7
Summary

Earthquakes pose the most dangerous natural threat to the built environment. The estimated economic loss from earthquakes exceeds $4 billion per year in the United States alone, and the risks are rising throughout the world as nations expand their urban centers and infrastructure in geologically active regions. In 1977, Congress established the National Earthquake Hazard Reduction Program (NEHRP) to counter this threat through scientific and engineering research. NEHRP-sponsored studies have significantly improved long-term forecasts of earthquakes and their site-specific effects, as well as the ability to respond rapidly to earthquake disasters. This research has clearly paid off in the formulation of more effective risk reduction strategies. However, there has been considerable controversy about how government and the private sector can best implement loss reduction measures through regulatory policies, economic incentives, long-term investments, and public education. Part of this debate concerns the role of scientific research in earthquake mitigation.

The Committee on the Science of Earthquakes, sponsored in part by a grant from the National Academy of Sciences, has conducted this study to appraise the needs for future research from four complementary perspectives: (1) the need to improve seismic safety and performance of the built environment, especially in highly exposed urban areas; (2) the requirements for disseminating information rapidly during earthquake crises; (3) the opportunities for exciting basic science, particularly in the context of current research on complex natural systems; and (4) the responsibility for educating people at all levels of society about the causes and effects of



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7 Summary Earthquakes pose the most dangerous natural threat to the built environment. The estimated economic loss from earthquakes exceeds $4 billion per year in the United States alone, and the risks are rising throughout the world as nations expand their urban centers and infrastructure in geologically active regions. In 1977, Congress established the National Earthquake Hazard Reduction Program (NEHRP) to counter this threat through scientific and engineering research. NEHRP-sponsored studies have significantly improved long-term forecasts of earthquakes and their site-specific effects, as well as the ability to respond rapidly to earthquake disasters. This research has clearly paid off in the formulation of more effective risk reduction strategies. However, there has been considerable controversy about how government and the private sector can best implement loss reduction measures through regulatory policies, economic incentives, long-term investments, and public education. Part of this debate concerns the role of scientific research in earthquake mitigation. The Committee on the Science of Earthquakes, sponsored in part by a grant from the National Academy of Sciences, has conducted this study to appraise the needs for future research from four complementary perspectives: (1) the need to improve seismic safety and performance of the built environment, especially in highly exposed urban areas; (2) the requirements for disseminating information rapidly during earthquake crises; (3) the opportunities for exciting basic science, particularly in the context of current research on complex natural systems; and (4) the responsibility for educating people at all levels of society about the causes and effects of

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earthquakes. The study comprises five elements, each presented as a chapter of the report: Survey of basic and applied earthquake science from ancient times to the present day, with a discussion of lessons drawn from past research Evaluation of the current status of seismic hazard analysis and its connections to earthquake engineering, loss estimation, and risk mitigation Examination of the new technologies in the main observational disciplines of seismology, geodesy, geology, and rock mechanics Technical assessment of the key issues for future earthquake science, including the application of a dynamical systems approach to integrate observations Analysis of research opportunities and requirements. 7.1 CURRENT CAPABILITIES The study of earthquakes, like the science of many other complex natural systems, is still in its juvenile stages of exploration and discovery. Research has been focused on two primary problems: (1) earthquake complexity and how it arises from the brittle response of the lithosphere to deep-seated forces, and (2) the forecasting of earthquakes and their site-specific effects. Investigations of the first problem began with attempts to place earthquake occurrence in a global framework and contributed to the discovery of plate tectonics, while work on the second addressed the needs of earthquake engineering and led to the development of seismic hazard analysis. The historical separation between these two lines of inquiry has been narrowed by recent progress on dynamical models of earthquake occurrence and strong ground motion. This research has transformed the field from a haphazard collection of disciplinary activities into a more coordinated system-level science that seeks to describe seismic activity not just in terms of individual events, but as an evolutionary process involving dynamical interactions within networks of interconnected faults. The bright prospect for “earthquake system science” is a major theme of this report. Experience shows that much can be learned from multidisciplinary investigations coordinated in the aftermath of large earthquakes, and it makes clear the importance of standardized instrumental data and geologic field work. During the last decade, research has been accelerated through the development of new observational and computational technologies. Subsurface imaging can now be applied with sufficient resolution to delineate the deep, three-dimensional architecture of fault systems. Neotectonic studies are improving constraints on fault geometries and long-term slip rates, and paleoseismology is furnishing an extended

