We will start with a ‘big picture’ that’s a bit simplified, then move in for a more detailed “close up” that is a bit more accurate. Don’t let that transition worry you…
If you look at that picture, you will notice that the North Island of Japan (Hokkaido) and about the top 1/3 of the South Island (Honshu) sit on the North American Plate (as does a nice chunk of Russia). In another posting, Pyromancer76 asked if Japan was being torqued due to the movement of Hokkaido 8 feet closer to the rest of North America? You can, IMHO, answer that by inspection of this diagram. Look closely and you can see little red arrows on the plate boundaries. This shows which way they are moving. Spreading, with the arrow heads pointed away from each other, or one plate being eaten by the other “Subducting” under it, when the arrows point at each other.
So we see for the whole top of the Pacific Plate that the arrows show subduction. The net force is pushing that “wedge” of the N.A. plate, upon which sits North Japan, toward the Pacific plate. But it tends to stick. So for long periods of time (100 year scale) it slowly gets pushed back and the land ‘bends’ a bit. Then, in a slipping thrusting movement, jumps a bit forward (and over) a bit of the Pacific plate. What that ought to mean is that for a long time northern Japan was slowly getting a bit bent away from the rest of N.America, and in the quake came back closer to ‘normal’.
There is an exception to this belief: Land deforms. Overtime, it deforms a lot. This is not a perfect process. So did it move 8 feet away, then spring 8 feet back, unbent? Very unlikely. The exact bendings and deformations are hard to know without measuring the land. But what we can say is that the subducting plate has some material come up again as the Volcanoes of Japan, making the land higher and wider over time, and that the pressure against that ‘wedge’ of the N.American plate with Japan on it will slowly bend and lift the land. North America and the Eurasian Plate are “locked” at the moment. There is a bit of mountains where they join, but not a lot. Most of the “impact” is being taken as subduction of the Pacific and volcano formation. So that’s where most of this last motion will show up. A “slow back”, then “fast forward” motion, a “springing” of Hokkaido and northern Honshu, with the ‘error’ or ‘delta’ of location showing up as mountains and the ‘excess’ from the subduction of the Pacific floor showing up as volcanoes.
(Yes, this is just the ‘big chunks’. If you look very closely you can find all kinds of detail that does not conform to this. Whole books can be written on the subtile “yes but”. I did NOT forget them, I have edited… Feel free to raise your favorite as a comment, if you like, then I won’t have folks complain that I made the posting too long by including it ;-)
OK, other “neat bits”.
Notice that we have arrows going up/down past each other near California. It looks like the Pacific Plate is sliding up toward Alaska (and that is how it is often presented here in the news. That “L.A. is sliding north and will someday be next to San Francisco”. But look at the bottom of the Pacific Plate. Being consumed from the SIDES but not the BOTTOM. The reality is that North America is moving southward more than that the Pacific plate is moving north. Golly, that’s a ‘bit different’… So it’s really that the Pacific Plate is ‘sitting still’ and getting swallowed up by the North American plate. (In very long time frames, the whole crust can wobble around, so we might all, net, end up rotating more northward. That’s a different question than the relative motions right now).
That the Filipino plate is shown as static is, IMHO, simplistic. There’s some action being ignored there… This page:
has a nice write up of the complexities. Basically, it’s being consumed from both sides, but in a complicated way…
So the conclusion for Japan? Since the rest of Honshu is on the Eurasian plate, locked to the North American plate, and since that is also pressing forward (but with the Filipino and Pacific plates taking the motion) it ought to hold more or less the same relative position to the rest of Japan as long as the two major plates stay locked (and the Pacific subducting).
However… this is happening on the face of a sphere. As the N.A. / Eurasian plates move south, they need to ‘extend’. Something needs to be getting ‘longer’ here for the geometry to work.
