4:45 PM - 5:00 PM
[SCG61-12] Physical interpretation of compensated linear vector dipole and rational decomposition of moment tensor
Keywords:Moment tensor, CLVD, shear faulting, crack opening, gravitational potential energy
Moment tensor is the most general and concise representation of indigenous sources in the solid Earth (Backus and Mulcahy, 1976). For extracting information about source mechanism from centroid moment tensor (CMT) data, we need to decompose it in some way. From a mathematical point of view, the way of moment tensor decomposition is not unique (Jost and Herrmann, 1989; Vavryčuk, 2015). Knopoff and Randall (1970) proposed decomposing moment tensor into the isotropic (ISO), double-couples (DC), and compensated linear vector dipole (CLVD) components. The physical interpretation of these components is clear for ISO and DC but not clear for CLVD.
In general, the moment tensor is defined by the volume integral of inelastic strain change over a source region. The generation of inelastic strain is accompanied by isotropic volume expansion (EX), shear faulting (SF), crack opening (CO), or a linear combination of them. Matsu'ura et al. (2019) derived the moment tensor representations corresponding to the three basic processes; that is, the EX, SF, and CO components of a moment tensor. The CMT of any seismic event can be uniquely decomposed into these three components, each of which has clear physical meaning. Here, it should be noted that the CO component has a nonzero isotropic part. Then, in comparison with the Knopoff-Randle notation, we obtain the following relations: MISO = MEX + the isotropic part of MCO, MDC = MSF, and MCLVD = the deviatoric part of MCO. In short, CLVD itself has no physical meaning.
After the occurrence of large earthquakes, the intrusion of high-pressure fluid into pre-existing faults from deep reservoirs may trigger seismic events. The intrusion of high-pressure fluid corresponds to the mechanical process of crack opening (Matsu'ura and Terakawa, 2021). From the viewpoint of energetics, shear faulting and crack opening are essentially different. The former is free from the change in gravitational potential energy, but the latter is not, because the crack opening is always accompanied by volume change (Matsu'ura, 2024). So, the triggering of seismic events by the intrusion of high-pressure fluid will be restricted in the shallow part of the Earth’s crust.
In general, the moment tensor is defined by the volume integral of inelastic strain change over a source region. The generation of inelastic strain is accompanied by isotropic volume expansion (EX), shear faulting (SF), crack opening (CO), or a linear combination of them. Matsu'ura et al. (2019) derived the moment tensor representations corresponding to the three basic processes; that is, the EX, SF, and CO components of a moment tensor. The CMT of any seismic event can be uniquely decomposed into these three components, each of which has clear physical meaning. Here, it should be noted that the CO component has a nonzero isotropic part. Then, in comparison with the Knopoff-Randle notation, we obtain the following relations: MISO = MEX + the isotropic part of MCO, MDC = MSF, and MCLVD = the deviatoric part of MCO. In short, CLVD itself has no physical meaning.
After the occurrence of large earthquakes, the intrusion of high-pressure fluid into pre-existing faults from deep reservoirs may trigger seismic events. The intrusion of high-pressure fluid corresponds to the mechanical process of crack opening (Matsu'ura and Terakawa, 2021). From the viewpoint of energetics, shear faulting and crack opening are essentially different. The former is free from the change in gravitational potential energy, but the latter is not, because the crack opening is always accompanied by volume change (Matsu'ura, 2024). So, the triggering of seismic events by the intrusion of high-pressure fluid will be restricted in the shallow part of the Earth’s crust.