Stress switching in subduction forearcs: Implications for overpressure containment and strength cycling on megathrusts
Stress switching in subduction forearcs: Implications for overpressure containment and strength cycling on megathrusts
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DOI：
10.1016/j.tecto.2013.02.035
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发表时间：
2013-07
期刊：
影响因子：
2.9
通讯作者：
R. Sibson
中科院分区：
文献类型：
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作者：
R. Sibson
俯冲界面剪切带（SISZ）内的孕震巨型逆冲构造在弧前垂壁下方普遍超压至接近静岩值（λv>0.9）。细粒材料内部的溶液沿较深层界面（150<T<350℃）转移有助于热液密封裂缝，降低整体渗透率。超压的下倾变化可能影响摩擦剪切阻力峰值的深度，这可能是影响特大逆冲破裂的主要因素。为了解释1995年智利Antofagasta Mw8.0大逆冲破裂后弧前上盘速度结构的震后变化，Husen和Kissling（2001）提出了大量跨大逆冲流体在界面上的释放。这种排放是“断层阀”作用的一种形式，在这种作用下，巨型逆冲断层本身就起到了密封作用，阻止了来自SISZ内部和下行板块脱水的超压流体。脆性破坏或断层再激活限制了流体超压，在低压差应力下，流体超压最高。2011年Mw9.0 tohokuo - oki特大逆冲断裂的大部分前上盘，震源机制显示应力状态从断裂前的挤压逆滑断裂转变为断裂后的伸展正滑断裂。平均应力和断层正应力由破坏前的大于垂直应力变为破坏后的小于垂直应力。在不断变化的应力场中，孔隙弹性效应和断层裂缝网络的流体损失共同提高了破坏后的渗透率，从而有望降低超压。通过俯冲界面的局部排水显著增加了摩擦强度，导致了强度凹凸度的破坏后分布。这种流体排放的强度变化幅度可能很大（<数百兆帕）。然后，流体超压的重新聚集以及沿俯冲界面的剪切应力影响到下一次破坏的时间。
Seismogenic megathrusts contained within subduction interface shear zones (SISZ) appear generally to be overpressured to near-lithostatic values (λv>0.9) below forearc hanging-walls. Solution transfer within fine-grained material along the deeper interface (150<T<350°C) contributes to hydrothermal sealing of fractures lowering bulk permeability. Down-dip variations in overpressuring likely affect the depth of the peak in frictional shear resistance which may serve as the prime asperity affecting megathrust rupture. To account for postseismic changes in the velocity structure of the fore-arc hanging-wall following the 1995 Antofagasta, Chile, Mw8.0 megathrust rupture, Husen and Kissling (2001) proposed massive trans-megathrust discharge of fluids across the interface. Such discharges are a form of ‘fault-valve’ action where the megathrust itself acts as a seal to overpressured fluids derived from within the SISZ and from dehydration of the descending slab. Brittle failure or fault reactivation limits fluid overpressure which is highest at low differential stress under a compressional stress regime. Over much of the forearc hanging-wall of the 2011 Mw9.0 Tohoku-Oki megathrust rupture, focal mechanisms show that the stress-state switched from compressional reverse-slip faulting prefailure to extensional normal-slip faulting postfailure. Mean stress and fault-normal stress thus changed from being greater than vertical stress prefailure, to less than vertical stress postfailure. Reductions in overpressure are expected from a combination of poroelastic effects and fluid loss through fault–fracture networks enhancing postfailure permeability in the changing stress field. Local drainage across the subduction interface increases frictional strength significantly, giving rise to a postfailure distribution of strength asperities. The amplitude of strength variations from such fluid discharge is potentially large (<hundreds of MPa). Time to the next failure is then affected by reaccumulation of fluid overpressure as well as shear stress along the subduction interface.