Estimating Magnitude and Location of Alaskan Earthquakes Using Intensity Data

Abstract

Abstract I have developed relationships describing the attenuation of Modified Mercalli Intensity (MMI) with distance for three regions of Alaska (south-central, interior, and southeast). The sparse intensity data precluded development of site-specific corrections similar to those determined in other regions of the United States. I tested the intensity attenuation relationships on sets of calibration events, events with well-determined locations and instrumentally determined magnitudes, for south-central and interior Alaska (sufficient events were not available for the southeast region). My comparisons indicate the attenuation relationships gave event intensity centers within 50 to 60 km of event epicenters and magnitude estimates within 0.35 to 0.44 magnitude units of instrumentally determined magnitude. I then used these attenuation relationships to analyze historic earthquakes occurring between 1899 and 1965 in the three regions that had poorly determined epicenters and/or magnitudes, including eight events with no previously determined magnitude values. Most intensity centers for earthquakes with poorly known epicenters lay within their estimated epicentral error ellipses. For historic earthquakes with known instrumental magnitudes, magnitude estimated from intensity differed by an average of 0.5 (south-central), 1.0 (interior), and 0.05 (southeast) magnitude units. These results indicate that although intensity data in Alaska are sparse, they can be used to estimate earthquake locations and magnitudes. As additional intensity data become available, the three broad regions of Alaska can be further subdivided to reflect areas of similar geology and stress orientation and new attenuation relationships can be developed. I also anticipate that additional intensity data will allow the development of site-specific corrections to further improve epicentral and magnitude estimates from MMI data.