基于單臺(tái)初至P波快速測(cè)定震級(jí)方法及應(yīng)用
苗慶杰1,2, 劉希強(qiáng)2, 崔鑫2
(1.中國(guó)科學(xué)技術(shù)大學(xué),安徽 合肥230026; 2.山東省地震局,山東 濟(jì)南250014)
摘要:通過函數(shù)形式Bt·exp(-At)來(lái)擬合單臺(tái)初至P波前2 s時(shí)間窗內(nèi)數(shù)據(jù)的包絡(luò)線,采用最小二乘法求出參數(shù)A和B,并提出三種繪制波形包絡(luò)線的方法。應(yīng)用實(shí)際地震記錄對(duì)三種方法進(jìn)行比較分析,得出一種繪制包絡(luò)線的最優(yōu)方法,改進(jìn)利用波形包絡(luò)求取震級(jí)的方法。利用山東數(shù)字化地震臺(tái)網(wǎng)小于100 km的垂直向記錄,經(jīng)過處理和統(tǒng)計(jì),得到震級(jí)與包絡(luò)參數(shù)B、P波前2 s最大速度值之間的統(tǒng)計(jì)關(guān)系,內(nèi)檢結(jié)果表明與實(shí)際震級(jí)的偏差較小,為0.36;lgB和震中距Δ存在較好的線性關(guān)系,與震級(jí)大小無(wú)關(guān)。因此,可以由P波觸發(fā)后2 s時(shí)間窗內(nèi)最大振幅和參數(shù)B通過經(jīng)驗(yàn)公式來(lái)快速估算地震震級(jí),有效增加預(yù)警時(shí)間,大大縮小預(yù)警盲區(qū)。
關(guān)鍵詞:?jiǎn)闻_(tái); 地震預(yù)警; 震級(jí)估算; 包絡(luò)線; 幅值
收稿日期:*2014-06-10
基金項(xiàng)目:國(guó)家支撐計(jì)劃課題(2012BAKB04);中國(guó)地震局臺(tái)網(wǎng)骨干專項(xiàng)(20130211);山東省地震局重點(diǎn)基金項(xiàng)目(編號(hào):JJ1407Y)
作者簡(jiǎn)介:苗慶杰(1980-),男,山東定陶人,研究生,工程師,主要從事地震波分析處理與應(yīng)用方面的研究工作.E-mail:qjmiao@163.com
中圖分類號(hào):P315.63文獻(xiàn)標(biāo)志碼:A
DOI:10.3969/j.issn.1000-0844.2015.01.0248
Rapid Determination Method for Magnitude Based on First-arrival
P Wave Recorded at the Single Station and its Application
MIAO Qing-jie1,2, LIU Xi-qiang2, CUI Xin2
(1.UniversityofScienceandTechnologyofChina,Hefei,Anhui230026,China;
2.EarthquakeAdministrationofShandongProvince,Jinan,Shandong250014,China)
Abstract:As an effective means of reducing casualties and economic losses, earthquake early warning systems have received considerable attention and many have been implemented around the world.A complete earthquake early warning system includes the identification of the earthquake location,estimation of its magnitude,estimation of its intensity at a target zone,and the dissemination of relevant information.The rapid determination of the earthquake magnitude is the most important part of such a system,after determining the location of the earthquake,because its accuracy affects predictions of ground motion of a target area and has influence on the issuance of warnings.An earthquake early warning system should provide the longest emergency response time possible.Full use must be made of the limited information from seismic stations nearest to the epicenter when calculating