The Richter Scale was devised by Charles Richter in $1940$ to compare the intensities of earthquakes. The intensity of an earthquake is determined by the amount of ground motion measured on a seismometer. Each increase of one unit in magnitude on the Richter scale corresponds to an increase of $10$ times the intensity measured on a seismometer.
[By expressing the intensity ($I$) as an exponential function of the magnitude ($M$), you can compare the intensities of earthquakes that do not differ by a whole integer. $I={10}^M$]

The two most costly earthquakes both occurred in Japan: the earthquake of $2011$ had a magnitude of $9.1$; the earthquake of $1995$ had a magnitude of $6.9$. If the earthquake in $2011$ was ${10}^k$  times more intense than the earthquake in $1995$, then find $k$.