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| 題名: | 集集地震之震前、同震及震後變形模式研究;Modeling studies on interseismic, coseismic and postseismic deformations associated with the 1999 Chi-Chi, Taiwan earthquake |
| 作者: | 許雅儒;Ya-Ju Hsu |
| 貢獻者: | 地球物理研究所 |
| 關鍵詞: | 黏彈性模型;延展全網逆推濾波法;震後變形;同震變形;間震期;地殼變形;viscoelastic;extended network inversion filter;postseismic;interseismic;coseismic |
| 日期: | 2004-06-07 |
| 上傳時間: | 2009-09-22 09:54:09 (UTC+8) |
| 出版者: | 國立中央大學圖書館 |
| 摘要: | 本論文採用大量GPS觀測資料進行各項研究，然而，受到大氣層水氣含量、季節變化、測站點位不穩定或其它因素影響，在GPS時間序列中通常隱含各種不同的雜訊。為了增加訊噪比，本研究利用一套系統化的分析方法處理GPS時間序列，分析結果顯示，雜訊較接近「白噪音＋閃爍雜訊」的模式。我們藉由最大可能估計法估算雜訊的大小，並修正其在時間序列造成之影響，進而求得較可靠之參數及其誤差。利用修正過後之地殼運動速度場，可直接計算地殼應變率之空間變化。本人改進前人方法，由內插之速度場直接推求應變率，並考慮測站和內插點之距離、測站速度之標準偏差及測站分佈，給予不同的權重來計算合理之地殼應變率。新模式針對測站密度不同，採用不同的特性長度，確保內插點受到良好包圍，避免資訊不足強作內插。同時，為避免內插時在沒有測站的地方產生異常訊號，最後得到的速度場再根據測站密度平滑化。但若測站幾何分佈不佳，內插的過程中還是容易產生假訊號，在解釋時必需特別小心。此外，我們以二維彈性半無限空間錯位模型來了解台灣造山帶間震期之地殼變形；利用投影至板塊運動方向之速度場，推求可能的斷層幾何形貌及滑移速率。結果顯示：台灣中部至南部的地質構造可能有滑脫面的存在，約位於深度10 km處。中部和南部剖面滑脫面上的滑移速率分別為2.7 cm/yr和4 cm/yr，顯示台灣南部似乎有較大的地震潛能，未來可能錯動的斷層應該位於觸口斷層以西。而中部剖面預測的斷層破裂位置，則與車籠埔斷層大致相同，集集地震的發生更驗證了模型的推論。不過模型並沒有考慮非彈性效應，再加上無法解釋中央山脈的成因，未來仍有待更深入之研究。 為了解集集地震同震及震後斷層滑移分佈，研究中使用彈性半無限空間模型及層狀模型進行逆推。同震滑移分佈顯示斷層錯動量最大約為10 m，位於石崗、豐原一帶，即斷層向東轉折處，如此大的滑移量可沿斷層面向下延伸約10 km。分析GPS觀測值及模型預測值之擬合度顯示，下盤測站之殘差存有系統偏差，隱含車籠埔斷層兩側可能存在之側向變化，可能比地層在緃向之變化重要。分析集集地震震後GPS資料顯示，震後變形並非斷層帶孔隙液壓之變化或黏彈性鬆弛現象所造成，主要機制應為震後滑移。分別對震後三個月及十五個月之資料進行逆推，最佳斷層模型顯示，在主斷層面下方深度10 km處，可能存在一傾角近似水平之滑脫面，與震前及同震變形之研究相符。此兩個時段之最大滑移量分別為25和46 cm，皆位於集集地震之震 源附近。根據斷層錯動量分佈，同震滑移大的地方無顯著震後變形，這表示大部份的應力已被釋放。十五個月之結果顯示震後滑移已轉往較深之滑脫面，其相對於整體釋放的能量由三個月的68% 增加至十五個月的80%。由震後滑移模型和集集地震餘震所計算的力矩也有所不同，在震後三個月餘震和震後滑移模型之力矩分別為1.6 x 1019 N-m和2.1x 1019 N-m，其量值大致相當；震後十五個月則為2.0 x 1019 N-m和4.7x 1019 N-m。換言之，震後滑移所釋放之能量已遠超過餘震之效應，其中隱含大部份的能量可能藉由無震滑移釋放。相較於同震滑移模型所計算之力矩，15個月震後滑移模型約為主震之15%。若採用層狀模型，滑脫面之深度比彈性半無限空間模型深約5 km，但所得滑移量分佈及與觀測資料之擬合度並無太大不同。 我們採用延展全網逆推濾波法（ENIF）探討震後滑移隨時空之變化。此法使用卡曼濾波預測斷層系統之行為，利用新的觀測資料去修正預測值，再加上資料在空間和時間之密集取樣，可精準地預測系統之行為。分析震後十五個月資料顯示，主斷層面所累積之滑移小於其下方之滑脫面，且滑移速率由剛開始的0.45 m/yr隨時間迅速遞減，在震後200天速率已小於0.1 m/yr；而滑脫面之滑移速率由起始的0.55 m/yr大致呈線性遞減，至震後450天才轉為不顯著。整個斷層滑移歷程，滑脫面累積之滑移量約為主斷層面之1.5倍。由於滑脫面之同震滑移並不顯著，再加上間震期之研究顯示此區在震前可能一直處於無震滑移之狀態，屬於速度強化地區，受到地震造成的應力影響產生加速滑移，使其震後滑移量大於主斷層面。 為了解下部地殼及上部地函在集集地震後可能產生之黏彈性變形，我們分析震後三年的資料發現，在車籠埔斷層以東之近斷層測站，震後位移衰減的速度大於更東邊的中央山脈及台灣東部之測站，這隱含震後斷層深部滑移或黏彈性變形二種可能性。但黏彈性模型預測之地表位移和GPS觀測資料之擬合度遠不如震後滑移模型。推測震後滑移對地表變形之貢獻可能大於黏彈性變形，需要更長的時間才能觀測到訊號較微弱的黏彈性變形。因此，研究中比較震後滑移及震後滑移加黏彈性變形之綜合模型，但在一維層狀模型的假設之下，是否有加入黏彈性變形對觀測資料之擬合度並無顯著區別。兩模式之預測值皆與觀測值存有顯著差距，尤其在垂直分量之擬合度並不佳，未來仍需探究側向變化之影響。 Plenty of GPS data were used in our seismic deformation studies. However GPS time series exhibit not only tectonic signals but also colored noises. In order to accurately estimate model parameters and their uncertainties, noises have to be evaluated using more realistic model. Some recent studies showed that continuous GPS data are best described as a combination of white noise and flicker noise. We use a maximum likelihood method to estimate amplitudes of these noises and remove them from GPS time series. After carefully corrections, secular velocities and their uncertainties are used to estimate crustal strain rates. We use a modified version of Ward (1998) for