I recently read a web article entitled “Ten Major Natural Disasters Predicted in the Near Future” …
…and I saw Japan’s Mt. Fuji listed as Number 6.
As you know, volcanoes have an arsenal of hazards at their disposal: Lahars, Lava Flows, Pyroclastic Blasts, Flank Collapse and Ash Fall. I’ve run computer simulations and blogged here at least once on each of the first four. Because ash fall looms as the most powerful weapon in Fuji's arsenal, it seems appropriate to focus on it today.
What do we need to run a computer simulation of ash fall?
(1) Because prevailing winds carry volcanic ash, the first requirement is a wind map. Winds vary with geographic location, season and altitude. The National Center for Atmospheric Research (NCAR) conveniently supplies this information…
The next two ingredients are:
(2) An ash injection rate into the atmosphere over the volcano at various altitudes, and
(3) A rate at which the ash drifts back down to Earth.
Using my (well greased) ‘Geophysical License’ and information from the previous eruption in 1707, I cooked up a plausible scenario.
--Inject 1 1/4 cubic kilometers of ash into the atmosphere above the mountain over ten hours.
--Let material from the highest injection level (16 km) fall to Earth over several hours.
Here is my prediction for the next Fuji eruption….
As you can see, prevailing west to east winds at nearly all altitudes over Fuji direct ash toward Tokyo Bay.
Now don’t expect to find Tokyo buried like Pompeii from Vesuvius -- most places might suffer accumulations of 2 to 40 cm. Hardly more than a small winter snowfall you’d think, but unlike snow, even one cm of ash will kill food crops and make life difficult for animals and people for years afterward. Good reason that risk modelers and insurance folks are deeply concerned about economic losses associated with potential volcanic eruptions…
Couple the potency and widespread impact of ash fall with the dense population and infrastructure concentration of the Tokyo Bay region, I can understand why Fuji made #6 on that list above.
Steven N. Ward Santa Cruz
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John Rundle is a Distinguished Professor of Physics and Geology at UC Davis and the Executive Director of the APEC Collaboration for Earthquake Simulations. He chaired the Board of Advisors for the Southern California Earthquake Center from 1994 to 1996. Read John's blog.