长白山火山区地壳结构研究及火山活动性分析
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摘要
长白山火山区位于中国东北部，其主峰天池火山位于中朝边界北纬41°20’-42°40’，东经127°00’—129°00’，高度为2744m。火山锥覆盖半径约20km，其顶部火山口积水湖泊天池水面高度2189.7m。在过去2000年中，天池火山至少有5次喷发，最大一次喷发在大约公元1000年左右，最近的主要喷发在公元1215年和1702年。总的岩石喷发量估计在9，000km~3。
长白山火山区的地球物理学和地球化学研究结果表明，长白山天池火山是一个亟待研究的火山。长白山火山区是一个地震活动异常区，该区小震活动每年达200次左右，为其周围地震活动的10倍。大地电磁测深剖面反演结果显示，天池火山下方10km左右存在一低阻带，其向北延伸约20km，底部深度可能达30km。区域S波资料层析成象研究表明，天池火山区下25-75km范围存在一速度异常区。区域地震面波和Moho反射波的模拟结果指出，该地区的地壳厚度为30-35km。近年来大量的人工地震资料和宽频带地震资料的研究表明，天池一带地壳内存在明显的低速体，地壳厚度由北西向南东方向逐渐增加，在天池附近达到最深，一般地壳厚度在32km左右，天池附近为39km左右。根据对地热和温泉释放气体的化学分析，由温泉释放的气体来自地壳内岩浆囊，并判断在5.4-5.8km以下存在一地热储库且该地热库位于岩浆喷发通道上。显然，对长白山地区的地球物理、地球化学、地质及地表形变的资料的进一步采集、研究将是十分必要的。
本研究利用接收涵数方法和射线追踪反演方法分别对长白山火山区宽频带地震资料和主动源地震折射/宽角反射资料进行了分析和研究，获得了不同资料模拟下的长白山火山区的地壳结构，并分析了结构的共同特征以及与长白山天池火山岩浆活动的可能关系。
一．接收函数对长白山火山区地壳速度结构的研究
1998年夏，中国地震局地球物理勘探中心与美国纽约州立大学Binghamton分校合作在长白山火山及临近地区安装了19套便携式宽频带地震仪，其目的是试图通过所记录到的远震资料获取该地区区域性的地壳速度结构。本研究采用接收函数的方法对这些资料进行了研究。
接收函数是由三分量地震图计算出来的时间序列，它所表示的是近接收仪区域地壳结构的相对反应。其波形是接收仪下方地壳结构内P波及P波转换为S波的组合。通过波形的模拟，我们可以获得接收仪下方的地壳结构信息。接收函数的振幅取决于入射波的入射角和产生转换波界面的速度对比度，而转换震相的到时则取决于转换界面的深度，P波与S波在转换界面和地表面间的速度，P波入射角度或射线参数。多次转换波的振幅和频率取决于转换速度的性质，如速度变化是宽域的或是比较单一的等。
宽频带地震资料的处理和分析
这项研究从41个地震事件中选取了29个地震做接收函数分析。对事件的选择原则，一是尽量避免复杂的近源响应，二是直达P波能清楚地辨认，再则是选用大震级地震以保证有足够的信号能量。所选择的大部分地震位于观测网以南，最集中的地区是印度尼西亚经巴布亚新归亚那到菲齐的地震活动带。其它地震中，有来自玛丽亚那海沟，阿流申海沟区和伊朗。共分析了来自19个观测仪器的109个地震纪录。
地震图中直达波到达后20-40秒左右的区间包含了近源，路径和近接收点的结构信息。在信号源附近，地震信号可以转换为P波或由S波再转换为P波。在接收点附近，信号为P波，P到S的转换波，以及P或S的反射波。对于远震而言，直达P波从Moho入射进入地壳几乎是垂直到达地表。因此，转换后的S波其水平分量较大，而P波的垂直分量较大。也就是说，通过Moho转换成的S波包含了接收点附近地壳结构的信息。这样一来我们就可以通过对源进行等效化，再把垂直分量通过反褶积与水平径向分量分离而获得径向分量接收函数。
由于接收函数方法利用地壳内的转换波和反射波确定地壳结构，其所能反映的地壳区域就取决于这些波在地壳内的穿透区域。而穿透区域又取决于射线参数和地壳厚度(或转换界面的深度)。对于射线参数p，我们采用P波走时的IASP91模型，根据源的深度和距离来估计。接收函数可采样的范围至少是直达P波在Moho界面的穿透点到观测点的水平距离。然而，界面越深这个采样分辨范围越小。基于本地区的地壳厚度的估计，P波对Moho界面的横向分辨率在10-15km。
在分析径向接收函数时，采用了三项分析技术：斜向叠加处理，直接搜索反演模拟和最小二乘反演。所有这三个方法中，都假设地壳是各向均匀，各向同性，Moho面没有倾斜。因为接收函数对地壳的采样只是接收点附近的局部区域，以上的假设也只针对接收点附近的局部区域。
接收函数获得的地壳速度结构
由斜向叠加所获得的Moho面深度在火山区外围为28-32km，在火山区为30-39km，其中天池附近地壳最厚39 km。两种方法，直接搜索反演和最小二乘反演，都给出了这样的地壳厚度趋势：在火山区及其附近，最厚的地壳厚度在长白县往北至二道白河，有可能包括漫江。
P波平均速度(?)_p在火山区为大约6.0-6.2km／s。最低的P波速度达到6.0km/s以下。获得最低速度的台站有二道白河(BACH)，天池火山南坡(CANY)和延边(YABA)，其中YABA站的P波速度达到约5.7km／s。这些台站的速度都是用叠加的接收函数所获得，因此其结果也是相对更可靠。所有用叠加的接收函数所获得的速度剖面都在10-15km存在一个中地壳边界，只有YABA站除外。YABA站的速度剖面在中地壳20-25km深有一个中地壳转换层。
二道白河(BACH)和天池火山的南坡(CANY)的叠加速度剖面显示，在上地壳存在一个明显的高速层，在中地壳存在一个低速异常。BACH站高速层在2-8km，CANY站高速层在2-12km。S波速度由上层的高速4 km／s减低到下层的3 km／s左右(或更低)，CANY的高速层比BACH的高速层更明显。两个站接收函数所反应的都是同一方向(南部)的地壳结构。
火山区附近最小二乘反演对单个台站接收函数的反演也表明，从长白(FROG站)到二道白河(BACH站)的区域存在一个局部低速度带。这个区域内台站接收函数的速度剖面显示，该速度带由二道白河(BACH)向长白(FROG)方向扩展，其低速带的速度增高，厚度加大并向深部倾斜。低速带在天池火山的南坡(CANY)比其在北坡(CBAI)要厚一些和深一些。天池火山的西坡(MANO)没有低速异常显示。往西更远一些的WUSU站，其速度剖面同样没有显示出低速异常的迹象，甚至中地壳的速度略微偏高。
二．地震折射／宽角反射对长自山火山区地壳速度结构的研究
与宽频带地震观测的同时，1998年8月，中国地震局地球物理勘探中心在长白山火山区进行了主动震源地震折射／宽角反射实验。该实验的目的是结合其它地球物理，地球化学和地质资料对长白山火山区的地壳结构，火山活动性进行进一步的研究。该实验在长白山火山区布设了2条主观测剖面，2条辅助观测剖面和一个台阵观测网，共投入200台高频三分量地震仪，进行了10次1.5吨级爆破作业。本项研究只对上述观测的两条主要剖面L1和L2进行了两点射线追踪的正演和反演模拟。
射线追踪反演
九十年代初，Zelt等发展了一套二维速度与界面同时反演的方法。这套方法中，模型参数化和射线追踪方法都是适合反演算法的正演步骤的。
走时的非线性反演要求必要的初始模型和一个适当的循环方法。一个实际有效的正演步骤对反演算法是十分关键的。Zelt等提出的模型参数化方法就是针对这一日的的。在Zelt的模型参数化方法中，模型的每一层由大小不同的四边或三边块体组成，这种参数化方法能够使模型的独立参数量达到最小。其射线追踪方法包含了对2-D射线追踪方程的数值解和射线出射角的自动确定。
Zelt等的射线追踪是通过运用零阶射线理论求解射线追踪方程进行的。对一组射线，出射角的确定是用一种循环搜索方法完成的。循环搜索方法首先确定一组射线的最小和最大出射角，然后确定每一条射线的出射角。在两点射线追踪中，则只确定离接收点最近的两条射线。反演中只用所确定的两条射线之间的计算走时和偏微分。在确定初始出射角时，总是假定源下边的介质是横向均匀介质。
反演是否继续下去，有以下两个标准判断：1)射线对所有观测走时进行了追踪；2)走时残差的RMS(root-mean-square)达到了所期望的值。实际资料中，这两个标准经常是互相制约的。如要追踪所有观测走时，就可能以增加RMS为代价，反之亦然。
速度模拟
为了给读者一个较为清晰的地理概念，根据它们的地理走向分别命名L1和L2这两条剖面为SN剖面和WE剖面。SN剖面起点在天池火山以南约70km的长白县，向北穿过天池火山区，终点在敦化境内，全长270km。WE剖面起点在天池火山以西约120km的靖宇县，向东穿过天池火山区北部，终点在延吉市，全长220km。沿SN剖面有66台观测仪，沿WE剖面有51台观测仪，这些观测仪一般间隔为4km。在SN剖面上进行了4次爆破作业，WE剖面上进行了3次爆破作业，爆炸量级在1.2-1.5吨TNT。
本项研究采用了垂直分量记录，对地壳的优势震相予以辩认。所辨认到的震相有，上地壳折射波(P_8)、中地壳反射波(P_1，P_2 and P_3)、Moho界面的反射波(PmP)和来自上地幔顶部的回折波(P_n)。总共在SN剖面上辨认出397个到时，在WE剖面上辨认出315个到时。
两条剖面最终的P波速度模型的走时拟合结果分别为，SN剖面RMS为0.1654，WE剖面RMS为0.1636。其对走时总数的拟合分别为93.7％和94.9％。总体上讲，射线从天池火山区向外方向的震相拟合的稍比相反方向好一些。这很可能是由于近火山区速度结构更加复杂的结果。然而，无论在何种情况下，对速度模型复杂程度的增加都尽量控制在最小量级。除两个模型中明显的低速层(LVZ)区域外，其它区域的速度呈现出相对平滑的垂向速度梯度和微弱的横向速度变化。
为了方便描述，我们称低速体LVZ以外的地壳为原地壳。可以明显看出，原地壳速度在上部几公里的范围内迅速增加到6.0km／s，然后梯度减小，在大约20km处增至6.4km／s。再往下，速度又较快地增加到Moho转换带附近的6.8-7.8km／s。从SN模型可以看出的另外一个明显特征是，Moho在天池火山下达到最深，而在低速体以下则最浅。天池火山下的Moho深度为大约40km，低速体下的Moho深度为大约30～35km，其它区域为35km左右。
