which is 2,489 square miles). In contrast, M 3 earthquakes typically have rupture areas of roughly 0.060 km2 (about 0.023 square miles or about 15 acres, equivalent to about 15 football fields). “Felt earthquakes” are generally those with M between 3 and 5, and “damaging earthquakes” are those with M > 5. The maximum velocity of ground shaking is a measure of how damaging the ground motion will be near the fault causing the earthquake. The intensity of shaking at any location is usually expressed using the Modified Mercalli scale and varies from III10 (felt by few people and would cause hanging objects to sway) for M 3, to X (when severe damage would occur). A large earthquake located onshore will generate intensity X near the fault rupture, intensity III at far distances, and all intensities between at intermediate distances.
Most earthquakes, whether natural or induced, that are recorded by seismometers are too small to be noticed by people. These small earthquakes are often referred to as microearthquakes or microseisms. This report adopts the latter term for all seismic events with magnitude M < 2.0. Microseisms as small as M -2 (see Appendix E for an explanation of negative magnitudes) are routinely recorded by local seismometer arrays during hydraulic fracturing operations used to stimulate oil and gas recovery. At M -2 the rupture areas are on the order of 1 m2 (a little less than 11 square feet).
Most naturally occurring earthquakes occur near the boundaries of the world’s tectonic plates where faults are historically active. However, low levels of seismicity also occur within the tectonic plates. This fact, together with widespread field measurements of stress and widespread instances of induced seismicity, indicate that the Earth’s crust, even in what we may consider geologically or historically stable regions, is commonly stressed near to the critical limit for fault slip (Zoback and Zoback, 1980, 1981, 1989). Because of this natural state of the Earth’s crust, no region can be assumed to be fully immune to the occurrence of earthquakes.
Induced seismicity may occur whenever conditions in the subsurface are altered in such a way that stresses acting on a preexisting fault reach the breaking point for slip. If stresses in a rock formation are near the critical stress for fault rupture, theory predicts and experience demonstrates that relatively modest changes of pore fluid pressures can induce seismicity. Generally, induced earthquakes are not damaging, but if preexisting stress conditions or the elevated pore fluid pressures are sufficiently high over a large fault area, then earthquakes with enough magnitude or intensity to cause damage can potentially occur.
Identifying whether a particular earthquake or microseism was caused by human activity or occurred naturally is commonly very difficult; often, inferences are made based on spatial and temporal proximity of the earthquake and human activity, on seismic history in the region, and on whether general models of induced seismicity would support a connection. For example, a small amount of fluid injected into the crust at shallow depths (e.g., during
10 The Mercalli scale uses Roman numerals.