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[SSS10-04] Development of a proxy of earthquake source complexities for representing temporal variation of focal mechanisms
Keywords:seismic source process models, inversion methods, temporal change of focal mechanisms
Field surveys and seismic waveform analyses often find the earthquake rupture involves changes in fault geometry and slip direction. For instance, the 2016 Kaikoura earthquake (MW 7.8) ruptured the multiple strike-slip and reverse faults with different geometries, and the 2007 Martinique earthquake (MW 7.4) exhibited coexistence of the down-dip extension and strike-slip fault ruptures. A ratio of non-double-couple (DC) component is often used as a proxy for the degree of spatiotemporal variation of the focal mechanisms for an earthquake. However, even when multiple-fault rupture occurs, the non-DC ratio remains small when the B-axes orient similarly, which indicates the non-DC ratio may not necessarily represent the complexity of the source process involving the temporal change of fault geometry and slip direction. To evaluate the complexity of source processes between earthquakes, we propose another proxy representing the temporal change of focal mechanisms.
Potency Density Tensor Inversion (PDTI), a recently developed method, can estimate a seismic source process with information on fault geometry, by projecting the various fault slips onto the model plane as potency density tensors obtained by dividing moment density tensors by rigidities. Moment rate tensors are calculated as spatial integration of the potency rate density tensors. The moment rate tensors are representative of the fault motion at each time, and from eigenvalues of each moment rate tensors, moment rates at each time (a moment rate function) are estimated. The seismic moment can be estimated from the eigenvalues of the moment tensor obtained by integrating potency density tensors over time and space. If a complex earthquake with the temporary changes in focal mechanisms, the value of time integration of moment rate becomes different from the seismic moment. From this context, we define γ; a time integral of moment rates normalized by the seismic moment as a proxy for the complexity of the source process and examine its validity using the PDTI models obtained from the real teleseismic body waveforms.
We calculated γ for the PDTI source process models of 28 earthquakes, including the published 22 earthquakes and the newly analyzed six earthquakes with the large non-DC ratios. We found that 20 earthquakes had γ less than 1.13, and other eight earthquakes had γ of 1.13<γ<1.55. There was no correlation between γ and non-DC ratios, and high values of γ were obtained even for the earthquakes with the small non-DC ratios. There were earthquakes with the non-DC ratios less than 20 % and γ higher than 1.13; the 2002 Denali earthquake (γ=1.135), the 2021 Haiti earthquake (γ=1.348), and the 2014 Thailand earthquake (γ=1.306). The first two were reported to rupture strike-slip and reverse faults. The 2014 Thailand earthquake included negative slip in the latter part of the source process model, which apparently contributed to the high γ value and may imply the destabilization of the solution due to the small MW (6.2) and the limited number of the seismic stations used for inversion. For an effective use of γ, it should be essential to be applied to the less destabilized solutions. The earthquakes with γ larger than 1.13 are the 2000 Sulawesi earthquake (γ=1.181), the 2007 Martinique earthquake (γ=1.228), and the 2016 Kaikoura earthquake (γ=1.263), which ruptured the strike-slip and reverse faults, and the 2013 Balochistan earthquake (γ=1.170) rupturing within the curved fault system, and the 2021 East Cape earthquake (γ=1.547) which ruptured normal, strike-slip and reverse faults in the slab. Even when earthquakes rupture multiple faults, the γ gets stably small when these faults are oriented similarly to some extent, for example the 2024 Noto Peninsula (γ=1.099) and the 2020 Caribbean (γ=1.040) earthquakes. These observations suggest that the developed proxy of γ can work better than non-DC ratio when measuring the temporal variation of faulting.
Potency Density Tensor Inversion (PDTI), a recently developed method, can estimate a seismic source process with information on fault geometry, by projecting the various fault slips onto the model plane as potency density tensors obtained by dividing moment density tensors by rigidities. Moment rate tensors are calculated as spatial integration of the potency rate density tensors. The moment rate tensors are representative of the fault motion at each time, and from eigenvalues of each moment rate tensors, moment rates at each time (a moment rate function) are estimated. The seismic moment can be estimated from the eigenvalues of the moment tensor obtained by integrating potency density tensors over time and space. If a complex earthquake with the temporary changes in focal mechanisms, the value of time integration of moment rate becomes different from the seismic moment. From this context, we define γ; a time integral of moment rates normalized by the seismic moment as a proxy for the complexity of the source process and examine its validity using the PDTI models obtained from the real teleseismic body waveforms.
We calculated γ for the PDTI source process models of 28 earthquakes, including the published 22 earthquakes and the newly analyzed six earthquakes with the large non-DC ratios. We found that 20 earthquakes had γ less than 1.13, and other eight earthquakes had γ of 1.13<γ<1.55. There was no correlation between γ and non-DC ratios, and high values of γ were obtained even for the earthquakes with the small non-DC ratios. There were earthquakes with the non-DC ratios less than 20 % and γ higher than 1.13; the 2002 Denali earthquake (γ=1.135), the 2021 Haiti earthquake (γ=1.348), and the 2014 Thailand earthquake (γ=1.306). The first two were reported to rupture strike-slip and reverse faults. The 2014 Thailand earthquake included negative slip in the latter part of the source process model, which apparently contributed to the high γ value and may imply the destabilization of the solution due to the small MW (6.2) and the limited number of the seismic stations used for inversion. For an effective use of γ, it should be essential to be applied to the less destabilized solutions. The earthquakes with γ larger than 1.13 are the 2000 Sulawesi earthquake (γ=1.181), the 2007 Martinique earthquake (γ=1.228), and the 2016 Kaikoura earthquake (γ=1.263), which ruptured the strike-slip and reverse faults, and the 2013 Balochistan earthquake (γ=1.170) rupturing within the curved fault system, and the 2021 East Cape earthquake (γ=1.547) which ruptured normal, strike-slip and reverse faults in the slab. Even when earthquakes rupture multiple faults, the γ gets stably small when these faults are oriented similarly to some extent, for example the 2024 Noto Peninsula (γ=1.099) and the 2020 Caribbean (γ=1.040) earthquakes. These observations suggest that the developed proxy of γ can work better than non-DC ratio when measuring the temporal variation of faulting.