Hotspots. There are several places in Yellowstone where I see quakes day after day after day. It's very interesting watching those spots and micro-analyzing the quake data around them. Usually, what happens is, there's a quake down deep somewhere, then a lot of other quakes ascending from the original spot, sometimes as much as a few miles, without quite making it to the surface. That's what happens at one of the spots, located just northwest of the caldera at 44 deg, 39 min N and 111 deg, 3 min, 36 sec W (44.65N, 111.06W), on a regular basis. Let's call it "Hotspot 1." Look at March 5th through 7th of 2006, for example, the ones with "YWB" in the right column (graphic version). Most of the 29 quakes stayed between 5 and 7 miles deep. That's good. Other places aren't so deep. That's not so good. Then there's January 19th-21st, 2001, another lovely burst at that spot; there were about 315 quakes over a 2-day, 15-hour period (graphic version). It's been around for a long long time. And after all these years, it's still percolating.
Hotspot 2 is just southwest of the lake, and inside the caldera. This one only appeared very recently. It's at around 44 deg, 25 min, 12 secs north and 110 deg, 25 min, 12 secs west. Yes, that's right; the spot is 25'12" off each major lat/lon line. That's 0.42 in decimal, so 44.42 N, 110.42 W. Isn't that odd? March 18, 2006 (graphic version) was especially memorable there. It started with a bang about 2 miles deep, and ended with an even bigger quake about 300 feet deep. It's like there's a vertical fault line there; the quakes (about 27 of them on that day) resonated up and down that fault (shaft maybe?) for about three hours before the final one that brought it all to a halt. Here's a seismogram for that day, from the closest station to that spot (The Promontory). It's very interesting. Given the depth of the final quake in the series, it almost looks like an eruption was just barely halted before it began. Remember, most quakes in Yellowstone are caused by magma movement putting pressure on different areas in different ways which change over time as things melt and move. There are long, low-amplitude tremors of long duration in that seismograph, and to me that means Harmonic Tremor, defined as "A continuous release of seismic energy typically associated with the underground movement of magma. It contrasts distinctly with the sudden release and rapid decrease of seismic energy associated with the more common type of earthquake caused by slippage along a fault." Look at it again. 11:34:30 to 11:35; 11:59 to 12:02. Harmonic tremors before and after the near-surface quake. That means after the quake happened, magma was still moving, but the new pressure was never released by another quake. There was a tiny quake the next day at 11:20, but at a depth of 4 miles. Then on March 23rd, another big quake. I don't know its coordinates yet, but note the protracted harmonic tremors after it. Where else would it be but right where we're studying? (Update, 4/1/06 [no joke]: according to Them, there was not one single quake in Yellowstone on 3/23/06, but that tremor is HUGE at the YTP station and much smaller at the LKWY and YLT stations. It COULD thus have non-seismic origins... but it sure LOOKS quakey, especially the harmonic-tremor-like traces after it.)
Another hotspot that's been less obvious until now is under the north side of Lake Yellowstone. From 12/27/08 to 1/4/09, almost 500 quakes occurred in a narrow band under the lake nowhere near any known fault lines (and yet, exactly coincident with the infamous Bulging Lake Bottom Scare from 2003 or so). And they weren't piddling little 1.2-magnitudes, either. A hundred of them were over 2.0; 18 were over 3.0. It's the largest swarm of "larger" quakes I've seen so far. That implies to me that the ground is moving more freely in that area now, and that means it's not as strong anymore. Now, does anyone know what would happen if a vertical shaft were to open up allowing the lake to drain into the magma chamber? Anyone? No? Well, it'd flash right into steam while trapped underground, that's what it'd do, and the pressure would become so tremendous so quickly that the ground above it would all give way and push the rest of the lake into orbit. Many, many volcanoes have exploded from exactly that trigger: water trapped in the rocks around its throat finds a path to the magma, turns instantly to steam, and blows the whole mountain apart. Yellowstone would super-blow. I wonder how much warning they'd get... "Hey, the lake's drai-" *BOOM*
Two of the animated images below show this Lake hotspot acting up. It's interesting to see its migration over the years; 9/29/1998, it looks like it was where it is now, but on 3/18/2006, it's farther to the southwest. Yet it never got really active until December of 2008, and then it went insane. Taking both the number and magnitude of the quakes into consideration, it's the closest we've come yet to a full blow-out, so I've devoted a whole section to that timeframe. (Just be glad I'm not disturbing you with what the Bible Code says about it. The summary: April of 2010. Buy enough food to last until Dec. 21st, 2012. After that you'll probably have no need for food.)
Those are just some of the obvious ones I know about right now. I'll probably find more later. For now, to whet your educational yearnings, here are some Animated Maps of hotspot activity (I can generate these for any date range on record without any effort, 'cause I'm a guru-level programmer; just ask if you have any requests).
1 year starting 9/1/1985
Month of March, 2006
3/4/1977 (Hotspot 1)
10/16/1985 and 10/19/1985
2/26/1990 (Hotspot 1)
8/12/1997 (Hotspot 1)
9/29/1998 (strange one; 2 spots 50 miles apart, back and forth)
6/16/1999 through 6/20/1999
8/8/1999 and 8/11/1999 (Hotspot 1)
1/25/2000 (Hotspot 1)
7/28/2000 and 7/29/2000
11/23/2000 and 11/24/2000
1/20/2001 (Hotspot 1)
September 9th, 2003, is the earliest date I have archived seismograms for. Sorry...
4/14/2004 - Seismogram
3/18/2006 (Hotspot 2) - Seismogram
3/18/2006 (same, non-cumulative)
4/5/2006 (A new hotspot?) - Seismogram
Week of 5/18/2006 - Seismogram