미국 캘리포니아 지역의 CCSE 관측망에서의 Lg파 감쇠현상의 공간적 변화 및 부지 응답 보정, 그리고 주파수 의존성

학위논문 (석사) -- 서울대학교 대학원 : 자연과학대학 지구환경과학부, 2021. 2. 김영희.We estimate lateral variation of the Lg Q at four center frequencies, 0.75 Hz, 1 Hz, 2 Hz and 2.75 Hz, based on the two-station method (TSM) along a great circle profile (A–A’) passing the Central California Seismic Experiment (CCSE) array. Relative site responses at each station are also obtained using the reverse two-station method. Positive site responses are obtained from 34 stations out of total 46 stations examined, and their responses are mostly correlated with geological features (i.e. sedimentary rocks) along the profile. Surficial lithology is likely to have a significant impact on the site response, rather than the thickness of sediments. The site responses also exhibit strong negative correlation to the Vs30 data. We then correct the Lg Q estimates for the site responses. The correction is effective in eliminating surficial properties, resulting in a shift of the peak locations, being more remarkable at lower frequencies (0.75 Hz and 1 Hz) than at higher frequencies (2 Hz and 2.75 Hz). With the site-response-corrected Q values, a power-law frequency dependence of Q(f)=(81±8)f^((0.62±0.11)) is obtained, reflecting the active tectonic setting and the presence of fluids in the region. We also compared the corrected Lg Q variation with several geologic properties. The effect of sediments becomes trivial after the correction, particularly at lower frequencies. Comparison to the Vs and Moho temperature showed that each frequency is capable of imaging distinct depth range. Finally, we compare the result to that of the Sacramento Valley. An analogous behavior at the sedimentary basin is obtained at both regions. However, the conflicting result in Sierra Nevada emphasizes a necessity of further investigation in the region with denser seismic networks.본 연구에서는 TSM(Two-Station Method) 방법을 활용하여 CCSE(Central California Seismic Experiment)관측망을 통과하는 대원상의 단면(A-A’)을 따라 Lg Q값이 공간적으로 변화하는 양상을 네 개의 중심 주파수 0.75 Hz, 1 Hz, 2 Hz, 그리고 2.75 Hz를 중심으로 추정하였다. 각 관측소에서의 상대적인 부지 응답 역시 RTSM(Reverse Two-Station Method) 방법을 활용하여 구하였다. 분석에 사용된 46개 관측소들 중 34개의 관측소로부터 양의 부지 응답 값이 얻어졌고, 이는 주로 퇴적암의 존재와 같은 이 지역의 지질학적 특성과 관련되어있다. 부지 응답 추정에 있어 퇴적물의 두께보다는 표층의 암상이 더 주요한 영향을 미치는 것으로 보인다. 또한 부지 응답 값은 Vs30값 분포와 강한 음의 상관관계를 나타냈다. 이렇게 측정된 부지 응답 값을 이용하여 Lg Q값 추정치를 보정했다. 보정 결과 피크 값이 나타나는 위치가 변화하였는데, 이는 보정 과정이 표층의 특성을 효과적으로 제거하기 때문으로 보인다. 피크 위치의 변동은 높은 주파수(2 Hz & 2.75 Hz)에서보다 낮은 주파수(0.75 Hz & 1 Hz)에서 더 두드러졌다. 부지 응답에 대해 보정된 Q값으로부터 Q(f)=(81±8)f^((0.62±0.11)) 이라는 주파수 의존성을 얻을 수 있었고, 이는 지구조적으로 활발한 지역적 특성과 유체의 존재성을 반영한다. 또한, 본 연구에서는 보정된 Lg Q값의 변화를 다양한 지질학적 특성들과 비교해보았다. 보정을 거치면 퇴적물의 영향은 줄어드는 모습을 보였고, 이는 특히 낮은 주파수에서 두드러지게 관찰되었다. S파 속도 및 모호면 온도 분포와의 비교를 통해 각각의 주파수들이 다양한 깊이 영역을 나타낼 수 있음을 확인하였다. 마지막으로, 이 지역에서의 결과를 사크라멘토 분지 지역에서의 결과와 비교했다. 퇴적 분지에서는 두 지역에서 유사한 거동이 관찰되었다. 그러나 시에라 네바다 산맥 인근에서는 두 지역에서 상반되는 결과를 보였기 때문에 추후에 더 촘촘한 관측망을 이용한 추가적인 연구가 이루어질 필요가 있다.Abstract ⅰ Chapter 1 Introduction 1 Chapter 2 Geological Setting 7 Chapter 3 Data Processing 11 Chapter 4 Methods 19 4.1. Two-Station Method (TSM) 20 4.2. Reverse Two-Station Method (RTSM) 23 Chapter 5 Results 30 5.1. 1-D Checker-board Resolution Test 30 5.2. Relative Site Response Result 31 5.3. Lateral Variation of Lg Q 33 5.3.1. TSM results 33 5.3.2. Passband Effect 34 Chapter 6 Discussion 46 6.1. Site Response 46 6.2. Frequency Dependence of Lg Q 47 6.3. Relation to the Regional Geology 48 6.4. Comparison to the Sacramento Valley Region 51 Chapter 7 Conclusions 62 References 64 초록 74Maste

