[Geysers] Seismographs

[Gordon Bower]

On Sun, 30 Jan 2005, Jeffrey Cross wrote:

> Several questions:
>
> 1) When Giantess registers on the seismograph, how big of an eartuquake
> does it create?  M 2.0?  1.0?

Around magnitude 0.

Here are two arguments for it. First, a calculation:

The Gutenberg-Richter relationship between the energy contained in seismic
waves and resulting magnitude is log10 E = 11.8 + 1.5M, E in ergs.

One of the good old-fashioned ways of testing a new seismograph after
installing it in the field was sending one member of the field crew some
distance away, having him swing a sledgehammer into the ground, and see if
you picked it up or not. A geyser's thump is a naturally occurring
sledgehammer. The mass of the hammerhead = the mass of the water above the
steam bubble; the distance the hammer falls = the thickness of the steam
bubble that collapses. How much energy is released when the water above
slams down onto the water below? (Some of it turns into heat and sound,
but its not a bad assumption to figure most of this energy is pounded into
the ground.)

A small geyser like West Triplet: less than 1 cubic meter of water,
falling at most 10 centimeters: less than 1000 Joules = 10^10 ergs =
magnitude -1.2 earthquake. Felt at a distance of only a few meters away.

A generous estimate for Giantess: let's say a 5x5m area of the pool,
lifted 30cm, by a bubble that collapses 3m from the surface. 220 kJ =
2.2x10^12 ergs = magnitude 0.3-0.4.

Hydrothermal explosions at Shishaldin volcano the winter of 1999-2000
caused earthquakes about M0.7-M1.1; the air-wave report showed up on the
seismograph traces 5km from the summit.

Now, an armwaving rule-of-thumb argument that gives the same answer:

With "real" earthquakes, M3s attract notice of people less than 10km away,
M5 less than 100km away. The same pattern would mean M1: 1km, M0: 300m;
M-1: 100m. Compare that to how far away from a geyser you are when you
detect its thumps:

West Triplet and Doublet: can only feel the thumbs right next to the pool;
can hear them from the Grand prediction board and Crystal Spring / from
Ear Spring and Sponge, under ideal conditions.

Sawmill start: audible from the Frog Ponds to Grand

Deep Blue: makes your knees wobble standing next to the pool, audible from
the Fountain overlook

Giantess: felt easily anywhere on Geyser Hill, audible from the Hamilton
parking lot.

GRB