8/4/2023 0 Comments Alaska quake aftershocksWhen we answer questions about larger earthquakes happening in the future, we choose our words carefully because we need to. This means that when the USGS aftershock forecast puts the probability of another magnitude 7 at less than 1 percent over the next year, that reflects how rare it is for an apparently normal aftershock sequence to precede a larger quake. There are different approaches to this, but they all draw from observations of past sequences and what they reveal about how aftershocks tend to play out. We are still recording magnitude 4+ earthquakes from the Offshore Kodiak magnitude 7.9 in January and the North Slope magnitude 6.3 in August.ĭoes the M4.9 increase the chances of an earthquake larger than the mainshock?Īftershock forecasts are based on statistical modeling. This is expected, and it would be far stranger if we did not experience a scattering of larger aftershocks as the sequence goes on. As an aftershock sequence goes on, it’s typical for strong aftershocks to mostly taper off but to still happen sporadically, sometimes in twos or threes and sometimes separated by weeks or months. The larger aftershocks are not evenly distributed, either. But that’s the long view, and that regular curve includes lulls and clusters of fewer and more earthquakes. If you look at a plot of the cumulative number of aftershocks over time, it forms an arc that begins steep and gradually flattens out as the aftershocks decrease. That magnitude 4.9 was the largest aftershock since November 30. But we know that the mainshock caused the very elevated rate of earthquakes in that area, so we can say that the great majority of those earthquakes are aftershocks. Some small number of those earthquakes would have happened anyway, and some presumed aftershocks just outside the rupture patch could be unrelated. Instead, we define them based on when and where they happen. For as long as the area around the rupture has an elevated rate of earthquakes, we will label the earthquakes in that area as aftershocks. The mainshock creates a new set of conditions, which causes the aftershocks.īecause aftershocks are just ordinary earthquakes, they have no special characteristics that help us to label them as aftershocks. What distinguishes aftershocks from ordinary earthquakes is this causal relationship. The aftershocks are smaller ruptures that happen in response to those new stresses. That rupture changed the distribution of stress in the rock throughout that area. 30-what we call the mainshock-ruptured an area inside the subducting Pacific plate roughly 20 miles deep and running from south of Point MacKenzie up to Big Lake. How do you know this was an aftershock? And what is the difference between an aftershock and an earthquake?Īftershocks are earthquakes. If we fail to answer your questions here, please seek us out on twitter or facebook and ask us there. In other words, things are moving in the right direction. Before yesterday’s magnitude 4.9, eleven days had passed since the previous magnitude 4. Consider that out of 40 aftershocks of magnitude 4 or greater, 17 happened in the first 72 hours. Also, the aftershocks have already slowed considerably. Taken together, the 6,000 aftershocks still account for only 10 percent of the energy released during the sequence, while the mainshock accounts for 90 percent. We cannot say when it will end, but we can say that the aftershocks have already grown far less frequent.Ī little perspective: While aftershocks can cause a great deal of anxiety for many, they are nothing compared to the mainshock in terms of destructive power. We are not surprised that strong aftershocks are still happening, and they do not suggest that a larger earthquake is on its way. Below, we will try to answer them as plainly and non-technically as possible.įirst, the short version: There is nothing unusual about this aftershock sequence. 30, and grew more insistent after the magnitude 5 aftershock on New Year's Eve. All of those questions popped up again Friday after the magnitude 4.9 aftershock, which was the largest since the night of Nov. 30 Anchorage earthquake, we’ve fielded far more questions about the aftershock sequence than we have about the magnitude 7.1 mainshock that started them.
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