Ambient noise as the new source for urban engineering seismology and earthquake engineering: a case study from Beijing metropolitan area

In highly populated urban centers, traditional seismic survey sources can no longer be properly applied due to restrictions in modern civilian life styles. The ambient vibration noise, including both microseisms and microtremor, though are generally weak but available anywhere and anytime, can be an ideal supplementary source for conducting seismic surveys for engineering seismology and earthquake engineering. This is funda- mentally supported by advanced digital signal processing techniques for effectively extracting the useful information out from the noise. Thus, it can be essentially regarded as a passive seismic method. In this paper we first make a brief survey of the ambient vibration noise, followed by a quick summary of digital signal processing for passive seismic surveys. Then the applications of ambient noise in engi- neering seismology and earthquake engineering for urban settings are illustrated with examples from Beijing metro- politan area. For engineering seismology the example is the assessment of site effect in a large area via microtremor observations. For earthquake engineering the example is for structural characterization of a typical reinforced con- crete high-rise building using background vibration noise.By showing these examples we argue that the ambient noise can be treated as a new source that is economical, practical, and particularly valuable to engineering seis- mology and earthquake engineering projects for seismic hazard mitigation in urban areas.

北京房山岩体附近随采矿活动增强的背景地震活动和动态触发事件研究

北京西部房山岩体多次发现动态触发事件,但之前的研究都受限于台站分布稀疏的影响。本文系统地分析了2011年3月11日日本东北近海MW9.1地震和2012年4月14日印度洋MW8.6地震之前和之后各40天内固定台站和流动台站记录的连续波形。首先,本文采用短时平均比长时平均的检测方法构建模板数据库;其次,本文运用波形模板匹配滤波技术对连续波形进行互相关来检测地震事件。最后,本文分别检测得到日本东北近海地震主震后和印度洋地震主震后1 956个和950个地震事件。多数检测到的地震属于浅源(<5km)地震并群集出现在北京西部房山岩体北岭向斜构造,这个地区邻近一个正在运行的煤矿。从两次主震的强振幅面波中分别检测出7个和10个地震事件,但对应的强前震和余震波形中则没有检测到类似的多事件爆发模式。多种统计检验表明两次主震后短时爆发事件是动态触发事件。本文认为采矿相关的活动可能扰动了地下应力环境,从而使该地区比其他地区更容易被动态应力触发。

Evaluation of 3D crustal seismic velocity models in southwest China:Model performance,limitation,and prospects

Southwest China is a tectonically and seismically active region,witnessing strong deformation due to the collision between the Indian and Eurasian plates.Constraining the subsurface velocity structure of this region is thus important in understanding the tectonics and geodynamic processes of continental collision and in mitigating seismic hazards.Numerous studies have provided various 3D seismic velocity models in southwest China.However,discrepancies exist among these models,and less effort has been made to quantify the reliability and accuracy of these existing velocity models.In this study,we use regional 3D waveform simulation to evaluate the performance of various regional crustal 3D velocity models in reproducing observed seismograms.We particularly focus on two recent earthquake sequence in the region,the 2021 Yunnan Yangbi MS_(6.4) earthquake sequence and the 2022 Sichuan Luding MS_(6.8) earthquake sequence.The tested 3D velocity models include the Southwest China Community Velocity model V1.0,the Unified Seismic Tomography Models for Continental China Lithosphere V2.0,the adjoint full waveform tomography model of the crustal and upper mantle beneath Eastern Tibetan Plateau,and the shallow seismic structure model beneath continental China.Our results show that the tested 3D velocity models generally capture well long-period(<0.2 Hz)waveforms,indicating that the 3D models adequately resolve overall large-scale subsurface structures.However,the 3D synthetics show discrepancies in higher frequencies(0.05–0.3 Hz)and the performance of the 3D velocity models varies from region to region,suggesting that smaller scale heterogeneities are not well constrained.Including shallow velocity structures(<10 km)can improve the waveform fitting,emphasizing the importance of incorporating shallow structures in waveform modeling.The full-waveform tomography model shows a slighter better performance than the other models,especially for the body-waves,highlighting the advantages of full-waveform method in achieving sub-wavelength resolution despite the usage of very long-period waveforms.In light of these comparison results of model performance,we identify the advantages and limitations of different seismic tomography models and methods,and we propose to incorporate different tomography methods and datasets to better constrain subsurface structures.While our target region in this study is southwest China,the analysis that we have conducted can be applied to other regions of various scales and tectonic settings for quantitative seismic model evaluation.

