Subduction earthquakes controlled by incoming plate geometry: The 2020 M>7.5 Shumagin, Alaska, earthquake doublet Yu Jianga, Pablo J. Gonzáleza,b, Roland Bürgmannc aCOMET, Department of Earth, Ocean and Ecological Sciences, School of Environmental Sciences, University of Liverpool, Liverpool, L69 3BX, United Kingdom, [email protected] bVolcanology Research Group, Department of Life and Earth Sciences, Instituto de Productos Naturales y Agrobiologıa (IPNA-CSIC), 38206 La Laguna, Tenerife, Canary Islands, Spain, [email protected] cDepartment of Earth and Planetary Science, University of California, Berkeley, CA, USA, [email protected] This manuscript is a preprint and has been submitted for publication in Earth and Planetary Science Letters. Please note that the manuscript is undergoing peer review and has not been accepted for publication. Subsequent versions of this manuscript may have slightly different content. If accepted, the final version of this manuscript will be available via the Peer-reviewed Publication DOI link on the right-hand side of this webpage. Please feel free to contact the corresponding author; we welcome feedback. Subduction earthquakes controlled by incoming plate geometry: The 2020 M>7.5 Shumagin, Alaska, earthquake doublet Yu Jianga, Pablo J. Gonz´aleza,b, Roland B¨urgmannc aCOMET, Department of Earth, Ocean and Ecological Sciences, School of Environmental Sciences, University of Liverpool, Liverpool, L69 3BX, United Kingdom bDepartment of Life and Earth Sciences, Instituto de Productos Naturales y Agrobiolog´ıa (IPNA-CSIC), 38206 La Laguna, Tenerife, Canary Islands, Spain cDepartment of Earth and Planetary Science, University of California, Berkeley, CA, USA Abstract In 2020, an earthquake doublet, a M7.8 on July 22nd and a M7.6 on Octo- ber 19th, struck the Alaska-Aleutian subduction zone beneath the Shumagin Islands. This is the first documented earthquake doublet, of considerable size, involving a megathrust event and a strike-slip event, with both events producing deeply buried ruptures. The first event partially ruptured a seis- mic gap, which has not hosted large earthquakes since 1917, and the sec- ond event was unusual as it broke a trench-perpendicular fault within the incoming oceanic slab. We used an improved Bayesian geodetic inversion method to estimate the fault slip distributions of the major earthquakes us- ing Interferometric Synthetic Aperture Radar (InSAR) wrapped phase and Global Navigation Satellite Systems (GNSS) offsets data. The geodetic in- versions reveal that the Shumagin seismic gap is multi-segmented, and the M7.8 earthquake ruptured the eastern segment from 14 km down to 44 km depth. The coseismic slip occurred along a more steeply, 26-degree dipping Preprint submitted to Earth and Planetary Science Letters May 16, 2021 segment, and was bounded up-dip by a bend of the megathrust interface to a shallower 8-degree dip angle connecting to the trench. The model for the M7.6 event tightly constrained the rupture depth extent to 23-37 km, within the depth range of the M7.8 coseismic rupture area. We find that the M7.6 event ruptured the incoming slab across its full seismogenic thickness, po- tentially reactivating subducted Kula-Resurrection seafloor-spreading ridge structures. Coulomb stress transfer models suggest that coseismic and/or postseismic slip of the M7.8 event could have triggered the M7.6 event. This unusual intraslab event could have been caused by accumulation and lo- calization of flexural elastic shear stresses at the slab bending region. We conclude that the segmented megathrust structure and the location of in- traslab fault structures limited the rupture dimensions of the M7.8 event and are responsible for the segmentation of the Shumagin seismic gap. Our study suggests that the western and shallower up-dip segments of the seismic gap did not fail and remain potential seismic and tsunami hazard sources. The unusual earthquake doublet provides a unique opportunity to improve our understanding of the role of the subducting lithosphere structure in the segmentation of subduction zones. Keywords: Subduction earthquake doublet, slab geometry, Shumagin seismic gap, Alaska subduction zone 1 1. Introduction 2 In 2020, a pair of large and similar sized earthquakes (doublet) occurred 3 along the eastern Aleutian subduction zone off the Alaska Peninsula (Fig. 4 1(a)). The first and largest earthquake of the doublet, with a magnitude (M) 2 5 of 7.8, occurred at 06:12:44 UTC (22:12:44 local) on July 22 2020. On Oct 6 19 2020 at 20:54:39 UTC (12:54:39 local), an anomalously large aftershock 7 (M=7.6) occurred 80 km southwest of the first event. According to the U.S. 8 Geological Survey (USGS) hypocentre catalog, both earthquakes are located 9 on the landward side of the subduction trench. The aftershocks of the first 10 event are distributed parallel to the trench, while those of the second event 11 are aligned perpendicular to the trench. The focal mechanism solutions from 12 the Global Centroid Moment Tensor (GCMT) catalog suggest the mechanism 13 of the M7.8 event is of thrust-faulting type, while the M7.6 event was a strike- 14 slip event. The centroid depths of both earthquakes were estimated as about 15 30-35 km. This suggests that the M7.8 event ruptured the buried megathrust 16 interface, but the M7.6 event was caused by an unusual strike-slip rupture 17 along an approximately trench-normal fault. 