[Geysers] Geysers and earthquakes

[Ron Keam]

I have just read through this article forwarded 
by Scott.  There are parallels with the effects 
of earthquakes on boiling springs / geysers.  The 
one example for which there is unambiguous 
evidence is the effect of the 1886 Tarawera 
eruption on the hot springs of the Rotorua 
geothermal system.  This has been spelt out in my 
book.  However, at the time of writing I 
deliberately refrained from presenting there an 
exhaustive analysis of the possible physical 
processes that had caused the enhanced (geyser) 
eruptive effects.

It was discovered that in the vicinity of 
Ohinemutu deformation in the form of land 
subsidence had occurred, while further south at 
Whakarewarewa no such deformation had occurred. 
And the observed unusual hydrothermal phenomena 
were different.  At Ohinemutu during the course 
of the volcanic eruption (ie. during the time the 
subsidence was occurring) a large number of 
springs and geysers broke out - most of them 
however quickly or gradually reverted to their 
state of relative quiescence over a period of a 
month or so afterwards.  At Whakarewarewa 
abnormal activities commenced eight days after 
the eruption but were persistent and still 
apparent six months later.  In order not to take 
up too much space and test the patience of 
readers I shall concentrate just on the effects 
at Ohinemutu.

I argued that at Ohinemutu one of the principal 
effects was the change in ground-water level 
which rose relative to the subsiding land 
surface.  Such would have induced springs to flow 
more rapidly, bringing hot water to the surface 
more rapidly, and indeed inducing extra boiling 
at higher levels in the geothermal plumbing 
system.  But this could not have been the cause 
of geyser displays triggered off-shore there in 
Lake Rotorua.  What I suspect was happening there 
(and also as an additional effect on land) is 
that during the passage of the earthquake waves 
steam bubbles produced during the rarefaction 
phases of the earthquakes were not entirely 
eliminated during the compression phases.  This 
could have led to amalgamation of bubbles and 
eventually to sizeable volumes of steam being 
formed in the plumbing.  Since steam has so much 
lower density than water, the pressures at the 
tops of the steam volumes would have been almost 
the same as the pressures at the bottom of the 
steam bubbles.  The general effect is therefore 
to reduce hydrothermal pressures within and below 
volumes where steam has formed, encourage 
boiling, and hence induce geyser action.

The proposed steam accumulation mechanism is not 
a million miles away from the way the authors 
(T.R. Walter and F. Amelung) explain the 
triggering of volcanic eruptions by megathrust 
earthquakes in the article Scott has drawn to our 
attention.  (See their figure 4.)

Ron Keam


>I might be sticking my neck out a bit by 
>attaching this article, but I'm sure that plenty 
>of gazers will find it of interest. Please note 
>the copyright by the Geological Society of 
>America.
>
>Downloaded online as a PDF because I am a member 
>of the GSA, published in the June 2007 issue of 
>"Geology." Even though it is short, tthe article 
>occupies 934KB due to graphics.
>
>Scott Bryan
>
>
>
>
>Get a sneak peak of the all-new 
><http://discover.aol.com/memed/aolcom30tour/?ncid=AOLAOF00020000000982>AOL.com.
>
>Attachment converted: Macintosh HD:GSA Geology 
>article.pdf (PDF /«IC») (00148112)
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-- 


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Ron Keam
The Physics Department
The University of Auckland
Private Bag 92-019
Auckland
New Zealand
Phone +64 9 373-7599 extension 87931
FAX +64 9 373-7445
EMail r.keam at auckland.ac.nz
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