低速度体(LVZ)位于SN和WE交叉点的南部、天池火山口以北的区域。两个剖面模型的速度在交叉点基本吻合，尤其是上部地壳。低速度体的主要特征在两条剖面上类似，其最低速度达到大约5.4km／s，比其周围低约0.6km／s。如果用6.0km／s的速度等值线作为低速度体的边界，该速度体位于SN剖面大约75-150km的范围内和WE剖面205-240km的范围内。低速度体的顶部深度为10-15km，其最厚部分大约10km，位于天池火山口与两剖面交叉点之间，但更靠近交叉点。
模型评估
最终速度模型的可靠性与许多因素有关，例如走时辨认的准确性、震相视速度的不确定性、震相类型的判别，以及射线覆盖区域不均匀等。本文研究的主要结果是天池火山以北地壳低速异常体的存在。该低速异常体既为本研究模型的特定特征。为了评估该模型的可依赖性，研究中采用SN剖面实际的源-接收点几何关系建立了评估模型，试图用此评估模型恢复相应的低速体以证明本研究结果的可靠性。
模型评估的方法可进行以下描述。假设模型A有一低速异常体，其综合射线走时为TA。假设另有一模型B除没有低速体外，其他参数与模型A完全一样。我们用模型A的综合走时TA对模型B进行综合走时反演，如果反演结果模型C与模型A一致，则说明这种反演方法对该特定的低速体的反演结果是可靠的。
三．长白山火山区的地壳低速体结构与岩浆体低速度体
地壳低速体结构
本文对两条折射／宽角反射剖面资料进行了处理和研究，获得了两个独立的速度模型。在两个剖面上均有低速度体存在，最低速度达到5.4km／s以下。低速度体以外区域，地壳速度梯度比较平滑，在6.0-5.5km／s左右。Moho转换带深度在天池火山下方为40km，低速度体下方为30km，其它区域为大约35km。由宽频带地震资料用接收函数方法所获得的结果也在天池与二道白河之间发现有低速度体的存在，最底速度在5.0-5.4km／s，两种方法获得了比较一致结果。
利用完全不同的两种方法的结果，检验了两种方法本身的一致性。分别在CANY和BACH两个站选择了来自Celcbes来的地震事件，求得接收函数后用折射，宽角反射的模型进行模拟。CANY站接收函数所采样的地壳在SN上大约10-60km的范围，BACH站接收函数采样的地壳在SN上大约75-125km。我们来看一下是否可以用SN的速度模型来模拟这两个站的接收函数。将SN剖面上接收函数所对应区间的速度分段采样后进行平均，建立一个初始一维模型。对每一个一维速度模型，固定P波速度不变反演S波速度。一维模型的层厚度为二维模型层厚度的10％。根据SN剖面所建立的一维模型对两个观测站的接收函数拟合的都很好。
在所得到的两个速度模型中，最明显的特征要算是低速度体(以最低速度点为中心)没有位于天池火山的正下方，而是偏向其北部。天池火山位于SN剖面上70km左右，6km／s等值线也正好延伸至天池火山峰的下方，不过这里的速度相对于低速度体内的最低速度较高。最低速度的中心在SN剖面的100-130km范围。接近两条测线的交叉点，两个模型的上地壳非常相似，但下地壳差别比较大。下地壳的这种差异有可能由速度随深度的模糊产生，或速度-深度这对变量的对立关系产生。一个不可忽视的因素是，接收点没有沿侧线直线布设。尤其是在两条测线交叉点以东WE剖面大约50km的范围内，观测点对侧线的偏离十分明显。
关于接收函数的讨论中描述过，在天池火山附近普遍存在一个低速体，尤其是在天池与二道白河(BACH站)之间，低速度层最明显。这个区域正是SN剖面低速度体所在范围。另外，接收函数的结论认为，低速度体以外的速度梯度整体上十分平稳，这也与SN和WE的结果一致。接收函数判断的低速体的深度在天池火山与BACH之间为5-10km，这与SN剖面低速体的深度不一致。
从接收函数的结果看，可以认为在WE上大约160km以西和280km以东的部分为原地壳。在SN剖面200km以北为原地壳，75-125km存在低速度体，其深度在8-20km，最低速度可达5.0-5.4km／s。这个结果与折射／宽角反射的结果基本一致，不过低速度体的深度不太重合，接收函数结果的速度偏低。由于接收函数方法对速度差更敏感，而对速度的绝对值和梯度相对不太敏感，因此在速度-深度解上存在相对不确定性，这种差别显然是可能存在的。
低速度体中心位于天池火山以北约50km，以代表低速度体范围的6.0km／s等值线为标志，扩展范围沿SN剖面从天池南坡的30km向北延伸到天池以北的75km，深度范围在10-25km／s左右。根据SN和WE两条测线的交叉点位置和低速度体在WE剖面上的范围判断，低速度体在东西方向的延伸范围约35km。
岩浆囊的可能性
只前普遍认为，引起低速体的原因有两个。温度的升高可能引起P波速度的降低，或者岩浆的熔融或半熔融也可能引起P波的速度降低。根据该地区地球化学的研究，天池火山区存在高温热源是不争的事实。根据Mainprice(1997)对岩浆玄武岩类，主要包括辉长岩，蛇绿岩及上地幔的harzburgite，进行的地震特性实验室模拟的结果，P波速度在3.5～4.7km／s有50～70％为熔融状态；P波速度在4.8～5.6km／s有35～50％为熔融状态。这里的研究结果指出，长白山火山区地壳低速体的最低速度可能达到5.4～5.6km／s，也就是说，该火山的岩浆体可能有35％左右为熔融状态。传统上认为，岩浆体有35％为熔融状态既有喷发的可能。然而，Nicolas(1996)等对玄武岩岩浆的流体机制及孔隙度研究后认为，20％为熔融状态的岩浆就可以流动而没有塑性形变。Nicolas的结果预示，即使20％熔融状态的岩浆也有喷发的可能性。
从近些年大量的洋中脊地震学和实验室模拟研究结果我们可以了解到，由于处于半熔融状态，岩浆囊的P波速度有一个较宽的范围，从3.5km／s到5.6km／s，S波速度也不为零。尽管大陆地壳岩浆囊和海洋地壳岩浆囊可能在年龄和传输过程上不同，因而岩浆囊的地震学特性(各向异性)有所不同，但它们应具有类似的特征。不过，大陆地壳岩浆囊半熔融状态P波速度与熔融比率仍然是一个待研究的课题。要确定长白山天池火山岩浆囊的性质，进一步的研究是十分必要的。
四．岩浆活动性分析—Dike破裂过程与地表垂直位移
火山活动最终要体现在岩浆囊及岩浆通道的活动。岩浆通道的活动标志着火山喷发的可能性，因此，研究岩浆通道的活动对火山喷发预测将是十分重要的。Dike是一种平面岩浆席，是普遍存在的岩浆通道之一。相对于其它岩浆通道，Dike的岩浆传输速度快，与围岩的热交换率低。
本文采用Coulumb软件对Dike类岩浆通道的不同参数及其发展过程对地面垂直位移的影响进行了模拟。只所以模拟Dike类岩浆通道，是因为这种类型的通道是最常见的岩浆通道。所模拟的Dike包括，垂直发展Dike，倾斜发展Dike，Dike的倾角与地面垂直位移，Dike的底部深度与地面垂直位移，Dike的厚度与地面垂直位移。从模拟结果发现，不同参数的Dike产生不同类型或量值的地面垂直位移，地面垂直位移对一些类型的Dike的参数在不同阶段的敏感性可能不同。根据长白山天池火山的地震活动性，本文认为，长白山天池火山存在2～3个可能的Dike发展的可能性。
The Changbaishan volcanic region is composed of several, mostly inactive, volcanoes, and is located on the border between NE China and North Korea. The basaltic base of the Changbaishan region was formed approximately 4.5 to 1.5 Ma, while the rylotitic and dacitic volcanic edifices are younger than about 3-1 Ma. The youngest volcano in the Changbaishan is the 2744m high Tianchi volcano. The Tianchi is capped by a summit lake (the Tianchi lake) and last erupted in 1215CE and 1702CE. There have been at least five large eruptions in the Changbaishan area during the common era. The eruption at about 1000CE was one of the largest eruptions in the last 2000 years, with ash deposits found up to 1200 km away in Japan. The total volume of rock erupted from the Changbaishan volcanic region is estimated to be about 9,000 km~3.
The Changbaishan region is seismically active, with an average of 18 earthquakes per month, although during the summer of 2003, more than 30 were detected per month. The majority of the hypocenters of these events cluster near the Tianchi caldera lake in the upper 5 km of the crust, and while the Changbaishan region is much more seismically active than the surrounding area, it is far less active than other volcanic regions. There are several hot-springs surrounding the Tianchi volcano, mostly concentrated on the northern slope, and they primarily discharge meteoric water that has circulated in regions of hot rock at upper crustal depths. Tang et al. (1998) resolved a low-resistivity body below about 10 km under the Tianchi, possibly reaching depths up to 30 km, and extending about 20 km to the north of the Tianchi summit.
