Causes of induced earthquakes
A variety of human activities can induce earthquakes. The graph below shows the proportions of cases each activity contributes to HiQuake.
Maximum magnitude earthquakes
What is the largest earthquake a project can induce? We do not know. In theory, earthquakes of any magnitude could be induced by human activity, since nearly all of the energy released by large earthquakes is natural in origin and the activity may only be the final trigger. Indeed, large earthquakes occur naturally with no human intervention. The graph below shows the maximum observed magnitudes to date for various types of project.
Number of cases per magnitude range
The graph below shows the number of cases in HiQuake by magnitude range. The different lines represent different project types as defined by the key. The most commonly reported induced earthquakes have magnitudes 3 ≤ M < 4. These cases are dominated by mining and surface water impoundment projects. The graph suggests underreporting of small-magnitude induced earthquakes.
Injection volumes and maximum magnitudes
One of the most successful theories for determining the maximum-magnitude earthquake an injection project may cause is that of McGarr (2014), who proposed that the maximum-magnitude earthquake for fluid-injection projects may be limited by the injected fluid volume. This theoretical limit holds for most cases, but it does not explain why some projects do not achieve their predicted maximum magnitude.