Earthquake Detective

Find the earthquake. Choose a scenario and investigate it using three seismic stations. Read the P and S waves, estimate source distance, test whether depth matters, locate the epicenter and experiment with how local ground conditions change the recorded shaking.

Teaching model. Simplified, approximately constant seismic-wave speeds keep the geometry visible. Real seismic-wave speeds vary through Earth.

1. Read the seismograms

Start an earthquake, then identify the first P-wave arrival and the later S-wave arrival at each station. Pick the beginning of each new phase, not the largest wiggle.

What are P and S waves?

P wave · Primary wave

P waves are compressional. Particles move back and forth in the same direction that the wave travels. P waves are fastest, so they arrive first.

They can travel through solids, liquids and gases. In this simplified model, P waves travel at about 6 km/s.

S wave · Secondary wave

S waves are shear waves. Particles move perpendicular to the direction that the wave travels. They are slower than P waves, so they arrive later.

S waves travel through solids, but not through liquids. In this simplified model, S waves travel at about 3.4 km/s.

The detective clue: P arrives first. S arrives later. The farther the earthquake is from a station, the larger the P–S arrival-time gap becomes.

Picking tip: Mark the start of a new seismic phase, not the largest wiggle. In a nearby case, the S wave can arrive while P-wave motion is still visible.

Station A

Relative recorded motion: —

ALP

Ground beneath Station A

Firm ground gives a moderate site response.

0 s 30 60 90 120 s

P: ? S: ? P–S gap: ?

Station B

Relative recorded motion: —

BRK

Ground beneath Station B

Firm ground gives a moderate site response.

0 s 30 60 90 120 s

P: ? S: ? P–S gap: ?

Station C

Relative recorded motion: —

CRS

Ground beneath Station C

Firm ground gives a moderate site response.

0 s 30 60 90 120 s

P: ? S: ? P–S gap: ?

Detective shortcut In this simplified model, source distance ≈ P–S gap × 8 km/s. The 8 km/s value is a conversion factor derived from the approximate P- and S-wave speeds. It is not the speed of either seismic wave.

2. Turn the P–S gap into source distance

The greater the separation between the P and S arrivals, the farther the waves have travelled.

If the earthquake source has depth, this is the sloping distance from the station to the hypocenter.

Model simplification: Earthquake Detective assumes approximately constant wave speeds of 6 km/s for P waves and 3.4 km/s for S waves so the relationship is easy to investigate. Real seismic-wave speeds vary through the Earth.

Small gap → nearer Large gap → farther

3. Draw the circles

The first map uses the measured source distances directly as circle radii. Scenario A should work cleanly. In deeper scenarios, something interesting happens.

A · Firm ground

B · Firm ground

C · Firm ground

N ↑

100 km

Site geology Bedrock Firm ground Soft sediment

Choose a scenario and start the earthquake.

What do the circles mean?
With a 0 km source depth, source distance and horizontal map distance are the same. Once the source moves underground, they are not.

4. Test the depth

Start at 0 km. If the circles already agree, no depth correction is needed in this idealized case. If they miss, increase the assumed depth and watch the horizontal-distance circles change.

Scenario Not started

Magnitude ?

Station A distance ?

Station B distance ?

Station C distance ?

Estimated depth ?

Epicenter ?

Hypocenter ?

Assumed source depth 0 km

0 km 130 km 260 km

Convert the P–S gaps first.

5. Locate the epicenter

Once you are satisfied with your depth estimate, use the corrected circles to place the epicenter.

Choose a scenario and start the earthquake.

A note about Scenario A Scenario A is deliberately an idealized surface-source model with a depth of 0 km. It is there to demonstrate the simple distance-circle method. Real tectonic earthquakes begin underground.

Earthquake scenarios

1. Read the seismograms

P wave · Primary wave

S wave · Secondary wave

3. Draw the circles

4. Test the depth

5. Locate the epicenter

Same earthquake. Different site response.

Take Earthquake Detective into the classroom.