The Western Geologist

I blogged about the 10/15/2006 earthquake in Hawaii last week here and here.

The image below shows the focal solutions for all of the earthquakes in the NEIC catalog. The focal solution for the Oct. 15 2006 event is shown in red.



The south end of the island seems pretty straight forward. All of the faults strike NE-SW. Thrust faults occur at depth (there's a rough transition at ~10 km, but the shallowest thrust event was 4 km), and normal faults occur above that. I'm picturing something like synorogenic collapse; Hawaii is a very thick pile of material and it's about as tall as it can be, so it makes sense to me that the upper part of that pile is extending. Extension would thin the pile making it more stable (something like the detachment in the upper part of the Himalayas).

The northern part of the island where the Oct. 15 event occurred is more complex. There are two significant thrust events in that area, but they occurred on faults that strike NW-SE (EDIT: I forgot to mention that one of these thrust events was a Mw 5.8 aftershock of the Oct. 15 quake) , or in other words, at a very high angle to the thrusts at the southern part of the island. So, it seems pretty clear that the stress field that's causing the thrust faults on the northern part of the island is different from the stress field that's causing the thrusts on the southern part of the island.

The orientation of the strike slip events in the northern part of the island isn't quite as easy to infer. There are two possibilities. The first is that the faults strike NE-SW (parallel to the faults on the southern part of the island and to the fracture zone that Chris mentioned - the blue line in the figure below). The second is that the faults strike NW-SE (as the lab lemming suggested, parallel to the thrust faults on the northern part of the island (the red lines in the figure below). I think that's more likely. If the strike slip faults strike NE-SW then there'd have to be an along-strike sense of motion (right lateral along part, left lateral along part), which seems physically unlikely. The lines in the figure below represent possible fault orientations inferred from the focal mechanisms:



I also favor a NW-SE orientation for both the strike slip and the thrust faults at the northern part of the island because something similar is observed in other parts of the world. For example, there are active thrust faults that are parallel to the San Andreas fault (a strike slip fault). In that case the two styles of faulting are caused by the oblique convergence of the Pacific and North American plates, so the SAF system isn't directly analogous to Hawaii. However it is an example of two different styles of faulting resulting from the same regional stress field.


I want to speculate now about the cause of the stresses in the northern part of the island. The image below shows the orientation of the P axes from the focal mechanisms (which should be equivalent to the maximum principal stress):




The black symbols are for the southern portion of the island, and they cluster pretty tightly. There's certainly a lot more scatter in the northern portion of the island. So, the stress field in the northern part of the island is a lot more heterogeneous than in the southern part of the island.

There does appear to be some overlap between the northern and the southern part of the island. The trend of the maximum principal stress for the Oct. 15 2006 quake is pretty similar to the trend of the maximum principal stress for the thrust events in the southern part of the island (shown below).



Here's a summary of my thoughts:

1) The thrust faults on the southern part of the island are resulted to the growth of the island, while the shallower normal faults are caused by extension resulting from something analagous to synorogenic collapse.

2) The stress field at the northern part of the island is much more heterogeneous than at the southern end of the island. As I discussed in an earlier message, I'd bet that the magnitude of the minimum and intermediate principal stresses at the northern end of the island are pretty similar. This means that both thrust and strike slip events would occur.

3) Both the thrust and the strike slip faults at the northern part of the island are oriented NW-SE at a high angle to the faults at the southern part of the island.

4) The northern part of the island is subjected to both local stresses related to the Hawaiian islands as well as more far field stresses associated with the movement of the Pacific plate. These far field stresses are swamped by the local stresses at the southern part of the island.

Here's my preferred interpretation of the faults on Hawaii:



The NE-SW faults are associated with local stresses while the NW-SE faults are driven by both local and far field stresses. At least that's my best guess.