郭祥云, 陳學(xué)忠, 李艷娥, 王生文

(1.中國(guó)地震局地球物理研究所,北京　100081; 2.中國(guó)地震局第一監(jiān)測(cè)中心,天津　300180)

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四川蘆山7.0級(jí)地震前中小地震P軸方位角CV值的變化①

郭祥云1, 陳學(xué)忠1, 李艷娥1, 王生文2

(1.中國(guó)地震局地球物理研究所,北京100081; 2.中國(guó)地震局第一監(jiān)測(cè)中心,天津300180)

摘要:通過對(duì)2003年1月1日—2013年4月1日蘆山地震前震源區(qū)中小地震震源機(jī)制解的分析,發(fā)現(xiàn)不同階段的震源機(jī)制解在一定程度上反映了強(qiáng)震孕育過程中構(gòu)造應(yīng)力場(chǎng)隨時(shí)間的變化。震源區(qū)中小地震的P軸方位角CV值在蘆山M7.0地震發(fā)生前有一個(gè)上升-下降-上升的過程,只是相比于汶川8.0級(jí)地震前CV值的下降-上升過程經(jīng)歷了更長(zhǎng)的時(shí)間,這表明四川蘆山M7.0地震的孕育經(jīng)歷了長(zhǎng)時(shí)間的應(yīng)力積累,與許多研究結(jié)果相一致。2007年1月1日—2014年4月1日CV值空間分布的非均勻性特征在龍門山斷裂帶南段有顯著的增強(qiáng)與減弱過程,對(duì)于發(fā)震地點(diǎn)可能有一定的指示意義。

關(guān)鍵詞:震源機(jī)制解; P軸方位角; CV值

0引言

地震是地下介質(zhì)不堪承受大于其強(qiáng)度的應(yīng)力而發(fā)生破裂的自然現(xiàn)象,孕震區(qū)的應(yīng)力狀態(tài)在地震周期中起著決定性的作用[1-3]。應(yīng)力狀態(tài)的時(shí)空變化和深部介質(zhì)特性與地震的孕育、發(fā)展以及地震強(qiáng)度關(guān)系密切。小于5.0級(jí)的中小地震發(fā)生頻率較高,其震源機(jī)制解對(duì)于描繪區(qū)域構(gòu)造、確定區(qū)域應(yīng)力方向、探討地震成因機(jī)制、開展地震預(yù)報(bào)等研究具有重要意義[4-10]。

2013年4月20日8時(shí)2分,四川省雅安市蘆山縣(30.3°N,103.0°E)發(fā)生M7.0強(qiáng)震。該地震位于龍門山前緣構(gòu)造帶南段,距離2008年5月12日汶川M8.0特大地震震中約70 km。本文擬利用震源區(qū)中小地震震源機(jī)制的P軸方位角CV值的變化,討論孕震過程中構(gòu)造應(yīng)力場(chǎng)的變化規(guī)律及其可能的前兆指示意義,提取震源區(qū)介質(zhì)物性的變化,以期為科學(xué)預(yù)測(cè)未來逆斷層型地震發(fā)生時(shí)間和地點(diǎn)提供可借鑒的依據(jù)。

1研究方法

陳颙[5]提出了用一群地震震源斷層面走向均值的標(biāo)準(zhǔn)偏差,或用震源機(jī)制解參數(shù)k=(n1-n2)/N(其中N為震源機(jī)制解總數(shù);n1為震源機(jī)制一致性的數(shù);n2為不一致的數(shù))來描述地震機(jī)制的一致或混亂程度,以探索孕震區(qū)內(nèi)背景構(gòu)造應(yīng)力場(chǎng)的動(dòng)力學(xué)過程,進(jìn)而確定區(qū)域內(nèi)未來地震的危險(xiǎn)性。該方法中震源機(jī)制一致性參數(shù)是個(gè)相對(duì)的概念。澤仁志瑪?shù)萚8]提出用震源機(jī)制一致性參數(shù)來研究應(yīng)力場(chǎng)取向的變化,以發(fā)現(xiàn)構(gòu)造運(yùn)動(dòng)的情況及強(qiáng)震的前兆信息,但該方法需要先確定背景應(yīng)力場(chǎng),具有很大的不確定性。近年來,郭祥云[11]發(fā)展了基于震源機(jī)制解統(tǒng)計(jì)特征的P軸方位角變異系數(shù)的方法,該方法不依賴于先驗(yàn)的背景應(yīng)力場(chǎng),同時(shí)也具有較好的穩(wěn)定性。因此本研究采用此方法來探索中小地震震源機(jī)制解在地震趨勢(shì)分析中的應(yīng)用。

