ISOSEISMALS: DISTRIBUTION OF APPARENT INTENSITY, 

INTRODUCTORY. 
In the study of earthquakes the distribution of the intensity of the shock over the region 
affected is usually an important part of the investigation. ‘The intensity is inferred, as a 
rule, from the records of instruments established for the purpose, and from the effect 
upon persons, loose objects, and structures. In the region affected April 18, 1906, how- 
ever, seismograph instruments were very few, and the distribution of the intensity of the 
shock has been determined largely by the effects noted. These effects are graded in 
various convenient scales and the gradation of intensity is indicated upon maps in the 
form of lines or curves, known as isoseismal curves, which express, as well as the data 
available will permit, zones or belts of equal intensity more or less concentric to the point 
or line above the seat of disturbance. The purpose of plotting such isoseismal curves 
is to locate approximately that portion of the earth’s surface immediately above the seat 
of the disturbance. In a discussion of the ideal case, the latter is supposed to be a point 
or centrum, and the place above it at the surface is called the epicentrum. The increase 
in our knowledge of earthquakes in recent years has, however, made it clear that the seat 
of disturbance is rarely if ever a point, but is usually distributed over a plane of rupture 
in the earth’s crust. When this plane of rupture is of small extent, as frequently happens, 
the terminology is little affected by the use of the expressions centrum and epicentrum 
in the discussion of the phenomena; but where, as in the larger earthquakes, the plane 
upon which movement in the earth’s crust takes place has a great horizontal extent, 
then these terms become misnomers and tend to obscure the facts. In the present case 
the inappropriateness of the terms centrum and epicentrum is glaringly apparent and they 
will, therefore, be avoided in this discussion. 
In the case of the California earthquake, the plotting of isoseismal curves for the pur- 
pose of discovering the region on the surface above the seat of the disturbance is in a large 
measure obviated by the fact that the rupture in the earth’s crust is revealed at the sur- 
face in the form of a fault traceable practically continuously for 190 miles, and probably 
continuously for 270 miles. This fault is undoubtedly the principal seat of the movement 
which caused the earthquake. Notwithstanding this fact, the study of the distribution 
of intensity isa matter of importance. It is highly desirable, where the plane of rupture 
is open to the surface and its trace is definitely ascertained, to plot the isoseismal curves, 
since their disposition under these circumstances may illuminate the general method of 
determining the position of a deep-seated fault which causes an earthquake, but is not 
apparent at the surface. It may at least contribute to a definition of the limitations of 
the method. 
It is, moreover, desirable that the distribution of the intensity of the shock should be 
determined as accurately as possible, since we can not safely assume that the main fault, 
which appears as a rupture of the earth’s crust from San Benito County to Humboldt 
County, is the only one which occurred on the morning of April 18. Indeed there are 
a priori grounds for believing that more than one dislocation of the earth’s crust occurred 
at the time of this great disturbance of the equilibrium of the stresses within it. If, in a 
region where stresses have accumulated to nearly the snapping point, a rupture is sud- 
denly effected in one place, it seems probable that the jar thus generated might precipi- 
tate ruptures in neighboring parts of the region under similarly high stresses. It appears, 
therefore, to be highly desirable to plot the gradations of intensity for the region affected ; 
not to discover the trace of the main fault, which is well known, but to see if such grada- 
tions indicate auxiliary faults in neighboring territory which did not appear as ruptures 
160 