18 The 2020 Shumagin earthquake sequence is interesting for several reasons. 19 Firstly, the mainshock is located within the Shumagin seismic gap. This 20 portion of the subduction thrust has been identified as a seismic gap since 21 the 1970s (Sykes, 1971, Davies et al., 1981). The seismic gap stretches ∼200 22 km along the Shumagin Islands and is bounded to the west by the 1946 23 Mw8.6 earthquake (L´opez and Okal, 2006) and to the east by the 1938 Mw8.2 24 rupture (Freymueller et al., 2021). The last earthquakes that are inferred 25 to have ruptured through part of or the whole Shumagin gap occurred in 26 1993, 1917, 1788, and possibly 1847 (Estabrook et al., 1994). Over the last 27 century, a few moderate (M6.5 to M7.0) events have occurred in the area at 28 depths greater than 30 km. However, the fault sections ruptured by those 29 earthquakes are relatively small compared to the 200 km-long seismic gap 3 30 (e.g., the estimated rupture area of the 1993 Ms6.9 earthquake is 40 km-long 31 and 15 km-wide, Lu et al. (1994)). If the whole Shumagin gap were fully 19 32 locked, the accumulated moment equates to 6.6×10 Nm/year, assuming a 33 plate convergence rate of 64 mm/year and a uniform rigidity of 50 GPa. This 34 would require a 7.5 event every 4 years, or a M8 event every 20 years. The lack 35 of historic M7.5+ earthquakes in the Shumagin region has been explained due 36 to substantial aseismic fault creep at seismogenic depths revealed by model 37 inversions of inter-seismic GNSS velocities (Fournier and Freymueller, 2007). 38 Fournier and Freymueller (2007) suggested that instead of rupturing in large 39 earthquakes, most of the seismic moment in the Shumagin gap is released 40 through steady creep. Thus, a moderate M7 earthquake every ∼40 years, as 41 observed in the last century, may be sufficient to accommodate the residual 42 slip deficit. To the west of the Shumagin gap, a recent interseismic coupling 43 model shows that the shallow portion along the Sanak segment, 240 km-long 44 and 115 km-wide, might be partially locked, with 15%-25% coupling (Drooff 45 and Freymueller, 2021). For the shallow portion along the Sanak segment, 46 if the estimated 1946 earthquake rupture area, 180 km-long and 115 km- 47 wide (L´opez and Okal, 2006), was fully locked, and the remaining area is 48 20% coupled, the seismic moment deficit would be accumulating at around 18 49 4.5×10 Nm/year. This calculation suggests that the seismic moment of the 21 50 1946 earthquake, 8.5×10 Nm (L´opez and Okal, 2006), releases 1900 years 51 of elastic strain accumulation along the Sanak segment. Large uncertainties 52 are associated with estimating earthquake recurrence intervals, including the 53 poorly constrained estimation of the 1946 earthquake slip and the assumption 54 of the highest slip deficit near the trench in the interseismic coupling model 4 55 (Drooff and Freymueller, 2021). Nevertheless, such long recurrence intervals 56 could be the reason for there only being one documented major earthquake 57 in the Sanak segment since 1700 (Estabrook and Boyd, 1992). 58 Secondly, the M7.6 slab-breaking aftershock had an unusual strike-slip 59 mechanism and was deeply buried. Large oceanic lithosphere strike-slip 60 events have previously occurred in the oceanic plate off subduction zones, 61 such as the 2018 Mw7.9 Gulf of Alaska earthquake (Lay et al., 2018) and 62 the 2012 Mw8.6 Wharton basin earthquakes off-Sumatra (Wei et al., 2013). 63 In addition, the subduction zone outer-rise region regularly hosts normal- 64 faulting mechanisms events. Outer-rise normal-faulting events are attributed 65 to plate bending stresses from slab pull, and can be modulated by the in- 66 terplate seismic cycle (Ammon et al., 2008). However, the occurrence of a 67 major intraplate earthquake in the oceanic lithosphere just landward of the 68 trench is rare, with only few reported examples, such as the October 4, 1994 69 Mw8.2 earthquake off Shikotan Island along the Kuril trench (Tanioka et al., 70 1995). Another notable example was a Mw7 strike-slip intraslab event lo- 71 cated beneath Kodiak Island, Alaska down-dip of the locked portion of the 72 Alaska-Aleutian megathrust (Hansen and Ratchkovski, 2001).