Changbaishan Tianchi volcano geothermal system:Magma chamber and hidden high-temperature geothermal resources
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摘要:
长白山天池火山是晚新生代中心喷发式复合型层状火山,全新世以来发生过多次大规模喷发,与深部岩浆热扰动活动相关的构造地震、地表形变、温泉气体组份等均显示该火山仍可能再次喷发,是我国东部潜在的高温地热资源区。长白山天池火山周边地下水十分丰富,具有形成高温水热活动的岩浆囊热源。为探索地壳浅层隐伏的高温地热资源,本文在野外考察基础上,利用地热学方法,计算了该区地层结构与热状态,分析了地表下的火山地热系统。结果表明:长白山天池火山区地下2 km深处的温度在66—110℃之间,12 km深处的温度在313—417℃之间;该区居里面深度较浅,平均深度为12.7 km,居里点温度为375℃,其中长白山天池火山喷发中心和望天鹅火山喷发中心为居里面上隆区;在人工地震基底速度约束下,通过沉积地层重力反演发现,在约3.5—5.5 km深度处存在密度梯级高压带,该高压带与12 km深度处的岩浆囊之间的区域是形成隐伏高温地热资源的有利区域。
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关键词:
- 长白山天池火山 /
- 浅层地热结构 /
- 沉积层密度梯级高压带 /
- 岩浆囊 /
- 高温地热资源
Abstract:The Changbaishan Tianchi volcano is a Late Cenozoic central eruptive composite layered volcano. It has experienced multiple large-scale eruptions since the Holocene, and there is still modern magmatic thermal disturbance in the depth. Therefore, it is a potential high-temperature geothermal resource area in China. The Changbaishan Tianchi volcano is an intraplate volcano which has undergone three forming stages of shield volcano, cinder cone and ignimbrite. The shield-forming stage: Late Pliocene-Early Pleistocene, basaltic magma erupted from Changbaishan Tianchi volcano and adjacent craters overflowed radially, forming a shield-shaped volcanic lava platform. The cone-forming stage: In the Mid-Late Pleistocene, the coarse rock lava formed by multiple eruptions overflowed from the centre of the Tianchi crater and accumulated on the volcanic shield basalt, forming the Tianchi volcanic cone. The ignimbrite-forming stage: In the Holocene, the activity of Tianchi volcanic reached a new peak, and an explosive eruption occurred at its centre. The pyroclastic flow accumulates and consolidates to form ignimbrite, which is covered on the volcanic cone in a sheet shape.
Active magma chambers still exist beneath Changbaishan Tianchi volcano. The shallow crustal magma chambers provide sufficient heat for underground hydrothermal activity. The groundwater surrounding Tianchi volcano is primarily composed of pore and fissure water in the Late Pleistocene basalt, which is characterised by thick aquifers, good connectivity and an abundance of water. At the periphery of Tianchi volcano, the thickness of basalt thins towards the periphery, and groundwater runoff from the Tianchi volcanic cone to the periphery is impeded by the surrounding bedrock, forming a ringed groundwater overflow zone. The groundwater is heated by high-temperature magma gas (CO2) at depth and then rises along fractures under stratigraphic pressure to form hot springs at the surface. There are three thermal spring groups associated with modern volcanic activity around the Tianchi crater lake in Changbaishan area. The genesis of all thermal spring groups is related to high-temperature magmatic gases from magma sacs. However, the maximum temperature at watering places of the hot spring groups is 82℃, and there is no high-temperature hydrothermal activity.
The hydrogeological conditions in Changbaishan Tianchi volcano are favourable for the formation of high-temperature hydrothermal systems. The presence of high-temperature upward-moving magma in the crust is evidenced by modern volcanic activity and shallow residual magma chambers serve as an ideal heat source for the formation of high-temperature geothermal systems. Although the Changbaishan Tianchi volcano did not form a high-temperature hydrothermal system on the surface, it is possible that a latent high-temperature geothermal system could form in the subsurface. With the support of the project, we analysed the crustal thermal structure and upward magmatic heat state of the region using geothermal methods. We also investigated the possible existence of a hidden shallow high-temperature geothermal system in the subsurface based on data from the field geothermal geological survey of Changbaishan area and the research results of previous researchers.
