Regional Earthquake Return-Time Maps
Probabilistic Seismic Hazard and Earthquake Recurrence Times
The maps in this area depict the mean return time of earthquakes of a given size or greater within a fixed distance of each location on the map. The fixed distance is 50 km for all earthquakes except for plate-subduction earthquakes. For those, the fixed distance is 200 km. The data used to prepare these estimates are discussed in the U.S.G.S. Probabilistic Seismic Hazard Maps, 2002 revision for conterminous U.S., and U.S.G.S. Probabilistic Seismic Hazard Maps for Alaska (OFR 99-36) for Alaska.
The mean return time is defined as “one over the mean rate of earthquakes having these properties.” For example, if the mean rate of magnitude 6 within 50 km of site S is 0.001, the mean return time of a M6 or greater is 1/0.001 or 1000 years.
Spatially, the mean return time of significant earthquakes is highly variable over the U.S.A. We have tried to make maps for the central and eastern U.S. that use the same color contour scale as the maps for the western U.S. However, we find that most of the action in California is in the 10 to 100 year time frame, which has been assigned one color, brown. To help you see the variation in the 10 to 100 year time frame for California and a few other states, a second map of the M6 and greater sources is presented here to complement the map that uses a common color scale. This second maps uses several colors to distinguish intervals in the 10 to 100 year time frame, and uses just one color for return times greater than 750 years.
Currently, these maps are available for the conterminous U.S. and for Alaska. We can make similar maps for Hawaii and Puerto Rico if we learn there is a demand for them.
In an approximate sense, and allowing for location uncertainty and for variations in site response, building codes, and other factors, you might expect some light damage from a magnitude 6 or greater earthquake in the western U.S. at some sites within 50 km of the source. Similarly, you might expect some light damage from a magnitude 5.5 or greater earthquake in the central and eastern U.S. at some sites within 50 km of the source. You can find our estimates of the mean return time for such events by choosing the appropriate map. Other maps are presented for completeness, such as C.E.U.S. sources within 50 km and with body-wave magnitude greater than 5. These maps show details of smaller sources that have some historical basis but which do not necessarily extrapolate to higher magnitude with any appreciable frequency or rate. For example, you can see some action along Lake Erie near Perry, Ohio that is not visible in the higher-magnitude maps.
These maps are close relatives of the earthquake probability maps, or EPMs, that we have provided since 2005. We have learned that many people can understand return time better than probability, and we present these maps for those people.
