Improved Relationships of Moment Magnitude with Regional and Local Magnitudes for Earthquakes in the United States

Abstract

ABSTRACT: The moment magnitude is the preferred magnitude to characterize the earthquake source. Moment magnitudes (Mw) are calculated from moment tensor inversion. In contrast, the regional short period surface-wave magnitude (Mblg) and local magnitude (ML) are calculated using the amplitude of the Lg surface waves and maximum horizontal amplitude measured by the seismograph, respectively. Moment magnitudes are inaccurate for small earthquakes (Magnitude < 3.5) and if the waveforms used in the moment tensor inversion are recorded in stations with inadequate azimuthal coverage for the earthquake. In such cases, the moment magnitude is estimated using other available magnitudes. Due to the difference in bedrock geology and anelastic attenuation coefficient, the National Earthquake Information Center (NEIC) uses different magnitudes for the Central Eastern (i.e., Mblg) and the Western (i.e., ML) United States. This study uses the Saint Louis University earthquake catalog to develop improved relationships of moment magnitude with the regional magnitude (Mblg) in the Central Eastern U.S., and the local magnitude (ML) for the Western U.S. to accurately estimate moment magnitude when the moment tensor inversion is unavailable. 1. INTRODUCTION Earthquake magnitude is a commonly accepted tool to quantitatively describe the size and severity of an earthquake. During an earthquake event, seismic energy at the hypocenter is radiated through the earth in different waveforms, i.e., body waves (P and S waves) and surface waves (Rayleigh and Love waves), which are recorded by the seismometers on the earth’s surface. In 1935, Charles F. Richter introduced the concept of measuring the earthquake magnitude based on the maximum amplitude of the waveform recorded in a particular type of seismograph (Woods - Anderson torsion instrument) in southern California (Richter (1935)). Richter magnitude (ML) developed by Richter (1935) was only valid for southern California for specific frequency and distance ranges. Thus, several magnitude scales were developed based on the amplitude of the specific type of seismic waves within specified frequency ranges to account for the regional anelastic attenuation of the waveforms and measure the earthquakes in regions other than southern California (e.g., USGS (2022a)).