Earthquake Report: Westernmost Aleutian Arc

This earthquake happened a couple weeks ago, but was interesting and I have been looking forward to following up on this with a report. Here is the USGS website for this M 6.8 earthquake.
The M 6.8 earthquake happened in a region where the Pacific-North America plate boundary transitions from a subduction zone to a shear zone. To the east of this region, the Pacific plate subducts beneath the North America plate to form the Alaska-Aleutian subduction zone. As a result of this subduction, a deep oceanic trench is formed. To the west of this earthquake, the plate boundary is in the form of a shear zone composed of several strike-slip faults. The main fault that is positioned in the trench is the Bering-Kresla shear zone (BKSZ), a right-lateral strike-slip fault. In the oceanic basin to the north of the BKSZ there are a series of parallel fracture zones, also right-lateral strike-slip faults.
My initial thought is that the entire Aleutian trench was a subduction zone prior to about 47 million years ago (Wilson, 1963; Torsvik et al., 2017). Prior to 47 Ma, the relative plate motion in the region of the BKSZ would have been more orthogonal (possibly leading to subduction there). After 47 Ma, the relative plate motion in the region of the BKSZ has been parallel to the plate boundary, owing to the strike-slip motion here. However, Konstantinovskaia (2001) used paleomagnetic data for a plate motion reconstruction through the Cenozoic and they have concluded that there is a much more complicated tectonic history here (with strike-slip faults in the region prior to 47 Ma and other faults extending much farther east into the plate boundary). When considering this, I was reminded that the relative plate motion in the central Aleutian subduction zone is oblique. This results in strain partitioning where the oblique motion is partitioned into fault-normal fault movement (subduction) and fault-parallel fault movement (strike-slip, along forearc sliver faults). The magmatic arc in the central Aleutian subduction zone has a forearc sliver fault, but also appears to have blocks that rotate in response to this shear (Krutikov, 2008).
There have been several other M ~6 earthquakes to the west that are good examples of this strike-slip faulting in this area. On 2003.12.05 there was a M 6.7 earthquake along the Bering fracture zone (the first major strike-slip fault northeast of the BKSZ). On 2016.09.05 there was a M 6.3 earthquake also on the Bering fracture zone. Here is my earthquake report for the 2016 M 6.3 earthquake. The next major strike-slip fault, moving away from the BKSZ, is the right-lateral Alpha fracture zone. The M 6.8 earthquake may be related to this northwest striking fracture zone. However, aftershocks instead suggest that this M 6.8 earthquake is on a fault oriented in the northeast direction. There is no northeast striking strike-slip fault mapped in this area and the Shirshov Ridge is mapped as a thrust fault (albeit inactive). There is a left-lateral strike-slip fault that splays off the northern boundary of Bowers Ridge. If this fault strikes a little more counter-slockwise than is currently mapped at, the orientation would match the fault plane solution for this M 6.8 earthquake (and also satisfies the left-lateral motion for this orientation). The bathymetry used in Google Earth does not reveal the orientation of this fault, but the aftershocks sure align nicely with this hypothesis.

Below is my interpretive poster for this earthquake.

I plot the seismicity from the past month, with color representing depth and diameter representing magnitude (see legend). I also include USGS earthquake epicenters from 1997-2017 for magnitudes M ≥ 6.5.

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. I plot moment tensors for the M 6.8 earthquake, as well as for the 2003 and 2016 earthquakes mentioned above. I also include moment tensors for earthquakes in 1999 and 2001 because these are also interesting earthquakes that I had not noticed before. It appears that perhaps the 1999 strike-slip earthquake led to an increased stress on the subduction zone, which slipped in 2001. I will need to consider this earthquake pair more later. Here are the USGS websites for the 1999 and 2001 earthquakes.
I also include the shaking intensity contours on the map. These use the Modified Mercalli Intensity Scale (MMI; see the legend on the map). This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations. The MMI is a qualitative measure of shaking intensity. More on the MMI scale can be found here and here. This is based upon a computer model estimate of ground motions, different from the “Did You Feel It?” estimate of ground motions that is actually based on real observations.
I include the slab contours plotted (Hayes et al., 2012), which are contours that represent the depth to the subduction zone fault. These are mostly based upon seismicity. The depths of the earthquakes have considerable error and do not all occur along the subduction zone faults, so these slab contours are simply the best estimate for the location of the fault. The hypocentral depth of the M 5.5 plots this close to the location of the fault as mapped by Hayes et al. (2012).

