昌馬斷裂帶古地震活動(dòng)特征的新認(rèn)識(shí)①

羅　浩,何文貴,袁道陽,邵延秀,,王慶民

(1.中國(guó)地震局地質(zhì)研究所活動(dòng)構(gòu)造與火山重點(diǎn)實(shí)驗(yàn)室,北京 100029; 2.中國(guó)地震局蘭州地震研究所,甘肅 蘭州 730000;3.蘭州地球物理國(guó)家野外科學(xué)觀測(cè)研究站,甘肅 蘭州 730000; 4.山東省地震局,山東 濟(jì)南 250014)

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昌馬斷裂帶古地震活動(dòng)特征的新認(rèn)識(shí)①

羅浩1,何文貴2,3,袁道陽2,3,邵延秀1,2,3,王慶民4

(1.中國(guó)地震局地質(zhì)研究所活動(dòng)構(gòu)造與火山重點(diǎn)實(shí)驗(yàn)室,北京 100029; 2.中國(guó)地震局蘭州地震研究所,甘肅 蘭州 730000;3.蘭州地球物理國(guó)家野外科學(xué)觀測(cè)研究站,甘肅 蘭州 730000; 4.山東省地震局,山東 濟(jì)南 250014)

昌馬斷裂位于祁連山西段,是祁連山系列次級(jí)斷裂與阿爾金斷裂東段的重要構(gòu)造轉(zhuǎn)換斷層之一,于1932年發(fā)生7.6級(jí)地震。位于昌馬斷裂中東段的臭水柳溝古地震探槽揭示了2次地震事件:一次為1932年昌馬地震事件,另一次為(902±44)a B.P.以來發(fā)生的事件,這彌補(bǔ)了昌馬斷裂全新世晚期古地震事件缺失的現(xiàn)狀。結(jié)合前人的研究結(jié)果可確定昌馬斷裂全新世至少發(fā)生7次古地震事件,推測(cè)地震復(fù)發(fā)間隔為1 ka左右,部分事件未能揭示。通過探槽揭示的低角度斷層、地層變形和部分?jǐn)嗔训牡孛蔡卣骺芍?受阿爾金斷裂NEE向擠出的影響,昌馬斷裂部分段落表現(xiàn)出低角度的逆沖推覆活動(dòng),形成其特有的低角度走滑現(xiàn)象,以吸收阿爾金斷裂東段的左旋位移。這也說明昌馬斷裂在承擔(dān)阿爾金斷裂與祁連山西段系列斷層的構(gòu)造轉(zhuǎn)換中起著重要作用。

古地震事件; 地震復(fù)發(fā)間隔; 低角度逆斷層; 昌馬斷裂

0　引言

F1:阿爾金斷裂;F2:旱峽—大黃溝斷裂;F3:昌馬斷裂;F4:大雪山北山斷裂;F5:鷹嘴山南緣斷裂;F6:野馬河—大雪山北緣斷裂;F7:野馬河南側(cè)斷裂;F8:肅北西南逆沖斷裂;F9:黨河南山逆沖斷裂;F10:后塘斷裂圖1　研究區(qū)構(gòu)造簡(jiǎn)圖Fig.1　Tectonic map of the study area

1932年沿昌馬斷裂帶發(fā)生的M7.6地震,引起了長(zhǎng)約120 km的地表破裂[7-8]。破裂帶主要由一系列地裂縫、張裂隙、壓性鼓包、逆斷層陡坎和水系與山脊斷錯(cuò)等變形組成[9]。地震引起最大同震位移水平向?yàn)?.5 m,垂直向?yàn)?.9 m[9],并觸發(fā)大量的山體滑坡等次生災(zāi)害[10]。前人曾針對(duì)昌馬斷裂開展過一些古地震研究工作?？祦硌竅11]認(rèn)為昌馬斷裂晚更新世以來主要發(fā)生4期活動(dòng),除1932年地震,最近一次地震事件活動(dòng)時(shí)間為(3 690 ±110)a以來,地震的平均復(fù)發(fā)周期在6 000 a左右。國(guó)家地震局蘭州地震研究所[9]至少開挖了9個(gè)探槽,揭示倒數(shù)第二次地震事件也在3 000多年前,這顯示了昌馬斷裂的長(zhǎng)復(fù)發(fā)周期。

然而,前人的探槽多開挖在較老階地或洪積扇上,由于缺少新沉積物,未能揭露斷層的最新活動(dòng)事件。本文選擇在沖溝的低級(jí)階地開挖探槽,補(bǔ)充了斷層的全新世晚期倒數(shù)第二次活動(dòng)事件,并整理前人的研究結(jié)果進(jìn)行分析,揭示了相對(duì)可靠的地震活動(dòng)周期。

