Collaborative Research: Quantifying Megathrust Earthquake Ruptures with Coastal Stratigraphy and Tsunami Simulations, South-Central Chile

2010年2月27日，智利海岸发生8.8级大地震。地震期间海底的隆起引发了席卷海岸的海啸。超过525人死亡，经济损失总额估计为150亿至300亿美元。在世界范围内，自1900年以来只有五次大地震比它更大，其中包括1960年5月发生在智利南部的有史以来最大的地震。基于100年前开始科学测量以来发生的少数几次大地震，很难完全理解大地震的行为和影响。中央华盛顿大学和罗格斯大学的一个研究小组正在利用智利过去地震和海啸的悠久历史记录（500年）和地质证据来调查大地震和海啸的影响。不寻常的沙子沉积物表明，在过去的2000年里，强大的海啸多次向陆地扫过。海啸沙层上下土壤的变化也表明地震期间海岸景观的上升或下沉。生活在沿海沉积物中的微小藻类（硅藻）对淡水到盐水的变化非常敏感，反之亦然。通过识别古代沉积层中硅藻化石的变化，可以精确地确定过去地震期间陆地水平变化的程度。研究人员还利用海啸波的计算机模型来计算断层破裂的位置和大小，这些断层破裂在这些结果的基础上产生了过去的地震。这些证据的结合将提高对古代地震的频率、位置和大小的理解，从而提高对产生地震的海底断层的长期行为的理解。研究小组将参与各种活动，以提高美国和智利公众对地震和海啸危害的认识和教育，其中包括：制作和分发西班牙语和英语的关于海啸准备和生存的公共服务手册；向公众、政府官员和当地利益攸关方进行介绍；美国小学儿童的地震和海啸活动；以及为在科学事业中代表性不足的西班牙裔和低收入学生举办的讲习班。研究生、本科生和博士后研究人员将在项目中获得宝贵的研究经验。了解俯冲带变形的物理性质，准确评估大型逆冲地震及其伴随的海啸的危害，需要对多个地震周期的地震和海啸历史进行相当详细的研究。准确和精确地估计多个地震周期的同震隆升和下沉的数量和时间对于理解俯冲带应变积累和释放的长期历史至关重要。智利中南部是两个地球的所在地？美国最大地震（1960年，里氏9.5级；2010年，里氏8.8级）。然而，在过去一个世纪之前，500年的地震和海啸的书面历史提供了有限的破裂程度和震级信息。该研究项目利用沿海地层沉降、隆起和海啸沉积物的证据，测量同震和震间垂直变形，并在智利中南部600公里的俯冲带沿线建立海啸年表。与海啸正演模拟的比较将确定最适合的破裂参数（长度、位置、深度、震级），用于过去2000年的大型逆冲地震。本项目将发展两种新的古地震方法。首先，利用硅藻与盐度、潮汐高程和生命形式之间的关系，利用贝叶斯硅藻传递函数重建大地震期间的垂直陆面变化记录。扩展的现代硅藻数据集，结合新的贝叶斯硅藻传递函数，将显著提高基于微化石的智利中南部同震和震间海岸变形重建的准确性和精度。其次，利用海啸模拟模型对海啸进行正演模拟，并将其与古海啸沉积物的分布相匹配，以区分过去海啸的破裂位置和长度。通过将变形重建与绘制的海啸沉积物淹没和纬度范围相结合，研究人员将使用数值模拟来评估过去大型逆冲地震的不同破裂情景。在智利中南部没有尝试过这样的破裂模型，也没有在其他地方发表过如此大的空间尺度。1960年、2010年和更早的海啸在各种海岸地貌环境下的沉积物地图将为识别史前海啸沉积物提供重要的校准比较，并利用它们来推断其他沿海地区受海啸危害的淹没程度。

On February 27, 2010, a great earthquake of magnitude 8.8 struck the coast of Chile. The uplift of the seafloor during the earthquake triggered a tsunami that swept over the coast. More than 525 people were killed and estimates of total economic losses were US $15-30 billion. Worldwide, only five great earthquakes since 1900 have been larger, including the largest earthquake ever recorded, which also occurred in southern Chile in May 1960. It is difficult to fully understand the behavior and effects of great earthquakes based on the handful that have occurred since scientific measurements began 100 years ago. A research team from Central Washington University and Rutgers University are using the long historical record (500 years) and geologic evidence of past earthquakes and tsunamis in Chile to investigate the effects of great earthquakes and tsunamis. Unusual sand deposits indicate that powerful tsunami waves swept landward multiple times in the last 2,000 years. Changes in the soil above and below the tsunami sand layers also indicate rising or sinking of the coastal landscape during earthquakes. Microscopic algae (diatoms) that live in coastal sediments are very sensitive to changes from fresh to salt water and vice versa. By identifying changes in the fossil diatoms present within ancient sediment layers, the amount of land-level change during past earthquakes can be precisely determined. The researchers are also using computer models of tsunami waves to calculate the locations and sizes of the fault ruptures that produced the past earthquakes based on these results. This combination of evidence will improve understanding of the frequency, location, and size of ancient earthquakes, and thus of the long-term behavior of the ocean-floor faults that