Rheological structure of lithosphere in northern Tibet inferred from postseismic deformation modeling of the 2001 MW7.8 Kokoxili earthquake
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摘要:
青藏高原岩石圈的流变学结构和形变机制是地学界长期争论的重大科学问题.2001年发生在东昆仑断裂带的MW7.8可可西里地震造成青藏高原北部地区岩石圈构造应力场的很大改变，引起下地壳与上地幔的快速弛豫形变，从而为研究这一问题提供了难得的机会.本研究采用该区域的GPS震后观测，反演这一地区岩石圈的流变学参数并探讨其形变机制.反演所采用的数据来自45个GPS观测点，其中包括一个中国地壳运动观测网络的基准站，数据最长时间跨度达6.4年.大地震震后形变场主要来源于地壳、上地幔的黏弹性松弛与断层面上的震后余滑，因此本研究同时反演介质的黏滞系数和断层的震后余滑.考虑到东昆仑断层南侧的巴颜喀拉-羌塘地区与北侧的柴达木盆地地区具有明显不同的地壳结构，断层南北两侧采用不同的Burgers体流变学结构，其下地壳-上地幔的短期和长期黏滞系数采用网格搜索法获得；断层震后余滑反演则同时施加近似正比于库仑应力的约束.最终结果显示：东昆仑断层北侧柴达木盆地地区下地壳-上地幔短期和长期黏滞系数分别为5×1018 Pa·s和1.5×1020 Pa·s；东昆仑断层南侧巴颜喀拉-羌塘地区下地壳-上地幔短期和长期黏滞系数分别为1.5×1018 Pa·s和1.5×1019 Pa·s.这一结果表明：巴颜喀拉-羌塘地区下地壳-上地幔黏滞系数显著低于柴达木盆地，意味着巴颜喀拉-羌塘地区下地壳可能存在部分熔融，其地壳形变模式更趋近于连续形变，而柴达木盆地形变模式更趋近于块体运动.研究区下地壳长期黏滞系数比下地壳流模型所主张的黏滞系数高2~3个数量级，表明下地壳流在本地区可能不存在.
Abstract:The rheological structure of lithosphere in northern Tibet has been debated for decades. The 2001 MW7.8 Kokoxili earthquake greatly changed the tectonic stress field in this area, providing an opportunity to address this issue by modeling the postseismic deformation at the Earth's surface. We collect GPS data observed after the quake, and use it to invert lithosphere rheological parameters and infer deformation mechanism in northern Tibet. GPS data from 45 sites, including one reference station from the Crustal Motion Observation Network of China, are processed to produce the time series. Data acquired after the 2001 Kokoxili earthquake and prior to the 2008 Yutian earthquake are used in the study, and most of the GPS sites are located in the near to mid field, and with at most 6.4 years of observation history. We perform a joint inversion to solve for the viscous relaxation in lithosphere and afterslip on the fault simultaneously. The Bayan Har-Qiangtang region and Qaidam Basin, located south and north of the east Kunlun fault, respectively, are assumed to have different rheological structures in the lithosphere, and viscosities of lower crust and upper mantle on each side are assumed to be the same and the values are inverted through grid search. Afterslip is constrained by both observations and the Coulomb stress distribution on the fault. Our results show the secular viscosities of 1.5×1019 Pa·s and 1.5×1020 Pa·s of lower crust/upper mantle for south and north of the fault respectively. The transient viscosities of 1.5×1018 Pa·s and 5×1018 Pa·s are also found respectively. These results reveal that viscosity of lower crust/upper mantle beneath the Bayan Har-Qiangtang region is much lower than that below the Qaidam Basin, implying possible partial melt in the lower crust of the Bayan Har-Qiangtang region. The deformation pattern in the Bayan Har-Qiangtang region agrees well with the distributed deformation model, while that of the Qaidam basin is more consistent with the block deformation model. Viscosity of lower crust in northern Tibet is 2~3 orders of magnitude higher than what the lower crustal flow model requires, suggesting that lower crustal flow likely does not exist in this region.
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Key words:
- Kokoxili earthquake /
- GPS data /
- Postseismic deformation /
- Rheologic structure /
- Lower crustal flow
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图 7
GPS观测点位分布与震后1年地表位移拟合
Figure 7.
Distribution of GPS sites and data fitting of first year observed and predicted postseismic displacements
图 1
青藏高原北部区域构造、地震活动和GPS震间水平运动速度场
Figure 1.
Regional tectonics, earthquakes and GPS derived interseismic horizontal velocity field in northern Tibetan Plateau
图 2
可可西里地震的同震破裂模型(万永革等，2008)
Figure 2.
Coseismic rupture model of the Kokoxili earthquake (Wan et al., 2008)
图 4
(a) 同震库仑应力变化；(b)同震库仑应力变化与震间累积应力之和
Figure 4.
Coseismic Coulomb stress change (a) and sum of Coulomb stress change and interseismic accumulated stress (b)
图 5
数据残差随黏滞系数变化的分布图
Figure 5.
Data residual chi-square distribution with respect to viscosity parameter realizations
图 8
GPS观测点震后位移时间序列(相对于第一个观测时刻)
Figure 8.
Postseismic displacement time series of GPS sites(relative to the first observation time)
表 1
基于可可西里地震震后形变的区域流变学参数估计
Table 1.
Viscosity estimates of northern Tibet based on postseismic deformation of the Kokoxili earthquake
模型 震后机制 黏滞系数(Pa·s) 备注(模型设置与数据) 断层北侧 断层南侧 Zhang et al., 2007 黏弹性松弛 η1=2×1017 黏弹性层20 km以下；4个GPS连续站震后1年数据 Diao et al., 2011 黏弹性松弛 η1=(5~6)×1017 黏弹性层32 km以下；沿青藏公路GPS流动站震后4个月数据 震后余滑+黏弹性松弛 η1=1×1018 Ryder et al., 2011 黏弹性松弛 η2= 9×1017
η1=1×1019黏弹性层16 km以下；沿青藏公路GPS流动站震后1年数据和震后2~5年InSAR数据 Wen et al., 2012 黏弹性松弛 η1=(2~5)×1019 黏弹性层深度15 km以下；震后2~6年InSAR数据 本研究模型A 震后余滑+黏弹性松弛 η2=5×1018
η1=1.5×1020η2=1.5×1018
η1=1.5×1019忽略介质分界面，断层北侧的介质分层和弹性参数都与南侧一致，黏弹性层20 km以下 本研究模型B 震后余滑+黏弹性松弛 η2=1×1018
η1=4×1019同上 本研究模型C 震后余滑+黏弹性松弛 η2= 5×1018
η1=1.5×1020η2=1.5×1018
η1=1.5×1019同时顾及断层两侧弹性介质的横向差异，但将断层视为介质分界，黏弹性层20 km以下 
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