Earthquake Report: Bering-Kresla Shear Zone (Russia, west of Aleutians)

If there is an earthquake and nobody is there to feel it, did it shake? Here is the USGS website for the M 6.3 earthquake that occurred in the far western Aleutians, so far west, it is called Russia.
Below is my interpretive poster for this earthquake. This map shows the slab contours (an estimate of the subduction zone plate interface). These contours are estimated by Hayes et al., (2012). The hypocentral depth is 12.3 km, which is shallower than the slab depth according to Hayes et al. (2012). This earthquake is clearly in the North America plate. Check out the Krutikov et al. (2008) figure below to see possible ways to interpret this moment tensor.
I placed a moment tensor / focal mechanism legend on the poster. There is more material from the USGS web sites about moment tensors and focal mechanisms (the beach ball symbols). Both moment tensors and focal mechanisms are solutions to seismologic data that reveal two possible interpretations for fault orientation and sense of motion. One must use other information, like the regional tectonics, to interpret which of the two possibilities is more likely. The moment tensor shows either right-lateral or left-lateral motion on a strike-slip fault.

    I include some inset figures.

  • In the upper right corner is a figure that shows the historic earthquake ruptures along the Aleutian Megathrust (Peter Haeussler, USGS). See more about this figure below.
  • To the left of that is a figure that shows (a) residual bathymetry modeled by Basset et al. (2015), (b) seismicity and moment tensors for historic earthquakes, and (c) earthquake slip patches for historic earthquakes.
  • In the lower left corner shows a low angle oblique view of the slab geometry for the Kuril-Kamchatka megathrust (Portnyagin and Manea, 2008).
  • To the right of that is a cross section of the Aleutian subduction zone (Saltus and Barnett, 2000) which shows an oblique cross section of the Aleutian subduction zone that is a part of the “Eastern Aleutian Volcanic Arc Digital Model.’

  • Here is a 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).

  • Here is a map that shows some of the large earthquakes in this region from 1996 through 2015. Refer to the moment tensor legend to help interpret the moment tensors for each earthquake. All, but one, are compressional solutions. Note how all the compressional earthquakes have roughly the same strike, oriented relative to the plate convergence vectors (blue arrows). The Aleutian arc may have slip partitioning that results in clockwise rotation of blocks instead of forearc sliver faults. I would have suspected that the strike of the thrust earthquakes would rotate with the strike of the subduction zone (like that occurs at the intersection of the New Britain and Solomon trenches).

  • Here is a map from Krutikov et al. 2008 (Active Tectonics and Seismic Potential of Alaska, Geophysical Monograph Series 179 Copyright 2008 by the American Geophysical Union. 10.1029/179GM07). Note that there are blocks that are rotating to accommodate the oblique convergence. There are also margin parallel strike slip faults that bound these blocks. These faults are in the upper plate, but may impart localized strain to the lower plate, resulting in strike slip motion on the lower plate (my arm waving part of this). Note how the upper plate strike-slip faults have the same sense of motion as these deeper earthquakes.

Here is the tectonic summary poster from the USGS. This shows epicenters for earthquakes from 1900-2014, plus the slab contours from Hayes et al. (2012). These slab contours are estimate for the location of the subduction zone fault and it is based upon the 3-D location of earthquakes. There is considerable uncertainty with this model, but it is the best that we have. Hayes and his colleagues are currently updating these global slab models.

The USGS prepared a more comprehensive summary poster for this region. This poster has some plots of seismicity in cross sectional view. Here is the poster, but I include some sections of the poster below that are relevant for this earthquake.

Here is a map for the southern Kamchatka Peninsula. Earthquakes are plotted with diameter scaled to magnitude. The cross section C-C’ is labeled, as are the Hayes et al. (2012) slab contours. I place the epicenter for this earthquake as a green circle. The diameter is scaled approximately to magnitude and the location is approximate.

Here I place the hypocenter for the 2016.01.30 earthquake on the cross section from the USGS poster. The location is approximate.

  • Here is the low-angle oblique map from Portnyagin and Manea (2008). I include the figure caption as a blockquote below.

  • Kamchatka subduction zone. A: Major geologic structures at the Kamchatka–Aleutian Arc junction. Thin dashed lines show isodepths to subducting Pacific plate (Gorbatov et al., 1997). Inset illustrates major volcanic zones in Kamchatka: EVB—Eastern Volcanic Belt; CKD—Central Kamchatka Depression (rift-like tectonic structure, which accommodates the northern end of EVB); SR—Sredinny Range. Distribution of Quater nary volcanic rocks in EVB and SR is shown in orange and green, respectively. Small dots are active vol canoes. Large circles denote CKD volcanoes: T—Tolbachik; K l — K l y u c h e v s k o y ; Z—Zarechny; Kh—Kharchinsky; Sh—Shiveluch; Shs—Shisheisky Complex; N—Nachikinsky. Location of profiles shown in Figures 2 and 3 is indicated. B: Three dimensional visualization of the Kamchatka subduction zone from the north. Surface relief is shown as semi-transparent layer. Labeled dashed lines and color (blue to red) gradation of subducting plate denote depths to the plate from the earth surface (in km). Bold arrow shows direction of Pacific Plate movement.

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