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record of past earthquakes, revealing evidence for the clustering of large events in “seismic storms.” The Global Positioning System (GPS) and interferometric synthetic aperture radar (InSAR) satellites are mapping with unprecedented resolution the crustal deformations associated with individual earthquakes, long-term tectonic loading, and the stress interactions among nearby faults. Networks of broadband seismometers have been deployed to record earthquake ground motions faithfully at all frequencies and amplitudes. By using high-performance computing and communications, scientists now have the means to process massive streams of observations in real time and, through numerical simulation, to quantify the many aspects of earthquake physics that have been resistant to standard analysis. Large earthquakes can be forecast on time scales of decades to centuries by combining the information from the geological record with data from seismic and geodetic monitoring. Earthquake scientists have begun to understand how geological complexity controls the strong ground motion during large earthquakes and, working with engineers, how to predict the site-specific response of buildings, lifelines, and critical facilities to seismic excitation. The long-term expectations for potentially destructive shaking have been quantified in the form of seismic hazard maps, which display estimates of the maximum shaking intensities expected at each locality in the United States. Once a large earthquake has occurred, automated systems can rapidly and accurately compute hypocenter location, fault-plane orientation, and other source parameters. Predicted distributions of the extent of strong ground motions can be broadcast in near real time, helping to anticipate damage and guide emergency response. In the case of distant, suboceanic earthquakes, post-event predictions of the earthquake-generated sea waves (tsunamis) can warn coastal communities with sufficient lead times to permit evacuation. Similarly, seismic activity can be used to warn about impending volcanic eruptions, as illustrated by the successful prediction of the Mt. Pinatubo eruption in 1991. 7.2 SCIENCE GOALS Despite these successes, many scientific questions about earthquakes remain to be answered. No available theory adequately describes the dynamical interactions among faults or the basic features of rupture nucleation, propagation, and arrest. From a practical perspective, the search for a comprehensive theory is motivated by the need to understand (1) active fault systems on time scales of days to centuries for the purpose of improving earthquake forecasting and (2) fault ruptures on time scales of seconds to minutes for the purpose of predicting strong ground motions.

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Extending the capabilities for long-term forecasting through better fault-system models is critical to improving seismic hazard analysis. On short time scales (hours to days), no method for event-specific earthquake prediction has yet demonstrated skill at a statistically reliable level; indeed, the chaotic nature of brittle deformation may imply that useful short-term prediction cannot be achieved, even with substantial improvements in the ability to detect precursory signals. Near-field observations before and during large earthquakes are too few and too limited, however, to rule out categorically the feasibility of short-term earthquake prediction. For example, aseismic “silent earthquakes” have recently been observed on the thrust interface of subduction zones by geodetic networks in Japan and Cascadia, but the interplay between these events and major earthquakes in subduction zones is not understood. Moreover, there are both observational and theoretical reasons to believe that large-scale failures within some fault systems may be predictable on intermediate time scales (years to decades), provided that adequate knowledge about the history and present state of the system can be obtained. It is not yet clear whether probabilistic forecasting methods can be devised that take advantage of this potential predictability, but such methods could contribute significantly to the reduction of earthquake losses. Fundamental understanding of earthquake predictability will likely come through a broad research program with the goals of improving knowledge of fault-zone processes; the nucleation, propagation, and arrest of fault ruptures; and stress interactions within fault networks. Better ground-motion prediction will depend on the ability to model fault ruptures, wave propagation, and the near-surface response to wave excitation. Three-dimensional simulation of the ground motions generated by dynamic fault ruptures is a challenging problem, owing to the complex physics of rock failure, the wide separation between the inner and outer scales of faulting, and the computational requirements for representing realistic geologic structures. Accurate prediction of strong ground motions requires detailed information about the heterogeneities in material properties and the stress field that govern high-frequency wave propagation. The committee identified specific long-term goals in nine areas of interdisciplinary research that offer exceptional opportunities to further the national effort in earthquake science: Fault Characterization: Document the location, slip rates, and earthquake history of dangerous faults throughout the United States. Global Earthquake Forecasting: Forecast earthquakes on a global basis by specifying accurately the probability of earthquake occurrence as a function of location, time, and magnitude, as well as the magnitude limits and other characteristics of likely earthquakes in a given place.

