NSFGEO-NERC Earthquake nucleation versus episodic slow slip: what controls the mode of fault slip?

地震是由断层上的快速滑动造成的，占一系列自然灾害死亡人数的大部分，全世界每年约有6万人死亡，其中约90%发生在发展中国家。断层上的滑动可能有三种方式。它们是(1)地震滑动；(2)由板块构造加载速率驱动的稳定断层蠕变；(3)幕式慢滑动事件，其中断层滑动自发加速，但从未达到地震滑动速度。间歇性的慢滑事件可以释放与地震相同的能量，但释放的时间是几天到几周，而不是几秒到几分钟。它们最常出现在俯冲带的某些区域，并与较高的孔隙压力有关。这三种断层滑动模式对理解断层滑动是至关重要的，因为幕式慢滑和断层蠕动缓解了应力积累并减少了地震危险，但也将应力从断层的一部分转移到另一部分，最终影响破坏性地震的成核。在这个项目中，我们将通过组合实验和数值模拟提供物理约束，以确定导致稳定的断层蠕动、幕式慢滑或地震的控制因素。到目前为止，还不清楚是什么阻止了一些造成缓慢断层滑动的不稳定性，但允许另一些人加速到引发地震的快速滑动速度。当滑动速度增加时，从不稳定的摩擦滑动(通常被视为导致地震)到稳定的摩擦滑动(通常被视为导致断层蠕变)的某种转变必须促进断层上的持续缓慢滑动。这种稳定过渡的性质引起了广泛的争论，它可能发生的条件范围也定义不清。我们将研究提出的解释这种稳定性转变和由此产生的慢滑事件的关键假设，其中包括(1)与高温下非常慢的滑动速率有关的摩擦性质的演变，(2)孔隙流体压力对稳定性转变的作用，其中断层中颗粒剪切物质的孔隙体积的微小增加导致孔隙压力的大幅下降，从而增加了剪切阻力(膨胀加强)，以及(3)断层性质和条件的空间变化，导致地震成核可以发生，但受到具有稳定摩擦性质的相邻区域的限制。这项工作将涉及综合实验室实验和数值模拟。受控实验室实验将测量与天然断层相关的以前未探索的温度、孔隙流体压力和滑动速率条件下断层摩擦力的演变。我们将量化摩擦性质的演变，从非常缓慢的构造断层滑动速率每年毫米，到那些通过插曲的缓慢滑动范围每天毫米，并到米每秒发生地震的滑动速率。滑动过程中由压实和膨胀所促进的流体压力变化也将被表征。实验室规模的实验的数值模拟将有助于确保在我们的数学描述中理解和捕捉所涉及的耦合物理机制。具有实验定义的性质的断层的大尺度行为将通过在天然断层尺度上的数值模拟来探索。数值模拟将把实验结果与幕式慢滑和地震成核的现场观测联系起来，并研究断层性质的空间变化对幕式慢滑事件与地震发生的作用。这项工作的一个关键成果将是确定断层条件和物理机制的范围，在这些断层条件和物理机制下，可能发生间歇性慢滑、断层蠕动或地震，最终导致改进地震危险预测。

Earthquakes, produced by rapid slip on faults, account for the majority of deaths from a range of natural disasters which amounts to about 60,000 people a year worldwide - around 90 percent of which occur in developing countries. Slip can occur in three ways on faults. These are (1) earthquake slip; (2) stable fault creep driven by plate tectonic loading rates; and (3) episodic slow slip events, where fault slip spontaneously accelerates but never reaches earthquake slip speeds. Episodic slow slip events can release the same amount of energy as earthquakes but over days to weeks rather than seconds to minutes. They most commonly occur in certain regions of subduction zones and have been linked to elevated pore pressures. These three modes of fault slip are vital to understand, as episodic slow slip and fault creep relieve stress build up and reduce seismic hazard, yet also transfer stress from one part of the fault to another, ultimately affecting the nucleation of destructive earthquakes.In this project, we will provide physical constraints from combined experiments and numerical modelling to determine the controlling factors leading to stable fault creep, episodic slow slip, or earthquakes. As yet, it is not understood what puts the brakes on some instabilities creating slow fault slip yet allows others to accelerate to rapid slip speeds that cause earthquakes. A transition of some sort from unstable frictional sliding (typically viewed as leading to earthquakes) to stable frictional sliding (typically viewed as leading to fault creep) while the sliding velocity is increasing must promote sustained slow slip on faults. The nature of this stability transition is widely debated and the range of conditions under which it may occur are ill defined. We will investigate the key hypotheses proposed to explain such stability transition and the resulting slow slip events, which include (1) evolution in friction properties related to very slow slip rates at elevated temperatures, (2) the role of pore fluid pressure on stability transitions, where small increases in pore volume of the granular shearing material in the fault produces a large decrease in pore pressure resulting in increase in the shear resistance (dilatant strengthening), and (3) spatial variation in fault properties and conditions leading to a situation where nucleation of an earthquake can occur but is limited by adjacent regions with stable frictional properties. The work will involve integrated laboratory experiments and numerical modelling. Controlled lab experiments will measure the evolution of fault friction under previously unexplored temperature, pore fluid pressure, and slip rate conditions relevant to natural faults. We will quantify the evolution of frictional properties from very slow, tectonic fault slip rates of millimetres per year, to those through the episodic slow slip range of millimetres per day, and into the slip rates of meters per second where earthquakes occur. Fluid pressure changes promoted by compaction and dilation during slip will also be characterized. Numerical modelling of the experiments at the laboratory scale will help to ensure that the coupled physical mechanisms involved are understood and captured in our mathematical descriptions. The large-scale behaviour of faults with the properties defined by the experiments will be explored by numerical modelling at the scale of natural faults. The numerical modelling will relate the experimental findings to field observations of episodic slow slip and earthquake nucleation and investigate the role of spatial variations in fault properties on the occurrence of episodic slow slip events vs. earthquakes. A key deliverable for this work would be identification of the range of fault conditions and physical mechanisms under which episodic slow slip, fault creep, or earthquakes can occur, leading ultimately to improved seismic hazard forecasting.