For most people, earthquakes are something that happens relatively close to the Earth's surface. They break through the crust, shake towns and cities, and leave behind faults that geologists can map. The deeper parts of the planet are generally thought to behave differently. Under the intense pressures and temperatures found far below the crust, rocks are expected to deform slowly rather than fracture suddenly.
That assumption is why a cluster of earthquakes beneath northern Utah and southwestern Wyoming has attracted such attention. Some of these events originated tens of miles beneath the crust, in a part of the Earth where conventional earthquake theory suggests seismic ruptures should be extremely difficult. A series of recent studies has strengthened the case that these unusual tremors are not measurement errors but genuine mantle earthquakes occurring deep beneath the western United States.