the earthquake magnitude,using special algorithms to extract the parameters that can better reflect the characteristics of earthquake magnitude from few data.Therefore,it is necessary to develop some method to calculate earthquake magnitude that is non-traditional, stable,and reliable.In this study,we used a simple function of the form Bt·exp(-At) and determined A and B in terms of the least-squares method by fitting this function with a 2-s time window of the first-arrival P waveform envelope.We proposed three methods for drawing waveform envelopes,which we then compared and analyzed using actual seismic records.An optimal method of drawing the envelopes was achieved,improving the method using the waveform envelope to obtain the magnitude.Based on 225 vertical recordings from the sensors of the Shandong Seismic Network,whose distances of separation are <100 km and that have an SNR ≥ 3 after processing and statistics,we obtained a statistical relationship with magnitude,envelope parameter B,and the maximum speed for the 2-s time window of the first-arrival P waveform.An inspection of the results shows that the difference between this and the actual magnitude is small (0.36).According to a computational analysis of the Shandong Seismic Network data,we found a good linear relationship between lgB and epicentral distance Δ, regardless of the magnitude.Therefore,we can quickly estimate the magnitude of an earthquake using empirical formulas with the maximum amplitude of the 2-s P waveform and parameter B,effectively increasing the warning time and greatly reducing the warning blind spot.The fitting formula used to estimate the magnitude in this study has lower error and higher accuracy compared with the existing empirical formula when estimating earthquake magnitude in the Shandong region.However, because of the lack of seismic data for earthquakes >M5,the inversion formula has some limitations, and in addition,parameter B has strong regional characteristics.This study is based on the Rushan earthquake in the Shandong region,for which the statistical relationship between magnitude, envelope parameter B,and the maximum speed for the 2-s time window of the first-arrival P waveform was based.Therefore,the empirical