Evidence of frictional melting observed in the fault rock drill cuttings from Pohang enhanced geothermal system (EGS) site

The 2017 Mw 5.5 Pohang earthquake in South Korea has been reported as one of the largest triggered earthquakes at an enhanced geothermal system (EGS) site. A fault that was ruptured in Pohang was not identified by geological investigations or geophysical surveys before the Mw 5.5 Pohang earthquake. “Mud balls” showing a fault gouge structure were reported in the Pohang EGS site only at the depth range of 3,790 – 3,816 m. In this study, we present new observation on the fault rocks retrieved from the Pohang EGS site as drill cuttings. The drill cuttings from 3,256 – 3,911 m interval contained mud balls similar to those observed at the depth of 3,790 – 3,816 m. Mud balls contained fine grains and showed foliated clay matrix with well-rounded clasts of quartz or feldspar, which are a typical fault gouge structure. In addition, mud balls retrieved from the depth of 3,256 and 3,260 m contained black fragments. SEM and TEM observation revealed that these black fragments consist of glassy matrix with sub-micrometer size clasts. Abundant vesicles were observed inside the black fragments, and some of the black fragments preserved foliation defined by compositional layering. TEM observation confirmed that the glassy matrix in the black fragments is amorphous material with a chemical composition similar to illite-smectite. These observations indicate that black fragments are resulted from the frictional melting during the coseismic slip.