Earthquake Report: Atka strikes again!

Today we had another cluster of seismic activity in the Aleutians, just west of the Amlia fracture zone. This series of earthquakes occurred just west of some earthquakes from last week or so. Here is my earthquake report for those earthquakes. I discuss the seismicity from this region from the past couple of years on that report page. Here is the USGS web page for today’s M 6.0 earthquake.
Below is a map that shows today’s earthquakes as orange circles, with diameter representing earthquake magnitude. I also place the USGS moment tensors for the three largest earthquakes (M 5.4, M 6.0, and M 6.3). Today’s earthquakes appear a little too shallow to be on the megathrust, so might be on faults in the North America plate. The depth estimates could be incorrect.
There is a legend that shows how moment tensors can be interpreted. Moment tensors are graphical solutions of seismic data that show two possible fault plane solutions. One must use local tectonics, along with other data, to be able to interpret which of the two possible solutions is correct. The legend shows how these two solutions are oriented for each example (Normal/Extensional, Thrust/Compressional, and Strike-Slip/Shear). There is more about moment tensors and focal mechanisms at the USGS.
These earthquakes are the result of north-northwest compression from the subduction of the Pacific plate underneath the North America plate to the north. These earthquakes occurred in the region of the subduction zone west of where the Amlia fracture zone is aligned. The AMZ is a left lateral strike slip oriented fracture zone, which displaces crust of unequal age, beneath the megathrust. The difference in age results in a variety of factors that may contribute to differences in fault stress across the fracture zone (buoyancy, thermal properties, etc). For example, older crust is colder and denser, so it sinks lower into the mantle and exerts a different tectonic force upon the overriding plate.

I include some inset figures.
• The lower right figure from Saltus and Barnett (2000) shows an oblique cross section of the Aleutian subduction zone that is a part of the “Eastern Aleutian Volcanic Arc Digital Model.’
• In the lower left corner, I place a map created by Peter Haeussler, USGS, which shows the historic earthquakes along the Alaska and Aleutian subduction zones.
• Above Hauessler’s map, I show a cross section of a subduction zone through the two main parts of the earthquake cycle. The interseismic part (in-between earthquakes) and the coseismic part (during earthquakes). This was developed by George Plafker and published in his 1972 paper on the Good Friday Earthquake.

This map is particularly useful on its own. This is the map that shows historic earthquake slip regions as pink polygons (Peter Haeussler, USGS). Dr. Haeussler also plotted the magnetic anomalies (grey regions), the arc volcanoes (black diamonds), and the plate motion vectors (mm/yr, NAP vs PP).


Speaking of the 1964 earthquake, here is a map that shows the regions of coseismic uplift and subsidence observed following that earthquake. The 27 March, 1964 M 9.2 earthquake is the second largest earthquake ever recorded on modern seismometers. This figure can be compared to the cross section below.


Here is the Plafker (1972)cross-section graphic on its own.


This figure shows a summary of the measured horizontal and vertical displacements from the Good Friday Earthquake. I include a figure caption from here below as a blockquote.

Profile and section of coseismic deformation associated with the 1964 Alaska earthquake across the Aleutian arc (oriented NW-SE through Middleton and Montague Islands). Profile of horizontal and vertical components of coseismic slip (above) and inferred slip partitioning between the megathrust and intraplate faults (below). From Plafker (1965, 1967; 1972)

Here is a graphic showing the sediment-stratigraphic evidence of earthquakes in Cascadia, but the analogy works for Alaska also. Atwater et al., 2005. There are 3 panels on the left, showing times of (1) prior to earthquake, (2) several years following the earthquake, and (3) centuries after the earthquake. Before the earthquake, the ground is sufficiently above sea level that trees can grow without fear of being inundated with salt water. During the earthquake, the ground subsides (lowers) so that the area is now inundated during high tides. The salt water kills the trees and other plants. Tidal sediment (like mud) starts to be deposited above the pre-earthquake ground surface. This sediment has organisms within it that reflect the tidal environment. Eventually, the sediment builds up and the crust deforms interseismically until the ground surface is again above sea level. Now plants that can survive in this environment start growing again. There are stumps and tree snags that were rooted in the pre-earthquake soil that can be used to estimate the age of the earthquake using radiocarbon age determinations. The tree snags form “ghost forests.


This is a photo that I took along the Seward HWY 1, that runs east of Anchorage along the Turnagain Arm. I attended the 2014 Seismological Society of America Meeting that was located in Anchorage to commemorate the anniversary of the Good Friday Earthquake. This is a ghost forest of trees that perished as a result of coseismic subsidence during the earthquake. Copyright Jason R. Patton (2014). (Please contact me for a higher resolution version of this image: quakejay at gmail.com)


Below is an educational video from the USGS that presents material about subduction zones and the 1964 earthquake and tsunami in particular.
Youtube Source IRIS
WMV file for downloading.
mp4 file for downloading.

Credits:
• Animation & graphics by Jenda Johnson, geologist
• Directed by Robert F. Butler, University of Portland
• U.S. Geological Survey consultants: Robert C. Witter, Alaska Science Center Peter J. Haeussler, Alaska Science Center
• Narrated by Roger Groom, Mount Tabor Middle School

References:
• Hayes, G.P., Wald, D.J., and Johnson, R.L., 2012. Slab1.0: A three-dimensional model of global subduction zone geometries in, J. Geophys. Res., 117, B01302, doi:10.1029/2011JB008524
• Plafker, G., 1972. Alaskan earthquake of 1964 and Chilean earthquake of 1960: Implications for arc tectonics in Journal of Geophysical Research, v. 77, p. 901-925.
• Saltus, R.W., and Barnett, A., 2000. Eastern Aleutian Volcanic Arc Digital Model – Version 1.0: U.S. Geological Survey Open-File Report 00