By Tiegan Hobbs, Ph.D., Postdoctoral Seismic Risk Scientist, Temblor (@THobbsGeo)
After a year of operating, the largest marsquakes recorded by NASA’s InSight Lander are associated with a nearby volcanic province: Elysium Bulge. Marsquakes have so far been too small to reveal the interior structure of the Red Planet. Scientists await larger events for even greater discoveries.
Citation: Hobbs, Tiegan, 2020, Shaking on Mars Causes Good Vibrations On Earth, Temblor, http://doi.org/10.32858/temblor.075
In April of 2019, the first ‘marsquake’ was recorded by NASA’s InSight lander. Now, the first batch of scientific papers has been released in the journal Nature Geoscience (Banerdt et al., 2020; Giardini et al., 2020; Lognonné et al., 2020). From these reports, we are getting our first glimpse into the structure of Mars and the likely volcanic sources of its seismicity. It’s an exciting time for the science community and the mission scientists, after years of build-up.
Working on our night moves
Like in a house with toddlers, we can only ‘hear’ on Mars during the evening when all is still. Wind storms during the Martian day generate a constant flurry of noise, drowning out any seismic activity detected by InSight. Further confounding attempts to listen to Mars shaking, InSight appears to have landed during a relatively quiet period for marsquakes. Seismic activity has picked up steadily between sol 72 and 299 (days since landing), although it is unclear why this is the case (Giardini et al., 2020). More marsquakes means more opportunities for scientists to learn about their origin.
Marsquakes fall into two categories, based on the frequency of their waves. High frequency events have energy in the 1-12 Hz range – meaning that up to 12 waves are measured each second. The low frequency events are more like a hum, where each wave generally stretches out for more than one second. On earth, we record earthquakes and seismic tremor at frequencies between about 0.01 Hz (one-hundredth of a wave per second) and 50-100 Hz (50-100 waves per second) – a much larger range than can be observed on Mars both due to differences in seismic sources, instrumentation, and the limited ability to transmit data through space.
Learn more: Listening to Earthquakes
Wave frequency matters to seismologists looking to understand the deep internal structure of the Red Planet. Seismic waves fade, or “attenuate”, with each cycle. A high frequency wave, which has a short wavelength, goes through more cycles to travel over the same distance as a low frequency wave with a longer wavelength. In other words: we need lower frequency waves to sample far-away or deep parts of Mars because the higher frequency waves fade too much to be useful.
Mars can’t shake it off
So far, there are far more high frequency events recorded than low:
“The 174 events detected until Sol 299 include 24 [Low Frequency] and 150 [High Frequency] events… Most events were detected during very quiet hours that were devoid of recorded wind perturbations” (Giardini et al., 2020)
The few low frequency marsquakes that have been large enough have occurred too deep in the crust to produce surface waves, a kind of seismic wave which is required for sophisticated analyses on Mars. This means there’s not yet been much of the right kind of seismic shaking to study the planet’s interior. But it’s been enough to get started.
To understand the structure of Mars, scientists are using any signal they can find. The hammer strokes designed to bury the heat-sensing mole probe supply a source of seismic energy. When winds blow over InSight, vibrations reveal the makeup of material in the top 100 meters (328 feet) of the ground. These measurements reveal that the soil around InSight is sandy, and has a lower seismic velocity than expected, suggests Lognonné and collaborators (2020).
How do seismologists locate marsquakes?
In order to identify where a marsquake occurred, scientists can’t use methods typically applied to locate quakes on Earth. Here, a network of seismic monitoring stations detect any seismic waves. Seismologists then use “triangulation” to determine where an earthquake occurred by determining the distance from the quake to three different stations. Geometrically, there is only one point that satisfies the condition of being the correct distance from three different spots: the epicenter. On Mars, however, the only station is InSight itself, so scientists have to get creative.
When identifying potential marsquakes, planetary seismologists must first exclude any other sources of noise – things like wind, the vibration of the lander or hammering from the heat probe. Then, seismologists pick out the individual elements of the marsquake seismogram, recorded by InSight. The polarization, which refers to the orientation of the components of the seismic signal, is used to determine the direction from which the wave travelled. Using a database of all possible models for the velocity structure of Mars, a range of allowable source-station distances is estimated with a very large uncertainty.
Some of the largest events, with magnitudes between 3 and 4 (Giardini et al., 2020), are thought to have occurred in the Cerberus Fossae region near Elysium Bulge (Ex: Jaeger et al., 2007). The Cerberus Fossae are deep, linear depressions, thought to be associated with development of the nearby volcanic Elysium Bulge. Although Mars has no plate tectonics anymore, it still has two plates. In the southern plate, a thick crust creates the southern hemisphere highlands. A thin crust in the northern hemisphere defines the other plate. Tectonic activity ceased on Mars as the planet cooled, or due to disruption from this thick southern hemispheric plate (Sleep, 1994; Lenardic et al., 2004). The large marsquakes, then, are likely related to thermal cooling or other stress-localization from the nearby volcano, according to Giardini and collaborators (2020).
So far, one aftershock was recorded on the Red Planet (Giardini et al., 2020). If a larger event should happen during the remainder of the InSight mission, recorded aftershocks may help describe the stress conditions driving the seismicity.
More to learn from InSight
Scientists will continue to record and analyze seismic signals from Mars, all of which are made available through IRIS, the Incorporated Research Institutions for Seismology, after embargo
Banerdt, W. B., Smrekar, S. E., Banfield, D., Giardini, D., Golombek, M., Johnson, C. L., et al., (2020). Initial results from the InSight mission on Mars. Nature Geoscience, 1-7. https://www.nature.com/articles/s41561-020-0544-y
Giardini, D., Lognonné, P., Banerdt, W. B., Pike, W. T., Christensen, U., Ceylan, S., et al., (2020). The seismicity of Mars. Nature Geoscience, 1-8. https://www.nature.com/articles/s41561-020-0539-8
Jaeger, W. L., Keszthelyi, L. P., McEwen, A. S., Dundas, C. M., & Russell, P. S. (2007). Athabasca Valles, Mars: A lava-draped channel system. Science, 317(5845), 1709-1711.
Lenardic, A., Nimmo, F., & Moresi, L. (2004). Growth of the hemispheric dichotomy and the cessation of plate tectonics on Mars. Journal of Geophysical Research: Planets, 109(E2).
Lognonné, P., Banerdt, W. B., Pike, W. T., Giardini, D., Christensen, U., Garcia, R. F., et al., (2020). Constraints on the shallow elastic and anelastic structure of Mars from InSight seismic data. Nature Geoscience, 1-8. https://www.nature.com/articles/s41561-020-0536-y
Sleep, N. H. (1994). Martian plate tectonics. Journal of Geophysical Research: Planets, 99(E3), 5639-5655.
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