Computation of seismograms and atmospheric oscillations by normal-mode summation for a spherical earth model with realistic atmosphere

We describe a theory to compute seismograms and atmospheric disturbances such as ionospheric oscillations or pressure variations in a realistic spherical earth model with atmosphere. This theory is valid for a source located either in the solid earth or in the atmosphere. Solid earth and atmospheric normal modes are computed for a radiation boundary condition that models the dissipation of acoustic signals in the high atmosphere of the Earth. We show that the coupling between ground and atmosphere occurs at a set of frequencies related to fundamentals and harmonics of atmospheric modes. Spheroidal modes near these frequencies have up to 0.04 per cent of their energy in the atmosphere, and thus may be strongly excited by atmospheric sources. This theory can be used for more accurate modelling of the seismic data from meteoritic events or volcanic eruptions as well as for the analysis of barograms or ionograms recorded after large earthquakes

Present-day Mars' seismicity predicted from 3-D thermal evolution models of interior dynamics

©2018. American Geophysical UnionThe Interior Exploration using Seismic Investigations, Geodesy and Heat Transport mission, to be launched in 2018, will perform a comprehensive geophysical investigation of Mars in situ. The Seismic Experiment for Interior Structure package aims to detect global and regional seismic events and in turn offer constraints on core size, crustal thickness, and core, mantle, and crustal composition. In this study, we estimate the present‐day amount and distribution of seismicity using 3‐D numerical thermal evolution models of Mars, taking into account contributions from convective stresses as well as from stresses associated with cooling and planetary contraction. Defining the seismogenic lithosphere by an isotherm and assuming two end‐member cases of 573 K and the 1073 K, we determine the seismogenic lithosphere thickness. Assuming a seismic efficiency between 0.025 and 1, this thickness is used to estimate the total annual seismic moment budget, and our models show values between 5.7 × 1016 and 3.9 × 1019 Nm

Modelling of the Total Electronic Content and magnetic field anomalies generated by the 2011 Tohoku-oki tsunami and associated acoustic-gravity waves,

International audienceIn this work, numerical simulations of the atmospheric and ionospheric anomalies are performed for the Tohoku-Oki tsunami (2011 March 11). The Tsunami-Atmosphere-Ionosphere (TAI) coupling mechanism via acoustic gravity waves (AGWs) is explored theoretically using the TAI-coupled model. For the modelled tsunami wave as an input, the coupled model simulates the wind, density and temperature disturbances or anomalies in the atmosphere and electron density/magnetic anomalies in the F region of the ionosphere. Also presented are the GPS-total electron content (TEC) and ground-based magnetometer measurements during the first hour of tsunami and good agreements are found between modelled and observed anomalies. At first, within 6 min from the tsunami origin, the simulated wind anomaly at 250 km altitude and TEC anomaly appear as the dipole-shaped disturbances around the epicentre, then as the concentric circular wave fronts radially moving away from the epicentre with the horizontal velocity ∼800 m s−1 after 12 min followed by the slow moving (horizontal velocity ∼250 m s−1) wave disturbance after 30 min. The detailed vertical-horizontal propagation characteristics suggest that the anomalies appear before and after 30 min are associated with the acoustic and gravity waves, respectively. Similar propagation characteristics are found from the GPS-TEC and magnetic measurements presented here and also reported from recent studies. The modelled magnetic anomaly in the F region ionosphere is found to have similar temporal variations with respect to the epicentre distance as that of the magnetic anomaly registered from the ground-based magnetometers. The high-frequency component ∼10 min of the simulated wind, TEC and magnetic anomalies in the F region develops within 6-7 min after the initiation of the tsunami, suggesting the importance of monitoring the high-frequency atmospheric/ionospheric anomalies for the early warning. These anomalies are found to maximize across the epicentre in the direction opposite to the tsunami propagation suggesting that the large atmospheric/ionospheric disturbances are excited in the region where tsunami does not travel

The Effects of the Atmospheric Pressure Changes on Seismic Signals or How to Improve the Quality of a Station

Seismic investigations are mainly limited by seismic noise. Two microbarometers have been installed in the seismic vault of two different GEOSCOPE stations, one at SSB and the other at TAM. All vertical components and most of the horizontal components show a significant correlation with pressure. In order to correct the seismic signals from the atmospheric pressure noise, a transfer function between the pressure data and the seismic data is inverted. Results show that, after correction, the noise levels reached on the horizontal components are similar between the two stations, and the vertical components display noise levels below the low-noise model as defined by Peterson (1993). This technique reduces part of the noise and allows detection of small earthquakes and a better extraction of normal modes. The analysis of the lowest normal modes of the Earth excited by the M_S = 8.2 Macquarie Island earthquake is given to illustrate the perspectives of the method

