U of U Seismograph Stations Research: The Tools for Constructing Earthquake Clocks- How Earthquake Recurrence Intervals are Estimated

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What is a "recurrence interval"? A recurrence interval is the average amount of time between individual earthquakes on any one fault. For instance, the recurrence interval for very large earthquakes (greater than magnitude 6.5) on the Wasatch Fault is about 400 years. That means about every 400 years an earthquake of this size occurs somewhere on the Wasatch Fault from Malad, Idaho to Gunnison, Utah. It is important to remember that this number is an average and that it has uncertainty associated with it. In this case, the average of 400 years has uncertainty of plus or minus 60 years. It is also important to remember that this 400 year recurrence interval is only for large magnitude 6.5 earthquakes. Smaller, but still damaging earthquakes such as magnitude 5.5, have shorter recurrence intervals. Measuring recurrence intervals by recording earthquakes using seismograph data The most reliable method of determining recurrence intervals between earthquakes is to have many years of seismograph data available. The accurate recording of earthquake events using instrumental monitoring allows scientists to determine the exact magnitude, time, and location of earthquakes. With this information, the earthquake can usually be assigned to the correct fault that it occurred on and the data used in determining the recurrence interval for that fault. This data is also the only way to find "blind faults" which are not exposed at the Earth's surface but are active and could have large earthquakes. Seismological data on recurrence intervals on the Wasatch Fault and in the Wasatch Front Region are both available because seismograph stations were established in Utah in the early 1960's. This type of data is the most reliable for determining recurrence intervals and estimating the risk of an earthquake. Historical Accounts of Earthquakes Prior to Seismographs Many earthquakes were recorded in Utah between the time of settlement and the introduction of seismograph stations in Utah. Seismologists have access to the information about time and general location by using newspaper accounts that describe the intensity of groundshaking in different parts of the same region and the time the individuals being interviewed felt the earthquake. The exact location of earthquakes cannot be determined by this method, particularly if the earthquake occurred in a remote region where no one was present to feel the effects and report them. Thus, this information gives scientists only a general idea of the earthquake magnitude and where it might have occurred. It is rare to be able to use historical accounts to assign the earthquake to a particular fault. Measuring recurrence intervals by detecting and dating prehistoric earthquakes On occasion, it is sometimes possible to use oral traditions of local Native Americans to determine if large earthquakes have happened in the recent prehistory of an area. Sometimes these accounts can be used to determine the year and the general region in which these earthquakes occurred. This information is of importance in determining long term intervals of major earthquakes for a region. More detailed information on recurrence intervals on specific faults, such as the Wasatch Fault, are gained by scientists studying large (greater than magnitude 6.5) prehistoric earthquakes that create small fault cliffs (or scarps). These cliffs form at the Earth's surface when the earthquake ruptures the fault all the way to the surface. The formation of these cliffs is often preserved in the rock record. Sometimes material datable by radiocarbon methods is preserved in the fault scarp. Scientists dig trenches along the fault to identify different earthquake events which have caused rupture of the Earth's surface and look to collect any datable material that might be present. By sampling the fault across many different fault cliffs that formed from several different earthquakes, the average time between these very large earthquakes can be determined. An example of how this method is used to read the rock record of faults can be found on the Wasatch Fault web page. The Big Unknown- the inability to determine recurrence intervals for highly damaging magnitude 6.0 to 6.5 earthquakes Earthquakes of magnitude 6.0 to 6.5 are potentially very damaging and could result in loss of life. However, because we have had only a few earthquakes of this size in historical time, and because they do not cause surface rupture and thus are not recorded in the rock record, we do not have a good estimate of their recurrence interval. These earthquakes often occur on "blind faults" that are not exposed at the Earth's surface. The existence of "blind" faults is often not known until they produce an earthquake and the seismograph recordings are used to determine their location within the Earth's crust. Constructing Earthquake Clocks All of the above techniques are used in combination to determine the recurrence intervals for a specific fault or for an entire region. The prehistoric data is vital for understanding the time periods in between the large earthquakes that occur infrequently. The historical accounts help to determine how frequently large earthquakes tend to occur within a region. The more recent recording of data by seismological instruments gives the most precise data that can be used to accurately determine size, time, and location of any earthquake and which provide the best data for determining recurrence intervals. The resulting calculations help scientists understand how the Earth works in that area and helps communities prepare and plan for possible earthquake events.