On 10 September, a massive Israeli military demolition in southern Lebanon sent seismic waves through the ground that registered as a 4.1-magnitude event near Nabatieh. The explosion did not trigger a separate tectonic earthquake: it was itself powerful enough to be detected and assigned a magnitude by seismic monitoring networks.
The Israeli military said the operation destroyed Hezbollah underground infrastructure beneath the Ali al-Taher Ridge. It said the underground network extended for more than two kilometres and contained rockets, missiles, drones, anti-tank weapons, mines and explosive devices. The site also included a command complex containing command centres, weapons-storage rooms and living quarters, according to the military, as according to Reuters
Israeli Prime Minister Benjamin Netanyahu and Defence Minister Israel Katz said the demolition completed Israel’s consolidation of what they described as a security zone in the area.
It was the latest in a months-long series of demolitions that have altered the physical landscape of southern Lebanon, cratering hillsides and reportedly causing parts of mountains to collapse.
Seismographs record vibrations travelling through the ground, regardless of what caused them.They measure the energy released and the waves that energy sends through rock.
Whether those vibrations come from rock slipping along a fault or explosives detonating underground, the instrument records the resulting ground motion, which seismologists use to calculate a magnitude.
Geophysicists have long converted between the two using standard energy-equivalence formulas. Seismologists have calculated that a magnitude-4 seismic event corresponds to roughly the energy released by 1,000 tons of TNT, while each additional whole point on the scale represents roughly a 32-fold jump in energy.
But that comparison describes energy, not the exact quantity of explosives involved. Only a portion of a blast’s energy is transferred into the ground, so magnitude cannot be used to calculate its explosive yield directly.
In August 2020, roughly 2,750 tons of ammonium nitrate detonated at the Port of Beirut, a blast with a TNT-equivalent yield estimated at around 1,100 to 1,500 tons. That explosion killed more than 200 people, leveled a swath of the capital, and was felt as far as Cyprus. Yet the USGS measured it at just magnitude 3.3.
So why did the south Lebanon blast register 0.8 magnitude points higher – a difference that represents approximately 16 times as much recorded seismic energy?
One likely factor is burial depth. Munitions engineers use a concept called the coupling coefficient to describe how efficiently an explosion’s energy transfers into ground shock versus dissipating as an airborne blast wave. Explosives detonated above ground or in open air couple only a small fraction of their energy into the earth. Beirut’s ammonium nitrate went off in a surface warehouse, and much of its force radiated outwards through the air, which is exactly why it was so lethal to buildings and people while remaining, geophysically, a relatively modest tremor.
Explosives detonated inside tunnels and surrounded by rock may transfer a greater proportion of their energy into the ground. Burial depth, local geology and the placement and timing of the charges can all affect the resulting seismic signal.
Seismologists can examine an event’s waveform to assess whether it was tectonic or human-made.
Natural earthquakes generate strong shear (S) waves as rock fractures and slips; explosions, by contrast, are overwhelmingly compressional, producing a much stronger pressure (P) wave relative to the S wave. A shallow depth, a short and abrupt signal, and the absence of a typical aftershock sequence can also point towards an explosion. However, the USGS listing does not identify the cause of this event, and confirming its seismic characteristics would require analysis of the underlying waveforms.
Ali al-Taher wasn’t a one-off. On 31 August, Lebanon’s National News Agency reported that a series of “powerful and consecutive” Israeli explosions caused part of a mountain in Qantara to collapse.
The demolitions have also taken place near culturally significant sites. Al Jazeera reported an Israeli explosion near Beaufort Castle, a protected 12th-century fortress in the Nabatieh district.