relationship could be applied to the Jiaodong Peninsula that has similar geological structural characteristics.When an earthquake next occurs in the Jiaodong Peninsula,we will be able to obtain parameter B quickly using the data for the 2-s time window of the first-arrival P waveform at the station nearest to the epicenter.From this,it will be possible to estimate the earthquake magnitude simply and quickly,and effectively increase the warning time,which will greatly reduce the warning blind spot.
Key words: single station;earthquake early warning;magnitude estimation; envelope curve; amplitude
0引言
在地震防災(zāi)中快速啟動(dòng)緊急求援和危機(jī)管理中快速獲取災(zāi)害信息方面,地震預(yù)警系統(tǒng)的作用日益凸顯,目前世界上許多國(guó)家和地區(qū)都已經(jīng)建立或正在建立地震預(yù)警系統(tǒng)[1]。一個(gè)完整的地震預(yù)警系統(tǒng)包括地震預(yù)警定位、地震震級(jí)的估算、預(yù)警目標(biāo)區(qū)烈度的估計(jì)和預(yù)警信息發(fā)布等幾個(gè)部分,其中震級(jí)估算是地震預(yù)警中一個(gè)基本問題,也是地震預(yù)警系統(tǒng)中最重要、最困難的一部分,在確定地震位置后,震級(jí)測(cè)定結(jié)果的準(zhǔn)確度對(duì)后續(xù)目標(biāo)地區(qū)的地震動(dòng)估計(jì)與警報(bào)發(fā)布具有極其重要的影響。地震預(yù)警系統(tǒng)應(yīng)該提供盡可能長(zhǎng)的應(yīng)急反應(yīng)時(shí)間,這就意味著對(duì)信息有高度的時(shí)效性要求,計(jì)算震級(jí)時(shí)必須充分合理利用離震中最近地震臺(tái)站的十分有限的信息,采用一些特殊的算法從這些信息中提取能夠較好反映地震震級(jí)特征的參數(shù)。
目前國(guó)內(nèi)外預(yù)警震級(jí)的測(cè)定有很多種方法,其所采用的相關(guān)參數(shù)也不盡相同,但方法都是基于下面兩個(gè)基本結(jié)論:其一是初始破裂的形態(tài)可以預(yù)測(cè)地震的最終規(guī)模;其二是P波攜帶著地震本身的信息,S波則攜帶地震能量的信息。
雖然對(duì)于測(cè)定預(yù)警震級(jí)的兩個(gè)基本結(jié)論的爭(zhēng)論仍在繼續(xù),到目前為止還沒有一個(gè)權(quán)威的結(jié)論,但是從近幾年發(fā)表的一系列相關(guān)研究來(lái)看,由初始破裂信息估算地震的大小是可行的。通過采用一些特殊的、穩(wěn)定的計(jì)算方法是可以得到比較滿意的地震震級(jí)估計(jì)結(jié)果,如τPMax方法[2-6]、τc方法[7-13]、ML10方法[14]、Pd方法[5-9,15-17]等。
1快速測(cè)定震級(jí)的方法
本文的震級(jí)計(jì)算方法采用Odaka于2003年提出的波形包絡(luò)算法[18],并對(duì)以往繪制包絡(luò)線的方法進(jìn)行改進(jìn),用f(t)=Bt·exp(-At)的函數(shù)形式對(duì)垂向速度記錄P波前2 s的波形包絡(luò)進(jìn)行擬合,用B值代表P波到來(lái)后地震動(dòng)幅值的增長(zhǎng)率,通過大量的地震數(shù)據(jù)統(tǒng)計(jì)分析得到B值和速度峰值與震級(jí)的相關(guān)關(guān)系。當(dāng)P波到達(dá)后,能夠在給定的很短時(shí)間內(nèi),通過完全經(jīng)驗(yàn)震級(jí)關(guān)系,快速?gòu)挠^測(cè)到的最大振幅和作為參數(shù)的B值估算出震級(jí)。
1.1三種繪制包絡(luò)線的方法
首先提出了三種繪制包絡(luò)線的方法(圖1)。
圖1 三種方法繪制的地震記錄P波前2 s的包絡(luò)線 Fig.1 The envelopes in the first 2 seconds of P-wave arrival drawed by the three methods
方法①(紅色折線)的包絡(luò)線繪制方法如下:
