<p>One station still measures the arrival of several wave types which originate from the same source (e.g. a Mars quake). Using the differences in the arrival time allows you to infer properties of the subsurface. The more quakes you have, thus the more directions you can map, the better you can constrain the underground beneath your station. However it requires you to make some model assumptions to understand what the arrivals mean.</p>
<p>The abstract of reference 10 in your cited paper, (<a href="https://doi.org/10.1126/science.abf8966" rel="nofollow noreferrer">Knapmeyer-Endrun et al, 2021</a>) reads</p>
<blockquote>
<p>Here, we determine the structure of the crust beneath the InSight landing site on Mars using both marsquake recordings and the ambient wavefield. By analyzing seismic phases that are reflected and converted at subsurface interfaces, we find that the observations are consistent with models with at least two and possibly three interfaces. If the second interface is the boundary of the crust, the thickness is 20 ± 5 kilometers, whereas if the third interface is the boundary, the thickness is 39 ± 8 kilometers.</p>
</blockquote>
<p>Here they take the seismic data (black) and compare them with 2-layer and 3-layer models where the rock properties are constrained by other measurements, e.g. mineralogy, gravimetry etc. Red and blue lines differ for the model aproach being used:
<a href="https://i.sstatic.net/82CYH2GT.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/82CYH2GT.png" alt="seismic wave data (black) and models" /></a></p>
<p>This method is also commonly employed on Earth itself, and especially useful to determine the immediate underground as well as the global structure of the body itself.</p>

CC BY-SA 4.0
2024-08-28T02:27:38.070Z
Thank you for taking the time to answer. Your explanation is very helpful.
John McCarthy
CC BY-SA 4.0
2024-08-26T22:02:18.993Z
2024-08-26T22:02:18.993Z
planetmaker
<p>The Insight lander touched down on Mars in 2018. It deployed its seismometer and gathered 4 years of data. Recently, a paper was published which, based on Insight data combined with modeling, concluded that &quot;A mid-crust composed of fractured igneous rocks saturated with liquid water best explains the existing data.&quot; Naturally this has made a splash in the news. The raw data consists of measurements (or estimates?) of the S-wave and P-wave seismic velocities and the bulk density. Not being a seismologist, I don't understand how one can measure a velocity with a single station. Can someone explain, or refer me to a source understandable to someone (like me) with a good grounding in physics but none in seismology?</p>
<p>The paper is &quot;Liquid water in the Martian mid-crust&quot;, Wright et al, 2024, PNAS, Vol. 121 No. 35. It can be found at <a href="https://www.pnas.org/doi/10.1073/pnas.2409983121" rel="nofollow noreferrer">https://www.pnas.org/doi/10.1073/pnas.2409983121</a></p>

CC BY-SA 4.0
2024-08-25T09:40:58.120Z
Did you follow any of the (extensive) references that paper gives on that topic?
planetmaker
CC BY-SA 4.0
2024-08-26T01:09:25.113Z
They're behind paywalls. If I were a student or a professor, I could read them. But even so, they're written for people who already understand seismology, which I don't.
John McCarthy
<seismology><mars>
Measuring seismic velocity with Insight lander's single seismometer
John McCarthy
# Measuring seismic velocity with Insight lander's single seismometer

(Asked by: username_0 on 2024-08-24)

The Insight lander touched down on Mars in 2018. It deployed its seismometer and gathered 4 years of data. Recently, a paper was published which, based on Insight data combined with modeling, concluded that "A mid-crust composed of fractured igneous rocks saturated with liquid water best explains the existing data." Naturally this has made a splash in the news. The raw data consists of measurements (or estimates?) of the S-wave and P-wave seismic velocities and the bulk density. Not being a seismologist, I don't understand how one can measure a velocity with a single station. Can someone explain, or refer me to a source understandable to someone (like me) with a good grounding in physics but none in seismology?

The paper is "Liquid water in the Martian mid-crust", Wright et al, 2024, PNAS, Vol. 121 No. 35. It can be found at <https://www.pnas.org/doi/10.1073/pnas.2409983121>

**Tags:** seismology, mars

> **Comments:**
> **username_1**: Did you follow any of the (extensive) references that paper gives on that topic?  
> **username_0**: They're behind paywalls. If I were a student or a professor, I could read them. But even so, they're written for people who already understand seismology, which I don't.  

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## 1 answers

### Answer by username_1 on 2024-08-26. Score: 1

One station still measures the arrival of several wave types which originate from the same source (e.g. a Mars quake). Using the differences in the arrival time allows you to infer properties of the subsurface. The more quakes you have, thus the more directions you can map, the better you can constrain the underground beneath your station. However it requires you to make some model assumptions to understand what the arrivals mean.

The abstract of reference 10 in your cited paper, ([Knapmeyer-Endrun et al, 2021](https://doi.org/10.1126/science.abf8966)) reads

> Here, we determine the structure of the crust beneath the InSight landing site on Mars using both marsquake recordings and the ambient wavefield. By analyzing seismic phases that are reflected and converted at subsurface interfaces, we find that the observations are consistent with models with at least two and possibly three interfaces. If the second interface is the boundary of the crust, the thickness is 20 ± 5 kilometers, whereas if the third interface is the boundary, the thickness is 39 ± 8 kilometers.

Here they take the seismic data (black) and compare them with 2-layer and 3-layer models where the rock properties are constrained by other measurements, e.g. mineralogy, gravimetry etc. Red and blue lines differ for the model aproach being used:
[![seismic wave data (black) and models](https://i.sstatic.net/82CYH2GT.png)](https://i.sstatic.net/82CYH2GT.png)

This method is also commonly employed on Earth itself, and especially useful to determine the immediate underground as well as the global structure of the body itself.

> **Comments:**
> **username_0**: Thank you for taking the time to answer. Your explanation is very helpful.  

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