The 2020 Sand Point Earthquake: (Disaster) Chart Party

6 minute read


The Storm Prediction Center

Seems a bit odd to open this part of the blog series with the Storm Prediction Center (SPC) with its own subheading. In a way, it is part of the explanation for why the Sand Point earthquake is important, and in another way, it provides a comparison for lay-people to compare against when I talk about how tsunami products are issued by the National Tsunami Warning Center (NWTC) and the Pacific Tsunami Warning Center (PWTC). Three centers, all under the NOAA umbrella, and they issue products under the same WEA framework. These comparisons were necessary for when I was talking to my mom about why the research I do is important. The first thing she asked me was, are tsunami warnings similar to tornado warnings?

An interesting question to be sure, it would be easy to say, “no.” But, that’s not what she was asking here. My mom and dad have been a bit spoiled having a child who can break down all the ominous weather messages these past years. They know how a tornado warning is issued! It usually starts with the SPC. The SPC will produce outlooks that detail what modes of severe weather: hail, wind, and tornados, will be affecting an area. These same outlooks will show the chances of severe weather with color coded systems. For them, I bypass the confusion of probability by telling them they’re more threat levels. Threat level 1 - Marginal … Threat level 5 - High. Imperfect? Yes. But, it gets them to respond appropriately; otherwise, my dad would say 30% chance means nothing will occur. Getting away from that tangent, these SPC outlooks give weather forecast offices (WFOs) a heads up. The SPC gives WFOs a further heads up by providing mesoscale discussions telling of the probability for issuing severe products like watches. It is an amazing system. Threats can start being relayed in a coherent matter, and for the most part, the messages stay the same. The public can be warned in a coherent way.

That is not quite how the NTWC and PTWC work. Instead of working together, they provide two different standards of messages that can be confusing for the public. Perhaps nothing is quite as telling as the product chart they use. Whereas, the NWS is in the process of eliminating the statement-watch-advisory-warning system, the NTWC and PTWC still make use of them, which can lead the public to interpret messages in a confusing manner. Is a tsunami watch worse than a tsunami advisory? When can we expect a tsunami? How long is it going to last? What are the impacts? Should I be worried? The NWS in recent years has addressed these concerns with their Hazard-Source-Impact style of warnings. For staying informed about storm hazards, these style of warnings do the job somewhat well. However, the NTWC and PTWC issue two different flavors of products. The kind of information you get depends on which TWC is in charge for your location, which can lead to confusion. To end this digression, I will link to two advisory cancellation messages for the 2020 Sand Point earthquake. The NTWC message differs from the PTWC message in that whilst the PTWC is canceling the advisory, they acknowledge that there may be lingering hazards. The NTWC is largely silent on these effects. People who are in their service region would be under the impression that all hazards are passed, and that there won’t be any lingering effects! Which would be wrong and potentially deadly for unexpecting mariners. I end this section by showing the TWC chart for how to warn about tsunamis.

Figure 1. TWC criteria for initial messages. Open as a new image to get the full experience!

Why Sand Point Matters

So, I spent some time knocking on the TWC for the US. However, knowing how they work is key for understanding the importance of Sand Point. The TWC only has 3 criteria for issuing products: location, depth, and magnitude. This unholy trinity leaves the US vulnerable to “tsunami” earthquakes. A “tsunami” earthquake is an earthquake which produces a much stronger tsunami than what its magnitude would suggest. Key examples of “tsunami” earthquakes are the 1896 Sanriku earthquake, the 1992 Nicaragua earthquake, and 2010 Mentawai earthquake. The PTWC owes its existence to a “tsunami” earthquake! The 1946 Aleutian Islands earthquake generated a tsunami that devastated Hilo, HI, which would envitably lead to the creation of what would become the PTWC in 1949. However, I am getting ahead of myself. It remains uncertain if the Sand Point earthquake is a “tsunami” earthquake, which is something to debate about later. At this time, I would say no. Those reasons to become better known later.

The chief thing to know here is that all earthquakes in the US TWC area of watch are diagnoised as a threat based on those 3 criteria mentioned above. The one I am concerned with, with regards to Sand Point is the magnitude. A M7.6 earthquake has a different set of criteria for initial messages than a M7.3 or a M8.0. Taking all things considered, the Sand Point earthquake seems to be larger than a Mw 7.6. The USGS finite fault for it described the rupture zone as being 125 km x 25 km, identifying it as a right-lateral strike-slip earthquake. My research has shown that this is not the whole story. The final initial sea-surface deformation plot I made for my publication (Santellanes et al., 2021, in review) has a potential megathrust rupture zone of 125 km x 125 km, and that it ruptured up-dip of the Simeonof earthquake rupture area. The final magnitude of which is debatable. Most estimates from the inversions run so far put the Mw between 8.0-8.1. Larger than the Simeonof, but still less than the Chignik.

Figure 2. Figure 1 from Santellanes et al. (2021), in review.

Figure 2 shows a proposed initial sea-surface deformation pattern from water-level inversions. From here, you can see the niceness of the deformation with regards to the rupture areas for the Simeonof and Chignik earthquakes. It seemingly fits in nicely with all that we currently know about the sequence so far. The big takeways from here is that Sand Point was potentially larger than expected from seismic data, and the tsunami waveforms can be better explained with this initital sea-surface deformation. All nice and well by far as science goes. But, we need to find the potential slip along the megathrust and/or a strike-slip fault. The sea-surface deformation basically navgigates away from that complexity by showing the end result deformation, which could be from a combination of ruptures.

Closing Thoughts

It has been shown that there is a possibility of a megathrust fault rupture during the 2020 Sand Point Earthquake. However, we cannot fool ourselves here at this step. The evidence appears pretty damning for a megathrust rupture, but we must keep with the tradition of several working hypothesises. After all, looks can be deceiving. In future blogs, these hypothesises will be explained in further detail. There is a lot to still be explained and things to be ruled out. At this point, the working hypothesises are:

  1. It is a strike-slip caused tsunami.
  2. It is a megathrust caused tsunami.
  3. It is a landslide caused tsunami.
  4. It is a combination of 1-3.

Many working hypothesises are hallmarks of good science. We cannot become too attached to an idea of something because we labored on it for so long. For now, at this point of explaining the paper, these were the open questions that needed to be answered or at least, answered in part. Attempting to answer these questions was the longest part of the science.


  1. Crowell, B. W., & Melgar, D. (2020). Slipping the Shumagin Gap: A Kinematic Coseismic and Early Afterslip Model of the Mw 7.8 Simeonof Island, Alaska, Earthquake. Geophysical Research Letters, 47(19), 1–7.

  2. “M 7.6 – 99 km SE of Sand Point, Alaska”. USGS. Retrieved October 8, 2021.

  3. Santellanes, S. R., Melgar, D., Crowell, B. W., & Lin, J. T. (2021). Potential megathrust co-seismic slip during the 2020 Sand Point, Alaska strike-slip earthquake. Pre-print on ESSOAr