QUESTION: Do those signals correspond to small earthquakes in
the USGS or Berkeley catalogs? If so, are those earthquakes located close
to the later epicenters such that they'd be classified as foreshocks?
If not, what distinguishes them from earthquakes?
REFERENCE: See Page 10, Paragraph 3 of Science Horizons Internal
Report #IR-07-0005.
ANSWER: We have not identified the precursors with any specific
physical event, e.g. earthquake or acoustic emission. What was
important was the identification of the physical characteristics of the
precursor. We have done that.
QUESTION: Since the array processing technique pinpoints an
azimuth but not a distance, there's a wide swath of earth that could
potentially have hosted the precursor source. For example, can you rule
out the possibility that you're seeing the P-wave signal from an Aleutian
earthquake? Fortunately the relevant earthquake catalogs are easily
searched for such information.
ANSWER: One way to solve this problem is to use multiple arrays to
determine the distance of the precursor.
QUESTION: What is the seismic magnitude (or equivalent
magnitude) of the precursor event? (This could be determined by
comparing the signal strength to that of the mainshock.) Given that
magnitude (i.e., strength of p wave at the source), would one expect to
see it recorded on high-gain seismic stations close to its epicenter?
ANSWER: We are working on the magnitude issue. It's not as easy as
one might think because the only relevant magnitude data that
SeismicRadar® gives us is the SNR. However, what we need to do is to
back out the signal strength of the precursor from the raw seismic
data.
I intend to install a seismic array in a tectonically active area and I
expect the number of precursors to increase substantially.
QUESTION: What is the frequency of the wave train that crossed
the Lajitas array? Given known attenuation of P-wave energy, would one
expect to those amplitudes to be observed at 1000 miles? Asking it the
other way around, given those amplitudes, what would the amplitudes have
had to be at the source? (This is similar to the previous set of
questions, asked a different way.)
ANSWER: We obtained the best results for the P-wave at
frequencies between 0.3 Hz to 1.2 Hz. For the precursors, the best
results were obtained between 0.3 Hz to 0.75 Hz. The P-wave was easy to
identify visually (See Figures 9 and 18 of Science Horizons' Internal
Report #IR-07-0005). Only SeismicRadar® was able to identify th
precursors
QUESTION: The Lajitas array has presumably collected data
before from numerous M6 events, including many off the Mexican and
Central American coast that are located closer than Parkfield (some
with much larger magnitude). Also the 2003 M6.4 San Simeon earthquake
was located very near Parkfield. Can precursors be identified for any
or all of those earthquakes?
ANSWER: Southern Methodist University is the operator of the
Lajitas array. They should answer this question. All they need is the
SeismicRadar® processing tool
QUESTION: Is there something unique about the 1000-mile
distance, or can the precursor signals be identified for earthquakes at
many thousands of miles distance?
ANSWER: There is nothing unique about the 1000 mile distance. I
chose the Gulf of Mexico earthquake because it was felt in Melbourne,
Florida. I chose the Parkfield earthquake because there allegedly were
no precursors. I chose TXAR because that was an AQUINAS® System arra
that Science Horizons delivered and we have worked well with colleagues
at SMU. It was a coincidence that both earthquakes were about 1,000 miles
from TXAR.
QUESTION: A devil's advocate would ask whether the precursors
might be either randomly correlated noise or reflective of waves passing
through the array whether or not there's a mainshock coming. It would be
interesting to "invent" some mainshocks that did NOT occur, and search
for precursors. This is commonly done (and necessary, really) in
pattern-recognition studies of earthquake signals.
ANSWER: So far we've only used a random 7 degree arc, and we
haven't been able to find any precursors in that interval.