Modeling seismic energy propagation in highly scattering environments

Abstract

Meteoroid impacts over millions to billions of years can produce a highly fractured and heterogeneous megaregolith layer on planetary bodies such as the Moon that lack effective surface recycling mechanisms. The energy from seismic events on these bodies undergoes scattering in the fractured layer(s) and generates extensive coda wave trains that follow major seismic wave arrivals. The decay properties of these codas are affected by the planetary body's interior structure. To understand the propagation of seismic waves in such media, we model the transmission of seismic energy in highly scattering environments using an adapted phonon method. In this Monte Carlo simulation approach, we track a large number of seismic wavelets as they leave a source and we record the resulting ground deformation each time a wavelet reaches a surface receiver. Our method provides the first numerical global modeling of 3-D scattering, with user-defined power law distributions of scatterer length scales and frequency-dependent intrinsic attenuation, under the assumption of 1-D background velocity models. We model synthetic signals for simple, but highly scattering interior models and vary the model parameters independently to assess their individual effects on the coda. Results show that the magnitude of the decay times is most affected by the background velocity model, in particular the presence of shallow low-velocity layers, the event source depth, and the intrinsic attenuation level. The decay times are also controlled to a lesser extent by the size-frequency distribution of scatterers, the thickness of the scattering layer, and the impedance contrast at the scatterers.

Expected Seismicity and the Seismic Noise Environment of Europa

“…Whatever the thickness, such a layer would likely introduce more intense 3‐D scattering than that modeled in section 6.4. Modeling such a layer is beyond the scope of the current study as it will require computationally intensive 3‐D numerical wave propagation codes or explorations using stochastic methods based on radiative transfer theory (e.g., Gillet et al, ) or the seismic phonon method (e.g., Blanchette‐Guertin et al, ), but we can consider qualitatively how such a layer may affect seismic data recovered from a landed Europa mission.…”

Effects of lateral variations in megaregolith thickness on predicted lunar seismic signals

“…The choice of model parameters was motivated by other recent lunar velocity studies [e.g., Garcia et al , ; Weber et al , ] and by the work in Blanchette‐Guertin et al []. The intensity of scattering reflects trade‐offs among the scattering layer thickness, the magnitude of intrinsic attenuation in that layer, and to a lesser extent, the size‐frequency distribution of scatterers for the frequency range investigated [ Blanchette‐Guertin et al , ]. Scattering is also intensified if shallow velocity layers and/or shallow and sharp velocity gradients are present, as these structures tend to focus seismic energy near the surface and toward the scattering layer.…”

“…The phonon method used here is detailed in Blanchette‐Guertin et al []. We track a large number of seismic packets as they travel through a two‐dimensional (2‐D) planetary interior from a source located at Δ = 0°, and we record the associated ground deformation each time a phonon packet hits the surface near a given receiver.…”

“…A consequence of the scattering is that secondary seismic phases, which could be used to detect major compositional and phase boundaries within the Moon, are almost entirely obscured in the APSE data set. In this work, we use a phonon method for generating synthetic seismograms [ Shearer and Earle , ; Blanchette‐Guertin et al , ] to investigate the effects of laterally varying megaregolith thickness on the propagation of seismic energy and on the resulting seismic signals recorded at various epicentral distances from the moonquake or impact source. A major objective is to identify conditions under which seismograms recorded at the lunar surface are less likely to be affected by high levels of scattering and for which secondary arrivals containing important information about interior structure can be more readily identified.…”

“…Using the phonon method developed in Blanchette‐Guertin et al [], we assess the effects of laterally varying megaregolith thicknesses, occurring in association with impact basins, on the seismic signals recorded at various locations along the surface. We investigated two simple basin models: (a) 4° diameter (∼120 km) basins centered at epicentral distances from the seismic source, Δ, of 20° and 40° and (b) a 40° diameter (∼1200 km) basin centered at Δ equal to 60°.…”