Living on an Active Earth Perspectives on Earthquake Science (2003) / Chapter Skim
Currently Skimming:
3. Facing the Earthquake Threat
3. Facing the Earthquake Threat
Pages 107-175
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 107... ...
Preparation and rapid emergency response are therefore the bulwarks of a good seismic defense. This chapter describes the context of current efforts to improve seismic safety and performance by summarizing what is known about the principal types of earthquake hazards, their distribution across the nation and the world, and the knowledge-based approaches to reducing earthquake risk.
|
From page 108... ...
When this happens, however, almost any structure built across the rupture path will be deformed by the severe strains characteristic of primary ground failure (Figure 3.1~. Predicting the magnitude and extent of fault rupture is therefore a major issue in seismic hazard analysis.
|
From page 109... ...
Structures that were built to withstand strong ground motion nevertheless did not survive severe dislocations along the fault. The left abutment of this bridge across the Ta-An River was constructed through the fault plane (top)
|
From page 110... ...
Shaking occurs during the passage of seismic waves as they propagate away from the rupturing fault. The most destructive shaking is usually the horizontal ground motion from S waves and surface waves, although the vertical component of motion can also excite a damaging structural response.
|
From page 111... ...
During the 1994 Northridge earthquake, the ground motion at frequencies below 2 hertz was observed to be highest at locations around the top edge of the fault to the north of the hypocenter, consistent with the directivity pulse expected from the Northridge rupture. At higher frequency, the radiation of waves from fault surfaces becomes less coherent, owing to small-scale fluctuations in fault slip and nearby material irregularities, causing the rupture directivity effect to become subdued and other effects such as
|
From page 112... ...
, not around the top edge of the fault, which experienced the longer-period directivity effects. Systematic differences in ground motion have been observed for different faulting types (7)
|
From page 113... ...
Earthquakes in the older, stronger regions of the continent generally have greater stress drops and therefore radiate more high-frequency energy for a given amount of fault slip; moreover, their seismic waves propagate with less attenuation compared to earthquakes in plate boundary deformation zones. The attenuation difference is probably attributable to lower temperature, reduced scattering, and more continuous waveguide for crustal shear en r~ -- - -1- - -- - r
|
From page 114... ...
Regional studies that deploy seismometers more densely will be needed to clarify these explanations and to understand how the vertical and lateral structure of the crust controls ground motions. Subsidence and Uplift Large thrust earthquakes in subduction zones can cause sudden, permanent elevation changes with damaging effects to coastal areas.
|
From page 115... ...
Geological Survey, |
From page 116... ...
SOURCE: National Oceanic and Atmospheric Administration, National Geophysical Data Center. tude and duration of the ground shaking (20~.
|
From page 117... ...
, resulting in numerous deaths. Lateral spreading or landsliding can also be caused by the shakinginduced loss of shear strength in certain types of "quick" or "sensitive" layers of salt-leached, clay-rich marine sediments.
|
From page 118... ...
In 1990, California enacted the Seismic Hazards Mapping Act, which significantly broadened the responsibilities of the state geologist to include mapping liquefaction and landslide hazards. Mitigation money from the Federal Emergency Management Agency (FEMA)
|
From page 119... ...
Additional systems have since been deployed, including those designed to provide rapid warnings of tsunami hazards from local earthquakes, when the runups occur soon after ground shaking (see below)
|
From page 120... ...
Fault ruptures that propagate at anomalously low velocity, and thus have a long source duration, appear to be responsible for some large tsunamis, whereas others are evidently due to landslides triggered by earthquakes or volcanic eruptions (26~.
|
From page 121... ...
A particular issue is the excitation of edge waves, which can result in large-amplitude late arrivals, as observed in a tsunami generated by the 1992 Cape Mendocino, California, earthquake (32~. 3.2 SEISMIC HAZARDS IN THE UNITED STATES A major task for earthquake science is to characterize the geographical distribution of seismic hazards.
|
From page 122... ...
This section describes seismic hazards in the United States, using the national seismic hazard maps as a guide. National Seismic Hazard Maps Seismic hazard maps depict the ground-motion intensity that will be exceeded with a specified probability during a specified exposure time (See section 2.7~.
|
From page 123... ...
These sources can then be used as scenarios for constructing ground-motion time histories needed to design critical facilities, conduct emergency management exercises, and estimate earthquake losses. California California has the highest levels of seismic hazard in the lower 48 states because more than 75 percent of the relative motion between the Pacific and North American plates occurs as active faulting within its borders.
|
From page 124... ...
