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.
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.
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.
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.
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