1　地質(zhì)背景

昌馬斷裂帶位于祁連山西段、山脈的北部,沿昌馬—西水峽盆地南緣展布,由4條次級(jí)斷裂組成[圖1(b)],總體走向NWW,各分段方向呈反S型延伸。昌馬斷裂形成于古生代加里東期,晚第三紀(jì)末斷裂再一次強(qiáng)烈活動(dòng)[8]。昌馬斷裂南側(cè)山脈主要由前震旦紀(jì)云母片巖、石英巖、結(jié)晶灰?guī)r和寒武紀(jì)變質(zhì)砂巖、千枚巖、板巖等組成;北側(cè)的山脈由奧陶紀(jì)火山巖、火山碎屑巖、厚層灰?guī)r組成。昌馬—西水峽盆地內(nèi)部主要沉積第四紀(jì)沉積物[12]。斷裂西側(cè)的昌馬盆地,寒武、奧陶系沖覆于第四級(jí)地層之上,僅第四系斷距就達(dá)560 m[7]。

昌馬斷裂曾多次強(qiáng)烈活動(dòng),斷錯(cuò)一系列不同時(shí)代的山脊及規(guī)模大小不等的沖溝。前人將斷裂分為四段,晚更新世以來斷裂的左旋滑動(dòng)速率具有由西向東增加的趨勢(shì)。位于西段的堿泉子附近左旋滑動(dòng)速率為(1.32±0.35)mm/a左右[13],而東段紅溝地區(qū)的滑動(dòng)速率達(dá)到(5.5±2.2)mm/a[7]。斷裂水平縮短速率由西向東具有減小的趨勢(shì),在堿泉子為(0.70±0.20)mm/a,而在東段的大泉口處斷層則表現(xiàn)為張性的特征[13]。

2　新古地震事件的限定

昌馬斷裂中東段的臭水柳溝地區(qū)具有較高的左旋滑動(dòng)速率,可達(dá)(3.68±0.41)mm/a[14]。洪積扇由北向南平緩下降,在洪積扇中部,由于斷層活動(dòng)形成明顯的反向陡坎(圖2)。沖溝主要發(fā)育4級(jí)階地,一級(jí)階地的陡坎高度為2.5 m左右,二級(jí)階地為4 m 左右[圖3(b)]。探槽開挖在洪積扇東端,沖溝的一級(jí)階地上(圖2),斷層陡坎前緣為1932年7.6級(jí)地震破裂帶[圖3(a)]。探槽垂直于陡坎開挖,尺寸為16 m(長(zhǎng))×2 m(寬)×2～3 m(深)。

圖2　臭水柳溝階地地貌解譯Fig.2　Geomorphic interpretation of Choushuiliugou terrace

圖3　臭水柳溝地貌特征Fig.3　Geomorphic features of Choushuiliugou area

臭水柳溝探槽東剖面斷層的上盤主要揭示4組地層(圖4):

U1砂礫層:灰黃色,層理不明顯,夾有少量的大塊礫石,直徑一般不大于10 cm。在上盤近斷層處,礫石定向排列,向下彎曲,形成逆斷層變形引起的拖曳褶皺;

U2黃土層:灰黃色,探槽南側(cè)中夾有砂礫層透鏡體,土黃色,礫石直徑多在2 cm以下,少見大礫石;

U3砂礫層:青灰色,礫石最大直徑可達(dá)10 cm,磨圓好,長(zhǎng)軸方向水平。在斷層上盤該層厚達(dá)20 cm,由于斷層的活動(dòng)在F2的上盤形成拖曳褶皺,在F1的下盤該層厚度在10 cm左右;

U4地表土:灰黃色,夾有草根、腐蝕質(zhì)。厚度不均勻,在變形處較薄,厚約20 cm。在F2的上盤,變形尾端厚度近50 cm,在F1的下盤,該層厚度也可達(dá)50 cm。

探槽揭示2條斷層,在探槽上部分為F1、F2,在探槽底部匯合成一條斷層,斷層走向320°,傾角20°,傾向SW。斷層上盤地層整體向NE方向傾斜,這與區(qū)域地形特征相反。

圖4　臭水柳溝探槽東壁剖面簡(jiǎn)要素描圖Fig.4　East section sketch of the Choushuiliugou trench

地震事件分析:

事件1:F1斷錯(cuò)了U1～U3層,U1層斷錯(cuò)的斜距為115 cm,其與U3層頂部的斜距相近,并在F1下盤形成有崩積楔,因此認(rèn)為其為一次事件。

事件2:F2斷錯(cuò)U1～U4層,為1932年昌馬地震。由于后期的上盤剝蝕U4層變薄,下盤斷塞塘持續(xù)接收沉積,U4層明顯厚于上盤。斷層F2破裂到地表,1932年昌馬7.6級(jí)地震引起U3層斷錯(cuò)斜距40 cm。U3層兩次事件的累積斜距明顯大于U1層,趨向于斷層的走滑相應(yīng)引起。

