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

双材料界面上微地震的（主要）观测研究

• 批准号: 1113579
• 负责人: Allan Rubin
• 金额: $ 18.85万
• 依托单位: Princeton University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2011
• 资助国家: 美国
• 起止时间: 2011-06-01 至 2014-05-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1113579&HistoricalAwards=false
• 关键词: mostly; Observational; Study; Microearthquakes; Bimaterial

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.

在三十年的时间里，人们已经预料到，沿界面沿着不同弹性特性的分离材料的界面破裂将有一个有利的传播方向，这是更兼容的材料运动的特征。但是，观察天然之道的这种趋势非常困难，在很大程度上是因为少量的大地震发生了，并且具有良好的速度对比度。 PI使用光谱比直接估计了在San Andreasfault的30公里部分的3,000多个小地震中，并具有较大且特征良好的速度对比度。光谱比与Asimple移动点源模型拟合，其中每个建模的地震具有四个参数：二爆长度（一个到SE，一个到NW）及其传播速度。将近900个Quakakes（大部分是大于70 m的）似乎已经解决了相当好的解决方案。反转结果是，有40％的良好事件大致是双侧的，尽管在144个事件中，超过80％的事件被归类为SE的强烈单侧破裂，这与理论预测一致。对于那些足够大的破裂半部分，可以使繁殖的速度生下解决方案，对于传播到SE的那一半而言，这种速度大约要大约10％，这与数值和实验室实验一致。他们发现（在时空和时间上）的前提事件往往会脱离那些预内壳，无论是在当时还是向SE脱离，这表明先前强调的历史的不对称性可以比材料对比更强大的影响力对比。所提议的工作的主要目标大大增加了它们在两个跨度和时间的数据库大小。 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.调查人员还将在复合地震中的子事件的位置进行系统的搜索，并在双重界面上进行地震成核的开始数字建模，并具有对前预测对骨骼对比的影响的特定目标，这些目标是对上述对比的影响，并构成了对骨架的影响，以及对骨质质量对比的影响。产生破坏性的地震也已滑倒，因此它们经常与不同的机械性能并列岩石。长期以来，人们一直认为，这种断层上的地震可能具有首选的繁殖方向。建立这种情况实际上与缓解危害有关，因为破裂传播的方向有更大的震动。甚至有人提出可以更改建筑物以反映这一点。这给我们的地震带来了我们正在研究的重要性，超过了它们的尺寸。它们的实用性在于他们的数量，因此结果具有统计意义，并且有能力教给我们有关将数值模型连接到一般地震的数值模型。