reply to post by zenius
Interesting, that. Good to be able to see ground motion displayed that way. Bookmarked the page as it's one I didn't already have in my ever-growing
lists of useful sites.
And yes, it looks a bit wavy. We'd need to know what the amplitude is. It could be in fractions of millimetres. In any case, they're quite
By the way folks, if you've been checking the Pacifc Northwest Seismic Network (PNSN) webicorders and you're wondering about the rather large and
longish trace that showed up on most of them just before 11:10 UTC today, don't be too concerned, as it's not a local quake. It's from a
magnitude 6.4 quake south of the Fiji Islands
occurred at 10:57:51 UTC. According to the theoretical P-Wave travel times map
this quake, they'd take about 12 minutes minutes to reach the PNW and the quake was certainly strong enough to be recorded by seismos there. Adding
12 minutes onto 10:57:51 gives us 11:09:51, which is near as dammit to the time it shows up on the PNSN webicorders.
(St Helens - West) shows a nice trace for this quake. How well
the individual webis show it depends on their settings, but this one (at 100 microvolts) is a fair example for any who are new to this. It's what is
commonly called a "teleseismic" trace. While these larger quakes can be terrible for people who live near them, the teleseismic records of them are
no big deal. We expect
to see these traces! In fact, they are helpful in confirming that equipment is working properly, especially when the
seismo is otherwise so "quiet" that we might wonder if it is actually set up properly!
I mention all this because we can't assume that everyone on here knows these things and they're useful things to know.
Also, because we
sometimes get new threads popping up -- with lots of UPPER CASE and exclamation marks!!!! -- to tell us of a sudden, "global seismic event" that has
shown up on webicorders all over the world (and that invariably
spells some kind of widespread gloom and doom), it's worth knowing that such
posters are usually either posting out of ignorance, or else they do
know the real story but they're just scaremongering and gathering flags
and stars from those who don't know.
These so-called "global" events are nearly always due to larger quakes in one place that are "felt" (by seismo equipment) over much of the world,
except for the "shadow zone" areas. Even mag six events are quite enough to do this and as they happen on average a few times every week, we get
quite a lot of these "global seismic events".
About the only exceptions to the above proviso are the true
global events, where the whole planet "rings like a bell" (vibrates) following a
massive quake. Thankfully, such huge quakes are rare: they generally need to be at least a mid to high-range magnitude 8 to do this. Following the
disastrous 2004 Asia quake, with a magnitude variously given at between 9.1 and 9.3, the whole planet vibrated as much as 1 cm. That may not sound
like much, but when you consider the energy required to do that it's really quite massive.