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Fault-System Dynamics: Understand the kinematics and dynamics of active fault systems on interseismic time scales, and apply this understanding in constructing probabilities of earthquake occurrence, including time-dependent earthquake forecasting. Fault-Zone Processes: Characterize the three-dimensional material properties of fault systems and their response to deformation through a combination of laboratory measurement, high-resolution structural studies, and in situ sampling and experimentation. Earthquake Source Physics: Understand the physics of earthquake nucleation, propagation, and arrest in realistic fault systems and the generation of strong ground motions by fault rupture. Ground-Motion Prediction: Predict the strong ground motions caused by earthquakes and the nonlinear responses of surface layers to these motions—including fault rupture, landsliding, and liquefaction—with enough spatial and temporal detail to assess seismic risk accurately. Seismic Hazard Analysis: Incorporate time dependence into the framework of seismic hazard analysis in two ways: (1) by using rupture dynamics and wave propagation in realistic geological structures to predict strong-motion seismograms (time histories) for anticipated earthquakes, and (2) by using fault-system dynamics to forecast the time-dependent perturbations to average earthquake probabilities. Seismic Information Systems: Develop reliable seismic information systems capable of providing (1) time-critical information about earthquakes needed for rapid alert and assessment of impact, including strong-motion maps and damage estimates, and (2) early warning of impending strong ground motions and tsunamis outside the epicentral zones of major earthquakes. Education and Outreach: Establish effective partnerships between earthquake scientists and other communities to reduce earthquake risk through research implementation and public education. 7.3 RESEARCH OPPORTUNITIES AND REQUIREMENTS In order to realize these goals, substantial investments will have to be made in the main observational disciplines of seismology, geodesy, geology, and rock mechanics. The seismological requirements include national and regional seismic networks capable of recording all earthquakes down to magnitude 3 (1.5 in urban areas of high seismic risk) with sufficient fidelity and density to determine the focal mechanisms and other source parameters of the smaller events. The recently completed Global Seismic Network (GSN) must be maintained to furnish data on source processes of major earthquakes around the world. Portable arrays of seismometers are needed to study aftershocks and other forms of earthquake

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clustering and to provide the high-resolution structural information necessary for investigating the architecture of fault systems and predicting ground motions. Probing the structure of sedimentary basins will require the extensive use of artificial-source reflection and refraction seismology, as well as seismographic data from deep boreholes to calibrate the effects of near-surface layering. Plausible 10-year objectives are to determine the structure of high-risk urban areas well enough to model the surface motions from deterministic seismic sources at all frequencies up to at least 1 hertz and to formulate useful, consistent, stochastic representations of surface motions up to at least 10 hertz. Geodetic instrumentation should be deployed for observing crustal deformations within active fault systems with enough spatial and temporal resolution to measure all significant motions, including aseismic events and the transients before, during, and after large earthquakes. This endeavor will require combining pointwise measurements using strainmeters and GPS with continuous deformation images from a dedicated U.S. InSAR satellite mission. Laser and radar altimetry are needed to produce the precise digital elevation models for investigating surface faulting and the deformations caused by buried faults. The determination of fault slip rates and rupture histories over many earthquake cycles will require the combination of geologic field study and high-precision age dating. At present, the long-term slip rates of most major faults in North America are either unknown or, at best, constrained by geologic measurements at only one or two sites. Slip-rate data are especially lacking in contractional provinces, where many questions still remain about how strain is partitioned among the major faults and between seismogenic faults and aseismic folding. New techniques of tectonic geomorphology can address these issues. To investigate the important problem of earthquake clustering, paleoseismologists must date slip episodes at particular points on a fault well enough to establish event sequences. The objective should be catalogs of large earthquakes spanning thousands of years on all major faults where such paleoseismic investigations are feasible. Better information on microscale processes is needed to formulate realistic macroscopic representations of the strength variations and the dynamic response of fault materials. Advances in laboratory-based techniques will be required to elucidate the dynamic phase of fault response, in which rapid large slips may cause large temperature excursions and weakening by pressurization of pore fluids and/or melt generation. Field examination of exhumed faults should explore the mechanical importance of shear localization structures and the evidence for fluids and/or local melting. Borehole data are needed to extrapolate laboratory results on laboratory-scale samples to natural faults, elucidate the generation of

