MOMENT MAGNITUDES OF LOCAL/REGIONAL EVENTS FROM 1-D CODA CALIBRATIONS IN THE BROADER MIDDLE EAST REGION

Abstract

Short-period body wave and long-period surface wave magnitudes are important for event characterization, and yield estimation applications. However, the uneven distribution of stations, complex heterogeneous structure, and high attenuation
of the Middle East make it difficult to calculate these magnitudes using simple 1-D path corrections. While reliable 1-D long-period regional moment magnitudes are routinely computed for the larger magnitude events, oftentimes the smaller events do
not have the required bandwidth and are thus not suitable for long-period waveform modeling that require periods of 5 seconds or greater. The complex structure of the lithosphere in the region causes significant variation in the recorded amplitude of body
and surface waves that travel along different paths, as well as phase blockage. These strong 2-D effects are most significant for source characterization of small and moderate magnitude events, which are often only observable at periods of less than 2
seconds. While the regional coda envelope method for moment magnitude estimation [22] has proven to be very robust in many studies, they still require 2-D path corrections for the shorter periods in laterally complex regions Iranian and Turkish
Plateaus. This current study will provide the necessary 1-D initial starting model for use in a future 2-D coda study. We are also very fortunate to have access to many new broadband stations that have become available in the region, through improved national networks as well as temporary deployments. This has given us the opportunity to make thousands of coda amplitude measurements to calculate source parameters, specifically we have estimated the source moment-rate spectra and moment magnitudes for roughly 2,900 events recorded by 74 stations over 13 narrow frequency bands ranging between
0.03-8 Hz. Moreover, this data will be crucial for 2-D path corrections for coda in the future. The absolute scaling of these spectra was calculated based on an independent waveform modeling solutions of the moment magnitudes for a subset of these events to
avoid circularity. Using our1-D path calibrations, we obtain fairly good agreement with waveform-modeled seismic moments for the larger events (Mw > 4.5), for the low frequencies (< 0.7 Hz). As expected, the coda-derived source spectra become very
scattered at higher frequencies (>0.7-Hz) because of uncorrected 2-D path effects as well as mixing of both Sn-coda and Lg-coda which are likely to have different attenuation.