Gorda plate earthquake 2015.02.24

Where else would we get a focal mechanism or moment tensor for an earthquake of such a small magnitude! We are so lucky. This is the USGS web page for the earthquake. We get Gorda plate earthquakes frequently and most of them (if not all) are probably aligned with the northeast striking strike-slip faults.

Here is a map showing the epicenter with the USGS focal mechanism.

The Gorda plate is deforming due to north-south compression between the Pacific and Juan de Fuca plates. There have been many papers written about this. The most recent and comprehensive review is from Jason Chaytor (Chaytor et al., 2004). Here is a map of the Cascadia subduction zone, as modified from Nelson et al. (2006) and Chaytor et al. (2004).

Here is the Chaytor et al. (2004) map that shows their interpretation of the structural relations in the Gorda plate.

This is also from Chaytor et al. (2004) and shows moment tensor solutions for earthquakes in the Gorda plate. Note how they could predominantly be interpreted as northeast striking strike-slip faults.

Here is a map from Rollins and Stein, showing their interpretations of different historic earthquakes in the region. This was published in response to the Januray 2010 Gorda plate earthquake. The faults are from Chaytor et al. (2004).

This map shows an earthquake swarm from 2014, which appears to align along another northeast striking strike-slip earthquake fault in the Gorda plate. This swarm is related to a Mw 6.8 earthquake. Check out my pages about the mainshock and the aftershocks. I made soem animations of these earthquakes here.

Here is a primer for those who want to learn more about focal mechanisms. This is from the USGS, where you can read more about them. Moment tensors are calculated differently, but their graphical representation is very similar to that of focal mechanisms.

References:

  • Chaytor, J.D., Goldfinger, C., Dziak, R.P., and Fox, C.G., 2004. Active deformation of the Gorda plate: Constraining deformation models with new geophysical data: Geology v. 32, p. 353-356.
  • Nelson, A.R., Kelsey, H.M., and Witter, R.C., 2006. Great earthquakes of variable magnitude at the Cascadia subduction zone: Quaternary Research, doi:10.1016/j.yqres.2006.02.009, p. 354-365.
  • Rollins, J.C. and Stein, R.S., 2010. Coulomb stress interactions among M ≥ 5.9 earthquakes in the Gorda deformation zone and on the Mendocino Fault Zone, Cascadia subduction zone, and northern San Andreas Fault: Journal of Geophysical Research, v. 115, B12306, doi:10.1029/2009JB007117, 2010.
Category(s): cascadia, Chemeketa Community College, College Redwoods, education, geology, humboldt, plate tectonics

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