Collaborative Research: Subduction Zone Segmentation over Multiple Seismic Cycles, South-Central Chile

俯冲带断层的可能分割是目前地震发生的物理地质和未来地震危险性评价的重要问题之一。未来的大地震将如何沿着断层排列？单个地震破裂如何以及为什么会停止？俯冲带断层形成了与世界上许多国家的海岸线平行的构造板块边界。近几十年来，仪器和建模的进步使人们对断层行为有了重要的了解，然而这些测量只涵盖了大地震之间时间间隔的一小部分。本研究项目的目标是利用保存在海岸地貌和沉积物中的跨学科证据阵列来研究俯冲带断层的分割是否在跨越数百年至数千年的多个地震周期中保持不变。该油田位于智利中南部的阿劳科半岛海岸，跨越了1960年5月22日（9.5兆瓦）和2010年2月27日（8.8兆瓦）两次世界历史上最大的俯冲带地震的破裂带边界。2010年之前，该项目的研究区域位于智利中南部，是秘鲁-智利海沟沿线最受认可的硬断层段边界之一；它被认为与几次历史上的断裂有关，包括1960年的智利大地震。2010年2月的智利地震对这一假设提出了质疑，当时断裂延伸到这一边界以南，与公元1835年的上一次地震只有部分重叠。过去4000年来地震的地质证据，频繁的大俯冲带地震，以及智利中南部异常长而全面的地震历史记录，使该地区成为解决断层分割基本问题的天然实验室。本研究项目将测试有关地震行为和海岸反应的多个假设：(1)多个地震周期的断层破裂可能终止于固定（硬）、可变（软）或随机边界，或者可能包括多个部分；(2)大俯冲带地震会在陆面突变的同时产生海啸沉积；(3)该地区全新世海岸演化反映了相对海平面的适度净变化，大地震期间的垂直隆升或下沉在地震间隔期间基本逆转。为了解决这些假设，研究小组将利用地貌学、沉积学和微化石分析，重建与多次地震周期相关的垂直陆地水平变化和海啸历史。他们将描述保存陆地水平变化和沉积序列中的海啸沉积物的最佳环境条件，并分析这些陆地水平变化对沿海景观的影响。俯冲带断层的破裂造成了世界上一些最大的地震和海啸，比如2011年日本发生的灾难性地震，造成数千人死亡，沿海城市被毁。在2004年苏门答腊、2010年智利和2011年日本发生的毁灭性地震中，不可预见的断层破裂程度和由此产生的地震震级强调了这项调查对评估其他类似地质环境中未来地震危害的实际重要性。该项目将建立在与智利科学家现有的强有力的合作基础上。研究结果将通过在智利建立的联系为地震和海啸风险地图的绘制提供信息，我们将安排关于地震和海啸危害的公开演讲。一般的概念将促进对地震行为的认识，这些地震行为超过了仪器记录的相对较短的时期。本项目由地球科学部的构造学项目、地貌学和土地利用动力学项目以及国际科学与工程办公室支持。

The possible segmentation of subduction zone faults currently presents one of the most significant questions to the physics and geology of earthquake occurrence and the assessment of future earthquake hazards. How will future large earthquakes be arrayed along a fault? How and why will individual earthquake ruptures stop? Subduction zone faults form the tectonic plate boundaries that parallel the coastlines of many countries around the world. Advances in instrumentation and modeling in recent decades have produced important insights into fault behavior, yet these measurements span only fractions of the time interval between major earthquakes. The goal of this research project is to use an interdisciplinary array of evidence preserved in coastal landforms and sediments to investigate whether segmentation of subduction zone faults is maintained over multiple earthquake cycles spanning hundreds to thousands of years. The field are, the Arauco Peninsula coast of south-central Chile, crosses the boundary between the rupture zones of two of the largest subduction zone earthquakes in the worldwide historic record, on May 22, 1960 (Mw 9.5) and February 27, 2010 (Mw 8.8). Prior to 2010, the study area for this project in south-central Chile was one of the most accepted hard fault segment boundaries along the Peru-Chile trench; it was believed to bound several historical ruptures, including the Great Chilean earthquake of 1960. The Chilean earthquake in February 2010 called this hypothesis into question, when the rupture extended south of this boundary and only partially overlapped with the previous earthquake in AD 1835. The combination of geological evidence of past earthquakes over the last 4000 years, frequent large subduction zone earthquakes, and an unusually long and comprehensive historic record of earthquakes in south-central Chile makes this region a natural laboratory to address fundamental questions about fault segmentation. This research project will test multiple hypotheses regarding earthquake behavior and coastal response: (1) fault ruptures over multiple earthquake cycles could terminate at fixed (hard), variable (soft), or random boundaries, or could include multiple segments; (2) large subduction zone earthquakes will produce a tsunami deposit simultaneous with an abrupt change in land level; and (3) the Holocene coastal evolution in this region reflects a modest net change in relative sea level in which the vertical uplift or subsidence during large earthquakes is largely reversed during the periods between earthquakes. To address these hypotheses the research team will reconstruct vertical land-level changes and tsunami history associated with multiple earthquake cycles using geomorphic, sedimentological and microfossil analysis. They will characterize the optimal environmental settings for the preservation of land-level changes and tsunami deposits in sedimentary sequences and analyze the impacts of these land-level changes on the coastal landscape.Ruptures on subduction zone faults have created some of the largest earthquakes and tsunamis in the world, such as the catastrophic earthquake in Japan in 2011 that killed thousands of people and destroyed coastal cities. The practical importance of this investigation for assessing future earthquake hazards in other similar geological settings was underscored by the unforeseen extent of fault ruptures and resulting earthquake magnitudes in the recent devastating earthquakes in Sumatra in 2004, Chile in 2010 and Japan in 2011. The project will build on strong existing collaboration with Chilean scientists. The results will inform the construction of earthquake and tsunami risk maps through established contacts in Chile, and we will arrange public presentations on earthquake and tsunami hazards. The general concepts will advance knowledge of earthquake behavior over time periods that exceed the relatively short period of the instrumental record.This project is supported by the Tectonics Program and Geomorphology and Land Use Dynamics Program in the Division of Earth Sciences and the Office of International Science and Engineering.