在統(tǒng)計(jì)學(xué)中,CV值為標(biāo)準(zhǔn)差與平均值的比值,即相對(duì)標(biāo)準(zhǔn)差方差,反映的是某個(gè)參數(shù)的非均勻分布特征,CV值越大,表示非均勻性越強(qiáng)；反之,非均勻性越弱。P軸方位角變異系數(shù)CV值或方差σ:

(1)

(2)

CV值的變化反映了P軸方位角空間取向的變化,間接地反映了孕震體應(yīng)力變化的過程。P軸方位角CV值上升-下降,表明非均勻性由增強(qiáng)過渡到減弱,間接反映了孕震體應(yīng)力場(chǎng)的變動(dòng)過程。上升-下降過程可能對(duì)應(yīng)了孕震體亞失穩(wěn)和臨失穩(wěn),應(yīng)力積累轉(zhuǎn)為應(yīng)力釋放,平穩(wěn)釋放轉(zhuǎn)為加速釋放,兩種階段對(duì)應(yīng)地震之前的孕育與臨震階段。

2數(shù)據(jù)資料

收集整理了四川省數(shù)字化區(qū)域地震臺(tái)網(wǎng)蘆山地震前龍門山構(gòu)造帶南段2003年1月至2013年3月的波形記錄資料。2003年1月—2013年3月蘆山震源區(qū)發(fā)生3.0級(jí)以上地震78次(圖1)。利用現(xiàn)在普遍流行的基于P波初動(dòng)和振幅比計(jì)算中小地震震源機(jī)制解HASH方法反演出滿足條件的72次地震的震源機(jī)制解(表1),其空間分布如圖1所示。

圖1　蘆山震源區(qū)2003年1月—2013年3月3.0級(jí)以上地震分布(藍(lán)色方框內(nèi)的地震被用于計(jì)算P軸方位CV值,紅色圓點(diǎn)表示有震源機(jī)制解,藍(lán)色圓點(diǎn)無震源機(jī)制解,三角表示蘆山地震震中位置)Fig.1　Distribution of earthquakes (ML≥3.0) from January,2003 to March,2013 in Lushan seismic area (Earthquakes in the blue box are used to calculate the CV value of P-axis azimuth red,and blue dot indicate focal mechanism and no focal mechanism,respectively,triangle indicates the epicenter location of Lushan M7.0 earthquake)

發(fā)震時(shí)間發(fā)震位置緯度/(°)經(jīng)度/(°)震源深度/km震級(jí)/MSPlaneⅠ走向/(°)傾角/(°)滑動(dòng)角/(°)P軸Az/(°)Pl/(°)2003-06-12T10:03:0029.97103.25153.224859-20212352003-09-19T09:12:0029.65103.3753.125646-159103422003-10-24T19:36:0030.49102.7553.2317538156142004-03-29T19:43:0030.00102.42143.711406330082004-06-14T01:23:0030.60102.6883.52822548223252004-07-14T05:08:0030.58102.8013.42113365139142004-09-16T07:12:0030.37102.9053.65529-91148742004-11-06T10:11:0029.48103.0283.033341-174183362004-12-26T09:52:0029.68102.97103.2283825043262005-04-20T11:02:0029.63102.97113.022273-6179162005-05-26T11:17:0029.62102.9753.035140-99135822005-06-14T01:03:0030.10102.7283.75561-175275232005-08-24T22:47:0029.37103.1553.09024-80161682005-09-30T13:32:0030.17103.2253.02088138334192006-02-20T04:16:0029.87102.68153.213566-1395262006-05-24T11:04:0029.88102.8353.2693496335112006-08-13T19:55:0029.90103.33143.11615018116172006-09-05T01:31:0029.82103.0553.61945899277132006-10-07T08:13:0029.82103.0353.132969928492006-10-20T21:22:0029.88102.5273.127345-156118452006-11-04T02:22:0029.73103.3273.0261418917242007-01-01T22:16:0029.57102.97123.5235999106142007-03-28T11:33:0030.53102.88133.012187157169142007-06-25T10:24:0030.53102.8253.143588275102007-07-02T19:07:0030.23103.3353.014356-3104252007-07-23T02:08:0030.73102.6763.35959174281182007-07-23T02:08:0030.73102.6763.35959174281182007-07-31T07:35:0029.53102.7384.38134-87160792007-08-02T06:06:0030.01102.6353.475389933972007-09-18T03:42:4430.22102.7753.332461-2283222008-08-12T02:49:3930.68102.65163.8339717482242008-09-14T01:41:2830.10103.10153.02656115313642008-11-15T01:55:0129.68103.5203.01686497253192008-12-02T04:01:4730.28102.23143.06668-116299592008-12-24T21:08:1230.17102.78183.32913520249272009-03-29T06:26:1429.73103.1353.220076-87114592009-05-12T06:44:3029.62103.62203.5295570255232009-06-10T14:18:5829.58102.77203.033367-2291182009-06-10T22:43:5629.88103.28103.6133471590202009-07-01T13:39:4429.70102.3853.510580-3558312009-08-04T04:04:0229.63103.28103.0309859237402009-09-19T12:16:3830.48102.65204.1938917613822009-10-17T15:53:1830.43103.0753.02696915731802009-11-25T22:01:1029.87103.28103.529238-155129482009-12-22T12:54:2729.85102.603.033641-170184382010-01-22T01:34:1429.48103.2793.120174-14159202010-02-20T02:57:4829.98103.38193.028350150154112010-03-05T06:32:1430.78102.75143.5661-3326222010-03-18T16:18:0930.83102.77143.112385-149351252010-04-29T17:37:3330.69102.5363.514482-9054532010-05-08T06:19:3930.13103.48173.73182-163256182010-06-27T11:21:4730.33103.28213.225019-48279572010-10-15T17:49:2329.63103.25143.121276-2916830

續(xù)表1

發(fā)震時(shí)間發(fā)震位置緯度/(°)經(jīng)度/(°)震源深度/km震級(jí)/MSPlaneⅠ走向/(°)傾角/(°)滑動(dòng)角/(°)P軸Az/(°)Pl/(°)2011-03-08T21:00:3730.84102.76123.132970-65273582011-03-24T23:18:1629.61103.18173.7159732329042011-03-30T13:53:0229.61103.20153.134148165204202011-07-08T17:06:2230.47102.88183.1936413415382011-07-25T15:33:5330.00103.14184.0415010512142011-08-08T05:42:0429.62102.72123.525509311352011-10-21T05:06:0130.02103.23163.522062110296152011-10-27T01:00:1929.64103.22154.013283-1287132012-07-01T09:53:4430.00102.42173.929226-170128442012-07-13T19:18:4929.35103.06183.92173498121112012-08-14T06:31:0930.07103.20153.132183-527782012-09-06T23:10:5330.03103.11163.1295987121142012-10-19T02:44:1630.14102.86264.126965-100159692012-10-22T21:27:0430.49102.99143.22618116830722012-12-03T02:13:5829.70103.35113.031152933972012-12-16T16:13:4330.46102.81173.1177801231122012-12-26T04:07:4529.01102.26143.33348617919922013-03-04T23:42:1730.98103.4464.1471-178205152013-04-01T06:21:1731.70103.2033.710067-11321261

