集集地震同震及震後應力演化與地震活動之相關性

Stress Evolution Associate with Seismicity during Coseismic and Postseismic Periods of the 1999 Chi-Chi, Taiwan, Earthquake

詹忠翰(Chung-Han Chan)

指導教授 ： 馬國鳳

國立中央大學/地球科學學院/地球物理研究所/博士(2006年)

摘要

為了比較在集集震前與震後不同時間區段間地震活動度之變化，避免人為或測站因素造成地震目錄不完整而影響估算之地震活動度變化，本研究搜尋自1995年後規模大於台灣地區地震規模完整度之地震。發現震後在車籠埔斷層上盤、北端以及南端延伸處，與嘉義地區地震活動具有上升之現象。更甚者，此處亦為在震後3個月之內活動度上升最多之處，此一地區正座落於主震應力上升之區域。而震後3-53個月其間亦反映在南山、高屏、花東外海與台中外海皆呈現地震活動下降之情況。此處亦為在震後3個月之內活動度上升相對較少之處，此一地區正座落於主震應力下降之區域。顯示該地區同時受到動態應力誘發與靜態應力下降造成地震陰影帶之影響。
研究中並以集集同震破裂模型計算對於鄰近區域造成應力重新分佈之狀態並且比較集集震後0-3個月之地震分佈。發現大部分之地震皆座落於主震影響而更趨於破裂之區域。然而並非所有地震皆能以靜態庫倫應力變化模型解釋之，在車籠埔斷層北端之地震活動並無法以應力變化之計算結果解釋之。造成此現象可能原因為逆推而得之錯動模型並無法忠實呈現集集破裂型態或是動態應力上升因而誘發該處斷層之破裂而產生餘震。為了從主震應力轉移之角度討論震後滑移之分佈，計算車籠埔斷層斷坡以及滑脫面上受到集集同震期間造成之應力變化。發現在斷坡部分，正向應力變化之結果較為符合震後滑移分佈因此推估斷層面上擁有較大之摩擦係數。在滑脫面部分，剪應力變化之結果較為符合震後滑移分佈因此推估斷層面上擁有較小之摩擦係數。文中亦檢視震後地震之震源位置與震源機制面上之庫倫應力變化。假設有效摩擦係數為零之情況下，主震造成震後地震剪應力之統計結果，對於逆衝、走向滑移，以及正斷層型態地震分別有74%、61%，以及63%受到同震應力增加超過0.01 bar。當有效摩擦係數為0.4，計算前、後地震二結點面上之應力變化。比較二者受到同震應力增加超過0.1 bar之比例，逆衝型態地震在震後上升26%；走向滑移型態地震在震後上升18%。而在集集地震後第1年間，陸續發生10次之大型地震。大部分較佳破裂面上之應力狀態受到主震及主震與相繼之大型地震累加而更趨於發生地震。
