ISOSEISMALS: DISTRIBUTION OF APPARENT INTENSITY. 225 
Grade B. Violent. — Comprizes fairly general collapse of brick and frame buildings 
when not unusually strong; serious cracking of brick work and masonry in excellent 
structures; the formation of fissures, step faults, sharp compression anticlines, and broad, 
wave-like folds in paved and asphalt-coated streets, accompanied by the ragged fissuring 
of asphalt; the destruction of foundation walls and underpinning structures by the 
undulation of the ground; the breaking of sewers and water-mains; the lateral displace- 
ment of streets; and the compression, distension, and lateral waving or displacement of 
well-ballasted street-car tracks. 
Grade C. Very strong. —Comprizes brick work and masonry badly cracked, with 
occasional collapse; some brick and masonry gables thrown down; frame buildings 
lurched or listed on fair or weak underpinning structures, with occasional falling from 
underpinning or collapse; general destruction of chimneys and of masonry, brick or 
cement veneers; considerable cracking or crushing of foundation walls. 
Grade D. Strong. —Comprizes general but not universal fall of chimneys; cracks in 
masonry and brick work; cracks in foundation walls, retaining walls, and curbing; a 
few isolated cases of lurching or listing of frame buildings built upon weak underpinning 
structures. 
Grade E. Weak. —Comprizes occasional fall of chimneys and damage to plaster, parti- 
tions, plumbing, and the like. 
This scale obviously is simply a classification of the phenomena observed. It defines 
as many grades as the facts seemed to express in this field. It is more finely subdivided 
than the Rossi-Forel scale and, for conditions in a modern city, the definitions are better 
framed. It has less intrinsic merit than the Omori scale, for both scales cover a similar 
range of destructive effects, but the subdivision is finer and more evenly spaced in the case 
of the Omori scale. Also the grades of the San Francisco scale can not be fixed by values 
of the acceleration, except approximately by comparison with the absolute scale. The 
fact, however, that it does not pretend to absolute values seems a point in its favor under 
the circumstances. And it is a practical scale for the phenomena dealt with. 
Altho rigorous values can not be obtained by such means, it is desirable to subject the 
grades to careful comparison with the numbers of the Omori scale in order to determine 
reasonably close acceleration values for them. 
A comparative study of the 3 scales is summarized diagrammatically in the accom- 
panying table at the top of next page.* 
Some of the effects which serve to define Grade A are weaker than the maximum effects 
defining No. 6 of the absolute scale; and nowhere, not even in the vicinity of the fault, 
were most buildings totally destroyed. 
Grade B covers a wide range. Perhaps if the initial shock had been a little stronger; 
it could have been subdivided with some certainty. 
Grades C and D cover each a slightly lower range of values than the scale numbers 3 
and 2, to which they correspond most closely. 
Grade E, as defined, is more narrowly limited than No. 1. 
These values, despite their lack of precision, constitute the best approximation to an 
absolute measure of energy developed, for each grade of intensity, which it appears prac- 
ticable to attain. There were no instruments of precision to record the character and 
amount of the motion of the shock, hence estimates of other sort than this seem difficult 
to make. The fact must not be lost sight of, however, that it is only an estimate, based 
upon the interpretation of a series of destructive effects produced in very variable media 
under variable conditions and then compared with a similar series of destructive effects 
produced in structures of a different sort, for which pretty accurate acceleration values had 
been determined experimentally. 


1 The definitions of the Omori absolute scale and the information about it are taken from the book 
on Earthquakes in the Light of the New Seismology, by Major C. E. Dutton. 
Q 
