Recent earth tides near Leonard Oklahoma

The above seismogram shows several days of three-component (cyan=vertical,
yellow=north-south, magenta=east-west) earth motion. The signals are from a Guralp
three-component. digital, borehole seismometer, model CMG1-TDB. The seismometer
is at 840 meters depth in the borehole, which puts it about 500 feet into
the Cambro-Ordivician Arbucke dolomite.

Earth tides, like ocean tides, are caused by the gravitational attraction
of the sun and moon on the rotating earth. The ocean tides are similar, but
they vary from place to place according to the shape of the shorline. What we
commonly call "TIDE" is the DIFFERENCE between the ocean tides and the earth
tides seen at shorelines.

The slow undulating curves, with a maximum about each 12 hours, 20 minutes
are the solid earth tides.

Earthquakes (some of which may be labelled) are so compressed on this
seismogram that they show a single spike, or for the larger quakes,
a spike tapering off for minutes or houra.




Earth tides near Leonard Oklahoma for 2003 JUL05 through JUL27



TWENTY-THREE DAYS FILTERED EARTH TIDES

The above seismogram has been decimated and filtered as if it had been
sampled only once each thousand seconds. Only the solid earth tides show;
earthquakes are eliminated by this filtering.

Notice:

1. Each component (vertical, north-south, and east-west ground motion) is
   different.

2. Each component is asymmetrical. The east-west is more like a series of Ms
   (MMMMMM) than it is like a sine wave. The vertical is similar but alternate
   peaks of the Ms are high and low. The north-south is nearest to symmetrical.

3. The shape and amplitude of the tides varies throughout the month, as the
   relative position of moon and sun changes. The highest tides are called
   "spring" tides. Spring is taken from a German word, and has nothing to
   do with the season of the year. Spring tides tend to occur at new and
   full moon. At new and full moon, the gravitation of sun and moon are in
   the same direction. The tide raising force of these two bodies adds, even
   at new moon, when the sun and moon are on opposite sides of the earth.
   Neap tides occur at quarter moon, when the sun and moon are at right angles
   with respect to the earth.

4. The time span between any two peaks or any two troughs is 12 hours and
   twenty minutes throughout the lunar month.

three-dimensional earth tide near Leonard, Oklahoma

The above seismogram shows three dimensional motion of the earth due to tides
near Leonard Oklahoma over a 25 hour period. The period is on July 12 and 13,
2003. This interval was cut from the 23 day seismogram previously shown. This
25 hour interval is near full moon. Instead of a seismogram showing motion
in one direction as time advances, the above seismogram shows the position of
the borehole seismometer. This type of seismogram is also called a "particle
motion plot".

The seismogram above can be turned to give two-dimensional motion
looking in different directions, as shown below.

two-dimensional earth tide near Leonard, Oklahoma


WHY ARE THERE TIDES IN A NORTH-SOUTH DIRECTION?

It may appear that as the moon revolves around the earth and the earth rotates,
the only differences would be east-west. However, even though they are smaller,
there ARE north-south tides. If the earth's and moon's path were all in the
same plane, and that plane passed through the sun, there might not be a north-
south tide. However the moon's orbit is inclined somewhere between 18.5
degrees and 28.5 degrees to the earth' equator. This means that at times
the moon will be have a northward force and sometimes a southward force on 
the earth. The moon's distance from the earth varies throughout the lunar
month.

Everything about the moon and sun which varies (direction of the moon, 
elevation of the moon above or below earth's equator, distance of the
moon from the earth, distance from sun to earth, etc. etc.), creates a 
different COMPONENT of the tidal force. Three hundered components have
been calculated by Doodsan. Usually at least the largest thirty or so must
be considered to calculate a realistic tidal curve by computer.


WHAT IS ACTUALLY RECORDED?

The borehole seismometer detects earth VELOCITY, not displacement or position.
When the vertical seismogram is at the top, that represents the fastest change
in position, not the highest level. The vertical change in position at Leonard
is about 30 centimeters during each six hours and ten minutes. We do not
notice this because the change is very slow and spread over a very large
area. 

The vertical recordings probably represent most importantly, the variation 
of gravity as the rising and falling earth tides carry the seismometer
farther, then closer to the earth's center. The actual highest position
of the earths surface is represented by the zero crossing of the trace as it
moves downward. The actual velocity of motion probably adds something to the
vertical trace.

The horizontal traces (north-south and east-west) probably represent tilt (under
one arc second), and perhaps some sideways motion. 


WHY THE EARTH TIDES ARE BEST RECORDED AT 100 OR MORE METERS BELOW
THE SURFACE

Near the surface, a seismometer can not usually detect earth tides, because
they are overwhealmed by temperature drifts inside the seismometer case. At the
depth of 840 meters (2755 feet), the Guralp CMG1-TBD seismometer is at
a constant internal temperature of 51.2 degrees celsius (124.2 degrees
fahrenheit). Measurements in a nearby borehole suggest that the rock 
temperature at 850 meters is about 42.8 degrees celsius (109 degrees
fahrenheit). In the top of the seismometer case, a small computer,
and the analog electronics in the seismometer modules, constantly use 119
milliamps of 12 VDC power. The heat produced by this power dissipation
raises the 42 degrees celsius in the surrounding rock to 51 degrees celsius
inside the seismometer.

In the link below, note that the gravimeter recording vertical earth tides
near Pecney, Czech Republic, has extremely precise electronic temperature
control, which allows it to operate in a near surface vault.



See last 48 hours vertical
earth tide detected at Pecny by the Czech Technical University

See a 48 hour tide record made by gravimeter in Tulsa, Oklahoma in 1939.

EARTHQUAKE CATALOG INDEX HOME SEISMOGRAM INDEX