Earthquake Epicenter
The earthquake epicenter is the point on Earth's surface directly above the hypocenter (or focus), which is the underground location where an earthquake rupture begins. The epicenter is located using seismic wave arrival time differences recorded at multiple seismograph stations, with the distance to the epicenter calculated from the S-P wave time interval. The epicenter is the reference point used in earthquake reporting, and ground shaking intensity is generally greatest near the epicenter, decreasing with distance according to attenuation relations.
Key Formula
d = v(S-P) × Δt(S-P)
LaTeX: d = v_{S-P} \cdot \Delta t_{S-P}
| Symbol | Meaning | Unit |
|---|---|---|
| d | Distance from seismograph to epicenter | km |
| v_{S-P} | Effective velocity factor for S-P wave method | km/s |
| \Delta t_{S-P} | Time difference between S-wave and P-wave arrival | s |
Worked Example
Problem
A seismograph records a P-wave arrival at 10:00:00 UTC and an S-wave arrival at 10:00:50 UTC. Using the simplified Omori formula where distance (km) ≈ 8 × (S-P time in seconds), calculate the approximate distance from the station to the epicenter.
Solution
Step 1: Calculate the S-P time difference. Δt(S-P) = 10:00:50 − 10:00:00 = 50 seconds Step 2: Apply the Omori approximation. d ≈ 8 km/s × 50 s = 400 km Step 3: This calculation would be repeated for at least two more stations; the epicenter lies at the intersection of the three circles (trilateration).
Answer
Approximately 400 km from the recording station
Epicenter Location Parameters and Seismic Wave Properties
| Parameter | P-Wave | S-Wave | Surface Wave | Notes |
|---|---|---|---|---|
| Wave type | Compressional | Shear | Love/Rayleigh | Different particle motion |
| Typical speed (km/s) | 6–8 | 3.5–4.5 | 2–4 | Varies with medium |
| Travel through fluids | Yes | No | Surface only | Key distinction |
| Damage potential | Low | Moderate | High | Surface waves most destructive |
| Used for epicenter? | Yes (arrival 1st) | Yes (arrival 2nd) | No | S-P gap used for distance |
Interactive Tools
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Related Terms
Seismograph
A seismograph (or seismometer) is an instrument that detects and records ground motion caused by seismic waves, volcanic activity, explosions, or other disturbances. The basic principle relies on inertia: a heavy mass suspended by a spring remains relatively stationary while the instrument frame moves with the ground, and this relative displacement is amplified and recorded as a seismogram. Modern broadband seismographs use electromagnetic sensors or force-balance accelerometers and can detect ground motions as small as 10⁻¹⁰ m over a frequency range of 0.001–50 Hz.
Geological Fault
A geological fault is a planar fracture or discontinuity in rock across which significant displacement has occurred due to tectonic stresses. Faults are classified by the direction of relative motion: normal faults (extension, hanging wall moves down), reverse or thrust faults (compression, hanging wall moves up), and strike-slip faults (horizontal shear motion along the fault plane). The sudden release of accumulated elastic strain energy along a fault produces earthquakes, and repeated fault movements over geological time can build mountain ranges, create rift valleys, and shape landscape topography.
Tectonic Plate
A tectonic plate is a massive, irregularly shaped slab of solid rock composed of oceanic or continental crust together with the underlying upper mantle (lithosphere) that moves atop the semi-fluid asthenosphere. Earth's lithosphere is divided into seven major plates and several minor ones that move relative to one another at rates of 2–15 cm per year, driven primarily by mantle convection, slab pull, and ridge push. The movement of tectonic plates is responsible for earthquakes, volcanic activity, mountain building, and the distribution of continents over geological time.
From Greek "epi-" (upon, above) and "kentron" (centre, point). The term was adopted into seismology in the late 19th century to distinguish the surface point from the deeper underground focus. The related term "hypocenter" (from Greek "hypo-", below) refers to the actual underground rupture point.