From Italy to Invercargill – why the Tongan explosion was heard around the world

NZ Herald,
Publish Date
Mon, 17 Jan 2022, 10:24am

From Italy to Invercargill – why the Tongan explosion was heard around the world

NZ Herald,
Publish Date
Mon, 17 Jan 2022, 10:24am

The violent eruption of underwater volcano Hunga Tonga Hunga Ha'apai on Saturday was heard in New Zealand and picked up as far away as the UK and Italy. What did people actually hear, and just how far did the shockwaves travel? We ask the experts. 

Brian Hamilton heard what sounded like the booming of distant cannons from his rural Awhitu home in southwest Auckland. 

The weather enthusiast had been monitoring satellite imagery and saw shockwaves ripple out from Hunga Tonga Hunga Ha'apai. 

A friend estimated the sound would reach them in the next half hour. 

He was right. The first rumbles started just after 7pm, and louder ones still at 8. 

"It was like distant cannons, a popping sound ... quite surreal because beautiful blue sky on a summer day, and you could hear like fireworks a long, long way away." 

Hamilton was hard of hearing and only caught the louder ones, but his wife heard more. "You could feel it in your eardrums." 

He is one of many across New Zealand who heard the submarine volcano exploding more than 2000km away. 

Social media was abuzz with people posting from Auckland, Tauranga, Napier, Wellington, and down south as far Invercargill. 

"Am I hearing the #tonga #eruption from my house in Wellington, New Zealand? Ninety per cent sure it's that," said Daniel from Lower Hutt. 

"That's so crazy. I was in the middle of a barbecue and it sounded like it was right above me! Heard two about three minutes apart," said CJ from Tauranga. 

"Sonic boom in Napier just observed #Tonga," tweeted Craig Ireson, saying it went on for about 20 minutes. 

Hunga's eruption injected a huge amount of energy into the atmosphere, said Richard Easther, Professor of Physics at the University of Auckland. "The bigger the explosion, the more air is stirred up, so the louder it's going to be." 

"It's not one continuous wall of sound, it's a pulse of sound that works its way out, spreads out effectively as a circle centred on the volcano." 

People in the North Island would have heard it before people in the South Island. 

Sound travels about one kilometre every three seconds, so what New Zealanders heard would have taken place about two hours earlier in Tonga, some 2400km away, estimates Dr Marcus Wilson, senior lecturer of Physics at the University of Waikato. 

A single blast would disperse as the sound travels, arriving as a lasting rumble that can take many seconds to pass. "It will be the really low sounds you're hearing, a really deep rumbling." 

"Some people will hear it more strongly than others ... these are pressure waves, basically squashing and stretching the air." 

Some social media posts called it a "sonic boom" but experts say that's a misnomer. 

A sonic boom is a shock wave produced by something travelling faster than the speed of sound, like the Concorde or high-performance fighter jets. 

Easther says the eruption may have produced a sonic boom at the source, but by the time the pressure wave arrived in New Zealand it would have been moving as regular sound. 

On the other side of the world, the UK Met Office picked up shockwaves of the Tonga volcano on its observation sites, confirming earlier readings from weather enthusiasts. 

Using readings from people's backyard weather stations, Dunedin dad David Orlovich tracked the pressure wave going around the world, to Sardinia and Sicily, Kingston, and the Florida Keys. 

Historically, large volcanic eruptions have made themselves seen and heard far away, like Indonesia's Krakatoa in 1883 and in New Zealand, Tarawera in 1886. 

"People heard (Tarawera) in the South Island and thought it was a shooting," said Easther. 

Hunga is part of a string of volcanoes forming the Tonga Arc, formed by the Pacific tectonic plate subducting under the Indo-Australian plate. 

The volcano has erupted several times over the past century, in 1912, 1937, 1988, 2009, and 2014. 

Little is known of those that took place in the early 1900s but of the last three, erupted volumes appear to be increasing, says senior Lecturer Marco Brenna at the University of Otago. Saturday's eruption appears to be much larger than what the volcano has done in recent history. 

A magma reservoir some 5 to 6km underground has been feeding previous eruptions, and likely the current one as well, he said. 

Details of the Saturday eruption are still emerging. Scientists agree it may not be the last and further eruptions of equal magnitude are possible. 

- by Qiuyi Tan, NZ Herald