本研究除了以主震應力轉移之角度討論震後滑移之分佈，更進一步以震後滑移之角度計算集集震後15個月之應力變化。顯示在震後期間主震以南之區域呈現應力上升之勢；在中央山脈地區則為應力下降；在東部外海部分，則呈現應力小幅上升。比較此結果與震後15-53個月與3-15個月地震活動度之差異性，發現在台灣島內大部分區域之地震活動度下降並無法以震後滑移造成之應力變化解釋之，此一現象說明集集震後一定時間內之地震行為仍然由餘震衰減所主導著。然而地震活動度衰減最劇烈之處為車籠埔斷層東方五十公里的中央山脈東側。比對震後應力變化，此區恰巧位於震後應力下降之處。
若考慮震後滑移與集集震後之黏彈性鬆弛效應造成之地表變形並且比較GPS在震後之觀測值。發現在上盤地區震後滑移之特性主導了震後變形，黏彈性效應則相對不明顯。在下盤地區，發現除了少部分位處於車籠埔斷層附近之測站，黏彈性滑移之效應無論在方向以及量級上皆更趨近於觀測結果。本研究進一步引用中央地質調查所之跨斷層重複精密水準測量之結果。發現在車籠埔斷層下盤之處大致上自西向東逐漸沈降，此一現象符合黏彈性鬆弛效應造成之地變形。然而在上盤地區顯示自西向東具有明顯抬升之趨勢，此一結果並不符合黏彈性鬆弛效應造成之地變形。根據集集主震震後之黏彈性回跳以集集及震後之震後滑移之特性，計算附近斷層面上在同震、震後15個月，以及震後50年之應力變化。在同震期間，彰化斷層沿線呈現剪應力上升、正向應力下降之趨勢。而大茅埔-雙冬-哮貓與水里坑斷層則呈現剪應力下降、正向應力上升之趨勢。在震後15個月，受到主震造成之黏彈性回跳以及震後滑移之影響，彰化斷層在淺部呈現剪應力下降、深部上升之趨勢，正應力則呈現附近所有斷層更趨向於破裂。在震後50年後，計算附近斷層受到主震及15個月震後滑移造成之黏彈性回跳以及震後滑移本身所造成之應力變化之總和。顯示彰化斷層在深部、大茅埔-雙冬-哮貓與水里坑斷層之大部分區域呈現剪應力上升，其餘地區則為下降；對於正向應力，除了彰化斷層在淺部呈現應力上升，其餘地區則為下降。
比較利用GPS資料逆推集集主震之破裂模型與將集集破裂簡化成斷坡與滑脫面上二個子斷層造成之靜態庫倫應力變化，二者皆顯示在滑脫面上與更深處，以及滑脫面終點處呈現應力上升之貌，此二區正為震後三個月之內地震活動度最為頻繁之區域。在台灣地區，當地震發生後數分鐘內，中央氣象局之速報系統便將地震之震源位置以及地震規模放置於網站中或藉由氣象局發布之電子報取得相關資訊。在震後數小時內，中央研究院地球所資料處理中心管理之 Broadband Array in Taiwan for Seismology 利用地震矩張量計算該地震之震源機制並放置於網站中或藉由氣象局發布之電子報取得相關資訊。根據台灣地區地震規模與斷層參數之經驗公式便可在地震發生後短時間內求得其可能之斷層參數並計算應力之變化。藉此便可預估餘震可能發生位置而得到餘震預警之效果。

