#Alaska quake aftershocks update#
We will update these pages as the sequence unfolds. The plots below show (left) the cumulative count of events with time and (right) magnitude of events with time for the earthquakes mapped above. More research is needed to better understand the interplay between these two large events. The M7.6 aftershock rupture originated outside the initial estimated rupture patch.
#Alaska quake aftershocks Patch#
The rupture patches of historical large events in the region are shown with the initial aftershock-based rupture patch for the M7.8 mainshock. The aftershock zone extends over an area 200 km long and 100 km wide (about 120 by 60 miles). Alaska earthquake: Anchorage rocked by aftershocks 1 December 2018 7.0 magnitude earthquake strikes Alaska A major 7.0 magnitude earthquake has struck Alaska, causing people to run from. The map below shows the distribution of the M7.8 mainshock, M7.6, M6.1, and M6.0 aftershocks as black stars and the remaining aftershocks in red. It was magnitude 7.6 earthquake and produced a vigorous aftershock sequence of its own. The largest aftershock in the sequence occurred on October 19, 2020. With more than 6,400 aftershocks recorded in the first 6 months of the M7.8 Simeonof Earthquake aftershock sequence, we're still watching it closely. This piece has been edited to reflect that revision.2020 M7.8 Simeonof Earthquake and M7.6 aftershock Note: On March 8, we revised the magnitude of the November 30 Anchorage earthquake upward, from 7.0 to 7.1. Earthquakes outside this small area are not aftershocks from the magnitude 7.1 earthquake-they’re just part of the background seismicity in other areas that haven’t ruptured recently, and should remind us that large earthquakes are possible anywhere and anytime in Southern Alaska. Remember, this only applies to the relatively small part of Southcentral Alaska that ruptured during the November 30 earthquake, from south of Point MacKenzie north to around Big Lake. Around that time, based on what we’ve seen so far, the aftershocks should blend into the activity we know and expect in that area. In other words, people in Southcentral should expect to keep feeling aftershocks through September, at least. Our current estimate suggests that aftershocks of magnitude 3 or larger will continue for around 300 days after the November 30 mainshock. As time passes and we collect more data, we can refine this analysis. By finding the point where that line crosses a line representing background seismicity (4), we can estimate when the rate of aftershocks will drop below pre-mainshock levels-in other words, when the aftershock sequence will effectively be over. If we also plot seven-day averages of aftershock counts over time (2), we can draw a line representing the rate of decrease in seismicity (3). Because of this relationship, we can plot the data on a log/log scale (1). In other words, the declining rate of aftershocks can be graphed as a curve that falls steeply at first and then levels off gradually without ever quite reaching zero.
Mathematically speaking, the decrease is logarithmic and asymptotic.
This is a defining characteristic of all mainshock-aftershock sequences. Shallow earthquakes associated with volcano processes and crustal faults within the overriding North American plate occur regularly and may produce vigorous aftershock or swarm-like sequences. When we look at plots showing the number of magnitude 3 and larger earthquakes each day, we can see that those daily counts start high and decrease rapidly. The largest recorded earthquake of this kind was the 2014 M7.9 Little Sitkin event. Because people mostly want to know when they will stop feeling aftershocks, we’ll estimate when we expect a return to background level for earthquakes greater than magnitude 3. We usually say that aftershocks have stopped when the seismicity in the ruptured area returns to its background level, which is the average rate of earthquakes prior to the mainshock. More than 1,000 aftershocks of magnitude 1.5 or greater have shaken Alaska since Friday’s big quak e knocked out power, ripped open roads and splintered buildings in Anchorage, US Geological. The burning question on the minds of many residents in Southcentral Alaska is, “When will the aftershocks stop?” This is a tricky question, but enough time has passed since the November 30 magnitude 7.1 earthquake for us to make an informed estimate.
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