地震而言，实际震源谱有存在多个凹陷频 率的现象，同一经验格林函数地震不同台站的震源谱的凹陷频率不同。 Fukushima-Ibaraki 地区部分地震的震源谱在低频部分出现幅值减小而偏离震源 谱模型，其原因还需要进一步研究。本文只计算了自停止破裂地震的拐角频率 和应力降。整体上，不同震级地震的拐角频率变化范围较大，拐角频率上限值 随震级增加而减少，应力降随地震矩大小增加而增加，相同地震矩大小地震的 应力降变化范围可达 2 个数量级。在 Bucaramanga nest 地区，大部分地震的应 力降都在几十到几百 Mpa 之间，随深度增加而略有增加，少数地震的应力降超 过 1000 Mpa，应力降在空间和空间上没有明显异常。在 Fukushima-Ibaraki 地区， 地震应力降都在几到几百 Mpa 之间, 随深度增加而略有增加，应力降在时间和 空间上没有明显异常。相比于 Brune 震源谱模型，Boatwright 震源谱模型计算 的应力降整体偏小。Boatwright 震源谱模型的最佳 n 值为 2，而 Brune 震源谱模 型的最佳 n 值大于 2 且最佳值不明显。

Rupture phase diagrams indicate that there are two rupture types for small earthquakes, magnitude of which are usually less than about 6 Mw. One is self-arresting rupture with smooth seismic spectra and simple source time function (STF). Another is runaway rupture with complicated STF, and holes exist on seismic spectra at specific frequency. In this paper, we calculate seismic spectra and source time function of small earthquakes, and estimate corner frequency and stress drop to understand rupture styles and rupture mechanism of small earthquakes. Empirical Green function (EGF) method is used to extract seismic spectra and STF from P waveform of multi-component of multiple stations. The Boatwright source model and Brune source model are used to estimate corner frequency and stress drop. There are a lot of factors to select a good EGFs including waveform correlation coefficient, EGFs earthquake magnitude, signal-to-noise ratio and fitting stability of seismic spectra. Waveform correlation coefficient, representing similarity of the source location and focal mechanism between target earthquakes and EGF earthquakes, is a significant factor. We apply multitaper spectral analysis method to obtain stable seismic spectra and Nelder-Meade method to fifing seismic spectra. We investigate seismic spectra and source parameters of the medium-deep earthquakes (125-165 km) with 3-5.7 Ml in Bucaramanga nest and the shallow earthquakes (less than 20 km) with 3-6.7 Mw in Fukushima-Ibaraki. In Bucaramanga nest, we totally obtain seismic spectra and STF of 714 earthquakes, 516 of which are self-arresting rupture and 198 of which are runaway rupture. In Fukushima-Ibaraki, we totally obtain seismic spectra and STF of 575 earthquakes, 283 of which are self-arresting rupture and 292 of which are runaway rupture. We find that there are more one holes exist on seismic spectra of a few runaway rupture earthquakes. Hole-frequency of seismic spectra calculated from different stations and same EGF earthquake are different. In Fukushima-Ibaraki, seismic spectra’s amplitude of some earthquakes reduces at low frequency, which deviate normal source model. In this paper, only corner frequency and stress drop of self-arresting rupture earthquakes are estimated. In these two regions, corner frequency of same magnitude earthquakes vary a lot and the highest corner frequency decrease with magnitude. Stress drop increase with seismic moment and the range variation of earthquake’s stress drop with same seismic moment is up to 2 order of magnitude. In Bucaramanga nest, stress drop of most of earthquakes are between tens and hundreds Mpa and increase slightly with depth, and only a few earthquakes’ stress drop are over 1000 Mpa, spatial and temporal distribution of which has no obvious characteristics. In Fukushima-Ibaraki, stress drop of all earthquakes are between a few and hundreds Mpa and increase slightly with depth, spatial and temporal distribution of which has no obvious characteristics. Stress drop estimated by Boatwright source model are generally less than that by Brune source model. The best n value of Boatwright source model is 2. The best n value of Brune source model is over 2 and it is not obvious.

经验格林函数 地震破裂类型 震源谱 拐角频率 应力降

empirical Green's function rupture styles seismic spectra corner frequency stress drop

中文

联合培养

南方科技大学

龚文正. 小地震震源参数和破裂机理研究[D]. 深圳. 哈尔滨工业大学,2020.