2015年7月3日新疆皮山MS6.5地震烈度與地震地質(zhì)災(zāi)害特點(diǎn)①

姚　遠(yuǎn),胡偉華,吳國棟,譚　明,阿里木江,常想德,羅　炬

(新疆維吾爾自治區(qū)地震局,新疆 烏魯木齊 830011)

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震災(zāi)報(bào)道

2015年7月3日新疆皮山MS6.5地震烈度與地震地質(zhì)災(zāi)害特點(diǎn)①

姚遠(yuǎn),胡偉華,吳國棟,譚明,阿里木江,常想德,羅炬

(新疆維吾爾自治區(qū)地震局,新疆 烏魯木齊 830011)

2015年7月3日新疆和田地區(qū)皮山縣發(fā)生的MS6.5地震極震區(qū)烈度達(dá)到Ⅷ度,砂土液化、地裂縫和崩塌等地質(zhì)災(zāi)害發(fā)育。極震區(qū)內(nèi)砂土液化發(fā)生范圍廣泛,地表發(fā)生噴砂冒水和邊坡的側(cè)向移動,對道路、水利等設(shè)施造成了嚴(yán)重的破壞;地裂縫斷錯多處路面,寬度最大達(dá)30 cm,裂縫多呈NWW走向;崩塌主要發(fā)生在高階地邊坡地帶,人煙稀少,未造成人員、財(cái)產(chǎn)損失。本次地震地質(zhì)災(zāi)害是新疆近十年來MS6以上較為嚴(yán)重的一次。

2015年皮山MS6.5地震; 地震地質(zhì)災(zāi)害; 砂土液化

0　引言

2015年7月3日9時(shí)7分,新疆維吾爾自治區(qū)和田地區(qū)皮山縣(37.6°N,78.2°E)發(fā)生MS6.5地震,震源深度10 km,震中烈度Ⅷ度。災(zāi)區(qū)總面積達(dá)到14 580 km2,涉及和田地區(qū)皮山縣及喀什地區(qū)葉城縣。

本次地震發(fā)生于西昆侖山地隆起區(qū)與塔里木盆地過渡地帶,震區(qū)中部以風(fēng)積、沖積平原及傾斜平原區(qū)為主,北部為塔里木盆地南緣沙漠地區(qū),南部為西昆侖山前的中、高山及丘陵區(qū)。新構(gòu)造運(yùn)動以來,西昆侖隨著帕米爾的隆升,塔里木盆地持續(xù)下降,西昆侖山前分布多條活動斷裂,在塔里木盆地邊緣地帶分布有澤普隱伏斷裂和皮山東南隱伏斷裂,此次地震的微觀震中位于這兩條隱伏斷裂之間(圖1)。澤普隱伏斷裂是塔里木盆地西南大型的隱伏斷裂,斷裂走向NW-NWW,從莎車縣西南經(jīng)澤普縣延至皮山縣東南。該斷裂是與澤普背斜相伴生的逆斷層,斷錯第四系—寒武統(tǒng),屬于第四紀(jì)隱伏活動斷層。

圖1　2015年7月3日皮山MS6.5地震區(qū)域構(gòu)造圖Fig.1　The regional structuring map of Pishan MS6.5 earthquake on July 3,2015

1　地震烈度判定

本次地震震中主體位于新疆和田地區(qū)皮山縣,震中海拔1 400 m。對災(zāi)區(qū)3個縣(皮山縣、墨玉縣、葉城縣)320個烈度調(diào)查點(diǎn)展開實(shí)地調(diào)查,烈度圖等震線長軸呈NWW走向分布(圖2)[1]。其中Ⅷ度極震區(qū)的長半軸為24 km,短半軸14 km,面積達(dá)到1 110 km2,涉及皮山縣固瑪鎮(zhèn)、皮西那鄉(xiāng)、科克鐵熱克鄉(xiāng)、木奎拉鄉(xiāng)及兵團(tuán)第十四師皮山農(nóng)場;Ⅶ度區(qū),長半軸達(dá)到45 km,短半軸為32 km,面積達(dá)3 410 km2,涉及皮山縣固瑪鎮(zhèn)、皮西那鄉(xiāng)、科克鐵熱克鄉(xiāng)、木奎拉鄉(xiāng)、喬達(dá)鄉(xiāng)、木吉鎮(zhèn)、桑株鄉(xiāng)、闊什塔格鄉(xiāng)、巴什蘭干鄉(xiāng)及兵團(tuán)第十四師皮山農(nóng)場;Ⅵ度區(qū)西北自葉城縣喀格勒克鎮(zhèn),東南至皮山縣皮亞勒瑪鄉(xiāng),長半軸為90 km,短半軸為51 km,面積10 060 km2。Ⅵ度區(qū)及以上總面積為14 580 km2[2]。

2　地震地質(zhì)災(zāi)害特征

皮山地震宏觀震中位于皮山縣皮西那鄉(xiāng),極震區(qū)烈度達(dá)到Ⅷ度,極震區(qū)長軸方向NWW。在極震區(qū)及Ⅶ度區(qū)內(nèi)發(fā)育有砂土液化(噴砂冒水)、地裂縫、崩塌等地震地質(zhì)災(zāi)害(表1)。

2.1砂土液化與噴砂冒水

砂土液化是飽和的粉砂、細(xì)砂在震動作用下,砂層中的孔隙水來不及排出,孔隙水壓力突然升高,有效應(yīng)力減小,致使砂土層出現(xiàn)液態(tài)的物理性狀。砂土液化的機(jī)制就是飽和疏松的粉砂、細(xì)砂土在震動力的作用下顆粒間的鏈接骨架喪失,顆粒移動、變密的過程。由于粉砂與細(xì)砂的滲透力較弱,骨架消失后孔隙水壓力會急劇增大,當(dāng)孔隙水壓力達(dá)到并超越總應(yīng)力時(shí),砂土體的有效應(yīng)力降為0,砂顆粒懸浮于水中,砂土體即發(fā)生液化[3-4]。

圖2　2015年7月3日皮山MS6.5地震烈度圖*新疆維吾爾自治區(qū)地震局，2015年7月3日皮山6.5級地震災(zāi)害損失評估報(bào)告.2015

表1　地震地質(zhì)災(zāi)害分布特征表

本次地震在極震區(qū)有大量的噴沙冒水現(xiàn)象,其中在皮西那鄉(xiāng)5隊(duì),農(nóng)田出現(xiàn)多處噴砂冒水現(xiàn)象,噴砂口直徑約30 cm,噴出錐直徑達(dá)到1.5 m,噴出物為細(xì)砂和粉砂(圖3)。

在皮山縣固瑪鎮(zhèn)雅普泉北側(cè)2.5 km處發(fā)現(xiàn)一系列的噴砂冒水口(圖4),呈線狀分布于農(nóng)田低洼積水處。砂土液化還導(dǎo)致雅普泉北側(cè)一水塘邊的黃土邊坡出現(xiàn)坍塌現(xiàn)象。本次地震中砂土液化造成的損失較大。

皮山縣處于昆侖山西麓,山前地帶北側(cè)為塔克拉瑪干沙漠,地基土主要由粉砂與細(xì)砂組成,分選級配較差。該區(qū)地下水位較淺,因此發(fā)生砂土液化的地點(diǎn)較多、情況較重。

2.2地裂縫

地裂縫一般指地震造成的沒有明顯錯動的地面裂縫[5]。在地震烈度Ⅷ度的皮山縣皮西那鄉(xiāng)庫木恰克村的果園內(nèi)見兩條地裂縫,其中西北側(cè)地裂縫壁面近直立,走向300°,長20 m,寬30 cm,裂縫中植被根系斜拉呈水平右旋錯動,錯距達(dá)7 cm;東南側(cè)地裂縫走向310°,長18 m,寬2～10 cm,植被根系斜拉右旋錯動,錯距為2 cm(圖5)。

在皮山縣皮西那鄉(xiāng)5隊(duì),地裂縫斷錯道路,呈5列平行展布,總體分布寬約15 m。地裂縫的寬度在0.5～1.5 cm,且延伸至路邊民院內(nèi),延伸長度達(dá)15～20 m,裂縫的總體走向?yàn)?10°(圖6)。

本次地震發(fā)現(xiàn)一些構(gòu)造作用形成的地裂縫,如皮山縣皮西那鄉(xiāng)鄉(xiāng)間公路上見柏油道路被擠壓折斷[圖7(a)],距該處約500 m,有一條拉張型裂縫寬度達(dá)到15 cm[圖7(b)]。以上現(xiàn)象說明,此次地震未在地表形成破裂帶,但還是有一定的擠壓變形,應(yīng)屬于同震變形。

2.3崩塌

皮山MS6.5地震發(fā)生在昆侖山前沖洪積扇前緣,極震區(qū)內(nèi)地質(zhì)災(zāi)害主要以砂土液化和地裂縫為主。在皮山縣城西南面的巴什蘭干鄉(xiāng),一些高階地受到河流的側(cè)向侵蝕形成臨空面[5],其主要物質(zhì)為膠結(jié)較為堅(jiān)硬的晚更新統(tǒng)礫石及黃土,在本次地震中出現(xiàn)了不同程度的崩塌(圖8)。

3　結(jié)論

通過對2015年7月3日皮山MS6.5地震的地質(zhì)災(zāi)害調(diào)查分析,得到以下初步認(rèn)識:

(1)本次地震的震級雖然不是特別大,但造成的地震地質(zhì)災(zāi)害卻非常豐富,尤其是砂土液化和地裂縫呈大面積片狀分布;

圖3　皮山縣皮西那鄉(xiāng)農(nóng)田中噴砂冒水口Fig.3　The sandboils and waterspouts in farmland in Pixina town

圖4　皮山縣固瑪鎮(zhèn)噴砂冒水、塌陷Fig.4　The sandboils and waterspouts,and collapse in Guma town

圖5　皮山縣皮西那鄉(xiāng)庫木恰克村果園內(nèi)地裂縫Fig.5　The ground fissures in orchard at Kumuqiake village

圖6　皮山縣皮西那鄉(xiāng)地裂縫Fig.6　The ground fissures in Pixina town

圖7　皮山縣皮西那鄉(xiāng)地裂縫斷錯公路Fig.7　Road broken by ground fissures in Pixina town

圖8　皮山縣巴什蘭干鄉(xiāng)高階地臨空面崩塌Fig.8　The free face of high terrace is collapse in Bashilangan town

(2)皮山縣極震區(qū)內(nèi)鄉(xiāng)鎮(zhèn)處于西昆侖山前沖洪積區(qū)前緣,地下水淺埋,沖積、淤積粉細(xì)砂為同震砂土液化提供了有利的地質(zhì)條件,伴隨著地表噴砂冒水、砂土體側(cè)向滑動等次生地質(zhì)災(zāi)害造成了道路開裂和路面下沉等破壞效應(yīng);

(3)同震砂土液化區(qū)主要分布在極震區(qū),屬于全新世粉細(xì)砂沖積、風(fēng)積地層區(qū)域,液化嚴(yán)重區(qū)為皮山縣皮西那鄉(xiāng)低洼、老河道所在范圍,而晚更新世的廢棄高階地由于地基穩(wěn)定,未出現(xiàn)砂土液化,僅在部分區(qū)域出現(xiàn)了一定的崩塌。

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Geological Hazard Characteristics and Seismic Intensity of July 3,2015,Pishan,Xinjiang,MS6.5 Earthquake

YAO Yuan,HU Wei-hua,WU Guo-dong,TAN Ming,Alimujiang,CHANG Xiang-de,LUO Ju

(Earthquake Administration of the Xinjiang Uygur Autonomous Region,Urumqi 830011,Xinjiang,China)

Based on field surveys,investigations,and seismo-geological hazards in relation to the Pishan,Xinjiang (MS6.5)earthquake that occurred on July 3,2015,this paper summarizes the associated distribution and development characteristics of sandy soil liquefaction,ground fissures,collapse,and ground deformation.The seismic intensity of the earthquake is described simply as follows.The earthquake occurred in Pishan county,Hetian City,Xinjiang,and the area of intensity of Ⅵ degrees ranged for 14 580 km2,from the west of the city to Kashi City in Yecheng County and to Moyu County,Hetian in the east.The meizoseismal area (Ⅷ degree)was 1 110 km2,the length of the major semi-axis was 45 km,and the length of the semi-minor axis was 32 km in the Ⅶ degree area.Seismo-geological hazards relating to this earthquake are mainly distributed in the meizoseismal area,where sandy soil liquefaction is particularly serious,and mainly occurred in Guma and Pixina Towns,Pishan county.The sand blasting nozzle was linearly distributed throughout a field of low-lying water,causing collapse of the loess side slope in the north of Yapu spring.Pishan county is located in the west of the Kunlun mountains and on the edge of the Tarim basin.The soil in this area is mainly composed of silt and fine sand,gradation is poor and groundwater shallow,thus sand soil liquefaction is very serious.Ground fissures and cracks occurring during the earthquake caused slight dislocation,and many can be seen in the meizoseismal area of Pixina town (ground fissures measuring 20 m long and 30 cm wide).Root systems show horizontal right lateral slip in these ground fissures,and the stagger distance is 2 to 7 cm.The road in Pixina Town was crushed and broken,which showed that the earthquake did not cause a rupture zone on the surface,but that compression deformation occurred,which is a type of co-seismic deformation.Although the magnitude of the Pishan earthquake was not large,it resulted in considerable seismo-geologic hazards,particularly sand soil liquefaction and ground fissures over a large area.Key words:the 2015 Pishan MS6.5 earthquake; seismic geological disasters; sand liquefaction

2015-07-29

新疆地震科學(xué)基金資助課題(201513);中國地震局地震應(yīng)急青年重點(diǎn)任務(wù)(CEA_EDEM-201512)

姚遠(yuǎn)(1988-),男,工程師,主要從事地震地質(zhì)研究。E-mail:yy8096658@126.com。

P315.9

A

1000-0844(2016)04-0663-06

10.3969/j.issn.1000-0844.2016.04.0663