I include some inset figures in the poster.

In the upper right corner is a figure that shows the historic earthquake ruptures along the Aleutian Megathrust (Peter Haeussler, USGS). I place a yellow star in the general location of this earthquake sequence (same for other figures here).
In the upper left corner is a figure from Gaedicke et al. (2000) which shows some of the major tectonic faults in this region.
In the lower right corner is a figure from Konstantnovskaia et al. (2001) that shows a very detailed view of all the faults in this complicated region.

Here is the interpretive poster from the 2016.09.05 M 6.3 #EarthquakeReport.

On 2017.05.08 there was an earthquake further to the east, with a magnitude M 6.2. Here is my interpretive poster for this earthquake, which includes fault plane solutions for several historic earthquakes in the region. These fault plane solutions reveal the complicated intersection of these two different types of faulting along this plate boundary. Here is my earthquake report for this earthquake sequence.

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). Note the fault plane solution and location for the 2014.06.23 M 7.2 earthquake. Do we see a trend here? This earthquake suggests the strike-slip faulting extends at least to the Bowers Ridge.

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 are several figures from Gaedicke et al. (2000) showing their tectonic reconstructions. I include their figure captions below in blockquote. The first map shows the general tectonic setting as in the poster above.

Map of the Aleutian–Bering region and location of the study area (rectangle). Lines with barbs indicate subduction zones: (1) Kamchatka Trench and (2) Aleutian Trench; lines with sense of displacement mark fracture zones (FZs): (3) Steller, (4) Pikezh and (5) Bering FZs. Single arrows show relative direction of convergence of the Pacific (P) and North American (NA) plates. Bathymetric contours are in meters.

This figure shows the complicated intersection of the BKSZ and the Kuril-Kamchatka Trench (a subduction zone).

The main tectonic features of the Kamchatka–Aleutian junction area modified from Seliverstov (1983), Seliverstov et al. (1988) and Baranov et al. (1991). The eastern side of the Central Kamchatka depression is bounded by normal faults. Contour interval is 1000 m. Lines A and B indicate the locations of profiles shown in Fig. 3; the rectangle marks the location of the area shown in Fig. 4.

This figure shows a medium scale view of the faults here, along with the major historic earthquakes. In this figure the BKSZ is labeled the Aleutian fracture zone (AFZ).

Rupture zones of the major earthquakes in the Kamchatka–Aleutian junction area [according to Vikulin (1997)]. Earthquakes with a magnitude of Mw>7 are shown.

Here is a great illustration that shows how forearc sliver faults form due to oblique convergence at a subduction zone (Lange et al., 2008). Strain is partitioned into fault normal faults (the subduction zone) and fault parallel faults (the forearc sliver faults, which are strike-slip). This figure is for southern Chile, but is applicable globally.

Proposed tectonic model for southern Chile. Partitioning of the oblique convergence vector between the Nazca plate and South American plate results in a dextral strike-slip fault zone in the magmatic arc and a northward moving forearc sliver. Modified after Lavenu and Cembrano (1999).

Here is a figure from Krutikov (2008) showing the block rotation and forearc sliver faults associated with the oblique subduction in the central Aleutian subduction zone. 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 a figure that shows the plate age for seamounts in the Hawaii-Emperor Seamount Chain (Torsvik et al., 2017).

