Instrumental seismic intensity

Seismic intensity at a given location that results from an earthquake is normally determined after post-earthquake investigations. On the other hand, with the growth of disaster prevention consciousness in recent years, there is an increasing need to facilitate emergency preparedness measures by identifying immediately after an earthquake the region in which damage is anticipated to occur with a high probability. This requires automatization, by avoiding human interpretation. Consequently, the relationship between the observed earthquake motion and the anticipated seismic damage is being studied.

Here, it is important underlining that the seismic intensity scale based on observation is relative, as addressed earlier, to the seismic capacity of the target structures, such as buildings. There is an issue, for example, that when the seismic capacity of the target structures improves as a result of changes in seismic design standards or rules, damage will decrease for the same level of earthquake motion level, resulting in a lower seismic intensity scale. Moreover, since it takes time to survey actual seismic damage, regular intensity scales are not adequate to determine the actions to be taken immediately after an earthquake. It is therefore desirable to establish a relation between seismic intensity scales and a physical parameter of earthquake motion.

Acceleration, velocity, etc, have been pointed out as candidates for physical quantities representing the characteristics of earthquake motions in lieu of seismic intensity scales.

However, although these types of physical quantities are easy to determine by means of seismic instrumentation systems, it is difficult for the public to understand what they actually mean. In this respect, the seismic intensity scales that have been used and generally accepted for a long time are more adequate from the viewpoint of preparation against earthquake disaster. Based on this background, instrumental seismic intensities, which estimate the seismic intensity level using seismic instrumentation, have been developed.

3.3.2.1. MMI instrumental intensity scale used in ShakeMaps of USGS

From the perspective of emergency preparedness, the United States Geological Survey (USGS) has published ‘ShakeMaps’ on which the earthquake motion intensity levels estimated immediately after an earthquake are shown in the MMI scale.

The USGS states in the ShakeMap manual [44] the following:

“That is not to say that instrumentally derived seismic intensity alone is sufficient for loss estimation. In fact, peak velocity and spectral response provide a more physical basis for such analyses. However, for the majority of users, we expect that the intensity map will be more readily interpreted than other maps of ground motion parameters and will be, therefore, more useful.”

The estimated intensity map is derived from ground motions recorded by accelerographs and it represents intensities that are likely to be associated with the recorded ground motions.

The MMI instrumental intensity scale is correlated by means of several equations with the observed peak acceleration or velocity of the earthquake motion, as shown below. Table 16 provides a graphic view of these correlations.

TABLE 16. SHAKEMAP INSTRUMENTAL INTENSITY SCALE TEXT DESCRIPTIONS (COURTESY OF U.S. GEOLOGICAL SURVEY)

ShakeMap Manual [44]

Converting from PGA and PGV to instrumental intensity:

“Wald and others (1999b) recently developed regression relationships between Modified Mercalli intensity I (Wood and Neumann, 1931, later revised by Richter, 1958) and PGA or PGV specifically for ShakeMap use by comparing the peak ground motions to observed intensities for eight significant California earthquakes. For the limited range of Modified Mercalli intensities V ≤ I ≤ VIII, Wald and others (1999a) found that for PGA,

I = 3.66 log(PGA) − 1.66 (standard deviation = sigma = 1.08) (2) and for peak velocity (PGV) within the range V ≤ I ≤ VIII,

I = 3.47 log(PGV) + 2.35 (standard deviation = sigma = 0.98) (3) Because we are also interested in estimating intensity at lower values, and our current collection of data from historical earthquakes does not provide constraints for lower intensity, we have imposed the following relationship between PGA and I :”

I = 2.20 log(PGA) + 1.00 (4)

Basically, in the lower range of the MMI scale, where the intensity is determined by perception of human senses, the maximum acceleration plays a key role; while in the upper range of intensities, where intensity is determined by the extent of seismic damage to structures, the peak velocity becomes more important. The manual of ShakeMap [44] contains the following description, which serves as a useful reference in considering one or the other parameter for indicating earthquake levels from the perspective of damage:

“In practice, we compute the I from the I versus PGA relationship (Eq. 2 and Eq.4), and if the intensity value determined from peak acceleration is ≥ VII, we then use the value of I derived from the I verses PGV relationship (Eq. 3). If the I determined from PGA is between V and VII, we weight both the PGA -derived and PGV-derived values, weighted by a factor linearly ramping from 1.0 for PGA at I V to 0.0 at I VII and vice versa. The switch to PGV for higher intensity insures that spurious high frequency acceleration spikes will not result in high intensities because the corresponding velocity for such a spike will be low. With our procedure, whereas the large acceleration peak would provide an abnormally high intensity, the much smaller

Using peak acceleration to estimate low intensities is intuitively consistent with the notion that lower (<VI) intensities are assigned based on felt accounts, and people are more sensitive to ground acceleration than velocity. Higher intensities are defined by the level of damage; the onset of damage at the intensity VI to VII range is usually characterized by brittle-type failures (masonry walls, chimneys, unreinforced masonry, etc.), which are sensitive to higher frequency accelerations. With more substantial damage (VII and greater), failure begins in more flexible structures, for which peak velocity is more indicative of failure.”

