increases in seismic activity following hydraulic fracturing along dormant or previously unknown faults are sometimes caused by the deep-injection disposal of hydraulic fracturing flowback (a byproduct of hydraulically fractured wells), and produced formation brine (a byproduct of both fractured and nonfractured oil and gas wells). for these reasons, hydraulic fracturing is under international scrutiny, restricted in some countries, and banned altogether in others. the european union is drafting regulations that would permit the controlled application of hydraulic fracturing.