PSN friends--I have noted several references to directional performance =
on a typical 15-20 sec "Lehman" design.  It was our observation that =
very few quakes  were blocked out due to orientation.  One summer when =
mictoseisms are lowest (here in Virginia) we ran two systems with booms =
at right angles, N-S & E-W.  We copied 66 events in two months.  There =
were signatures in both orientations--(excepting for a half-dozen small =
events) and we noted the predictable comparisons with incoming P,S, & L =
wave fronts.  P waves show strong coming in broadside.  S waves show =
strongest coming in off the end.  Likewise the Surface waves -S show a =
distinctive difference whether they be Love waves or Raleigh.  You =
simply cannot hide from a readable event due to orientation.
   This test was made with systems having no filtering excepting to =
remove mechanical room noise typical in a building, and 60 hz AC.  We =
were satisfied the generalizations above were verified, as we copied =
several events (Alaskan or thereabouts--coming into our booms at 45 =
degrees) which appeared identical on our n-s,e-w readouts.  Running two =
long period systems is a luxury now, but in earlier days of the big =
bulky worldwide sensors, dual copy was standard--with few stations to =
get a tag on direction of event.
  Here in VA. we usually ran our system broadside a few degrees off E-W =
to accent the events of South Pacific, but any South American events of =
size would boom right in with lower P waves and heavy S wave signatures.
   Another side effect of noting amplitude of S or surface wave =
arrivals--one can tell whether the event has a shallow focus--less than =
30 km, or a deep focus of several hundred km--even before you are told!!
   I hope this is informative.  Jim Lehman







PSN friends--I have noted several =
references to=20
directional performance on a typical 15-20 sec "Lehman" design.  It =
was our=20
observation that very few quakes  were blocked out due to=20
orientation.  One summer when mictoseisms are lowest (here in =
Virginia) we=20
ran two systems with booms at right angles, N-S & E-W.  We =
copied 66=20
events in two months.  There were signatures in both=20
orientations--(excepting for a half-dozen small events) and we =
noted the=20
predictable comparisons with incoming P,S, & L wave fronts.  P=20
waves show strong coming in broadside.  S waves show strongest =
coming=20
in off the end.  Likewise the Surface waves -S show =
a distinctive=20
difference whether they be Love waves or Raleigh.  You simply =
cannot hide=20
from a readable event due to orientation.
   This test was made with =
systems having=20
no filtering excepting to remove mechanical room noise typical in a =

building, and 60 hz AC.  We were satisfied the generalizations =
above were=20
verified, as we copied several events (Alaskan or thereabouts--coming =
into our=20
booms at 45 degrees) which appeared identical on our n-s,e-w =
readouts. =20
Running two long period systems is a luxury now, but in earlier days of =
the big=20
bulky worldwide sensors, dual copy was standard--with few stations =
to get a=20
tag on direction of event.
  Here in VA. we usually ran our =
system=20
broadside a few degrees off E-W to accent the events of South Pacific, =
but any=20
South American events of size would boom right in with lower P waves and =
heavy S=20
wave signatures.
   Another side effect of =
noting=20
amplitude of S or surface wave arrivals--one can tell whether the event =
has a=20
shallow focus--less than 30 km, or a deep focus of several hundred =
km--even=20
before you are told!!
   I hope this is =
informative.  Jim=20
Lehman
   =