Seismic refraction: imaging the lower crust and upper mantle: PART 2

***This blog post follows on from the previous post (Part 1), so if anything sounds unfamiliar, check that one out too!***

We have been using three different sources of seismic energy during the cruise, all of which our ocean-bottom seismographs (OBSs) have recorded:

i) a GI-gun array to image the sediment layers and the upper part of the crystalline oceanic crust at high resolution;

ii) a Bolt-airgun array to propagate signals laterally through the mid-to-lower crust; and

iii) a G-gun array on the RV Sonne to generate the long distance, deep travelling signals that reach the lower crust and the mantle below. 

 

A view over the aft A-frame of the Cook. The Bolt airgun array is on the right and the GI-gun array on the left, with the air bubbles for both sources just breaking the surface by their towing floats.

In our work area the oceanic crust is ~10 km thick so the crust-mantle boundary – or Moho as it is named after the eminent seismologist Mohorovicic – is ~13 km below the sea surface (10 km of crust plus 3 km of water). To image this boundary we need to propagate seismic signals to more than 13 km below the surface and to at least 50 km laterally to see these signals returning from depth where they have travelled through the mantle, to our instruments located on the seabed. (Have a look at the diagram in Part 1 to see how an OBS further from the source will record signals that have penetrated deeper in the Earth.)

 

An example G-gun source refraction data plot from an OBS, showing signals which have travelled through the sediment cover, through oceanic igneous crust, through the mantle, and direct to the OBS through the water.

The OBSs also record earthquakes travelling through the work area, and we have recorded several of these from as close as Panama to as far away as Japan. The arrival of the magnitude 6 Panama earthquake of the 31^st January is shown below superimposed on top of some of the RRS Cook’s airgun array seismic arrivals at an OBS located within the northern ridge-axis grid.

OBS data showing Cook airgun arrivals, and the rather larger arrivals from the Panama earthquake

We've come to the end of our seismic activities now, after shooting seismic lines over the Costa Rica Rift, around borehole 504B, and also out to the west over the Ecuador Fracture Zone and Ecuador Rift spreading centre. We're just recovering the last of our OBSs deployed in the south of our study zone, before heading off to map some interesting areas of the seafloor for a few days.

Navigation plots of JC114 (up to date on the 25th Feb), showing our seismic tracks, path from Puerto Caldera, and the locations of the nearest global earthquakes to our work area which our OBSs may have recorded, including the Panama earthquake shown above.