Overpressure and earthquake initiated slope failure in the Ursa region, northern Gulf of Mexico

We use fluid flow and slope stability models to study the evolution of overpressure and slope stability in the Ursa region, northern Gulf of Mexico. Our predictions match measured overpressures (pressures in excess of hydrostatic) from Integrated Ocean Drilling Program Expedition 308 Site U1324 above 200 m below seafloor (mbsf) but overpredict deeper (200-610 mbsf) overpressures by 0.4-1.1 MPa. Modeled overpressure at Site U1322 matches measurements for the entire section (0-240 mbsf) with exception of the measurement at 240 mbsf. Slope stability models that integrate modeled overpressure, vertical stress, and effective stress during deposition predict slope failure at 61 ka on the eastern end of the region. This failure corresponds to the base of a mass transport deposit that has been interpreted as a retrogressive failure initiated by high overpressure. Overpressure alone could not drive failure of a second mass transport deposit (MTD2) that has its base along the 27 ka horizon. With an earthquake acceleration model coupled with our slope stability model, we predict that horizontal acceleration from a magnitude 5 earthquake within 140 km of the Ursa region at 27 ka would initiate the failure that created MTD2; the same earthquake at 20 ka would have to be within 40 km for failure. This magnitude and maximum rupture distance are consistent with seismicity near the Ursa region. We therefore propose that in some cases, overpressure drives failure on low-angle slopes; however, earthquakes, even on passive margins, may play a critical role in initiating slope failure in sediments weakened by overpressure.