3.3.2.2. JMA instrumental seismic intensity

The seismic intensity scale used in Japan is based on the measurement with a specific seismometer, that is, with a seismic intensity measuring device. A network of such devices has been set up all over the country. The JMA seismic intensity scale is indicative of the general phenomena due to an earthquake and of the damage situation at each grade of seismic intensity, as shown in Table IV-4 in Annex IV.

Before introducing this instrumental scale, seismic intensity was determined by observation of the extent of damage to buildings, the perception of meteorological observatory personnel and other elements, using a table describing each of the intensity levels. However, from the experience of strong earthquakes, an issue arose in that delays in announcing the seismic intensity were likely to occur (a mobile observation team from the JMA Seismological Division would need to conduct a field investigation). In this regard, from April 1996, seismic intensity has been determined and officially announced by using seismic intensity measuring devices.

JMA instrumental seismic intensity is calculated from the observed earthquake ground motion wave form as follows [34].

Calculation procedure of the JMA instrumental seismic intensity

 Step 1

Calculate Fourier spectra for the three spatial components, two horizontal and one vertical, of the recorded earthquake acceleration time-histories

 Step 2

Correct the influence of frequency (period) contents of earthquake motion using three filters shown in Fig. 18.

 Step 3

Compute the time-histories with inverse Fourier transform

 Step 4

Calculate vector synthesis of time-histories of the three spatial components (absolute value of acceleration)

 Step 5

Calculate ‘𝐴 ’ value (acceleration) so that the total time during which the absolute acceleration is higher than ‘𝐴 ’ becomes equal to 0.3 seconds (Fig. 19).

FIG. 18. Filters for calculation of the JMA instrumental seismic intensity [34]. (Courtesy of Japan Meteorological Agency)

 Step 6

Calculate the instrumental seismic intensity ‘𝐼 ’, as;

𝐼 = 2 𝑙𝑜𝑔 𝐴 + 0.94 (Round the thousandth digit and round off the hundredth digit) (5) Relationship between ‘JMA seismic intensity scale’ and ‘instrumental seismic intensity’ 𝐼 is shown in Table 17.

After long studies on instrumental seismic intensity, the intensity scale calculated from the instrumental records as mentioned above has been used as the official intensity scale since 1996, based on the experience in the 1995 Southern Hyogo prefecture earthquake. Subsequently, no changes have been made although the computation formulas have been reviewed based on the damage found after the occurrence of strong earthquakes.

What needs to be noted here is that JMA specifies seismometers for its monitoring of instrumental seismic intensity (referred to as instrumental seismic intensity meters). Since acceleration data significantly depend on the ground and geological features where the seismometers are installed, the locations of installation are specified as given below.

Blue line: low frequency (<0.5Hz) cut Red line: high frequency (>10Hz) cut

Green line: frequency effect (considering acceleration and velocity) Black line: total (normalized to 1.0 at 1.0Hz)

Frequency (Hz)

Modification Factor

 Intensity meters need to be installed on flat land that is entirely made of the same type of geology, avoiding for example cliffs.

 Intensity meters need to be installed away from structures, so that they are not affected by the vibrations of such structures.

 Intensity meters need to be installed in a manner that the intensity meters or their foundations are rigidly connected with the supporting ground, so that the intensity meters will shake the same as the ground surface.

FIG. 19. Vector synthesis of three spatial components time-history. (Courtesy of Japan Meteorological Agency)

TABLE 17. RELATIONSHIP BETWEEN JMA SEISMIC INTENSITY SCALE AND INSTRUMENTAL SEISMIC INTENSITY 𝐼

JMA seismic intensity Scale Instrumental seismic intensity 𝐼

0 less than 0.5

1 larger or equal to 0.5 and less than 1.5

2 larger or equal to 1.5 and less than 2.5

3 larger or equal to 2.5 and less than 3.5

4 larger or equal to 3.5 and less than 4.5

5 Lower larger or equal to 4.5 and less than 5.0 5 Higher larger or equal to 5.0 and less than 5.5 6 Lower larger or equal to 5.5 and less than 6.0 6 Higher larger or equal to 6.0 and less than 6.5

7 larger or equal to 6.5

𝐴

Total duration exceeding A = 0.3 sec

From the viewpoint of the physical meaning of AJMA, the characteristics of the frequency filter applied in Step 2 needs to be pointed out first. The high-cut filter corresponds to the common thinking that the high frequency component of earthquake motion has less influence on structural damage. The frequency filter with a -1/2 gradient on double logarithmic chart, means that damage likely happens due not only to acceleration but also to velocity. This may correspond to MMI instrumental seismic intensity in the Table 16.

A second insight is that the time used in Step 5 results from consideration of the impulse caused by the earthquake inertial load that is effective to produce structural damage.

4. DAMAGE INDICATING PARAMETERS AND EARTHQUAKE MOTION