Earthquake Rupture Sizes

Earthquakes occur on faults, but usually only part of the fault slides during one particular earthquake. We say that the segment of the fault that moved "ruptured". In general, larger earthquakes produce larger ruptures.

But how large is an earthquake rupture? The long answer is it depends on factors such as the fault type, the fault hsitory, and the geologic environment. So we can't write down an accurate formula that would work in all cases. We can provide typical (in the ballpark) values based on some empirical relationships. For example, an early formula for subduction zones [based on observations from near Japan and described in Utsu and Seki (1955)] suggests that the

log10(Rupture Area) ~ M – 4.0

where the Rupture Area is in square kilometers and M is the magnitude. The values are rough, so don't agnonize over factors of two in these estimates, and don't imagine that earthquakes actually rupture perfectly circular patches. Sometimes they are roughly circular, sometimes they are much longer than they are wide. For example, magnitude 7 and larger earthquakes on faults such as the San Andreas and magnitude 8 and larger earthquakes on subduction zone plate boundaries are usually better envisioned as roughly rectangular ruptures. Here are some typical equivalent diameters:

Mw Diameter
0 10 m
1 35 m
2 100 m
3 350 m
4 1 km
5 3.5 km
6 10 km
7 35 km
8 100 km

 

For experts, you can compute similar vaules by using the standard formulas for a circular fault and assuming a 3 MPa stress drop earthquake occuring in a material with a shear-modulus of 30 GPa.

 

For more information, please see the list of Seismology Texts or the list of popular-science books on earthquake science.