Improved Seismic Monitoring - Improved Decision-Making Assessing the Value of Reduced Uncertainty (2006) / Chapter Skim
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4 Benefits from Improved Earthquake Hazard Assessment and Forecasting
4 Benefits from Improved Earthquake Hazard Assessment and Forecasting
Pages 77-104
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From page 77... ...
These hazards include earthquake ground shaking, tsunamis generated by earthquakes, and volcanic eruptions. It then discusses the central role that seismic monitoring plays in the development of ground motion prediction models, which are a vital input into all aspects of earthquake engineering (see Chapter 6)
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From page 78... ...
Seismograph networks supply earthquake parameter and waveform data that are essential for the real-time evaluation of tectonic activity for public safety (e.g., volcanic eruptions, tsunamis, earthquake mainshocks and aftershocks) , the development of earthquake hazard maps and seismic design criteria used in building codes and land-use planning decisions (e.g., characterization of seismic sources, ground failure, strong ground motion attenuation)
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From page 79... ...
Changes in technology during the last two decades have resulted in improvements in the detail, quality, and usefulness of seismic data through the deployment of three-component digital and strong motion sensors capable of reliable on-scale recordings over a range of earthquake sizes. The majority of strong motion networks in the United States were established prior to the ready availability of digital technology and,
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From page 80... ...
. Digital waveform data, either weak or strong motion, are used to further improve earthquake locations, characterize seismic source and wave propagation effects, measure the state of stress in the brittle crust, and develop ground motion attenuation models.
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From page 81... ...
and within structures, and obtaining a better understanding of ground response in urban areas, have become critical elements in the national goal of reducing seismic risk. The existing ground motion hazard maps -- as illustrated in Figure 4.1 -- provide information on a national scale, and these will increasingly have to be supplemented by more detailed maps representing seismic zonation for urban areas.
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From page 82... ...
The recent provision of funding to enable the USGS to expand seismic instrumentation for tsunami warning and response,1 following the Indian Ocean tsunami of 2004, represents an explicit recognition by Congress of the value of seismic networks for emergency response. Volcano Monitoring Nearly every recorded volcanic eruption has been preceded by an increase in earthquake activity beneath or near the volcano.
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From page 83... ...
MONITORING FOR GROUND MOTION PREDICTION MODELS Earthquake engineering practice uses ground motion prediction models to estimate ground motion levels for the design of structures. These
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From page 84... ...
This large degree of variability is reflected in the irregular distribution of both damage and ground shaking intensity patterns observed following earthquakes. Use of Ground Motion Prediction Models for Building Codes and Seismic Design Because of the uncertainty in the location and timing of future earthquakes, engineers generally take a probabilistic approach to characterizing the strength of future earthquake ground motions for seismic design at a given site.
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From page 85... ...
However, given the variability in the ground motion level, we would expect the largest ground motion level from the occurrence of 10 earthquakes to be higher than that from one, because with 10 earthquakes there is a higher probability that one of them would produce a ground motion level that is much higher than the average value for that earthquake.
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From page 86... ...
These instruments will be installed both on the ground, to measure the strength of the shaking that enters structures, and within structures, to measure the behavior of the structures in response to ground shaking. The ground instruments will have the capability of recording both strong ground motion (i.e., motions having potentially damaging levels, directly important for earthquake engineering)
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From page 87... ...
The strong ground motions recorded during recent large earthquakes in other countries, including Turkey, Taiwan, and Japan -- the latter two of which have strong motion recording systems that are vastly superior to those in the United States -- all point to the likelihood that our current ground motion models are too conservative (Somerville, 2003)
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From page 88... ...
To a large extent, these analyses can use information from quite small earthquakes, which occur with sufficient frequency to provide useful results. These small earthquakes do not themselves generate strong motions, but the weak motions that they generate can be used to understand earthquake source and wave propagation characteristics, which can then be extrapolated for the prediction of strong ground motions from larger earthquakes.
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From page 89... ...
Information about the severity of ground shaking at specific bridge loca tions and the capacity of those bridges allows CalTrans to prioritize post earthquake inspection and repair activities. Reduced traffic delays through efficient rerouting impact both the immediate emergency response and the 14 395 34.5 5 101 5 405 101 134 210 110 101 10 1 10 34 110 15 710 KILOMETERS 610 1 5 0 15 405 MILES 1 3 5 0 10 Amplification 5 33.5 119 118 117 FIGURE 4.3 Map showing earthquake ground motion amplification in Southern California.
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From page 90... ...
FIGURE 4.4 Cost function graph, showing increased construction costs as a function of design ground motions. SOURCE: Ketchum et al.
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From page 91... ...
The 1995 Kobe earthquake provided further evidence from recorded strong motion data, supported by wave propagation modeling using basin-edge structures, that ground motions may be particularly large at the edges of fault-controlled basins. The Kobe earthquake caused severe damage to buildings in a zone about 30 km long and 1 km wide, and offset about 1 km southeast of the fault on which the earthquake occurred (Pitarka et al., 1998)
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From page 92... ...