calculating strain rates in spherical geometries. Our particular method considers three different types of weights: observational errors, distance between the observation and the location of estimation, and an additional weight to account for variable station density. We also consider spatial coverage of stations and apply a smoothing operator to avoid unreasonable velocity variations without data constrained. Our approach provides more stable and interpretable strain estimates than previous studies, especially when stations are irregularly distributed or when we use larger spatial length scale of estimation. We use a GPS-derive surface velocity field of Taiwan for the time period between 1993 and 1999 to infer interseismic slip rates on subsurface faults. We adopt a composite elastic half-space dislocation model constrained by GPS horizontal velocities projected into the direction of plate motion (306º). The model fault geometry includes a shallowly dipping décollement, in western Taiwan, and a two-segment fault representing the Longitudinal Valley Fault (LVF) in eastern Taiwan. The décollement is composed of two fault segments, one extending west under the Central Range (CR) and one extending east of the LVF, with estimated slip rates of about 35 mm/yr and 80 mm/yr, respectively. The optimal geometry of décollement is subhorizontal (2º~11º) at a depth of 8~9 km. The inferred surface location of the western end point of dislocation in the northern profile is located 15 km east of the Chelungpu fault, while in the southern section, it is located beneath the Chukou fault. Our model successfully match the horizontal velocity field and predict the location of possible future rupture, while cannot explain the vertical data and thus fail to predict the active mountain building processes in Taiwan. This failure indicates a more complex rheological model that incorporates inelastic behavior is needed. GPS measurements of coseismic displacements from the 1999, Chi-Chi, Taiwan earthquake are modeled using both elastic half-space and layered models. The optimal slip distribution shows maximum slip is 11 m, concentrated at the northern bend of the fault and extended about 10 km in down-dip direction from the ground surface. Both models show only one or two meters of net slip at the hypocenter. We also find the large and spatially coherent residuals, which may be attributed to elastic lateral heterogeneity, topography, as well as inelastic deformation, all of which are ignored in this simplified model. Similar strategies are adopted to estimate postseismic slip distributions and fault geometries using 3-month and 15-month GPS data after mainshock. Preliminary analyses show afterslip is the main mechanism of postseismic deformation. Assuming the shallow fault dips 24~26° E, as determined by numerous studies of the mainshock, we invert for the deeper fault structure. Our results show that the fault dip shallows with depth below the hypocenter, merging into a nearly horizontal décollement at a depth of 10 km, consistent with interseismic and coseismic modeling studies. The afterslip distributions for two time periods show