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

2010年2月27日，智利沿海发生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人遇难，估计总经济损失为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这更大，包括有记录以来最大的一次地震，1960年5月也发生在智利南部。从100年前科学测量开始以来发生的几次大地震来看，很难完全理解大地震的行为和影响。来自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长期历史记录(500年)和地质证据来调查大地震和海啸的影响。不同寻常的沙子沉积表明，在过去的2000年里，强大的海啸波多次席卷陆地。海啸沙层上下土壤的变化也表明地震期间沿海地貌的上升或下降。生活在沿海沉积物中的微小藻类(硅藻)对从淡水到咸水的变化非常敏感，反之亦然。通过识别存在于古代沉积层中的硅藻化石的变化，可以精确地确定过去地震期间陆地平面的变化量。研究人员还使用海啸波的计算机模型，根据这些结果计算导致过去地震的断层破裂的位置和大小。这些证据的结合将提高对古代地震的频率、位置和大小的了解，从而提高对产生地震的海底断层的长期行为的了解。研究小组将开展各种活动，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版的公共服务手册，介绍如何在海啸中做好准备并在海啸中幸存下来；向公众、政府官员和当地利益攸关方介绍情况；为美国小学生开展地震和海啸活动；以及为在科学职业生涯中代表性不足的西班牙裔和低收入学生举办讲习班。研究生、本科生和博士后研究人员将在该项目中获得宝贵的研究经验。了解俯冲带变形的物理学，并准确评估巨型逆冲地震及其伴随的海啸的危险，需要在多个地震周期内相当详细的地震和海啸历史。准确和准确地估计多个地震周期的同震隆起和下沉的数量和时间，对于了解俯冲带应变积累和释放的长期历史至关重要。智利中南部曾发生过两次地球上最大的地震--S地震(1960年，9.5级；2010年，8.8级)。然而，在上个世纪之前，关于地震和海啸的500年书面历史提供的关于破裂程度和强度的信息有限。这项研究项目利用沿海的沉陷、隆起和海啸沉积的地层学证据来测量同震和地震间的垂直形变，并在智利中南部俯冲带600公里处的地点建立海啸年表。与海啸的正演模拟进行比较，将确定过去2000年来大地震的最佳破裂参数(长度、位置、深度、震级)。该项目将开发两种新的古地震方法。首先，利用硅藻与盐度、潮汐高度和生命形式之间的关系的贝叶斯硅藻传递函数将被用来重建大地震期间陆地平面垂直变化的记录。扩展的现代硅藻数据集，与新的基于贝叶斯硅藻的传递函数相结合，将显著提高基于微化石的智利中南部同震和地震间海岸变形重建的准确性和精确度。其次，海啸模拟模型将被用来创建海啸的正演模拟，并将其与古海啸沉积物的分布相匹配，以区分导致过去海啸的破裂位置和破裂长度。通过将形变重建与绘制的海啸沉积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地震的不同破裂情景。这样的破裂模型在智利中南部还没有尝试过，也没有在其他地方以如此大的空间尺度发表过。1960年、2010年和更早的海啸在各种沿海地貌环境中的沉积物地图将为确定史前海啸沉积物并利用它们推断其他易受海啸危害的沿海地点的淹没程度提供重要的校准比较。

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.