Without detailed knowledge of the deeper structure (provided in the case of Santa Monica by seismic exploration for oil) , it is difficult to predict the spatial variation of ground motion levels due to deeper geological structure.
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From page 93... ...
The few strong motion instruments located in urban regions in the United States are insufficient to provide an adequate description of the spatial distribution of ground motion. The proposed ANSS instruments will record damaging earthquakes on scale, providing the data that are needed for understanding the distribution of ground shaking level and its relationship to the distribution of damage (e.g., Figure 4.5)
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From page 94... ...
The ground motions recorded by seismic monitoring instruments from both small and large earthquakes provide data that can be used to identify the deep geological structure beneath urban regions. The adequacy of crustal structure models can then be tested to assess whether seismological ground motion simulation techniques are able to predict the observed patterns of ground motion amplitudes.
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From page 95... ...
. Earthquake predictions can be divided into short-term predictions (hours to weeks)
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From page 96... ...
Three examples of long- and intermediate-term earthquake forecasts in the United States -- based on applications of elastic rebound theory and the seismic gap method -- include the Parkfield, California, earthquake prediction experiment; the 1989 Loma Prieta, California, earthquake; and the 2002 San Francisco Bay area earthquake forecast. Experience with long- and intermediate-term predictions based on simplified or basic models of earthquake occurrence, however, has shown that accurate forecasts can be difficult even for plate boundaries that have seemingly regular historical sequences of earthquakes, as the following example demonstrates.
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From page 97... ...
occurred along an area of the San Andreas Fault where long- or intermediateterm forecasts had been made by a number of seismologists (Lindh, 1983; Sykes and Nishenko, 1984; Scholz, 1985)
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From page 98... ...
They go beyond the current national seismic hazard maps in also addressing the time-dependent hazard of the region, using information about the time elapsed since the last large earthquake in the region. In contrast, the national seismic hazard maps are time independent, in that they do not change depending on the time from some specific earthquake in the region.
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From page 99... ...
BENEFITS FROM IMPROVED EARTHQUAKE HAZARD ASSESSMENT 99 Sacramento ROODG R D G SA Santa ER N S Rosa C R EE 1 K Napa FFAAULT Sonoma AND Petaluma REAS 101 Novato Nov 27% Vallejo FAULT San Rafael P HA YW acific San ARD MT .DIABL GREENVILLE GREE Danville DIABLO O San Francisco FOakland FAAUULLT F NVILLE O 3% 580 Tracy cea 21% rancisco Hayward Livermore Pleasanton n 3% Pacifica San Mateo Bay FAUL T 880 Half Moon 101 Bay Palo Alto CALA 10% VERAS SSanan 280 JoseJose N 11% 0 20 MILES FAUL 0 20 KILOMETERS SA T N 17 EXTE EXTENT GR IN % Probability of magnitude LOMANT Gilroy 6.7 or greater quakes EG O PRIOF 101 before 2032 on the R Santa Cruz PRI IO ETARUPTURE indicated fault Watsonville QUAKE FA 1 U Monterey Increasing probability LT Bay along fault segments Salinas Expanding urban areas Monterey FIGURE 4.7 Long-term earthquake forecast for the San Francisco Bay area.
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From page 100... ...
As a possible long-term indicator of seismic potential, the seismicity surrounding the dormant southern section of the San Andreas agrees with independent assessments of the long-term estimates derived from paleoseismology (Ellsworth, 1990; see also Southern California Earthquake Center [SCEC] Southern California earthquake forecast model in WGCEP, 1995)
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From page 101... ...
Our responsibilities to the public dictate that we maintain up-to date scientific assessments of earthquake predictability and prediction methodologies, and carefully explore their implications for seismic risk. At the state level, evaluating earthquake predictions from a public policy perspective is the responsibility of the California Earthquake Prediction Evaluation Council, and it is my understanding that the Keilis-Borok et al.
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From page 102... ...
, and the 2004 Mw 6.5 San Simeon earthquake within a 9-month window. The developers predicted that an earthquake of magnitude 6.4 or larger would occur in southeastern California before September 5, 2004, but no such earthquake occurred.
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From page 103... ...
argues that a justification for pursuing earthquake prediction without a tested scientific basis is that society is willing to accept the uncertainties that come with the learning process in which seismologists are engaged because the potential benefits of predicting an earthquake might greatly outweigh the costs of that uncertainty. According to this alternative view of earthquake prediction, earthquakes present such a dire threat that society expects seismologists to attempt earthquake prediction even in the face of great uncertainties, in much the same way as it expects medical doctors to attempt to treat an illness even if its cause is difficult to diagnose.
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From page 104... ...
Such observations could be the basis for more focused monitoring of particular regions, providing some prospect for the development of an earthquake prediction capability. Regional networks of seismic recording instruments, such as those planned for the ANSS, provide an important component of the earthquake monitoring system needed for the potential development of such a prediction capability.
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