Living on an Active Earth Perspectives on Earthquake Science (2003) / Chapter Skim
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6. Research Opportunities and Requirements
6. Research Opportunities and Requirements
Pages 350-383
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From page 350... ...
As a practical matter, the nation must counter the threat by redoubling its efforts to prepare for and respond to earthquakes. The Committee on the Science of Earthquakes posed three questions regarding the Earth science component of this national effort: (1)
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From page 351... ...
Fault characterization at the detail required for comprehensive seismic hazard analysis will require nationwide efforts to improve capabilities in the three main observational areas of seismology, geodesy, and geology: · a national seismic network capable of recording all earthquakes down to moment magnitude (M) 3 with fidelity across the entire seismic bandwidth and with sufficient density to determine the source parameters, including focal mechanisms, of these events; the location threshold for regional networks should reach M 1.5 in areas of high seismic risk; · geodetic instrumentation for observing crustal deformation 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; and · programs of geologic field study to quantify fault slip rates and determine the history of fault rupture over many earthquake cycles.
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National Seismographic Network from 56 to 100 stations and modernize regional seismic networks (Box 6.1~. These components of the ANSS plan, if brought into full operation, would furnish the instrumental system needed to satisfy the seismological objective stated above.
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GPS receivers located with millimeter precision over baselines of thousands of kilometers will be deployed to map long-term strain rates across the width of the Pacific-North American plate boundary, while arrays of GPS stations will be used to measure the short-term deformations associated with volcanoes and earthquakes (Figure 6.1~. PBO will also augment the presently sparse array of borehole strainmeters and seismometers along the main active faults from Alaska to Mexico to better characterize transient tectonic strain signals.
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From page 355... ...
Existing sites are shown in green. Right panel: Strawman distribution of continuous GPS and strainmeters for an instrument cluster to cover the San Andreas fault system, comprising 400 new GPS receivers and 175 new strainmeters.
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From page 356... ...
The geologic mapping of faults and the measurement of fault slip rates and prehistoric events is coordinated by the USGS, state geological surveys, and multi-institutional research organizations, such as the Southern California Earthquake Center (SCEC)
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To date, most earthquake research has relied on data from arrays of seismometers, geodetic positioning of benchmarks, and geologic mapping confined to land areas. A major objective of earthquake science should therefore be to extend observational systems and data bases into the offshore environments (Box 6.3~.
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. This type of long-range forecasting is essential for seismic hazard analysis, and further work on the problem should receive a high priority.
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From page 359... ...
For example, although the Cascadia subduction zone has not produced a great earthquake in historical times, much can be inferred about the prospects of future events from the study of subduction-zone seismicity in other regions. Likewise, an improved understanding of the San Andreas fault will likely come from new knowledge about analogous strike-slip faulting in New Zealand, Turkey, and China, or perhaps from the study of the anomalous, slow earthquakes that characterize oceanic transform faults (Figure 6.2~.
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From page 360... ...
Major devastation and loss of life are to be expected if earthquake activity migrates westward into this urban area. It is not known if similar patterns in earthquake migration might occur along the San Andreas fault, but this question could be addressed through paleoseismological studies of San Andreas earthquakes over the last 10,000 years.
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From page 361... ...
Paleoseismic techniques can be used to identify sequences of slip events at particular points on a fault, but these events must be precisely correlated in time and space to investigate clustering. The dense sampling and precise dating needed for this task are still lacking even along well-studied faults such as the San Andreas.
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From page 362... ...
If segment boundaries play a key role in the termination of ruptures, then highly segmented faults may tend to be more characteristic in their behavior or at least more predictable in the lateral extent of future ruptures. A long, smooth fault such as the San Andreas may not have any "hard" segment boundaries, making the size of ruptures more sensitive to timedependent stress heterogeneities.
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Physically based simulations such as these serve as useful platforms for hazard analysis and data assimilation.
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, detailed images of subsurface structures that control stress and strain heterogeneity (USArray) , and better knowledge of deformation processes at depth (the San Andreas Fault Observatory at Depth [SAFOD]
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From page 365... ...
and should be employed more widely in laboratory earthquake studies. Detailed investigations of fault properties and processes, in particular fluid-dominated processes, are needed on spatial dimensions from centimeters to kilometers to fill the scale gap between laboratory studies of rock mechanics and regional geophysical studies of active faulting.
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From page 366... ...
It also provides a long-term basis for capitalizing on field-based research. If FIGURE 6.4 Schematic cross section of the San Andreas fault zone at Parkfield, showing the SAFOD drill hole proposed as part of the EarthScope project, and the pilot hole being drilled in 2002.
