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A Study of Thermoelastic Effects in Seismic Monuments and Their Signature in Low-Frequency Seismic Data Collected on Earth and on Mars

Session: Insight Seismology on Mars: Results From the First Martian Year of Data and Prospects for the Future [Poster]

Type: Poster

Date: 4/23/2021

Presentation Time: 11:30 AM Pacific

Description: 

The NASA mission InSight to Mars observed a number of Phobos transits which resulted in a transient tilt of the VBB broad-band seismometer (Stähler et al., GRL, 2020). Triggered by these eclipse events which each last at most 30 seconds we conducted experiments at Black Forest Observatory to corroborate the interpretation that the seismometer tilts result from changes in illumination and subsequent thermoelastic ground deformations.

While both the VBB on Mars and observatory grade terrestrial seismometers are furbished with extensive thermal shielding it is not a priori clear how a change in outside irradiation can lead to a seismometer tilt with a delay of only 5 to 10 seconds, as observed on Mars.

Our experimental setup consists of a 100 Watt incandescent light bulb with which we illuminate piers with thermally shielded seismometers in the BFO mine. By placing a bulb directly on the concrete pier and switching on the bulb for only 1 minute at each location we have mapped out the tilt response as a function of the bulb location. To first order the seismometer tilts away from the bulb location and the degree of tilting decays with distance away from the bulb.

While the skin depth for a heat pulse of 30s duration is only 2 mm for concrete and less than 0.5 mm for Martian regolith, this thin surface layer is still elastically coupled to the volume below and if the surface layer expands (light bulb experiments) or contracts (Phobos transit) the volume below must follow based on Hooke's law.

We find that only pier instruments show a clear response to illumination events while sensors installed in 160 cm deep post holes show almost no reaction. Our experiments also lead us to suggest that to further improve low-frequency horizontal component seismic recordings it may be beneficial to thermally isolate not only the sensors but also the pier on which they are installed.

Finite element calculations are planned to simulate these experiments and to further confirm the interpretation.

Presenting Author: Rudolf Widmer-Schnidrig

Student Presenter: No

Authors