For this question, I gave you a table which had the number of earthquakes with magnitudes of 7 or larger in each year from 1900 through 1996. You were to make a graph of these numbers with time running along the X axis and the number of earthquakes running along the Y axis. You were then to decide, based on your graph, if the number of large earthquakes worldwide is going up, down, or staying basically the same.
My graph for this question is shown in Figure 1. The red circles are the numbers of earthquakes in each year, while the green line which connects these dots is there to make the trends easier to see. The long-dashed blue line is the mean number of earthquakes of this size worldwide in a given year, which is 20. The blue dotted lines are the maximum number (41 in 1943) and minimum number of quakes (6 in 1986), respectively.
From this graph, my conclusion is that the number of earthquakes of magnitude 7 or larger worldwide varies quite a bit from year to year, but seems to be remaining basically constant. There may also be some sort of cycle of more quakes followed by fewer, but saying anything more than that is pretty risky. It's also clear that the mid-1940s to early 1950s was a pretty active time for large earthquakes, and that the 1980s and early 1990s had fewer earthquakes than the average.
You might argue that, on a smaller scale, the number of earthquakes is going up (since we are rising out of a dip in the 1980s) and that the number of earthquakes is actually going down, since there appears to be a very slight ``downhill'' trend over time. I accepted either of these answers, provided you said why you believed the answer to be correct.
The real answer, however, is that even 100 years is probably not a long enough time period to look at the number of large earthquakes to decide if there are any trends over time. The best we can say at the moment is that there is no strong evidence that the number of large earthquakes worldwide is going up over time.
You may well be wondering why I didn't give you a table for smaller earthquakes. The answer is that it would have been deceptive. Our ability to detect and locate earthquakes depends on how many seismometers there are and how well they are scattered about the globe. Due to concerns over verifying global nuclear test ban treaties, the US Government has spent loads of money placing seismometers all around the world, and so the number of seismometers worldwide has steadily gone up over the years. As a result, the number of earthquakes we can detect has gone up, and the minimum magnitude earthquake which we can see has gone down.
So if you hear people saying very silly things like, ``the world is going to end soon, because there are more and more earthquakes happening all around the world,'' you now can look them in the eye and tell them that's baloney. The number of earthquakes is actually remaining more-or-less constant (to the best of our knowledge), it's just that we are detecting more and more of them. Doomsday may or may not be coming with the new millenium, but seismology certainly doesn't support the idea that the world's going to end anytime soon.
By the way, the next millenium starts on 1 January 2001, not on 1 January 2000. I know that people like me will never convince the general population of that, but it really annoys me to have people get that wrong. So please don't.
Greg Anderson
