Geodynamical simulation of the effects of ridge subduction on the scale of the seismogenic zone south of Chile Triple Junction
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摘要: 智利三联点以北地震较多，智利三联点以南地震很少且热异常显著。为探究洋脊俯冲对由温度定义的孕震区的影响，以智利三联点区域的地质背景为基础建立二维有限元数值模型，对洋脊俯冲过程进行数值模拟，并将俯冲角度和汇聚速率等因素对孕震区的影响进行了对比。结果表明，洋脊俯冲过程中孕震区宽度减小，导致智利三联点以南的地震远少于智利三联点以北。剖面附近的观测数据与数值模拟结果的对比表明，数值模拟可以大致反映智利三联点区域板块间的孕震区宽度和地表热流特征。当俯冲汇聚量相同时，板块间的汇聚速率越大，洋脊俯冲过程中孕震区则越宽且其下边界越深，海沟附近的地表热流越高。与汇聚速率相比，俯冲角度等因素对地表热流的影响较小。俯冲角度越大，洋脊俯冲过程中孕震区越窄。当数值模型包含剪切生热时，洋脊俯冲过程中孕震区宽度大约可减小至15 km，且孕震区很浅。这可导致智利三联点以南部分区域难以发生地震，出现观测不到和达-贝尼奥夫带的现象。Abstract: Earthquakes are much more frequent to the north of Chile Triple Junction than to the south, where the thermal anomaly is also more significant. To study the effects of ridge subduction on the thermally defined seismogenic zone, two-dimensional finite element models were established based on the geology of Chile Triple Junction, the process of ridge subduction was simulated, and the effects of the initial slab dip and the convergence rate on the seismogenic zone were compared. The results show that the width of the seismogenic zone decreases during the ridge subduction, inducing earthquakes to occur much less to the south than to the north of Chile Triple Junction. By comparing the observed data in the vicinity of the profiles with the numerical simulation results, we find that the numerical simulation can roughly reflect the width of the interplate seismogenic zone and the surface heat flow in the area of Chile Triple Junction. At the same convergence distance, a larger convergence rate comes with a wider seismogenic zone and the deeper the downdip limit of the seismogenic zone, the higher the surface heat flow in the vicinity of the trench. Compared with the convergence rate, factors such as slab dip have little effect on the surface heat flow. In the process of ridge subduction, larger the slab dips leads to narrower seismogenic zones. When the effect of shear heating is included in the simulation, the width of the seismogenic zone in the process of ridge subduction can shrink to about 15 km and the depth of the seismogenic zone is small. Such a narrow and shallow seismogenic zone makes it hard for earthquakes to occur and for the Wadati-Benioff plane to be observed in some areas south of Chile Triple Junction.
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Key words:
- Chile Triple Junction /
- ridge subduction /
- seismogenic zone /
- numerical simulation
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图 1 智利三联点区域构造背景
（a）智利三联点区域1906—2021年地震分布，数据来自ISC （2021）（Bondár，Storchak，2011；Storchak et al，2017；2020）；（b）智利三联点区域热流数据分布（Lucazeau，2019）
Figure 1. Geotectonic background of the Chile Triple Junction
（a） Earthquake distribution in the area of CTJ from 1906 to 2021. The seismicity data are from the ISC （2021）（Bondár，Storchak，2011；Storchak et al，2017；2020）；（b） The heat flow data in the area of CTJ （Lucazeau，2019）
图 2 参考模型初始设置及边界条件
这些参数由干橄榄岩（Hirth，Kohlstedt，2013）的位错蠕变和扩散蠕变的流变实验获得
Figure 2. Initial setups and boundary conditions of the reference model
All these parameters come from the rheological experiments on dislocation creep and diffusion creep of dry olivine （Hirth，Kohlstedt，2013）
图 3 参考模型成分场（a）和黏滞系数η场的演化（b）
图中橙色实线为等温线，最低为100 ℃，最高为1 250 ℃
Figure 3. Evolutions of the material field （a） and the viscosity field η （b） of the reference model
Orange solid lines are isotherms，with a minimum of 100 ℃ and a maximum of 1 250 ℃
图 4 不同汇聚速率模型在俯冲汇聚量相同时的成分场（a）和黏滞系数场η（b）
