Magnetotelluric Investigation of the Causative Fault of the 2016 Mw 5.8 Pawnee, Oklahoma Earthquake

On September 1, 2016 a Mw 5.8 earthquake struck the Pawnee area of north central Oklahoma. The rupture occurred in a region of complex fault interactions at 4 -6 km depth, along a previously unmapped fault called the Sooner Lake Fault. Seismic studies suggest this fault is a near vertical (70-90 degree dip), left lateral strike-slip fault. Despite broad agreement between activities related to hydrocarbon production and induced seismicity, we do not fully understand the linkages between injection, subsurface fluid migration, and pressure build-up along faults. Not every injection well is associated with seismicity, and not every fault in a newly seismic region is reactivated by oil and gas production activities. This study deployed magnetotelluric (MT) instruments to better understand the relationship between induced seismicity and waste water injection. The MT data was acquired across 18 MT stations along a profile line perpendicular to the geoelectric strike of the Sooner Lake Fault. This data was imported into the WingLink software with which an inversion was carried out to image the rupture zone. The resulting MT cross-section clearly imaged an upper layer of conductive sediments and a conductive wedge associated with the rupture zone of the Sooner Lake Fault within the resistive basement. The ~4 km wide base of this conductive wedge is found at ~12 km depth and tappers out upward to a depth of ~6 km. The hypocenters of the earthquakes coincide with the tip of this conductive wedge.Geolog

Optimization of Tensor Controlled-Source Electromagnetic Exploration Methods: Case Study from Trachyte Mesa Intrusion, Henry Mountains, Utah

The Henry Mountains in Utah are home to several small igneous intrusions from the Late Oligocene to Early Miocene. One of the most critical small satellite bodies to the main intrusions of the Henry Mountains is the Trachyte Mesa intrusion. What is interesting about the Trachyte Mesa intrusion is that geologist and geophysicist are able to observe outcrops on the surface of Earth in order to characterize the mesa. An important question that is continuously being researched about this area concerns the emplacement of these intrusions. Scientist believe that by understanding the internal structure of these intrusions, answers regarding the emplacement of these intrusions will appear. There has been debate, however, regarding this internal structure. The main dispute is between two structural geologists who have competing ideas regarding Trachyte Mesa after conducting magnetic surveys in the area. Sven Morgan (2008) has concluded that the Trachyte Mesa intrusion internal structure is based on a sheet magma stacking model. Whereas Paul Wetmore (2009) believes that the mesa took the shape of the surrounding rock structures. In order to shed further light upon this dilemma, the original aim of the project was to conduct a tensor control-source electromagnetic characterization survey of the area and produce a model of the Trachyte Mesa intrusion. Due to several complications with responses from the equipment, the aim of the project was modified. The new goal is to discover the cause of the unusual signals from the equipment, whether it be from equipment malfunctions or the resistive environment present at Trachyte Mesa. With the extensive testing of equipment, a controlled-source electromagnetic survey process that will pave the way for future projects using the same methodology will also be established