Seismic evidence for a Holocene Mw 6.5-7.1 earthquake along the Littoral Fault Zone, off the Guangdong-Hong Kong-Macao Greater Bay Area

The Guangdong-Hong Kong-Macao Greater Bay Area (GBA) is one of the most economically developing and densely populated areas in China. It is spatially only ~20-100 km north of the central segment of the largest regional seismogenic fault-the Littoral Fault Zone. However, this segment is absent of major earthquakes with M_w =>7 in documented history (since 1600) while the west and east segments of Littoral Fault Zone have experienced several M_w=>7 events, it remains unclear whether this segment is aseismic, or has longer earthquake recurrence interval. The answer to this question matters greatly to the precise assessment of earthquake potential for the GBA. To address this issue, we used newly acquired >4000-km-long seismic reflection profiles, in combination with 1 shallow borehole data, to examine the detailed fault structures and evaluate the Holocene activity of the Littoral Fault Zone central segment. Our new results show that this central segment is approximately 157-km long with decreasing width from 25 km to 6 km westward. It comprises a NEE-trending and SE-dipping main fault with two sub-segment and a series of secondary normal faults. In the west sub-segment of the main fault, the 115-km en-echelon segmented sidewall faults consist of the 80-km segmented late Pleistocene faults and the 35-km Holocene fault, with maximum 2.7 m displacement of the Holocene activity. In its east sub-segment, the main fault is a 42-km strike-slip fault (transtensional faults) with negative flower structures and a fault bend transitioning from the NEE trend (N70°) to the SEE trend (N110°). The Holocene fault directly cuts through the Holocene strata with tips reaching 6.6 m from the seafloor, while the Late Pleistocene faults cause the overlying strata to be deformed and folded. Based on the Holocene rupture displacement and active length, we estimated that the main fault may have triggered earthquakes with magnitude up to M_w 6.6-7.1.