US Earthquake Threats

 The largest earthquake on record was a magnitude 9.4-9.6 that struck in Valdivia, Chile, in 1960. Lists of largest earthquakes are generally only reliable since the early 1900s, when the use of scientific measuring equipment began to be used and records were kept.

Large earthquakes since the 1700s can have magnitudes reasonably deduced due to reports of the earthquake. However, if nobody was around to feel or record it, did it really happen?

Joking aside, the reality is there have been many extremely large and destructive earthquakes that we don’t have records for, or which were not adequately recorded. Except for digging trenches and examining the geologic record, we don’t have much historical records for determining size and frequency.

None of the notable earthquakes previously discussed were in North America. That doesn’t mean there isn’t a high risk in the United States.

First, let’s look at the mega-quakes from around the world.

The Wikipedia website has a list of the top 36 largest earthquakes in the world. The largest quake registered was as high as 9.6, with the bottom of the list being 15 magnitude 8.5 earthquakes. The earliest quake listed was in 1575, and the most recent (as of this writing) being in 2012.

Of those quakes, one was listed in the sixteenth century, and three for the seventeenth century. Nine occurred in the eighteenth century. Seven happened in the nineteenth century. Ten were recorded through the twentieth century. For the twenty-first century we have already had six, and we haven’t even gotten out of the second decade.

Earthquake Threats over the Last Century

While there may not be accurate records for past mega-quakes, it would appear, at least over the last few centuries, that we are in an upward trajectory and can expect more mega-quakes.

Of the 36 earthquakes, only four occurred in the United States, with three of those in Alaska. The only non-Alaska quake was the Cascadia rupture on January 26, 1700, that had an estimated magnitude range of 8.7 to 9.2. The December 16, 1811, rupture on the New Madrid fault in Missouri didn’t even make the list with its mere 8.1 magnitude.

While North America hasn’t put in high ranking earthquake numbers, it’s not like the earth is quiet. 

Since the January 26, 1700, Cascadia earthquake there have been more than 40 earthquakes of magnitude 6.5 or higher that rocked different areas of the United States. Seven of those happened in the 1800’s. A whopping 23 occurred in the 1900’s. From 2000 to July 5, 2019, there were ten magnitude 6.5 or higher quakes.

During the twentieth century, the first three decades saw only one of these 6.5 or larger earthquakes per decade. The 1930’s didn’t have any. Then three happened in the 40’s, four more in the 50’s, and three in each of the decades from the 60’s to 80’s. Five more happened in the 90’s. 

In the first decade of the 21st century, four more magnitude 6.5 quakes shook areas of Alaska and California. have happened in the first decade of the new century. Nine and a half years into the second decade has already seen six more.

Rising Earthquake Threats?

Staying in the United States, when we look at magnitude 7.0 and larger, there was one in the 1700’s and four in the 1800’s. It’s very likely there could have been more, but we don’t have any record of them.

The 1900’s saw 14 of these large earthquakes, with one in each of the first two decades, and none in the 20’s and 30’s. There was one in the 40’s (a monstrous 8.6 in Alaska), four in the 50’s (including another 8.6 in Alaska), two in the 60’s (one was a devasting 9.2 in Alaska), and another two in the 70’s. Only one magnitude 7.0 or larger happened in the 80’s, and another two happened in the 90’s.

The U.S. had a 7.9-magnitude quake strike Alaska during the first decade of this century. However, there have been five of magnitude 7.0 or greater during the first nine and a half years of the second decade, with four of those in Alaska.

The following table shows the number of magnitude 7.0 or larger earthquakes per time period. Note that the 18th and 19th centuries have their own rows, while the 20th and 21st centuries are divided into decades. The data used to compile this table was for my book that was completed in fall 2019.

If you didn’t notice, Alaska is the most earthquake prone state. However, earthquakes in the Last Frontier state seldom make the news as there isn’t the damage and destruction that would make them newsworthy.

Looking at the numbers, it appears that the frequency of large earthquakes is increasing. 

When we look into regions more closely, you will discover that big earthquakes are overdue in each region, with “big” being a magnitude 7.0 or greater rupture.

Something I mention in chapter 4 of my book, which is starting to be discovered through studies, is a big earthquake in one area could trigger earthquakes in another. This triggering could either come within a very short time following the first earthquake. Or, the first earthquake could result in increasing stresses on other faults and raise the likelihood of a big earthquake happening on those faults in the near future.