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Summary Magnetic minerals form or alter in the presence of hydrocarbons, making them a potential magnetic proxy for identifying hydrocarbon migration pathways. In this paper we test this idea by magnetically measuring core samples from the Tay Fan in the Western Central Graben in the Central North Sea. In a companion paper, 3D petroleum systems modelling has been carried out to forward model migration pathways within the Tay Fan. Rock magnetic experiments identified a range of magnetite, maghemite, iron sulphides, siderite, goethite and titanohematite, some of which are part of the background signal, and some due to the presence of hydrocarbons. Typical concentrations of the magnetic minerals were ∼10–200 ppm. Importantly, we have identified an increasing presence of authigenic iron sulphides (likely pyrite and greigite) along the identified lateral hydrocarbon migration pathway (east to west). This is likely caused by biodegradation resulting in the precipitation of iron sulphides, however, though less likely, it could alternatively be caused by mature oil generation, which subsequently travelled with the migrating oil to the traps in the west. These observations suggest mineral magnetic techniques could be a rapid alternative method for identifying the severity of biodegradation or oil maturity in core sample, which can then be used to calibrate petroleum systems models. 

Predicting rainfall‐induced landslide motion is challenging because shallow groundwater flow is extremely sensitive to the preexisting moisture content in the ground. Here, we use groundwater hydrology theory and numerical modeling combined with five years of field monitoring to illustrate how unsaturated groundwater flow processes modulate the seasonal pore water pressure rise and therefore the onset of motion for slow‐moving landslides. The onset of landslide motion at Oak Ridge earthflow in California’s Diablo Range occurs after an abrupt water table rise to near the landslide surface 52–129 days after seasonal rainfall commences. Model results and theory suggest that this abrupt rise occurs from the advection of a nearly saturated wetting front, which marks the leading edge of the integrated downward flux of seasonal rainfall, to the water table. Prior to this abrupt rise, we observe little measured pore water pressure response within the landslide due to rainfall. However, once the wetting front reaches the water table, we observe nearly instantaneous pore water pressure transmission within the landslide body that is accompanied by landslide acceleration. We cast the timescale to reach a critical pore water pressure threshold using a simple mass balance model that considers variable moisture storage with depth and explains the onset of seasonal landslide motion with a rainfall intensity‐duration threshold. Our model shows that the seasonal response time of slow‐moving landslides is controlled by the dry season vadose zone depth rather than the total landslide thickness.