Tectonic relations of carbon dioxide discharges and earthquakes

CO₂‐rich springs occur worldwide along major zones of seismicity. They are mostly in young orogenic belts, but some are in areas of rifting continental platforms. Analyses of ¹³C content indicate that much of the CO₂ is derived from the mantle and that other important sources are the metamorphism of marine carbonate‐bearing sedimentary rocks and the degradation of organic material. The presence of calc‐silicate minerals, such as pumpellyite in metagraywacke, is evidence of former conversion of carbonate‐bearing rocks into calc‐silicate minerals and release of CO₂. The CO₂ pressure in fractured rocks of a fault zone reduces the effective normal stress and, if it is sufficiently great, allows the fault to slip. If the pressure were maintained at a sufficiently high level, the fault behavior might be characterized by frequent small earthquakes and aseismic creep such as occur along active segments of the San Andreas system. The presence of CO₂‐rich springs may indicate a potentially hazardous seismic region. Monitoring of CO₂ discharges could be useful in earthquake prediction.