The shallow thermal structure was calculated by using the Fourier heat conduction equation, based on the two geothermal wells of CR1 and CR2 thermophysical parameters data and the regional heat flow data. For depths between 5 km and the depth of the Curie point (DC), we calculated the depth of the Curie point (DC) based on aeromagnetic anomolies data and used geothermal heat flow constraints to calculate the temperature of the Curie point (TC) , then we calculate the geothermal temperature gradient (dT/dZ). At last, we obtained the thermal structure between 5 km and the Curie points, combining the temperature at 5 km depth with the calculated values. The shallow density structure of the Tianchi volcano in Changbaishan area was obtained by using the gravitational anomaly genetic-finite cell method. This was achieved through parametric substitution of thermal and density parameters and inversion of the density difference at the stratigraphic interface. Poisson's equation and steady-state Fourier heat conduction equation were found to have formal similarities. The obtained results are as follows:
The temperature at a depth of 2 km below the Changbaishan Tianchi volcanic zone ranges from 66 to 110℃, with an average of 78℃. At a depth of 12 km, the temperature ranges from 313 to 417℃, with an average of 372℃. The depth of the area is shallow, with an average of 12.7 km, and the temperature at this depth is 375℃. The eruption center of Changbaishan Tianchi volcanic and Wangtiane volcano is the uplift area of Curie geothermal surface. The ground temperatures in the shallow and middle layers of the crust are centred on Changbaishan volcano and Wangtianwan volcano on the southwest side, forming a connected high-temperature area in the northeast-southwest direction. The north-south temperature profile reveals a deep crustal-mantle heat source beneath the eruption centre of Changbaishan volcano. As heat transfers from downward to upward, the curie surface slowly uplifts from south to north, forming a low and slow high-temperature uplift at a depth of 12−15 km below the Tianchi volcano. This uplift continuously provides heat upward to the magma chambers.
Gravity inversion was used to study sedimentary strata based on artificial seismic velocity data. The results show that around the eruption centre of Tianchi volcano, the residual densities of the layers at different depth are positively and negatively interspersed. This indicates that the geological bodies of each stratum and the surrounding rocks have a structural pattern of interpenetration in a transversal direction. The average surface density is 1.90 g/cm3, which is consistent with the low-density characteristics of the accumulation rock based on pyroclastic flow formed by multiple eruptions overlying the surface. The average density values of the subsurface layers at various depths are 1.96 g/cm3, 2.21 g/cm3, 2.63 g/cm3, and 2.66 g/cm3, respectively. There is a large jump in the density value between the second and third layers. The density jump zone corresponds to a depth of approximately 3.5−5.5 km (including surface elevation) on the profile. The difference in density between the upper and lower layers is approximately 0.42 g/cm3, which is supposed to be a layer of density step high pressure zone formed during rapid deposition caused by volcanic eruption.
Based on the above calculations, we have analysed that the depth of the Changbaishan Tianchi volcanic area is approximately 8.6 km, shallower than the average depth of the global crust. This indicates that the heat source in this area is shallow and located in a shallow layer of the crust. This finding is consistent with the geological fact that the Changbaishan Tianchi volcano is an active volcano that has had eruptive activities since the Holocene. The upper uplift zone of the Changbaishan Tianchi volcanic area, along the eruption centres of Changbaishan and Wangtiane, is oriented in a northeast-southwest direction. This reflects the influence of deep geodynamic processes on the thermal state of the shallow crust. Due to the scarcity of heat flow measurement points in the Changbaishan Tianchi volcanic area, the curie surface is crucial for studying the shallow thermal structure of the region. It also provides a theoretical basis for investigating the trajectory of underground magma source storage and transport in the Changbaishan crater.
Furthermore, we consider the shallow surface density step high pressure zones to be a crucial factor in the development of the deep latent high-temperature geothermal system of the Changbaishan Tianchi volcano. The pressure and material in the deep part of the eruption centre are instantly released during the eruption of volcanic material on the surface, causing the volcanic neck to instability and collapse. This forms rapid sedimentary tectonic conditions, creating a special density step high pressure zone at a depth of 3.5−5.5 km. The sedimentary fluid between the density step high pressure zone and the high-temperature gas-liquid channel experiences abnormally high fluid stress due to the high pressure of the overlying high-pressure zone and the high temperature of the underlying high-temperature gas-liquid channel, causing the high-temperature and high-pressure thermal fluid to converge along the favourable area at the bottom boundary of the density step high pressure zones, forming a hidden high-temperature geothermal system. The hydrothermal activity of the hot springs beneath the surface is not directly linked to the magma chambers. Instead, it is connected to the high-temperature geothermal fluid through the top interface of the density step high pressure zones.
Main conclusions: ① The geothermal geological and hydro-geological conditions of Changbaishan Tianchi volcano are favourable. Although there is no high-temperature hydrothermal activity on the surface, it is assumed that a high-temperature geothermal system is concealed underground. There is an obvious density step high pressure zone at a depth of 3.5−5.5 km in the area around the eruption centre of Tianchi volcano, which is a favourable area for the formation of hidden high-temperature geothermal resources. This area is favourable for the formation of concealed high-temperature geothermal resources. ② The high-pressure zone is crucial to the formation of geopressured–geothermal resources. Anomalous high-pressure zone may only appear during rapid deposition. During the Late Cenozoic era, the volcanic neck of Changbaishan Tianchi volcano area collapsed due to the instantaneous release of deep pressure and material from the eruption centre. This event created the necessary dynamic condition for rapid deposition and the formation of density step high pressure zones. The high-pressure zone caused by the density step is also a significant factor contributing to the absence of high-temperature hydrothermal activity on the surface of Changbaishan Tianchi volcano.
