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

三十年来，人们一直认为地震破裂沿着不同弹性材料的分界面会有一个有利的传播方向，即更柔顺的材料的运动方向。然而，在天然断层上观察这种趋势是困难的，这在很大程度上是因为在具有良好特征的速度对比的断层上发生的重大地震数量很少。PI使用频谱比直接估计了圣安德烈斯断层30公里长的一段沿着3,000多个小地震的目录中的方向性，这些地震具有很大的且特征良好的速度对比。谱比用简单的移动点源模型拟合，其中每个模拟地震有四个参数：结构长度（一个向东南方向，一个向西北方向）和它们的传播速度。近900次地震，大多数是大于70米的地震，似乎得到了相当好的解决。反演结果表明，40%的良好解决的事件大致是双边的，虽然超过80%的144个事件被归类为强烈的单侧破裂到SE，与理论预测一致。对于那些破裂的一半，是足够大的传播速度besomewhat解决，该速度是由大约10%的那些一半传播到SE，定性与数值和实验室实验一致。他们发现，无论是向西北方向还是向东南方向，在前震附近（在空间和时间上）发生的地震倾向于在远离前震的地方破裂，这表明先前应力历史的不对称性比材料对比度对破裂方向性的影响更大。这将使他们能够探索前震位置和主震方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，我们观察到的主震方向性和余震不对称性之间明显缺乏相关性是否经得起更大的数据样本，以及这些行为如何与当地跨断层速度对比相关。研究人员还将对复合地震中子事件位置的不对称性进行系统的研究，并对双材料界面上的地震成核进行数值模拟，具体目标是了解前震对主震方向性影响的衰减，以及双物质对比对地震成核的影响。能够产生破坏性地震的大断层，也会滑动很长的距离，所以它们经常将具有不同机械性能的岩石并置在一起。长期以来，人们一直预测，这种断层上的地震可能有一个优选的传播方向。确认这是否真的与减灾有关，因为在破裂传播的方向上有更强的地面震动;甚至有人建议可以修改建筑规范以反映这一点。这使我们正在研究的地震具有超越其小规模的重要性。它们的有用之处在于数量庞大，因此结果在统计上是有意义的，而且它们能够教会我们如何将地震破裂的数值模型与一般的真实的地震联系起来。

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.