Strainmeter
Encyclopedia
A strainmeter is an instrument used by geophysicists to measure the
deformation of the Earth.
Linear strainmeters measure the changes in the distance between two points,
using either a solid piece of material (over a short distance)
or a laser
interferometer (over a long distance, up to several hundred meters).
The type using a solid length standard was invented by Benioff in 1932,
using an iron pipe; later instruments used rods made of fused quartz
.
Modern instruments of this type can make measurements of length changes over
very small distances, and are commonly placed in boreholes to measure
small changes in the diameter of the borehole.
Another type of borehole instrument detects changes in a volume filled with
fluid (such as silicone
oil).
The most common type is the dilatometer
invented by Sacks and Evertson in the USA
(patent 3,635,076);
a design that uses specially shaped volumes to measure the strain tensor
has been developed by Sakata in Japan.
All these types of strainmeters can measure deformation over frequencies
from a few Hz
to periods of days, months, and years. This allows them to measure
signals at lower frequencies than can be detected with seismometers.
Most strainmeter records show signals from the earth tide
s, and seismic wave
s
from earthquake
s.
At longer periods, they can also record the gradual accumulation of stress (physics)
caused by plate tectonics
, the release of this stress in earthquake
s,
and rapid changes of stress following earthquakes.
The most extensive network of strainmeters is installed in Japan
;
it includes mostly quartz
-bar instruments in tunnels and borehole strainmeters,
with a few laser instruments.
Starting in 2003 there has been a major effort (the Plate Boundary Observatory)
to install many more strainmeters along the Pacific/North-America plate boundary
in the United States
.
The aim is to install about 100 borehole strainmeters,
primarily in Washington, Oregon
and California
, and five laser strainmeters,
all in California
.
deformation of the Earth.
Linear strainmeters measure the changes in the distance between two points,
using either a solid piece of material (over a short distance)
or a laser
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
interferometer (over a long distance, up to several hundred meters).
The type using a solid length standard was invented by Benioff in 1932,
using an iron pipe; later instruments used rods made of fused quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
.
Modern instruments of this type can make measurements of length changes over
very small distances, and are commonly placed in boreholes to measure
small changes in the diameter of the borehole.
Another type of borehole instrument detects changes in a volume filled with
fluid (such as silicone
Silicone
Silicones are inert, synthetic compounds with a variety of forms and uses. Typically heat-resistant and rubber-like, they are used in sealants, adhesives, lubricants, medical applications , cookware, and insulation....
oil).
The most common type is the dilatometer
Dilatometer
A dilatometer is a scientific instrument that measures volume changes caused by a physical or chemical process. A familiar application of a dilatometer is the mercury-in-glass thermometer, in which the change in volume of the liquid column is read from a graduated scale...
invented by Sacks and Evertson in the USA
(patent 3,635,076);
a design that uses specially shaped volumes to measure the strain tensor
has been developed by Sakata in Japan.
All these types of strainmeters can measure deformation over frequencies
from a few Hz
Hertz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....
to periods of days, months, and years. This allows them to measure
signals at lower frequencies than can be detected with seismometers.
Most strainmeter records show signals from the earth tide
Earth tide
Earth tide is the sub-meter motion of the Earth of about 12 hours or longer caused by Moon and Sun gravitation, also called body tide which is the largest contribution globally. The largest body tide contribution is from the semidiurnal constituents, but there are also significant diurnal...
s, and seismic wave
Seismic wave
Seismic waves are waves of energy that travel through the earth, and are a result of an earthquake, explosion, or a volcano that imparts low-frequency acoustic energy. Many other natural and anthropogenic sources create low amplitude waves commonly referred to as ambient vibrations. Seismic waves...
s
from earthquake
Earthquake
An earthquake is the result of a sudden release of energy in the Earth's crust that creates seismic waves. The seismicity, seismism or seismic activity of an area refers to the frequency, type and size of earthquakes experienced over a period of time...
s.
At longer periods, they can also record the gradual accumulation of stress (physics)
Stress (physics)
In continuum mechanics, stress is a measure of the internal forces acting within a deformable body. Quantitatively, it is a measure of the average force per unit area of a surface within the body on which internal forces act. These internal forces are a reaction to external forces applied on the body...
caused by plate tectonics
Plate tectonics
Plate tectonics is a scientific theory that describes the large scale motions of Earth's lithosphere...
, the release of this stress in earthquake
Earthquake
An earthquake is the result of a sudden release of energy in the Earth's crust that creates seismic waves. The seismicity, seismism or seismic activity of an area refers to the frequency, type and size of earthquakes experienced over a period of time...
s,
and rapid changes of stress following earthquakes.
The most extensive network of strainmeters is installed in Japan
Japan
Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...
;
it includes mostly quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
-bar instruments in tunnels and borehole strainmeters,
with a few laser instruments.
Starting in 2003 there has been a major effort (the Plate Boundary Observatory)
to install many more strainmeters along the Pacific/North-America plate boundary
in the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
.
The aim is to install about 100 borehole strainmeters,
primarily in Washington, Oregon
Oregon
Oregon is a state in the Pacific Northwest region of the United States. It is located on the Pacific coast, with Washington to the north, California to the south, Nevada on the southeast and Idaho to the east. The Columbia and Snake rivers delineate much of Oregon's northern and eastern...
and California
California
California is a state located on the West Coast of the United States. It is by far the most populous U.S. state, and the third-largest by land area...
, and five laser strainmeters,
all in California
California
California is a state located on the West Coast of the United States. It is by far the most populous U.S. state, and the third-largest by land area...
.