Re: Earthquakes and moon

From: Bigdakine (bigdakine_at_aol.comGetaGrip)
Date: 01/29/05 

Date: 29 Jan 2005 18:50:19 GMT

>Subject: Earthquakes and moon
>From: "Zinc Potterman" zincnews@tiscali.c123o.uk.(delete digits to reply)
>Date: 1/9/05 2:37 AM Hawaiian Standard Time
>Message-id: <crr8is$i9f$1@titan.btinternet.com>
>
>Apologies as I'm sure this has been asked before, but is there any research
>done into the connection between the moon and earthquakes/volcanoes.
>Surely this must be the largest disturbance to the earth likely to trigger
>the sudden slippages at faults/plates.

For small earthquakes, yes.

http://gore.ocean.washington.edu/posterspapers/Wilcocketal_GRL_2001.pdf

And
http://quake.wr.usgs.gov/research/deformation/modeling/papers/nature.html

"The seismic waves excited by earthquakes produce dynamic Coulomb stress
changes that, at distances more than about one source dimension from the fault,
can be an order of magnitude larger than the static stress changes. How can one
distinguish whether the static or dynamic stresses control seismicity? In other
words, is it strong shaking or the weak permanent stress changes that promote
seismicity? Because the dynamic stresses oscillate, they are everywhere
positive at some point in time. All sites are shaken, and thus the dynamic
stresses cast no stress shadows and should produce no antishocks (seismicity
rate decreases), at odds with observations. Belardinelli et al calculated the
dynamic stress evolution in the 1980 Mw=6.9 Irpinia, Italy, sequence in which
nearby faults ruptured 20 s apart. The second event was not triggered at the
time of the dynamic peak. Rather, a delayed triggering mechanism must be
involved irrespective if static or dynamic stresses are responsible, because
the second rupture nucleated 12 s after the dynamic peak and 6 s after the
static value had been reached. Other evidence, however, suggests that at larger
distances from the rupture, dynamic stresses may explain the distribution of
seismicity rate changes better than the static stresses. Kilb et al find that
the pattern of dynamic Coulomb stress changes bears similarities to that for
static stress changes. While the peak dynamic stress field lacks shadows, it
does exhibit lobes with small stress change in roughly the same positions as
the static stress shadows. The key difference is that the dynamic stress
increases are an order of magnitude larger in the direction of rupture
propagation. The Landers rupture propagated unilaterally to the northwest, and
produced more aftershocks in this direction. The observed seismicity rate may
thus be influenced both by static and dynamic effects.

If static stress changes influence earthquake occurrence, then seismicity rates
might be modulated by the solid earth tides. Unlike earthquakes, the tides
produce no strong motion (shaking), but they do alter the stress on faults. The
tidal Coulomb stress range, dominated by the normal-stress component, is only
about ±0.01 bar, or 1/10 the threshold of detection in the most sensitive
aftershock studies. Vidale et al calculated the tidal stresses on the fault
planes of 13,000 earthquakes along the creeping portions of the San Andreas and
Calaveras faults, finding that the seismicity rate is higher at times when the
tidal stresses unclamped the fault, but not significantly so. Lockner and
Beeler cycled stress in a laboratory sample to simulate the tides, and found
that stress changes 30.1 bar caused strong correlations in the timing of
stick-slip events, in accord with aftershock studies. They estimated that if
detection increased with the square root of the sample size, >20,000
earthquakes would be needed to find a statistically significant association
with tidal stresses, in which case ~1.5% of the seismicity would be correlated.
Vidale et al repeated their experiment with 27,500 quakes, and found that the
rate of seismicity during the peak tidal unclamping is 1.0% higher than
average, a difference significant at the 95% confidence level. Thus the tides
perceptibly alter the rate of seismicity, suggesting that the much larger
off-fault stress changes associated with earthquakes are indeed one cause of
seismicity rate changes. "

THese documents have other references.

Stuart
Dr. Stuart A. Weinstein
Ewa Beach Institute of Tectonics
"To err is human, but to really foul things up requires a creationist"
  

"Creationists aren't impervious to Logic: They're oblivious to it."

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