臺灣博碩士論文加值系統

中-大型地震前後地殼速度變化可以透過地震波走時層析成像法解析，然而，兩個時期不相等的震源-測站之波線路徑在空間中的分佈會使層析成像的速度變化結果產生偏差。為了測試由於地震波線分佈不相等對於解析速度構造變化造成的偏差效應，本研究將地震前與地震後兩時期的走時資料，各別考量相對應地震對的地震位置、震央距差值以及相對應波線對的波線方位角差值，挑選出地震前後兩時期相似路徑的地震波線，並將走時資料依空間分佈分為三種：(1)不相等波線、(2)相等波線不包含測站修正以及(3)相等波線包含測站修正，進行一系列的測試，研究走時層析成像在時間-空間上的解析情形，檢驗不相等的地震波線空間分佈如何影響三維速度構造。並以2013年10月31日發生於臺灣東部芮氏規模6.4的瑞穗地震，檢驗使用震波走時層析成像法解析地殼速度變化的可行性。透過棋盤格測試、解析能力測試以及反演程度差異的確認，解析出速度變化在空間中的可信區域。
速度變化的結果顯示，P波速度在震源區域有約2%正值速度變化出現在深度10公里處，往下可延伸至15公里包圍震源並伴隨著量值增加至約5%。在深度20公里處，正值速度變化以北北東-南南西的趨勢集中在距離震源位置大約10-30公里的北北東區域，而此正值區域包圍斷層主要滑移區的周圍。此外，在深度10公里處有約3-8%負值速度變化出現在餘震集中之區域的東北邊。大多數餘震發生在正值與負值速度變化的邊界。S波速度在深度10公里處，有負值速度變化(~5-10%)出現在震源位置的西側及東北側，而位於震源位置西側的負值速度變化向下延伸至15公里伴隨著量值下降至約3%。此外，在震源深度的正值速度變化(5-10%)出現在距離震源大約10-30公里的北北東區域，與震源主要破裂區域符合，而此正值速度變化向下延伸、向北集中在離震源位置約30公里處。速度變化的結果顯示，在主震區域的地震P波和S波速度沿著斷層破裂方向變快、大多數餘震集中在速度變化梯度大的區域，而在斷層破裂終止、距離震源位置約30公里處，速度在約深度12-15公里以上的區域變慢、20公里區域變快。我們認為這樣的速度變化是由於地震前後震源區域及其周圍應力調整、裂隙密度改變以及斷層下盤地塊的彈性回跳所致。
值得注意的是不單只是地震波線在空間上的分佈會影響速度構造反演的結果，走時資料在挑波時的權重也會造成解析速度變化的偏差。然而，當我們考慮了兩時期的地震波線在空間上具有相同分佈位置，而各時期走時資料具有各自反演速度構造的能力，並且兩時期走時資料解析速度構造的能力相當，地震走時層析成像法即可用以解析三維空間速度構造隨時間的變化。

Using seismic travel time tomographic method, changes of seismic wave velocity in crust associated with moderate-to-large sized earthquakes could potentially be resolved with the tomographic images before and after the occurrence of an earthquake. However, unequal ray distribution during two time periods can also cause the artifacts in resulting of temporal seismic wave velocity changes. Thus, to selected the two travel time datasets with equal ray distribution, we considered the location distance of each event pair, the differences of epicentral distance and the gap of azimuths for each station-source pair. We then conducted a series of tests to investigate the temporal-spatial resolution of tomographic results, especially for examining how the unequal ray distributions can influence the three-dimensional VP and VS crustal structures. We demonstrated the feasibility of time-dependent tomographic method by applying it to image the velocity changes before and after an earthquake occurred on 2013 October 31, in Rueisuei in eastern Taiwan (ML = 6.4), which was well recorded by the Central Weather Bureau Seismic Network (CWBSN) stations, Taiwan Strong Motion Instrumental Program (TSMIP) and Broadband Array in Taiwan for Seismology (BATS) stations. In our tomographic results, quite different patterns were found between the results of equal and unequal type of ray distributions. Through investigating the checkerboard resolution tests, resolution maps and discrepancy between the checkerboard-like models within two time periods, the reliable region were revealed.
In our results, in the source region of the 2013 Rueisuei earthquake, a positive velocity changes (the model difference, ΔM) in VP (~2-10%) appeared surrounding the source location at 15 km depth. This anomaly of velocity changes downwardly extended to ~10-30 km away in the northern region of source at 20 km depth with a tendency in the NNE-SSW direction, which rupture of the fault plane propagated. In the north-eastern region of aftershocks, a negative ΔM (~3-8%) appeared in the north-eastern region at 10 km depth and downwardly stretched to 15 km with decreased amplitude (~1-2%). For ΔM in VS, two negative ΔM (~5-10%) appeared in the north-eastern region and the west side of the source location, and these anomalies downwardly extended to 15 km depth with decreasing in intensity. In the region where most aftershocks located, a strong positive anomaly (~5-10%) was shown with the same trending in the NNE-SSW direction just located at the major slip area. This anomaly downwardly stretched and further localized in the north where there was ~30 km away from the source location at 20 km depth. We suggested that the observed ΔM in the source region were mainly caused by increasing in stress due to the rupture of the mainshock and elastic rebounded of crust.
It is worth noting that not only will the distribution of rays affect the results of time-dependent travel time tomographic inversion but also different weighting value of the arrival-pickings from travel time data in the pre-seismic and the post-seismic periods bias the structures in tomographic inversion procedure. When the comparable resolution before and after a moderated-to-large sized earthquake can be achieved with identical ray distribution, the travel time tomographic method is then able to resolve the seismic wave velocity changes.

論文口試委員會審定書 i
誌謝 ii
中文摘要 iii
Abstract v
Table of Contents vii
List of Figures ix
List of Tables xiii
Chapter 1 Introduction 1
Chapter 2 Data and analysis procedure 5
2.1 The 2013 Rueisuei earthquake 5
2.2 Data 5
2.3 Selection of ray distribution 6
2.4 Flowchart analysis 11
Chapter 3 Method 14
3.1 Determination of earthquake location 15
3.2 Travel time tomographic inversion 16
3.3 Parameter setting 17
3.4 Model difference 18
Chapter 4 Resolution verification 21
4.1 Checkerboard resolution tests 21
4.2 Discrepancy of checkerboard-like structures 22
4.3 Summary 26
Chapter 5 Results and discussion 31
5.1 Model difference in VP and VS 31
5.2 Discussion 34
Chapter 6 Conclusions 40
References 42
Appendices 49
A. Initial 3-D velocity models for tomographic inversion 49
B. Testing for different value of smoothing and damping factors 53
C. Time-expending tomographic inversion 56
D. Tomographic inversion with an initial 1-D velocity model and equal-type travel time data 63
E. Synthetic test considering an initial 3-D velocity model and equal-type travel time data 65
F. Testing the influence of weighting value of arrival-pickings in travel time data on tomography with the checkerboard-like structure considering an initial 3-D velocity model and equal-type travel time data 68

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