maximum slips of 25 cm and 46 cm in the hypocentral region at 7-12 km, respectively. Afterslip is notably absent in the region of maximum coseismic slip, consistent with the afterslip being driven by the mainshock stress change. The afterslip on the décollement in the first 3 months and over 15 months contribute 68% and 80% of the total modeled moment release, respectively. It is worth noting that the slip on the lower décollement becomes more prominent over the longer time period. The afterslip moment inferred from the 15-month GPS observation is 4.7 x 1019 N m, 44% of which occurred in the first 3 months. In contrast, the seismic moment released by the aftershocks in the same period is approximately 2.0 x 1019 N m and 85% of that moment was released before the end of 1999. This indicates that although part of the GPS-observed moment may be due to aftershocks, there is still a large amount of postseismic deformation which is aseismic. Then we adopt a more realistic layered model to estimate slip distribution. The optimal model show the depth of lower décollement is 5 km deeper than previous one, but the slip distribution and misfit are very similar. We employed the Extended Network Inversion Filter（ENIF）to invert for the evolution of afterslip with time and space. The modeling algorithm is based on recursive liner Kalman filtering, which is used to estimate system processes at each epoch given all past and present data and forecast future observations. The prediction and update process are iterated through the entire data set to precisely describe the system. The postseismic GPS data over 15 months is used to infer the history of afterslip, it shows shallow slip decays more rapidly on the main fault than deeper décollement. The slip rate on the main fault began about 0.45 m/yr and downed to 0.1 m/yr in 200 days, while it on the décollement started form 0.55 m/yr and took about 450 days decreasing to 0.1 m/yr. The accumulated slip on lower décollement over 15 months is 1.5 times larger than it on the main fault. The coseismic slip on the décollement is not significant, while afterslip is prominent. This is consistent with the experiment result of a velocity-strengthening fault zone rheology. To explain the possible deformation with viscous flow in the lower crust and upper mantle, we examine postseismic GPS data over 3 years. The moving directions of stations are congruent in different time periods and the magnitudes of displacements decrease with time, the rate decrease is large near the fault but small in eastern Taiwan. This might imply either a deepening of the slip or the effect of viscous flow. We adopted a viscoelastic model with 1-D layer structure, but the model predictions are discordant with GPS observations. Misfits are similar for combination of afterslip and viscoelastic models, and afterslip model alone. In either case, the discrepancy between predictions and observations still exits, especially for vertical displacements. We conclude a more realistic 3D model, included lateral heterogeneity of viscosity is really needed in the future. |
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