A (mostly) Observational Study of Microearthquakes on a Bimaterial Interface

三十年来，人们一直预计，地震破裂沿着具有不同弹性特性的材料的界面将有一个有利的传播方向，即更柔顺的材料的运动方向。然而，在自然断层上观察这种趋势是很困难的，这在很大程度上是因为在速度对比明显的断层上发生的重大地震很少。PI利用频谱比直接估计了沿着圣安德烈斯断层30公里部分的3000多个小地震的指向性，这些地震具有较大且特征清晰的速度对比。用简单的移动点源模型拟合频谱比，其中每个模拟地震有四个参数：两个断裂长度（一个向东南和一个向西北）和它们的传播速度。近900次地震，大多是大于70米的地震，似乎得到了相当好的分辨。反演结果表明，尽管144个事件中有80%以上被归类为东南方向的强烈单侧破裂，但40%的良好解析事件大致是双侧的，这与理论预测一致。对于那些大到足以使传播速度在某种程度上得到解决的破裂一半，传播到东南方向的一半的速度大约大10%，定性地与数值和实验室实验一致。他们发现，在附近（在空间和时间上）前震的事件倾向于远离这些前震破裂，无论是向西北方向还是向东南方向，这表明先前应力历史的不对称性比材料对比对破裂方向性产生更大的影响。提出的工作的一个主要目标是在空间和时间上大大增加他们的数据库的大小。这将使他们能够探索前震位置和主震指向性之间的相关性如何随着前震和主震之间的空间和时间距离的增加而衰减，我们所观察到的主震指向性和余震不对称性之间明显缺乏相关性是否符合更大的数据样本，以及这些行为如何与局部断层间速度对比相关联。研究人员还将系统地寻找复合地震中子事件位置的不对称性，并开始在双材料界面上进行地震成核的数值模拟，其具体目标是了解前面提到的前震对主震指向性影响的衰减，以及双材料对比对地震成核的一般影响。能够产生破坏性地震的大断层也会滑动很远的距离，因此它们经常把具有不同力学特性的岩石放在一起。长期以来，人们一直预测，在这样的断层上发生的地震可能有一个优先的传播方向。确定这种情况是否属实与减轻危险有关，因为在破裂传播的方向上有更强的地面震动；甚至有人建议修改建筑规范以反映这一点。这使得我们正在研究的地震的重要性超过了它们的小规模。它们的有用之处在于它们的数量很大，因此结果在统计上是有意义的，并且它们能够教会我们如何将地震破裂的数值模型与实际地震联系起来。

For three decades it has been anticipated that earthquake ruptures along an interface separatingmaterials with different elastic properties will have a favored propagation direction, that being thedirection of motion of the more compliant material. However, observing this tendency on naturalfaults has been difficult, in large part because of the small number of significant earthquakes onfaults with a well-characterized velocity contrast. The PI has used spectral ratios to directly estimatedirectivity in a catalog of over 3,000 small earthquakes along a 30-km section of the San Andreasfault with a large and well-characterized velocity contrast. The spectral ratios were fitted with asimple moving point source model in which each modeled earthquake has four parameters: tworupture lengths (one to the SE and one to the NW) and their propagation velocities. Nearly 900earthquakes, mostly those larger than 70 m, appear reasonably well resolved. The inversion resultssuggest that 40% of the well-resolved events are roughly bilateral, although more than 80%of the 144 events classified as strongly unilateral rupture to the SE, consistent with the theoreticalprediction. For those rupture halves that were large enough for the propagation speed to besomewhat resolved, that speed was greater by roughly 10% for those halves propagating to the SE,qualitatively consistent with numerical and laboratory experiments. They find that events withnearby (in space and time) foreshocks tend to rupture away from those foreshocks, whether to theNW or to the SE, indicating that asymmetry of prior stressing history can exert a stronger influenceon rupture directivity than the material contrast.A major goal of the proposed work is to greatly increase the size of their database in bothspace and time. This will allow them to explore how the correlation between foreshocklocation and mainshock directivity decays with increasing spatial and temporal distance betweenforeshock and mainshock, whether the apparent lack of correlation between mainshock directivityand aftershock asymmetry we have observed stands up to a larger data sample, and how thesebehaviors correlate with the local across-fault velocity contrast. The investigators will also undertake a systematic search for asymmetry in the location of sub-events in compound earthquakes, and beginnumerical modeling of earthquake nucleation on a bimaterial interface, with the specific goals ofunderstanding the aforementioned decay of the influence of foreshocks on mainshock directivity,and the influence of the bimaterial contrast on earthquake nucleation generally.Large faults that are capable of producing damaging earthquakes have also slipped large distances,and so they often juxtaposes rocks with different mechanical properties. It has long beenpredicted that earthquakes on such faults could have a preferred propagation direction. Establishingwhether this is actually the case is relevant to hazards mitigation because there is much strongerground shaking in the direction that the rupture propagates; it has even been proposed that buildingcodes could be altered to reflect this. This gives the earthquakes we are examining an importancewhich surpasses their small size. Their usefulness lies in their large number, so that the resultsare statistically meaningful, and in their ability to teach us about connecting numerical models ofearthquake rupture to real earthquakes generally.