22 research outputs found
High-quality lowest-frequency normal mode strain observations at the Black Forest Observatory (SW-Germany) and comparison with horizontal broad-band seismometer data and synthetics
International audienceWe present spectra concentrating on the lowest-frequency normal modes of the Earth obtained from records of the invar-wire strainmeters and STS-1 broad-band seismometers located in the Black Forest Observatory, Germany after the disastrous earthquakes off the NW coast of Sumatra in 2004 and off the coast near Tohoku, Japan in 2011. We compare the spectra to ones obtained from synthetic seismograms computed using a mode summation technique for an anelastic, elliptical, rotating, spherically symmetric Earth model. The synthetics include strainâ strain-coupling effects by using coupling coefficients obtained from comparisons between Earth tide signals recorded by the strainmeters and synthetic tidal records. We show that for the low-frequency toroidal and spheroidal modes up to 1 mHz, the strainmeters produce better signal-to-noise ratios than the broad-band horizontal seismometers. Overall, the comparison with the synthetics is satisfactory but not as good as for vertical accelerations. In particular, we demonstrate the high quality of the strainmeter data by showing the Coriolis splitting of toroidal modes for the first time in individual records, the first clear observation of the singlet 2 S 0 1 and the detection of the fundamental radial mode 0 S 0 with good signal-to-noise ratio and with a strain amplitude of 10 â11. We also identify the latter mode in a record of the Isabella strainmeter after the great Chilean quake in 1960, the detection of which was missed by the original studies
High-quality lowest-frequency normal mode strain observations at the Black Forest Observatory (SW-Germany) and comparison with horizontal broad-band seismometer data and synthetics
We present spectra concentrating on the lowest-frequency normal modes of the Earth obtained from records of the invar-wire strainmeters and STS-1 broad-band seismometers located in the Black Forest Observatory, Germany after the disastrous earthquakes off the NW coast of Sumatra in 2004 and off the coast near Tohoku, Japan in 2011. We compare the spectra to ones obtained from synthetic seismograms computed using a mode summation technique for an anelastic, elliptical, rotating, spherically symmetric Earth model. The synthetics include strainâstrain-coupling effects by using coupling coefficients obtained from comparisons between Earth tide signals recorded by the strainmeters and synthetic tidal records. We show that for the low-frequency toroidal and spheroidal modes up to 1 mHz, the strainmeters produce better signal-to-noise ratios than the broad-band horizontal seismometers. Overall, the comparison with the synthetics is satisfactory but not as good as for vertical accelerations. In particular, we demonstrate the high quality of the strainmeter data by showing the Coriolis splitting of toroidal modes for the first time in individual records, the first clear observation of the singlet 2S0/1 and the detection of the fundamental radial mode 0S0 with good signal-to-noise ratio and with a strain amplitude of 10^â11. We also identify the latter mode in a record of the Isabella strainmeter after the great Chilean quake in 1960, the detection of which was missed by the original studies
The mechanical properties of the Martian soil at the InSight landing site
The InSight mission is a NASA geophysical mission aimed at better understanding the structure of Mars and of the other rocky plan-ets of the solar system. To do so, various instruments are used, including a very sensitive seismometer (SEIS) and a dynamic self-penetrating heat probe (HP3) that have been placed on the Mars surface by the Instrument Deployment Arm (IDA). Besides geophys-ical data (which have definitely enriched and completed existing knowledge on the structure of Mars), the InSight instruments, togeth-er with orbiter observations and tests carried out on the soil with the IDA, have significantly increased the knowledge of the geologi-cal and geotechnical characteristics of the surface material at the InSight site, which is made up of a basaltic sand. In-situ data were also successfully compared with terrestrial previous estimates from terrestrial lab tests, carried out on various soil simulants. Small strain (elastic) parameters at small strains were derived from wave velocity measurements between the self-penetrating probe and the seismometer. Strength data were derived from both IDA operations and penetration data. The soil includes some pebbles within a somewhat cohesive sandy matrix, limiting the heat probe penetration to only 40 cm length. Thermal data were also obtained, allowing for some thermo-elastic modelling of the effect of the Phobos (one of the âMoonsâ of Mars) eclipses. Elastic data were also derived from the effects of wind on the ground, detected by SEIS
Constraints on the shallow elastic and anelastic structure of Mars from InSight seismic data
Marsâs seismic activity and noise have been monitored since January 2019 by the seismometer of the InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) lander. At night, Mars is extremely quiet; seismic noise is about 500 times lower than Earthâs microseismic noise at periods between 4 s and 30 s. The recorded seismic noise increases during the day due to ground deformations induced by convective atmospheric vortices and ground-transferred wind-generated
lander noise. Here we constrain properties of the crust beneath InSight, using signals from atmospheric vortices and from the
hammering of InSightâs Heat Flow and Physical Properties (HP3) instrument, as well as the three largest Marsquakes detected
as of September 2019. From receiver function analysis, we infer that the uppermost 8â11 km of the crust is highly altered and/
or fractured. We measure the crustal diffusivity and intrinsic attenuation using multiscattering analysis and find that seismic
attenuation is about three times larger than on the Moon, which suggests that the crust contains small amounts of volatiles
SEIS: Insightâs 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 Marsâ 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 Vikingâs 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 Marsâ 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 Mw ⌠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
Geophysical Observations of Phobos Transits by InSight
International audienceSince landing on Mars, the NASA InSight lander has witnessed eight Phobos and one Deimos transits. All transits could be observed by a drop in the solar array current and the surface temperature, but more surprisingly, for several ones, a clear signature was recorded with the seismic sensors and the magnetometer. We present a preliminary interpretation of the seismometer data as temperatureâinduced local deformation of the ground, supported by terrestrial analog experiments and finiteâelement modeling.The magnetic signature is most likely induced by changing currents from the solar arrays. While the observations are not fully understood yet, the recording of transitârelated phenomena with high sampling rate will allow more precise measurements of the transit times, thus providing additional constraints for the orbital parameters of Phobos. The response of the seismometer can potentially also be used to constrain the thermoelastic properties of the shallow regolith at the landing site
The mechanical properties of the Martian soil at the InSight landing site
The InSight mission is a NASA geophysical mission aimed at better understanding the structure of Mars and of the other rocky planets of the solar system. To do so, various instruments are used, including a very sensitive seismometer (SEIS) and a dynamic self-penetrating heat probe (HP3) that have been placed on the Mars surface by the Instrument Deployment Arm (IDA). Besides geophysical data (which have definitely enriched and completed existing knowledge on the structure of Mars), the InSight instruments, together with orbiter observations and tests carried out on the soil with the IDA, have significantly increased the knowledge of the geological and geotechnical characteristics of the surface material at the InSight site, which is made up of a basaltic sand. In-situ data were also successfully compared with terrestrial previous estimates from terrestrial lab tests, carried out on various soil simulants. Small strain (elastic) parameters at small strains were derived from wave velocity measurements between the selfpenetrating probe and the seismometer. Strength data were derived from both IDA operations and penetration data. The soil includes some pebbles within a somewhat cohesive sandy matrix, limiting the heat probe penetration to only 40 cm length. Thermal data were also obtained, allowing for some thermo-elastic modelling of the effect of the Phobos (one of the âMoonsâ of Mars) eclipses. Elastic data were also derived from the effects of wind on the ground, detected by SEIS