A seismogram is interpreted when geologists look at squiggly lines made by a seismograph on a piece of paper, according to Michigan Technological University. Initial waves, called P waves, are small and close together because they travel quickly. More intense waves, called S waves, are next followed by surface waves that show up as the largest markings on a seismogram.
Before an earthquake occurs, a seismograph detects microseisms, very small, regular waves that nobody feels from far away. Heavy traffic, ocean waves, subway trains and the wind can also be the cause of microseisms.
When an earthquake begins, the seismograph picks up P waves. These waves are small and fast, but they are larger than microseisms. A seismogram indicates S waves next, which are larger and more intense than P waves.
The most prominent part of a seismogram during an earthquake is the area that shows surface waves. These types of markings are more spread out because the waves are a lower frequency than P waves and S waves. Surface waves travel more slowly yet are taller than previous marks on the seismogram. Earthquakes that occur near the Earth's surface manifest as the most intense waves recorded by a seismograph.
Seismographs are the actual mechanical sensors that detect earthquakes. A seismogram refers to the markings on a piece of paper made by a seismograph. A seismogram is what geologists interpret when determining whether or not an earthquake happened. Seismographs can detect earthquakes from thousands of miles away.