為確定古地震發(fā)生的年代,在探槽下盤b層底部采集1個(gè)14C樣品CSLG-C14-1,為U2層斷錯(cuò)前的年代。該樣品結(jié)果為(902±44)a B.P.。因此事件1的活動(dòng)時(shí)間在(902±44)a B.P.之后,并接近該時(shí)間點(diǎn)。

4　討論

4.1古地震復(fù)發(fā)間隔

結(jié)合前人的研究資料,利用逐次限定法重新分析了昌馬斷裂的古地震活動(dòng)特征[14]。國(guó)家地震局蘭州地震研究所[9]開挖的探槽共揭示了8次古地震事件(圖5)。各事件活動(dòng)時(shí)間分別為:事件Ⅰ(20 790±1 440)a B.P.、事件Ⅱ(18 800±1 800)a B.P.、事件Ⅲ(12 995±420)a B.P.、事件Ⅳ(9 450±160)a B.P.、事件Ⅴ(8 590±230)a B.P.、事件Ⅵ(6 670±80)a B.P.、事件Ⅶ(5 270±260)a B.P.及事件Ⅷ約(3 270±95)a B.P.。本文將臭水柳溝探槽揭示的事件命名為Ⅸ事件,活動(dòng)時(shí)間約為(902±44)a B.P.以來,1932年昌馬地震則認(rèn)為是第Ⅹ次事件。由此可見全新世以來斷裂共發(fā)生7次事件,間隔分別約為:900 a、1 900 a、1 400 a、2 000 a、2 200 a及900 a。

該點(diǎn)斷裂的左旋滑動(dòng)速率為(3.68±0.41)mm/a[13],1932年地震的最大同震水平位移為4 m[9],因此可估計(jì)昌馬斷裂的強(qiáng)震復(fù)發(fā)間隔為1 000 a左右,這與部分古地震事件間隔一致。假設(shè)昌馬斷裂的大震復(fù)發(fā)模式符合特征地震模型,那么具有長(zhǎng)復(fù)發(fā)間隔的兩次相鄰古地震之間可能存在1次未知地震事件,例如第Ⅷ和Ⅸ事件之間。由此可推測(cè)全新世以來昌馬斷裂可能發(fā)生了10次地震事件,這些事件的復(fù)發(fā)間隔在1 ka左右。

圖5　昌馬斷裂古地震事件年代分布圖Fig.5　Distribution map of paleoearthquakes along the Changma Fault

4.2低角度走滑斷層

走滑斷層剖面上多顯示為高傾角斷層,主要表現(xiàn)為花狀構(gòu)造樣式。青藏高原大型走滑斷層的多條探槽揭示了類似的構(gòu)造性質(zhì),如阿爾金斷裂[15-17]、東昆侖斷裂[18]及鮮水河斷裂[19]。大型走滑斷層甚至由下往上表現(xiàn)為多組花狀構(gòu)造的疊加[20]。青藏高原周邊的中小型壓扭性走滑斷層其傾角多大于60°[21-24],然而臭水柳溝探槽揭示昌馬斷裂在該點(diǎn)主要表現(xiàn)為低角度的逆沖斷層。這并非個(gè)例,位于斷裂西段和中西段的堿泉子探槽和捷達(dá)板溝探槽揭示斷層的傾角分別為40°和30°[9],明顯有別于東段的高角度左旋走滑斷層的活動(dòng)特征[25]。

臭水柳溝探槽揭示斷層上盤地層傾向與正常地層形成傾向相反,說明其受到斷層的推覆作用發(fā)生傾斜,近斷層處地層表現(xiàn)為斷層的拖曳變形,遠(yuǎn)離斷層更可能表現(xiàn)為平緩的褶皺變形。在西段堿泉子地區(qū)也有類似的現(xiàn)象,部分?jǐn)鄬颖憩F(xiàn)為平緩的陡坎,影響范圍超過100 m,推測(cè)為斷層在厚層洪積扇中形成斷彎褶皺所致[13]。臭水柳溝一級(jí)階地陡坎的高度為(2.5±0.1)m左右(圖6),同級(jí)階地的左旋走滑位移為(16±1)m。水平縮短與走滑比值約為1∶5,堿泉子附近該比值為1∶2,說明從地表變形看斷裂的中東段-西段表現(xiàn)出低角度左旋走滑斷層的特征。我們推測(cè)形成這種地表變形的原因有2個(gè):一是昌馬斷裂整體表現(xiàn)為低角度走滑斷層,斷層上盤沿傾角為20°～45°的斷面發(fā)生水平滑動(dòng)引起上盤地層的強(qiáng)烈變形,且這種現(xiàn)象十分少見;其次是受阿爾金斷裂活動(dòng)的影響,該區(qū)域發(fā)生NEE或EW向擠出,由于部分段落與區(qū)域應(yīng)力夾角不同,盡管仍以左旋走滑為主,但局部水平縮短分量明顯增大,表現(xiàn)出低角度推覆作用。我們更趨向于第二種推測(cè)。假如推測(cè)成立,說明昌馬斷裂晚更新世以來水平縮短作用明顯,吸收大量的阿爾金斷裂東段左旋位移,在阿爾金斷裂與祁連山西段構(gòu)造轉(zhuǎn)換作用中承擔(dān)越來越重要的作用。