In this study, both broadband seismic data and seismic refraction/wide-angle reflection data were analyzed by Receiver Function method and Ray Tracing Inversion method, respectively. The velocity structures of Changbaishan region are presented and the possible relationship with magma chamber activity is analyzed.
Crustal structure determined by modeling receiver functions
In the summer of 1998, the State University of New York at Binghamton, in collaboration with the Research Center of Geophysical Exploration (RCGE) of the Chinese Seismological Bureau, installed 19 portable, broadband seismic stations in NE China, in the region of the Changbaishan volcanic area. The data from this seismic network are analyzed in this study.
Out of 41 earthquakes considered, 29 events below 48 km depth (one event was shallow, with a depth listed as 33 km) and at a range of 30-908 were used in this study. Two criterions were employed for event selection. The first criterion was used to avoid complex near-source structure effects, while the second was used to ensure that the direct P arrived well before secondary P phases. To ensure that the signal was strong only large events were selected. The minimum magnitude event found to equalize well was 5.3 Mb, although this depended on the signal-to-noise ratio of a given seismogram. Most of the accepted events detected by the temporary broadband seismometers were located south of the network, since the highest concentration of seismicity meeting the above criterion was in the seismic belt from Indonesia through Papua New Guinea to the Fiji islands. One event was from the Mariana Trench, three events from the north (the Aleutian Trench region) and one from the west Oocated in Iran). Due to equipment failures, power outages, periods of unusually high anthropogenic noise levels, and differing durations of deployment not all of the events were recorded at all of the stations. In this study, total 109 seismograms recorded at the 19 stations were used for modeling.
The first 20 or so seconds following direct P in a teleseismic seismogram principally contains rupture effects, source-side structure, and receiver-side structure. The source-side signal results from near source reflected P waves and S to P conversions. Receiverside signal consists of P to S conversions and P and S reflections near the receiver, principally in the crust. Direct P is incident on the Mono nearly vertically for a teleseismic distance source. Therefore, the amplitudes of S conversions are larger on the horizontal components, while the P signals are larger on the vertical component. Since the horizontal components of the seismogram principally contain receiver-side effects, it is possible to equalize the source effects and retain the receiver-side effects by deconvolving the vertical component from the radial component leaving a radial receiver function. In order to avoid division by small numbers, the equalization procedure of uses a water-level deconvolution where signal with spectral power below some threshold, called the water level, is discarded. This equalization method includes a Gaussian filter in the deconvolution, which acts as a low-pass filter removing noise in the seismograms.
Because the receiver function method uses crustal converted and reflected phases to determine crustal structure, the region of crust sampled depends on the offset of these crustal reverberations. The offsets of the reverberations depend on the ray parameter (p) and the crustal thickness (or more generally the depth to the deepest interface). The p was estimated from the event depth and range using the IASP91 model of P global seismic travel times. The lateral extent from the station that the crust is sampled by a receiver function is at least the distance to the penetration point of the first bounce P arrival. In this region where the crust is expected to be about 30-35 km thick, with an average P wave velocity of 6.0-6.5 km/s, and for the data set used, the receiver functions sample the Moho about 40-50 km from the stations. In this area, the lateral resolution of P on the Moho is about 10-15 km.
