Frequency of earthquakes Are earthquakes getting less frequent across the centuries? I know that more seismic stations have register more earthquakes in the last century, but that doesn't imply there were more. I am interested on the geophysical side of it.
The logic is that things settle down, so there is less stuff to shuttle. Add to it that the earth (at its core) get colder with the passage of time. 
 A: There is no evidence that the global earthquake frequency has changed significantly over that last few centuries. Because the physical mechanism responsible for earthquakes is motion along faults, we believe there have been earthquakes for as long as the earth has had a crust and lithosphere comparable to today.   
The oldest continental rocks dated, and ample geological evidence, indicate that plate tectonics has been ongoing for over 4 billion years. There may have been some variation in the rate of tectonic processes over time, but there is no evidence that plate tectonics is 'slowing down' because the earth is cooling.
You also should not assume that the temperature of the earth's core has been decreasing over this time. The radioactive decay and gravitational differentiation of the core could provide heat source in the earth's interior. 
Prior to the formation of the crust, billions of years ago, the processes of the primordial earth would have been different than the processes responsible for earthquakes today.    
A: Yes, indeed. Earthquakes have been on a steady rise since the earliest seismographs. Hard data from the United States Geological Service (USGS), Richter 6 earthquakes and above, 1895 thru 2015:


*

*1895 - 1905 = 11.

*1905 - 1915 = 48.

*1915 - 1925 = 281.

*1925 - 1935 = 524.

*1935 - 1945 = 446.

*1945 - 1955 = 906.

*1955 - 1965 = 1352.

*1965 - 1975 = 1285.

*1975 - 1985 = 1067.

*1985 - 1995 = 1525.

*1995 - 2005 = 1539.

*2005 - March 31, 2015 = 1640.


Note the two (2) decade "earthquake pause" (warming pause). 1995 to 2005.
Causation is strongly in support of internal heating within the planet. The Vostok Ice Core data also indicates in all past warming periods, the warming preceded the uptick in CO2. This makes sense if the earth is heating from within and producing the additional CO2.
