Focus (hypocenter)The rock first breaks here, on the fault. Scientists call it the hypocenter; focus is the older name.
EpicenterThe epicenter is on the ground directly above the focus. It is the place a map shows for an earthquake.
FaultA break along which blocks of rock on either side have moved. The surface they slip on is also called the fault plane.
Fault scarpA fault scarp looks like a step in the land. It is made by slip on the fault.
FootwallThe footwall lies under the fault. In this earthquake it stayed higher.
Hanging wallThe hanging wall lies over the fault. Here it has dropped, which makes this a normal fault.
Seismic wavesWhen the fault slips, the stored energy moves outward in all directions. It moves as seismic waves, like ripples on a pond.
SeismographIts base is fixed in the ground. A heavy weight hangs free. The record it writes is a seismogram, shown in the second view.
On the record 3
P wavesThe fastest seismic waves, so they reach a seismograph first. P stands for primary.
S wavesS waves are slower than P waves, so they arrive second. S stands for secondary. The gap between the two tells how far away the earthquake was.
Surface wavesThey travel along the ground, more slowly than P and S waves. They usually shake the hardest.
How this list was compiled: Every term checked on 2026-09-30 against the U.S. Geological Survey: the Earthquake Hazards Program glossary (hypocenter or focus, epicenter, fault, fault plane, fault scarp, seismic waves, body and surface waves), https://www.usgs.gov/glossary/earthquake-hazards-program; The Science of Earthquakes (fault, hypocenter, epicenter, seismic waves, P and S waves, seismograph and seismogram), https://www.usgs.gov/programs/earthquake-hazards/science-earthquakes; Seismographs: Keeping Track of Earthquakes, https://www.usgs.gov/programs/earthquake-hazards/seismographs-keeping-track-earthquakes; and the USGS block diagram of a normal fault (hanging wall and footwall), https://www.usgs.gov/media/images/normal-fault. Second reference: Wikipedia, Epicenter, Hypocenter, Fault (geology), Fault scarp, Seismic wave and Seismometer, text only. Licence: own work
The drawing: Drawn for this site on 2026-09-30 from plain geometry; nothing traced. The record is a drawing, not a real seismogram. Licence: own work
How it works
Label the parts of an earthquake on a block of ground cut open along a fault. The second view is the record a seismograph writes, with the three kinds of wave in the order they arrive.
Read the full guide
Underground first, then the record
The star is where the earthquake starts and the dot on the surface is straight above it. The sloping break is the fault; the block above it has dropped and left a step. Rings spreading from the star stand for the shaking. On the record, read left to right: the first small waves, the larger second waves, then the largest.
Focus is underground; epicenter is on top. Epi means upon.
On a sloping fault, the hanging wall is the block above the break and the footwall the block below it.
Questions and answers
What is the difference between the focus and the epicenter?
The focus, or hypocenter, is the place below the surface where the earthquake starts. The epicenter is the point on the surface directly above it.
What are the three types of seismic waves?
P waves, S waves and surface waves. P and S waves travel through the inside of the Earth. P waves are faster and arrive first. Surface waves travel along the ground and usually shake the hardest.
What is a fault?
A break in the rock along which the blocks on either side have moved. An earthquake happens when two blocks suddenly slip past one another along a fault.
What does a seismograph do?
It records the shaking of the ground. Its base moves with the ground while a heavy hanging weight stays still, and the difference is written down as a seismogram.