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引言
地震动的方向性效应会改变地震动的空间分布特征,进而加剧特定区域的地震破坏。地震破裂方向性、震源辐射模式、局部场地放大效应、传播路径衰减均可能引起地震动的方向性效应,而地震破裂方向性是最主要的诱因。Benioff (1955)在加利福尼亚州科恩县MW7.5地震中发现了地震动方向性效应,并称之为“地震多普勒效应”。地震在断层上以接近剪切波速的破裂速度朝一个主要方向破裂时,辐射的地震能量几乎同时到达破裂前方,由于能量的积累效应在破裂前方一般会产生大速度脉冲的速度时程,破裂前方的地震动具有幅值更大、持时更短的特点。辐射的地震能量陆续到达破裂后方,其地震动具有持时更长、幅值更小的特点。从地震动和震害空间分布的角度分析,国内外破坏性大地震中已经观察到明显的方向性效应,例如:1992年美国加州兰德斯MW7.3地震(Velasco et al,1994)、1994年美国北岭MW6.7地震(Wald et al,1996)、1999年中国台湾集集MW7.7地震(Phung et al,2004)、2008年中国四川汶川MW7.8地震(胡进军,谢礼立,2011)和2016年意大利中部MW6.2地震(Ren et al,2017)等。McGuire等(2002)已证实大地震的破裂方式以单向破裂为主,地震动很可能呈现方向性效应,基于多种观测数据(远场测震、近震动态数据、大地测量)的震源运动学破裂模型反演可以清晰地揭示出大地震的破裂方向性特征(岳汉等,2020)。
中小地震一般被视作均匀破裂的点源,不考虑单向破裂或不均匀破裂,但近年来的一些研究结果表明,多数小地震的破裂方式具有单向破裂或不均匀破裂的特点(Folesky et al,2016;Yoshida,2019;Yoshida et al,2019;Ross et al,2020)。Ross等(2020)利用谱分解技术研究了美国加州地区四组小震群的震源破裂方向性,发现超过60%的小地震具有单向破裂特征。Lin和Lapusta (2018)提出了环状震源模型来解释小地震的单向破裂。小地震破裂方向性对地震动峰值参数、震源谱或视震源时间函数等的空间分布也具有不可忽视的影响(Cultrera et al,2009;Courboulex et al,2013),目前国内外有多种估计小地震破裂方向性的方法,例如高阶矩张量法(McGuire,2004)、破裂方向性效应拟合方法(Boatwright,2007;Convertito et al,2012;Kane et al,2013;Wen et al,2015;Wang et al,2019)、谱分解方法(Calderoni et al,2015;Ross et al,2020)以及重复地震序列视持时变化观测(Lengliné,Got,2011)等。
据中国地震台网测定,2021年5月21日云南省大理州漾濞县(25.70°N,99.88°E)发生MS6.4地震,震源深度为10 km。自2021年5月18日以来在此次地震震中附近累计发生了45次M>3.0地震,此次漾濞地震序列是典型的前震-主震-余震序列,大多数地震为走滑型地震。根据地震序列重定位结果,此次地震序列的破裂断层较为复杂,包括走向北西-南东、倾向西南、长约20 km的右旋走滑未知断裂和走向北东-南西或北北东-南南西的多个长约5 km的左旋走滑次级断裂(段梦乔等,2021;李大虎等,2021;龙锋等,2021;Yang et al,2022),该未知断裂大致平行于其东北方向数千米处的维西—乔后断裂。漾濞MS6.4地震造成了大理州56个乡镇受灾,研究漾濞地震序列的震源破裂方向性,有助于理解地震的发震机制,更准确地评估地震引起的地面运动及破坏分布特征。本文基于强震动观测记录,拟建立漾濞地震序列地震动参数预测方程,并根据记录事件内残差的方位角相关性,通过拟合破裂方向性效应,分析记录数量充足且空间分布均匀的九次地震的破裂方向性特征,确定地震破裂方式、破裂方向、破裂速度等,以探讨方向性效应对周期的依赖性。
1. 方法
Ben-Menahem (1961)将震源简化为具有一致滑动分布和恒定破裂速度的线源模型,以破裂方向φ和破裂速度vr作为破裂方向性的特征参数,并用方向性系数Cd来表示单侧破裂线源模型引起的与观测场址方位角相关的破裂方向性效应。Boatwright (2007)进一步改进Cd以用来表示不对称双侧破裂的线源模型的方向性效应,并引入特征参数k表示在某一个破裂方向上的破裂占整个破裂的比重,Wen等(2015)、Wang等(2019)和Colavitti等(2022)均采用改进的Cd来表示中小地震的破裂方向性效应,表达示为