Figure 4. The material field （a） and the viscosity field η （b） of different convergence rate models when the convergence distances are the same
图 5 不同初始俯冲角度模型在t＝19.5 Ma时的成分场（a）和黏滞系数场η（b）
Figure 5. The material field （a） and the viscosity field η （b） of models of different initial slab dips at t＝19.5 Ma
图 6 对比模型孕震区宽度的演化
Figure 6. Evolution of the width of the seismogenic zone of contrast models
图 7 海沟附近地表热流分布
（a）剖面1各模型热流结果；（b）剖面1不同初始俯冲角度模型热流结果（汇聚速率＝4.2 cm/a）；（c）剖面2不同初始俯冲角度模型热流结果（汇聚速率＝1.8 cm/a）
Figure 7. Surface heat flow in the vicinity of the trench
（a） Heat flow result of different models of profile 1；（b） heat flow result of models of different initial slab dips of profile 1 （convergence rate＝4.2 cm/a）；（c） heat flow result of models of different initial slab dips of profile 2 （convergence rate＝1.8 cm/a）
图 8 剖面1（左）和剖面2（右）的P波速度扰动（Simmons et al，2010）
Figure 8. Vertical cross sections of P wave velocity perturbations （Simmons et al，2010） along the profile 1 （left） and the profile 2 （right）
图 9 剖面1 （a）和剖面2 （b）的地震分布及孕震区
白色圆点为地震活动性数据。图a汇聚速率为4.2 cm/a；图b汇聚速率为1.8 cm/a
Figure 9. Seismogenic zones and earthquake distributions of the profile 1 （a） and the profile 2 （b）
Seismicity data are indicated by the white dots. The convergence rate of fig. a is 4.2 cm/a and 4.2 cm/a for fig. b
表 1 数值模型的相关参数
Table 1. Relevant parameters of numerical model
物质
名称厚度
/km热扩散率κ
/（m2·s−1）比热容Cp
/（J·kg−1·
K−1）密度ρ 热膨胀
系数α/K−1内摩擦角
φ/°黏聚力
C/Pa生热率
A/（W·m−3）流变
准则a晶粒大小
指数m前因子b/ Pa−n·m−p·s−1
diff/disl应力指数n 活化能E 活化体积V diff disl diff disl diff disl diff disl 地幔 to 660 9.89×10−7 750 3 300 2×10−5 20 20×106 — 干橄榄岩 3 2.37×10−15 6.52×10−16 1 3.5 375×103 530×103 4×10−6 13×10−6 大陆上
地壳20 1.21×10−6 750 2 800 2×10−5 20 20×106 10−6 湿石英 1 1×10−50 8.57×10−28 1 4.0 0 223×103 0 0 大陆下
地壳20 1.15×10−6 750 2 900 2×10−5 20 20×106 4×10−7 湿钙长石 1 1×10−50 7.13×10−18 1 3 0 345×103 0 0 岩石层
地幔0—87 9.87×10−7 750 3 300 2×10−5 20 20×106 — 干橄榄岩 3 2.37×10−15 6.52×10−16 1 3.5 375×103 530×103 4×10−6 18×10−6 沉积层 4 1.21×10−6 750 3 000 2×10−5 5 10×106 — 辉长岩 1 1×10−50 1.12×10−10 1 3.4 0 497×103 0 0 大洋
地壳8 1.15×10−6 750 3 100 2×10−5 20 10×106 — 辉长岩 1 1×10−50 1.12×10−10 1 3.4 0 497×103 0 0 薄弱带 1.21×10−6 750 3 300 2×10−5 0.03 1×106 — 辉长岩 1 1×10−50 1.12×10−10 1 3.4 0 497×103 0 0 注：a流变准则：干橄榄岩来自Hirth和Kohlstedt （2013）；湿石英来自Gleason和Tullis （1995）；湿钙长石来自Rybacki （2006）；辉长岩来自Wilks和Carter （1990）。b黏滞系数的前因子来自Ranalli （1995）的平面应变单轴试验结果（Naliboff，Buiter，2015）（刘梦雪等，2019）. 
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表 2 参考模型孕震区宽度演化
Table 2. Evolution of the width of the seismogenic zone of the reference model
时间/Ma 闭锁区宽度/km
x（350℃）－x（100℃）过渡带宽度/km
x（450℃）－x（350℃）孕震区宽度/km
x（350℃）－x（100℃） ＋0.5×（x（450℃） －x（350℃））洋脊与海沟横
坐标差/km0 86 36 104 −164 3 70 31 86 −60 6 89 34 106 −13 10 68 44 90 32 15 52 39 72 78 19.5 49 34 66 112
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表 3 不同初始俯冲角度模型孕震区宽度演化
Table 3. Evolution of the width of the seismogenic zone of models of different initial slab dips
时间/Ma 孕震区宽度/km 洋脊与海沟横坐标差/km 15° 30° 45° 15° 30° 45° 0 111 104 66 −145 −164 −158 3 96 86 76 −67 −60 −36 6 85 106 74 −33 −13 8 10 94 90 60 14 32 36 15 90 72 51 46 78 67 19.5 80 66 52 77 112 92
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表 4 部分模型孕震区深度演化
Table 4. Evolution of the depth of the seismogenic zone of some models
时间/Ma 参考模型 汇聚速率＝6.6 cm/a 俯冲角度＝45° 剪切生热 孕震区上边界
深度/km孕震区下边界
深度/km孕震区上边界
深度/km孕震区下边界
深度/km孕震区上边界
深度/km孕震区下边界
深度/km孕震区上边界
深度/km孕震区下边界
深度/km0 7 47 7 47 8 39 8 43 3 23 70 23 70 12 74 24 67 6 14 74 18 94 7 71 11 73 10 17 71 17 91 10 60 11 66 15 17 69 − − 5 51 9 51 19.5 19 70 − − 1 47 19 26
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