From: stead@skadi.CSS.GOV (Richard Stead)
Newsgroups: ca.earthquakes
Subject: Re: Alamo, Ca EQ's & conjugate faults
Date: 16 Nov 92 19:18:48 GMT

In article <1992Nov16.093222.1@novax.llnl.gov>, bjones@novax.llnl.gov writes:
> I live in Alamo Calif. There is an interesting fault structure in our area
> that is called a conjugate fault. It runs east west, linking the southern end of

For others who may not know this term - a conjugate fault is a strike-slip
fault (also called wrench fault - any fault like the San Andreas in which
the motion is one side slides horizontally with respect to the other - little
or no movement up and down) which is at about right angles to the dominant
strike slip faults in a region.  Thus, in CA, most strike slip faults are
NW-SE in trend.  So conjugate faults would be NE-SW in trend.  In addition,
the sense of motion is opposite that of the dominant faults.  For example,
the San Andreas is "right-lateral".  This means that if you stand on one
side of the fault and look to the opposite side after a quake, it looks
like everything on that side moved to your right.  A conjugate fault to
the San Andreas would be left-lateral.

(In a more general, technical sense, a conjugate fault plane is the plane
which intersects another fault plane at right angles in space, and the
line of intersection is parallel to the intermediate stress axis.  That's
a bit harder to put into everyday terms.)

> the Concord fault and the northern end of the Calaveras fault (as I understand

I'm afraid I'm not familiar with the Concord fault or the conjugate structure
in question.  Must have been away from CA too long.

> EQs were weird in that they were shallow and you could always hear them before
> you felt them. There would be a low frequency boom then a few seconds later you

That boom is the P-wave.  Most likely, you did not hear the P-wave itself,
but rather harmonics it induced in the building you were in or nearby
structures.  Generally, you would have to be within less than a mile of
the epicenter to have a chance of hearing the P-wave itself.  Any energy it
has in the range of human hearing diminishes very quickly with distance.
It is fast, short in duration, low in energy, but generally higher in frequency
than later waves.

> felt the shock wave. The initial shocks were very violent due to the shallow 

This later wave is actually several types of waves.  The S-wave is the first,
but is generally still to small in size to do much, then come the Love and
Rayleigh waves (two types of surface waves).  These are the waves carrying
the most energy, and they do the damage and the shaking.  These are normally
much longer in duration than the P-wave and much lower frequency.

I assume by "violent" you are referring to the abruptness and sharpness of the
motion you felt.  This is primarily due to how close you were to the quakes,
not so much how deep they were.  All quakes in CA are "shallow", generally
occurring between 5 and 15 km deep (though some as deep as 25 km).  However,
15 km is not all that far (10 miles).  So if you are 20 miles from the fault,
that can be close to the fault, but still much larger than the depth of the
quake.  But you have the right idea - the further away you are, the less
sharp the motion is, because that sharpness is really higher frequency
energy, and higher frequencies diminish more rapidly than the lower
frequencies.


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