$$ {{C}_{\text{d}}=\sqrt{\frac{{k}^{2}}{{\left[1-\dfrac{{v}_{\text{r}}}{\beta }\mathrm{cos}\psi \right]}^{2}}+\dfrac{ ( 1-k{ ) }^{2}}{{\left[1+\dfrac{{v}_{\text{r}}}{\beta }\mathrm{cos}\psi \right]}^{2}}} }\;\;, $$ (1) 式中:ψ为离源地震波与破裂方向之间的夹角(Joyner,1991),对于直达波,ψ=φ-θ,θ为观测台站的方位角;vr/β为马赫数,β为震源处介质的剪切波速;k为沿着破裂方向φ的破裂占整个破裂的比例,是判断破裂方式以及φ是否是主破裂方向的指标,当k=1时,表示沿破裂方向φ的单侧破裂,当k=0时,表示背离破裂方向φ的单侧破裂,当k介于0—1之间时,表示双侧破裂,k>0.5则φ为主破裂方向,k<0.5则与φ相反的方向(φ-180°或者φ+180°)为主破裂方向。
地震动参数预测方程通常采用与震源、传播路径和局部场地相关的预测变量来预测地震动参数(峰值、反应谱、持时等),为方便工程应用,地震动预测方程中采用的预测变量一般不能完全描述震源、传播路径和局部场地的影响,这些不被预测方程表示的其它效应包含在残差中。尽管目前已有预测方程采用了方向性效应校正项来体现破裂方向性的影响(Spudich,Chiou,2008),但绝大多数常用的衰减关系仅采用震级、断层破裂类型等简单的预测变量表示震源的影响,如果某次地震的破裂方式为单侧破裂或不对称的双侧破裂,那么破裂方向效应可能存在且表现为与方位角相关的事件内残差。理论上,方向性系数Cd可表示事件内残差随方位角的变化规律,即:
$$ \sum\limits_{i = 1}^N {{{ [ {\delta {W_i} - {{\lg }}C_{\rm{d}}^n + C_i^{{\rm{un}}}} ] }^2}} = {\rm{min}} ,$$ (2) 式中:δWi表示第i个观测记录的事件内残差;N表示记录数量;Ciun是N个服从正态分布的随机数,正态分布的均值为0,标准差为地震动预测方程的标准差,用来反映地震动预测方程中一些不确定性因素的影响;n取值范围一般为0—2,与震源模型有关,目前n的取值仍存在争议(Ruiz et al,2011),现有研究采用了不同的n值解释了破裂方向性对地震动和视震源谱的影响,例如n=1 (Bernard et al,1996;Boatwright,2007;Wen et al,2015)或n=0.5 (Gallovič,2016;Ren et al,2017),Gallovič (2016)研究表明n=0.5可以更好地表示近断层区域内频率>1.0 Hz地震动的方向性效应,Ren等(2017)讨论了n的取值对k和vr/β的影响,并发现n=0.5时vr/β的结果更符合地震学认知,因此本文取n=0.5。
2. 强震动数据及地震动预测方程
27个自由场强震动台站和117个用于地震预警和烈度速报的一般台站在86次MS(ML)0.0—6.4漾濞地震中共获取了2470组加速度时程记录,其中37次ML<3.0地震每次仅获得了1—2组记录,其它地震中均获取了不少于10组记录。尽管强震动观测台站和用于地震预警和烈度速报的一般台站的传感器类型、仪器安装方式均不同,但同台址或相邻台址两类台站的观测记录对比结果显示,加速度记录波形、峰值和反应谱无显著差异(Qiang et al,2023)。为确保数据质量,首先对强震动记录进行数据处理,包括波形人工质量检查、基线校正、四阶非因果巴特沃斯带通滤波等,低通滤波频率统一为30 Hz,高通滤波频率则根据记录信噪比(信噪比阈值为3.0)确定(Wang,Wen,2021)。为减小采样偏差,采用震级相关的截断距离对记录进行筛选,截断距离通过多次试算确定,当M=3.0时截断距离为40 km,M=6.4时截断距离为80 km,M∈(2.8,6.4)时采用线性插值确定各震级的截断距离。本文最终选用MS2.8—6.4地震中725组强震动记录来建立地震动预测方程,选用记录的震级-距离分布如图1a所示。不同周期可用的记录数与高通滤波频率有关(Abrahamson,Silva,1997),如图1b所示,随周期增大可用记录数量减少。本文使用的地震动参数有峰值加速度(peak ground acceleration, 缩写为PGA)、峰值速度(peak ground velocity, 缩写为PGV)和0.1—15 s周期段内的14个周期点的5%阻尼比加速度反应谱(pseudospectral acceleration,缩写为PSA),水平向地震动参数是两个水平分量地震动参数的几何平均值。
本文以震级、Joyner-Boore距离(破裂面水平投影最近的距离)、vS30作为预测变量,包含了震级的二次项函数、震级相关的几何扩散、非弹性衰减项和vS30相关的线性场地放大效应,地震动预测方程可表示为
$$ \lg Y = {a_1} + {a_2}M + {a_3}{M^2} + ( {a_4} + {a_5}M ) \lg \sqrt {{R^2} + {h^2}} + {a_6} ( \sqrt {{R^2} + {h^2}} - 1 ) + {a_7}\lg {v_{{\text{S30}}}} + \varDelta, $$ (3) 式中:Y为水平向地震动观测值;M为震级(MS或ML);R表示Joyner-Boore距离,采用了杨九元等(2021)基于InSAR数据反演确定的MS6.4漾濞地震的破裂面计算其观测记录的R值,对于其它地震以震中距近似代替R值;vS30值表示地下30 m土层的等效剪切波速,本文基于坡度相关的全球vS30值通过最近邻域差值估计了各观测台站的vS30值,大多数台站的vS30值在360—760 m/s的范围内;h是用来表示近场饱和效应的虚拟深度;Δ是总残差;a1—a7为回归系数。
