Last week was really weird for earthquake geologists. We have seen one of the strongest earthquakes ever measured and another handfull of major events, all of them showing strike-slip fault movement. Manuel came up with the perfect description at his Planeet Aarde Geoblog: It’s strike-slip week on Planet Earth.
Mw8.6 Sumatra, 11 April
The most outstanding quake was of course the Mw8.6 Sumatra event of 11 April. Judging on the epicentre location and the magnitude, an ocean-wide tsunami was expected and an alert was published for the entire Indian Ocean coasts. Soon it became clear that the tsunami threat was overestimated and the warnings were cancelled. However, the systems performed well, and this is really good news. Naturally, everyone expected an earthquake of this magnitude to be a subduction megaquake. This would have lead to a significant offset of the seafloor and a tsunami. (I have been in Oman at this time and already looked for the best place to watch the waves approaching, really!) Then it turned out that we had a strike-slip event without significant movement of the water column. Only a small tsunami was observed. This event ranges number 11 in the list of the strongest events ever recorded by instrumental seismology.
Moment tensor solutions (EMSC image). Clear strike-slip with only minor vertical movement.
Slip distribution for the NE-SW fault plane solution: up to 7 m offset, up to 300 km rupture length (USGS image).
Mw8.0 Sumatra, 11 April
Only some 6 hours after this event, an Mw8.0 (or even more, see image) quake rocked the same area. What an aftershock – or was a second fault triggered? This will be interesting. I am expecting a lot of discussion on that. Again, we had a nice strike-slip movement.
The moment tensor solutions show a clear strike slip mode (EMSC image).
Just to summarize: This area experienced an incredible series of earthquakes since 2004.
- 2004 Mw9.1 (3rd largest event)
- 2005 Mw8.6 (9th largest)
- 2012 Mw8.6 (11th largest)
- 2007 Mw8.5 (13th largest)
- 2012 Mw8.0 (still large)
What an incredible release of energy.
M6.0 Oregon, 12 April
The US westcoast experienced an interesting M6.0 strike-slip event on the same day offshore Oregon:
M6.0 off Oregon. Yes, strike-slip.
Mw6.0 Gulf of California, 12 April
At 7:06 UTC Baja California was shaken by a moderate event, and again strike slip:
Magnitude Mw6.0, nice strike-slip in Baja California (EMSC image)
USGS even says it was M6.2.
Mw7.0 Gulf of California, 12 April
Ten minutes later, a Mw7.0 event occurred. I also expect some discussion on this and some really nice data. USGS reported the quake to have M6.9.
10 minutes after the 6.0 another earthquake rocked the Baja California area, again strike-slip.
I have to admit that this is an unusual accummulation of moderate to mega strike-slip events. Unusual? Yes, meaning that I don’t know about any comparable series. NOT meaning that the things we observed would be out of the statistical range. NOT meaning that Earth is trying to kill us. NOT meaning that the end is near. NOT meaning that we can not explain that. NOT meaning that HAARP or any other non-existing technology has been used. It’s just interesting and the data will be analyzed for lots of papers.
By the way, another strong EQ happened in Mexico (M6.5), but not strike-slip, and today is the 20th anniversary of the Roermond Earthquake that hit Germany, Belgium and the Netherlands with a magnitude of 5.9.
Have a nice and safe weekend.
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