Underwater Volcanic Explosion Near Tonga Sets Off Pacific Wide Tsunami

 Dang! CNN Tsunami advisories lifted in US after waves hit Tonga following volcanic eruption

An underwater volcano in the South Pacific erupted violently on Saturday, causing tsunamis to hit Hawaii, Japan, and Tonga's largest island, Tongatapu -- sending waves flooding into the capital.

The Hunga-Tonga-Hunga-Ha'apai volcano, about 30 kilometers (18.6 miles) southeast of Tonga's Fonuafo'ou island, first erupted on Friday and a second time on Saturday around 5:26 p.m. local time, according to CNN affiliate Radio New Zealand (RNZ).

The eruptions sent a plume of ash, gas and steam 20 kilometers (12.4 miles) into the air, according to RNZ. Satellite imagery showed a massive ash cloud and shockwaves spreading from the eruption. Ash was falling from the sky in the Tonga capital, Nuku'alofa, Saturday evening and phone connections were down.

The eruption caused a severe tsunami on Tongatapu, where the capital is located, with waves flowing onto coastal roads and flooding properties on Saturday.

There's a satellite video of the explosion at the link which is just stunning. A slightly less awesome, but at least embeddable one:

The explosion sent a tsunami out over the whole Pacific region, including the US Lori Dengler at the Eureka Time Standard: An unprecedented tsunami from the Tonga Islands

In the 74-year history of US tsunami warning centers, there had never been a volcanic-generated tsunami worthy of issuing a bulletin about. The software for sending alerts is predicated on an earthquake source and the warning center folks had to put something in the earthquake magnitude field in order to send it out.
Normally, the earthquake triggers the tsunami alerting process. An audible alert goes off and computer screens flash in the tsunami warning centers whenever an earthquake above a certain magnitude occurs and that initiates the analysis and bulletin dissemination. For US earthquakes, it typically takes five minutes or less to get an initial bulletin out. If the earthquake is further away, that initial bulletin may take 10 minutes.

This morning, the first tsunami bulletin from the National Tsunami Warning Center was issued at 3 a.m. PST. It was a statement — an earthquake of M1 had occurred in the Tonga Islands area at 8:27 p.m PST the day before and “Earthquakes of this size are known to generate tsunamis potentially dangerous to coasts outside the source region.” At 4:53 a.m. bulletin #2 announces that a Tsunami Advisory is in effect for all the US and Canadian West Coast and Alaska. The Pacific Tsunami Warning Center issued similar advisories for Hawaii. This time the volcanic source was included, and Ryan at the NWS deemed it worthy of waking me up.
. . .
By the time the US tsunami centers issued their first bulletin, the tsunami was approaching Hawaii. By the time I got my computer up and running, I could pull up coastal tide gauges and underwater deep pressure data and watch the tsunami progressing across the

Pacific.

After an hour or two, it became clear to me that this tsunami looked very different than typical earthquake caused ones. It was particularly evident on the DART instruments. Beginning in the late 1990s, NOAA began deploying pressure sensors on the ocean floor. These instruments now called DART, are located in deep water far from coastal topography so that they can measure the true tsunami signal unaffected by coastal amplification. They are an important tool in the warning arsenal for forecasting likely tsunami heights.

I have looked at a lot of DART graphs for earthquake tsunamis. They tend to start out with larger amplitudes and then decay and disappear over the next hour. Today’s tsunami is lasting much longer on the deep-sea instruments – four hours now and still counting. We don’t know for sure yet why it is lasting so long – whether it has to do with the eruption itself or complex interaction with the sea floor. But it likely means that today’s tsunami will last even long than usual, and my guess is that I will still be seeing traces in a few days.

On the California coast, Port Luis recorded the highest water level so far – at just over 4 feet with Crescent City a close second at 3.9. 

The vulnerability of Crescent City to tsunamis has been well known since the Good Friday 1964 quake in Alaska led to the flooding, damage and death in Crescent City. Apparently, the hydrography nearby helps to bring out the best in them.

The signal of the tsunami was also easily visible in the Aleutian Islands:

But even more than the tsunamis, which beyond Tonga, seem to have had only minor consequence, I wonder about the effect of this eruption on the climate in the medium term. Large volcanic eruptions have been observed to have strong cooling impacts on the whole earth several times in recorded history, and down through geologic time. In recent events, it was considered to be the result of sulfate aerosols blocking sunlight. 

Well, several cubic kilometers of sulfate laden seawater just had a violent encounter with a whole lot of hot lava. Just sayin.'