Empirical Distance Attenuation and the Local-Magnitude Scale for the Central United States

Abstract

Seismic waveform data from the Cooperative New Madrid Seismic Network (CNMSN), the U.S. National Seismograph Network (USNSN), and stations operated by the U.S. Geological Survey and the Incorporated Research Institutions for Seismology (IU) are corrected to the theoretical Wood–Anderson response, and the peak amplitudes are used to determine a local-magnitude scale for the central United States. The distance-correction function can be expressed as ![Graphic][1] , with amplitude A in millimeters and hypocentral distance r in kilometers, showing a weak attenuation with distance. Synthetic seismograms are calculated based on a detailed crustal model obtained from local well log acoustic data in the Mississippi embayment to examine the effect of structure on station factors. The results show that the Mississippi embayment sediment, as a whole, may amplify seismic waves at some specific frequencies, such as 0.5 and 5.0–6.0 Hz, with a factor less than 4.0, and may not amplify seismic waves in the frequency band from 0.5 to 9.9 Hz on average, even though the amplification factors do increase with the source magnitude. Estimations for b -values and magnitudes of earthquakes with a return period of 500 yr in the central United States are performed, based on the original New Madrid catalog ( M D or m bLg ) and the newly created M L catalog. Results show that the b -value is smaller for the M L catalog (0.790) yielding one M 7.5 event every 500 yr, a result that converges with the paleoearthquake estimate for the region. On average, the relation between M L and M D or m bLg (for 1.5≤ M D≤5.0) can be expressed as M L=1.008 M D+0.0714. [1]: /embed/inline-graphic-1.gif