Seismological data suggests that six blasts preceded the Beirut port explosion, the last of them a combustion of fireworks that apparently set off a warehouse full of ammonium nitrate, an Israeli analyst said on Thursday.
The six blasts were at 11-second intervals during the August 4 incident, with the main explosion following the last by around 43 seconds, Boaz Hayoun of Israel's Tamar Group told Reuters.
Hayoun, a former military engineering officer whose current roles include overseeing safety standards for explosives use in Israel, said his analysis was based on data from seismological sensors stationed across the region.
"I cannot say categorically what caused this, but I can say these blasts were at the same location," he told Reuters.
Among the sensors cited by Hayoun was an array installed about 70 km (43 miles) off Lebanon's coast by the international geological project IRIS – which cast doubt on his conclusions.
IRIS said its sensors picked up more than five "small bursts" at intervals of around 11 seconds before the main Beirut explosion, a sequence that continued after the incident.
"I do not believe that they are associated with the large explosion in Beirut," Jerry Carter, director of IRIS data services, told Reuters.
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"They could be from a seismic survey," he added, referring to geologists carrying out airgun bursts for underwater mapping.
Lebanese officials have blamed the explosion, which killed at least 172 people and left much of the capital in ruins, on a huge stockpile of ammonium nitrate catching fire after being stored unsafely at the port for years.
President Michel Aoun has said investigators would also look into the possibility of "external interference" such as a bomb, as well as negligence or an accident as causes.
Hayoun assessed that the Beirut incident involved underground explosions. The 43-meter (140-foot) deep crater at the port could not have been left by the explosion of the amount of ammonium nitrate reported by Lebanese authorities, he said:
"It would have been shallower, maximum 25 or 30 metres."
The main explosion, of the ammonium nitrate warehouse, was preceded by a nearby fire.
Hayoun said that having seen footage of that fire he was convinced it was caused by the combustion of fireworks – and that this would have been sufficient to set off the ammonium nitrate.
Israel Defense, a leading private online journal with close ties to the Israeli military establishment and which first reported Hayoun's findings, described his analysis of a possible blast sequence as consistent with munitions detonations.
Such a sequence could be consistent with "weapons systems that are activated in a chain" and which might have been stored in the port and set off accidentally or deliberately, said Israel Defense.
However, it did not provide evidence to suggest sabotage.