S ~ ~ An a ~ no of o ._ a u, 0) a o of of I:, Am_ oft\ / 'of A, FIGURE 3.9 Probabilistic seismic hazard maps for the conterminous United States.
|
From page 125... ...
Therefore, restricting land use in active fault zones is the primary strategy for mitigating fault rupture as a seismic hazard (Box 3.2; Figure 3.244. These policies are less effective where the hazard is distributed across a broad area and all sites face significant seismic hazards; here the primary mechanisms for reducing risk are good engineering design and construction standards and effective emergency management.
|
From page 126... ...
~ Wit 2.. -- ....' FIGURE 3.10 Disaggregated seismic hazards for Knoxville, Tennessee, for response-spectrum acceleration at 1 hertz (left panel)
|
From page 127... ...
. At 5 hertz, the seismic waves from the more distant sources are more severely attenuated, and the hazard is dominated by the smaller, more local events (blue bars near Knoxville)
|
From page 128... ...
published a major assessment of earthquake hazards that, for the first time, merged results from geodetic measurements, neotectonic slip rates, and historic seismicity into a probabilistic seismic hazard analysis for southern California (40~. That report concludes that earthquakes of M 7.2 to 7.6 have occurred and will recur in the Los Angeles region to relieve the contractional strain accumulating across the "Big Bend" of the San Andreas.
|
From page 129... ...
Pacific Northwest The band of high seismic hazard paralleling the Pacific coast, which includes the Portland and Seattle-Tacoma metropolitan areas, comes from three tectonically distinct sources: great earthquakes on the main thrust of the Cascadia subduction zone; shallow earthquakes in the upper crust above the subduction zone; and deeper earthquakes within the subducting lithosphere of the Juan de Fuca plate. The 1000-kilometer-long subduction interface that runs along the continental margin from Cape Mendocino to the northern tip of Vancouver Island appears to be locked and accumulating strain, rather than slipping aseismically as previously believed (42~.
|
From page 130... ...
~ ~ ., , The Seattle fault is one of several shallow thrust faults accommodating north-south compression in the Puget lowlands (Figure 3.12~. The FIGURE 3.12 Photograph of uplifted beach from prehistoric earthquake on the Seattle fault, looking east across Puget Sound from Bainbridge Island toward Seattle, Washington.
|
From page 131... ...
The late Cenozoic normal faults are distributed relatively uniformly in the Basin and Range, but the historic and instrumental seismicity is concentrated in the central Nevada seismic belt, along the western margin of the province in eastern California and western Nevada, and the intermountain seismic zone, along the eastern edge of the province, from southern Nevada across central Utah to southwestern Montana and central Idaho. From 1915 to 1954, a sequence of five large earthquakes (M 6.8 to 7.7)
|
From page 132... ...
Central United States Seismic activity decreases markedly in the stable continental interior, east of the Rockies. However, seismic waves propagate more efficiently through the colder, thicker lithosphere that underlies this region than through the hotter crust and upper mantle of the western United States (48~.
|
From page 133... ...
Hough, On the Modified Mercalli intensities and magnitudes of the 1811-1812 New Madrid earthquakes, I Geophys.
|
From page 135... ...
Such an earthquake would likely cause substantial damage. Unlike plate boundary zones, where the source of the deformation is fairly clear, the seismicity of the central and eastern United States arises from deformations that are poorly described and not well understood.
|
From page 136... ...
The largest historic earthquake in the United States (M 9.2) was generated in 1964; its ground shaking and liquefaction caused spectacular damage to Anchorage (Box 2.3~.
|
From page 137... ...
Convergent Environments Most of the global seismic energy release is in subduction-zone earthquakes (Figure 3.17~. Subduction zones are marked by intermediate- and deep-focus seismicity that defines the Wadati-Benioff zones of the subducted lithospheric slabs, which dip beneath volcanic arcs in places such as Japan, the Aleutian Islands, and the Cascadian province of the western United States (57~.
|
From page 140... ...
Steeper subduction interfaces, such as that beneath the Mariana Island arc, usually produce smaller earthquakes (58~; they commonly involve older, denser oceanic lithosphere and trench-normal extension of the overriding plate, sometimes in the form of back-arc spreading. Shallow-dipping subduction zones, like that beneath Chile, tend to produce larger earthquakes and commonly involve younger oceanic lithosphere.
|
From page 141... ...
