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Jupiter's moon Io is the most geologically active celestial body in our system. It has the strongest volcanoes and quakes. Of course in practice these quakes wouldn't be as dangerous as on Earth for someone on the surface due to Io's low surface gravity (0.183 g). Let's ignore that however, do we know some of the strongest Ioquakes, what would be their magnitude on the Richter scale?

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    $\begingroup$ Nobody put a seismometer on Io yet, so we don't know. The only other bodies except for Earth, where seismometers have been placed so far are the Moon and Mars. $\endgroup$ Feb 15, 2020 at 16:50
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    $\begingroup$ @AtmosphericPrisonEscape Probes could measure a quake from outer space. $\endgroup$
    – user30007
    Feb 15, 2020 at 16:52
  • $\begingroup$ How do you propose to do that? To my knowledge no mission has done so. On Earth, for exceptionally strong earthquakes we can detect density disturbances propagating from the quake into the upper atmosphere, but that is only possible due to dedicated instruments on the dedicated Earth monitoring fleets by ESA and NASA. None of those factors are given on Io. $\endgroup$ Feb 15, 2020 at 16:59
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    $\begingroup$ "With a laser or something" - very vague proposal. No, Juno doesn't have any geological mapping tools. Furthermore Juno is a mission dedicated to infer Jupiter's interior structure. To achieve this, it has a high-precision radio antenna on board which has to point at specific angles between Jupiter and Earth. There is no space there for much Io science, even if the instrument were there. $\endgroup$ Feb 15, 2020 at 17:07
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    $\begingroup$ This is a good question, but for now the only possible answer is "we don't know." Moreover, we won't know for at least another decade-plus because (1) as noted in other comments, a seismometer is needed, (2) missions that far out require over a decade of planning, development, and operation, and (2) there are no proposed Io lander missions in this decade. $\endgroup$ Feb 16, 2020 at 3:22

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I did not find any Richter-scale numbers for Io, probably since there are no direct measurements yet, or how atmosphericprisonescape puts it:

Nobody put a seismometer on Io yet, so we don't know. The only other bodies except for Earth, where seismometers have been placed so far are the Moon and Mars.

But I found some interesting comparison at on NASA's SpacePlace: High Tideo on Io!

On planet Jupiter's moon Io ("EYE-oh"), the ground itself moves up and down like an elevator taking people to the top and bottom of a 30-story building!

This would mean amplitudes for the tidal waves (and therefore also for regular Io-quakes) of ${\rm 60 \dots 90 m}$! I am not sure though where that number comes from, but it compares with ${\rm 0.2m}$ tidal waves of the Earth's solid crust which is not really measurable on Earth, except with interferometers like e.g. GEO600.

References

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  • $\begingroup$ @ConnorGarcia You mentioned an equation $\log E=5.24+1.44M$ to relate energy $E$ to magnitude $M$, but is that universal? Could we somehow get from wave high to Io-quake magnitude? $\endgroup$
    – B--rian
    Mar 4, 2021 at 10:49
  • $\begingroup$ These kind of equations come from the definition of the seismic moment. Often the magnitude as defined by Richter is used so that the moment matches in Californian earthquakes (thus shallow strike-slip quakes). Energy then comes from the energy of a seismic shear wave with the amplitudes as-is. en.wikipedia.org/wiki/Moment_magnitude_scale $\endgroup$ Mar 4, 2021 at 10:54
  • $\begingroup$ @planetmaker I do not fully understand your comment, but I understand that there are multiple ways of definining magnitude. If we ignore magnitude for a minute: Do you agree with my idea that we could in general estimate the energy of the tidal "quakes" on Io, given the wavelength (and some assumptions)? $\endgroup$
    – B--rian
    Mar 4, 2021 at 11:01
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    $\begingroup$ Yes, you need the wavelength, the amplitude and some physical properties like young's modulus, poisson number and compression modulus. These can reasonably be estimated given roughly known chemistry $\endgroup$ Mar 4, 2021 at 11:33
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    $\begingroup$ Amplitude is half the crest to trough, so it might be 30-45 meter amplitude. But is this difference from Io-quakes (plate slip/shifts) or pure tidal compression? The original Richter scale is just $M_L=\log_{10}(A)$ where $A$ is the quake amplitude (I don't know the amplitude units). The modern formula, as planetmaker points out, require more physical properties to evaluate. $\endgroup$
    – Connor Garcia
    Mar 4, 2021 at 16:48

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