Field-based Quantification of Empirical Relationships Between Pore-pressure Change and Seismic Shaking Using Data from the Chi-Chi (Mw = 7.6) Earthquake, Taiwan

世界各地的冲积扇经常受到大地震的震动，这些地震导致了一些历史上最严重的山体滑坡，即使在非常平缓的斜坡上也是如此(例如，1964年阿拉斯加安克雷奇的图恩高地山体滑坡)。然而，直到1999年台湾集集地震后，才有系统的现场证据表明冲积扇中孔隙压力的同震变化。1999年台湾中部集集(Mw=7.6)地震是由现代地震学、大地测量学和水文学仪器组成的密集网络记录的。由70个水文站组成的网络中的188个水文井首次捕捉到了广泛的同震孔隙压力变化，这些变化发生在地震震中以西的一个大型冲积扇(浊水河扇)上。用80个宽带强震加速度计以高采样率记录了集集地震主震和几次大(6级)余震期间该风扇上的地面运动，并及时公布了数据。从数字化的水位记录中出现了一种复杂但系统的孔压同震变化模式，这导致了一种假设，即同震孔压变化是由于沉积物对地震震动的非线性机械反应造成的。然而，这一假设是基于对小样本进行的少量实验室实验的经验关系，所施加的应力的大小和频率与地震震动时的大小和频率有很大不同。集集地震的地面运动数据和水文数据的可获得性为定量现场测试沉积物中的孔压变化与地震震动之间的关系创造了难得的机会。这一项目结合了集集地震的水文资料和地震学资料，探索了这两个数据集之间的经验关系，这将是孔压变化与地震震动之间的第一个基于现场的关系，将极大地促进我们对地震期间冲积扇液化和滑坡潜力的理解和评估，因此对地震工程界具有重要价值。

Alluvial fans over the world are often shaken by large earthquakes that have led to some of the worst historical landslides even on very gentle slopes (e.g., the Turnagain Heights landslide, Anchorage, Alaska, in the 1964 Alaskan earthquake). However, systematic field evidence for coseismic change in pore-pressure in alluvial fans was not available until after the 1999 Chi-Chi, Taiwan, earthquake. The 1999 Chi-Chi (Mw=7.6) earthquake in central Taiwan was recorded by a dense network of modern seismological, geodetic and hydrologic instruments. Widespread coseismic pore-pressure changes, captured for the first time by 188 hydrologic wells in a network of 70 hydrologic stations, occurred across a large alluvial fan (the Choshui River fan) west of the epicenter of the earthquake. Ground-motion on this fan, both during the main-shock and during several large (M 6) aftershocks of the Chi-Chi earthquake was recorded by 80 broadband strong-motion accelerometers at high sampling rate, and the data was promptly published. A complex, yet systematic, pattern in the coseismic change in pore-pressure emerged from the digitized water-level records that led to a hypothesis that the coseismic pore-pressure change was due to the nonlinear mechanical response of sediments to seismic shaking. The hypothesis, however, was based on an empirical relationship from a small number of laboratory experiments performed on small samples under applied stresses with magnitude and frequency greatly different from that during earthquake shaking. The availability of both the ground-motion data and the hydrological data from the Chi-Chi earthquake has created a rare opportunity for a quantitative field test of the laboratory-based relationship between the pore-pressure change in sediments and seismic shaking. This project combines both the hydrological data and the seismological data from the Chi-Chi earthquake in a search of the empirical relationship between the two data sets that would be the first field-based relationship between pore-pressure change and seismic shaking, and would greatly facilitate our understanding and assessment of liquefaction and landslide potential in alluvial fans during earthquakes, and would thus be of great value to the seismic engineering community.