After identifying all usable seismograms, the receiver functions were determined using a constant Gaussian width parameter of 2.4 Hz and varying water levels (from 5×10~(-5) to 10~(-2)). Water level was determined largely by the quality of the original seismogram and stability of the equalization. The radial receiver functions were interpreted using three analysis techniques which estimate differing sets of model parameters: a slant-stacking procedure, a direct search forward modeling scheme, and a least-squares inversion (LS). In all of these methods, that the crust is locally homogeneous, isotropic, and that the Moho is not dipping was assumed. While using the above three techniques does not provide independent measurements, due to the differing methods and model parameters estimated, the three methods indicate robustness of the final results.
Receiver functions recorded at MANG do not suggest a low velocity anomaly and we concluded that the western extent of the low velocity anomaly is located under the western flank of the Changbaishan. One velocity profile determined at WUSU station also did not contain a midcrust low velocity zone. With these data we were unable to discern the eastern extent of this low velocity region; however, receiver functions sampling the crust to the south of DRAG are not indicative of a midcrust low velocity anomaly.
P-Wave Velocity Structure from Wide-Angle Reflection and Refraction Data
In 1998, Geophysics Exploration Center of China Earthquake Administration conducted seismic refraction/wide-angle reflection experiment in Changbai volcano area. This study also analyzes seismic arrivals collected by receivers placed along two primary lines in this experiment, the first trending from south to north, called line SN, and the second trending west to east, called line WE. Lines SN and WE are 270 and 203 km long and are composed of 66 and 57 recorders, respectively. There were three explosions along line WE, and four along line SN; however, the northernmost explosion on line SN was not used, since the signals from this source were not detected by enough recorders. The explosions were composed of 1.2-1.5 tons of TNT distributed among several shallow wells separated by 3-15 m.
Based on apparent velocity analysis, following phases were identified: the upper crust refracted phase (P_g), three mid-crust refractions/reflections (P_1, P_2 and P_3), the Moho reflection (PmP), and the upper mantle refracted phase (P_n). In total, 397 and 315 travel-time picks along lines SN and WE are used, respectively.
Throughout the inversion process, a graphical user interface of the inversion program of Zelt and Smith (1992), RayGUI, that was developed by J. Song during this study was used based on the modeling approach outlined by Zelt (1999). The 2D velocity models consist of several layers containing smooth velocity gradients
Travel-times predicted by the final P-wave velocity (v_p) models for lines SN and WE fit the observed travel-times with an RMSE of 0.1654 and 0.1636 seconds, and predict 93.7% and 94.9% of the travel-time picks, respectively. In general, the observed travel-times are fit better away from the Tianchi volcano. This is most likely due to an increased complexity of velocities near the Tianchi.
Let's refer to the crust away from the LVZ as unperturbed crust, and the velocities in the unperturbed crust increase with depth rapidly to 6.0 km/sec in the upper several kilometers. The velocities then increase less rapidly to about 6.4 km/sec at depths of about 20 km, increasing slightly faster to about 6.8-7.8 km/sec near the Moho transition. The Moho is deepest under the Tianchi and shallowest under the LVZ. The depth of the Moho is at about 40 km under the Tianchi, 30-35 km under the LVZ, and about 35 km elsewhere.
The LVZ is located roughly between the Tianchi summit and the intersection of the two lines. The velocities in the LVZ are as low as 5.4 km/sec, roughly 0.6 km/sec slower than the velocities immediately adjacent to the LVZ. Taking the 6.0 km/sec velocity contour as a proxy for the edge of the LVZ, the LVZ is located between about 75-150 km along line SN and 205-240 km along line WE, with the top of the LVZ about 10-15 km below the surface. The LVZ is at most about 10-15 km thick, and it is thickest to the north of the Tianchi volcano.
The reliability of the final P velocity models depends on a number of factors, including the accuracy of the individual travel time picks, the uncertainty of the apparent velocities of the phases, the type of phases, and the ray coverage. However, the resolving power of data can be appraised by attempting to recover a particular model feature by inverting synthetic travel-times. The most prominent result of this study is the presence of anomalous low velocities in the crust to the north of the Tianchi volcano, and to assess whether this result is robust, the source/receiver geometry of line SN was used to attempt to recover a similar anomaly in a trial model.
Crustal Structure and Magama Chamber
Receiver Function results also inferred two regions of low velocities in the upper to middle crust, one to the south and one to the north of the Tianchi. In the velocity models of Receiver Function, the crust is unperturbed south of about 20 km and north of about 175 km along line SN, and that west of about 160 km and east of about 280 km along line WE the crust is unperturbed, similar to in these 2D velocity models. The northern LVZ of Receiver Function is located in the depth range of about 8-16 km and between about 75-125 km along line SN, and v_p is as low as 5.0-5.4 km/sec. The southern LVZ of Receiver Function is located approximately between 20-60 km along line SN, with v_p as low as about 5.0 km/sec, and extends from depths of about 15 km to the Moho. The northern LVZ of Receiver Function is roughly consistent with these results, although the velocities and the depth of the LVZ do not overlap; however, a low velocity anomaly in the velocity model of line SN that would correspond to the southern LVZ of Receiver Function is not resolved. The northern LVZ of Receiver Function was better constrained than the southern LVZ of Receiver Function, the latter was only constrained with two receiver functions from the broadband station CANY (slightly to the south of the Tianchi summit. Even though there are not as many crossing rays-paths under the Tianchi compared to the more northern regions of line SN, resolutions tests indicate that the velocities are relatively well constrained in this region of the line. Hence, the differences between refraction/wide-angle reflection results and those of Receiver Function are most likely due to the relatively unconstrained velocity-depth trade-off in the receiver function method, which is less sensitive to absolute velocities and more sensitive to velocity impedance.
Dike Modeling
This study also investigated the relationship between ground deformation from dike emplacement and geometries using analytical models. A free surface half space model of Coulomb 2.5 was used in the investigation. Basic dike parameters investigated are: dip, depth of the top, depth of the bottom, and thickness of dilation, and the cross-sections are presented to illustrate temporal vertical deformation. In vertical dike (dip 90°) models, vertical ground deformation increases while the dike develops upward, and the deformation pattern is symmetric about the dike, forming two peaks at both sides of the dike where the lateral extent of surface vertical deformation increases as the dike develops. For a non-vertical dike, vertical deformation is asymmetric about the dike and extends further laterally when compared to the vertical case. Deformation amplitude is greater as well. Surface expression is negligible when the depth of dike bottom is greater than 6 km. Vertical deformation reaches 0 and is downward at the location where the dike will open on the surface. For models with dikes dipping greater than 45°, along a perpendicular bisector starting from center of the dike vertical deformation decreases on the dipping side of the dike to negative values and then back to positive values, forming a second peak. In models with dikes with dip less than 45°, vertical deformation is negative after the initial decrease near the dike location. Finally, vertical deformation increases when the thickness of dike dilation increases. These models can help guide the initial interpretation of leveling observations made in volcanic areas.
