underground losses, for example damage to nearby petroleum wells, could also be analyzed. The concept of risk involves predicting the effect of induced ground motions, and perhaps fault slip, on structures and humans. If structures can incur moderate or heavy damage, risk involves predicting the effect of that damage (e.g., structural collapse) on humans in the vicinity.
Note that risk involves loss caused by structural damage, including effects on humans. If no structures or other constructed facilities are present, for example because the causative earthquakes occur in an uninhabited area, there is no risk. (Exceptions always exist to these general statements. One case would be an earthquake causing a rock slide that injures hikers in a national park, with no structure involved, but such cases would be rare exceptions.) The concept of risk could also be extended to include frequently occurring ground shaking that is a nuisance to humans (in the general, rather than legal, sense).
Factors Affecting Hazard
A set of questions can be addressed to understand and possibly quantify the hazard and risk associated with induced seismicity associated with energy technologies (Figure 5.1). Descriptions of each question are as follows:
1. Does an energy technology at a particular location generate apparent seismic events (meaning those that are felt at the surface)? The large majority of activities associated with hydrocarbon production do not cause any apparent seismic events. If no seismic events are recorded, this may be because the seismic events are too small (e.g., M < 0.0) to be recorded by regional seismic instruments, but the effect is the same: there is no apparent seismic activity.
2. Does an energy technology at a particular location generate just microseisms, or microseisms and earthquakes? This question involves the size of seismic events that are associated with the energy technology. Microseisms (by definition, seismic events with M < 2.0) generally do not produce ground motions strong enough to have an effect on structures, but they can in cases of close proximity be felt by humans at the surface. For example, two shallow (~2 km deep [1.2 mile deep]) seismic events of M 1.5 and M 2.3 in Blackpool, England, were reported by a number of people to have been felt in April and May 2011 (BGS, 2011).
3. Can earthquake shaking be felt at the surface? Not all earthquakes are felt at the surface. Earthquake ground motions at the surface depend on the size (magnitude) of the event and its depth, among other factors. The deeper the earthquake, the larger it must be to cause ground motions at the surface that can be felt by humans. Very shallow seismic activity (e.g., 2 km) has a higher hazard of causing felt ground motions than deeper activity.