A series of moderate earthquakes occurred in northwest region of Alaska in April-June of 2014. It began with a magnitude 5.7 earthquake on Friday, April 18 at 10:44 am AKDT (18:44 UTC). This event was followed by another magnitude 5.7 earthquake 12 minutes later. On May 3 and June 7 two more earthquakes, magnitudes 5.7 and 5.8, respectively, occurred in the same source region. And lastly, on June 16 there was another magnitude 5.7 earthquake. All five events are shown by blue stars on the above map. These events were located approximately 20 km (~12 miles) NE of Noatak and 40 km (25 miles) S of the Red Dog Mine site. Vigorous aftershock sequences followed each magnitude 5.7 earthuqke, including about 20 events with magnitude 4 or greater. Overall, over 500 aftershocks (yellow circles) have been reported by AEC through end of September. As of October 22, the last reported earthquake in this swarm was magnitude 2.9 on September 25. Red circles on the above map show better constrained aftershock locations.
The plot above shows cumulative number of events that occurred during the Noatak earthquake swarm. You can see significant rate increase in aftershock occurrence immediately after each magnitude 5+ event (red stars). In each case the aftershock rate decreased notably after about two weeks.
This is a time-magnitude plot of events that occurred during the Noatak earthquake swarm. You can see that more aftershocks occur immediately after each magnitude 5+ event. Aftershock activity slows down considerably after one to two weeks. Also, you can see smaller aftershocks were being recorded after the May 3 M5.7 event, reflecting installation of additional seismic sensors in Noatak and Kotzebue. As of October 22, the last reported earthquake in this swarm was magnitude 2.9 on September 25.
The mainshocks and largest aftershocks were felt strongly in Noatak and at the Red Dog Mine. It was also felt as far as Kotzebue. No major structural damage has been reported, however some residents reported items falling off the shelves and cracks appearing in the walls and ceilings. Click here for DYFI map.
Seismicity in the region forms diffuse cloud with several clusters. It has been suggested that extensional tectonics in northwest Alaska is result of interaction between smaller Bering crustal plate to the west and North American plate to the east and south. The broad distribution of seismic activity suggests that deformation is distributed over many active structures and not concentrated on a single major fault system. The localized normal faulting may be associated with the formation of pull-apart structures. Geological mapping in the area revealed older faults and various assemblages of volcanic and sedimentary rocks, but no quaternary faults with surface expression have been identified. These efforts are hampered by swampy alluvial cover in the river valleys and limited bedrock exposure in the hillsides. Current events are the largest in the region since the magnitude 5.5 earthquake on 12 July, 1981.
The elastic-wave radiation pattern of the April, May and June events indicate normal faulting, typical for this region. SE-NW orientation of fault planes is consistent with the trend of best located aftershocks (red circles on the map). It's unclear if these events occurred on the same fault, or on a series of smaller unconnected structures.
Page composed by N.Ruppert: October 22, 2014