Friday(ish) Focal Mechanisms

Posted on July 24, 2010 by Chris Rowan

A post by Chris Rowan Yes, yes, I know it’s Saturday. I was running a bit late this week anyway, and then there were 3 magnitude 7s beneath the Philippines to have a look at. Indeed, there has been lots of seismic action in the Papua New Guinea region this week. The area is very complicated tectonically: it’s a collisional belt involving a number of small microplates interacting with each other.

At the beginning of the week, there were three large shocks beneath New Britain, where the Solomon Sea plate is subducting northward beneath the Bismarck Sea plate.


1. 18th July, Magnitude 6.9; Depth 42 km
Focal mechanism indicates E-W compression.
USGS Page

2. 18th July, Magnitude 7.3; Depth 35 km
Half-hour after the M6.9. Focal mechanism indicates N-S compression.
USGS Page

3. 20th July, Magnitude 6.3; Depth 54 km
Focal mechanism indicates N-S convergence
USGS Page
Given the extremely close proximity of these 3 earthquakes, they could be regarded as a foreshock-main shock-aftershock sequence (although obviously, there are many smaller aftershocks associated with the sequence as well). The latter two earthquakes are consistent with shortening at or near the subduction interface; the first earthquake, though, indicates convergence perpendicular to this. This seems a little odd; the rupture may be deep enough to be in the subducting plate, and appears to show some kind of lateral squashing. However, I actually blogged last September about a similar earthquake beneath Indonesia, so perhaps it’s not so unusual in this region after all.

Later in the week, there were also large shocks a few 100km to the east and west of the New Britain sequence. I note these with the usual caveat that while nearby large earthquakes might have given a little nudge to areas that were already on the verge of rupturing, proving any direct connection is exceedingly difficult.


4. 21 July, Magnitude 6.1, N of Halmahera, Indonesia; Depth 102 km
E-W compression, but with a large strike-slip component. The latter might be related to the fact that this earthquake is associated with a subduction zone (the Philippine plate moving westward beneath the Sunda plate) that appears to be offset by a transform fault just to the south of the rupture.
USGS Page

5. 22 July, Magnitude 6.2, Vanuatu; Depth 35 km
Focal mechanism indicates E-W compression, so is clearly associated with eastward subduction of the Australian plate beneath the arc.
USGS Page
Then, last night, seismometers witnessed 3 large >M 7 earthquakes, very deep in the mantle, below the Moro Gulf near the Philippines, with a smaller 6.5 in the early hours of this morning:


6. 23 July, Magnitude 7.3; Depth 605 km

7. 23 July, Magnitude 7.6 Depth 576 km
50 minutes after M7.3

8. 23 July, Magnitude 7.4; Depth 616 km
25 minutes after M7.6.

9. 24 July, Magnitude 6.5; Depth 564 km
All four focal mechanisms indicate NW-SE extension. Like the Halmera earthquake, this sequence appears to be linked to the westward subduction of the Philippine plate beneath the Sunda plate, with the earthquakes taking place in a deeply subducted part of the Philippine slab. The extension is probably the result of down dip tension as the slab sinks into the mantle, with the first shock apparently triggering similar events above and below it.

Notes:

Categories: earthquakes, focal mechanisms, tectonics
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