From: stead@skadi.CSS.GOV (Richard Stead)
Newsgroups: ca.earthquakes
Subject: Re: AGU Annual Meeting Brief in Science Magazine
Keywords: Landers, Andy Michael
Date: 9 Jan 93 00:58:38 GMT
Distribution: na

Andy makes a great post, but I thought I'd expand on a couple points, just
for general information.

In article <1ikqc8INNav9@morrow.stanford.edu>, andy@pangea.Stanford.EDU (Andy Michael USGS Guest) writes:
> And its rupture direction directed a lot of energy
> toward the east side of the Sierra intensifying its effects at many of
> these areas.  The Cape Mendocino mainshock ruptured out toward the ocean
> and so did not transfer as much energy to these regions.  Basically what

Just want to make a rather nit-picky point here.  Rupture direction
changes the power experienced at a particular remote point, not the total
energy.  For example, if you are exactly in the direction that a
strike-slip fault propagates, then the S-wave energy all arrives in a very
short wave packet of large amplitude.  This represents a lot of energy
in a short amount of time - large power.  If the fault propoagated in
the other direction, then the S-wave duration is increased by about
twice the duration of the quake.  But the amplitude is much reduced and the
energy is spread across that entire time - lower power.  For a rigid
structure, the long duration shaking can be resisted, but the high power
pulse could break it.  For a resilient structure, the high power pulse
can be shrugged off, but the long, repetitive lower amplitude pulses
may weaken it.  A quake can also initiate in the middle of a fault and
propagate in both directions - then either end gets about the worst of
both types (a high-power pulse followed by a long-duration low-power pulse).

Note that the rupture direction mainly affects the properties of S-waves.
This is because fracture propagation procedes at just a little less than
the S-wave speed of the material.  This leads to constructive reinforcement
in the direction of propagation (the S-wave from the first bit of fault
arrives at the second bit just as the second bit is making its S-wave, etc.)
P-waves are traveling faster and are affected differently.

The total S energy received can vary according to the direction you are
from the fault.  45 degrees from the fault (either end, either side) are
"nodal lines" for S.  This means that an ideal fault would generate no
S energy along those directions.  The energy increases rapidly to the
maximum as you move away from 45 degrees.  because faults are not ideal,
there is energy along the nodal lines, but it is much lower than other
directions.

> >What exactly is microseismicity? Also is the computer program from the
> 
> Small earthquakes.  Its a fairly loosely used term for anything under
> M4 or so.

I'll also add that microseismicity is to be distinguished from microseism
(even though the words are very similar).  The microseism is noise at about
7 seconds that is due primarly to waves hitting the shores of the continents.
It is detected all over the world.  Just wanted to head off any potential
confusion since both words are often used by seismologists without explanation.


--
Richard Stead
Center for Seismic Studies
Arlington, VA
stead@seismo.css.gov
Newsgroups: ca.earthquakes