(a) The thickness of this shell of Earth's crust is 0.1 meter,
much smaller than the radius of Earth.  Thus, we can figure the
volume of this shell to be the surface area of Earth, multiplied
by 0.1 meter.  So the volume is

   V = 4 * pi * (6378000 m)^2 * 0.1 m = 5.1 * 10^13 cubic meters.

If your calculator is precise enough, you can also compute the volume
of this shell by subtracting the shrunken size of Earth from the 
pre-shrunk size, in which case

        4 * pi
   V =  ------ * ( (6378000 m)^3 - (6377999.9 m)^3 ) = 5.1 * 10^13 m^3.
          3

In either case, the density of the crust is 3.0 g/cm^3, or 3000 kg/m^3,
so the mass is M = DV = 3000 * 5.1 * 10^13 = 1.5 * 10^17 kg.  The 
gravitational acceleration of Earth on this layer is about 10 m/sec^2.
Finally, this layer falls a height of 0.1 meter.  So the gravitational
potential energy of this total fall is

   E = m * a(grav) * h = 1.5 * 10^17 Joule.

(b) Again, the total energy of the Hiroshima atomic bomb was about 
1.8 * 10^13 Joule.  Thus this shrinkage of Earth would release

   1.5 * 10^17
   -----------  = 8300 times the energy of the Hiroshima bomb as heat.
   1.8 * 10^13

Would we notice it?  Well, this energy is spread out over the entire
surface of the planet.  So each square meter of Earth's surface would
be exposed to

      1.5 * 10^17 Joule
   ---------------------- = 290 Joules.  This is equivalent to 
   4 * pi * (6378000 m)^2

290 / 4.2 = 69 calories, which is not quite enough heat to melt one gram
of ice into liquid water (remember that the heat of fusion for water is
80 calories per gram).  If this amount of heat were released gradually,
say over the course of months or years, we would probably not notice it.
If it happened very quickly, however, say in a second or so, it might
get a little warm.  And if the Earth shrank at this rate for thousands
or millions of years, it would eventually give off a fair bit of heat.