3結(jié)果分析

3.1P軸方位角CV值隨時(shí)間的演化

利用計(jì)算得到的中小地震震源機(jī)制解,選取圖1中藍(lán)色矩形框范圍內(nèi)所有地震的P軸方位角進(jìn)行歸一化,以5個(gè)為一組計(jì)算一個(gè)CV值,以3個(gè)數(shù)據(jù)進(jìn)行滑動(dòng),可以看到P軸方位角的CV值隨時(shí)間的變化(圖2)。從圖中可以看出,從2003年1月—2013年3月蘆山地震前CV值有一個(gè)上升-下降-上升的過程。CV值從2002年開始上升,2004年6月上升到歷史最高值,之后緩慢下降,直到2008年12月份下降到最低值,下降過程長(zhǎng)達(dá)4年多,顯示該地區(qū)經(jīng)歷了很長(zhǎng)時(shí)間的應(yīng)力積累過程;2009年開始緩慢上升回返,在這個(gè)過程中發(fā)生了蘆山M7.0地震,地震前CV值變化過程與汶川地震類似[11],只是持續(xù)時(shí)間更長(zhǎng)。許多研究也表明四川蘆山M7.0地震是一個(gè)高角度逆沖斷破裂。如此高角度的逆斷層錯(cuò)動(dòng),只有在極大的差應(yīng)力下和長(zhǎng)期應(yīng)變能積累后才能產(chǎn)生[12]。龍門山斷裂帶位于青藏高原東南緣,由于青藏高原隆升引起地殼物質(zhì)東流,在此遇到四川盆地阻擋,因此應(yīng)變能長(zhǎng)期積累。定量計(jì)算模擬表明,整個(gè)龍門山斷裂帶上地殼底部都是應(yīng)力高增長(zhǎng)率區(qū)[13]。從圖3同樣可以看出從2009年10月開始CV值高值區(qū)域逐漸在龍門山斷裂南段集中擴(kuò)展,表明該地區(qū)應(yīng)力軸的空間取向非均勻性在增強(qiáng),應(yīng)力有明顯的調(diào)整過程。雖然2008年汶川大地震使龍門山斷裂南段的應(yīng)力得到了有效的緩解,但地震造成的破裂主要集中在龍門斷裂帶北段,而南部基本沒有發(fā)生錯(cuò)動(dòng),余震活動(dòng)也較弱,因此其上積累的應(yīng)變能并沒有完全釋放。

圖2　蘆山震源區(qū)2003—2013年標(biāo)準(zhǔn)差和CV值隨時(shí)間的演化Fig.2　Variation of standard deviation and CV value with time from 2003 to 2013 in Lushan seismic area

圖3　川滇地區(qū)CV值空間分布Fig.3　Spatial distribution of CV value in Sichuan—Yunnan region

3.2P軸方位角CV值空間的演化

以2年為窗長(zhǎng)、1年步長(zhǎng),分為6個(gè)不同時(shí)段:2007—2008年、2008—2009年、2009—2010年、2010—2011年、2011—2012年和2012—2013年4月,對(duì)蘆山震源區(qū)中小地震震源機(jī)制解P軸方位角CV值的空間分布特征隨時(shí)間的變化進(jìn)行研究(圖3)。結(jié)果表明,CV值空間分布的非均勻性有一個(gè)顯著的增強(qiáng)與減弱過程,從2007—2012年CV值高值區(qū)域在29°～31°N,101.5°～103.0°E一帶有一個(gè)顯著的擴(kuò)展過程,2012—2013年3月此區(qū)域明顯減小,隨后蘆山地震在其附近發(fā)生,這也許表明CV值對(duì)發(fā)震地點(diǎn)有一定的指示意義。

4結(jié)論與討論

地應(yīng)力是導(dǎo)致各種構(gòu)造變形的力學(xué)因素,測(cè)定其方向和強(qiáng)度,研究其空間分布及隨時(shí)間的變化,對(duì)于探索地震的孕育及發(fā)生是非常必要的。震源機(jī)制解分析是目前了解地殼深部現(xiàn)今應(yīng)力狀態(tài)最有效、最直接的方法之一。根據(jù)地震波輻射花樣理論基礎(chǔ)得到的震源機(jī)制解的本質(zhì)是彈性應(yīng)變給出的三維應(yīng)力狀態(tài)的描述,大量中小地震震源斷層的錯(cuò)動(dòng)表現(xiàn)本身呈現(xiàn)的是區(qū)域或較局部的一種應(yīng)力狀態(tài)。在一次強(qiáng)震孕育過程中,在潛在震源區(qū)及其附近,由于局部構(gòu)造活動(dòng)加劇,構(gòu)造應(yīng)力場(chǎng)增強(qiáng),在相對(duì)穩(wěn)定的區(qū)域應(yīng)力場(chǎng)背景上會(huì)出現(xiàn)應(yīng)力場(chǎng)時(shí)空調(diào)整變化。研究震源區(qū)應(yīng)力狀態(tài)對(duì)于認(rèn)識(shí)地震的形成過程、判斷地震危險(xiǎn)性和發(fā)展地震預(yù)測(cè)方法都具有重要意義[14-15]。