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fault-zone structure, and clarify why some faults creep while others slip in large earthquakes. High-resolution methods focused on imaging fault structure and deformation processes should be combined with systematic programs for in situ sampling and experimentation at an established set of long-term natural laboratories. The diversity of earthquakes and their geological environments necessitates a global approach. Global scope is particularly important for capturing enough earthquakes to test the controversial assumptions of earthquake forecasting schemes under different tectonic and stress conditions. A truly comprehensive theory of earthquakes must be able to explain the similarities and differences of earthquakes at different depths, including the perplexing occurrence of events to depths of nearly 700 kilometers in descending lithospheric slabs. The GSN and temporary deployments of portable seismometer arrays above active subduction zones will yield the requisite data on intermediate-focus and deep-focus earthquakes, while laboratory studies of deformation will furnish information on the microscale physics of earthquake instabilities at the high pressures and temperatures of descending slabs. Pioneering international efforts such as the Global Seismic Hazard Assessment Program, the Global Strain Rate Map Project, the Global Fault Mapping Project, and the Working Group on Earthquake Recurrence Through Time are providing uniform standards and data access for seismic hazard analysis on a global scale, and they should be expanded with aggressive data-gathering efforts that exploit the new technologies described in this report. A major objective of earthquake science should be to extend observational systems and data bases into the oceans to understand the distribution of offshore faulting and its seismogenic and tsunamigenic potential. Progress toward understanding fault-system dynamics will depend on the ability to integrate the disparate observations regarding stress, strain, and rheology into self-consistent models that can be tested against observations not yet collected. Dynamical simulations at various scales are needed to assess the discrepancies among laboratory-based friction laws, observed fault-system behaviors, and seismological data on large earthquakes. Simulations of earthquake occurrence on fault networks are required to understand the behavior of natural fault systems and to address fundamental questions relating to earthquake occurrence, such as the effects of stress interactions and prior earthquakes in determining earthquake probability. The practical objective of this research is to develop procedures that can assimilate all information on fault-system behaviors into probabilistic forecasts and can update these forecasts consistently based on seismic activity and other new information. Reliable procedures for simulating ground-motion time histories have to be developed, rigorously tested against the available strong-motion

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data, and then applied in earthquake engineering research and practice. As richer data sets are collected, a primary challenge will be to set up a computational framework for the systematic refinement of three-dimensional wave-speed and attenuation models and the use of these models in the calculation of synthetic seismograms. A consistent interface will be needed for coupling ground-motion calculations to nonlinear soil-modeling codes. Significant research is needed on how to use waveform modeling to characterize the probability distributions of ground-motion time histories, or parameters derived from those time histories, in a way that properly accounts for both aleatory and epistemic uncertainties. The transition of earthquake science to a systems-oriented, physics-based approach has important ramifications for the types of cooperative research activities and organizational structures that will be most effective in addressing the basic and applied problems of earthquake research. In particular, additional support is needed for scientific centers and distributed collaboratories with advanced information technology infrastructures, where the disciplinary activities of many research groups can be coordinated, evaluated, and synthesized into system-level models of earthquake behavior. In addition to their key role in basic earthquake science, such centers have proven to be effective in disseminating earthquake information and research results, formulating science-based strategies for loss reduction, and educating groups at all levels about the role of science in disaster mitigation and loss reduction. 7.4 RESOURCE REQUIREMENTS The technological investments and programmatic support required for earthquake research during the next 10 years will outstrip the resources currently available through NEHRP and other federal programs. Major initiatives by the two NEHRP science agencies illustrate this situation. The U.S. Geological Survey (USGS) has proposed the deployment of an Advanced National Seismic System (ANSS), which would upgrade the U.S. National Seismographic Network, modernize regional networks, and deploy 6000 strong-motion stations in high-risk urban areas. A fully implemented ANSS would upgrade regional networks to modern seismic information systems and provide the framework for developing real-time warning systems. The ANSS plan, if brought into full operation, would greatly improve seismological instrumentation in the United States and would contribute substantially to the objectives outlined in this report. This system will require capital investments of approximately $170 million, and its annual operational costs are estimated to be about $47 million. In comparison, the congressional appropriation for the entire USGS component of the NEHRP budget was only $50 million for FY 2001.

