Are earthquakes becoming more frequent? What are the long term trends?
I find an interesting page here, with this graph showing a pretty convincing increase since the 1980's.
But this increase could be due to better instrumentation - wider distribution of more sensitive seismographs. However, this explanation does not accord with the recent downward trend.
The writer also collects all earthquakes >6.9 magnitude, which would be recorded by less widespread, less sensitive instruments.
Again, a trend is apparent:
DATES FROM & TO PERIOD NO. EARTHQUAKES (Mag. > 6.99) --------------------------- ----------- ------------------------------ 1863 to 1900 incl 38 yrs 12 1901 to 1938 incl 38 yrs 53 Reference list 1901 to 1938 1939 to 1976 incl 38 yrs 71 Reference list 1939 to 1976 1977 to 2014 incl * 38 yrs 144 (to Sept. 2009) predict >180 in total.
So here we have evidence that the earthquake frequency is rising.
However, we find the opposite view here "Although it may seem that we are having more earthquakes, earthquakes of magnitude 7.0 or greater have remained fairly constant throughout this century and, according to our records, have actually seemed to decrease "in recent years".
By "This century", they mean the 21st century. Not a very large dataset. Reminds me of the methods used by climate change deniers, who try to use a few years' data to contradict much longer data series.
The match between earthquake activity and global temperatures is interesting - right down to the recent easing up of both temperature and earthquakes.
Hotter rocks expand more, which would mean more seismic activity.
Emphatically, this is not a claim that global warming is causing increasing earthquake activity. But that is a reasonable hypothesis for seismologists to pore over for a few decades.
If earthquakes are increasing, what are volcanoes up to?
Here is a page on volcano activity trends. It shows an increase over 200 years, but puts it down to better reporting. However it is generally agreed that if global warming causes glaciers and icecaps to melt, the "unloading" (reduced pressure on earth's mantle) will cause an increase in volcanism. Which will help to reduce global warming, by reflecting more sunlight back into space.
It is argued by Prof Ben McGuire of UCL that unloading of land-based ice due to global warming could affect the earth's crust, triggering earthquakes and volcanoes.
So there is evidence that both earthquakes and volcanoes are increasingly common, and there are plausible mechanisms to relate them to observed global warming. The relationship at the moment is speculative, and it will take many years for scientists to come to a consensus on the question. In the meantime we should set about decarbonising the world economy.
See also: Earthquake resistant buildings
Earthquakes may be connected.