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Observed intensity-epicentral distance relations in earthquakes

TECHNICAL UNIVERSITY, ISTANBUL, Turkey

Abstract

The observed intensity-distance relation for a number of earthquakes that occurred in Turkey and elsewhere was studied. Observed results indicate that these curves fit one of the curves given by formula

for either n equal to 3 or 5. There are earthquakes with n equal to 5 for which the curve has an inflection point at some distance. This change in slope is explained by the arrival of totally reflected body waves from some discontinuity within the crust the M-discontinuity and discontinuities within the upper mantle as well as the concentration of rays resulting from the focusing effect of the increase in velocity gradient also in the crust or the upper mantle. The M-discontinuity has the largest contribution. There are cases where the decay of intensity with distance is azimuth dependent which may be attributed to crustal structure being different in different directions. This type of reasoning should help to explain why earthquakes in eastern Canada and Missouri are felt over very large distances at some azimuth.

The difference in the observed values of n in the above formula may be result of different type of motion at the focus and/or the crustal structure of the region.

The effect of the ground material on the intensities is not considered in this paper. They are to play a role within limited areas and to be of irregular nature.

This article has been cited by other articles:

				B. F. Howell Jr. and T. R. SCHULTZ Attenuation of Modified Mercalli intensity with distance from the epicenter Bulletin of the Seismological Society of America, June 1, 1975; 65(3): 651 - 665. [Abstract] [PDF]

				B. F. HOWELL Jr. Seismic regionalization in North America based on Average Regional Seismic Hazard Index Bulletin of the Seismological Society of America, October 1, 1974; 64(5): 1509 - 1528. [Abstract] [PDF]