most mineral vein systems are a result of repeated natural fracturing during periods of relatively high pore fluid pressure. the impact of high pore fluid pressure on the formation process of mineral vein systems is particularly evident in "crack-seal" veins, where the vein material is part of a series of discrete fracturing events, and extra vein material is deposited on each occasion. one example of long-term repeated natural fracturing is in the effects of seismic activity. stress levels rise and fall episodically, and earthquakes can cause large volumes of connate water to be expelled from fluid-filled fractures. this process is referred to as "seismic pumping".