Hydraulic fracturing method is used to determine the major principal stress (\( \sigma_1 \)) in an underground rock strata having tensile strength of 6 MPa. The minor principal stress (\( \sigma_3 \)) in the strata is 8 MPa. If fluid pressure of 10 MPa is required to fracture the vertical borehole in that strata, the magnitude of \( \sigma_1 \), in MPa, is: