Earthquakes - Geotechnical Engineering Investigation - page 973

Site Response Study
General
The following applies to: (1) areas where seismic coefficient data ( Figures 11.14 , 11.15 , and
11.44) are not available, or (2) the reconnaissance phase of a site-specific study.

Historical Seismicity
Catalogs of worldwide earthquakes are available from the USGS Earthquake Hazards
Program, or from the Advanced National Seismic System (ANSS) hosted by the Northern
California Earthquake Data Center. Information is provided in terms of intensity and mag-
nitude, and in some cases, the magnitude vs. the number of times exceeded for a particu-
lar time interval.
Depending on the region and information available, the first step in a site response
study is to estimate site intensity I from an analysis of earthquake history data. All known
events of I IV or greater, occurring within 320 km of the site (NRC requirement), are

located and zoned by intensity as a seismicity map, such as given in Figure 11.11.
Alternatively, relationships between various magnitudes ( M

4.0) and exceedance for var-
ious earthquake locations within some site distance are prepared. From this, the distance
of earthquakes of various magnitudes from the site are determined. For example, one may
find that an event of M 4.2 occurred 3 mi from the site and the largest event of M 7.5

occurred 60 km from the site.
Recurrence analysis (Section 11.2.8) is performed to determine the probable return of
events of various magnitudes to locations where they have occurred in the past (the source,
usually given as the epicenter). Events of significant magnitudes with return periods of 50
or 100 years are usually selected, depending on the importance and type of the structure.
“Engineering Lifetimes” for various types of constructions are given in Table 11.15. Events
of the highest magnitude and those with a recurrence interval closest to the economic life
of the structure are of major interest. Judgment is required to evaluate the results of the
recurrence analysis. If the maximum intensities of some of the historical data are overesti-
mated, the results might indicate that either the region is overdue for the occurrence of a
damaging shock, or that there is a regional change toward a lower level of intensity.
Site intensity I s , is estimated by the application of attenuation laws and relationships
(see Section 11.2.5) such as Figure 11.16, or it is imposed on a “capable” fault and then
attenuated to the site.

Geologic Study of Fault Structures
Geologic study is performed to locate fault structures and their lengths, and to identify
capable faults (Section 11.3.1) . Fault systems are correlated with intensity distributions.

TABLE 11.15
Engineering Lifetimes

Type of Construction Lifetime (years)

Nuclear powerplants 40-80

Buildings and pipelines 50

Bridges, tunnels, flood-control structures, and navigation locks 100

Dams 100-150

Solid-waste disposal in landfills 250

Repositories for hazardous nuclear wastes 10,000

From Civil Engineering , ASCE, November 1993.