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Numerical Simulations Reproduce Characteristics of Distributions of Non-double-couple Components in Global Moment Tensor Catalogs

Description: 

Non-double-couple (NDC) components of seismic moment tensors (MTs) describe earthquake source processes beyond the double-couple components representing slip on planar faults. MTs from global and regional catalogs find pervasive NDC components with a mean deviation from a double-couple (DC) source of around 20%. Their distributions vary only slightly with magnitude, faulting mechanism, or geologic environments. This consistency suggests that for most earthquakes, especially smaller ones, NDC components are largely artifacts of the MT inversion procedure. We generate synthetic seismograms for the DC components of earthquakes around the world using one Earth model and inverting them with a different model. To match the waveforms with a different Earth model, the inversion changes the mechanisms to include a substantial NDC component while largely preserving the fault geometry (DC component). The resulting NDC components have a size and distribution similar to those reported in global catalogs. MT inversions not adequately accounting for the effects of laterally varying Earth structure can also cause the NDC components of MTs to have a preferred polarity for large shallow thrust- and normal-faulting earthquakes. MT catalogs show that strike-slip earthquakes have predominantly negative NDC components, whereas simulations show equal likelihood for negative and positive NDC components. Hence the preferentially negative NDC components for strike-slip earthquakes could reflect global fault geometry. These NDC components could also be additive, reflecting near-simultaneous strike-slip and dip-slip faulting, such that more common normal-faulting contributions give rise to negative NDC components whereas contributions of thrust-faulting yield positive NDC components. The mean NDC components of earthquakes of all faulting types tend to zero with increasing magnitude, supporting the hypothesis that the different polarities for earthquakes with different faulting types is largely due to lateral heterogeneity of Earth structure to which the longer wavelengths used in the inversion of larger earthquakes are less sensitive.

Session: Advances in Reliable Earthquake Source Parameter Estimation - II

Type: Oral

Date: 4/16/2025

Presentation Time: 10:45 AM (local time)

Presenting Author: Boris

Student Presenter: No

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