EarthScope RAPID: Geodetic and seismological response to the Mineral, VA Earthquake, 23 August, 2011

这项研究的总体目标是用地震仪和全球定位系统（GPS）接收器分别记录一年的余震和微妙的地面运动，对弗吉尼亚中部地震带（CVSZ）进行测量。这些测量结果使地球科学家能够看到2011年8月23日破裂的断层，并更好地了解导致地震的地壳应力状态。北美东部的地震历史悠久，虽然不频繁，但有时会发生破坏性大的地震。与集中在板块边界的地震活动不同，北美东部被动边缘的地震没有被清楚地识别为活动断层，也没有很好地理解驱动地震的过程。北美东部的地震聚集在明确的地震带中，比如CVSZ，那里每隔几十年就会发生一次小型到中等规模的地震，包括弗吉尼亚州的Mineral地震。地震带散布着地震间隙，其中一个围绕着美国国会大厦，那里几乎没有地震活动的历史。对这些观察结果的一种可能解释是，地震之所以集中在某些地区，是因为地壳中有一个局部的、潜在的地质弱点，周期性地释放累积的应力。另外，观测到的地震，包括Mineral， VA事件，都是史前地震的余震，不常见，但非常大的地震，代表了几个世纪以来地壳断裂愈合的过程。估计震级为7.0级或以上的地震的例子是南卡罗来纳州查尔斯顿（1886年）和大浅滩（1929年）事件，这两个地点现在都是公认的地震带，其特征是频繁发生小到中等规模的地震。本研究使用的工具是地震学联合研究机构（IRIS）仪器库提供的7台地震仪，以及将按照板块边界天文台（PBO）规格建造的2台新的GPS接收器。现在发生在弗吉尼亚州矿物地区的小地震都是2011年8月23日主地震的余震，这些地震正在从破裂处转移，沿着断层向地表移动。当这种情况发生时，地震检波器网络精确定位它们的位置，建立了断层面的三维图像。同时，定位GPS接收器，以毫米精度记录疑似断裂面两侧地面的水平和垂直位置。地震学和GPS大地测量数据一起记录了地壳和陆地表面在一次中等规模地震后的一年中是如何变形的。这些数据为地壳应力状态的地质学思考提供了信息，用于约束地震复发和灾害的模型。北美东部需要更多的这类研究，因为从阿巴拉契亚山脉的构造和大西洋的开放中继承下来的许多旧断层嵌入地壳中，显然受到载荷，能够产生地震，造成结构破坏，威胁到人类的生命。地质学界有能力向基础设施工程师、规划人员和社区通报与北美东部断层和地震有关的相对危害，这将极大地有助于对弗吉尼亚州矿物事件进行仔细而详细的研究。

The overall goal of this research is to instrument the central Virginia seismic zone (CVSZ) in the region surrounding the Mineral, VA earthquake with seismometers and global positioning system (GPS) receivers that are designed to record aftershocks and subtle ground movements respectively for a year. These measurements allow geoscientists to visualize the fault that ruptured on 23 August 2011 and better understand the state of stress in the crust that lead to the earthquake. Eastern North America has a long history of infrequent, but sometimes large and damaging earthquakes. Unlike the seismic activity that is concentrated along plate boundaries, earthquakes in the passive margin of eastern North America are not clearly identified with active faults nor are the processes that drive them well understood. Earthquakes in eastern North America are clustered into well-defined seismic zones, like the CVSZ where small to moderate-sized events including the Mineral, VA earthquake seem to occur every few decades. The seismic zones are interspersed with seismic gaps, one of which surrounds the nation's capitol, where there is little history of earthquake activity. One possible explanation of these observations is that the earthquakes cluster in areas because there is a localized, underlying geologic weakness in the crust that periodically releases accumulated stresses. Alternatively, the observed earthquakes, including the Mineral, VA event are all aftershocks of pre-historic, infrequent, but very large earthquakes and represent a crustal fracture healing over a period of several centuries. Examples of such earthquakes with estimated magnitudes of 7.0 or greater are known from the Charleston, SC (1886) and Grand Banks (1929) events and both of these locations are now recognized seismic zones characterized by frequent small to moderate-sized earthquakes. The tools that are being employed in this research are seven seismometers available from the Incorporated Research Institutions for Seismology (IRIS) instrument pool and two new GPS receivers that will be constructed following Plate Boundary Observatory (PBO) specifications. The small earthquakes that are now occurring in the Mineral, VA area are all aftershocks of the 23 August, 2011 main event that are migrating away from that rupture and traveling up along the fault plane towards the surface. As this happens, precise pinpointing of their location with a seismometer network builds a three-dimensional picture of the fault plane. At the same time, the GPS receivers are being positioned to record, with millimeter accuracy, the horizontal and vertical position of the ground surface on either side of the suspected fault plane. Together, the seismological and GPS geodesy data document how the crust and land surface deforms in the year following a moderate-sized earthquake. These data inform geological thinking on the state of stress in the crust that is used to constrain models of earthquake recurrence and hazards. Eastern North America requires more of this kind of research because many old faults inherited from the construction of the Appalachian Mountains and opening of the Atlantic Ocean are embedded in the crust and are evidently loaded and capable of generating earthquakes that cause structural damage and threaten human life. The ability of the geologic community to inform infrastructural engineers, planners, and communities as to the relative hazards associated with eastern North American faults and earthquakes will be greatly aided by a close and detailed study of the Mineral, VA event.