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

EarthScope RAPID：对 VA 地震的大地测量和地震学响应，2011 年 8 月 23 日

• 批准号: 1202798
• 负责人: Frank Pazzaglia
• 金额: $ 2.83万
• 依托单位: Lehigh University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2012
• 资助国家: 美国
• 起止时间: 2012-01-01 至 2014-12-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1202798&HistoricalAwards=false
• 关键词: EarthScope; RAPID; Geodetic; seismological; response

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.

这项研究的总体目标是使用地震仪和全球定位系统 (GPS) 接收器对弗吉尼亚州矿产地震周围地区的弗吉尼亚州中部地震带 (CVSZ) 进行监测，这些接收器旨在分别记录一年的余震和微妙的地面运动。 这些测量结果使地球科学家能够直观地看到 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。 北美东部地区长期以来一直发生过罕见但有时规模较大且具有破坏性的地震。 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并没有被明确地识别为活动断层，也没有很好地理解驱动它们的过程。 北美东部的地震聚集在明确的地震带中，例如 CVSZ，其中包括弗吉尼亚州矿产地震在内的中小型地震似乎每隔几十年就会发生一次。 地震带中散布着地震间隙，其中之一围绕着美国国会大厦，那里几乎没有地震活动的历史。 对这些观测结果的一种可能的解释是，地震集中在一些地区，因为地壳中存在局部的潜在地质弱点，会定期释放累积的应力。 另外，观测到的地震，包括弗吉尼亚州矿产地震，都是史前罕见但特大地震的余震，代表了几个世纪以来地壳断裂的愈合过程。 估计震级为 7.0 级或以上的此类地震的例子可以从南卡罗来纳州查尔斯顿 (1886) 和大班克斯 (1929) 事件中得知，这两个地点现在都被认为是频繁发生小到中型地震的地震带。 这项研究中使用的工具是地震学联合研究机构 (IRIS) 仪器库中提供的七个地震仪和两个将按照板块边界观测站 (PBO) 规范建造的新型 GPS 接收器。目前在弗吉尼亚州 Mineral 地区发生的小地震都是 2011 年 8 月 23 日主要事件的余震，这些余震正从破裂处迁移并沿着断层面向地表移动。 当这种情况发生时，用地震仪网络精确定位它们的位置就可以构建断层面的三维图像。 与此同时，GPS 接收器正在定位，以毫米级精度记录可疑断层面两侧地面的水平和垂直位置。地震学和 GPS 大地测量数据共同记录了中型地震后一年内地壳和陆地表面的变形情况。 这些数据为地质学思维提供了关于地壳应力状态的信息，用于约束地震复发和灾害的模型。 北美东部需要更多此类研究，因为阿巴拉契亚山脉的建设和大西洋开口所遗留下来的许多古老断层都嵌入地壳中，并且具有明显的荷载，能够产生地震，造成结构破坏并威胁人类生命。 对弗吉尼亚州矿产事件的密切而详细的研究将极大地帮助地质界向基础设施工程师、规划者和社区通报与北美东部断层和地震相关的相对危害。