Improving the determination of moment tensors, moment magnitudes and focal depths of earthquakes below Mw 4.0 using regional broadband seismic data

Abstract

Determining accurate source parameters of small magnitude earthquakes is important to understand the source physics and tectonic processes that activate a seismic source as well as to make more accurate estimates of the probabilities of the recurrences of large earthquakes based on the statistics of smaller earthquakes. The accurate determination of the focal depths and focal mechanisms of small earthquakes is required to constrain the potential seismic source zones of future large earthquakes, whereas the accurate determination of seismic moment is required to calculate the sizes (best represented by moment magnitudes) of earthquakes. The precise determination of focal depths, moment magnitudes and focal mechanisms of small earthquakes can help greatly advance our knowledge of the potentially active faults in an area and thus help to produce accurate seismic hazard and risk maps for that area. Focal depths, moment magnitudes and focal mechanisms of earthquakes with magnitudes Mw 4.0 and less recorded by a sparse seismic network are usually poorly constrained due to the lack of an appropriate method applicable to find these parameters with a sparse set of observations. This dissertation presents a new method that can accurately determine focal depths, moment magnitudes and focal mechanisms of earthquakes with magnitudes between Mw 4.0 and Mw 2.5 using the broadband seismic waveforms recorded by the local and regional seismic stations. For the determination of the focal depths and the moment magnitudes, the observed seismograms as well as synthetic seismograms are filtered through a bandpass filter of 1-3 Hz, whereas for the determination of the focal mechanisms, they are filtered through a bandpass filter of 1.5-2.5 Hz. Both of these frequency passbands have a good signal-to-noise ratio (SNR) for the small earthquakes of the magnitudes that are analyzed in this dissertation. The waveforms are processed to their envelopes in order to make the waveforms relatively simple for the modeling. A grid search is performed over all possible dip, rake and strike angles and as well as over possible depths and scalar moments to find the optimal value of the focal depth and the optimal value of the scalar moment. To find the optimal focal mechanism, a non-linear moment-tensor inversion is performed in addition to the coarse grid search over the possible dip, rake and strike angles at a fixed value of focal depth and a fixed value of scalar moment. The method of this dissertation is tested on 18 aftershocks of Mw between 3.70 and 2.60 of the 2011 Mineral, Virginia Mw 5.7 earthquake. The method is also tested on 5 aftershocks of Mw between 3.62 and 2.63 of the 2013 Ladysmith, Quebec Mw 4.5 earthquake. Reliable focal depths and moment magnitudes are obtained for all of these events using waveforms from as few as 1 seismic station within the epicentral distance of 68-424 km with SNR greater or equal to 5. Similarly, reliable focal mechanisms are obtained for all of the events with Mw 3.70-3.04 using waveforms from at least 3 seismic stations within the epicentral distance of 60-350 km each with SNR greater or equal to 10. Tests show that the moment magnitudes and focal depths are not very sensitive to the crustal model used, although systematic variations in the focal depths are observed with the total crustal thickness. Tests also show that the focal mechanisms obtained with the different crustal structures vary with the Kagan angle of 30o on average for the events and the crustal structures tested. This means that the event moment magnitudes and event focal mechanism determinations are only somewhat sensitive to the uncertainties in the crustal models tested. The method is applied to some aftershocks of the Mw 7.8, 2015 Gorkha, Nepal earthquake which shows that the method developed in this dissertation, by analyzing data from eastern North America, appears to give good results when applied in a very different tectonic environment in a different part of the world. This study confirms that the method of modeling envelopes of seismic waveforms developed in this dissertation can be used to extract accurate focal depths and moment magnitudes of earthquakes with Mw 3.70-2.60 using broadband seismic data recorded by local and regional seismic stations at epicentral distances of 68-424 km and accurate focal mechanisms of earthquakes with Mw 3.70-3.04 using broadband seismic data recorded by local and regional seismic stations at epicentral distances of 60-350 km.