Liquefaction

Liquefaction of the ground during earthquakes has long been documented and has drawn much attention from earthquake engineers because of its devastation to engineered structures. In this chapter we review a few of the best studied field cases and summarize insights from extensive experimental data critical for understanding the interaction between earthquakes and liquefaction. Despite the progress made in the last few decades, several outstanding problems remain unanswered. One is the mechanism for liquefaction beyond the near field, which has been abundantly documented in the field. This is not well understood because, according to laboratory data, liquefaction should occur only in the near field where the seismic energy density is great enough to cause undrained consolidation leading up to liquefaction. Another outstanding question is the dependence of liquefaction on the frequency of the seismic waves, where the current results from the field and laboratory studies are in conflict. Finally, while in most cases the liquefied sediments are sand or silty sand, well-graded gravel has increasingly been witnessed to liquefy during earthquakes and is not simply the result of entrainment by liquified sand. It is challenging to explain how pore pressure could build up in gravely soils and be maintained at a level high enough to cause liquefaction.

Epilogue

We identify some common threads and trends in the observations of hydrological responses to earthquakes. We suggest that seismic energy density is a useful metric for interpreting observations and relating different types of responses. We conclude with a summary of outstanding questions and new opportunities.

Groundwater Temperature

Changes of temperature in response to earthquakes have long been documented and, in the case where systematic patterns of change can be discerned, may reveal important hydrogeologic processes. Progress in our understanding of these processes, however, has been slow, largely because systematic measurements are relatively scarce. In this chapter we review some cases where earthquake-induced changes of groundwater temperature were documented and interpreted. More importantly, we show that most interpretations are under-constrained and accurate explanation of the measured changes is often difficult. In order to better constrain the interpretation, co-located measurement of groundwater flow from conductive fractures or formations intersecting the wells is needed to interpret temperature measurements. An often neglected mechanism is turbulent mixing of water in wells, which may occur frequently during earthquakes because the water column in a well at thermal equilibrium with the local geotherm is usually in a state of mechanical disequilibrium.

Hydro-Mechanical Coupling

We summarize the basic principles that couple rock deformation and fluid flow. Topics covered include linear poroelasticity, consolidation, liquefaction, rock friction, and frictional instability. Together, these are the processes that serve as a starting point for understanding how water and earthquakes influence each other.

Groundwater and Stream Composition

Changes of groundwater chemistry have long been observed. We review some studies of the earthquake-induced changes of groundwater and streamflow composition. When data are relatively abundant and the hydrogeology is relatively simple, the observed changes may provide valuable insight into earthquake-induced changes of hydrogeological processes. Progress in this aspect, however, has been slow not only because systematic measurements are scare but also because of the distribution of chemical sources and sinks in the crust are often complex and unknown. Most changes are consistent with the model of earthquake-enhanced groundwater transport through basin-wide or local enhanced permeability caused by earthquake-induced breaching of hydrologic barriers such as aquitards, connecting otherwise isolated aquifers or other fluid sources, leading to fluid source switching and/or mixing. Because the interpretation of earthquake-induced groundwater and stream compositions is often under-constrained, multi-disciplinary approaches may be needed to provide a better constrained interpretation of the observed changes.

Hydrologic Precursors

Predicting earthquakes is a long-desired goal. The main challenge is to identify precursory signals that reliably predict the impending earthquake. Since hydrological and hydrogeochemical properties and processes can be very sensitive to minute strains, the hope is that measurements from hydrological systems might record precursory rock deformation that would otherwise be undetectable. Of the many hundreds of studies, we review a subset to illustrate how signals can be challenging to interpret and highlight questions raised by observations—examples come from China, Japan, Taiwan, India, the USA, Russia, France, Italy and Iceland. All are retrospective studies. Some signals seem to have no other explanation than being precursory, however, rarely is enough data available to undertake a thorough analysis. Some hydrological precursors might be recording deformation events that are slower than traditional earthquakes (and hence usually harder to detect). Long times series of data are critical for both identifying putative precursors and assessing their origin and reliability.

Introduction

Abstract“ … The waters that seem to have been cut off on the land of all communion with the sea, the springs, the lakes, were in extraordinary agitation in many distant lands at the same time.

Stream Flow

Changes in stream discharge after earthquakes are among the most interesting hydrologic responses because they are visible at Earth’s surface and can be dramatic. Here we focus on changes that persist for extended periods but have no obvious source. Such increases have been documented for a long time but their origins are still under debate. We first review some general characteristics of streamflow responses to earthquakes; we then discuss several mechanisms that have been proposed to explain these responses and the source of the extra water. The different hypotheses imply different crustal processes and different water–rock interactions during the earthquake cycle. In most instances, these hypotheses are under-constrained. We suggest that multiple mechanisms may be activated by an earthquake.

Response to Tides, Barometric Pressure and Seismic Waves

Groundwater responses to Earth tides and barometric pressure have long been reported and increasingly used in hydrogeology to advance our understanding of groundwater systems. The response of groundwater to seismic waves has also been used in recent years to study the interaction between earthquakes and fluids in the crust. These methods have gained popularity for monitoring groundwater systems because they are both effective and economical. This chapter reviews the response of groundwater system to Earth tides, barometric pressure, and seismic waves as a continuum of poroelastic responses to oscillatory forcing across a broad range of frequency.

Groundwater Flow and Transport

We summarize the basic principles of, and governing equations for, groundwater flow and transport. Topics covered include the concepts of pressure and hydraulic head, Darcy’s law, permeability, and storage. We compare saturated and unsaturated flow. We provide an introduction to heat and solute transport.