首先對(duì)原始垂直向速度進(jìn)行適當(dāng)?shù)靥幚砗妥儞Q。對(duì)原速度記錄做零線調(diào)整,并對(duì)調(diào)整后的結(jié)果取絕對(duì)值;其次,將P波到達(dá)后2 s內(nèi)樣點(diǎn)的y坐標(biāo)值排成一個(gè)行向量,D=[d1,d2,d3,…,d200],然后再把這2s時(shí)間段所對(duì)應(yīng)的包絡(luò)線上的數(shù)據(jù)點(diǎn)的y坐標(biāo)排成另外一個(gè)行向量,M=[m1,m2,m3,…,m200],于是包絡(luò)線上的第一個(gè)數(shù)據(jù)點(diǎn)的值m1=d1,對(duì)于第二個(gè)數(shù)據(jù)點(diǎn),如果d2≥d1,那么m2=d2,如果d2=d1,那么m2=m1,可以根據(jù)包絡(luò)線的定義推導(dǎo)出包絡(luò)線上的數(shù)據(jù)點(diǎn)的通項(xiàng)公式:M(i)=max(d1,d2,……,di);
方法②(藍(lán)色折線)的包絡(luò)線繪制方法是在方法①繪制的包絡(luò)線的基礎(chǔ)上依次連接相鄰極值點(diǎn),包絡(luò)線的數(shù)據(jù)點(diǎn)為極值點(diǎn);
方法③(藍(lán)色折線)的包絡(luò)線繪制方法是對(duì)方法②繪制的包絡(luò)線進(jìn)行線性插值后得到的。
1.2信號(hào)仿真檢驗(yàn)
為了檢驗(yàn)用于本研究所編寫的程序方法的正確性,故在研究開始之前先進(jìn)行信號(hào)仿真檢驗(yàn),先給漸變模型函數(shù)f(t)=Bt·exp(-At)中的B和A賦值,比如B=42,A=4(圖2),然后再用所寫的程序進(jìn)行反演圖示2中的曲線,得出B和A的值(圖3),比較兩者的B和A值是否與事先賦予的值一致,如果一致,就說(shuō)明所編寫的程序是正確的。
從圖2和圖3可以看出,通過程序反演計(jì)算出的B和A值與事先賦予的值一致,誤差為0,說(shuō)明本程序沒有問題。
圖2 模型函數(shù)當(dāng)B=42,A=4時(shí)所顯示的圖示 Fig.2 Model function(B=42,A=4)
圖3 反演圖2中的曲線,求得B和A值 Fig.3 Obtaining values of B and A by inverting the curve in Fig.2
1.3對(duì)三種包絡(luò)線擬合分析及比較
圖4為分別使用漸變模型函數(shù)f(t)=Bt·exp(-At)對(duì)地震記錄HAY201312091338.00U的三種包絡(luò)線進(jìn)行擬合的計(jì)算結(jié)果。
為了驗(yàn)證使用哪種方法所繪制的包絡(luò)線進(jìn)行擬合計(jì)算出的震級(jí)精度更高,選取17個(gè)ML≥2.5,信噪比大于3,垂向記錄數(shù)為92條的乳山地震進(jìn)行計(jì)算檢驗(yàn),見表1。分別對(duì)每條垂向記錄的三種類型的包絡(luò)線使用f(t)=Bt·exp(-At)的函數(shù)形式進(jìn)行擬合,計(jì)算選定記錄的B值、速度峰值,對(duì)這些記錄的B值、速度峰值和它們的震級(jí)做線性回歸,求出回歸方程,然后反演出地震震級(jí), 并統(tǒng)計(jì)震級(jí)偏差情況。
對(duì)這些地震事件里的垂直向的記錄,分別使用上面介紹的三種方法繪制P波前2s的包絡(luò)線并進(jìn)行擬合計(jì)算相關(guān)參數(shù),并對(duì)相關(guān)參數(shù)做線性回歸,分別得到估算震級(jí)方程為:
擬合方法①繪制的包絡(luò)線得到a=0.943,b=-0.348,c=2.768,預(yù)警震級(jí)估計(jì)公式為:
擬合方法②繪制的包絡(luò)線得到a=1.525,b=-0.815,c=3.155,預(yù)警震級(jí)估計(jì)公式為:
表 1驗(yàn)證三種包絡(luò)線的方法所選用的乳山地震資料
Table1The data of Rushan earthquake chosed to verify the methods of drawing three envelopes
編號(hào)發(fā)震時(shí)間緯度/(°)經(jīng)度/(°)震級(jí)/ML12013-10-11T0.6∶36∶01.5536.842121.6862.722013-10-17T02∶36∶10.0736.836121.6962.632013-10-17T03∶55∶46.1036.827121.6882.942013-10-24T0.8∶01∶45.0736.850121.6842.652013-11-07T0.7∶01∶25.6036.842121.6892.562013-11-17T20∶18∶49.2636.849121.6792.572013-12-09T13∶38∶03.8736.830121.6803.282014-01-04T23∶31∶02.9036.834121.6892.692014-01-07T22∶24∶06.6036.820121.6704.7102014-01-08T0.1∶46∶30.5536.841121.6942.5112014-01-05T0.2∶39∶02.1436.836121.6962.6122014-01-08T19∶27∶20.2636.838121.6382.5132014-01-09T16∶54∶30.6536.829121.6813.5142014-01-10T19∶45∶30.4736.842121.6982.8152013-10-01T12∶07∶55.2736.828121.6963.6162013-10-05T03∶49∶11.9336.839121.6952.7172013-10-05T11∶30∶02.2236.829121.7013.2
