A series of moderate earthquakes occurred in northwest Brooks Range 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+ earthuqke, including about 20 events with magnitudes 4 or greater. Overall, nearly 500 aftershocks (yellow circles) have been reported by AEC through end of the year. Activity has slowed down significantly by September. 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. Seismic activity decreased significantly by September.
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 the so called Trans-Bering Seismic Belt that extends from west to east from Seward Peninsular in Alaska, through Bering Strait and into Chukotka region of Russia. It has been suggested that extensional tectonics in northwest Alaska is result of interaction between smaller Bering crustal plate to the west and south and North American plate to the east. 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 Noatak region 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 1981 earthquake activity also exibited a swarm-like behavior and lasted for up to 6 months.
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).
Earthquake swarms can be defined as sequences of events that cluster in time and space, with few dominant events (mainshocks) that reach similar magnitudes and occur throughout the course of the earthquake sequence. The 2014 Noatak earthquake sequence had five magnitude 5.7-5.8 mainshocks that were minutes to weeks apart. Earthquake swarms are common in volcanic and geothermal areas or ocean ridges. Intraplate continental swarms without active volcanism have been reported in continental rift zones, like Rio Grande in North America, Kenya in Africa, Baikal in Asia and Bohemia region in Europe. These have been attributed to a weakened crust that allow intrusion of upper mantle material into the crust layers. Migration of fluids have been indicated as a possible triggering mechanism of the earthquake swarms. Some migration of the earthquake sources in southeast direciton has been observed throughout duration of the Noatak sequence. Waveform cross-correlation of all detected earthquakes in this swarm revealed that only small percentage of events occurred as repeating sources, i.e. occurred in nearly the same location. These families of repeating sources stayed active for only days to few weeks. This indicates that activity was not confined to a stationary source in both time and space. Migration of hypocenters and waveform dis-similarity have been suggested as evidence of role of pressurized fluids in the swarm triggering, but without high precision measurements of hypocenter migration, we cannot differentiate between fluids, aseismic slip, or static triggering mechanisms.
Triggering mechanisms of swarms in the Noatak region is probably a combination of a result of heterogeneous stress field in the absence of a well-developed fault plane (internal triggering) and an external force associated with the fluid pressure and migration. However, particular mechanism of interaction between fluids, stress field and existing fractures or faults is a complex problem which is not fully understood yet. Our hypothesis is not easily verified in the absence of a dense seismic network to provide hypocenteral locations with high accuracy, gas and isotope geochemistry studies, heat flow measurements, detailed geological structural mapping and subsurface seismic imaging. Further interdisciplinary studies are needed to understand earthquake behavior in the Noatak region in particular, and in the Trans-Bering Seismic Belt in general.
Page composed by N.Ruppert. Last updated December 22, 2014