本文通過震源機(jī)制解P軸方位角CV值的時(shí)空演化來考察地殼應(yīng)力狀態(tài)的變化。研究發(fā)現(xiàn):蘆山7.0級(jí)地震前孕震區(qū)內(nèi)一系列中小地震P軸方位角CV值震前有一個(gè)上升-下降-上升的過程,表明應(yīng)力軸的空間取向發(fā)生了變化,強(qiáng)震發(fā)生前應(yīng)力場(chǎng)有明顯的調(diào)整過程;CV值空間分布的非均勻性特征在龍門山斷裂帶南段有顯著的增強(qiáng)與減弱過程,這對(duì)于發(fā)震地點(diǎn)可能有一定的指示意義。

中小地震的P軸方位角CV值在蘆山地震前的時(shí)間演化特征,似乎比較客觀地反映了震源區(qū)應(yīng)力場(chǎng)變動(dòng)情況。震源區(qū)及其附近應(yīng)力場(chǎng)在震前很長(zhǎng)時(shí)間里,有一個(gè)應(yīng)力逐步增強(qiáng)和釋放的過程,這種現(xiàn)象和巖石破裂試驗(yàn)中的得到的結(jié)果非常一致,但在實(shí)際應(yīng)用中取多大范圍內(nèi)的中小地震,CV值參數(shù)多大起伏為孕震階段特征,還需要對(duì)長(zhǎng)期大量的資料進(jìn)行積累分析才能得到。本文僅探討了這一分析方法及應(yīng)用問題,具體的震情判斷指標(biāo)還有許多工作要做。未來如果能在基于應(yīng)力狀態(tài)演化的物理背景下結(jié)合其他觀測(cè)手段去進(jìn)行地震預(yù)報(bào)和預(yù)測(cè),將會(huì)更加行之有效。當(dāng)然還需要更多的震例研究或更長(zhǎng)時(shí)間的檢驗(yàn)。

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CVValue Variation of P-axis Azimuth for Small to Moderate Earthquakes before the LushanMS7.0 Earthquake in Sichuan

GUO Xiang-yun1, CHEN Xue-zhong1, LI Yan-e1, WANG Sheng-wen2

(1.InstituteofGeophysics,CEA,Beijing100081,China; 2.FirstCrustMonitoringandApplicationCenter,CEA,Tianjin300180,China)

Abstract:In the preparation process of a strong earthquake,at the potential source area and its vicinity,the tectonic stress field can be enhanced by local tectonic activities.There will be a space-time adjustment of the stress field with the background of a stable regional stress field.To detect the crustal stress state of the study area,in this paper we analyze the focal mechanism data of small to moderate earthquakes that occurred at the epicenter of the Lushan MS7.0 earthquake and its surrounding areas more than ten years before the Lushan earthquake.We obtained the focal mechanism solutions of the small to moderate earthquakes using the Hardebeck and Shearer (HASH) method,then used the CV value of the P-axis azimuth (the ratio of the standard deviation to its average) to identify the stress field change.The results show that the stress field obviously changed at the seismic source and its surrounding areas with time.The P-axis azimuth CV values of the small to moderate earthquakes exhibited a rise-decline-rise process prior to the Lushan M7.0 earthquake,which means that there was a long period of stress accumulation before the M7.0 earthquake,consistent with the results of many previous studies.The CV values and apparent stress had the same tendency changes,which objectively reflect the changes in the stress field in the process of earthquake preparation.This study demonstrates that there is a certain relationship between the occurrence of strong earthquakes and the change in the P-axis azimuth CV values of small and moderate earthquakes.

Key words:focal mechanism solution; P-axis azimuth; CV value

DOI:10.3969/j.issn.1000-0844.2016.02.0242

中圖分類號(hào):P315.3

文獻(xiàn)標(biāo)志碼:A

文章編號(hào):1000-0844(2016)02-0242-07

作者簡(jiǎn)介:郭祥云(1975-),女,山西洪洞人,高級(jí)工程師,現(xiàn)主要從事地震監(jiān)測(cè)與預(yù)報(bào)方面的研究工作。E-mail:guoxiangyun@cea-igp.ac.cn

基金項(xiàng)目:中國(guó)地震局行業(yè)專項(xiàng)(201408014);中國(guó)地震局地球物理研究所基本科研重大專項(xiàng)(DQJB11C11)

收稿日期:①2014-10-31