长白山火山区的地球物理学和地球化学研究结果表明，长白山天池火山是一个亟待研究的火山。长白山火山区是一个地震活动异常区，该区小震活动每年达200次左右，为其周围地震活动的10倍。大地电磁测深剖面反演结果显示，天池火山下方10km左右存在一低阻带，其向北延伸约20km，底部深度可能达30km。区域S波资料层析成象研究表明，天池火山区下25-75km范围存在一速度异常区。区域地震面波和Moho反射波的模拟结果指出，该地区的地壳厚度为30-35km。近年来大量的人工地震资料和宽频带地震资料的研究表明，天池一带地壳内存在明显的低速体，地壳厚度由北西向南东方向逐渐增加，在天池附近达到最深，一般地壳厚度在32km左右，天池附近为39km左右。根据对地热和温泉释放气体的化学分析，由温泉释放的气体来自地壳内岩浆囊，并判断在5.4-5.8km以下存在一地热储库且该地热库位于岩浆喷发通道上。显然，对长白山地区的地球物理、地球化学、地质及地表形变的资料的进一步采集、研究将是十分必要的。
本研究利用接收涵数方法和射线追踪反演方法分别对长白山火山区宽频带地震资料和主动源地震折射/宽角反射资料进行了分析和研究，获得了不同资料模拟下的长白山火山区的地壳结构，并分析了结构的共同特征以及与长白山天池火山岩浆活动的可能关系。
一．接收函数对长白山火山区地壳速度结构的研究
1998年夏，中国地震局地球物理勘探中心与美国纽约州立大学Binghamton分校合作在长白山火山及临近地区安装了19套便携式宽频带地震仪，其目的是试图通过所记录到的远震资料获取该地区区域性的地壳速度结构。本研究采用接收函数的方法对这些资料进行了研究。
接收函数是由三分量地震图计算出来的时间序列，它所表示的是近接收仪区域地壳结构的相对反应。其波形是接收仪下方地壳结构内P波及P波转换为S波的组合。通过波形的模拟，我们可以获得接收仪下方的地壳结构信息。接收函数的振幅取决于入射波的入射角和产生转换波界面的速度对比度，而转换震相的到时则取决于转换界面的深度，P波与S波在转换界面和地表面间的速度，P波入射角度或射线参数。多次转换波的振幅和频率取决于转换速度的性质，如速度变化是宽域的或是比较单一的等。
宽频带地震资料的处理和分析
这项研究从41个地震事件中选取了29个地震做接收函数分析。对事件的选择原则，一是尽量避免复杂的近源响应，二是直达P波能清楚地辨认，再则是选用大震级地震以保证有足够的信号能量。所选择的大部分地震位于观测网以南，最集中的地区是印度尼西亚经巴布亚新归亚那到菲齐的地震活动带。其它地震中，有来自玛丽亚那海沟，阿流申海沟区和伊朗。共分析了来自19个观测仪器的109个地震纪录。
地震图中直达波到达后20-40秒左右的区间包含了近源，路径和近接收点的结构信息。在信号源附近，地震信号可以转换为P波或由S波再转换为P波。在接收点附近，信号为P波，P到S的转换波，以及P或S的反射波。对于远震而言，直达P波从Moho入射进入地壳几乎是垂直到达地表。因此，转换后的S波其水平分量较大，而P波的垂直分量较大。也就是说，通过Moho转换成的S波包含了接收点附近地壳结构的信息。这样一来我们就可以通过对源进行等效化，再把垂直分量通过反褶积与水平径向分量分离而获得径向分量接收函数。
由于接收函数方法利用地壳内的转换波和反射波确定地壳结构，其所能反映的地壳区域就取决于这些波在地壳内的穿透区域。而穿透区域又取决于射线参数和地壳厚度(或转换界面的深度)。对于射线参数p，我们采用P波走时的IASP91模型，根据源的深度和距离来估计。接收函数可采样的范围至少是直达P波在Moho界面的穿透点到观测点的水平距离。然而，界面越深这个采样分辨范围越小。基于本地区的地壳厚度的估计，P波对Moho界面的横向分辨率在10-15km。
在分析径向接收函数时，采用了三项分析技术：斜向叠加处理，直接搜索反演模拟和最小二乘反演。所有这三个方法中，都假设地壳是各向均匀，各向同性，Moho面没有倾斜。因为接收函数对地壳的采样只是接收点附近的局部区域，以上的假设也只针对接收点附近的局部区域。
接收函数获得的地壳速度结构
由斜向叠加所获得的Moho面深度在火山区外围为28-32km，在火山区为30-39km，其中天池附近地壳最厚39 km。两种方法，直接搜索反演和最小二乘反演，都给出了这样的地壳厚度趋势：在火山区及其附近，最厚的地壳厚度在长白县往北至二道白河，有可能包括漫江。
P波平均速度(?)_p在火山区为大约6.0-6.2km／s。最低的P波速度达到6.0km/s以下。获得最低速度的台站有二道白河(BACH)，天池火山南坡(CANY)和延边(YABA)，其中YABA站的P波速度达到约5.7km／s。这些台站的速度都是用叠加的接收函数所获得，因此其结果也是相对更可靠。所有用叠加的接收函数所获得的速度剖面都在10-15km存在一个中地壳边界，只有YABA站除外。YABA站的速度剖面在中地壳20-25km深有一个中地壳转换层。
二道白河(BACH)和天池火山的南坡(CANY)的叠加速度剖面显示，在上地壳存在一个明显的高速层，在中地壳存在一个低速异常。BACH站高速层在2-8km，CANY站高速层在2-12km。S波速度由上层的高速4 km／s减低到下层的3 km／s左右(或更低)，CANY的高速层比BACH的高速层更明显。两个站接收函数所反应的都是同一方向(南部)的地壳结构。
火山区附近最小二乘反演对单个台站接收函数的反演也表明，从长白(FROG站)到二道白河(BACH站)的区域存在一个局部低速度带。这个区域内台站接收函数的速度剖面显示，该速度带由二道白河(BACH)向长白(FROG)方向扩展，其低速带的速度增高，厚度加大并向深部倾斜。低速带在天池火山的南坡(CANY)比其在北坡(CBAI)要厚一些和深一些。天池火山的西坡(MANO)没有低速异常显示。往西更远一些的WUSU站，其速度剖面同样没有显示出低速异常的迹象，甚至中地壳的速度略微偏高。
二．地震折射／宽角反射对长自山火山区地壳速度结构的研究
与宽频带地震观测的同时，1998年8月，中国地震局地球物理勘探中心在长白山火山区进行了主动震源地震折射／宽角反射实验。该实验的目的是结合其它地球物理，地球化学和地质资料对长白山火山区的地壳结构，火山活动性进行进一步的研究。该实验在长白山火山区布设了2条主观测剖面，2条辅助观测剖面和一个台阵观测网，共投入200台高频三分量地震仪，进行了10次1.5吨级爆破作业。本项研究只对上述观测的两条主要剖面L1和L2进行了两点射线追踪的正演和反演模拟。
射线追踪反演
九十年代初，Zelt等发展了一套二维速度与界面同时反演的方法。这套方法中，模型参数化和射线追踪方法都是适合反演算法的正演步骤的。
走时的非线性反演要求必要的初始模型和一个适当的循环方法。一个实际有效的正演步骤对反演算法是十分关键的。Zelt等提出的模型参数化方法就是针对这一日的的。在Zelt的模型参数化方法中，模型的每一层由大小不同的四边或三边块体组成，这种参数化方法能够使模型的独立参数量达到最小。其射线追踪方法包含了对2-D射线追踪方程的数值解和射线出射角的自动确定。
Zelt等的射线追踪是通过运用零阶射线理论求解射线追踪方程进行的。对一组射线，出射角的确定是用一种循环搜索方法完成的。循环搜索方法首先确定一组射线的最小和最大出射角，然后确定每一条射线的出射角。在两点射线追踪中，则只确定离接收点最近的两条射线。反演中只用所确定的两条射线之间的计算走时和偏微分。在确定初始出射角时，总是假定源下边的介质是横向均匀介质。
反演是否继续下去，有以下两个标准判断：1)射线对所有观测走时进行了追踪；2)走时残差的RMS(root-mean-square)达到了所期望的值。实际资料中，这两个标准经常是互相制约的。如要追踪所有观测走时，就可能以增加RMS为代价，反之亦然。
速度模拟
为了给读者一个较为清晰的地理概念，根据它们的地理走向分别命名L1和L2这两条剖面为SN剖面和WE剖面。SN剖面起点在天池火山以南约70km的长白县，向北穿过天池火山区，终点在敦化境内，全长270km。WE剖面起点在天池火山以西约120km的靖宇县，向东穿过天池火山区北部，终点在延吉市，全长220km。沿SN剖面有66台观测仪，沿WE剖面有51台观测仪，这些观测仪一般间隔为4km。在SN剖面上进行了4次爆破作业，WE剖面上进行了3次爆破作业，爆炸量级在1.2-1.5吨TNT。
本项研究采用了垂直分量记录，对地壳的优势震相予以辩认。所辨认到的震相有，上地壳折射波(P_8)、中地壳反射波(P_1，P_2 and P_3)、Moho界面的反射波(PmP)和来自上地幔顶部的回折波(P_n)。总共在SN剖面上辨认出397个到时，在WE剖面上辨认出315个到时。
两条剖面最终的P波速度模型的走时拟合结果分别为，SN剖面RMS为0.1654，WE剖面RMS为0.1636。其对走时总数的拟合分别为93.7％和94.9％。总体上讲，射线从天池火山区向外方向的震相拟合的稍比相反方向好一些。这很可能是由于近火山区速度结构更加复杂的结果。然而，无论在何种情况下，对速度模型复杂程度的增加都尽量控制在最小量级。除两个模型中明显的低速层(LVZ)区域外，其它区域的速度呈现出相对平滑的垂向速度梯度和微弱的横向速度变化。
