Overestimation of epicentral distances for earthquakes in the middle-America region from Ann Arbor, Michigan, seismograms

Abstract

A re-examination of Ann Arbor, Michigan (AAM), seismograms for 18 earthquakes occurring in the middle-America region (between latitudes 5° and 20°N and longitudes 80° and 105°W) during 1967 and 1968 reveals that, on the long-period seismograms, more than one distinct arrival is recorded within 30 sec of the Jeffreys-Bullen (J-B) S arrival time. The earliest of these arrivals, at the J-BS time, has the smallest amplitude. For earthquakes of magnitude (m_b) smaller than about 5.5, the amplitude of the arrival at the J-BS time on the AAM long-period seismograms (magnification 1,500) is so small that it is frequently overlooked during routine interpretation of seismograms. In such cases, one of the subsequent arrivals is interpreted as the S phase, and the epicentral distances are overestimated by 3° to 5° using J-BP and S tables. Examination of seismograms published in the literature suggests that epicentral distances of small magnitude earthquakes occurring in the middle-America region may be overestimated at other North American stations during routine interpretation of seismograms using J-BP and S tables.