Tomographic Inversion for Regional Phase Attenuation and Source Parameters

Our ability to monitor seismic events that are too small to be observed at teleseismic distances depends critically on the accurate characterization of the effects of source, path and site on regional phases. Magnitude, yield and event identification procedures rely on path and site corrections, while identification schemes such as MDAC (magnitude and distance amplitude correction) additionally rely on source correction via a regionally appropriate scaling model. Independently determined Mw can drive the source correction. Here, we focus on the use of Lg amplitudes to obtain a laterally varying attenuation model with power law frequency dependence, site amplification terms, and source scaling parameters such as apparent stress, using data from the USArray deployment. We collected amplitudes from broadband vertical channels of 605 USArray stations for 986 western US PDE events through June 2008. The IRIS DMC provided waveform data and instrument response information. We measured RMS amplitudes in eleven overlapping octave width bands between 0.25 and 16 Hz, for windows defined by group velocities 3.6 to 3.0 km/s. Over 320,000 amplitudes passed a pre-phase signal-to-noise threshold of 2. Berkeley moments were used to set absolute levels. We found 1-Hz attenuation (Qo) to be high in stable regions and across batholiths, and low in areas of recent volcanism. The power law exponent (eta) varied from 0.4 to 0.9 and was generally lower in the high Q regions. We explore methods that break the tradeoff between attenuation and stress by damping mean Qo and eta to values determined by inverting for frequency dependent source terms, and by applying ground truth scaling information from relative coda studies for particular events. Tests that evaluate regionalization of apparent stress are underway, although we acknowledge that the limited bandwidth and deployment period are not ideal for this purpose.