EarthScope and Tectonics
Recently NSF’s EarthScope program office put out a media announcement with the top ten discoveries they attributed to the soon-to-end program. (EarthScope, for those unfamiliar with the program, originally had three main legs: the Transportable Array (TA) + Flex Array collection of seismometers, the Plate Boundary Observatory (PBO) network of GPS stations, and the San Andreas Fault Observatory at Depth (SAFOD), a drill hole through the fault). What struck GG about this collection was just how little we learned about tectonics, which was a selling point of sorts for the program prior to its start.
Now some of the “discoveries” are not discoveries at all–one listed is that there is a lot of open data. Folks, that was a *design*, not a discovery. A couple are so vague as to be pointless–North America is “under pressure” and there are “ups and downs” in drought–stuff we knew well before EarthScope, so these bullets give little insight to what refinements arose from EarthScope. And then the use of LIDAR to look at displacements of the El Mayor-Cucapah earthquake was hardly a core EarthScope tool or goal even as the program might have contributed funds. So the more substantive stuff might amount to 5 or 6 points.
Arguably PBO has more than delivered and SAFOD disappointed, but GG would like to consider the TA’s accomplishments–or non-accomplishments. TA-related “discoveries” in this list are actually a single imaging result and two technique developments (ambient noise tomography, which emerged largely by happy coincidence, and source back projection for earthquake slip, which is largely a continued growth of preexisting techniques). So in terms of learning about the earth, we are really looking at one result worthy of inclusion.
That result is summarized as “Imaged new slabs of the ancient Farallon plate and its predecessors, following the trail in the mantle of the plate breaking up under North America, showing it extends from the Pacific Coast subduction zone to the eastern part of the continent.” Now ignoring the typo “new” (presumably they meant “Newly imaged”, not “Imaged new”), how much of a discovery is this? Frankly, as worded, the answer is none at all. Steve Grand basically posited this in tomography he did in 1994. While the imaging has improved with new techniques and data, there is still quite a bit of controversy over exactly what the high velocity blobs really represent.
Frankly, it kind of feels like we struck out–we already knew that the Farallon plate had subducted under North America. Questions that were in play at the start of TA seem to still be kicking about. Have we figured out the mechanisms creating the Rockies? The Great Plains? Have we determined what Yellowstone really represents? (Do we even agree on where its deeper plumbing lies?) Do we know what causes intracontinental basins? Do we understand the causes of intraplate seismicity, things like Charleston in 1886 and New Madrid in 1811-12? Look, there are papers claiming answers on some of these, but the contribution of the TA has been as much to garble things as clarify them. So let’s take a big step back and ask ourselves, where were we going, what did we get, and so what? After that, maybe we’ll know better just what the TA has meant for tectonics.
Where were we going? In essence, the TA was designed to image the upper mantle under the continent in a uniform manner. Previous attempts to get high resolution (~50 km or better) images by stitching together individual arrays and experiments proved difficult and often conflicted with other constraints; putting uniform equipment at a uniform ~80 km spacing would end that problem. On a side note, we would get a couple years of uniform earthquake coverage across the country, which might turn up something interesting. And by good fortune, ambient noise tomography came along as the TA started up, allowing for imaging of the crust using surface waves in places we had had no hope of doing any such thing.
What did we get? From a raw imaging standpoint, we got quite a bit. We have a plethora of seismic wavespeed models of the crust and upper mantle, dominantly shear waves in the crust but both P- and S-waves in the mantle, some examining radial anisotropy, some azimuthal anisotropy and a couple trying to do both. We also have images of discontinuities, generally from the Moho down into the transition zone, and some of these consider anisotropy as well. In a few places we have some new seismicity catalogs. And TA data has been used to try to image other parts of the Earth, a thread we’ll just point out.
So what? This is where we may have gotten stuck. We’ve reconfirmed that the eastern U.S. has much higher wavespeeds than the west and mapped the boundary a bit better–but again, we mostly knew this going in. We’ve reconfirmed that Airy isostasy is too primitive a model to understand most of the nation’s topography. A lot of this feels like apple polishing. Frankly, what stands out to a grumpy geophysicist is just how much of what we see we don’t understand. And frankly, getting our arms around our ignorance is potentially the greatest gift of the TA.
So what does GG mean by this? Well, we’ve seen lots of blue blobs and red blobs in the crust and mantle wavespeed models. In the west, this was hardly a surprise and so folks gleefully interpreted mantle drips and plumes and convective upwellings and so on because, well, in the west so much has gone on that such things seem quite plausible. The problem is that when you go east, you see some of the same stuff under geology that looks frighteningly dull. What this may mean remains largely unclear–some interpret these features the same way as out west (so, for instance, Levin et al. interpret low wavespeeds as upwelling asthenosphere in New England), but others stop and stare at some of these anomalies and are uncomfortable in making such pronouncements.
The biggest source of discomfort turned up by the TA has got to be the lithosphere itself. Seismologists have their own definition of the lithosphere: it is to extend to a depth where you encounter a decrease in wavespeed with depth. Thinning or thickening lithosphere seems a promising way of deforming the crust. Finding this boundary is tricky and usually relied on surface wave studies. In the early days of the TA it seemed a refinement of receiver function analysis was yielding good results. This modification uses incoming S-waves converting to P-waves as a tool for finding the so-called lithosphere-asthenosphere boundary (LAB). The Sp waves arrive before the S-wave and so are not interfered with by reverberations and other complications common to more classical Ps receiver functions. So this was applied across the continent, first to some of the permanent stations in the east but then to more stations as the TA moved across. And clarity became confusion.
You see, in the continental interior we have a couple extra tricks. One is heat flow, and it puts the base of the lithosphere a couple hundred kilometers down or more. Another are surface waves, which because of lateral smearing are kind of suspect in the west but seem more reliable in the dull and uniform east. This too puts the LAB 200 km or more down. But the Sp receiver functions were putting a major discontinuity that sure looked like the LAB at maybe 100 km down or less. Renamed the mid-lithospheric discontinuity (MLD), workers have puzzled over this and found it to be present on other continents as well. At the moment it is probably fair to say that its origin remains disputed. As a result, interpretations in the west are also feeling a bit suspect.
Although the MLD was recognized without TA stations, the search for the LAB that turned it up was in no small part a result of trying to get a continental image of the lithosphere that was the motivation for the TA. It would be fair game for EarthScope to claim it as its own discovery, but evidently the voters on this weren’t really convinced. Perhaps the reason is that we don’t really understand what we are looking at still. And frankly that is largely true of a lot of the stuff we now see in the crust and upper mantle. The temptation is to heave everything into some kind of meta-model and hope it tells us what is going on; frankly that seems naive.
Realistically what the TA has done is force us to confront our ignorance. Are planar high-wavespeed objects slabs? There are some features kind of hard to interpret that way. Are all low-wavespeed objects warm? Or are they wet? Or maybe even full of carbon? Or maybe anisotropic? And how is all this connected to making mountains–or not making mountains–at the surface? There were some late additions to the TA approach: magneotelluric experiments were belatedly added in some areas, and infrasound was added to TA sites in the latter part of the deployment. Some of the logic was the recognition that just plain old red-and-blue seismic tomography wasn’t going to get all the answers.
So perhaps the greatest discovery of the TA, and perhaps even EarthScope, is that there is complexity in the upper mantle and crust that we don’t have a good handle on, and until we do get a grip, we might be more deluded than we thought (or even currently think) about how we make mountains. So the biggest discoveries might well be in front of us even as EarthScope fades into the past. That might be the discovery that should have been touted.