SEISMIC INTENSITY SCALES AND DAMAGE INDICATING PARAMETERS . 94

defined based on seismic damage observations. The seismic intensity at the point at which the earthquake motion time-history is observed indicates the damage level in its vicinity. However, since there are few seismic damage observations of specific SSCs for which actual earthquake motion data is available, the relationship between seismic intensity scales and the DIPs selected during the preparation of this publication, is presented here as an approximate evaluation.

4.2.1. Example of evaluation in the United States: MMI scale

The standardized CAV which was originally developed from a study of over 200 earthquake time-history records, which were correlated with the observed damage to SSCs in the immediate vicinity of the recording instrument [43].

Almost all correlations of damage were for residential and light commercial SSCs that had not been designed to any engineering-based industry standard and, much less, received any seismic resistant design. A value of 0.16 g-sec standardized CAV was associated with the threshold of damage. This value is one of the DIPs listed in US NRC Regulatory Guide 1.166 [46] as a threshold value for damage to a nuclear power plant, in relation with the SL-1 earthquake exceedance.

The relationship between the computed standardized CAV and the MMI determined in the study is plotted in Fig. 26 using the data listed in Ref. [43]. As can be seen in the figure, the Standardized CAV has a wide range of variation for the same MMI intensity, when the MMI intensity is larger than grade V. In addition, it is seen that the threshold of 0.16 g-sec is an extremely conservative value, which corresponds with the minimum computed Standardized CAV at an MMI intensity of VII.

FIG. 26. Standardized CAV vs. MMI intensity scale.

0.16 g-sec

The qualitative definition of level VII in the MMI intensity scale is as follows (see also Table IV-1 in Annex IV):

 Damage negligible in buildings of good design and construction; slight to moderate damage in well-built ordinary buildings; considerable damage in poorly built or badly designed buildings, adobe houses, old walls (especially if laid up without mortar), spires, etc.

 Cracked chimneys to considerable extent, walls to some extent.

 Fall of plaster in considerable to large amount, also some stucco.

 Numerous windows break, furniture to some extent.

 Loosened brickwork and tiles shake down.

 Weak chimneys at the roofline break (sometimes damaging roofs).

 Fall of cornices from towers and high buildings.

 Dislodged bricks and stones.

 Overturned heavy furniture, with damage from breaking.

 Damage is considerable to concrete irrigation ditches.

The damage threshold for buildings of good design and construction is 2.8 times higher than the 0.16 g-sec threshold. For industrial/power generating facilities that actually experienced damage, the smallest computed standardized CAV value was 0.768 g-sec, which is 4.8 times higher than the 0.16 g-sec threshold [43].

4.2.2. Example of evaluation in India: MSK scale

In the working group, the results of DIP computations for seismic motions observed in India were reported [49]. For the four earthquakes shown in Table 18, the DIPs of the seismic motions observed at a total of 69 points were calculated and the seismic damage in the vicinity of these points was identified in relation to the MSK seismic intensity scale.

What is interesting here is the relationship between the MSK seismic intensity and the JMA instrumental seismic intensity. JMA instrumental seismic intensities were calculated using the equations in Section 4.1.2, even though the type of seismometers and the guidance for installation environments (ground and geological features) are specified for the JMA seismic intensity scale and these specifications may have not been followed in the recording stations shown in Table 18. The results are shown in Fig. 27. A large scatter in JMA instrumental intensity is obtained for each value of MSK intensity when the later is larger than grade IV.

Hence, the expected correlation between the two intensities, as shown in Table 9, is not found in these cases. Regression lines are shown in Fig. 27, for each of the four earthquakes. The line corresponding to the Chamoli earthquake shows an abnormal behaviour, such as a negative slope.

TABLE 18. EXAMPLE OF EVALUATION IN INDIA - SUMMARY OF EARTHQUAKE DATA

No EQ Date Magnitude Recording

stations

Epicentre Acceleration

Range

Lat (°) Long (°) Gal

1 Sikkim, at India

Nepal Border Sep.18, 2011 Mw = 6.9 13 27.6 N 88.2 E 0.61 – 201.65

2 Chamoli (NW

Himalaya)

Mar. 29, 1999 Mb = 6.3/6.8 MS = 6.6/6.5 (USGS/IMD)

10 30.408 N 79.416 E 9.56 – 352. 84

3 Uttarkashi Oct. 20, 1991 Mb = 6.1 (IMD) MS = 7.1 (USGS)

13 30.780 N 78.774 E 17.4 – 288.80

4 NE India, at Indo Myanmar

border region

Aug. 6, 1988 Mb = 6.8

MS = 7.3 33 25.149 N 95.127 E 38.6 – 331.3

FIG. 27. Example of evaluation in India - MSK intensity vs. JMA instrumental intensity.

The Chamoli earthquake observation data is from a small magnitude earthquake with a shallow hypocentre that occurred in the Garhwat region of the Western Himalayas. According the report [49], significant disparities in damage occurred even in adjacent regions due to the effect of river terraces formed by alluvial deposits contacting sand and boulder. From the fact that earthquake motions are significantly affected by ground and geological features, the disparities at the point at which the acceleration data was observed and at the point at which the seismic damage occurred leads to the result mentioned above. While the JMA seismic intensity scale specifies the environments in which seismometers need to be installed, the measurements taken in Chamoli did not meet these requirements (see Section 3.3.2.2), making it impossible to accurately estimate the JMA instrumental seismic intensity. It can be said that this is a good example that shows the effect of ground and geological features on earthquake motions and also on damages. Seismic intensity scales are affected by the locations, observers and design standards. Thus, the result of intensity is not absolute, but relative to these parameters, as mentioned in Section 3.2.

The results of DIP computation for the four earthquakes shown in Table 18, including Chamoli earthquake are shown in Fig. 28 through Fig. 31.

As shown in Table 9, the MSK intensity grade tends to be larger than the MMI intensity grade for the same seismic damage. Taking an approach similar to the one in Ref [43], the minimum standardized CAV value is calculated for an MSK intensity of VII. This value is 0.15 g-sec, which is a slightly smaller than the 0.16 g-sec value given in Ref [46] as an operating basis earthquake exceedance criterion. However, when excluding the data of Chamori earthquake, the minimum CAV value for MSK VII for the earthquakes considered in India becomes 0.18 g-sec. Consequently, the value in Ref [46] to judge whether an observed earthquake is significant or not, that is, a standardized CAV of 0.16 g-sec, can be judged as reasonable, even for the case of India.

In addition, some scatter in the equivalent effective maximum acceleration AJMA can be seen in Fig. 29. This may be due to the environments in which the seismometers were installed, the structural seismic capacity realized by seismic design requirements and the locational influence on seismometers. It is important to be careful about the instrumentation environments, for example, to install them in the vicinity of the target facilities and with a rigid foundation.

FIG. 29. Example of evaluation in India - AJMA vs. MSK.

FIG. 30. Example of evaluation in India - Mean response acceleration (2-10 Hz, h=0.05, SRSS) vs. MSK.

FIG. 31. Example of evaluation in India - Mean response acceleration (10-20 Hz, h=0.05, SRSS) vs. MSK.

4.3. DAMAGE TO CONVENTIONAL INDUSTRIAL FACILITIES AND DAMAGE