Early this month my mom E-mailed me to ask what I thought of the 7.1 earthquake in the South Pacific. So I went to the USGS Seismology Page and took a look. Actually I had seen it already. That site is a regular stop on my daily walk across the web. The quake was fifteen miles deep and located in the Solomon Islands region on the far Southwest side of the Pacific plate.

When I looked at on a map of geologic plates then superimposed on a map of Google Earth, it is easy to see the subduction zone. The quake is one of a series that is ongoing along the trench that runs Northwest-Southeast. It is where the Pacific plate is overriding the Australian plate. Both sides of the trench are littered with transverse faults and ridges. It is a very active section. The Pacific plates being shoved up over the Australian and the Australian plate is being pushed under, subducted by, the Pacific plate. That region is, tectonically speaking, quite active. The various fault lines are easy to discern even without the overlay.

There are other places where quakes happen, but it is easy to see why an event would happen there. Another one such quake was a 6.5 on the tenth of January, it was at a depth of 13 miles off the northern coast of California. It was north of an East-West ridge that marks the north end of the San Andreas fault (a transverse fault). This quake was also along a subduction zone. It is where the Gorda plate is being pushed under the North American plate. It is called the Cascadia subduction zone and runs north along the coast to Vancouver Island and beyond.

The difference between the two, other than magnitude and depth, is that the first was on the leading edge of the Pacific plate overriding the Australian plate. The second was on the leading edge of the Gorda plate being pushed under the North American plate.

Another kind of quake is the kind that occurs on a transverse fault. It happens when the stress and tension between two plates that are grinding past each other is released. The plates snap and jump past each other . The recent quake in Haiti was just such an event. The magnitude was determined to be 7.0 at a depth of eight miles. That depth was where the energy was released from, it was the center of the quake. I know that there have been a tremendous number of aftershocks. I have not studied them to find out the depth or where they sit on the fault. I do know there have been other fair sized quakes (5.0+) further north and west along the same set of faults that run under Haiti.

Alas, poor Haiti, rides on what is called a micro plate. It is caught between the westward moving Atlantic plate and being ground into from the west by the Caribbean plate. Again easy to see in Google earth even without marked fault lines. There are other quakes that are more difficult to grasp. One such is a series of quakes that have taken place in Oklahoma,USA.

Beginning on the fourteenth of January a group of four quakes has moved the earth near Oklahoma City. The first was a 3.3 magnitude at the depth of three miles. The second and third were 3.8 and 3.7. They were both 4.8 miles deep. Then another 3.7 on the twenty fourth at three miles down. The four are within a few miles of each other. Two were on the East side of the North Canadian river and two on the West side.

This is odd to me. The events happened in the middle of the North American plate, east-west wise. They were also relatively shallow. Is there an ancient fault under laying a million years or two of sediment? Is the sediment just settling? Or is the ground settling because the Ogalla Aquifer is being pumped out? In any case it is curious.

There is one other thing concerning geological events. Since the seventeenth of this month Yellowstone has been alive with a swarm of quakes. They range in magnitude from 0.5 to 3.8 and the depth seems to be just about six miles. As of this past Saturday there have been 1,033 of them. The scientists involved report that the quakes are not connected to the underlying volcano but are associated with ancient faults that lay outside of the caldera. For more information you can pull up the volcanoes at Yellowstone Volcano Observatory.

What does any of this mean? What do I make of it? Damned if I know but I do enjoy watching the planet breathe.

Michael Sherer