» A M7.7 earthquake in the Sea of Okhotsk and two M6.2 events in Iran

2012-08-14 | Christoph Grützner in Earthquake

This morning at 03:00 UTC an earthquake with magnitude M_W7.7 occurred in the Sea of Okhotsk. Due to its great depth of more than 600 km no tsunami was triggered and surface shaking was pretty low, however, the event was felt in a wide area. Moment tensor solutions indicate a thrust event. Historical seismicity tells us that significant earthquakes are likely to happen in the region and mostly occur in great depths, but at this certain location only few events have been registered during the last decades.

EMSC graphic: Moment tensor solutions of the M7.7 Sea of Okhotsk event

EMSC graphic: Historical seismicity in the Sea of Okhotsk region.

The major plate boundary to the Pacific plate is located to the SE, as the seismicity image nicely illustrates. The recent event is situated pretty deep, so I suspect it does fit well to the subduction slab geometry, if you consider the subduction angle there. The Okhotsk plate boundary to Eurasia is further to the NW. This observation, among others, some years ago led to the question whether the Okhotsk plate does really exist or if it’s rather a part of the North American plate. Seno et al. (1996) found that a separate Okhotsk plate better fits the observed earthquake pattern and the plate motion rates.

Seno, T., Sakurai, T., Stein, S., 1996. Can the Okhotsk plate be discriminated from the North American plate? Journal of Geophysical Research, 101, B5, 11,305-11,315.

By the way, this was only the tenth M>7 event in 2012, we saw seven M>7 EQs and two M>8 events so far, and none of them caused major damages or great live losses.

Two much smaller earthquakes, however, hit Iran on 2012-08-11 with a magnitude of M_W6.2 and left reportedly more than 300 people dead. The events happened within 15 minutes, were very shallow (~10 km depth), and caused intensities up to VI in the city of Tabris and surrounding areas. Given the poor building standards, this shaking resulted in a disaster.

USGS graphi: Shake map for the first shock.

The earthquakes were almost pure strike-slip events at the Alpine-Himalayan-Collisional Zone:

EMSC graphic: Moment tensor solutions

Until now, Iran refuses to accept international aid as media report. In 2003, a moderate event of magnitude 6.6 destroyed the historical city of Bam in Iran and left more than 25,000 people dead. The collapse of the traditional adobe buildings led to the immense life loss. This illustrates dramatically that not the mere magnitude of earthquakes is responsible for the risk, but the depth of the event, the sense of motion, and the local conditions like building standards, site amplification effects, infrastructure, and disaster prevention make the difference.