Changing impacts of Alaska-Aleutian subduction zone tsunamis in California under future sea-level rise

Abstract

The amplification of coastal hazards such as distant-source tsunamis under future relative sea-level rise (RSLR) is poorly constrained. In southern California, the Alaska-Aleutian subduction zone has been identified as an earthquake source region of particular concern for a worst-case scenario distant-source tsunami. Here, we explore how RSLR over the next century will influence future maximum nearshore tsunami heights (MNTH) at the Ports of Los Angeles and Long Beach. Earthquake and tsunami modeling combined with local probabilistic RSLR projections show the increased potential for more frequent, relatively low magnitude earthquakes to produce distant-source tsunamis that exceed historically observed MNTH. By 2100, under RSLR projections for a high-emissions representative concentration pathway (RCP8.5), the earthquake magnitude required to produce >1 m MNTH falls from ~M_w9.1 (required today) to M_w8.0, a magnitude that is ~6.7 times more frequent along the Alaska-Aleutian subduction zone.

Fig. 1. Alaska-Aleutian subduction zone tectonic setting and distant-source tsunami modeling.

a Plate tectonic setting of Alaska showing the locked and creeping sections of the Alaska-Aleutian subduction zone, earthquake section boundaries, and approximate historical earthquake extents. Red circles show sites with paleoseismic evidence supporting multi-section earthquake ruptures. Ch Chirikof Island, Si Sitkinak Island, St Sitkalidak Island. b Light gray shaded area shows the approximate extent of slip used in the U.S. Geological Survey (USGS) Science Application for Risk Reduction (SAFRR) scenario magnitude 9.1 Semidi section earthquake underlain by a grid of the NOAA unit sources used in this paper. c Map of the Ports of Los Angeles and Long Beach showing the location of the long-term tide gauge (est. 1923) measuring water levels at the ports (TG1) and the synthetic tide gauge (TG2) where maximum nearshore tsunami heights (MNTH) were measured in this study. d Plot showing the probability density function (PDF) of the MNTH from our suite of modeled earthquake magnitudes in the year 2000 with no tidal variability included (blue histogram), the PDF of the tidal variability at TG1 (green histogram), and the combined MNTH and tidal variability PDF (red histogram). The dashed line shows the 1-m amplitude SAFRR scenario tsunami striking at high tide (MHW), resulting in a MNTH of 1.5 m at the ports.

Fig. 2. Local probabilistic sea-level projections for the Ports of Los Angeles and Long Beach from 2000 to 2100.

Projections are calculated using RCP2.6 (blue) and RCP8.5 (orange) projections and for projections combining enhanced Antarctic Ice Sheet (AIS) contributions from with the RCP2.6 (purple) and RCP8.5 (red) projections from (DP16). Solid lines in the shaded areas show the median of each projection and dashed lines bordering the shaded area and vertical lines on the right show the 95% credible intervals.

Fig. 3. Normalized distributions of maximum nearshore tsunami heights (MNTH) at the Ports of Los Angeles and Long Beach.

The MNTH distribution for 2000 reflects the range of possible modeled MNTH, including tidal variability and does not include sea-level rise. MNTH distributions for 2050, 2070, and 2100 are calculated by combining the same range of modeled MNTH with a low-emissions (RCP2.6) sea-level projections with the K14 Antarctic Ice Sheet (AIS) contribution and DP16 AIS contribution. b high-emissions (RCP8.5) sea-level projections with K14 AIS contribution and DP16 AIS contribution.

Fig. 4. Plots showing the earthquake magnitude that produces tsunamis that have a 50% chance of exceeding defined maximum nearshore tsunami heights (MNTH) as a function of time.

We examined three MNTH: a 0.5 m (measured multiple times during the historical period), b 1.0 m (two times larger than any historical event), and c 1.5 m (similar to a previous tsunami scenario conducted at the ports and the highest storm-driven extreme sea level recorded at the ports).