California earthquakes

2016-03-28 by

Originally published on Blogspot Tuesday, September 29, 2015

Having grown up in California, I experienced many earthquakes.
The first one I remember was the 7.5 magnitude quake in Kern county in 1952. (I realize I'm dating myself here.) I was too young to realize what was happening, but I was playing in our back yard at the time and remember seeing the fence sway. I thought it was something strange. It wasn't until that night that I learned it was an earthquake - not that it meant anything to me at the time. We were a hundred miles or so away from the epicenter, and didn't have any damage.
I experienced several more small to moderate quakes over the years in Los Angeles. Then experiencing the 6.6 San Fernando quake in 1971, when I lived only miles from the epicenter, was quite a shock - in multiple senses of the word.
I had just awakened to my alarm to get up for school when it hit. I was still in bed. I thought the building I was in was going to come down on top of me. Fortunately, it didn't.
A few months after that, I had a brief dream. I still remember that dream today. I was down near the coast when a tremendous earthquake hit. Suddenly, after the shaking, I was about 25 feet under water, with broken-up freeway concrete all around, trying to get up to the surface.
I don't know whether that dream was due to a holdover stress/emotion (PTSD in today's terms) from the San Fernando quake or something else.
Earthquakes are difficult if not impossible to predict. At the time, if I remember correctly, someone predicted a high probability of a major earthquake on the San Andreas fault within the next 30 years. We're now 40-some years later. It obviously hasn't happened.
My schooling and work took me away from Southern California.
But ever since then, I've tracked earthquakes around the world. Today, I try to check the USGS site for US and world earthquakes occasionally. I've noticed the quake numbers increase in the midwest US around Oklahoma.

So to the reason for this post.
In tracking earthquakes, I've also noticed a couple of other things.
I wonder ...great quakes since 1900 - USGS

First, there is a large gap along the western coast of North America in recent (i.e. last 100 years or so) great earthquakes (magnitude 8,0 or above). There have been great earthquakes in this region in the past.
You may have heard of the "Ring of Fire" around the Pacific Ocean. It's a roughly horseshoe-shaped feature surrounding much of the ocean where the earth is very active - lots of volcanoes and other geological activity.
Several great earthquakes have occurred around this ring since 1900:
- Chile (9.5, 8.8, 8.5, and just recently an 8.3)
- Ecuador (8.8)
- Indonesia, with the 2004 quake which killed an estimated 250,000 people from that and the tsunami (9.1, 8.6, 8.5)
- Japan with the 2011 quake which resulted in the tsunami still discussed today because of the Fukushima nuclear plant destruction (9.0)
- Kamchatka and the Kuril Islands, north of Japan (9.0, 8.5)
- The Aleutian Islands (8.7, 8.6)
- Alaska with the 1964 quake and tsunami (9.2).
But there is that gap at North America. USGS CA quakes 2015-0912

Second, the number of earthquakes greater than magnitude 2.5 in Southern California around the San Andreas Fault, especially near the Salton Sea, seems to have slowed over the last few weeks - at least by my observations. For several weeks now, the little dots which appear around there to the Mexican border seem less numerous. (Link is for the current date if you want to check.)
Now one geological hypothesis is that smaller quakes can relieve the strain so bigger ones don't occur. I saw a recent news report where that lower section of the San Andreas Fault has not slipped in a major way since the 1600s.
USGS CA quakes as of 2015-0925To the right I've inserted two weeks of earthquake data points. The first for the week preceding 9-12-2015; the second for the week preceding 9-25-2015. (Unfortunately, I don't have images of earlier data showing the higher point count.)

Is this recent reduction of quakes in this section a precursor to the "big one" in Southern California?
And recently there was another prediction that there was a high probability of a major San Andreas quake within the next 30 years. So what else is new?
Only time will tell.
But I wonder ...

Credits: images are from USGS.
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