r/Earthquakes • u/BlankVerse • Oct 06 '22
Slight Shifts in Magnetic Field Preceded California Earthquakes — Magnetometers detected faint signals that with further study, may improve our understanding of what happens before earthquakes and offer promise for early detection. Article
https://eos.org/research-spotlights/slight-shifts-in-magnetic-field-preceded-california-earthquakes4
u/subdep Oct 07 '22
The part I’m curious about is:
Are there not local magnetic field changes all the time with no earthquakes occurring nearby? Or do these “modest” magnetic changes only occur 24-72 hours before some sort of earthquake?
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u/alienbanter Oct 07 '22
I've just been reading through the actual study - it's pretty complicated, but my understanding of their basic method is that they split the time before an earthquake into periods of equal length, with one precursor p period before the earthquake and seven preceding quiescent q periods (with the periods being a multiple of 24 hours long), and then compared the signal to noise ratios in the q versus p periods. So they were using earthquake catalogs and specifically looking at the times before known earthquakes, not all time periods (in my understanding). In the discussion, they also have this paragraph:
Overall, this analysis suggests that there is an elevated 98th percentile in the p-period as compared to the q-periods, but this is not the case for every earthquake, even after the Āp correction. We do not know if this means that some earthquakes have a signal associated and others do not. We are analyzing a stochastic process and scatter is expected, especially if a real effect were masked by noise from other, unrelated processes. Also, it is important to note that the strict 72–24 hr p-period (controlled by the tuning parameters β and λ) was a tool for our hypothesis test. Plausibly, antecedent signals happen before and after this interval. We simply show that this period tends to have greater magnetic activity than its quiescent cousins.
I think this basically says that the elevated signal to noise ratio they observed wasn't present in all earthquakes either (and they also only looked at 19 events).
I think this part of the intro is a good summary:
We observed a modest association between the magnetometer measurements and earthquake occurrence 2–3 days later. The signal is not strong enough to provide useful predictions, but it does suggest a relationship that could be explored to understand a physical link. We do not address practical earthquake prediction in the form of probabilistic estimation of magnitude, timing and location as discussed for example, in Jackson (1996), nor do we provide an alarm-based criterion against which a prediction method can be evaluated (e.g., Zechar & Jordan, 2008).
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u/subdep Oct 07 '22
Yeah, that was my concern; that they weren’t looking at time periods where no earthquakes occurred in the month before or after measurement. If that same “signal” they are detecting happens just as often during non-earthquake periods, then it means they are just detecting a signal that has nothing to do with earthquakes. They already admit many of the earthquakes didn’t have a signal before it, so… further study is needed.
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u/alienbanter Oct 07 '22
Yep, and luckily they're pretty clear about this not being a "quick build a magnetic earthquake prediction machine" kind of study. I hope eventually someone can come up with study to show a plausible physical explanation for precursors like this, if they do exist.
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u/subdep Oct 07 '22
Probably has a lot to do with the rock types, strata topology, fault type, water table, etc. Under certain conditions the pressure does something to the combination of materials’ ability to conduct electricity which has an effect on the magnetic field in the local area. Some areas are probably more conducive to this effect than others.
I would be interested to see if the same effect/lack there of happens at the same locations over time during different earthquakes.
I’m just spit balling for fun here and I, too, look forward to seeing the actual scientific results.
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u/alienbanter Oct 07 '22
Since this can be a bit of a controversial topic, I'll just preemptively remind any future commenters to please follow the subreddit rules or your comments will be removed!