为了方便描述，我们称低速体LVZ以外的地壳为原地壳。可以明显看出，原地壳速度在上部几公里的范围内迅速增加到6.0km／s，然后梯度减小，在大约20km处增至6.4km／s。再往下，速度又较快地增加到Moho转换带附近的6.8-7.8km／s。从SN模型可以看出的另外一个明显特征是，Moho在天池火山下达到最深，而在低速体以下则最浅。天池火山下的Moho深度为大约40km，低速体下的Moho深度为大约30～35km，其它区域为35km左右。
低速度体(LVZ)位于SN和WE交叉点的南部、天池火山口以北的区域。两个剖面模型的速度在交叉点基本吻合，尤其是上部地壳。低速度体的主要特征在两条剖面上类似，其最低速度达到大约5.4km／s，比其周围低约0.6km／s。如果用6.0km／s的速度等值线作为低速度体的边界，该速度体位于SN剖面大约75-150km的范围内和WE剖面205-240km的范围内。低速度体的顶部深度为10-15km，其最厚部分大约10km，位于天池火山口与两剖面交叉点之间，但更靠近交叉点。
模型评估
最终速度模型的可靠性与许多因素有关，例如走时辨认的准确性、震相视速度的不确定性、震相类型的判别，以及射线覆盖区域不均匀等。本文研究的主要结果是天池火山以北地壳低速异常体的存在。该低速异常体既为本研究模型的特定特征。为了评估该模型的可依赖性，研究中采用SN剖面实际的源-接收点几何关系建立了评估模型，试图用此评估模型恢复相应的低速体以证明本研究结果的可靠性。
模型评估的方法可进行以下描述。假设模型A有一低速异常体，其综合射线走时为TA。假设另有一模型B除没有低速体外，其他参数与模型A完全一样。我们用模型A的综合走时TA对模型B进行综合走时反演，如果反演结果模型C与模型A一致，则说明这种反演方法对该特定的低速体的反演结果是可靠的。
三．长白山火山区的地壳低速体结构与岩浆体低速度体
地壳低速体结构
本文对两条折射／宽角反射剖面资料进行了处理和研究，获得了两个独立的速度模型。在两个剖面上均有低速度体存在，最低速度达到5.4km／s以下。低速度体以外区域，地壳速度梯度比较平滑，在6.0-5.5km／s左右。Moho转换带深度在天池火山下方为40km，低速度体下方为30km，其它区域为大约35km。由宽频带地震资料用接收函数方法所获得的结果也在天池与二道白河之间发现有低速度体的存在，最底速度在5.0-5.4km／s，两种方法获得了比较一致结果。
利用完全不同的两种方法的结果，检验了两种方法本身的一致性。分别在CANY和BACH两个站选择了来自Celcbes来的地震事件，求得接收函数后用折射，宽角反射的模型进行模拟。CANY站接收函数所采样的地壳在SN上大约10-60km的范围，BACH站接收函数采样的地壳在SN上大约75-125km。我们来看一下是否可以用SN的速度模型来模拟这两个站的接收函数。将SN剖面上接收函数所对应区间的速度分段采样后进行平均，建立一个初始一维模型。对每一个一维速度模型，固定P波速度不变反演S波速度。一维模型的层厚度为二维模型层厚度的10％。根据SN剖面所建立的一维模型对两个观测站的接收函数拟合的都很好。
在所得到的两个速度模型中，最明显的特征要算是低速度体(以最低速度点为中心)没有位于天池火山的正下方，而是偏向其北部。天池火山位于SN剖面上70km左右，6km／s等值线也正好延伸至天池火山峰的下方，不过这里的速度相对于低速度体内的最低速度较高。最低速度的中心在SN剖面的100-130km范围。接近两条测线的交叉点，两个模型的上地壳非常相似，但下地壳差别比较大。下地壳的这种差异有可能由速度随深度的模糊产生，或速度-深度这对变量的对立关系产生。一个不可忽视的因素是，接收点没有沿侧线直线布设。尤其是在两条测线交叉点以东WE剖面大约50km的范围内，观测点对侧线的偏离十分明显。
关于接收函数的讨论中描述过，在天池火山附近普遍存在一个低速体，尤其是在天池与二道白河(BACH站)之间，低速度层最明显。这个区域正是SN剖面低速度体所在范围。另外，接收函数的结论认为，低速度体以外的速度梯度整体上十分平稳，这也与SN和WE的结果一致。接收函数判断的低速体的深度在天池火山与BACH之间为5-10km，这与SN剖面低速体的深度不一致。
从接收函数的结果看，可以认为在WE上大约160km以西和280km以东的部分为原地壳。在SN剖面200km以北为原地壳，75-125km存在低速度体，其深度在8-20km，最低速度可达5.0-5.4km／s。这个结果与折射／宽角反射的结果基本一致，不过低速度体的深度不太重合，接收函数结果的速度偏低。由于接收函数方法对速度差更敏感，而对速度的绝对值和梯度相对不太敏感，因此在速度-深度解上存在相对不确定性，这种差别显然是可能存在的。
低速度体中心位于天池火山以北约50km，以代表低速度体范围的6.0km／s等值线为标志，扩展范围沿SN剖面从天池南坡的30km向北延伸到天池以北的75km，深度范围在10-25km／s左右。根据SN和WE两条测线的交叉点位置和低速度体在WE剖面上的范围判断，低速度体在东西方向的延伸范围约35km。
岩浆囊的可能性
只前普遍认为，引起低速体的原因有两个。温度的升高可能引起P波速度的降低，或者岩浆的熔融或半熔融也可能引起P波的速度降低。根据该地区地球化学的研究，天池火山区存在高温热源是不争的事实。根据Mainprice(1997)对岩浆玄武岩类，主要包括辉长岩，蛇绿岩及上地幔的harzburgite，进行的地震特性实验室模拟的结果，P波速度在3.5～4.7km／s有50～70％为熔融状态；P波速度在4.8～5.6km／s有35～50％为熔融状态。这里的研究结果指出，长白山火山区地壳低速体的最低速度可能达到5.4～5.6km／s，也就是说，该火山的岩浆体可能有35％左右为熔融状态。传统上认为，岩浆体有35％为熔融状态既有喷发的可能。然而，Nicolas(1996)等对玄武岩岩浆的流体机制及孔隙度研究后认为，20％为熔融状态的岩浆就可以流动而没有塑性形变。Nicolas的结果预示，即使20％熔融状态的岩浆也有喷发的可能性。
从近些年大量的洋中脊地震学和实验室模拟研究结果我们可以了解到，由于处于半熔融状态，岩浆囊的P波速度有一个较宽的范围，从3.5km／s到5.6km／s，S波速度也不为零。尽管大陆地壳岩浆囊和海洋地壳岩浆囊可能在年龄和传输过程上不同，因而岩浆囊的地震学特性(各向异性)有所不同，但它们应具有类似的特征。不过，大陆地壳岩浆囊半熔融状态P波速度与熔融比率仍然是一个待研究的课题。要确定长白山天池火山岩浆囊的性质，进一步的研究是十分必要的。
四．岩浆活动性分析—Dike破裂过程与地表垂直位移
火山活动最终要体现在岩浆囊及岩浆通道的活动。岩浆通道的活动标志着火山喷发的可能性，因此，研究岩浆通道的活动对火山喷发预测将是十分重要的。Dike是一种平面岩浆席，是普遍存在的岩浆通道之一。相对于其它岩浆通道，Dike的岩浆传输速度快，与围岩的热交换率低。
本文采用Coulumb软件对Dike类岩浆通道的不同参数及其发展过程对地面垂直位移的影响进行了模拟。只所以模拟Dike类岩浆通道，是因为这种类型的通道是最常见的岩浆通道。所模拟的Dike包括，垂直发展Dike，倾斜发展Dike，Dike的倾角与地面垂直位移，Dike的底部深度与地面垂直位移，Dike的厚度与地面垂直位移。从模拟结果发现，不同参数的Dike产生不同类型或量值的地面垂直位移，地面垂直位移对一些类型的Dike的参数在不同阶段的敏感性可能不同。根据长白山天池火山的地震活动性，本文认为，长白山天池火山存在2～3个可能的Dike发展的可能性。
The Changbaishan volcanic region is composed of several, mostly inactive, volcanoes, and is located on the border between NE China and North Korea. The basaltic base of the Changbaishan region was formed approximately 4.5 to 1.5 Ma, while the rylotitic and dacitic volcanic edifices are younger than about 3-1 Ma. The youngest volcano in the Changbaishan is the 2744m high Tianchi volcano. The Tianchi is capped by a summit lake (the Tianchi lake) and last erupted in 1215CE and 1702CE. There have been at least five large eruptions in the Changbaishan area during the common era. The eruption at about 1000CE was one of the largest eruptions in the last 2000 years, with ash deposits found up to 1200 km away in Japan. The total volume of rock erupted from the Changbaishan volcanic region is estimated to be about 9,000 km~3.