Okhotsk By Gosh(k)
OK, that’s the ‘simple view’. But we can go into a bit more detail… That “wedge” of the “North American Plate” is being redefined by some folks. (I think they are likely correct, but we’re starting to get an awful lot of dinky plates being defined around the world and I’m suspecting, for some of them, some folks just needed a Ph.D. thesis or a new publication on the record …)
This article, for example, finds that the part of the Philippine plate near Japan is a ‘new plate’ and everthing will need to be rethought…
But he’s already “Chief Researcher” so it must be true…
So here we can see that there is a chunk that is locked to the Eurasia plate at a piece that is now being redefined as the “Amurian Plate), but the top part is sliding just a bit (the “transform fault”) relative to the North American plate. IMHO, likely a chunk of the N.American plate that’s been busted off rather than bend or subduct under Eurasia. There is also a bit that is spreading (the red line) where it’s being pulled a bit away from the main part of the Eurasian Plate.
Doesn’t really change the analysis much, other than to note that there may be a bit of ‘mountain building’ at the “convergent boundary” in the middle of Honshu. I’d expect the Philippine plate to just keep on getting swallowed until the Pacific plate is directly subducting under the Eurasian/Amurian Plate with southern Japan on it, but things do sometimes change on this planet ;-)
And WHY is North America headed south
So, why is North America headed south? We know that the Atlantic is getting wider, and that can account for the spreading out and for the subduction of the Pacific plate under the Eurasian / Okhotsk / Philippine and down in the southern hemisphere. But that southward component? That part that gives me earthquakes right now (and volcanoes ‘not so much’… unless it changes ;-)? Where is THAT coming from?
This would work better on a sphere, but this is what we get to work with on 2-D computer screens, so just remember the actual geometry is spherical, OK?
Notice how the active spreading zone in the Atlantic wraps right around the Arctic and “ends” near Siberia? The crust is “younger” near it, and ages as you move out. The Arctic was once smaller, and as it spreads and gets bigger around it, the continental plates get moved out. Not a lot (the color band is narrower than in other places) but ‘enough’.
The ridge is the slowest known spreading ridge on earth, with a rate of less than one centimeter per year. Until 1999, it was believed to be non-volcanic; that year, scientists operating from a nuclear submarine discovered active volcanos along it. In 2001 two research icebreakers, the German Polarstern and the American Healy, with several groups of scientists, cruised to the Gakkel Ridge to explore it and collect petrological samples. Among other discoveries, this expedition found evidence of hydrothermal vents. In 2007, Woods Hole Oceanographic Institution conducted the “Arctic Gakkel Vents Expedition” (AGAVE), which made some unanticipated discoveries, including the unconsolidated fragmented pyroclastic volcanic deposits that cover the axial valley of the ridge (whose area is greater than 10 km2). These suggest volatile substances in concentrations ten times those in the magmas of normal mid-ocean ridges. The AGAVE expedition also discovered on the Gakkel ridge, using “free-swimming” robotic submersibles, what they called “bizarre ‘mats’ of microbial communities containing a half dozen or more new species”.
But hey, I’m sure all the Arctic science is now “settled science” ;-)
Personally, I suspect that the spreading zone doesn’t just end there, but gets covered up as it dives under Siberia (see: http://en.wikipedia.org/wiki/Laptev_Sea_Rift ) and “pops up” again over in Alaska (and maybe also that skinny line headed toward Canada below Greenland). Just, at the moment, it’s under a continental land mass that it can’t pull apart and the Atlantic spreading is keeping the bits shoved together. Basically, for now, the Atlantic spreading is beating it at that location. If I’m correct, that means any ‘bending’ or ‘stretching’ as things ‘head south’ ought to come out of Alaska and that end of Siberia rather than Japan.
If you notice, the color bands in the Pacific are much wider than anywhere else. The Pacific is spreading faster than the other places. (Though Australia has been moving North at a good clip!)
