This diagram shows an earthquake along a fault. The focus of the
earthquake is where the energy is released underground. The
epicenter is the spot on the Earth’s surface directly above the
focus. USGS and NPS What Is an Earthquake? The expression “on
solid ground” is often used to describe something as stable.
Usually the solid ground underfoot seems very stable. But
sometimes it is not. "The ground seemed to twist under us like a
top while it jerked this way and that, and up and down and every
way," wrote a person describing the experience of being in the
large 1906 earthquake in San Francisco, CA. Earthquakes happen
as large blocks of the Earth’s crust move suddenly past one
another because of the force of plate tectonics. These blocks of
the Earth’s crust meet at cracks called faults. Sometimes those
pieces do not slide smoothly past one another. There can be
friction along the fault – jagged edges that snag the blocks of
rock. This makes it difficult for them to move past each other.
Sometimes they get st