5:15 PM - 7:15 PM
[PPS09-P07] Numerical simulation of seismic waveforms to evaluate Martian subsurface heterogeneity inferred from seismic records of a meteoroid impact
Keywords:marsquake, meteoroid impact, regolith, numerical simulation of seismic wave, coda wave, scattering
NASA's Mars InSight lander deployed the SEIS seismograph to record many marsquakes. The observed seismic waveforms include long and strong coda waves that obscure the P- and S-wave onsets. These coda waves are generated by scattering from medium heterogeities, including the regolith and megaregolith—fractured layers on the Martian surface and bedrock, respectively. Mars’ coda waves last longer than Earth’s but shorter than the Moon’s. Many studies have been conducted on the interior structure of Mars based on the waveform records of various volcanic tremors, but most of them assume of a horizontally stratified structure. Scattering in the crust has also been studied, but the waveforms have not been fully synthesized considering the regolith layer and its heterogeneity.
Accurate source location is essential for modeling Mars’ internal structure based on waveform modeling. Due to the single station observation, however, the hypocenters of many marsquakes remain uncertain. Thus, we attempted to numerically reproduce the waveforms of S1034a, a marsquake event caused by a meteoroid impact, for which a crater can be used as a marker of the exact hypocenter location.
Observed S1034a’s traces showed unclear P- and S-wave onsets, gradual P-wave coda amplification, and abundant high-frequency scattering. These features may be due to the influence of short-wavelength heterogeneous structure in the regolith layer. In addition, the amplitude of the surface waves was small even though the epicenter was at the ground surface. This suggests the existence of structures that attenuate surface waves in shallow areas. We attempted to reproduce the waveform of S1034a by numerical simulation using the seismic wave propagation code OpenSWPC. We performed 2-D P-SV wavefield simulations, with a model with a low-velocity layer and a regolith-like inhomogeneous layer at 0-2 km dept in addition to the previously proposed stratigraphic model. This initial attempt resulted in simulated waveforms that differ considerably from the observed ones, particularly in overly fast coda decay and the impulsive arrivals of surface waves.
Next, new parameters were introduced to remedy these differences: a stronger heterogeneity and weaker attenuation. The random media with a shorter characteristic scale length to reproduce surface sediment. Velocity fluctuations were also made stronger to simulate the gradual amplitude amplification of P-wave coda. The random medium of the megaregolith layer is based on the parameters of previous lunar studies. Quality factors are on the order of 1-2 orders larger than those on the earth's surface to reduce the attenuation of seismic waves near the surface. A gradually changing velocity with depth was also introduced to better reproduce the observed surface waves.
This sequence of improvements disperses the surface waves and attenuates the pulse-like amplitude significantly. However, the coda wave amplitude duration is longer than in the previous model, but still shorter than the observed waveform, and features such as an indistinct P-wave onset shape and a slowly amplifying P-wave coda have not yet been reproduced. This suggests that the scattering in the regolith layer may be stronger than in the current model, or that the quality factors may be higher. In addition, the scattering of the megaregolith layer also needs to be investigated. It is expected that the construction of a more appropriate scattering model and 3D wavefield simulation to reproduce more detailed marsquake waveforms in the future will help clarify the crustal structure and scattering characteristics of Mars.
Accurate source location is essential for modeling Mars’ internal structure based on waveform modeling. Due to the single station observation, however, the hypocenters of many marsquakes remain uncertain. Thus, we attempted to numerically reproduce the waveforms of S1034a, a marsquake event caused by a meteoroid impact, for which a crater can be used as a marker of the exact hypocenter location.
Observed S1034a’s traces showed unclear P- and S-wave onsets, gradual P-wave coda amplification, and abundant high-frequency scattering. These features may be due to the influence of short-wavelength heterogeneous structure in the regolith layer. In addition, the amplitude of the surface waves was small even though the epicenter was at the ground surface. This suggests the existence of structures that attenuate surface waves in shallow areas. We attempted to reproduce the waveform of S1034a by numerical simulation using the seismic wave propagation code OpenSWPC. We performed 2-D P-SV wavefield simulations, with a model with a low-velocity layer and a regolith-like inhomogeneous layer at 0-2 km dept in addition to the previously proposed stratigraphic model. This initial attempt resulted in simulated waveforms that differ considerably from the observed ones, particularly in overly fast coda decay and the impulsive arrivals of surface waves.
Next, new parameters were introduced to remedy these differences: a stronger heterogeneity and weaker attenuation. The random media with a shorter characteristic scale length to reproduce surface sediment. Velocity fluctuations were also made stronger to simulate the gradual amplitude amplification of P-wave coda. The random medium of the megaregolith layer is based on the parameters of previous lunar studies. Quality factors are on the order of 1-2 orders larger than those on the earth's surface to reduce the attenuation of seismic waves near the surface. A gradually changing velocity with depth was also introduced to better reproduce the observed surface waves.
This sequence of improvements disperses the surface waves and attenuates the pulse-like amplitude significantly. However, the coda wave amplitude duration is longer than in the previous model, but still shorter than the observed waveform, and features such as an indistinct P-wave onset shape and a slowly amplifying P-wave coda have not yet been reproduced. This suggests that the scattering in the regolith layer may be stronger than in the current model, or that the quality factors may be higher. In addition, the scattering of the megaregolith layer also needs to be investigated. It is expected that the construction of a more appropriate scattering model and 3D wavefield simulation to reproduce more detailed marsquake waveforms in the future will help clarify the crustal structure and scattering characteristics of Mars.