Williams Randolph T, Goodwin Laurel B, Sharp Warren D, Mozley Peter S
Department of Geoscience, University of Wisconsin-Madison, Madison, WI 53706;
Department of Geoscience, University of Wisconsin-Madison, Madison, WI 53706.
Proc Natl Acad Sci U S A. 2017 May 9;114(19):4893-4898. doi: 10.1073/pnas.1617945114. Epub 2017 Apr 24.
Our understanding of the frequency of large earthquakes at timescales longer than instrumental and historical records is based mostly on paleoseismic studies of fast-moving plate-boundary faults. Similar study of intraplate faults has been limited until now, because intraplate earthquake recurrence intervals are generally long (10s to 100s of thousands of years) relative to conventional paleoseismic records determined by trenching. Long-term variations in the earthquake recurrence intervals of intraplate faults therefore are poorly understood. Longer paleoseismic records for intraplate faults are required both to better quantify their earthquake recurrence intervals and to test competing models of earthquake frequency (e.g., time-dependent, time-independent, and clustered). We present the results of U-Th dating of calcite veins in the Loma Blanca normal fault zone, Rio Grande rift, New Mexico, United States, that constrain earthquake recurrence intervals over much of the past ∼550 ka-the longest direct record of seismic frequency documented for any fault to date. The 13 distinct seismic events delineated by this effort demonstrate that for >400 ka, the Loma Blanca fault produced periodic large earthquakes, consistent with a time-dependent model of earthquake recurrence. However, this time-dependent series was interrupted by a cluster of earthquakes at ∼430 ka. The carbon isotope composition of calcite formed during this seismic cluster records rapid degassing of CO, suggesting an interval of anomalous fluid source. In concert with U-Th dates recording decreased recurrence intervals, we infer seismicity during this interval records fault-valve behavior. These data provide insight into the long-term seismic behavior of the Loma Blanca fault and, by inference, other intraplate faults.
我们对于时间尺度超过仪器记录和历史记录的大地震发生频率的理解，主要基于对快速移动的板块边界断层的古地震研究。直到现在，对板内断层的类似研究还很有限，因为相对于通过挖沟确定的传统古地震记录，板内地震复发间隔通常很长（数万年到数十万年）。因此，人们对板内断层地震复发间隔的长期变化了解甚少。为了更好地量化板内断层的地震复发间隔，并检验相互竞争的地震频率模型（例如，时间依赖型、时间独立型和聚类型），需要更长的板内断层古地震记录。我们展示了美国新墨西哥州里奥格兰德裂谷洛马布兰卡正断层带中方解石脉的铀钍测年结果，这些结果限制了过去约55万年中大部分时间的地震复发间隔——这是迄今为止任何断层记录的最长地震频率直接记录。这项工作确定的13次不同的地震事件表明，在超过40万年的时间里，洛马布兰卡断层发生了周期性的大地震，这与地震复发的时间依赖模型一致。然而，这个时间依赖序列在约43万年时被一系列地震打断。在这次地震群期间形成的方解石的碳同位素组成记录了二氧化碳的快速脱气，表明存在异常流体源的间隔。结合记录复发间隔缩短的铀钍年代，我们推断这个间隔期间的地震活动记录了断层阀行为。这些数据为洛马布兰卡断层以及由此推断的其他板内断层的长期地震行为提供了见解。
