On Monday, a major earthquake occurred south of the Alaska Peninsula. The 7.6 magnitude earthquake is currently the third largest earthquake to occur in 2020. The good news about this earthquake is that the impacts were minimal. But all large earthquakes have lessons to teach and this was no exception at first glance, the earthquake was not surprising. It was located south of the Alaska Peninsula, 60 miles southeast of Sand Point, and only 52 miles southwest of the largest (so far) earthquake of 2020, July 22 M7.8. The July 22 earthquake caused 20 aftershocks of magnitude 5 and greater before last week, including two earthquakes of 6.1 seconds. Technically, 7.6 is another follower – it lies within a sub-earthquake zone and is clearly associated with stress changes of 7.8, but 7.6 differs from most aftershocks in several ways. It is almost the size of the main earthquake. The general rule of seismology is that the largest satellite of an earthquake is approximately one unit less than the main earthquake. Was at a different error with a different error coupon than the one leading to 7.8. It’s been nearly three months since 7.8 and the largest aftershocks most often occur within the first few weeks.
General rules do not carry much weight in seismology. It is not uncommon for several earthquakes of approximately the same size to occur in a short period of time and to various but related faults. In 1992, we encountered 7.2, 6.5, and 6.6 over an 18 hour period, all on different faults. Watching earthquake activity in the western United States this year shows that months could occur among major earthquakes potentially regionally related. But I have to admit that Monday’s earthquake was a real surprise to me.
The U.S. Geological Survey analyzes all large earthquakes quickly and usually produces location, time of origin, magnitude, and potential fault within 15 to 20 minutes of their occurrence. In another half hour, a model of the potential error is produced and an estimate of the strength of the vibration and the impacts. It’s impressive how much detailed information is available so quickly. Back in 1992, it took hours to get a strength of 7.2.
Seismologists describe defects in the coordination mechanism. Informally, we call them beach balls because they are a bit like a four-color beach ball with dark areas representing areas around the earthquake source where the rocks are compressed and bright areas where they stretch. It doesn’t take long to learn about the three main types of faults from the focal mechanism, and 7.6 on Monday showed the classic quadrupole pattern of a blow earthquake. USGS seismologists create models of potential slip and fault trends and see how the models match the observed seismic records. In this case, the north-northwest directional slip fault was most appropriate.
The surprise is that the north and northwest are perpendicular to the Alaska – Aleutian subduction zone. The July 22 earthquake was at a thrust fault that appears to coincide with the main subduction façade. That’s what I would expect the well-behaved M7 or M8 top to look like. What is the great slip event taking place here against the tide, so to speak? It is another lesson that subduction zones are not simple. There is evidence of a shift in the earthquake directions around where 7.6 was centered, a major rupture in the plane of the subduction zone. 7.8 added pressure to it and 7.6 fired it.
As July 22, the M7.6 triggered a tsunami warning. An initial notice was issued by the National Tsunami Warning Center (NTWC) five minutes after the earthquake, putting the Alaska Peninsula, the Eastern Alutians and the Cook Inlet into warning status. The tsunami threat to the United States and the Canadian western coasts was under analysis. It took them more than two hours to announce that the tsunami was unlikely to affect us.
I assumed any resulting tsunami would be smaller than last July. The larger the size, the larger the tsunami (usually), and driving earthquakes are more likely to produce measurable tsunamis than a blow tsunami. The thrust results in a vertical deformation of the sea floor and the movement of sliding is mainly horizontal. The obvious conclusion is that the 7.6 tsunami must be much smaller than that produced by the larger earthquake.
My conclusion was wrong. The M7.8 July produced a 9-inch tsunami at Sandpoint. It has not been observed in any other coastal tide scale. On Monday, 7.6 reached 2.3 feet at Sandpoint and was recorded on 13 tide gauges in Alaska and Hawaii and in Crescent City as it reached 8 inches. I wasn’t the only one who was surprised. Scientists at the Pacific Tsunami Warning Center issued a preliminary notice stating that the earthquake poses no risk of a tsunami on Hawaii. After five hours and 40 minutes, they changed status to consultant after increases in tide gauges had already been recorded in Hawaii.
I don’t have an answer as to why this week’s tsunami was so much bigger. The good thing is that it did no harm, and again provides fodder for research and a better understanding of tsunami generation. But it also provides a much-needed opportunity to assess the warning system and make sure it works best when the next devastating tsunami approaches our path.
Monday’s earthquake explains why the tsunami from Alaska is my least favorite field event. It only takes four to five hours to reach us, and if there is no deep sea sensor operating near the source, it is difficult for tsunami warning centers to make a final call. By the time NTWC made the call on Monday, we had less than two and a half hours to coordinate the evacuation if it were necessary.
It also confirmed my pet’s extreme dissatisfaction with the US tsunami warning system – two centers and five different types of flyers. Each post is directed to different operational areas but most people find it difficult to sort by why someone mentioned a warning in place while another states that there was no threat. We can do better.
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