圖6　臭水柳溝斷層陡坎剖面Fig.6　Scarp profile of the fault in Choushuiliugou area

5　結(jié)論

(1)臭水柳溝探槽揭示2次地震事件,活動(dòng)時(shí)間分別為(902±44)a B.P.之后,及1932年昌馬7.6級(jí)地震。

(2)通過對(duì)前人開挖探槽揭示的古地震事件和樣品年代的重新整理分析,認(rèn)為昌馬斷裂至少發(fā)生了10次古地震事件,其中至少有7次發(fā)生在全新世。基于特征地震模型推測(cè)昌馬斷裂的強(qiáng)震復(fù)發(fā)間隔可能為1 ka左右,部分古地震事件現(xiàn)有探槽未能揭示。

(3)昌馬斷裂部分段落表現(xiàn)為低角度的逆沖斷層,形成其特有的低角度走滑現(xiàn)象。以此形式斷裂吸收了阿爾金斷裂東段部分左旋走滑位移,在阿爾金斷裂與祁連山西段一系列次級(jí)斷層的構(gòu)造轉(zhuǎn)換中起重要的作用。

致謝:文中涉及到的14C年代學(xué)樣品由蘭州大學(xué)西部環(huán)境教育部重點(diǎn)實(shí)驗(yàn)室測(cè)試,在此致謝。

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New Insight on Paleoearthquake Activity along Changma Fault Zone

LUO Hao1,HE Wen-gui2,3,YUAN Dao-yang2,3,SHAO Yan-xiu1,2,3,WANG Qing-min4

(1.Key Laboratory of Active Tectonics and Volcano,Institute of Geology,CEA,Beijng 100029,China;2.Lanzhou Institute of Seismology,CEA,Lanzhou 730000,Gansu,China;3.Lanzhou National Observatory of Geophysics,Lanzhou 73000,Gansu,China;4.Earthquake Administration of Shandong Province,Jinan 250014,Shandong,China)

The Changma fault consists of four sub-faults,and is an important transform fault located between the Altyn Tagh fault and the western segment of the Qilian Mountains.In 1932,a M7.6 earthquake occurred in the fault,forming a rupture with a length of approximately 120 km that is composed of a series of fissures,fault scarps,offset terraces,and gullies.The maximum horizontal and vertical coseismic displacement was up to 5.5 m and 1.9 m,respectively.A trench excavated at Choushuiliugou in the middle-eastern segment of the Changma fault shows evidence of two paleoearthquakes:one is the Changma earthquake (1932)and the other earthquake occurred after 902 ± 44 a B.P..Combined with the results of previous studies,it was determined that seven paleoearthquakes have occurred in the Holocene. Base on characteristic earthquake model,we speculate an earthquake recurrence interval of approximately 1 ka,and part of paleoearthquake events are not revealed by the trench.The trench also shows that the dip direction of the layer on the hanging wall is opposite to that of the down wall and low-angle fault.Based on a survey of dislocation landforms,the ratio of horizontal shortening and left strike slip was found to be 1∶5.Eastern extrusion occurred in relation to the influence of Altyn Tagh fault activities in the study area.The change in fault trend has resulted in regional stress and differing angles between some of the fault segments,particularly the tip of sub-faults.Active fault characteristics are shown predominantly as sinistral strike-slip,but there is an evident increase in the component of horizontal shortening,which forms nappe activities on the hanging wall.It also forms a specific low-angle strike-slip phenomenon,which illustrates that the Changma fault has prominently presented crustal shortening since the late Pleistocene,in addition to absorbing much of the sinistral displacement of the Altyn Tagh fault,and playing a significant role in the tectonic transition between the Altyn Tagh fault and a series of faults in the western segment of the Qilian mountains.

paleoearthquake; earthquake recurrence interval; low-angle thrust fault; Changma fault

2015-07-30

中國(guó)地震活斷層探察-南北地震帶北段項(xiàng)目(20140802306);國(guó)家自然基金項(xiàng)目(40872132)

羅浩,男,博士后,主要從事地震地質(zhì)與地質(zhì)災(zāi)害方面的工作。E-mail:hy-luo@163.com。

P546

A

1000-0844(2016)04-0632-06

10.3969/j.issn.1000-0844.2016.04.0632