Imaging of active faults from the temporal and spatial distributions of relocated seismicity induced beneath an EGS site in the Pohang region of southeastern Korea

<p>The 2017 Pohang earthquake (M_L 5.4) is the second largest earthquake occurred in an intraplate in modern Korea and is considered the largest induced earthquake from an EGS system around the world.&#160; The mainshock was proceeded by a few foreshocks and followed by a few thousands of aftershocks.&#160; Numerous densely distributed seismic stations in local and regional distances were deployed to monitor this earthquake sequence. &#160;Original hypocenters in the Pohang region were located using HYPODD that is independent on crustal structures.&#160; A comprehensive crustal Vp and Vs model was recently available from an invited committee of foreign experts based on well logs and regional seismic data.&#160; This model is then revised, especially the uppermost few hundred meters, based on results from a study of S to P converted waves from shallow interfaces beneath various stations, from the traditional Wadati plots analysis, and from the interpretation of two short seismic reflection/refraction profiles.&#160; From continuous data, 5 to 10 folds of additional earthquakes than the original manually picked events can be identified and located. P and S arrival times from all earthquakes are re-picked from continuous data and are relocated using the revised model and Hypoellipse program.&#160; Temporal and spatial distribution of relocated seismicity at depths range from 3 to 7 km are more clustered and confined than that from the original catalog.&#160; A few thin vertical cross-sectional views of hypocenters parallel and perpendicular to the seismicity reveal that seismicity propagates along multiple NE-SW trending faults beneath the Pohang basin and extending NE offshore into East Sea.&#160; These fault system is sandwiched between the Yongshan fault and a few other secondary faults to the south.&#160; The main shock (5.4) and the two largest aftershocks (4.3 and 4.6) as well as their associated aftershocks show predominantly NE-SW strike-slip with reverse faulting propagating along three different adjacent faults.&#160; Geometry of active faults and their tectonic implications will be presented and discussed in the meeting.</p>