An apparatus and method for generating high-pressure pulses in a subterranean dielectric medium are provided. The method includes providing an electrode assembly in the medium, the electrode assembly having first and second electrodes that define a gap therebetween. A shaped electric current pulse is delivered to the electrode assembly. The electric current pulse has a duration greater than 100 μs so that an electric arc is formed between the first and second electrodes, thereby producing a pressure pulse in the medium.