2008年汶川地震与龙门山断裂带的深浅部变形及启示

概述了2008年汶川8.0级地震后近10年有关龙门山断裂带的深浅部变形研究结果,通过较为系统的综合对比分析,归纳得出的基本共识为:龙门山断裂带的深部构造形态和速度结构均十分复杂,2008年汶川8.0级地震是由多次子事件构成的十分复杂破裂过程,其主要滑动量在深浅部都有展布.综合分析认为:龙门山断裂带中北段的深浅滑动速率存在明显差异,孕震深处的滑动速率约为浅部的2~3倍,以重复地震分析给出的深部滑动速率估算的汶川地震复发间隔约为500~4500 a.针对探测程度十分有限的大陆内部断裂带,应充分发挥重复地震的原位探测优势,集成地震学、大地测量学和地质地貌学的各自优势进行深浅部构造变形的有效探测分析,对断裂闭锁段和深浅构造变形差异显著地区的强震危险性尤应重视.

Applications of the Hilbert-Huang Transform for Microtremor Data Analysis Enhancement

In this paper we discuss the use of the Hilbert-Huang transform（HHT） to enhance the time-frequency analysis of microtremor measurements. HHT is a powerful algorithm that combines the process of empirical mode decomposition（EMD） and the Hilbert transform to compose the HilbertHuang spectrum that contains the time-frequency-energy information of the recorded signals. HHT is an adaptive algorithm and does not require the signals to be linear or stationary. HHT is advantageous for analyzing microtremor data, since observed microtremors are commonly contaminated by nonstationary transient noises close to the recording instruments. This is especially true when microtremors are measured in an urban environment. In our data processing HHT was used to（1） eliminate the unwanted short-duration transient constituents from microtremor data and use only the coherent portion of the data to carry out the widely used horizontal to vertical spectral ratio（H/V） method;（2） identify and eliminate the continuous industrial noise in certain frequency band; and（3） enhance the H/V analysis by using the Hilbert-Huang spectrum（HHS）. The efficacy of this proposed approach is demonstrated by the examples of applying it to microtremor data acquired in the metropolitan Beijing area.

Overview of deep structures under the Changbaishan volcanic area in Northeast China

The Changbaishan volcano is an active and considerably hazardous volcano located on the border of China and North Korea. This paper summarizes a series of geophysical surveys as well as seismological and volcano-observational networks around the Changbaishan volcanic area. We characterize deep structures related to the Changbaishan volcanic area. The prominent low-velocity anomalies and low-resistance bodies associated with the magma system under the Changbaishan volcano were detected in the crust and upper mantle, and high-velocity anomalies were imaged within the mantle transition zone,suggesting that the origin of the Changbaishan volcano is related to the subducted Pacific slab. However, there exist a few major obstacles for comprehensively elucidating the deep structure of the Changbaishan volcano as well as for the preparedness for and response toward future volcanic unrest and activity. It is essential to collect data from both China and Korean Peninsula to image the deep structure beneath the Changbaishan volcanic area. A multi-disciplinary approach comprising seismological investigations, deformation information from GNSS and InSar, and gravity and magnetotelluric surveying is a reliable manner for imaging high-resolution structures and fluid movement for the spatial distribution and variation of the volcanic magma chamber.An effective volcano-monitoring network system is considerably important to improve hazard assessments and characterize the potential future eruption of the Changbaishan volcano.