, but are also found behind ocean-continent subduction zones, such as the foreland fold-and-thrust belts in the eastern foothills of the Andes. The style of deformation ranges from block motions along large reverse faults that penetrate deep into the crystalline basement, as in the Pampean Ranges in northwestern Argentina and the Zagros of Iran and ~ ~ six It; ~ Ski ~ sS > ~ ~ s ~ ~ ~ ~~ T )
|
From page 142... ...
Moreover, with the exception of a few places such as Iceland, most oceanic spreading centers are far removed from areas of human habitation, and their seismic activity poses little danger. Subaerially exposed spreading centers are rare and atypical, but examples in Iceland and Djibouti have offered particularly good opportunities to study this class of normal faults and their associated earthquakes (62~.
|
From page 143... ...
Extension in the hanging-wall block above subduction zones, which is well documented, has produced significant earthquakes behind some circum-Pacific and Mediterranean arcs (64~. In Tibet and the South American Altiplano, compression during plate collision has built large plateaus so high that they may be gravitationally collapsing toward their margins, resulting in active normal faulting within their interiors (65~.
|
From page 144... ...
A major class involves the trench-parallel strike-slip faults associated with the strain partitioning in oblique subduction zones, discussed above. Other strike-slip faults accommodate the considerable horizontal motions associated with the lateral advection of crust escaping continent-continent collision zones.
|
From page 145... ...
(73~. Intraplate Earthquakes Not all major earthquakes have occurred at plate boundaries or even within the broad plate boundary zones of distributed continental deformation (Figure 3.17~.
|
From page 146... ...
A large earthquake hit Omit on August 17, 1999, killing over 17,000 people and causing 56 billion in direct economic losses.
|
From page 147... ...
Paleoseismic investigations of the surficial fault ruptures associated with the earthquakes reveal that the fault had not moved for 50,000 to 100,000 years or more before the recent event (77~. Though they can occur far from plate boundaries, most intraplate earthquakes are still caused by plate-tectonic forces.
|
From page 149... ...
The requirements included a nationally consistent inventory of buildings and infrastructure classified according to structural class; accurate estimates of the properties of potential earthquake hazards; a standardized methodology to describe and compute seismic damage to and replacement costs for buildings and lifelines, as well as secondary damage from floods, fire, and hazardous material release; and estimates of social and economic losses. This program has resulted in a publicdomain software package and database termed Hazards U.S.
|
From page 150... ...
Requirements for improved loss estimation are driving a broad research agenda to understand the vulnerability of society to earthquakes. This will involve collecting data on the full inventory of vulnerable structures, characterizing the complete range of impacts from earthquake disasters, accurately characterizing the fragility of the built environment, and increasing the accuracy of hazard assessments for the probable earthquake sources.
|
From page 151... ...
A sound strategy for the reduction of earthquake risk has four basic components: better characterization of seismic hazards; land-use policies to limit exposure to seismic hazards; preparation of the built environment to withstand future earthquakes; and rapid response to earthquake disasters. Seismic Hazard Characterization Characterizing and mapping seismic hazards has progressed substantially in recent years, but much work remains to be done in collecting information on active faults and incorporating new results from earthquake research into national and global seismic hazard maps.
|
From page 152... ...
These policies are less effective where the hazard is distributed across a broad area and all sites face significant seismic hazards; here the primary mechanisms for reducing risk are good engineering design and construction standards and effective emergency management. Life Safety U.S.
|
From page 153... ...
Among the most important are the Structural Engineers Association of California, the Building Seismic Safety Council (BSSC)
|
From page 154... ...
~) 9 I.` FIGURE 3.24 Alquist-Priolo zones for active faults in a region south of Hollister, California, mapped by the California Division of Mines and Geology.
|
From page 155... ...
Realistic estimates of the earthquake ground motion are also required to optimally design these systems. For example, for base isolated structures, the emphasis is on the prediction of long-period ground motions; thus, fault-rupture directivity and basin response are potentially important effects that should be considered in the estimation of these motions.
|
From page 156... ...
and, two years later, with the release of a major study on design guidelines for new steel momentframe buildings (93~. In FEMA 273, performance-based engineering is implemented by matching the desired performance objectives, such as the four NEHRP categories described in Table 3.2, with different levels of ground motion along a seismic hazard curve, ranging from weak and frequent to intense and rare.
|
From page 157... ...