本文采用Abrahamson和Youngs (1992)提出的随机效应回归方法进行分析,回归系数及标准差见表1,本文给出的预测中位值可以很好地预测地震动参数随距离的衰减规律(图2)。由于近场数据有限,通过回归分析确定的h值会造成近源地震动预测值并不完全满足震级越大预测值越大的基本认识,Yenier和Atkinson (2015)基于全球范围内观测数据丰富的地震确定的h值,建立了震级相关的h值经验关系,本文在此经验关系初步给出的h值的基础上,给出了修正的h值以确保震级越大近距离的地震动强度指标越大,本文给出的h值介于1.0—3.9范围内(表2)。
表 1 地震动参数预测方程回归系数及标准差Table 1. Regression coefficients and standard deviations of the parameter prediction equations of ground motion地震动强度指标 回归系数 a1 a2 a3 a4 a5 a6 a7 φ τ σ PGA 2.907 4 −0.089 5 0.028 5 −1.719 0 0.142 1 −0.009 6 −0.085 5 0.340 8 0.192 4 0.391 4 PGV 2.226 0 −0.106 2 0.046 6 −1.481 1 0.099 0 −0.008 6 −0.521 2 0.358 9 0.244 6 0.434 3 PSA T=0.10 s 2.581 5 0.066 6 0.026 1 −1.456 6 0.043 9 −0.005 4 −0.023 1 0.357 2 0.171 4 0.396 2 T=0.15 s 2.650 7 0.082 0 0.025 9 −1.081 5 0.041 3 −0.008 2 −0.241 3 0.368 8 0.205 0 0.421 9 T=0.20 s 2.818 1 0.090 8 0.029 2 −1.117 1 0.027 4 −0.006 6 −0.333 8 0.419 9 0.230 2 0.478 9 T=0.26 s 2.902 3 0.030 4 0.042 1 −0.963 7 −0.000 5 −0.006 7 −0.405 1 0.456 1 0.237 0 0.514 0 T=0.30 s 2.764 9 0.046 3 0.044 1 −0.900 2 −0.013 7 −0.006 5 −0.426 5 0.448 0 0.230 3 0.503 7 T=0.36 s 3.021 2 0.068 7 0.043 0 −1.003 7 −0.008 1 −0.005 4 −0.556 9 0.424 4 0.224 9 0.480 3 T=0.40 s 3.165 0 0.103 1 0.039 3 −1.1328 0.005 1 −0.005 3 −0.624 6 0.420 0 0.223 8 0.475 9 T=0.46 s 2.810 6 0.198 6 0.037 9 −0.977 2 −0.050 5 −0.003 7 −0.637 0 0.404 7 0.230 9 0.465 9 T=0.50 s 2.657 4 0.251 2 0.034 4 −0.949 4 −0.059 6 −0.004 0 −0.646 8 0.403 3 0.232 5 0.465 5 T=0.60 s 2.190 3 0.343 8 0.036 4 −0.735 3 −0.130 6 −0.001 5 −0.654 1 0.402 5 0.244 5 0.470 9 T=0.70 s 2.026 6 0.344 0 0.035 3 −0.619 7 −0.124 0 −0.003 1 −0.677 0.400 3 0.241 0 0.467 2 T=0.90 s 1.713 3 0.364 3 0.034 4 −0.429 3 −0.142 6 −0.002 0 −0.721 1 0.403 5 0.246 0 0.472 6 T=1.00 s 1.416 7 0.430 2 0.028 6 −0.294 7 −0.147 7 −0.003 0 −0.749 4 0.411 8 0.232 4 0.472 9 T=1.50 s 1.015 6 0.548 5 0.018 0 −0.079 2 −0.155 4 −0.001 9 −0.919 5 0.457 7 0.260 7 0.526 7 表 2 震级相关的h值Table 2. Magnitude-dependent h valuesM h M h M h M h 2.8—3.5 1.00 3.6—3.7 1.05 4.0—4.5 1.10 4.6—4.7 1.20 5.0 1.50 5.2 1.50 5.6 2.00 6.4 3.90 3. 结果及讨论
