Plate Tectonics and Earthquake Simulator
Equations in this simulation
| v | Plate speed v (cm per year) | as fast as fingernails grow: the plates never stop, but the fault stays locked between earthquakes | |
|---|---|---|---|
| μ | shear modulus of rock | 30 GPa | |
| W | width of the fault that slips | 15 km down a vertical transform fault; 100 km down a sloping subduction zone, which is why subduction earthquakes are the largest | |
| Δσ | Stress the fault can hold Δσ (MPa) | earthquakes release 1 to 10 MPa of stress, whatever their size | |
| s | slip deficit (how far the fault has fallen behind the plates) | released as slip d in the earthquake; the time it takes to build up again is the recurrence interval |
With the current values:
| L | Rupture length L (km) | the length of fault that breaks at once | |
|---|---|---|---|
| M₀ | seismic moment, N·m | the size of an earthquake as energy, not as shaking | |
| Mw | moment magnitude | each step of 1 is 32 times more energy; 2 steps is 1,000 times |
With the current values:
| v_P, v_S | speeds of the P and S waves | P (pressure) waves 6.0 km/s, S (shear) waves 3.5 km/s in the crust | |
|---|---|---|---|
| D | Distance from the epicenter to the seismometer (km) | a single station knows how far away the quake was from the gap between the P and S arrivals; three stations find where |
With the current values:
How to use the earthquake simulator
- Two tectonic plates creep past each other a few centimeters a year, but the fault between them is locked. The road and fence across it bend as strain builds up, and the charts show the slip deficit and the stress on the fault growing year by year.
- When the stress reaches what the fault can hold, it slips all at once: an earthquake. The road snaps into an offset, the elastic rebound, and P waves (blue) and S waves (red) spread out from the epicenter. The seismometer records them, and the seismogram on the chart shows the P wave arriving first and the stronger S wave after it.
- Change the plate speed, the stress the fault can hold, the rupture length and the type of boundary, and read the magnitude from the seismic moment and the time between earthquakes. A subduction zone breaks a much wider fault, which is why the largest earthquakes, magnitude 9, happen there. Move the seismometer and see how the S − P time tells its distance.
Frequently asked questions
What causes an earthquake?
The plates of the Earth's outer shell move a few centimeters a year, but friction locks the faults between them. The rock on either side bends elastically, storing energy, until the stress overcomes the friction and the fault slips in seconds, releasing the strain built up over decades or centuries. This is the elastic rebound theory, proposed after the 1906 San Francisco earthquake, when fences and roads were found offset by up to 6 m.
How is earthquake magnitude measured?
Today by the moment magnitude, Mw = (2/3)(log₁₀ M₀ − 9.1), where the seismic moment M₀ = μ × area × slip: the rigidity of the rock times the area of fault that broke times how far it moved. Each step of 1 in magnitude is about 32 times more energy. A 200 km rupture with 1.5 m of slip is about Mw 7.4; the 2011 Tōhoku earthquake broke about 500 km by 200 km with up to 50 m of slip, Mw 9.1.
What are P and S waves?
The two kinds of seismic wave that travel through the body of the Earth. P (primary) waves are compressions, like sound, and arrive first at about 6 km/s in the crust. S (secondary) waves shake the ground sideways at about 3.5 km/s and are usually stronger. S waves cannot pass through liquid, which is how the Earth's outer core was found to be molten.
How is the distance to an earthquake found?
From the gap between the P and S arrivals at a seismometer. Both left at the same moment, so the gap grows with distance: D = (t_S − t_P) ÷ (1/v_S − 1/v_P), about 8 km for every second of gap in the crust. One station gives a distance, a circle on the map; three stations give three circles that cross at the epicenter.
It says WebGL is turned off.
The 3D view needs WebGL, which every current browser has. It can be switched off by hardware acceleration being disabled in the browser settings, or by a very old graphics driver. Turn hardware acceleration on, or try another browser.
Is anything uploaded?
No. The simulation is drawn by your own browser with WebGL; nothing is sent anywhere, and it keeps working offline once the page has loaded.