From: stead@skadi.CSS.GOV (Richard Stead)
Newsgroups: ca.earthquakes
Subject: Re: Ferndale shocks south
Date: 1 May 92 23:54:04 GMT
Distribution: ca

I'll take this as an opportunity to try and explain faulting to those
who may be novices.

In article <tmtb6INNndv@agate.berkeley.edu>, jmcd@sag4.ssl.berkeley.edu (John McDonald) writes:
> 
> Is it just me or did anyone else notice that it seems
> that only one of the three >6.0 shocks up north was
> widely felt in the Bay Area and points south?  
> Since the first 6.9 temblor was during a busy part of
> a beautiful Saturday, i can see why so many didn't feel
> it in the Bay Area.
> The second shock, the 6.0 at 12:45 am on Sunday morning
> didn't appear to be felt by anyone around here, but the
> 6.5 three and a half hours later was felt by seemingly
> everyone (but me).  Are the wave actions different?
> Does the proximity to the San Andreas have anything to
> do with it? (ie, do waves propagate down a fault in 
> an enhanced fashion?)
> 
> John McDonald

Those are definitely part of it.  The problem here is the earthquake mechanism.
An earthquake (with a very small number of bizarre exceptions) always
occurs on a planar fault and always involves one side sliding in some
direction relative to the other.  However, the plane can be in any
orientation (vertical, horizontal or at some angle to the surface), and
the slip direction can be at any angle.  We can divide all that up into
three basic cases for shallow events (events occurring within 15 kilometers
of the surface).  These are thrust, normal and strike-slip.  A thrust
occurs on a plane that is at an angle to the surface such that one side
slides up over the other.  Normal faults (so named by their appearance
in mines in England) are also on angled fault planes, but one side moves
down and away from the other.  Strike-slip events occur on vertical
faults and one side slides by the other.

 ----------/                     /-----
 |        /                     /     |       ---------|--------
 |       /-------      --------/      |       |        |       |
 |      /       |      |      /       |       |        |       |
 ------/        |      |     /---------       |        |       |
      /         |      |    /                 ---------|--------
      -----------      -----

    thrust                normal                strike-slip

(In the doodles, the top of the page is the surface of the earth and the bottom
is deep in the earth, so it's like you were in a deep trench looking at the
fault from the side.  The strike slip shows no motion, because on side
moved toward you and the other moved away).

There is a good reason why there are three types for shallow events.
The surface of the earth is called a "free surface", basically, no one
prevents the surface of the earth from moving (the atmosphere doesn't
provide much resistance), whereas other surfaces in the earth have rock
on both sides.  A free surface means that the force in the vertical
direction is the smallest force.  In each of these fractures, the force
is strongest in a direction parallel to the surface.

Anyway, regardless of orientation, the wave field generated when a fault
slips is a "quadrupole".  All you EE's will know that one.  All a quadrupole
means is that there are four regions away from the qauke that will get
strong waves and four regions that get weak waves.  (For the EE's who haven't
heard this about quakes before, there are actually 2 quadrupoles, one for
P and one for the more substantial S waves.  The P-wave quadrupole is oriented
such that the four poles lie in a plane perpendicular to the fault plane
that includes the direction of motion.  One node coincides with the
direction of motion.  For the S-wave one peak coincides with the direction
of motion).

Anyway, the first quake was a thrust on one fault, the next two were strike-slip
on another.

More things affect the pattern of perceptible shaking.  One is "directivity".
This occurs when a very large portion of fault surface breaks (generally,
directivity has been measured on quakes of M5 and larger).  The break
starts on one part of the fault and moves at about the S-wave speed along
the fault.  Like a sonic boom, this causes reinforcement of energy in the
driection that the fault ruptured.  There is also "focusing" - afterall,
these are waves we're talking about, and certain structures deep in the
earth act exactly like lenses.  Since the first quake was in a somewhat
different spot than the later two, it is possible that it's waves are
focused slightly differently than thsoe of the other two.

Finally, there's just our own human perception capability.  We are most
sensitive to motion at certain frequencies.  But earthquakes can have
different distributions of energy in different frequency ranges, even
when they're the same magnitude (this is related ot the "stress drop"
you've heard other seismologists mention).

Hope this helps explain things a bit.
--
Richard Stead
Center for Seismic Studies
Arlington, VA
stead@seismo.css.gov


From lockhart@mothra.nts.uci.edu  Thu Jun 11 13:38:51 1992
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From: "Jack C. LOCKHART" <LOCKHART@uci.edu>
To: eugene@amelia.nas.nasa.gov (Eugene N. Miya)
Subject: AVAILABILITY OF USGS EARTHQUAKE DATA VIA MODEM
Date: Thu, 11 Jun 92 13:37:57 -0700
Status: R


Eugene,

Please include this information in the frequently asked questions that
you post every month or so to the "ca.earthquakes" and/or
"sci.geo.geology" newsgroups.  A lot of people without FTP access
desire this data and are willing to get it via modem.

Thanks,

~jack_


>From noiro.acs.uci.edu!mothra.nts.uci.edu!lockhart Thu Jun  4 13:18:29 PDT 1992
Article: 2970 of ca.earthquakes
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