Resolvability of the Centroid‐Moment‐Tensors for Shallow Seismic Sources and Improvements From Modeling High‐Frequency Waveforms

Abstract

Shallow earthquakes in the depth range 0–30 km make up more than 60% of all world's earthquakes. However, resolving their seismic source parameters such as the depth and moment tensor components presents a challenge. Here, we investigate the effect of frequencies higher than 0.025 Hz on centroid‐moment‐tensor inversion for the earthquakes occurring in the top 10 km of the Earth's crust. For a synthetic source located at the depth of 1 km, the maximum amplitude of ground motion due to a vertical dip‐slip mechanism from the waveforms filtered at 0.01–0.15 Hz is about 1,400 times larger than that filtered at 0.01–0.025 Hz. We quantify the effect of this dramatic difference and other waveform differences by introducing the “balance of amplitudes” and “waveform similarity” functions for different depths and frequencies. They present a simple and fast way to estimate the resolvability of seismic sources at a given depth and frequency band. For the 20 May 2016, Mw = 5.9 Petermann Ranges earthquake in Central Australia analyzed at 0.01–0.025 Hz, a high uncertainty accompanies the estimated source parameters. When the frequency band is 0.01–0.15 Hz, the centroid depth is well constrained at 1 km and the mechanism is a thrust fault striking ~314°N and dipping ~30°NE. These simulations require accurate Earth models. Our result, obtained at higher frequencies, is in a great agreement with various other studies that have been carried out for this earthquake and confirms a 20‐km long, shallow rupture.

Long-period ground motion simulation using centroid moment tensor inversion solutions based on the regional three-dimensional model in the Kanto region, Japan

“…However, in regions with strong heterogeneities, such as thick sediments and subducting oceanic plates, focal mechanisms could be incorrectly estimated using conventional CMT methods with a 1D velocity model. To address this issue, the CMT inversion based on Green's functions using the local/regional 3D model has been developed in such regions (e.g., Lee et al 2013;Hejrani et al 2017;Okamoto et al 2018;Takemura et al 2018aTakemura et al , 2018bTakemura et al , b, 2020Wang and Zhan 2020;Hejrani and Tkalčić 2020). By using the 3D CMT results of moderate earthquakes along the Nankai Trough, Takemura et al (2020) demonstrated that the differences in centroid depths and focal mechanisms between 1D and their 3D CMT solutions were significant for offshore earthquakes due to offshore heterogeneities.…”

Moment Tensors of Ring‐Faulting at Active Volcanoes: Insights Into Vertical‐CLVD Earthquakes at the Sierra Negra Caldera, Galápagos Islands

“…detailed information of ring-faulting, shorter-period seismic waves may be utilized with heterogeneous 3-D velocity structures around the calderas (e.g., Contreras-Arratia and Neuberg, 2019;Hejrani and Tkalčić, 2020). Also, recent studies demonstrated that inversion analysis using a simple 1-D Earth model can affect the MT inversion results (Hjörleifsdóttir and Ekström, 2010;Hejrani et al, 2017); hence, effects of the 3-D velocity structures on the resolvable moment tensors need to be further examined.…”

Inflation and Asymmetric Collapse at Kilauea Summit during the 2018 Eruption from Seismic and Infrasound Analyses

“…The correlated waveforms make it hard to distinguish source processes such as reservoir pressurization, crack opening or closing, or shear slip around a ring fault (Fukao et al, 2018;. The combined issues of indeterminate focal depth and weak excitation for shallow source can be overcome using higher frequency waves up to 0.15 Hz; However, the trade-off issue between the volumetric component and vertical CLVD still remains (Hejrani and Tkalčić, 2020). Characterization of non double-couple sources can be improved by increasing the coverage of the source focal sphere.…”