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When watching news about earthquakes, I think most people are used to hear about the Richter scale, a logarithmic scale used to measure earthquake intensity. A very strong earthquake, like the 2011 one leading to the Fukushima meltdown, happens about a couple times per century, and ranks over 9.0 in this scale. Small ones happen all the time. For example, here in Brazil we had one recorded a few days ago, at 2023-11-05 in Bahia state, ranking 2.7 in this scale.

A less know fact is that we can calculate an energy equivalent for an earthquake, from its Richter magnitude. Wolframalpha gives us the following formula to calculate it:

$$ E = 10^{1.5M + 4.8} $$

Where E is energy, in joules, and M the Richter magnitude.

Using this formula I wrote a small Python script to calculate energies over the scale, but in tons of TNT equivalent, a unit typically used to describe the energy released in explosions, together with its multiples (kiloton = 1000 tons, megaton = 1,000,000 tons, et cetera):

ton = 4.184e9 #One ton of TNT corresponds to 4.184 billion joules

for i in range(2, 20):
    print('Richter',
          (richter_magnitude := i/2),
          '~=',
          round(pow(10, 1.5*richter_magnitude + 4.8)/ton, 3),
          'tons of TNT.')

Running it I got these results:

Richter 1.0 ~= 0.0 tons of TNT.
Richter 1.5 ~= 0.003 tons of TNT.
Richter 2.0 ~= 0.015 tons of TNT.
Richter 2.5 ~= 0.085 tons of TNT.
Richter 3.0 ~= 0.477 tons of TNT.
Richter 3.5 ~= 2.682 tons of TNT.
Richter 4.0 ~= 15.08 tons of TNT.
Richter 4.5 ~= 84.802 tons of TNT.
Richter 5.0 ~= 476.879 tons of TNT.
Richter 5.5 ~= 2681.688 tons of TNT.
Richter 6.0 ~= 15080.242 tons of TNT.
Richter 6.5 ~= 84802.435 tons of TNT.
Richter 7.0 ~= 476879.138 tons of TNT.
Richter 7.5 ~= 2681688.466 tons of TNT.
Richter 8.0 ~= 15080242.459 tons of TNT.
Richter 8.5 ~= 84802435.285 tons of TNT.
Richter 9.0 ~= 476879138.377 tons of TNT.
Richter 9.5 ~= 2681688466.305 tons of TNT.

With this data now we can compare earthquakes to explosive power. For example, the nuclear weapon used to level Hiroshima released an estimated energy (yield) of 15,000 tons (15 kilotons). In the table we see this is roughly the energy of an earthquake of 6.0 Richter magnitude. The first hydrogen bomb ever detonated in the Castle Bravo test had an estimated yield a thousand times higher, 15 megatons, worth close to a magnitude 8.0 in our table. In a answer I gave previously to a question in the physics stackexchange, I estimated the implosion energy of the failed Titan submersible expedition at roughly 30kg TNT equivalent, or 0.03 tons, corresponding to a small quake between 2.0 and 2.5 magnitude.

Now, for the last weeks I'm following the harrowing news and social media posts documenting the ongoing Gaza Strip siege. Amidst all the human suffering there, one thing that impressed me was the degree of buildings destruction by air raids and shelling, with imagery reminiscent of post-apocalyptic movies, or historical photos from World War II, as shown in those before and after satellite images (source):

enter image description here enter image description here enter image description here enter image description here

A new analysis by two United States-based researchers, Jamon Van Den Hoek and Corey Scher, and Al Jazeera’s AJ Labs unit showed that overall at least 16 percent of all buildings in the Gaza Strip have been destroyed. In Gaza City alone, the building destruction reached at least 28 percent.

According to the latest data from the United Nations Office for the Coordination of Humanitarian Affairs (OCHA), the World Health Organization (WHO) and the Palestinian government, and as of November 7, Israeli attacks have damaged at least 222,000 residential units, with more than 40,000 completely destroyed. (source)

Despite no nuclear weapons being used so far, the sheer amount of conventional chemical explosives already dropped there is impressive. As of November 9, 2023:

According to the Euro-Med Human Rights Monitor, Israel has dropped more than 25,000 tonnes of explosives on the Gaza Strip since October 7, equivalent to two nuclear bombs.

In comparison, the Little Boy nuclear bomb dropped by the United States on Hiroshima during World War II yielded 15,000 tonnes of high explosives and destroyed everything within a one-mile (1.6km) radius. (source)

Some of the individual bombs used are quite large, leaving craters over 10 meters wide, as shown in this photo of the aftermath of an air raid in the Jabalia refugee camp (source):

enter image description here

But a visual analysis by the Guardian has identified at least five craters in the densely populated refugee camp, which weapons experts said were left by the use of multiple JDAMs – joint direct attack munitions – in the airstrike.

Marc Garlasco, a former Pentagon chief of high value targeting during the Iraq war in 2003, told the Guardian the biggest crater at the site looked like a hit with a GBU-31. “There are other bombs it could be. That one just fits best with all the evidence. The estimated crater is around 40ft [12 metres].”