P And S Wave Velocity Determination

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Seismic anisotropy of Precambrian lithosphere : Insights from Rayleigh wave tomography of the eastern Superior Craton

The seismic data used in this study are freely available from the CNDC (Canadian National Data Centre for Earthquake Seismology and Nuclear Explosion Monitoring) and IRIS DMC (Data Management Center) via their data request tools. The Leverhulme Trust (grant RPG-2013-332) and National Science Foundation are acknowledged for financial support. L.P. is supported by Janet Watson Imperial College Department Scholarship and the Romanian Government Research Grant NUCLEU. F.D. is supported by NSERC through the Discovery Grants and Canada Research Chairs program. We also thank two anonymous reviewers and the Associate Editor for insightful comments that helped improve the manuscript.Peer reviewedPublisher PD

Application of surface-wave tomography to mineral exploration : a case study from Siilinjarvi, Finland

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Surface-wave tomography for mineral exploration : a successful combination of passive and active data (Siilinjärvi phosphorus mine, Finland)

Surface wave (SW) methods offer promising options for an effective and sustainable development of seismic exploration, but they still remain under-exploited in hard rock sites. We present a successful application of active and passive surface wave tomography for the characterization of the southern continuation of the Siilinjarvi phosphate deposit (Finland). A semi-automatic workflow for the extraction of the path-average dispersion curves (DCs) from ambient seismic noise data is proposed, including identification of time windows with strong coherent SW signal, azimuth analysis and two-station method for DC picking. DCs retrieved from passive data are compared with active SW tomography results recently obtained at the site. Passive data are found to carry information at longer wavelengths, thus extending the investigation depth. Active and passive DCs are consequently inverted together to retrieve a deep pseudo-3D shearwave velocity model for the site, with improved resolution. The southern continuation of the mineralization, its contacts with the host rocks and different sets of cross-cutting diabase dikes are well imaged in the final velocity model. The seismic results are compared with the latest available geological models to both validate the proposed workflow and improve the interpretation of the geometry and extent of the mineralization. Important large-scale geological boundaries and structural discontinuities are recognized from the results, demonstrating the effectiveness and advantages of the methods for mineral exploration perspectives.Peer reviewe

Metabolism Modeling in Rivers With Unsteady Flow Conditions and Transient Storage Zones