So “what happened”? Why is the Atlantic spreading ‘winning’ at the moment? For the simple reason that the Pacific plate is subducting but the Atlantic sea floor is not. For more, see:
California into Nevada
Look at where that Pacific spreading zone hits North America. Notice that it’s splitting off Baja California? It continues up into the area of The Salton Sea and Death Valley (which is below sea level as the spit has caused the bottom to drop about 9000 feet. It’s only as high as it is because about 9000 feet of sediment has fallen in to fill the ‘crack’…) Then it wanders on up toward the rest of the Basin and Range Provence where the crust is rather thin and flat. Why is the “Basin and Range” thin? Because as the continental USA is shoved south it gets geometrically stretched and thinned out. It can take a while to stretch a continent in two… (remember that point about things need to get wider as they get shoved ‘around the bend’ and into a wider part of a spherical shape?)
It is generally accepted that basin and range topography is the result of extension and thinning of the lithosphere, which is composed of crust and upper mantle. Extensional environments like the Basin and Range are characterized by listric normal faulting, or faults that level out with depth. Opposing normal faults link at depth producing a horst and graben geometry, where horst refers to the upthrown fault block and graben to the down dropped fault block.
If you start seeing a whole lot of quakes out there, and with some drop of elevation, well, it’s just nature asking if you would like sea front property in a few tens or hundreds of millions of years ;-)
Volcanoes and Me
So, IMHO, the answer to “why am I not getting volcanoes?” is pretty simple. Further north (Alaska), the land is on an east /west line, but moving south. Subduction and volcanoes. When the land lies North / South: At Cascadia, the spreading is back off shore, so subduction ashore with quakes and volcanoes. Further south, Mexico, spreading is in the Gulf, so volcanoes inland. For me, I’ll only get the Volcanoes return when the spreading rifts enough to give more subduction (millions of years). Or if the other side of the Pacific Plate gets stuck and stops subducting quite so much… the whole plate could get shoved a bit more this way instead, but I think the Philippine plate has to be consumed before that happens. Then again, a little pickup in either the Atlantic spreading rate OR the Pacific rate (under me) and we could “pop one”. The deep red end of the scale is “zero to 9.7 million years” and we had the Sutter Buttes pop up just 1.5 Million years ago. Lassen erupted in 1914 (though it’s just enough north of me that I don’t need to worry…much… “this behaviour is by design” ;-) Things can “hiccup” from time to time. After all, it was only a SMALL volcano ;-)
So while it’s fairly unlikely for that to happen again, the spreading age shows that things have not really changed much between then, and now…
One speculative bit: I note that Mammoth Lakes / volcano is just about where one would project the spreading zone to be. I have to wonder if it represents the place where the rifting has started to break through the surface…
The wiki gives an idea how much attention is put on this idea:
The Gulf of California Rift Zone (GCRZ) is the northernmost extension of the East Pacific Rise which extends some 1300 km from the mouth of the Gulf of California to the southern terminus of the San Andreas Fault at the Salton Sink.
The GCRZ is an incipient rift zone akin to the Red Sea Rift where continental crust associated with the North American Plate is being pulled apart by tectonic forces and being replaced by newly-formed oceanic crust and seafloor spreading.
That’s it, the whole thing. 2 Sentences. And they have it ending at the Salton Sea. Nevermind that Mammoth Supervolcano caldera or all those other volcanic features in central California… Everything is either ‘past’ or ‘earthquake fault’ from there to Cascadia…
Those folks who are looking at it, tend to look in Mexico. This paper has a 2003 date citation in it, so is fairly new:
The Gulf of California (GofC) extends
~1300 km from the fast-spreading midocean
ridge system of the East Pacific
Rise to the San Andreas Transform Fault
zone in southern California. One result
of the last 6 myrs of deformation, which
has been active over the full length of
the Gulf, is that the Baja California
peninsula is now part of the Pacific
Plate. On the Sonora conjugate margin,
there is a zone of active basin and range
type extension. Crustal deformation
within the Gulf itself ranges from classic
ridge-transform structures in the south to
diffuse deformation in the north. The
GofC is an excellent place for studying
the rifting process because both sides of
the rift are accessible, and the history of
plate interactions in this area is well
Geophysically, the GofC is largely
Mammoth Mountain was formed in a series of eruptions that ended 57,000 years ago
The volcano was formed from a long series of eruptions that started about 110,000 years ago and lasted to perhaps 57,000 years ago, although the volcano is still active with minor eruptions; the largest of which was a minor phreatic (steam) eruption 700 years ago.