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A second example is the EarthScope initiative, proposed by the National Science Foundation (NSF). This facility-oriented program includes the Plate Boundary Observatory (PBO), which would expand existing geodetic networks with additional permanent GPS stations and campaign-style observations and fill major gaps in measurements of plate boundary deformation in the western United States. The second geodetic component—a satellite-based InSAR imaging system—would map decimeter-level deformations of fault ruptures continuously over areas tens to hundreds of kilometers wide, as well as a range of nonseismic phenomena such as volcano inflation, glacial flow, and ground subsidence. USArray, the seismological component of EarthScope, would map lithospheric structure nationwide on scales of tens of kilometers and provide new capabilities for active-source imaging of specific features, including sedimentary basins where seismic risk is often high. EarthScope would also construct a San Andreas Fault Observatory at Depth (SAFOD) that would, for the first time, sample the fault by deep drilling, monitor its seismicity and strain, and perform in situ experiments to depths of 4 kilometers. Deployment costs for the EarthScope instrument systems are estimated to be $91.3 million for PBO, $245 million for InSAR, $64 million for USArray, and $17.4 million for SAFOD. Data analysis and management will require an additional $15 million to $20 million per year during the first decade of EarthScope operations. In comparison, total FY 2001 geoscience expenditures by the NSF in support of NEHRP were about $12 million. Geologic field work will be an important part of EarthScope; yet even if this NSF initiative were fully funded, it would not boost resources for earthquake geology to the levels envisaged in this report. The research opportunities for characterizing the structure and history of active fault systems warrant a severalfold increase in the neotectonic and paleoseismic studies currently supported by the USGS, as well as the NSF. Work in this area is limited by the small number of earthquake geologists engaged in this type of research, underlining the need for increasing efforts in geoscience education at both the undergraduate and the graduate levels. Experience gained during NEHRP demonstrates that the willingness of society to invest in risk reduction is best achieved through an active collaboration among scientists, engineers, government officials, and business leaders, working together among an informed populace. A corollary is that earthquake research will contribute to risk reduction more effectively when it is carried out in a context that recognizes the problem’s engineering, economic, and political dimensions. No agency is responsible for ensuring an integrated approach to research problems in earthquake science and engineering, and better mechanisms should be developed for bringing the two fields together to exploit potential synergies. Cooperation among the NSF and USGS earthquake science centers and

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the NSF earthquake engineering centers will be critical to this goal. Science participation in engineering programs such as the Network for Earthquake Engineering Simulation should be encouraged. Research to understand earthquakes and their effects is central to continuing efforts to decrease earthquake risk. The technological and conceptual developments documented in this report have positioned the field of earthquake science for major advances. Investments made now will eventually pay off in terms of saved lives and reduced damage. These returns can be realized sooner by encouraging unconventional lines of research; coordinating scientific activities across disciplines and organizations, especially between scientists and engineers; and supporting international programs to investigate the global diversity of earthquake behavior. Few problems are more challenging to science or strategically relevant to the nation, and few have a greater potential for elucidating the fundamental geological processes that shape the face of the planet.

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