Sobering

As if losing 4 instruments to the ocean wasn't enough, my advisor and I started looking at the data, and it is not good news. Three of the Scripps instruments and one of the Lamont ones did not provide any useful data at all. So, that leaves us with 8 functioning seismometers. Four of these 8 did not record for the whole time they were deployed so what it boils down to is this:

The colored circles show the seismometers that actually worked. Those that have a portion of black indicate how long they were working (black shows the proportion of time they weren't working). If you look closely, there are small black triangles. Those are the stations we should have had but don't due to loss at sea or crappy data.





So, in reality, this is less than 50% data return, the worst Don has ever had (which is just perfect for me and my thesis!). We won't be able to do a lot of what we set out to do. But, there are glimmers of hope. In going through the data, we have found some interesting features that may unfold into a unique story that we weren't planning on telling. So, we might have some unexpected blessings. I guess you have to run with what you've got.

As far as what we were planning on doing with the data, I'll try to give you a quick run down of the problems we were trying to address. First, you need to know a little bit about earth's structure. If you were to take a slice of the earth, you would see that it is made up of layers. The brittle and thin outermost layer is the crust. Beneath the crust is the lithosphere, which is also rigid but thicker than the crust. The crust and the lithosphere make up the plates that move around the surface of the earth. Beneath the lithosphere is a region called the asthenosphere. The asthenosphere is more ductile, and accommodates the motion of the rigid plates above. Think of it like the consistency of silly putty, it's not liquid but it deforms. If you were to keep traveling down to the center of the earth, you would go through the mantle, the liquid outer core, and into the solid inner core. We are mostly interested in learning about the lithosphere and the asthenosphere associated with old oceanic plates.

Oceanic plates are made at mid ocean ridges (sometimes referred to as spreading centers). Here, melts from the mantle rise up to the crest of the mid ocean ridge, as the two plates on either side of the mid ocean ridge pull away from the ridge crest, and the melts erupt at the surface, cool and make new crust. The region of the mantle that the melt came from is now cold and chemically depleted (since it lost the melt), this is the lithosphere. With time, this newly formed crust and lithosphere move away from the spreading center and continue to cool off. With more and more cooling over time, the lithosphere thickens (as you are adding more cold material).

So, this is what we know. A big question that remains in the earth sciences is whether the oceanic lithosphere continues to thicken through all time, or does it reach some steady state thickness around 80 million years, as some data suggest. We wanted to try to answer this question by studying really old oceanic lithosphere. In our study area, the crust is about 150 million years old. So, although we would not be able to say how the lithosphere changes through time, we would be able to provide a great snap shot of what the upper earth structure looks like in this old oceanic region.

We were going to try to answer this question in a couple of different ways. We can use seismic waves that travel through the earth to basically get a cat scan of the region. By measuring differences in travel times (or phases and amplitudes of seismic waves) from earthquakes coming from all around the world, we can determine what the seismic velocity is in the material that lies under the seismometers. Since the lithosphere is cold, it would have a faster seismic velocity than the underlying asthenosphere. This is one way we could look at the thickness of the lithosphere.

Another unique way we were going to look at it involves seismic anisotropy. Sorry if this is getting too complicated and convoluted. The mineral olivine, that makes up most of the upper earth structure (aside from the crust) is highly anisotropic. So, seismic waves travel at different speeds on it's different crystallographic axes. When groups of olivine crystals are subject to stress (such as at the mid ocean ridge crest where the plates are pulling apart), they tend to all align in a certain way. Here, all of the axes along which seismic waves travel the fastest align in the direction of stress.

This is also where the choice of the location of our experiment comes in. The crust and lithosphere that underly the two groups seismometers were formed by two different mid ocean ridges. There are faint red lines on the first graphic with the stations plotted that show the orientation of the mid ocean ridge that formed those two regions. If you can't see them, these lines are nearly perpendicular to each other. We can measure seismic anisotropy with our seismometers, which tells us about the direction of the stress field that is responsible for the anisotropy. What we would expect to see is a difference in seismic anisotropy beneath the two groups of seismometers, reflecting the fossil spreading direction of the mid ocean ridge that formed these two different regions. But, as we go deeper into the asthenosphere, we expect the anisotropy to tell us something different. Here, the stress field is dictated by the current plate motion, as the crust/lithosphere plate that rides above the asthenosphere kind of shears it in the direction that it is moving. So, although these two different regions should have different directions of seismic anisotropy shallow in the lithosphere, in the asthenosphere it should be in the same direction since they are both a part of the Pacific plate that is moving coherently as one to the north west. This is the other way we would try to answer the question about lithospheric thickness, by looking at what depth the direction of seismic anisotropy changes.

Sorry for all the science. Hopefully most of you can understand this, but if not leave me a comment or something and I can try to better explain it. Needless to say, I was very excited about this project, and if the data set had come out more favorably, we would have been able to answer these questions in a lot of different ways using different techniques. Now we are going to be very very limited. But, like I said, hopefully we will stumble upon some unexpected scientific breakthroughs. You never know.

On the upside, food today was GREAT. After that near sob session of looking at the data with my advisor, we went down for lunch and much to my delight it was Mexican! Some delicious beef tamales, Kalua pork taquitos  (they called them enchiladas, but they were really taquitos), jalepeno poppers, black beans and rice. Just what the doctor ordered. And of course, Aloha Friday = pizza night! I had Hawaiian and a slice of artichoke feta. I am sure going to miss Aloha Friday.

Hopefully I can get my spirits back up to enjoy the last few days at sea!