Influence of anthropogenic landscape modifications and infrastructure on the geological characteristics of liquefaction

Many large cities worldwide are built on natural and engineered geological
materials that are highly susceptible to liquefaction and associated
ground failure in earthquakes. Constitutive equations describing
relationships between sediment geotechnical characteristics, seismological
parameters, and liquefaction susceptibility of natural and engineered
sediments are well established. What is less understood is the role of
anthropogenic landscape modifications (e.g., river channel modifications,
sediment engineering and re-distribution) and infrastructure (e.g.,
buildings, buried infrastructure such as drainage systems) on the spatial
distributions and severity of liquefaction and ground deformation. Here we
use stratigraphic studies, ground penetrating radar (GPR), and analyses of
high-resolution aerial photographs to evaluate surface and subsurface
geological manifestations of recurrent liquefaction in
anthropogenically-modified landscapes during the 2010-2011 Canterbury
earthquake sequence in New Zealand. Engineered fill layers provided low
density, high permeability traps that captured fluidized sediment and
promoted the formation of a unique assemblage of liquefaction-induced
sediment intrusions that differ from those preserved in proximal natural
sediment. Subsurface drainage systems imparted significant influence on
the location, size and orientations of liquefaction ejecta features.
Sediments adjacent to engineered stream channels experienced large lateral
strains that are unlikely to have occurred in the absence of channel
modifications. Spatial variations in pre-Anthropocene topography and
liquefaction-susceptible sediments exerted strong influence on the
characteristics of liquefaction hazards, even in highly engineered
environments. Collectively, these observations highlight important
interactions between concomitant Anthropocene and pre-Anthropocene
environments that should be carefully considered when interpreting the
geologic effects of contemporary earthquakes and / or using
pre-Anthropocene geological records to forecast future hazards.

全球诸多大型城市均建于天然与人工工程地质体之上，此类地质体在地震中极易发生液化（liquefaction）及相关地面破坏。描述天然与人工沉积物的岩土工程特性、地震学参数与液化敏感性（liquefaction susceptibility）之间关系的本构方程（constitutive equations），目前已得到充分建立与验证。但目前对人为景观改造（anthropogenic landscape modifications，如河道改造、沉积物工程化处理与再分配）及基础设施（如建构筑物、排水系统等地下基础设施）对液化与地面变形的空间分布及严重程度的影响，仍缺乏足够认知。本研究采用地层学研究、探地雷达（ground penetrating radar, GPR）及高分辨率航空影像分析，针对2010-2011年新西兰坎特伯雷地震序列（Canterbury earthquake sequence）期间，人为改造景观中反复发生的液化现象的地表与地下地质表现开展评估。人工填土层作为低密度、高渗透性的捕获体，可捕获流态沉积物（fluidized sediment），进而形成独特的液化诱发沉积物侵入体（sediment intrusions）组合，该组合与近源天然沉积物（proximal natural sediment）中保存的侵入体存在显著差异。地下排水系统（subsurface drainage systems）对液化喷出物特征（liquefaction ejecta features）的位置、规模与产状具有显著影响。紧邻人工改造河道的沉积物承受了较大的侧向应变（lateral strains），若无河道改造，此类应变大概率不会发生。即使在高度人工工程化的环境中，人类世前（pre-Anthropocene）地形与易液化沉积物的空间分布差异，仍对液化灾害（liquefaction hazards）特征具有显著控制作用。综合来看，上述研究结果揭示了共存的人类世（Anthropocene）与人类世前地质环境之间的重要相互作用，在解读现代地震的地质效应，或利用人类世前地质记录预测未来灾害时，需充分考虑该相互作用。