本文选取了记录数量充足且空间分布均匀的9次M4.2—6.4地震,包括主震、三次前震和五次余震,用来分析这些地震的震源破裂方向性,地震基本信息及记录数详见表3。除了8号地震外,其它地震在R<100 km内的可用记录数均不少于50组,由于预测方程的截断距离在40—80 km范围内,在分析破裂方向性时选用了100 km内的记录,为消除远场可能的非弹性衰减差异,在拟合估计破裂方向性参数时,对事件内残差进行远场距离项校正,以保证事件内残差与距离无明显相关性。本文选用的9次地震在漾濞地震序列中的空间位置如图3a所示,图3b—j分别给出了9次地震中R<100 km的记录的台站空间分布情况。
表 3 选用的9次地震的基本信息及记录数Table 3. Basic information and the number of recordings for the nine events considered地震编号 发震时刻(北京时间) MS 北纬/° 东经/° 震源深度/km 记录数 年-月-日 时:分:秒 1 2021-05-19 20:05:56 4.6 25.65 99.91 10 50 2 2021-05-21 21:21:25 5.6 25.65 99.92 10 96 3 2021-05-21 21:21:57 4.2(ML) 25.63 99.96 10 50 4 2021-05-21 21:48:34 6.4 25.70 99.88 10 100 5 2021-05-21 21:55:28 5.0 25.67 99.89 9 62 6 2021-05-21 22:31:10 5.2 25.61 99.97 8 97 7 2021-05-21 23:23:34 4.5 25.59 99.98 9 72 8 2021-05-22 02:28:43 4.2(ML) 25.63 99.92 19 37 9 2021-05-22 20:14:36 4.7 25.60 99.92 10 63 9次地震中PGA和PGV的距离校正事件内残差随方位角的变化如图4,5所示:1,8,9号地震的PGA和PGV距离校正事件内残差对方位角有很强的依赖性,说明这三次地震可能存在明显的破裂方向性效应;4号地震的PGV距离校正事件内残差随方位角变化十分明显,但PGA距离校正事件内残差整体上随方位角无明显变化;3号和6号地震的距离校正事件内残差整体上在0附近,随方位角无明显变化。针对每次地震的PGA和PGV距离校正事件内残差,采用网格搜索方法确定式(2)中Cd的各震源破裂方向性参数,包括φ,vr/β及k,每次地震均进行了500次网格搜索,每次搜索中均采用了N个随机数来表示预测值的不确定性。表4给出了分别利用PGA和PGV距离校正事件内残差估计的破裂方向性参数,以$C_{\rm{d}}^{0.5} $的最大值与最小值的比值($\max C_{\rm{d}}^{0.5}/\min C_{\rm{d}}^{0.5} $)作为判断破裂方向性强弱的指标(Wang et al,2019)。1,8和9号地震破裂方向性效应明显,基于PGA和PGV的$\max C_{\rm{d}}^{0.5}/\min C_{\rm{d}}^{0.5} $均不小于2,基于PGA和PGV分别估计的破裂方向性参数也十分接近;对于4号地震(主震),基于PGA和PGV的$\max C_{\rm{d}}^{0.5}/\min C_{\rm{d}}^{0.5} $均不超过2,但基于PGV的破裂方向性参数的不确定性很小,明显低于基于PGA估计结果的不确定性,说明4号地震的破裂方向性效应较弱,且主要对PGV具有影响;其它几次地震的$\max C_{\rm{d}}^{0.5}/\min C_{\rm{d}}^{0.5} $值均不超过2,且基于PGA,PGV分别估计的破裂方向性参数的不确定性很大,说明这些地震无明显破裂方向性效应,此时采用方向性效应拟合来确定的破裂方向性参数的不确定性很大,不能明确给出这几次地震的震源破裂方向性参数。
表 4 基于PGA,PGV分别估计的地震破裂方向性参数Table 4. Rupture directivity parameters estimated based on PGA and PGV,respectively地震编号 峰值参数 φ/° vr/β k $ {\max C_{\rm{d}}^{0.5} }$ ${\min C_{\rm{d}}^{0.5} } $ ${{\max C_{\rm{d} }^{0.5} }/{\min C_{\rm{d} }^{0.5} } }$ 1 PGA 316.0±11.4 0.93±0.04 0.96±0.03 3.73 0.75 4.95 PGV 309.0±33.6 0.70±0.16 0.86±0.10 1.70 0.80 2.14 2 PGA 296.1±51.4 0.61±0.16 0.68±0.10 1.32 0.83 1.59 PGV 220.6±38.4 0.64±0.10 0.88±0.14 1.56 0.79 1.97 3 PGA 119.0±69.9 0.58±0.17 0.27±0.16 1.31 0.83 1.59 PGV 130.3±104.2 0.64±0.19 0.27±0.15 1.44 0.82 1.75 4 PGA 