. any repairs are mmor Life Safety Structure remains stable and has significant reserve capacity; hazardous nonstructural damage is controlled Collapse Prevention Building remains standing, but only barely; any other damage or loss is acceptable High Low SOURCE: Building Seismic Safety Council and Applied Technology Council, Guidelines for the Seismic Rehabilitation of Buildings, Federal Emergency Management Agency Report FEMA-273, Washington, D.C., 400 pp., October, 1997.
|
From page 158... ...
The performance-based approach increases the demands on accuracy on the engineering side of the equation as well. To achieve performance objectives, the calculation must go well beyond the standard pseudostatic lateral-force considerations prescribed by traditional building codes.
|
From page 159... ...
If ground motions within the specified probabilities are found by structural analysis to cause building drifts that exceed those required to meet the associated performance objective, the design of the building has to be modified (e.g., by increasing its stiffness, strength, or ductility capacity) until the performance objective is met.
|
From page 160... ...
As a point of reference, in central Los Angeles the annual probability of exceeding 0.42g 1-hertz spectral acceleration, which is the intensity measure being used here, is 0.1 percent. If this level of shaking were to occur, the maximum story ductility expected is about 4, implying displacements four times the elastic limit, major damage to nonstructural elements, and permanent distortions or possibly fracture of local beam-column connections in the building.
|
From page 161... ...
During the September 14, 1995, Guerrero earthquake (M 7.3) , its warning arrived 72 seconds before the strong ground motion (98~.
|
From page 162... ...
ShakeMap interpolates between observed ground motions recorded at seismograph sites using empirical ground-motion relations and maps of site conditions. As more dense instrumentation is deployed in urban areas under the Advanced National Seismic System, ShakeMap will be implemented in other areas.
|
From page 163... ...
, the conventional hypocenter, and the spatial distribution of ground motion. With these requirements, detection and location algorithms for real-time warning will differ from standard procedures at traditional seismographic stations.
|
From page 164... ...
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 (105~. An example of a public-private partnership designed to bridge the implementation gap is the lifelines program conducted by the Pacific Gas and Electric Co., Caltrans, the California Energy Commission, and the Pacific Earthquake Engineering Research Center (106~.
|
From page 165... ...
The Federal Emergency Management Agency leads NEHRP and holds the federal responsibility for seismic risk mitigation. Although FEMA is deeply involved in both risk assessment and emergency management, it is primarily a user, not a coordinator, of earthquake research.
|
From page 166... ...
Yoshimura, The influence of critical Moho reflections on strong ground motions recorded in San Francisco and Oakland during the 1989 Loma Prieta earthquake, Geophys.
|
From page 167... ...
Seed and I.M. Idriss, Ground Motions and Soil Liquefaction During Earthquakes.
|
From page 168... ...
Hopper, National Seismic-Hazard Maps: Documentation June 1996, U.S. Geological Survey Open-File Report 96-532, USGS Federal Center, Denver, Colo., 111 pp.
|
From page 169... ...
ground motion, and the level of shaking defined by PE50 = 2 percent is called BSE-2 ground motion. In the NEHRP Recommended Provisions for Seismic Regulations for New Buildings and Other Structures (Building Seismic Safety Council, FEMA Report 368, Washington, D.C., 374 pp., March 2001)
|
From page 170... ...
Hopper, National Seismic-Hazard Maps: Documentation June 1996, U.S. Geological Survey Open-File Report 96-532, Denver, Colo., 111 pp.
|
From page 171... ...
Although late Cenozoic normal faults are distributed relatively uniformly across this region, historical and instrumental seismicity is concentrated in two zones: the central Nevada seismic zone, which extends along the western margin of the province in eastern California and western Nevada, and the intermountain seismic zone, along the eastern edge of the province from southern Nevada across central Utah to southwestern Montana. The large (>M 7)
|
From page 172... ...
Geological Survey Bulletin 2032A, Reston, Va., p.
|
From page 173... ...
The BSSC promotes the development and adoption of seismic safety provisions in building codes suitable for use throughout the United States.
|
From page 174... ...
92. Guidelines for the Seismic Rehabilitation of Buildings (Building Seismic Safety Council and Applied Technology Council, Federal Emergency Management Agency Report FEMA-273, Washington, D.C., 400 pp., October, 1997~.
|
From page 175... ...
Scrivener, and B Worden, Trinet "shakemaps": Rapid generation of peak ground motion and intensity maps for earthquakes in Southern California, Earthquake Spectra, 15, 537-555, 1998.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.