西索羅門群島地震速度構造研究及探討

Seismic Velocity Structure and Its Tectonic Implications for the Western Solomon Islands

古進上(Chin-Shang Ku)

指導教授 ： 吳逸民

國立臺灣大學/理學院/地質科學研究所/博士(2020年)

https://doi.org/10.6342/NTU202000886

摘要

本文旨在利用分析環境噪聲訊號及遠場地震訊號為西索羅門群島的地下速度構造提供新的貢獻。在2007年發生了一個地震矩規模8.1的地震，這個地震引發了海嘯並在西索羅門群島造成嚴重的傷亡，同時也促使了在2007年地震的破裂帶周圍安裝地震監測網。在論文的第一部份，利用分析環境噪聲訊號並應用基因演算法來搜尋當地的一維速度模型，該模型由上下地殼及部分上部地函所構成。反演所得到的一維速度構造，分別在上下地殼求得6.9及13.5公里的厚度，而上下地殼及上部地函的剪力波速度分別為2.62, 3.54及4.10 km/s，而相對應的P波對S波的速度比分別為1.745, 1.749及1.766。經由驗證理論到時與實際到時的差距，本研究所求的一維速度模型（WSOLOCrust）相較於全球一維速度模型（iasp91）及從CRUST 1.0所截取的區域一維速度模型，均可求得較佳的平均到時殘差，分別有0.85及0.16秒的改善。這也代表此一維速度模型未來可以當成是一個較佳的參考模型，以利用於地震定位。在論文的第二部分，本研究選擇該地震監測網所記錄到的遠場地震波形，進一步分析該區域地震速度構造的橫向變化。透過接收函數及表面波頻散曲線的聯合反演，求得各測站底下的一維速度模型。所求得的速度模型顯示整個區域地殼模型的劇烈變化。莫何面的深度則在範圍25-40公里之間。在大多數的地震站底下，也觀察到低速帶的存在。本研究為該區域提出各測站底下的初步一維速度模型，未來藉由更多測站的佈放以求得更多觀測資料，將有助於區域三維速度模型的建構。

關鍵字

索羅門群島 ； 地震觀測網 ； 環境噪聲訊號 ； 接收函數 ； 表面波頻散曲線 ； 聯合反演

並列摘要

This thesis aims at exploiting information in the ambient noise field and teleseismic seismograms to provide new constraints on the crustal and uppermost mantle structure of the western Solomon Islands. An MW 8.1 earthquake occurred in 2007 that induced tsunami to hit the western Solomon Islands and caused casualties, which initiated the deployment of a seismic network around the rupture zone of the 2007 event. In the first part of the thesis, the ambient noise records were analyzed, and a genetic algorithm (GA) scheme was developed to search a one-dimensional (1-D) crustal velocity model, which constitutes two layers (upper and lower crust) and a half-space (uppermost mantle). The resulted thickness values for the upper and lower crust are 6.9 and 13.5 km, respectively. The shear-wave velocities (VS) of the upper crust, lower crust, and uppermost mantle are 2.62, 3.54, and 4.10 km/s with VP/VS ratios of 1.745, 1.749, and 1.766, respectively. The differences between the predicted and observed travel times show that the 1-D model (WSOLOCrust) has an average of 0.85- and 0.16-sec improvements in travel time residuals compared to the results of global iasp91 and local CRUST 1.0 model, respectively. This layered crustal velocity model could, therefore, serve as a better reference velocity model to locate earthquakes and tremor sources in the local area. In the second part, the seismograms of teleseismic events recorded at the seismic network are used to investigate the lateral variations of the seismic velocity structure. Joint inversion of the P-wave receiver functions and surface-wave group velocity dispersion curves is used to estimate station-based 1-D velocity models. The resulted velocity models show a highly variable crustal structure across the region. The Moho depths beneath stations are ranging from 25 to 40 km. The low-velocity zone (LVZ) is observed at most seismic stations. This study provides the preliminary station-based seismic velocity models for the study region, and more stations will be deployed in the following project. An integrated 3-D velocity model will be determined in the future.

並列關鍵字

Solomon Islands ； seismic network ； ambient noise ； receiver functions ； surface-wave dispersion curve ； joint inversion

參考文獻

Acton, C. E., K. Priestley, S. Mitra and V. K. Gaur (2011). Crustal structure of the Darjeeling—Sikkim Himalaya and southern Tibet, Geophys. J. Int. 184, 829–852.

Ammon, C. J., G. E. Randall and G. Zandt (1990). On the nonuniqueness of receiver function inversions, J. Geophys. Res. 95, 15303–15318.

Back, T. (1996). Evolutionary algorithms in theory and practice, Oxford University Press, New York.

Bath, M. (1974). Spectral Analysis in Geophysics, Elsevier, Amsterdam.

Bensen, G. D., M. H. Ritzwoller, M. P. Barmin, A. L. Levshin, F. Lin, M. P. Moschetti, N. M. Shapiro, and Y. Yang (2007). Processing seismic ambient noise data to obtain reliable broad-band surface wave dispersion measurements, Geophys. J. Int. 169, 1239–1260.