2018 Anchorage Earthquake The 30 November 2018 M w 7 . 1 Anchorage Earthquake

Cite this article as West, M. E., A. Bender, M. Gardine, L. Gardine, K. Gately, P. Haeussler, W. Hassan, F. Meyer, C. Richards, N. A. Ruppert, et al. (2019). The 30 November 2018 Mw 7.1 Anchorage Earthquake, Seismol. Res. Lett. 91, 66–84, doi: 10.1785/0220190176. Supplemental Material The Mw 7.1 47 km deep earthquake that occurred on 30 November 2018 had deep societal impacts across southcentral Alaska and exhibited phenomena of broad scientific interest. We document observations that point to future directions of research and hazardmitigation. The rupture mechanism, aftershocks, and deformation of the mainshock are consistent with extension inside the Pacific plate near the down-dip limit of flat-slab subduction. Peak ground motions >25%g were observed across more than 8000 km2, though the most violent near-fault shaking was avoided because the hypocenter was nearly 50 km below the surface. The ground motions show substantial variation, highlighting the influence of regional geology and near-surface soil conditions. Aftershock activity was vigorous with roughly 300 felt events in the first six months, including two dozen aftershocks exceedingM 4.5. Broad subsidence of up to 5 cm across the region is consistent with the rupture mechanism. The passage of seismic waves and possibly the coseismic subsidence mobilized ground waters, resulting in temporary increases in stream flow. Although there were many failures of natural slopes and soils, the shaking was insufficient to reactivate many of the failures observed during the 1964 M 9.2 earthquake. This is explained by the much shorter duration of shaking as well as the lower amplitude long-period motions in 2018. The majority of observed soil failures were in anthropogenically placed fill soils. Structural damage is attributed to both the failure of these emplaced soils as well as to the ground motion, which shows some spatial correlation to damage. However, the paucity of instrumental ground-motion recordings outside of downtown Anchorage makes these comparisons challenging. The earthquake demonstrated the challenge of issuing tsunami warnings in complex coastal geographies and highlights the need for a targeted tsunami hazard evaluation of the region. The event also demonstrates the challenge of estimating the probabilistic hazard posed by intraslab earthquakes. Introduction On the morning of 30 November 2018, southcentral Alaska experienced the most societally significant earthquake in the region in half a century. TheMw 7.1 earthquake occurred nearly 50 km beneath Anchorage inside the subducting slab as a result of tensional forces near the transition from flat to steeply dipping slab. Strong to severe shaking was felt by more than half of Alaska’s population. Because the earthquake impacted so many sectors of society, it is arguably the best earthquake learning experience in Alaska since the Mw 9.2 Great Alaska earthquake in 1964. The purpose of this article is to provide an introduction to the observations and impacts across disciplines. Anchorage and southcentral Alaska experience frequent shaking from earthquakes occurring on the Alaska–Aleutian subduction zone interface. But earthquakes inside the subducting slab and in the overlying crust add to the hazard. Magnitude 4 and 5 earthquakes are felt routinely, albeit lightly, by themajority of Alaskans. Even large earthquakes occur with some regularity. More than 80% of the M 6+ earthquakes in the United States occur in Alaska and surrounding waters. Averaged over decades, M 7+ earthquakes occur somewhere along the arc every other year, though the past few years have exceeded this rate. 1. Geophysical Institute, University of Alaska Fairbanks, Fairbanks, Alaska, U.S.A.; 2. Alaska Science Center, U.S. Geological Survey, Anchorage, Alaska, U.S.A.; 3. National Tsunami Warning Center, Palmer, Alaska, U.S.A.; 4. Civil Engineering Department, University of Alaska Anchorage, Anchorage, Alaska, U.S.A.; 5. Department of Geosciences, University of Alaska Fairbanks, Fairbanks, Alaska, U.S.A.; 6. Golder Associates Inc., Anchorage, Alaska, U.S.A. *Corresponding author: mewest@alaska.edu © Seismological Society of America 66 Seismological Research Letters www.srl-online.org • Volume 91 • Number 1 • January 2020 Downloaded from https://pubs.geoscienceworld.org/ssa/srl/article-pdf/91/1/66/4910771/srl-2019176.1.pdf by University of Alaska Fairbanks user on 23 January 2020 From this perspective, theMw 7.1 earthquake was not exceptional and did not surprise anyone familiar with the region’s tectonics. What made this earthquake unusual was its proximity to human population. The population of Alaska is small compared with other parts of the United States and is clustered in a handful of locations separated by hundreds of kilometers. Hence, the vast majority of earthquakes occur at considerable distance from human population. The earthquake ground motions felt routinely are nearly always