EAR-PF: The spatiotemporal scales of transient slow slip on the San Andreas fault near San Juan Bautista, central California, and the implications for seismic hazard

Heather Shaddox博士被授予NSF EAR博士后奖学金，将地震和大地测量观测相结合，以表征加州中部圣胡安鲍蒂斯塔附近圣安德烈亚斯断层的断层滑动和地震危险的性质。这项工作将在加州大学伯克利分校与导师罗兰·伯格曼合作进行。断层上的滑动以频谱的形式出现，从缓慢、连续的滑动(即无地震滑动)到地震期间的快速运动(即地震破坏)。无震滑动可以暂时缓解断层上的应力，但也可能在断层的相邻部分引发破坏性的大地震。因此，检测和了解这种暂时释放的应力(称为非地震滑动瞬变)是地震危险性评估的关键，并具有直接的社会影响。由于无地震滑动瞬变不会产生地震波，因此需要像陆地全球定位系统(GPS)这样具有观测极限的大地测量仪器来检测这些运动。该项目将使用相关的小地震(微震活动)来推断无地震滑动瞬变的位置，利用现有的和新安装的仪器，以前所未有的分辨率创建地震活动目录。该项目的结果将提供有关断层滑动的空间和时间尺度的信息，并对确定地震危险性具有重要意义。这种详细的分析还可能揭示基本断层滑动问题的答案，包括地震前无地震滑动的频率，以及地震触发无地震滑动瞬变的频率。我们还可以通过研究这个自然实验室来洞察其他直接观测有限的构造环境(例如，俯冲带的近海部分)。该项目还涉及为加利福尼亚州旧金山湾区的小学生提供有关地震的教育材料，旧金山湾区是地震危险性很高的地区。该项目的总体目标是以前所未有的分辨率研究加利福尼亚州中部圣胡安鲍蒂斯塔附近圣安德烈亚斯断层上的瞬时无震滑动的时空尺度，以最终获得对断层滑动性质的更广泛的理解。圣胡安·鲍蒂斯塔位于圣安德烈亚斯断裂蠕动段的西北边界和1906年旧金山7.9级地震的东南终止点，形成了该断层由蠕动向闭锁的过渡。圣胡安包蒂斯塔附近密集的仪器设备和瞬时无震滑动的历史记录使其成为研究地震和无震滑动相互作用的极好的天然实验室。我们将通过结合地震(一般地震活动、近重复地震)和大地测量(CGPS、InSAR、钻孔应变仪、蠕滑仪)观测来创建圣胡安鲍蒂斯塔附近圣安德烈亚斯断层上自发和触发的无震滑动瞬变的目录，以帮助回答以下问题：无震滑动瞬变的空间尺度、时间尺度和震级是什么？无震性滑动导致中小地震的频率和机制(S)？这些地震多久会导致无地震滑动一次？根据瞬时无震滑动的规模，圣胡安·鲍蒂斯塔附近的圣安德烈亚斯断层是否有能力发生6兆瓦的大地震？我们将进一步研究浅层和深层无震滑动的关系，以及在这种锁定过渡中蠕变速率的时空变化，并评估近重复地震作为瞬时无震滑动的替代作用。我们还希望通过研究这个自然实验室来深入了解其他构造环境，特别是直接观测有限的俯冲带近海部分。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Dr. Heather Shaddox has been awarded an NSF EAR Postdoctoral Fellowship to combine seismic and geodetic observations to characterize the nature of fault slip and earthquake hazard for the San Andreas fault near San Juan Bautista in central California. This work will be conducted at University of California Berkeley in collaboration with mentor Roland Burgmann. Slip on faults occurs as a spectrum, ranging from slow, continuous sliding (i.e., aseismic slip) to rapid movement during earthquakes (i.e., seismic failure). Aseismic slip can temporarily relieve stress on a fault but may also trigger large, damaging earthquakes on adjacent segments of the fault. Therefore, detecting and understanding this temporary release of stress (called aseismic slip transients) is key in seismic hazard evaluation and has direct societal impacts. Because aseismic slip transients do not generate seismic waves, geodetic instruments like land Global Positional System (GPS) stations with observational limits are required to detect these movements. This project will use related small earthquakes (microseismicity) to infer the location of aseismic slip transients, taking advantage of existing and newly installed instrumentation to create a catalog of seismicity at unprecedented resolution. The results of this project will provide information about spatial and temporal scales of fault slip and have implications for determining seismic hazard. This detailed analysis may also uncover answers to fundamental fault slip questions including how often aseismic slip precedes earthquakes and how often earthquakes trigger aseismic slip transients. We can also gain insights into other tectonic environments where direct observations are limited (e.g., the offshore portion of subduction zones) by studying this natural laboratory. This project also involves educational materials about earthquakes for elementary school students in the San Francisco Bay Area of California, a region with high seismic hazards. The general objective of this project is to study the spatiotemporal scales of transient aseismic slip on the San Andreas fault near San Juan Bautista in central California at unprecedented resolution to ultimately gain a broader understanding of the nature of fault slip. San Juan Bautista, located at the northwest boundary of the creeping section of the San Andreas fault and the southeast termination point of the Mw 7.9 1906 San Francisco earthquake, forms a creeping-to-locked transition of the fault. The dense instrumentation and historic record of transient aseismic slip near San Juan Bautista make it an excellent natural laboratory to study the interplay of seismic and aseismic slip. We will create a catalog of spontaneous and trigged aseismic slip transients on the San Andreas fault near San Juan Bautista by combining seismic (general seismicity, near-repeating earthquakes) and geodetic (cGPS, InSAR, borehole strainmeter, creepmeter) observations to help answer the following questions: What is the spatial scale, temporal scale, and magnitude of aseismic slip transients? How often and by what mechanism(s) does aseismic slip lead to small and moderate-sized earthquakes? How often do these earthquakes lead to aseismic slip? Based on the scales of transient aseismic slip, is the San Andreas fault near San Juan Bautista capable of large (Mw 6) earthquakes? We will further study the relationship between shallow and deep aseismic slip, spatiotemporal changes in creep rate in this locking transition, and evaluate the utility of near-repeating earthquakes as a proxy for transient aseismic slip. We additionally hope to gain insights into other tectonic environments, particularly the offshore portion of subduction zones where direct observations are limited, by studying this natural laboratory.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.