Mammoth lies on the South end of the Mono-Inyo chain of volcanic craters, some of which erupted as recently as 250 years ago. Mammoth Mountain is on the southwestern edge of Long Valley Caldera, a large area that subsided after an enormous eruption 760,000 years ago. The Mammoth Mountain magmatic system is considered distinct from that of Long Valley caldera and Inyo Craters.
Sure like how they think it’s all ‘distinct’… just because the rift that runs under all of them circles 1/2 the globe, that’s no reason to think they are connected ;-)
Near the center of the caldera there is a mound called the “resurgent dome” that was formed by magmatic uplift. The area is still volcanically active and has periodic rhyolitic lava flows.
The history and deposits of the Mono and Inyo Craters overlap with Long Valley Caldera in time and space. The Mono-Inyo Craters volcanic field developed along a 30-mile-long (50 km) fissure system that extends northward from Mammoth Mountain on the southwestern rim of the caldera to Mono Lake.
The interesting bit, here, is that these volcanoes are not subduction volcanoes… While we’re not quite up there with Iceland, this is an interesting place as it is where the “rift meets the land” and mid-ocean spreading heads to the mountains…
The fun bit to ponder: Where does it go from there? It ends up back out to sea off the coast of Northern California, so you can draw a line from Mono Lake to the Cascadia Subduction Zone. What do you find? Lassen, Shasta, Eagle Lakes… and the volcanic fields of Northern California, plus more: http://vulcan.wr.usgs.gov/LivingWith/VolcanicPast/Places/volcanic_past_california.html ( Though note that they call it the “past” having embraced the “new normal” of the length of a couple of human lifetimes…)
The Cascades include Shasta, and head off inland (over the subduction area) from northern California on through Oregon and Washington. Yes, they matter. But that they are subduction features does not mean you can ignore the rifting zone south of it, nor our ability to “pop a volcano” in unexpected places, should nature decide to give it a go… The Sutter Buttes, where I grew up, are just about where you would project that rift from the coastline toward Mono lake crossing that wide empty very non-volcanic valley. The Geysers and a hot springs region is on the other side of it. The most fascinating part of it all, for me, was growing up in this odd place where subduction is so near, and rifting so near, and they hold in balance for so long some times. But don’t ever think it’s over or that “this time is different”. It’s just a moment to digest past excesses…
So could California have a new volcano just show up, anywhere along that rifting line from the coast to Mono lake? Could it have an old one give a mighty burp from there on down to the Gulf of Mexico? “You Betcha!”. (Though rifting could also ‘just stop’). And that’s why I watch that gulf area for quakes so much, and the Mammoth Mountain area too. Because just after that, the old rift ‘makes a hard left’ heads over the fields and valley, and very near to me (in volcano terms ;-) Basically, as long as the Arctic is pushing things south “enough” to overcome the desire to spread here, no volcano. But if it ever gets ‘out of balance’ or if the spreading zone ‘straightens out’ a little as the land spreads ‘going around the bend’, or if the San Andreas doesn’t keep that little block shoved up against the rift zone enough, well “things change” some times…
So for now, I get to enjoy life “balanced on the edge” of the forces of subduction, geometric stretch, land spreading, rifting, volcanism, and slip quakes. Ah, “Life On The Cusp!” Gotta Love It!