Calculator · S–P time · classroom seismology

How far away was the earthquake? S–P time to distance

As of 2 November 2026, the classroom rule used by Saint Louis University's earthquake outreach exercise is to multiply the S–P time in seconds by 8 to get the distance to the earthquake in kilometres: a 10-second gap means about 80 km (50 miles), and distances from three stations locate the epicentre.

Work out the distance Compare magnitudes Earthquakes today

S–P time

About 80 km (50 miles)

10 seconds × 8 km per second of S–P lag.

Rule: Saint Louis University, Earth & Atmospheric Sciences outreach, Earthquake location exercise (multiply S–P by 8), checked 2026-11-02

How the rule works

P waves arrive first; S waves are slower and arrive later. The further away the quake, the bigger the gap.
Distance = S–P × (Vp × Vs) ÷ (Vp − Vs). With Vp 6 km/s and Vs 3.5 km/s that is S–P × 8.4 km.
Classrooms round the factor to 8 km (about 5 miles) per second.
One station gives a circle. Three circles from three stations cross at the epicentre.

Worked examplesS–P time to distance (× 8 km)

S–P time	Distance (km)	Distance (miles)	With Vp 6, Vs 3.5 (km)
1 s	8	5	8
2 s	16	10	17
5 s	40	25	42
10 s	80	50	84
15 s	120	75	126
20 s	160	99	168
25 s	200	124	210
30 s	240	149	252
40 s	320	199	336
50 s	400	249	420
60 s	480	298	504

A rule for nearby earthquakes. For distant ones, real wave speeds change with depth and seismologists use travel-time tables instead.

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What this page shows, and what it misses

It turns an S–P time into a distance from one station with the classroom factor of 8 km per second, or with wave speeds you choose. It does not read seismograms, find the direction, or work out depth or magnitude. Published locations of real earthquakes come from agencies such as the USGS and EMSC: see today's earthquakes.

Rule from Saint Louis University's earthquake outreach exercise.

FAQQuestions people ask

How do you find the distance to an earthquake from the S-P time?

Read the arrival time of the first P wave and the first S wave on a seismogram, subtract to get the S-P time in seconds, and multiply by 8 to get the approximate distance in kilometres (about 5 miles per second of lag). A 25-second S-P time means about 200 km.

Why multiply the S-P time by 8?

P waves travel faster than S waves, so the gap between them grows with distance. Distance = S-P time x (P speed x S speed) / (P speed - S speed). With typical crust speeds of about 6 km/s for P and 3.5 km/s for S, that factor is about 8.4 km per second, which classrooms round to 8.

How many stations do you need to locate an epicentre?

Three. One station gives only a distance, which is a circle around that station on the map. Draw circles from three stations and the epicentre is where they cross.

Is this how agencies such as the USGS locate earthquakes?

Not exactly. The 8 km rule is a teaching shortcut that works best for nearby earthquakes. Agencies use many stations and full travel-time models of the Earth, because wave speeds change with depth and the shortcut drifts at long distances.