關鍵字

集集地震 ； 黏彈性回彈 ； 庫倫應力

並列摘要

In the past decade, many studies had proved the influences of static stress changes on spatial or temporal distribution of the aftershocks and further large earthquakes. By contrast, other studies have deepened the argument by resolving stress changes on aftershock focal mechanisms, which removes the assumption that the aftershocks are optimally oriented for failure.
The 21st September 1999 Chi-Chi, Taiwan, earthquake (Mw=7.6) produced a remarkable set of data and clearly exhibits stress transfer. Large amount of GPS static measurements and strong motion acceleration records recorded this event. Such a data set offers a unique opportunity to understand the earthquake process and the generation of ground motions. Furthermore, 7 of continuous monitoring GPS stations were set up near the Chelungpu fault, mostly within three weeks after the occurrence of Chi-Chi mainshock and about 80 campaign-surveyed stations were resurveyed up to 7 times from September 1999 to December 2000.
We explore how Coulomb stress transfer might control aftershock distribution, long-term seismicity, and postseismic slip in a ramp-flat thrust fault system. The Mw=7.6 Chi-Chi shock, with a surface-cutting 30°-dipping ramp fault merging into a near-horizontal d?collement, is representative of continental thrust systems throughout the world, and so inferences drawn from this uniquely well-recorded event may be widely applicable elsewhere. The 3D distribution of aftershocks and their focal mechanisms are consistent with the calculated spatial distribution of Coulomb stress changes. Here one compares the percentage of planes on which failure is promoted after the main shock relative to the percentage beforehand. For Chi-Chi we find a 28% increase for thrust and an 18% increase for strike-slip mechanisms, commensurate with increases reported for other large main shocks. However, perhaps the chief criticism of static stress triggering is the difficulty in observing predicted seismicity rate decreases in the stress shadows, or sites of Coulomb stress decrease. Detection of sustained drops in seismicity rate demands a long catalog with a low magnitude of completeness and a high seismicity rate, conditions that are met at Chi-Chi. We find four lobes with statistically significant seismicity rate declines of 40–90% for 50 months, and they coincide with the stress shadows calculated for strikeslip faults, the dominant faulting mechanism. The rate drops are evident in uniform cell calculations, 100-month time series, and by visual inspection of the M≧3 seismicity. An additional reason why detection of such declines has proven so rare emerges from this study: there is a widespread increase in seismicity rate during the first 3 months after Chi-Chi, and perhaps many other main shocks, that might be associated with a different mechanism. And nearly all the M≧6 aftershocks are found to be promoted by several bars as a result of the mainshock.
We further consider whether the stresses imparted by the coseismic slip could have triggered postseismic slip on the fault. We find a fair correlation between the inferred postseismic slip and regions of calculated stress increase on the ramp and d?collement. The correlation of stress with slip is best if the fault friction is very high (μ?=0.8) along the uppermost 5 km of the ramp, and if friction is exceedingly low (μ?=0.0) along the d?collement. Finally, we search for a change in aftershock distribution and rate caused by the postseismic d?collement slip. We find a marked decrease in aftershocks with respect to Omori decay where the postseismic slip is calculated to further depress the Coulomb stress, and an increase of seismicity and the rate of M ≧ 5.0 earthquakes in the corresponding positively stressed zones.
The GPS observations suggest significant slip on the hanging wall of the Chelungpu fault, while little surface deformation is observed on the footwall. Repeated precise leveling survey across central Taiwan also shows significant uplift on the hanging wall of the Chelungpu faults during Aug. 2002 to Mar. 2004. we consider both of the afterslip and viscoelastic rebound behaviors during postseismic period to compare with geodesy data. They show good correlation between GPS observation on the hanging wall with the calculation deformation based on afterslip. On the footwall, by contrast, the calculation deformation based on viscoelastic rebound seems fit the observation better.
We also estimate the stress evolution along the faults near the Chelungpu fault. During coseismic period, the shear stress along Changhua fault seems promoted by mainshock, while the normal stress is dropped. For the Shiaomao and Hsuilikeng faults, the normal stress is promoted, while the shear stress is dropped. During 15 mo. after Chi-Chi, the shear stress is dropped at shallow part of the Changhua fault, while it is enhanced at deep part. It also shows the normal stress along all of the faults is promoted them to failure. 50 years after Chi-Chi, the shear stress at deeper part of the Changhua, Shiaomao and most part of Hsuilikeng is promoted to failure. For the normal stress, except at shallow part of the Changhua fault, it shows stress dropped at rest of the faults.

並列關鍵字

Chi-Chi earthquake ； Coulomb stress change ； visco-elastic rebound

參考文獻

蔡宜純（2006），1999集集大地震後之黏彈性鬆弛效應，國立中央大學地球物理研究所碩士論文。in prep.

Anderson, E.M. (1951), The dynamics of faulting and dyke formation with applications to Britain, Edinburgh, United Kingdom, Oliver and Boyd. 206 p.

Chan, C. H. and K. F. Ma (2004), Association of Five Moderate-Large Earthquakes to the Faults in Taiwan. Terr. Atmos. Oceanic Sci., 11, 97-110.

Cheng, W. B. (2000), Three-dimensional crustal structure around the source area of the 1999 Chi-Chi earthquake in Taiwan and its relation to the aftershock locations, Terr. Atmos. Oceanic Sci., 11, 643-660.

Chi, W.-C., and D. Dreger (2004), Crustal deformation in Taiwan: Results from finite source inversions of six Mw > 5.8 Chi-Chi aftershocks, J. Geophys. Res., 109, B07305, doi:10.1029/2003JB002606.