Chris Cobb-Smith, a former United Nations weapons inspector, said: “The munition is almost certainly JDAM, either a GBU 31 (Warhead Mark 84) general purpose bomb or possibly a GBU 56 (Warhead BLU 109) bunker buster. Both about 2,000lb [900kg].” (source)

So, with individual bombs containing from at least a hundred of kilograms of explosives, up to a ton, I think we should expect resulting tremors from air raids ranging approximately from 2.0 up to 3.5 in the Richter scale, according to our table.

So I ask, are seismometers over the world able to record the small earthquakes generated by these explosions? Is their sensibility enough to count or pinpoint individual strikes, or at least to record the noise? Another thing I wonder is if all this noise interfere with real earthquake detection.

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    $\begingroup$ I don't know if they are sensitive enough or not, but part of the issue is the baseline natural seismic noise level of the Earth due to all kinds of "magma displacements" (what I learned watching submarine action/adventure movies) regular low-level grinding of plates and, depending on where the seismometers are, possibly even big waves crashing (thought oit might be infrasound that I'm thinking of for that one...) $\endgroup$
    – uhoh
    Commented Nov 14, 2023 at 8:50
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    $\begingroup$ I think the key difficulty here is range. I don't know the exact numbers but I would expect that one can see the impacts on a seismometer placed for example in Tel Aviv but not in Europe or the US. I also seem to recall that if leftover WWII bombs are detonated in Germany one can see that on seismometers in Germany but I don't have a source for that. $\endgroup$
    – quarague
    Commented Nov 20, 2023 at 11:16
  • $\begingroup$ I'm not sure about your calculation : not an expert here, but I think a lot of the energy of munitions' explosions will go into the air, but into the earth. In the extreme cases of the nuclear explosions you mention, these were airbursts hundreds of metres above the ground. And of what does go into the earth, a lot will be absorbed by soil rather than making it to bedrock, wheras most earthquakes are deep enough to start there. $\endgroup$ Commented Dec 11, 2023 at 7:19

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Bomb explosions are detectable as seismic events, but the energy falls off with distance, so for quakes of less than magnitude 3 you'd have to be with a few 10s of kilometers, depending on your equipment.

The closest such equipment would be part of the Israel Seismic Network and its map at https://eq.gsi.gov.il/en/earthquake/lastEarthquakes.php of the last 30 quakes in the region isn't currently showing anything in Gaza.map of the eastern Mediterranean Sea and the surrounding land regions showing earthquakes of magnitude 2 or greater, none near Gaza

Individual bomb blasts are also short bursts of energy on the surface, often on top of or into compacted earth, unlike seismic activity which is commonly a large, longer release at depth along a faultline radiating through rock. A nuclear weapon, for example, has a much different seismic signature from the equivalent amount of TNT or modern high explosives.

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    $\begingroup$ Are you sure quakes less a magnitude 3 are detectable only up a few 10s of kilometers? For the 2.7 Bahia quake I gave as example, looking at the detailed view, one of the detection stations is given as SNDB, at 11.97°S 51.29°W, almost 900km far from the event, at 13.07°S 43.47°W. $\endgroup$
    – ksousa
    Commented Nov 15, 2023 at 18:17
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    $\begingroup$ From The Brazilian Seismographic Network: Historical Overview and Current Status at isc.ac.uk/iscbulletin/agencies/VAO-ops.pdf: "As a rule of thumb we assumed that an earthquake with magnitude mR 2.5 (M w = 2.0) is recorded to a maximum distance of 150 km, mR 3.5 (M w = 3.0) to 500 km and finally that an earthquake with magnitude mR 4.0 (M w = 3.5) can be detected out to a distance of 1200 km." So, more than a few, but not much farther for the small quakes. Note: There's a distinction between old Richter scale magnitude (mR) and the more modern scale of moment magnitude (Mw) $\endgroup$ Commented Nov 16, 2023 at 0:13
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    $\begingroup$ Thank you. Also, do the stations publish raw data, or do they perform some pre-filtering, say, eliminating surface occurrences as noise, before publishing? I thought maybe the explosions are recorded, by don't show up because surface signals are filtered. $\endgroup$
    – ksousa
    Commented Nov 16, 2023 at 0:43
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    $\begingroup$ @ksousa That's going to vary a bit. An individual station generally gives you just strength and broad directional information. They automatically feed into a central system that takes data from across a network and calculates the location, which for any region takes tuning. This can include filtering out smaller magnitude quakes which are less of a concern for immediate public reporting and response, but the data are retained for further analysis. In recent years there has been a lot of work identifying weaker seismic signals from noise with machine learning. $\endgroup$ Commented Nov 16, 2023 at 20:29
  • $\begingroup$ Great explanation. I'll wait a few days before awarding the bounty, just in case somebody else wants to weigh in. $\endgroup$
    – ksousa
    Commented Nov 16, 2023 at 21:24

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