233.1±58.6 0.63±0.14 0.78±0.14 1.45 0.82 1.78 PGV 167.1±6.8 0.62±0.04 0.82±0.04 1.42 0.81 1.82 5 PGA 247.1±108.1 0.51±0.14 0.71±0.15 1.21 0.82 1.47 PGV 197.4±82.4 0.68±0.18 0.72±0.18 1.50 0.83 1.81 6 PGA 99.4±55.6 0.66±0.17 0.19±0.17 1.55 0.81 1.92 PGV 130.2±64.9 0.70±0.17 0.24±0.17 1.59 0.82 1.94 7 PGA 114.6±41.9 0.63±0.13 0.34±0.14 1.34 0.83 1.61 PGV 97.1±51.4 0.64±0.15 0.22±0.14 1.47 0.82 1.80 8 PGA 149.8±21.9 0.76±0.10 0.87±0.11 1.90 0.80 2.39 PGV 155.6±19.9 0.84±0.12 0.93±0.09 2.43 0.77 3.16 9 PGA 244.4±11.2 0.65±0.08 0.87±0.06 1.57 0.80 1.98 PGV 242.5±18.7 0.70±0.13 0.87±0.08 1.73 0.79 2.18 1,4,8和9号地震的破裂方式均为不对称双侧破裂,4号地震的优势破裂方向约为东南方向,与主要余震分布长轴方向(李大虎等,2021;龙锋等,2021;苏金波等,2021)、地震烈度图的长轴方向(卢永坤等,2021)较为一致,破裂起始点(震中)约位于整个破裂长度80%的位置,vr/β=0.62,根据CRUST1.0模型给出的震源处剪切波速3.55 km/s,震源破裂速度约为2.2 km/s。Gong等(2022)利用经验格林函数方法确定了主震具有东南方向(约144°)破裂传播的特征,破裂速度约为2.1—2.4 km/s;Chen等(2022)也证实了漾濞地震主震震源主要朝东南方向破裂传播,且破裂速度约为2.2 km/s。8号地震震源破裂主要向东南侧发展,东南侧破裂长度约占整个破裂的90%,vr/β约为0.80,破裂速度较大,约为2.8 km/s。1号地震的主破裂方向为西北向(约为310°),主破裂方向破裂长度约占整个破裂的90%,基于PGA和PGV估计的vr/β差距较大,分别为0.93和0.70,破裂速度可能不低于2.5 km/s。9号地震的破裂面走向近似垂直于1,4,8号地震,主破裂方向为西南向(约为240°),主破裂方向破裂长度约占整个破裂的87%,vr/β=0.65—0.70。前震(1号)和余震(8号、9号)的破裂速度均大于主震(4号)破裂速度。Zhou等(2022)根据平行断层方向和垂直断层方向观测台站的视震源谱拐角频率判断了漾濞MS6.4地震序列中3次前震的破裂方向性,包括1号和2号地震,1号地震优势破裂方向为西北方向,与本文结果一致,2号地震优势破裂方向为东南方向,而本文结果则认为2号地震无明显破裂方向性。雷兴林等(2021)也估计了漾濞地震序列中多次地震的破裂方向性,漾濞地震主震和部分强余震主要为北西向断层上的南东向破裂,而中强前震和多数余震主要为北东向断层上的双向破裂,本文判断的多个地震的破裂方向性也明显不同。
已有研究发现破裂方向性效应的周期相关性(Bernard et al,1996;Pacor et al,2016)。本文基于不同周期的PSA分别估计了9次地震的震源破裂方向性参数,并以$\max C_{\rm{d}}^{0.5}/\min C_{\rm{d}}^{0.5} $作为定量判断破裂方向性效应强弱的指标,如图6所示。2,3,5,6和7号地震几乎在所有周期上均未观察到明显的方向性效应,8号和9号地震在T<0.7 s时地震动的破裂方向性效应很明显,4号地震则近似对T>0.6 s地震动有较为明显的破裂方向性效应,1号地震的破裂方向性对T>0.6 s和T<0.3 s的地震动均有一定影响。同时我们也注意到,对于破裂方向性在较宽周期段上有影响的地震,基于不同周期估计的破裂方向的不确定性较小,不超过20° (图7),但对于无明显破裂方向性效应的地震,基于不同周期PSA通过拟合Cd估计的破裂方向的不确定性很大,这也说明Cd并不适合估计破裂方向性效应不明显的地震的破裂方向参数。
4. 结论
本文采用2021年云南漾濞地震序列的强震动观测记录,在建立该地震序列地震动参数预测方程的基础上,基于破裂方向性效应拟合方法估计了九次观测记录丰富且空间分布均匀的地震的震源破裂方向性特征参数,得到如下结论:
1) 尽管漾濞地震主震具有不均匀双向破裂特征,优势破裂方向为东南方向(φ=167.1°N,k=0.82),但其破裂方向性效应并非十分明显,且主要影响PGV,这可能与其破裂速度较低有关(约为2.2 km/s)。
2) 1号(前震)、8号(余震)、9号(余震)地震表现出显著的破裂方向性效应,这三次地震均为不均匀双侧破裂(k>0.85),但优势破裂方向不同,分别为西北、东南和西南,这也说明了本次地震序列地震破裂的复杂性,前震、主震、余震差异明显的优势破裂方向可能与漾濞地震序列复杂的触发过程有关,这三次地震的破裂速度均明显大于主震。