low-frequency rumbles from distant earthquakes or the abrupt modest tremors of small local earthquakes. Even larger recent earthquakes, such as the 2002Mw 7.9 Denali fault earthquake (Eberhart-Phillips et al., 2003), the 2013 Mw 7.5 Craig earthquake (Yue et al., 2013), the 2014Mw 7.9 Little Sitkin earthquake (Macpherson and Ruppert, 2015), and the January 2018Mw 7.9OffshoreKodiak earthquake (Ruppert et al., 2018), had only minor impacts. In each case, the impact of these earthquakes was tempered by distance from human population and the absence of significant tsunamis. The most notable exception for Anchorage is the 1964 Mw 9.2 Great Alaska earthquake. The 700 km long rupture generated violent ground motions across southern Alaska and generated a tsunami with fatal impacts from Alaska to California. Although the majority of damage and fatalities resulted from the tsunami (Lander, 1996), which did not extend to Anchorage, the damage from strong ground motion was extensive (Hansen, 1965). The shaking caused widespread damage to buildings and infrastructure that had been constructed with limited regard for seismic resilience. The most significant failures were not caused by the amplitude of the shaking. Instead, the ground motions, which lasted many minutes and were rich in longperiod energy, triggered widespread ground failures (Hansen, 1965). The repeated strain cycling of wet unconsolidated soils (i.e., loosely arranged, lacking strong bonds) and clays caused slumping, landslides, liquefaction, and in some more notorious cases the complete sloughing of steep bluffs. The reconstruction in the decade following the 1964 earthquake coincided with a boom in Alaska development tied to the discovery of oil. The rapid growth and optimism about the future—buoyed by statehood in 1958—led the state, and Anchorage in particular, to adopt a proactive stance toward safe development. With the memories of 1964 still raw, Anchorage adopted notably progressive building codes for the time. Even the state legislature wrote explicit seismic requirements into many of its laws. Anchorage today is a product of this history. Themunicipality of Anchorage is home to 300,000 of the state’s 740,000 residents. The greater Anchorage region, including theMatanuska–Susitna (Mat-Su) valley, adds another 100,000. Anchorage infrastructure, especially in its outlying areas, is generally young. The population of Anchorage has grown more than threefold since 1964. Although a portion of the infrastructure predates the worldwide introduction of seismic construction details in the early 1980s, much of the development boom occurred after the introduction of these standards. Oversight and code enforcement vary by location but are good in downtownAnchorage. Taken together, these various factors have led to a city that has made an honest effort at seismic resilience. The 30 November earthquake represents the first critical test of these efforts. There has been no magnitude 6 or larger earthquakes within 100 km of Anchorage in the past half a century. In 2012, anM 5.8 earthquake 30 km north–northwest of Anchorage generated ground motions of ∼5%g in the downtown area, and an M 6.4 in 1983 caused damage to a school that had previously been flagged for poor construction. In 2016, theMw 7.1 Iniskin earthquake produced ground motions in the 10%–15%g range (Grapenthin et al., 2018). Isolated cases of damage were recorded, primarily from secondary influences such as ruptured natural gas lines. However, the earthquake occurred 250 km away from Anchorage, and the damage was light enough so that no systematic effort was undertaken to compile and assess damages. The Iniskin earthquake highlighted the urban hazard potential of earthquakes generated inside the subducting Pacific plate and the possible effects of nearby sedimentary basins. Although the Anchorage earthquake was far from a worstcase scenario, its impact was profound. It tested the region’s earthquake preparedness and shortcomings more than any earthquake in recent history. This makes it a rare learning experience. Many of these topics will be examined in detail by later studies. By providing a broad multidisciplinary overview, the authors hope to catalyze subsequent research that is both insightful and societally relevant. Tectonic Setting and Earthquake Source This earthquake occurred at the northeastern end of the Alaska–Aleutian subduction zone, where the subducting Pacific plate is moving to the north-northwest at about 5:1 cm=yr (Fig. 1). The tectonics of the region include the collision and subduction of the Yakutat terrane, which has characteristics of an oceanic plateau (Christeson et al., 2010). The subduction zone has a very shallow dip, which is attributed to the high buoyancy and thickness of the subducted Yakutat slab compared with a typical oceanic slab (Ferris et al., 2003; Eberhart-Phillips et al., 2006; Abers, 2008; Haeussler, 2008). Seismicity follows the slab to a depth of about 200 km, below which the slab appears to des