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From page 367... ...
. 6.5 EARTHQUAKE SOURCE PHYSICS Better knowledge of earthquake source physics on the short time scales of fault rupture will improve the understanding of how strong ground motions are generated, as well as the processes that lead up to the fault
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More certainly, improvements in long-term forecasting and the still-bright prospects for intermediate-term prediction will depend on understanding the dynamical connection between the evolution of the stress field on interseismic time scales of decades to centuries and the stress heterogeneities created and destroyed during the few seconds of an earthquake. Goal: Understand the physics of earthquake nucleation, propagation, and arrest in realistic fault systems and the generation of strong ground motions by fault rupture.
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6.6 GROUND-MOTION PREDICTION Seismic shaking is influenced heavily by the details of how seismic waves propagate through complex geological structures. Strong ground motions can be enhanced by resonances in sedimentary basins and wave
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In particular, past earthquakes have demonstrated that areas of damage are often localized in highly populated sedimentary basins near active faults. Site-specific information about the time histories of shaking will be needed for performance-based design of structures in such settings.
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Small earthquakes recorded on such networks will furnish the dense data coverage needed for constructing three-dimensional models of sedimentary basins, while the strongmotion data will be essential for validating the ability of numerical models to predict ground shaking in future large earthquakes.
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From page 372... ...
These simulations demonstrate that the peak velocity amplification pattern in the basin depends on the specifics of the faulting. For example, two earthquakes on the San Andreas fault, the same in every respect except for the direction of propagation, can produce very different patterns of amplification depending on how seismic waves interact with basin structure.
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About half of these instruments will be free-field and half will be installed in buildings and other structures. This network will improve seismic hazard maps and will also enable engineers to correlate ground motions with building performance.
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Ground-motion prediction has the potential to enhance greatly local seismic zonation by including effects of rupture directivity, the orientation of the fault (e.g., the hanging-wall effect) , and structures such as sedimentary basins, basin edges, and buried folds and faults.
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From page 376... ...
This input is critical to earthquake engineering and design, as well as to the predictions of human casualties, damage to the built environment, and economic losses that can be expected from future earthquakes. Much applied research is still needed to improve the earthquake forecasts, attenuation relations, and site-response factors needed to apply seismic hazard analysis techniques.
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From page 377... ...
The notion of full waveform modeling of scenario earthquakes is not new to seismic hazard analysis of course, but opportunities exist for a greater level of interdisciplinary research, including collaboration with earthquake engineers, to develop a methodology appropriate for design and risk management applications. Over the long term, the most effective strategy for reducing the economic losses in earthquakes will be through the design of structures to withstand seismic shaking at specified levels of performance (see Section 3.5~.
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Such solutions are unacceptable in standard practice. It will be more efficient and effective to combine seismic hazard analysis for a structure-specific, ground-motion intensity measure (or vector of such indicators)
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early warning of impending strong ground motions and tsunamis outside the epicentral zones of major earthquakes. The ANSS, with its planned nationwide network of on-line broadband seismographs and strong-motion sensors, can fulfill some of these objectives.
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From page 380... ...
Interdisciplinary collaborations are critical in establishing a firm technical basis for civic action and strengthening the resolve of public officials to improve mitigation strategies. Adapting probabilistic seismic hazard analysis to the needs of performance-based design is an example of where more cooperation between scientists and engineers could pay off in a big way.
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From page 381... ...
Other educational objectives include · studies of earthquakes in the laboratory and field to enrich the educational experiences of students from all backgrounds and help them appreciate the excitement of basic and applied science; · scientist-mentored summer internships for undergraduates, such as those funded through the NSF Research Experiences for Undergraduates program; · work with museums to create novel and interactive learning environments; · development of K-12 earthquake curricula in accordance with the National Science Education Standards and state-sponsored efforts, such as California's Earthquake Loss Reduction Plan; guiding these efforts should be specific objectives to (1) structure these Earth science curricula
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From page 382... ...
6.10 RESOURCE REQUIREMENTS The programmatic support required for earthquake research during the next 10 years will outstrip the resources currently available through NEHRP and other federal programs. The ANSS and EarthScope initiatives, for example, would greatly improve the observational capabilities for earthquake science in the United States and would contribute substantially to the objectives outlined in this report.
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From page 383... ...
E.H. Field and the SCEC Phase III Working Group, Accounting for site effects in probabilistic seismic hazard analyses of Southern California: Overview of the SCEC Phase III report, Bull.
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