擬合方法③繪制的包絡(luò)線得到a=1.270,b=-0.656,c=2.917,預(yù)警震級(jí)估計(jì)公式為:
分別用三種震級(jí)估計(jì)公式估算出這些地震記錄的震級(jí),并計(jì)算誤差。由圖5可見,對(duì)按方法①繪制的包絡(luò)線進(jìn)行擬合,計(jì)算的震級(jí)平均偏差為0.375 3;對(duì)方法②繪制的包絡(luò)線進(jìn)行擬合,計(jì)算的震級(jí)平均偏差為0.346 5;對(duì)方法③繪制的包絡(luò)線進(jìn)行擬合,計(jì)算的震級(jí)平均偏差為0.376 6。由此可以看出,擬合采用方法②繪制的包絡(luò)線計(jì)算的震級(jí)精度高于其他兩種方法,故本文將按方法②進(jìn)行相關(guān)研究。
圖4 三種包絡(luò)線擬合結(jié)果 Fig.4 Fitting results of three envelopes
圖5 擬合三種方法繪制的包絡(luò)線估算震級(jí)的誤差分布直方圖 Fig.5 Histogram of errors distribution of magnitudes estimated by the three envelopes
2資料處理及結(jié)果分析
2.1資料選取
本文收集了山東數(shù)字地震臺(tái)網(wǎng)記錄到的2009年1月1日至2014年4月1日共56個(gè)ML2.0~4.7的乳山地震事件,所用地震事件的發(fā)震時(shí)刻、震級(jí)大小等詳細(xì)資料如表2和圖6所示。由于地震事件文件全部都是SEED格式文件,所以我們編制了轉(zhuǎn)化程序把SEED格式的波形數(shù)據(jù)文件轉(zhuǎn)換成ASSIC碼格式的文本文件,并對(duì)地震記錄進(jìn)行基線校正等基本處理。
理論分析表明,利用垂向地震記錄進(jìn)行相關(guān)研究,震中距超過100 km時(shí),由于折射、反射等原因,地震震相變得非常復(fù)雜,首先到達(dá)的也不是Pg震相,所以不容易提取特征參數(shù),震級(jí)的估算也會(huì)產(chǎn)生較大誤差。鑒于以上原因,考慮地震預(yù)警系統(tǒng)時(shí)效性要求高,本研究?jī)H使用震中距在100 km內(nèi)的地震臺(tái)站波形記錄參與統(tǒng)計(jì)分析,在此震中距范圍內(nèi),P波成份比較簡(jiǎn)單,能夠滿足時(shí)效性要求,充分保證地震震級(jí)估計(jì)結(jié)果的準(zhǔn)確性和可靠性。
選用的地震波形記錄需要滿足信噪比的要求,因此,本研究只選用P波觸發(fā)后2 s時(shí)間內(nèi)信噪比大于3的地震記錄參與相關(guān)統(tǒng)計(jì),根據(jù)以上地震波形記錄的挑選原則,選出225條垂直向地震記錄參與統(tǒng)計(jì)分析。
表 2 所選用的乳山地震資料
圖6 震中及臺(tái)站分布圖 Fig.6 Distribution of epicenters and stations
2.2資料處理及結(jié)果
采用第1.3節(jié)所述的最優(yōu)繪制包絡(luò)線的方法對(duì)所挑選出的地震記錄P波到達(dá)后前2 s的數(shù)據(jù)進(jìn)行包絡(luò)線的繪制,然后再用f(t)=Bt·exp(-At)形式的函數(shù)對(duì)包絡(luò)線上的點(diǎn)進(jìn)行擬合,求出系數(shù)B,即為擬合曲線在P波到時(shí)點(diǎn)的斜率。圖7為HAY臺(tái)地震記錄做出的包絡(luò)線和擬合曲線及計(jì)算出的相關(guān)特征參數(shù)。
對(duì)全部地震記錄的特征參數(shù)做線性回歸,得到a=1.699,b=-0.993,c=3.057,估算震級(jí)方程為:
用式(4)估算出所有地震記錄的震級(jí),并計(jì)算偏差(圖8(a)),其平均偏差為0.36。
2.3lgB與lgΔ的關(guān)系
有學(xué)者[19-21]對(duì)大量的地震數(shù)據(jù)記錄研究分析后得到震中距l(xiāng)gΔ與lgB成線性負(fù)相關(guān)的關(guān)系,與震級(jí)的大小無(wú)關(guān)。因此可以通過計(jì)算P波初至階段振幅的增長(zhǎng)率B來(lái)估算出震中距。本文擬用山東省數(shù)字化地震臺(tái)網(wǎng)記錄到的ML≥2以上震中距在100 km以內(nèi)的垂直向P波記錄數(shù)據(jù)來(lái)反演A和B值,并分析檢驗(yàn)lgB與lgΔ存在何種關(guān)系。
