Seven identifible earthquakes are superimposed on the tides. The above seismogram was recorded at one sample per second from a Geotech KS54000-0103 broadband seismometer at 119 meter depth in the Glastnost Borehole near Leonard, Oklahoma. The trace represents vertical ground acceleration. Like Ocean tides, earth tides are caused by the gravitational attraction of the sun and moon, which, at any given location, changes as the earth rotates.
Several earthquakes show up as spikes.
The earthquake's individual waves are only seen if the trace is spread out. The Ocean generated microseisms, with a repitition period of only about six seconds, are different from tides, which repeat in 12 hours and 20 minutes. The constant up and down of oceanic microseisms thickens the tide trace into a band. This leaves only tides and long period free vibrations of the earth. This trace shows almost exclusively earth tides.