Cascadia Seismic Event Planning for the Maritime Community

Transcription

1 Cascadia Seismic Event Planning for the Maritime Community Jonathan Allan, Oregon Dept. of Geology and Mineral Industries Presentation to Annual Pacific Northwest Waterways Association Meeting (PNWA) October 7th, 2015 Embassy Suites, Portland

2 Presentation Outline Introduce concept of local vs distant tsunami sources; Discuss recent Oregon mapping efforts, results and end products; What are we doing to address maritime safety in response to tsunami; Upcoming focused modeling and mapping efforts in the Columbia R.; and, Conclusion.

3 Cascadia Subduction Zone (CSZ) North American Plate Overrides Juan de Fuca Plate Along Cascadia Subduction Zone at a rate of 1.5 inches/year

4 Using paleoseismology as the foundation for tsunami modeling 10,000 year record of deep sea turbidites Coastal evidence of coseismic subsidence and tsunami inundation Tsunami deposits in Oregon estuaries and coastal lakes

5 A 10,000 year history of ~40 great Cascadia earthquakes in Oregon The last big Cascadia earthquake and local tsunami in the Northwest was January 26, 1700 at ~9:00 PM. (Satake et al., 2003, Journ. Geophys. Res., v 108, no. B11, p )

6 Subduction Zone Fault Slip Displaces the Pacific

7 Tsunami genesis Courtesy Professor Miho Aoki, University of Alaska-Fairbanks Art Department, University of Alaska-Fairbanks

8 Tsunamis are NOT like wind waves Wind waves break on shore in a few seconds Tsunamis are a higher ocean surface that just keeps coming for tens of minutes

9 WHERE DO OREGON TSUNAMIS COME FROM? Subduction Zones (blue lines). Local Oregon Tsunamis Distant Tsunamis LSAC Meeting, Newport 2015

10 Distant vs Local Tsunami Distant: Arrives 4 + hours after the earthquake Lower damage and flooding than local tsunamis National Tsunami Warning System can warn you Local Tsunami: Arrives minutes after the earthquake Much higher waves Much further inland penetration NOAA Tsunami Warning System ineffective Earthquake = Only Warning

11 Tohoku, Japan, March 11, 2011 The tsunami, seen crashing into homes in Natori, Miyagi prefecture. AP AP Disaster Management Building

12 Oregon Tsunami Inundation Hazard Team Acknowledgments: Rob Witter Joseph Zhang Kelin Wang Chris Goldfinger George Priest Laura Stimely

13 Qualitative Explanation of Cascadia Tsunami Scenarios shown on published tsunami inundation maps (TIMs) = 100% of local tsunamis (~5000-yr event) Local Tsunami Evacuation Zone for Oregon = 98% of local tsunamis (~5000-yr event) = 95% of local tsunamis (~2500-yr event) SB 379 (approximately) = 79% of local tsunamis (~500-yr event)

14 Summary: Cascadia Tsunami Scenarios Paleoseismology constrains results Cascadia earthquakes: M w 8.7 to M w 9.2 Coseismic slip range: 9 to 44 m Tsunami runup: 4 to >25 m Max shoreline wave amplitude: 4 to >25 m Inundation distance: 1 to >3 km Tsunami arrival: 15 to 20 minutes Tsunami Inundation Map (TIM)

15 STATEWIDE DISTANT TSUNAMI SCENARIOS AK64: Largest historical tsunami (1964 Mw 9.2 earthquake in Gulf of Alaska) AKmax: Maximum-considered tsunami (Hypothetical Mw 9.2 earthquake in Gulf of Alaska)

16 AKmax = Distant Tsunami Evacuation Zone for Oregon Has maximum directivity to the Oregon coast. (Source 3 illustration from Tsunami Pilot Study Working Group (2006))

17 27 fathoms (164 ft) Shoreline Tsunami Height is Strongly Affected by Bathymetry Designing for Tsunamis

18 Open coast XXL1 mean = 62 ft

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22 Maritime Tsunami Dangers High velocity currents that tear up port facilities and tethered vessels Wind wave amplification by tsunami currents Rapidly reversing velocity Minimum flow depth (grounding of vessels) Debris field from withdrawing tsunamis & currents Tsunami wave steepness (only very close to shore) Vorticity (whirlpools)

23 Crescent City Near Brookings, Oregonian

24 Thresholds Vorticity, Minimum Flow Depth, Velocity, Wind Wave Amplification Velocity threshold >3 knots Large amplification of PNW wind waves in open ocean Damaging to port facilities in estuaries Minimum flow depth for grounding vessels <50 ft for statewide guidance Could be much less for estuaries with only smaller vessels Threshold vorticity >0.1 for very strong shear Could be as low as >0.01 for any shear

25 PORT DAMAGE vs TSUNAMI CURRENT VELOCITY (NTHMP Mapping and Modeling Subcommittee (2014) modified from Lynett and others (2014) 3-KNOT THRESHOLD

26 STATEWIDE MARITIME GUIDANCE Distant Tsunamis Warning: NOAA broadcasts 4 hours to take action Offshore: go to >30 fathoms; Guidance during event: USCG Tied to dock: Check with local officials Explore options in advance (e.g. go upriver? Out to sea?) On Land: Go to evacuation site Wait until local officials say it is safe to return. Local Tsunamis Warning: Ground shaking Ocean roar Water receding or surging 10 minutes to take action Offshore: go to >100 fathoms; Guidance during event: USCG Tied to dock or on land: Go to evacuation site Wait until local officials say it is safe to return. Plan to be out to sea for days with nearby ports out of commission (fuel, food, etc.).

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29 Newport-Toledo Maritime Guidance for Distant Tsunamis Guidance is for distant tsunamis, because there are more options for these smaller tsunamis than for local tsunamis which arrive in tens of minutes after the earthquake and always destroy or heavily damage port assets. Process: Organize Local Tsunami Advisory Committee (LSAC) Present key tsunami hazard data Formulate straw man guidance options LSAC reviews and then meets to decide

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31 Major Damage to Complete Destruction Moderate to Major Damage Minor to Moderate Damage Time history data is from Oregon Department of Geology and Mineral Industries Open-File Report O-13-16, observation Station 81, Grid B.

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33 Threshold Assumed tide = MHHW (mean higher high water) 7.6 ft NAVD88 LSAC Meeting, Newport 2015

34 LSAC Meeting, Newport 2015

35 Summary Cascadia will be devastating for the PNW; Coastal communities will be isolated and likely will have to survive for a number of weeks on few supplies; Ports and harbors will be destroyed, and many estuaries (including critical navigational channels) will likely be filled with debris and impassable; Jetties will likely be destroyed and controlled entrances will be free to migrate again; For a distant tsunami, and time permitting, mariners may choose to evacuate to deep water (30 fathom line); For a local tsunami, there is no time to evacuate from ports; and, Vessels already out on the ocean should evacuate to either the 30 fathom line (distant event) or 100 fathom line for a local tsunami.