Emerged Coral Reefs Record Holocene Low‐Angle Normal Fault Earthquakes

Abstract

Low-angle normal faults (LANFs; dip <30°) accommodate kilometers of crustal extension, yet it remains unclear whether these faults can host large earthquakes or if they predominantly creep aseismically. Most active LANFs typically slip at rates of <3 mm/year. Here, we report U-Th ages from a series of distinct levels of formerly shallow-living corals killed by uplift-induced emergence of the footwall of one of the world's fastest-slipping LANFs, the Mai'iu fault in Papua New Guinea, which slips at rates of 8-12 mm/year. Coral ages and coastal morphology indicate punctuated episodic uplift events consistent with seismic slip on the Mai'iu fault. Maximum episodic uplift increments of 0.5-1.8 m imply earthquakes of M w > 7. We present the first coral paleoseismological record of normal fault earthquakes, which constrain the timing and surface uplift patterns of multiple LANF seismic cycles and confirm that LANFs can slip in large (M w > 7) earthquakes. Plain Language Summary The Earth's crust breaks along fractures and faults, including "normal faults," where tectonic plates pull apart. Subsequent slip on these faults enables plates to move away from each other and can occur via slow fault creep or infrequent fast earthquakes. Although normal faults typically intersect the surface of the Earth at a steep angle, some low-angle normal faults intersect the Earth's surface at a shallower angle. It remains unclear whether these faults typically creep gradually or slip in large earthquakes because active low-angle normal faults are uncommon and slip at low long-term slip rates. This study uses uplifted fossilized coral reefs to reconstruct the timing and style of fault slip near one of the world's fastest-slipping low-angle normal faults, the Mai'iu fault in Papua New Guinea. Isotopic and geomorphological evidence from corals killed by fault-related coastal uplift show that the Mai'iu fault slips infrequently and episodically, revealing a history of large earthquakes in this region.

High‐ and Low‐Angle Normal Fault Activity in a Collisional Orogen: The Northeastern Granada Basin (Betic Cordillera)

“…The highest magnitude events occur on the HANFs of the hangingwall (Boncio et al., 2004). However, the seismicity recorded in instrumental and historical periods of the above scenarios is far from the Mw 6.3 and 6.7 earthquakes (Abers et al., 1997), and larger events that possibly took place during the Holocene in Woodlark Basin (Biemiller et al., 2020).…”

“…Recently, Biemiller et al. (2020) described the possible occurrence of Mw > 7.0 earthquakes related to LANFs during the Holocene in this basin. Extensional detachment systems are usually accompanied by high‐angle normal faults (HANFs) that affect the hangingwall of LANFs (Wernicke, 1981).…”

“…In turn, in the Woodlark Basin, two recorded events of Mw 6.7 and 6.3 had respective focal mechanisms with nodal planes of 24° and 32° dip (Abers et al, 1997). Recently, Biemiller et al (2020) described the possible occurrence of Mw > 7.0 earthquakes related to LANFs during the Holocene in this basin. Extensional detachment systems are usually accompanied by high-angle normal faults (HANFs) that affect the hangingwall of LANFs (Wernicke, 1981).…”

The Dynamics of Unlikely Slip: 3D Modeling of Low‐Angle Normal Fault Rupture at the Mai'iu Fault, Papua New Guinea

“…Wernicke (1995) posited that LANF earthquakes are particularly rare because these faults typically accommodate relatively slow extension rates (a few mm/yr) and thus host only infrequent earthquakes; however, faster extension rates ≥1 cm/yr across the Mai'iu fault (Biemiller, Boulton, et al., 2020; Wallace et al., 2014; Webber et al., 2018) and the Banda detachment (Cummins et al., 2020) suggest these LANFs should host more frequent earthquakes. Nonetheless, paleoseismically recorded recurrence intervals from the Goodenough portion of the Mai'iu fault range from 482 to 1,590 years (Biemiller, Taylor, et al., 2020). It is plausible that large LANF earthquakes have not been recorded simply because their recurrence intervals are much longer than the relatively short instrumental record.…”

Using Syntectonic Calcite Veins to Reconstruct the Strength Evolution of an Active Low‐Angle Normal Fault, Woodlark Rift, SE Papua New Guinea

“…We conclude that the Mai'iu fault evolves by a rolling‐hinge style mechanism. Slip on this fault is partially accommodated by infrequent earthquakes (Biemiller, Taylor, et al., 2020) at ∼6–12 km depth ( T ≈ 130–275°C) where the fault is relatively strong (μ > 0.25–0.62). At the shallowest crustal levels (<6 km; T < 150°C), the Mai'iu fault is a true low‐angle normal fault, dipping ∼22 ± 2°, and is highly misoriented for slip.…”