Whole-stream metabolism characterizes energy and carbon transformations, thus providing an estimate of the food base and CO2 emission sources from streams and rivers. Metabolism models are generally implemented with a steady flow assumption that does not hold true for many systems with sub-daily flow variation, such as river sections downstream of dams. The steady flow assumption has confined metabolism estimation to a limited range of river environments, thus limiting our understanding about the influence of hydrology on biological production in rivers. Therefore, we couple a flow routing model with the two-station stream metabolism model to estimate metabolism under unsteady flow conditions in rivers. The model's applicability is further extended by including advection-dispersion processes to facilitate metabolism estimation in transient storage zones. Metabolism is estimated using two approaches: (a) an accounting approach similar to the conventional two-station method and (b) an inverse approach that estimates metabolism parameters using least squares minimization method. Both approaches are complementary since we use outputs of the accounting approach to constrain the inverse model parameters. The model application is demonstrated using a case study of an 11 km long stretch downstream of a hydropower plant in the River Otra in southern Norway. We present and test different formulations of the model to show that users can make an appropriate selection that best represents hydrology and solute transport mechanism in the river system of interest. The inclusion of unsteady flows and transient storage zones in the model unlocks new possibilities for studying metabolism controls in altered river ecosystems.publishedVersio

Exploiting seismic signal and noise in an intracratonic environment to constrain crustal structure and source parameters of infrequent earthquakes

In many regions of the world characterized by a relatively low rate of seismicity, the determination of local and regional seismic source parameters is often restricted to an analysis of the first onsets of P waves (or first motion analysis) due to incomplete information about Earth structure and the small size of the events. When rare large earthquakes occur in these regions, their waveforms can be used to model Earth structure. This, however, makes the nature of the earthquake source determination problem circular, as source information is mapped as structure. Presented here is one possible remedy to this situation, where through a two-step approach we first constrain Earth structure using data independent of the earthquake of interest. In this study, we focus on a region in Western Australia with low seismicity and minimal instrument coverage and use the CAPRA/LP temporary deployment to demonstrate that reliable structural models of the upper lithosphere can be obtained from an independent collection of teleseismic and ambient noise datasets. Apart from teleseismic receiver functions (RFs), we obtain group velocities from the cross-correlation of ambient noise and phase velocities from the traditional two-station method using carefully selected teleseismic earthquakes and station pairs. Crustal models are then developed through the joint inversion of dispersion data and RFs, and structural Green's functions are computed from a layered composite model. In the second step of this comprehensive approach, we apply full waveform inversion (three-component body and surface waves) to the 2007 M L= 5.3 Shark Bay, Western Australia, earthquake to estimate its source parameters (seismic moment, focal mechanism, and depth). We conclude that the full waveform inversion analysis provides constraints on the orientation of fault planes superior to a first motion interpretation

Multimodal surface-wave tomography to obtain S- and P-wave velocities applied to the recordings of unmanned aerial vehicle deployed sensors

Exploration seismic surveys in hard-to-access areas such as foothills and forests are extremely challenging. The Multiphysics Exploration Technologies Integrated System (METIS) research project was initiated to design an exploration system, facilitating the acquisition in these areas by delivering the receivers from the sky using unmanned aerial vehicles. Air dropping of the sensors in vegetated areas results in an irregular geometry for the acquisition. This irregularity can limit the application of conventional surface wave methods. We have developed a surface wave workflow for estimating the S-wave velocity (VS) and P-wave velocity (VP) models and that supports the irregular geometry of the deployed sources and receivers. The method consists of a multimodal surface-wave tomography (SWT) technique to compute the VS model and a data transform method (the wavelength/depth [W/D] method) to determine the Poisson's ratio and VP model. We applied the method to the METIS's first pilot records, which were acquired in the forest of Papua New Guinea. Application of SWT to the data resulted in the first 90 m of the VS model. The W/D method provided the Poisson's ratio averaged over the area and the VP model between 10 and 70 m from the surface. The impact of the acquisition scale and layout on the resolution of the estimated model and the advantages of including the higher modes of surface waves in the tomographic inversion are assessed in detail. The presence of shots from diverse site locations significantly improves the resolution of the obtained model. Including the higher modes enhances the data coverage and increases the investigation depth