February 2019
SPECFEM: Probing Mars interior with 3D seismic wave simulations
InSight launched on May 5, 2018 from Vandenberg Air Force Base on the coast of California, and successfully landed in the Elysium Planitia, the second largest volcanic region on Mars surface, on November 2, 2018 after a 300-million-mile journey. The mission is the first to gather geophysical measurements from surface-installed instruments to explore the internal structure and dynamics of a solar system object other than the Earth or Moon. Understanding Mars’ interior and its dynamics will also help us understand the formation of the Earth and how our planet, together with our solar system, has evolved over time.
The lander’s geophysical payload includes a very broad band seismometer to listen to the seismic activity on Mars. To better characterize and interpret seismic signals recorded by the single broad-band seismometer deployed to Mars, we run numerical seismic wave simulations using a global 3D wave propagation solver, SPECFEM3D_GLOBE (Komatitsch & Tromp 2002). The simulations have been initiated by implementing a 1D reference model for Mars, followed by superimposing topography and crustal thickness variations to analyze the distinct crustal dichotomy between the southern and northern hemispheres specifically on surface waves (Figure 1 & 2). Following Earth simulations, attenuation, Mars ellipticity, rotation and gravity (Cowling approximation) are all taken into account during simulations. All Mars models will be soon integrated into the SPECFEM3D_GLOBE package available through CIG.
Every wiggle from Mars is invaluable, thus 3D wave simulations, both at regional and global scales, are complementary to other modelling techniques to reveal Mars' mysteries. Future steps consist of implementing a set of crustal models as well as 3D mantle models derived from thermal evolution simulations and Mars’ seismic sources - marsquakes, meteorite impacts, etc. Adjusting and refining these models based on observed seismic waveforms from InSight will add to our understanding of the Mars interior – fingers crossed.
See also:
Bozdağ, E., Ruan, Y., Metthez, N., Khan, A., Leng, K., van Driel, M., Wieczorek, M., Rivoldini, A., Larmat, C., Giardini, D., Tromp, J., Lognonne, P. & Banerdt, B. W., 2017. Simulations of seismic wave propagation on Mars, Space Science Reviews, Volume 211, Issue 1-4, pp 571–594, doi: 10.1007/s11214-017-0350-z.
Contributed by Ebru Bozdağ and Daniel Peter
Komatitsch, D. and Tromp, J. (2002), Spectral‐element simulations of global seismic wave propagation—I. Validation. Geophysical Journal International, 149: 390-412. doi:10.1046/j.1365-246X.2002.01653.x
Figure 1. A movie snapshot of 3D wave simulations on Mars showing the vertical-component velocity at the surface. The simulation is performed for the 2011 Virginia earthquake (Mw = 5.8, depth = 12 km) placed at the location of the InSight seismometer. Background model is 1D Sohl & Spohn (1997) with Mars topography from the Mars Orbiter Laser Altimeter (MOLA) (Smith et al. 1999) and 3D crustal thickness variations based on inversions of martian gravity and topography data (e.g., Neumann et al. 2004). Rotation, ellipticity, attenuation, gravity are all taken into account. Simulations were performed on “Tiger” system of the Princeton Institute for Computational Science & Engineering (PICSciE)
Movie credit: shakemovie by Ebru Bozdağ and Daniel Peter [YouTube]
Figure 2. Comparison of vertical (Z) and transverse (T) components of displacement seismograms computed for a 1D (red) and 3D (black) models. The 3D model and source are the same as used in movie simulations. The map in the middle shows crustal thickness variations, station & source locations, and ray paths. Seismograms are bandpass filtered between 20-250 seconds. Vertical- and transverse-component seismograms are normalised by their maximum displacements of 7.2 x10−6 and 2.1 x10−5 meters, respectively.