On Jun 12, 8:35*pm, wrote:

Keep it coming, Mr McNeil.....!!

I'm sorry you don't fooly appreciate my gift but here goes:

Earthquake size is expressed by its magnitude.

Magnitudes usually are measured from the amplitude and period of
seismic signals as they arrive and are recorded at a seismic station.
For a given earthquake, the amplitude decreases with increasing
distance (due to attenuation of the signals) and a distance dependent
correction is applied when computing magnitude to result in one
magnitude value for each station.

Earthquake size does not depend on where an earthquake was recorded,
this is contrary to felt effects - the intensity - which decreases
with distance from the earthquake source.

Several methods exist on how to compute magnitude - in principal, all
methods provide the same or a similar value. However, there are
fundamental differences on how these magnitudes are computed
(sometimes resulting differing magnitudes). Here is a short summary,
describing various magnitude types:

ML:
The local magnitude ML is computed for earthquakes, which occurred
relatively close to the recording stations. Typically this is done for
earthquakes within a few hundred kilometers between the earthquake and
the recording station. The first magnitude scale developed 1935 by
Richter (the 'Richter-Magnitude') is such a local magnitude; even
today earthquake size is commonly given as 'Richter-Magnitude'.

mb:
The body-wave magnitude mb is typically recorded for earthquakes that
occurred more than about 2000 kilometers away from the recording
station. It can be computed relatively fast, because its value relies
on the amplitude of the so-called P-phase of an earthquake. P-phases
are waves travelling through the body of the earth's interior and are
the first signal that reaches a seismic station.

For large earthquakes (magnitude larger than 6), mb 'saturates',
meaning that even if the actual size of the earthquake is larger, the
value of mb does not increase any more. In such cases, seismologists
have to rely on other magnitude types.

MS:
The surface wave magnitude MS is measured from surface waves. These
waves travel along the surface of the earth with a velocity much
slower than P-waves travel through the earth. Therefore, one has to
wait a longer time, until these waves arrive at a distant station and
MS cannot be computed as rapidly as mb. Depending on distance, it may
take up to 1 or 2 hours until surface waves arrive, compared to a
maximum of 20 minutes of P-waves.

MS is measured from 20 s period waves (compared to 1 s for mb) and
'saturation' begins only for very large (magnitude larger than 8)
earthquakes. The slow surface wave speed is the reason, why
seismologists cannot distinguish quickly between a strong and very
strong (magnitude 6) earthquake.

Earthquakes close to the earth's surface (say, the upper 30
kilometers) generate large surface waves compared to a same-size
earthquake at larger depth (this has to do with how surface waves are
generated).

Shallow earthquakes are more prone to cause damage than deep ones; a
high MS-value compared to the mb-magnitude thus indicates that strong
damage might have occurred for an earthquake close to a major urban
area.

The ratio between MS- and mb-magnitude is also a good measure to
distinguish earthquakes from (nuclear) explosions. Explosions have a
much smaller source-volume than similar sized earthquakes and
explosions typically cause less shearing motion (which mainly generate
surface waves) than earthquakes.

Explosion MS-values are thus typically much smaller than for an
earthquake of the same size. For shallow seismic events, the mb/MS
ratio is thus a good discriminant (large ratios pointing to an
explosion).

Mw:
The moment magnitude Mw is the only magnitude that is directly related
to the physics at the earthquake source. Mw is derived (based on
theoretical considerations) from the seismic moment M0, which is the
product of the fault area times average displacement at the fault
times material rigidity.

In theory, Mw does not saturate since M0 includes the complete
earthquake rupture.

Several ways exist to determine Mw; often Mw is obtained by fitting
seismic waveforms or spectral amplitudes by scaling synthetic
seismograms to match observed seismogram amplitudes. The procedures
are (a bit) more time consuming than simple seismogram amplitude
measurements (ML, mb, MS) and Mw for larger events globally are
currently available several hours after an earthquake.

M:
Whenever the magnitude type in one of our lists is given as 'M', this
means, that the seismological observatory reporting the specific
magnitude did not specify how the magnitude was computed. Often, these
are magnitude values from the NEIC.

You may assume, that such a magnitude value represents 'their best
effort', and for strong earthquakes such magnitudes often are
magnitudes of the type Mw.

http://www.seismo.ethz.ch/redpuma/magnitudes.html

*******

What this all does not say is that the background noise is 5.5 M
"whatevers" for negative North Pacific and Atlantic anomalies and for
positive anomalies according to the height and depth of the sea level
highs and lows, the likelihood for runs of greater than 6 M. increases
with the contrast of air pressure systems at sea level.

(I have no idea how upper air levels affect the convergence of sub
soil acoustics. No doubt that which accompanies severe storms
accompanies severe earthquakes.)

*******

Some more background:

MAP 5.1 2009/06/12 14:32:56 -17.355 167.637 35.9 Vanuatu
MAP 4.1 2009/06/12 11:42:52 53.090 172.766 15.6 Near
Islands
MAP 6.0 2009/06/12 09:44:20 -17.611 167.748 52.6 Vanuatu
MAP 5.0 2009/06/12 09:24:45 -17.540 167.677 53.6 Vanuatu
MAP 5.0 2009/06/12 08:47:43 -17.491 167.650 53.2 Vanuatu

Having removed everything below 4M. (blasted Alaskan 4.1!!!) we have 4
medium sized quakes showing up consecutively at one place. (OK, I'm
hoping the Alaskan will be marked down, they usually drop a couple of
magnitudes on closer inspection.)

I haven't checked the Atlantic sea level charts but I have no doubt
that there were a sequence of parallel occlusions accompanying one or
two Lows predicted before the Vauatuans occurred.

A pair indicates that the meteorological storms are ended. That is,
there will be no more tornadoes; and three indicates that the storms
extant are ended but that another is due to scale up. 4 of them, on
the other hand, is something quite rare. And it is a pity that that
bloody Aleutian took place when it did to muck things up.

But there you go.

Or not as the case may be.