Comprehensive geostress observation station on the seabed of Bohai strait

A 106km dual road/rail carriageway across the Bohai Straits is under planning in China, but there are two active faults in the southern Bohai Strait where occurred M7.5 (A.D. 173) and M7.0（A.D. 1548）earthquakes. Therefore, to study the fault activity induced microearthquakes and rock deformations on the upper and lower of faults， several monitoring stations were set up on both sides of the faults. By measure the fracture locations, accumulation and release of the seismic strain energy in different depths，it gives us a better understanding of the structural, and mechanical properties of the seabed faults.

The technologies involved include offshore drilling, geostress measurement, microseismic observation and geostress monitoring. The drilling ship starts from the seabed with a depth of 35 meters. The drilling depth is 150 meters of which the upper part is 105 meters thick sediments of the quaternary period, and the lower part is 45 meters thick granite. A comprehensive geostress monitoring instrument is installed in this section.

Within the depth of 25.00 142.50m in the southern Bohai Strait, the maximum horizontal principal stress is 1.80MPa to 10.88MPa and the minimum horizontal principal stress is 0.81MPa to 7.31MPa. The orientation of horizontal principal stress varies from 26.60 to 72 degrees, with an average of 44.97 degrees.

After the field geostress measurement, a comprehensive geostress monitoring system was installed. The system consists of a sea surface buoy and a borehole submarine strain gauge. The total length of the strain gauge is 8.5 meters. The seabed monitoring data can be sent to the server via WIFI or GPRS to accurately monitor the elastic deformation of surrounding rocks.

Since 2011, the active faults in the Bohai Strait have been monitored for long-term ground stress and microearthquakes, and 8 significant slow earthquakes and more than 250 microearthquakes have been recorded. Among them, there were 5 earthquakes of grade 3-4, 22 of grade 2-3, and 200 of grade 0-2. Most earthquakes occurred at a depth of 2-8 kilometers, and the epicenter was basically distributed along the fault. The results can provide a basis for the study of fault activity and earthquake prediction.