通過對(duì)收集到的225個(gè)樣本進(jìn)行計(jì)算分析(圖9),發(fā)現(xiàn)lgB與震中距Δ存在近似線性關(guān)系,這個(gè)關(guān)系不受震級(jí)的影響,因此,可以把lgΔ換成lgB,地震震級(jí)可以由P波觸發(fā)后2 s時(shí)間窗內(nèi)振幅絕對(duì)值的最大值和參數(shù)B通過下面的經(jīng)驗(yàn)公式來(lái)進(jìn)行確定:
M=algPmax+blgB+c
公式中的a、b、c是可以通過最小二乘法確定的常數(shù)。
2.4與相關(guān)研究結(jié)果對(duì)比
周彥文[22]使用該方法利用山東臺(tái)網(wǎng)2000年1月至2007年5月記錄到的ML≥2以上41個(gè)數(shù)據(jù)擬合得出計(jì)算震級(jí)的反演公式
用經(jīng)驗(yàn)公式(5)反演估算上述225條地震記錄的震級(jí),得出相應(yīng)的誤差分布,平均偏差為0.38;而本研究得出的經(jīng)驗(yàn)公式反演上述地震記錄得出的平均偏差為0.36(圖8)。
圖7 記錄HAY201312091338.00U在P波到達(dá)后2 s內(nèi)的包絡(luò)線和擬合曲線及計(jì)算出的特征參數(shù) Fig.7 The envelope and its fitting curve in 2 s after the arrival of P-wave and the calculated characteristic parameters at the record of HAY201312091338.00U
圖8 擬合震級(jí)誤差分布直方圖 Fig.8 Histogram of errors distribution of the fitted magnitudes
圖9 參數(shù)B與震中距Δ的經(jīng)驗(yàn)關(guān)系 Fig.9 The relationship between B and Δ
3結(jié)論
(1) 提出了繪制波形包絡(luò)線的三種方法,利用實(shí)際地震數(shù)據(jù)統(tǒng)計(jì)分析了分別以它們求取震級(jí)的誤差指標(biāo),確立繪制包絡(luò)線的最優(yōu)方法,改進(jìn)了利用波形包絡(luò)求取震級(jí)的方法。
(2) 經(jīng)過對(duì)山東數(shù)字化地震臺(tái)網(wǎng)的地震數(shù)據(jù)計(jì)算分析,發(fā)現(xiàn)參數(shù)B值與震中距Δ具有近似線性關(guān)系,這個(gè)關(guān)系不受震級(jí)的影響,因此,地震震級(jí)可以由P波觸發(fā)后2s時(shí)間窗內(nèi)最大振幅和參數(shù)B快速測(cè)定。
(3) 基于山東數(shù)字化地震臺(tái)網(wǎng)小于100km的垂向記錄初至P波2s時(shí)間窗內(nèi)的最大振幅和參數(shù)B值,經(jīng)過處理和統(tǒng)計(jì),得到了震級(jí)與包絡(luò)參數(shù)B、P波前2s最大速度值之間的統(tǒng)計(jì)關(guān)系:
Mest=1.699lgPmax-0.993lgB+3.057
依據(jù)此震級(jí)統(tǒng)計(jì)公式,對(duì)地震數(shù)據(jù)進(jìn)行了預(yù)測(cè),表明與實(shí)際震級(jí)的偏差較小,為0.36。
(4) 通過和山東地區(qū)已有的估算震級(jí)經(jīng)驗(yàn)公式對(duì)比發(fā)現(xiàn),本研究所得出的擬合公式反演估算出的震級(jí)誤差更低,精度更高。
4討論
由于缺乏5級(jí)以上的地震數(shù)據(jù),震級(jí)反演公式有一定的局限性,今后在有關(guān)震例增加后,需要做進(jìn)一步的分析研究。參數(shù)B具有強(qiáng)烈的區(qū)域性特點(diǎn),本研究是以山東地區(qū)的乳山地震數(shù)據(jù)為樣本進(jìn)行擬合求出參數(shù)B值,然后統(tǒng)計(jì)回歸得出震級(jí)與2 s時(shí)間窗內(nèi)的振幅最大值和參數(shù)B值之間的相關(guān)關(guān)系。因此,這個(gè)經(jīng)驗(yàn)關(guān)系適用于地質(zhì)結(jié)構(gòu)特性相近的膠東半島地區(qū)的所有臺(tái)站,當(dāng)膠東半島地區(qū)再次發(fā)生地震時(shí),就可用離震中最近的地震臺(tái)站記錄到的P波前2 s內(nèi)的數(shù)據(jù)快速求出參數(shù)B,即可估算出震級(jí),簡(jiǎn)單快速,有效增加了預(yù)警時(shí)間,并大大縮小了預(yù)警盲區(qū)。
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