Source Parameters of the Upper-Mantle September 21, 2013 Mw4.8 Wyoming Earthquake

Abstract:

On September 21, 2013, a Mw4.8 earthquake occurred at more than 70 km deep in the Wind River Range (western Wyoming). Moment tensor inversions show dominant strike-slip faulting with a small reverse component, with nodal planes striking nearly ENE and NW. While intermediate-depth earthquakes are common in a subduction context, there have been very few upper-mantle intraplate events. The 2012 Mw8.6 off-shore Sumatra event is an example of a « deep » intraplate strike-slip earthquake which occurred in the low ocean crust or mantle. The Wyoming event is thus a very unique example of deep intracontinental earthquake. The physical mechanism responsible of intermediate-depth earthquakes remains under debate. This work aims at determining robust source parameters to investigate any distinct characteristic from that for shallow earthquakes or subduction zone intermediate-depth events.

We adopt the empirical Green’s function (EGF) approach to empirically correct for path and site effects. The nearby Ml3.0 aftershock which occurred 2 hours after the mainshock and at similar depth, is considered as EGF earthquake. This only aftershock is located at ~5 km from the mainshock and turns out to be a good EGF since both earthquakes generate high-correlated waveforms. Using the multitaper approach we are able to deconvolve the EGF from the mainshock with robust spectral and temporal representations. We use both the spectral (spectral ratio) and the temporal (source time function) information to obtain a more robust estimate of the source duration and the corresponding corner frequency, but require an adaptive window in order to have good signal to noise ratios in as wide a frequency band as possible. We propose a semi-automatic, adaptive time window approach, that allows for a robust measurement at each single station, thereby enhancing the analysis of the azimuthal distribution of corner frequencies.

We additionally investigate the radiated seismic energy of the Mw4.8 earthquake, in order to test whether the seismic efficiency provides additional constraints to explain any differences with respect to crustal earthquakes or other intermediate-depth earthquakes.