Epicenter — Science definition

Defined in 2 courses:

Where it is taught

In use

The exact spot underground where the rupture begins is the focus (also called the hypocenter), and the point on the surface directly above it — usually where the shaking is strongest — is the epicenter.

Earthquakes & Volcanoes

Questions using epicenter

Problem 1

What is the epicenter of an earthquake?
  1. The point underground where the rupture starts
  2. The point on the surface directly above the focus
  3. The instrument that records shaking
  4. The slowest-moving seismic wave
Answer and explanation

The point on the surface directly above the focus

The epicenter is the point on Earth's surface located directly above the focus (hypocenter), where the rupture actually begins underground.

Problem 2

Two seismograph stations record the same earthquake. Station A shows a big time gap between the P-wave and S-wave arrivals. Station B shows almost no gap. Which station is farther from the epicenter, and why?
Answer and explanation

Station A is farther away. P-waves always travel faster than S-waves, so the longer the waves travel, the more the P-wave pulls ahead — creating a bigger gap between their arrival times.

Problem 3

In a stated travel-time model, P arrives at 50 s and S at 86 s at one station. Calculate the S–P gap. Explain why neither that gap nor the epicenter alone establishes shaking damage at every town.
Answer and explanation

The S–P gap is 36 s. Distance inference needs the appropriate travel-time model; direction requires other stations. Damage also depends on local intensity, depth, ground and structures, so one gap or surface location is insufficient.

The S–P gap is 36 s. Distance inference needs the appropriate travel-time model; direction requires other stations. Damage also depends on local intensity, depth, ground and structures, so one gap or surface location is insufficient.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the physical mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

Problem 4

Independent chapter check: In a stated travel-time model, P arrives at 50 s and S at 86 s at one station. Calculate the S–P gap. Explain why neither that gap nor the epicenter alone establishes shaking damage at every town.
Answer and explanation

The S–P gap is 36 s. Distance inference needs the appropriate travel-time model; direction requires other stations. Damage also depends on local intensity, depth, ground and structures, so one gap or surface location is insufficient.

The S–P gap is 36 s. Distance inference needs the appropriate travel-time model; direction requires other stations. Damage also depends on local intensity, depth, ground and structures, so one gap or surface location is insufficient.

Response criteria

  • Use the stated measurements or conditions, with correct units.
  • Connect the result to the physical mechanism or evidence relationship.
  • Identify the assumption, model limit or further evidence requested.

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