Fluidization and melting of fault gouge during seismic slip: Identification in the Nojima fault zone and implications for focal earthquake mechanisms
Fluidization and melting of fault gouge during seismic slip: Identification in the Nojima fault zone and implications for focal earthquake mechanisms
复制标题
DOI：
10.1029/2001jb001711
复制
发表时间：
2003-04-10
影响因子：
3.9
通讯作者：
Nagase, T
中科院分区：
文献类型：
--
作者：
Otsuki, K;Monzawa, N;Nagase, T
[1] 地震滑动事件期间发生的物理过程的记录在日本野岛活动断层的断层岩石中得到了很好的保存。断层岩形成于约 3 公里深度，由源自花岗岩的极细碎岩和假速晶岩的薄交替层组成。每层厚度都小于几毫米，对应于一次地震滑动事件。非常薄的滑移带宽度表明存在某些滑移减弱机制，我们对断层岩的研究表明泥岩的流化和熔融尤其重要。使用碎片对应方法的检测概率来区分流化和非流化凿孔。从颗粒材料科学可知，颗粒材料从颗粒摩擦状态到流化的相变只能通过固体颗粒体积分数的非常小的减少来发生。一旦发生流化，即使在热加压达到其极限状态之前，摩擦阻力也会突然降低至接近于零。当断层泥熔化时，摩擦阻力由熔化物的粘度决定，熔化物的粘度主要取决于温度、化学成分、H2O浓度和未熔化颗粒的体积分数。对于每个假速电解质层，我们使用各种温度指数估计熔化温度，并测量未熔化颗粒的体积分数。我们综合这些数据来重建地震滑动事件期间粘度的变化。由于低温（750℃～800℃）和固体颗粒体积分数大的综合作用，初始熔融时熔体粘度非常高（10（7-9）Pas）。因此，地震滑动不可避免地被瞬间抑制。一旦克服了这种机械障碍，粘度就会随着滑移的增加而持续显着降低。在1000℃时，粘度降低至10(4Pas)，并最终在1280℃时降低至10Pas。因此，应力几乎完全下降，破裂往往会消失。
[1] The record of physical processes that occur during seismic slip events is well preserved in fault rocks from the active Nojima fault in Japan. The fault rocks formed at about 3 km depth, and comprise thin alternating layers of very fine gouge and pseudotachylyte derived from granite. Each layer is thinner than a few millimeters, and corresponds to one seismic slip event. The very thin slip zone width suggests that some mechanisms of slip weakening operated, and our studies of the fault rocks suggest that fluidization and melting of gouge were particularly important. Fluidized and nonfluidized gouges were distinguished using the detection probability of fragmented counterparts method. It is known from granular material science that the phase transition from a grain friction regime to fluidization of granular materials can occur only by a very small decrease in volume fraction of solid grains. Once fluidization occurs, the frictional resistance decreases abruptly to nearly zero even before thermal pressurization reaches its extreme state. When fault gouge is melted, frictional resistance is governed by the viscosity of melt which depends mainly on temperature, chemical composition, H2O concentration and the volume fraction of unmelted grains. For each pseudotachylyte layer, we estimated the temperature of melt using various temperature indices, and measured volume fraction of unmelted grains. We synthesized these data to reconstruct the change in viscosity during seismic slip events. The melt viscosity is very high (10(7-9) Pas) during initial melting due to the combined effect of low temperature (750degrees-800degreesC) and large volume fraction of solid grains. Thus seismic slip is inevitably restrained instantaneously. Once this mechanical barrier is overcome, the viscosity reduces continuously and dramatically as slip increases. At 1000degreesC, the viscosity reduces to 10(4Pas) and eventually to 10Pas at 1280degreesC. Thus stress drops almost completely and rupturing tends to run away.