The Changbaishan region is seismically active, with an average of 18 earthquakes per month, although during the summer of 2003, more than 30 were detected per month. The majority of the hypocenters of these events cluster near the Tianchi caldera lake in the upper 5 km of the crust, and while the Changbaishan region is much more seismically active than the surrounding area, it is far less active than other volcanic regions. There are several hot-springs surrounding the Tianchi volcano, mostly concentrated on the northern slope, and they primarily discharge meteoric water that has circulated in regions of hot rock at upper crustal depths. Tang et al. (1998) resolved a low-resistivity body below about 10 km under the Tianchi, possibly reaching depths up to 30 km, and extending about 20 km to the north of the Tianchi summit.
In this study, both broadband seismic data and seismic refraction/wide-angle reflection data were analyzed by Receiver Function method and Ray Tracing Inversion method, respectively. The velocity structures of Changbaishan region are presented and the possible relationship with magma chamber activity is analyzed.
Crustal structure determined by modeling receiver functions
In the summer of 1998, the State University of New York at Binghamton, in collaboration with the Research Center of Geophysical Exploration (RCGE) of the Chinese Seismological Bureau, installed 19 portable, broadband seismic stations in NE China, in the region of the Changbaishan volcanic area. The data from this seismic network are analyzed in this study.
Out of 41 earthquakes considered, 29 events below 48 km depth (one event was shallow, with a depth listed as 33 km) and at a range of 30-908 were used in this study. Two criterions were employed for event selection. The first criterion was used to avoid complex near-source structure effects, while the second was used to ensure that the direct P arrived well before secondary P phases. To ensure that the signal was strong only large events were selected. The minimum magnitude event found to equalize well was 5.3 Mb, although this depended on the signal-to-noise ratio of a given seismogram. Most of the accepted events detected by the temporary broadband seismometers were located south of the network, since the highest concentration of seismicity meeting the above criterion was in the seismic belt from Indonesia through Papua New Guinea to the Fiji islands. One event was from the Mariana Trench, three events from the north (the Aleutian Trench region) and one from the west Oocated in Iran). Due to equipment failures, power outages, periods of unusually high anthropogenic noise levels, and differing durations of deployment not all of the events were recorded at all of the stations. In this study, total 109 seismograms recorded at the 19 stations were used for modeling.
The first 20 or so seconds following direct P in a teleseismic seismogram principally contains rupture effects, source-side structure, and receiver-side structure. The source-side signal results from near source reflected P waves and S to P conversions. Receiverside signal consists of P to S conversions and P and S reflections near the receiver, principally in the crust. Direct P is incident on the Mono nearly vertically for a teleseismic distance source. Therefore, the amplitudes of S conversions are larger on the horizontal components, while the P signals are larger on the vertical component. Since the horizontal components of the seismogram principally contain receiver-side effects, it is possible to equalize the source effects and retain the receiver-side effects by deconvolving the vertical component from the radial component leaving a radial receiver function. In order to avoid division by small numbers, the equalization procedure of uses a water-level deconvolution where signal with spectral power below some threshold, called the water level, is discarded. This equalization method includes a Gaussian filter in the deconvolution, which acts as a low-pass filter removing noise in the seismograms.
Because the receiver function method uses crustal converted and reflected phases to determine crustal structure, the region of crust sampled depends on the offset of these crustal reverberations. The offsets of the reverberations depend on the ray parameter (p) and the crustal thickness (or more generally the depth to the deepest interface). The p was estimated from the event depth and range using the IASP91 model of P global seismic travel times. The lateral extent from the station that the crust is sampled by a receiver function is at least the distance to the penetration point of the first bounce P arrival. In this region where the crust is expected to be about 30-35 km thick, with an average P wave velocity of 6.0-6.5 km/s, and for the data set used, the receiver functions sample the Moho about 40-50 km from the stations. In this area, the lateral resolution of P on the Moho is about 10-15 km.
After identifying all usable seismograms, the receiver functions were determined using a constant Gaussian width parameter of 2.4 Hz and varying water levels (from 5×10~(-5) to 10~(-2)). Water level was determined largely by the quality of the original seismogram and stability of the equalization. The radial receiver functions were interpreted using three analysis techniques which estimate differing sets of model parameters: a slant-stacking procedure, a direct search forward modeling scheme, and a least-squares inversion (LS). In all of these methods, that the crust is locally homogeneous, isotropic, and that the Moho is not dipping was assumed. While using the above three techniques does not provide independent measurements, due to the differing methods and model parameters estimated, the three methods indicate robustness of the final results.
Receiver functions recorded at MANG do not suggest a low velocity anomaly and we concluded that the western extent of the low velocity anomaly is located under the western flank of the Changbaishan. One velocity profile determined at WUSU station also did not contain a midcrust low velocity zone. With these data we were unable to discern the eastern extent of this low velocity region; however, receiver functions sampling the crust to the south of DRAG are not indicative of a midcrust low velocity anomaly.
P-Wave Velocity Structure from Wide-Angle Reflection and Refraction Data
In 1998, Geophysics Exploration Center of China Earthquake Administration conducted seismic refraction/wide-angle reflection experiment in Changbai volcano area. This study also analyzes seismic arrivals collected by receivers placed along two primary lines in this experiment, the first trending from south to north, called line SN, and the second trending west to east, called line WE. Lines SN and WE are 270 and 203 km long and are composed of 66 and 57 recorders, respectively. There were three explosions along line WE, and four along line SN; however, the northernmost explosion on line SN was not used, since the signals from this source were not detected by enough recorders. The explosions were composed of 1.2-1.5 tons of TNT distributed among several shallow wells separated by 3-15 m.
