The Influence of the Rate of Increase of Ground Vibration Accelerations During Earthquakes on the Value of the Observed Macroseismic Effect

Aims:

It is known that along with the traditionally considered amplitudes and durations of ground vibrations, the rate of increase in the intensity of ground vibrations in time can also affect the level of macroseismic effects caused by earthquakes. According to the previously obtained correlations, the differences between the observed macroseismic effects during earthquakes with slow and fast increases in the amplitude level of oscillations can reach one point of the macroseismic scale. The purpose of these study is to obtain, on the basis of a significantly (almost 9 times) larger than before, the volume of initial data (in combination with a more effective method of analysis) new and more accurate quantitative estimates of the studied dependences, as well as their possible interpretation.

Background:

The work continues the research began in 1985-1989. A representative statistical material was used, including 1250 accelerograms of earthquakes that occurred in different regions of the world, with magnitudes M = 2.5-7.7, distances of 5-230 km and independent estimates of macroseismic intensities I = 3-10 points by the MSK or MMI.

Objective:

Correlations between the absolute and relative rates of increase of ground vibration accelerations during earthquakes with different magnitudes and distances, on the one hand, and macroseismic effects caused by these vibrations, on the other, are considered.

Methods:

The study was carried out in the form of a direct statistical comparison of the parameters describing the form of ground vibrations during earthquakes with the characteristics of variations in macroseismic effects caused by these vibrations. A sample was formed and analyzed, including 1250 accelerograms of sensible and strong earthquakes recorded in various regions of the world and having independent estimates of the macroseismic intensity of shaking at instrumental registration sites.

Results:

It is shown that the macroseismic intensity of shaking can depend on the relative rate of increase of acceleration amplitudes in the general wavetrain of ground vibrations. An increase in the macroseismic intensity of shaking was observed with an increase in the relative rate of increase of the amplitudes and, conversely, it decreases with a slowdown in the rate of increase of the acceleration intensity. Similar constructions, made according to the data of the Time-Frequency Signal Analysis (TFSA) of 50 accelerograms of earthquakes with M = 3.3-6.2, a distance of 7-139 km and a macroseismic intensity of 4-7 MMI points, showed the same dependence, but clearer and with large coefficients of regression and correlation. The difference between earthquakes with “fast” and “slow” accelerations in the intensity I can reach one MSK point.

Conclusion:

The results of this study indicate that the rate of increase in the acceleration of ground vibrations during earthquakes can in a certain way affect the macroseismic effects. Earthquakes with slowly increasing amplitudes of ground vibration accelerations form average less macroseismic effects than those with rapidly growing accelerations.

Variations in the shaking intensity, at the same time, are quite significant and can be compared with variations associated with differences in soil-geomorphological conditions, focal mechanisms, general seismotectonic conditions and other factors that are traditionally taken into account in detailed assessments of seismic hazard. Therefore, this factor should also be taken into account when conducting such studies.

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