[Sca-cooks] small earthquakes

[Daniel Phelps]

Stefan wrote:

I'm sorry to hear of your discomfort. But I suspect that these small
earthquakes are actually good, since they let off some of the
building pressure.


These small quakes do minimal damage, even if they
are disconcerting. However, if the pressure between the sliding
tectonic plates is not occasionally relieved, it continues to build
until it overcomes the friction holding the plates back and that is
when you get the much bigger 6, 7 and 8 magnitude (on the Richter(?)
scale) quakes. Those are the ones that do major damage and in which
people get hurt and killed.

Yes but there are even smaller movements on some faults, albeit more or less
constantly, in such places as Houston.  Such movement occurs with little or
no notice at all.  People drive over the fault scarp of the Long Point fault
in Houston every day without knowing it is there.  Every few years the
vertical displacement is sufficient that a little road work needs be done.
Such faults are called growth faults.

Regarding the theory of small quakes relieving building strain on earthquake
associated faults some might be interested in an experiment originally
proposed back in the early 70's.  It seems IIRC that injection of fluid in
certain areas out west was shown to trigger minor earth quakes, microseis.
Data showed that they could, albeit crudely, turn such on and off  with
pumping.  Some bright boffins suggested placing injection wells on certain
California faults and thus, through pumping, dissipate the strain building
toward "the big one" with "minor" quakes.  Nice in theory; but just where to
place the wells, how much to pump, who get hung out to dry if "the big one"
occurs after work begins or even if the "minor" quakes turn out to not be so
minor... all these questions gave everyone pause.  A nice moral quandary,
should we attempt to prevent a disaster or say "we do not have enough data
to act" and let the chips fall where they may.

Got me.  I do all my geologic research in and around Florida these days.

Daniel C. Phelps, P.G.