Evening star., 1932-10-10
BY N. ii. HECK,

BY n. IL. HECh,
Chief Division of Terrestrial Magnetism and
Selsmology, United States Coast and
Geo0etic Survey.
In many parts of the earth forces are
at work producing stresses which will
eventually grow SO great that they will
exceed the strength of the rocky crust.
The resulting break, or, more probably,
slipping along the surface of previous
break, will cause an earthquake and
will also relieve the stress, at least
partially
Prediction can go little farther and
state that such occurrences will be more
probably in some places than others,
but when it comes to definite time and
place it is another matter.
One difficulty is that in many cases
the breaking point is approached SO
slowly that there is long period when
an accidental application Of small
force can cause an actual earthquake.
However. by selection of places where
severe earthquakes have occurred and
where repetition would cause great dam
age it becomes economically possible to
study the earthquake activity and re-
lated phenomena which may tell us
eventually whether or not there Is a
state of stress which may be dangerous.
Statistics Important.
In order to 6eal with earthquakes,
they must be better understood. Col
lection of accurate statistics and accu-
rate location of earthquakes Is impor-
tant. Earthquake insurance must be
based on statistics and accurate loca-
tion is necessary to the association of
earthquake occurrence with geological
formation.
Satisfactory attaining of these ends
Is accomplished through the use of
highly sensitive seismographs and sys-
tematic collection of observations Of
witnesses.
Artificial earthquakes. due to eXplo-
sion as well as the earthquakes them
selves, tells us much about the earth
itself, which is not only the site of the
earthquakes but also of the structures
erected by man.
Earthquake study has begun to show
such possibilities that intelligent peo-
pie even in regions where it has been
customary to suppress reference to
earthquakes, are beginning to see that
such an attitude is Improbable and
dangerous.
A well known engineering magazine
Is carrying almost weekly reference to
QSSlSu VI UuiIllIllSS, UllOSCo diio OoAio
to resist earthquake damage. Insurance
companies are making minimum re-
quirements of earthquake resistance be-
fore irsuring large buildings.
Two Kinds of Stress.
Buildings are subject to two kinds Of
stress from earthquakes, and we are
not yet sure which is the more serious,
though in most cases both types are
present.
The first may be compared to ham
mer blow followed by rapid vibrations
of considerable extent. The other is
continued slower vibration of the earth,
with periods to which the buildings re-
spond so that they get into resonance
and sway to dangerous degree.
Principles of design to meet these
conditions are gradually being evolved,
both from observation of what has oc-
Cured, estimates of the forces at work
and testing of working models on a
large shaking platform, as at Stanford
University Work in the latter field
is still in its infancy.
Numerous buildings to resist earth
quakes have been constructed in Japan,
Italy, Greece, Mexico and the United
States None of these has as yet been
fully tested by earthquake.
One difficulty with interpreting
earthquake effects has been the lack of
exact measure of the forces acting.
This, it is hoped. will soon be corrected,
as instruments to record strong earth
motions have recently been installed by
the United States Coast and Geodetic
Survey in regions with history OF strong
earthquakes Hereafter when a build
ing designed to resist earthquakes has
passed through one we may hope to
know Just what forces it has withstood.