What is the wavelength of an earthquake wave if it had a speed of 5km/s and a frequency of 12 hz?

For ANY wave,

Wavelength = (wave speed) / (frequency)

Wavelength = (5,000 m/s) / (12 / sec)

Wavelength = (5,000 / 12) meters

Wavelength = 416.7 meters

electric field is a function of 1/r^2

yes, the force per unit charge for a point charge is inverse square law.

the ima (higher than 1m) will be greater than the ima with the height of 1m.