3) 四次地震(1,4,8,9号)的破裂方向性效应均有一定的周期相关性,两次余震(8号和9号)对<0.7 s地震动的破裂方向性效应明显,主震(4号)则近似对>0.6 s地震动有一定影响,前震(1号)则对<0.6 s和<0.3 s的地震动均有一定影响。
中国地震局工程力学研究所“国家强震动台网中心”为本文提供了强震动台站观测数据,云南省地震局为本文提供了地震预警和烈度速报台站的观测数据,作者在此一并表示感谢。
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图 6 长白山天池火山周边剩余布格重力异常图
(a)反演区剩余布格重力异常;(b)剖面CD自由空气异常本文与前人观测对比;(c)剖面CD剩余布格重力异常特征
Figure 6. Residual Bouguer gravity anomaly around Changbaishan Tianchi volcano
(a) Residual bouguer gravity anomaly map of inversion area;(b) Comparison of free air anomalies in profile CD between this article and previous observations;(c) Residual bouguer gravity anomaly characteristic of profile CD
图 5 长白山天池火山地壳浅层温度剖面与岩浆囊结构特征
(a) 南北向温度剖面(剖面AB位置见图4a,单位:℃);(b) 地壳浅层岩浆囊的电性结构 (汤吉等,2001;阮帅等,2020)(剖面SN位置见图4a,单位:Ω·m)
Figure 5. Shallow crustal temperature profile and magmatic chamber structure of Changbaishan Tianchi volcano
(a) Temperature profile of North-South direction (see Fig.4a for the location of profile AB,unit:℃);(b) The electrical structure of magma chamber in the shallow crust (Tang et al,2001;Ruan et al,2020) (see Fig.4a for the location of profile SN,unit:Ω·m)
图 8 地壳浅层密度剖面与电阻率剖面
(a) 浅层密度剖面 (单位:g/cm3);(b) 浅层剩余密度剖面 (单位:g/cm3);(c) 浅层电性结构 (汤吉等,2001)(剖面EF及SSW−NNE剖面位置见图7a)
Figure 8. The profile of shallow density and resistivity
(a) Density profile of shallow layer (unit:g/cm3);(b) Residual density profile of shallow layer (unit:g/cm3);(c) Electrical structure of shallow layer (Tang et al,2001);(See Fig.7a for the location of section EF and SSW−NNE section)
表 1 天池火山周边温泉地热地质条件
Table 1 Geothermal geological conditions of hot springs around Tianchi volcano
温泉群 出露地段 东经
/°北纬
/°泉口温度
/℃热储温度/℃ 热储深度
/km热水循环深度
/km范围 平均 湖外 聚龙温泉群 天池瀑布北侧第四纪
粗面岩中128°03′30″ 42°02′47″ 40—82 134.9—157.9 146.4 4.15—4.17 4.53—4.56 锦江温泉群 长白山西坡锦江峡谷
第四纪粗面岩中127°59′33″ 41°56′22″ 58—60 118.5—162.0 125.8 3.68—4.30 4.06—4.36 湖内 湖滨温泉群 第四纪粗面岩中 128°03′45″ 42°01′12″ 42—73 表 2 天池火山区地层热物性参数表
Table 2 Geothermal physical parameters of stratum around Tianchi volcano
地热井 东经/° 北纬/° 井深/m 井中水温/℃ 大地热流
/(mW·m−2)热导率
W/(m·K)地温梯度
/(℃·km−1)生热率
/(μW·m−3)CR1 127.6 41.9 2500 31—34 79.9 2.72 29.38 1.26 CR2 127.5 42.1 2300 30—31 70.9 2.80 25.33 1.26 -
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