The Seismic Moment and Seismic Efficiency of Small Impacts on Mars

Since landing in late 2018, the InSight lander has been recording seismic signals on the surface of Mars. Despite nominal prelanding estimates of one to three meteorite impacts detected per Earth year, none have yet been identified seismically. To inform revised detectability estimates, we simulated numerically a suite of small impacts onto Martian regolith and characterized their seismic source properties. For the impactor size and velocity range most relevant for InSight, crater diameters are 1–30 m. We found that in this range scalar seismic moment is 106–1010 Nm and increases almost linearly with impact momentum. The ratio of horizontal to vertical seismic moment tensor components is ∼1, implying an almost isotropic P wave source, for vertical impacts. Seismic efficiencies are ∼10−6, dependent on the target crushing strength and impact velocity. Our predictions of relatively low seismic efficiency and seismic moment suggest that meteorite impact detectability on Mars is lower than previously assumed. Detection chances are best for impacts forming craters of diameter >10 m

SEIS: Insight's Seismic Experiment for Internal Structure of Mars.

By the end of 2018, 42 years after the landing of the two Viking seismometers on Mars, InSight will deploy onto Mars' surface the SEIS (Seismic Experiment for Internal Structure) instrument; a six-axes seismometer equipped with both a long-period three-axes Very Broad Band (VBB) instrument and a three-axes short-period (SP) instrument. These six sensors will cover a broad range of the seismic bandwidth, from 0.01 Hz to 50 Hz, with possible extension to longer periods. Data will be transmitted in the form of three continuous VBB components at 2 sample per second (sps), an estimation of the short period energy content from the SP at 1 sps and a continuous compound VBB/SP vertical axis at 10 sps. The continuous streams will be augmented by requested event data with sample rates from 20 to 100 sps. SEIS will improve upon the existing resolution of Viking's Mars seismic monitoring by a factor of ∼ 2500 at 1 Hz and ∼ 200 000 at 0.1 Hz. An additional major improvement is that, contrary to Viking, the seismometers will be deployed via a robotic arm directly onto Mars' surface and will be protected against temperature and wind by highly efficient thermal and wind shielding. Based on existing knowledge of Mars, it is reasonable to infer a moment magnitude detection threshold of M w ∼ 3 at 40 ∘ epicentral distance and a potential to detect several tens of quakes and about five impacts per year. In this paper, we first describe the science goals of the experiment and the rationale used to define its requirements. We then provide a detailed description of the hardware, from the sensors to the deployment system and associated performance, including transfer functions of the seismic sensors and temperature sensors. We conclude by describing the experiment ground segment, including data processing services, outreach and education networks and provide a description of the format to be used for future data distribution.Electronic supplementary materialThe online version of this article (10.1007/s11214-018-0574-6) contains supplementary material, which is available to authorized users

Field Measurements of Terrestrial and Martian Dust Devils

Surface-based measurements of terrestrial and martian dust devils/convective vortices provided from mobile and stationary platforms are discussed. Imaging of terrestrial dust devils has quantified their rotational and vertical wind speeds, translation speeds, dimensions, dust load, and frequency of occurrence. Imaging of martian dust devils has provided translation speeds and constraints on dimensions, but only limited constraints on vertical motion within a vortex. The longer mission durations on Mars afforded by long operating robotic landers and rovers have provided statistical quantification of vortex occurrence (time-of-sol, and recently seasonal) that has until recently not been a primary outcome of more temporally limited terrestrial dust devil measurement campaigns. Terrestrial measurement campaigns have included a more extensive range of measured vortex parameters (pressure, wind, morphology, etc.) than have martian opportunities, with electric field and direct measure of dust abundance not yet obtained on Mars. No martian robotic mission has yet provided contemporaneous high frequency wind and pressure measurements. Comparison of measured terrestrial and martian dust devil characteristics suggests that martian dust devils are larger and possess faster maximum rotational wind speeds, that the absolute magnitude of the pressure deficit within a terrestrial dust devil is an order of magnitude greater than a martian dust devil, and that the time-of-day variation in vortex frequency is similar. Recent terrestrial investigations have demonstrated the presence of diagnostic dust devil signals within seismic and infrasound measurements; an upcoming Mars robotic mission will obtain similar measurement types

