we were give a puzzle problem to solve and here it is..

"And the Earth Moved"
Seismographs are sensitive instrument for detecting earthquakes. They consists of large mass suspended by springs. When an earthquake happens, the disturbance travels through the earth, making the instrument vibrate. However, the mass tends not to move because of its inertia. This difference in movement is simplified and written out as a trace.

can you explain why most places in the world receive two traces for each single earthquake event? Can you also explain why a few places detect only one?

Thanks in advance 🙂

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