US-Mexico Planning Visit: Collaborative Study of Active Faulting in Baja California, Mexico

美国-墨西哥计划访问：对墨西哥下加利福尼亚州活动断裂的合作研究项目摘要NSF-CNIC资金将支持德克萨斯大学奥斯汀分校的地球科学家与高等教育研究中心(CICESE)之间的合作。这项合作的长期研究重点将是更好地了解下加利福尼亚州北部的活动断层速率和地震危险性，这些断层是圣安德烈亚斯断层系统南部的一部分。初步规划会议的主要目标将是召集一个跨学科的科学家团队，他们将致力于实现这些研究目标，首先(1)检查现有的数据集，以突出关键的科学差距，(2)汇编现有的西班牙语和英语已出版和未出版的文献，(3)确定如何结合资源来回答关键的科学问题和开发新的研究方法，(4)进行探索性实地工作，以及(5)设计向当地传播我们所学的战略。从危险的角度来看，合作将集中在恩塞纳达市，因为它位于阿瓜布兰卡断层上方和以北，阿瓜布兰卡断层是下加利福尼亚州北部两条断层中最大的一条，这两条断层将位移从位于东部的太平洋-北美构造板块边界转移出去。尽管它在半个多世纪前就被认为是一座重要的建筑，但由于它位于墨西哥西北部的圣安德烈亚斯系统的外围，它几乎没有引起人们的注意。然而，最近附近的断层提醒人们，鉴于这部分板块边界系统在板块边界变形中的作用，人们对其了解得太少了。Agua Blanca断层上的地震是一个主要的问题，但我们计划收集的第一类测量结果也将有助于阐明它在将离岸滑动转移到其他活动断层中的作用，并将有助于洞察构造板块运动的比例，这一比例表示为地震生成的脆性断层与更良性的扩散褶皱。更详细地，该项目的重点将是测量Agua Blanca断层的新的地质滑动速率，并通过这样做来填补我们对圣安德烈亚斯南部(SSAF)板块边界系统理解的三个现有空白。首先，新的滑移率将为对这一断层对美国和墨西哥城市构成的地震危险的估计提供信息。现有的ABF滑移率估计是高度不精确的，但表明它可能容纳多达14%的板块边界滑移。这些估计以及ABF的地貌表达暗示了一个能够产生破坏性地表破裂的断层，但需要新的地质滑移率来证实这一点。其次，ABF的新滑动速率限制了南加州和下加利福尼亚州北部近海鲜为人知的断层网络所容纳的板块边界滑移量。以前的估计表明，板块边界滑移总量的2%-20%可能来自近海，但通过收集海底滑移率来证实这一点是复杂和昂贵的。然而，由于ABF是近海主要的断层馈送滑动，其滑动速率制约着近海的最小滑动速率。第三，新的ABF滑动速率将有助于揭示板块边界滑动被断层外分布变形吸收的比例，而不是脆性断层吸收的比例。大地测量和地质滑动速率表明，该地区的主要陆上断裂容纳了大约80%的整体相对板块运动。由于新的ABF滑动速率将限制离岸板块边界滑动的容纳程度，在考虑到陆上和离岸滑动后，20%的滑动赤字的任何剩余都可以归因于未被检测到的断层外分布变形。通过这些方式，我们希望利用来自美国和墨西哥大学的多名科学家的力量，在圣安德烈亚斯断层系统南部一个突出但鲜为人知的部分回答有关地震危险和滑动分割的关键悬而未决的问题。

US-Mexico Planning Visit: Collaborative Study of Active Faulting in Baja California, MexicoProject Summary NSF-CNIC funds will support the initiation of a collaboration between geoscientists at the University of Texas, Austin, and the Centro de Investigación Científica y de Educación Superior de Ensenada (CICESE). The long-term research focus of this collaboration will be working toward a better understanding of rates of active faulting and seismic hazard for faults in northern Baja California, which are part of the southern San Andreas Fault system. The primary goal of the initial planning meeting will be to bring together an interdisciplinary team of scientists that will work towards addressing these research goals by first (1) examine existing datasets to highlight critical scientific gaps, (2) compile existing Spanish- and English-language published and unpublished literature, (3) determine how to combine resources to answer key scientific questions and develop new research methods, (4) conduct exploratory field work, and (5) design strategies for disseminating what we learn to local. From a hazard perpective, the collaboration will be focused on the city of Ensenada since it sits above and just north of the Agua Blanca Fault, the largest of two faults in northern Baja California that transfer displacement away from the Pacific-North American tectonic plate boundary that lies to the east. Despite having been recognized as an important structure over half a century ago, because of its location in northwestern Mexico on the periphery of San Andreas system it has garnered little attention. However, recent faulting nearby has served as a reminder that this part of the plate boundary system is too poorly understood given its role in plate boundary deformation. Earthquakes on the Agua Blanca Fault are a primary concern, but the first type of measurements we plan to collect will also help to illuminate its role in transferring slip off shore to other active faults and will lend insight into the proportion of tectonic plate motion that is expressed as earthquake-generating brittle faulting versus more benign diffuse folding.In greater detail, the focus of this project will be to measure new geologic slip rates for the Agua Blanca Fault and by doing so to fill three existing gaps in our understanding of southern San Andreas (sSAF) plate boundary system. First, the new slip rates will inform estimates of the earthquake hazard this fault poses for US and Mexican cities. Existing slip rate estimates for the ABF are highly imprecise, but suggest that it may accommodate as much as 14% of total plate boundary slip. These estimates as well as the geomorphic expression of the ABF are suggestive of a fault that is capable of producing damaging surface ruptures, but the new geologic slip rates are needed to confirm this. Second, new slip rates for the ABF constrain on the amount of plate boundary slip accommodated by the poorly understood network of faults off shore of southern California and northern Baja California. Previous estimates suggest a 2-20% of total plate boundary slip may be accommodated off shore, but confirming this by collecting sub-marine slip rates is complex and expensive. However, since the ABF is the primary fault feeding slip offshore, its slip rate constrains the minimum offshore slip rate. Third, new ABF slip rates will help to reveal the proportion of plate boundary slip that is absorbed by off-fault distributed deformation rather than brittle faulting. Geodetic and geologic slip rates indicate that the major onshore faults in this region accommodate roughly 80% of total relative plate motion. Since the new ABF slip rates will constrain how much plate boundary slip is accommodated off shore, whatever remains of the 20% slip deficit after both on- and offshore slip is accounted for can be attributed to undetected off-fault distributed deformation. In these ways, this we hope to harness the strengths of multiple scientists from universities in both the US and Mexico to answer key outstanding questions regarding seismic hazard and slip partitioning in a prominent but poorly understood section of the southern San Andreas Fault system.