Overview of first atmospheric results from InSight

The InSight spacecraft landed in the flat regions of Elysium Planitia on November 26th 2018. The instruments on board InSight make it capable of acting as a meteorological station at the surface of Mars. A pressure sensor (PS), two temperature and wind sensor booms (TWINS), along with the InSight FluxGate (IFG) magnetometer, form the Auxiliary Sensor Payload Suite (APSS). This is complemented by capabilities to measure surface brightness temperature by the radiometer in the Heat-Flow and Physical Properties Package (HP3) suite, to explore the impact of atmospheric processes on seismic measurements by SEIS, and to use InSight cameras to estimate atmospheric opacity (notably caused by suspended dust particles) and other atmospheric phenomena such as clouds and dust devils. We will discuss results drawn from atmospheric measurements on board InSight over the first two months of operation, highlighting new perspectives permitted by the high-frequency, continuous nature of the InSight acquisitions. Surface pressure measurements record global-to-local atmospheric phenomena: CO2 condensation (annual), dust cycle and storms (seasonal), baroclinic waves (weekly), thermal tides (daily), gravity waves (thousands of seconds), convective cells (hundreds of seconds), convective vortices (tens of seconds, leading to dust devils if dust particles are transported in the vortex). Two main large-scale wind regimes were expected from Global Climate Modeling at the InSight landing site during a typical year: towards the northwest in northern spring and summer, then in the opposite direction in southern summer. Existing in-situ measurements on Mars and Large-Eddy Simulations indicate that daytime convective vortices and cells not only impact pressure, but also temperature and winds; the nighttime atmosphere on Mars is comparatively much less turbulent and dominated by shear-driven turbulence, in contrast to the buoyancy-driven turbulence active in daytime. All such existing measurements and model predictions will be compared and challenged with InSight measurements. Seismic signatures associated with atmospheric phenomena will also be discussed, with a particular emphasis on the knowledge gained by the unprecedented measurements performed by InSight’s seismometers

Influence of Body Waves, Instrumentation Resonances, and Prior Assumptions on Rayleigh Wave Ellipticity Inversion for Shallow Structure at the InSight Landing Site

Based on an updated model of the regolith’s elastic properties, we simulate the ambient vibrations background wavefield recorded by InSight’s Seismic Experiment for Interior Structure (SEIS) on Mars to characterise the influence of the regolith and invert SEIS data for shallow subsurface structure. By approximately scaling the synthetics based on seismic signals of a terrestrial dust devil, we find that the high-frequency atmospheric background wavefield should be above the self-noise of SEIS’s SP sensors, even if the signals are not produced within 100–200 m of the station. We compare horizontal-to-vertical spectral ratios and Rayleigh wave ellipticity curves for a surface-wave based simulation on the one hand with synthetics explicitly considering body waves on the other hand and do not find any striking differences. Inverting the data, we find that the results are insensitive to assumptions on density. By contrast, assumptions on the velocity range in the upper-most layer have a strong influence on the results also at larger depth. Wrong assumptions can lead to results far from the true model in this case. Additional information on the general shape of the curve, i.e. single or dual peak, could help to mitigate this effect, even if it cannot directly be included into the inversion. We find that the ellipticity curves can provide stronger constraints on the minimum thickness and velocity of the second layer of the model than on the maximum values. We also consider the effect of instrumentation resonances caused by the lander flexible modes, solar panels, and the SEIS levelling system. Both the levelling system resonances and the lander flexible modes occur at significantly higher frequencies than the expected structural response, i.e. above 35 Hz and 20 Hz, respectively. While the lander and solar panel resonances might be too weak in amplitude to be recorded by SEIS, the levelling system resonances will show up clearly in horizontal spectra, the H/V and ellipticity curves. They are not removed by trying to extract only Rayleigh-wave dominated parts of the data. However, they can be distinguished from any subsurface response by their exceptionally low damping ratios of 1% or less as determined by random decrement analysis. The same applies to lander-generated signals observed in actual data from a Moon analogue experiment, so we expect this analysis will be useful in identifying instrumentation resonances in SEIS data

First Focal Mechanisms of Marsquakes

Since February 2019, NASA's InSight lander is recording seismic signals on the planet Mars, which, for the first time, allows to observe ongoing tectonic processes with geophysical methods. A number of Marsquakes have been located in the Cerberus Fossae graben system in Elysium Planitia and further west, in the Orcus Patera depression. We present a first study of the focal mechanisms of three well-recorded events (S0173a, S0183a, S0235b) to determine the processes dominating in the source region. We infer for all three events a predominantly extensional setting. Our method is adapted to the case of a single, multicomponent receiver and based on fitting waveforms of P and S waves against synthetic seismograms computed for the initial crustal velocity model derived by the InSight team. We explore the uncertainty due to the single-station limitation and find that even data recorded by one station constrains the mechanisms (reasonably) well. For the events in the Cerberus Fossae region (S0173a, S0235b) normal faulting with a relatively steep dipping fault plane is inferred, suggesting an extensional regime mainly oriented E-W to NE-SW. The fault regime in the Orcus Patera region is not determined uniquely because only the P wave can be used for the source inversion. However, we find that the P and weak S waves of the S0183a event show similar polarities to the event S0173, which indicates similar fault regimes