17:00 〜 17:15
[S02-6-03] Experiment to Trigger a Moderate-sized Earthquke
We are proposing an experiment to understand the initiation of large earthquakes by inducing seismic events on a shallow fault with water injection. Increasing the fluid pressure near an active fault will reduce normal pressure on a fault and bring it closer to failure, according to the classic Coulomb failure criterion. A study to monitor the water pressure and subsequent triggered earthquakes can help answer some fundamental questions in seismology about the stress levels that cause earthquakes and the physical conditions that are necessary for a large earthquake to occur.
Possible sites for such an experiment would be transform faults near mid-ocean ridges, such as the East Pacific Rise. Seismicity in these regions is quite shallow and accessible with ocean boreholes of 2 to 3 km depth. In such settings, moderate (M5 to M6) earthquakes occur at repeat intervals of 5 to 15 years. We would like to conduct a water injection experiment at one of these sites a few years before the expected earthquakes recurrence, to try to trigger an early occurrence of the event. Also, moderate earthquakes in these regions are usually accompanied by active foreshock sequences.
What is the water pressure needed to trigger an earthquake ? Assuming a recurrence time of about 15 years and a stress drop of about 2 MPa for an M5 earthquake, to trigger an earthquake 2 years early, we would need to increase the shear stress 0.3 MPa. Using the Coulomb failure criterion, this would correspond to a decrease of normal stress (or increase of pore pressure) of about 0.5 MPa. This increase is relatively small, however, the absolute pore pressure at 1 km depth is about 8 MPa. So pumping pressure of about 8.5 MPa would be necessary to reduce the normal stress on the fault. Such pumping pressures are possible using the current riser drilling technology.
Possible sites for such an experiment would be transform faults near mid-ocean ridges, such as the East Pacific Rise. Seismicity in these regions is quite shallow and accessible with ocean boreholes of 2 to 3 km depth. In such settings, moderate (M5 to M6) earthquakes occur at repeat intervals of 5 to 15 years. We would like to conduct a water injection experiment at one of these sites a few years before the expected earthquakes recurrence, to try to trigger an early occurrence of the event. Also, moderate earthquakes in these regions are usually accompanied by active foreshock sequences.
What is the water pressure needed to trigger an earthquake ? Assuming a recurrence time of about 15 years and a stress drop of about 2 MPa for an M5 earthquake, to trigger an earthquake 2 years early, we would need to increase the shear stress 0.3 MPa. Using the Coulomb failure criterion, this would correspond to a decrease of normal stress (or increase of pore pressure) of about 0.5 MPa. This increase is relatively small, however, the absolute pore pressure at 1 km depth is about 8 MPa. So pumping pressure of about 8.5 MPa would be necessary to reduce the normal stress on the fault. Such pumping pressures are possible using the current riser drilling technology.