Hawaiian-Emperor Chain. White dots are the locations of radiometrically dated seamounts, atolls and islands, based on compilations of Doubrovine et al. and O’Connor et al. Features encircled with larger white circles are discussed in the text and Fig. 2. Marine gravity anomaly map is from Sandwell and Smith.

Here are several figures from Konstantnovskaia et al. (2001) showing their tectonic reconstructions. I include their figure captions below in blockquote. The first figure is the one included in the poster above.

Geodynamic setting of Kamchatka in framework of the Northwest Pacific. Modified after Nokleberg et al. (1994) and Kharakhinov (1996)). Simplified cross-section line I-I’ is shown in Fig. 2. The inset shows location of Sredinny and Eastern Ranges. [More figure caption text in the publication].

Here are 4 panels that show the details of their reconstructions. Panels shown are for 65 Ma, 55 Ma, 37 Ma, and Present.

The Cenozoic evolution in the Northwest Pacific. Plate kinematics is shown in hotspot reference frame after (Engebretson et al., 1985). Keys distinguish zones of active volcanism (thick black lines), inactive volcanic belts (thick gray lines), deformed arc terranes (hatched pattern), subduction zones: active (black triangles), inactive *(empty triangles). In letters: sa = Sikhote-aline, bs = Bering shelf belts; SH = Shirshov Ridge; V = Vitus arch; KA = Kuril; RA = Ryukyu’ LA = Luzon; IBMA = Izu-Bonin-Mariana arcs; WPB = Western Philippine, BB = Bowers basins.

Alaska | Kamchatka | Kurile

Earthquake Reports

2017.06.02 M 6.8 Aleutians

2017.05.08 M 6.2 Aleutians

2017.05.01 M 6.3 British Columbia

2017.03.29 M 6.6 Kamchatka

2017.03.02 M 5.5 Alaska

2016.09.05 M 6.3 Bering Kresla (west of Aleutians)

2016.04.02 M 6.2 Alaska Peninsula

2016.03.27 M 5.7 Aleutians

2016.03.12 M 6.3 Aleutians

2016.01.24 M 7.1 Alaska

2015.11.09 M 6.2 Aleutians

2015.11.02 M 5.9 Aleutians

2015.11.02 M 5.9 Aleutians (update)

2015.07.27 M 6.9 Aleutians

2015.05.29 M 6.7 Alaska Peninsula

2015.05.29 M 6.7 Alaska Peninsula (animations)

1964.03.27 M 9.2 Good Friday

References

Gaedicke, C., Baranov, B., Seliverstov, N., Alexeiev, D., Tsukanov, N., Freitag, R., 2000. Structure of an active arc-continent collision area: the Aleutian-Kamchatka junction. Tectonophysics 325, 63–85
Hayes, G. P., D. J. Wald, and R. L. Johnson, 2012. Slab1.0: A three-dimensional model of global subduction zone geometries, J. Geophys. Res., 117, B01302, doi:10.1029/2011JB008524.
Konstantnovskaia, 2001. Arc-continent collision and subduction reversal in the Cenozoic evolution of the Northwest Pacific: an example from Kamchatka (NE Russia) in Tectonophysics, v. 333, p. 75-94.
Krutikov, L., et al., 2008. Active Tectonics and Seismic Potential of Alaska, Geophysical Monograph Series 179, doi:10.1029/179GM07
Lange, D., Cembrano, J., Rietbrock, A., Haberland, C., Dahm, T., and Bataille, K., 2008. First seismic record for intra-arc strike-slip tectonics along the Liquiñe-Ofqui fault zone at the obliquely convergent plate margin of the southern Andes in Tectonophysics, v. 455, p. 14-24
Torsvik, T. H. et al., 2017. Pacific plate motion change caused the Hawaiian-Emperor Bend in Nat. Commun., v. 8, doi: 10.1038/ncomms15660
Wilson, J. Tuzo, 1963. “A possible origin of the Hawaiian Islands” in Canadian Journal of Physics. v. 41, p. 863–870 doi:10.1139/p63-094.