produce them. The research team will engage in diverse activities to increase public awareness and education about earthquake and tsunami hazards in both the U.S. and Chile that include: production and distribution of a public service handbook in Spanish and English on preparing for and surviving a tsunami; presentations to the public, government officials, and local stakeholders; earthquake and tsunami activities for U.S. grade school children; and workshops for Hispanic and low-income students who are underrepresented in science careers. Graduate, undergraduate, and postdoctoral researchers will gain valuable research experience in the project.Understanding the physics of subduction-zone deformation and accurately assessing the hazards from megathrust earthquakes and their accompanying tsunamis requires earthquake and tsunami histories of considerable detail over multiple earthquake cycles. Accurate and precise estimates of the amounts and timing of coseismic uplift and subsidence over multiple earthquake cycles are critical to understanding the long-term history of strain accumulation and release at subduction zones. South-central Chile was the site of two of Earth?s largest earthquakes (1960, Mw 9.5; 2010, Mw 8.8). However, prior to the past century, the 500-year written history of earthquakes and tsunamis provides limited information on rupture extent and magnitude. This research project uses coastal stratigraphic evidence of subsidence, uplift, and tsunami deposits to measure coseismic and interseismic vertical deformation and construct tsunami chronologies at sites along 600 km of the south-central Chilean subduction zone. Comparisons with forward simulations of tsunamis will identify best-fit rupture parameters (length, location, depth, magnitude) for megathrust earthquakes during the last 2000 years. Two new paleoseismic methods will be developed in this project. First, Bayesian diatom transfer functions that employ the relation between diatoms and salinity, tidal elevation, and life form will be used to reconstruct records of vertical land-level change during large earthquakes. The expanded modern diatom dataset, combined with new Bayesian diatom-based transfer functions, will significantly increase the accuracy and precision of microfossil-based reconstructions of coseismic and interseismic coastal deformation in south-central Chile. Second, tsunami simulation models will be used to create forward simulations of tsunamis and match them with the distributions of paleotsunami deposits to differentiate the rupture locations and lengths responsible for past tsunamis. By combining the deformation reconstructions with mapped tsunami-deposit inundation and latitudinal extent, the researchers will use numerical simulations to evaluate different rupture scenarios for past megathrust earthquakes. Such rupture modeling has not been attempted in south-central Chile, or published elsewhere at this large spatial scale. Maps of the deposits of the 1960, 2010, and earlier tsunamis in a variety of coastal geomorphic settings will provide important calibration comparisons for identifying prehistoric tsunami deposits and using them to infer inundation extent at other coastal sites subject to tsunami hazards.