Based on apparent velocity analysis, following phases were identified: the upper crust refracted phase (P_g), three mid-crust refractions/reflections (P_1, P_2 and P_3), the Moho reflection (PmP), and the upper mantle refracted phase (P_n). In total, 397 and 315 travel-time picks along lines SN and WE are used, respectively.
Throughout the inversion process, a graphical user interface of the inversion program of Zelt and Smith (1992), RayGUI, that was developed by J. Song during this study was used based on the modeling approach outlined by Zelt (1999). The 2D velocity models consist of several layers containing smooth velocity gradients
Travel-times predicted by the final P-wave velocity (v_p) models for lines SN and WE fit the observed travel-times with an RMSE of 0.1654 and 0.1636 seconds, and predict 93.7% and 94.9% of the travel-time picks, respectively. In general, the observed travel-times are fit better away from the Tianchi volcano. This is most likely due to an increased complexity of velocities near the Tianchi.
Let's refer to the crust away from the LVZ as unperturbed crust, and the velocities in the unperturbed crust increase with depth rapidly to 6.0 km/sec in the upper several kilometers. The velocities then increase less rapidly to about 6.4 km/sec at depths of about 20 km, increasing slightly faster to about 6.8-7.8 km/sec near the Moho transition. The Moho is deepest under the Tianchi and shallowest under the LVZ. The depth of the Moho is at about 40 km under the Tianchi, 30-35 km under the LVZ, and about 35 km elsewhere.
The LVZ is located roughly between the Tianchi summit and the intersection of the two lines. The velocities in the LVZ are as low as 5.4 km/sec, roughly 0.6 km/sec slower than the velocities immediately adjacent to the LVZ. Taking the 6.0 km/sec velocity contour as a proxy for the edge of the LVZ, the LVZ is located between about 75-150 km along line SN and 205-240 km along line WE, with the top of the LVZ about 10-15 km below the surface. The LVZ is at most about 10-15 km thick, and it is thickest to the north of the Tianchi volcano.
The reliability of the final P velocity models depends on a number of factors, including the accuracy of the individual travel time picks, the uncertainty of the apparent velocities of the phases, the type of phases, and the ray coverage. However, the resolving power of data can be appraised by attempting to recover a particular model feature by inverting synthetic travel-times. The most prominent result of this study is the presence of anomalous low velocities in the crust to the north of the Tianchi volcano, and to assess whether this result is robust, the source/receiver geometry of line SN was used to attempt to recover a similar anomaly in a trial model.
Crustal Structure and Magama Chamber
Receiver Function results also inferred two regions of low velocities in the upper to middle crust, one to the south and one to the north of the Tianchi. In the velocity models of Receiver Function, the crust is unperturbed south of about 20 km and north of about 175 km along line SN, and that west of about 160 km and east of about 280 km along line WE the crust is unperturbed, similar to in these 2D velocity models. The northern LVZ of Receiver Function is located in the depth range of about 8-16 km and between about 75-125 km along line SN, and v_p is as low as 5.0-5.4 km/sec. The southern LVZ of Receiver Function is located approximately between 20-60 km along line SN, with v_p as low as about 5.0 km/sec, and extends from depths of about 15 km to the Moho. The northern LVZ of Receiver Function is roughly consistent with these results, although the velocities and the depth of the LVZ do not overlap; however, a low velocity anomaly in the velocity model of line SN that would correspond to the southern LVZ of Receiver Function is not resolved. The northern LVZ of Receiver Function was better constrained than the southern LVZ of Receiver Function, the latter was only constrained with two receiver functions from the broadband station CANY (slightly to the south of the Tianchi summit. Even though there are not as many crossing rays-paths under the Tianchi compared to the more northern regions of line SN, resolutions tests indicate that the velocities are relatively well constrained in this region of the line. Hence, the differences between refraction/wide-angle reflection results and those of Receiver Function are most likely due to the relatively unconstrained velocity-depth trade-off in the receiver function method, which is less sensitive to absolute velocities and more sensitive to velocity impedance.
Dike Modeling
This study also investigated the relationship between ground deformation from dike emplacement and geometries using analytical models. A free surface half space model of Coulomb 2.5 was used in the investigation. Basic dike parameters investigated are: dip, depth of the top, depth of the bottom, and thickness of dilation, and the cross-sections are presented to illustrate temporal vertical deformation. In vertical dike (dip 90°) models, vertical ground deformation increases while the dike develops upward, and the deformation pattern is symmetric about the dike, forming two peaks at both sides of the dike where the lateral extent of surface vertical deformation increases as the dike develops. For a non-vertical dike, vertical deformation is asymmetric about the dike and extends further laterally when compared to the vertical case. Deformation amplitude is greater as well. Surface expression is negligible when the depth of dike bottom is greater than 6 km. Vertical deformation reaches 0 and is downward at the location where the dike will open on the surface. For models with dikes dipping greater than 45°, along a perpendicular bisector starting from center of the dike vertical deformation decreases on the dipping side of the dike to negative values and then back to positive values, forming a second peak. In models with dikes with dip less than 45°, vertical deformation is negative after the initial decrease near the dike location. Finally, vertical deformation increases when the thickness of dike dilation increases. These models can help guide the initial interpretation of leveling observations made in volcanic areas.
引文
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刘启元，范会吉，1992。震源区和接收区结构不同情况下体波合成地震图的矩阵- 射线方法。地球物理学报，第35卷，第2期，第193—203页。
刘启元，1994。大陆岩石圈的宽频带地震台阵研究，现今地球动力学研究及其应用，刘若新主编，地震出版社，第143—150页。
刘启元，Kind R．，2004。分离三分量远震接收函数的多道最大或然性反褶积方法，地震地质，第26卷，第3期，第416-425页。
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刘启元，Kind R．，1992。远震接收函数及非线性复谱波形反演，中国地球物理学会年刊，地震出版社，第18页．
刘启元，范会吉，1992。震源区和接收区结构不同情况下体波合成地震图的矩阵- 射线方法。地球物理学报，第35卷，第2期，第193—203页。
刘启元，1994。大陆岩石圈的宽频带地震台阵研究，现今地球动力学研究及其应用，刘若新主编，地震出版社，第143—150页。
刘启元，Kind R．，2004。分离三分量远震接收函数的多道最大或然性反褶积方法，地震地质，第26卷，第3期，第416-425页。
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汤吉，马明志，刘若新，1998，天池火山区岩浆囊的大地电磁探测及近期微震观测，长白山天池火山近代喷发，刘若新主编，科学出版社，第108-122页。
李继泰，杨清福，李春峰，马明志，1995，长白山天池火山地质及近期动态观测结果，火山作用与人类环境，刘若新主编，地震出版社，第28-35页。
张先康，1999，长白山天池火山区岩浆系统、地壳结构的三维深地震测深探测研究，中国地震局地球物理勘探中心项目报告。
张先康，2002，长白山天池火山区岩浆系统深部结构的深地震探测研究，地震学报，第24卷，第2期，第135-143页。
郭履灿，马石庄，张禹慎，1996，应用地震CT技术研究长白山火山的岩浆囊，CT理论与研究，第5卷，第1期，第47-52页。
魏海泉，杨清福，1998，长白山天池火山地质，长白山天池火山近代喷发，刘若新主编，科学出版社，第6-18页。
Aki, K., Richards, P.G., 1980, Quantitative seismology, vol. 2, Freeman, San Franciso.
Aloisi, A., O. Cocina, et al., 2002. Seismic tomography of the crust underneath the Etna volcano, Sicily. Physics of the Earth and Planetary Interiors 134(3-4): 139-155.
Ammon, C. J., Randall, G. E., Zandt, G., 1990. On the nonuniqueness of receiver function inversions. J. Geophys. Res. 95, 15303-15318.
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