Pacific Northwest Earthquakes

Transcription

1 Last Time- Earthquake Hazards What were the earthquake hazards we talked about last time? Which hazard commonly causes the most deaths? Name two different ways we saw that buildings can fail in an earthquake. Name two ways you can safeguard your home against ground shaking. What does liquefaction do and how does it work? Why might a landslide occur during an earthquake? Modified from a presentation by Carla Wittington Pacific Northwest Earthquakes The Cascadia Subduction Zone The PNW has 3 Sources of Earthquakes The 3 Sources of Earthquakes: 1. Deep (subducting slab) earthquakes (45-60 km in the Juan de Fuca subducting plate) 2. Subduction zone (interplate) earthquakes [aka megathrust] (at the contact between the 2 plates) 3. Shallow crustal earthquakes (faults in the North American plate) Deep (subducting slab)earthquakes Where? In the Juan de Fuca plate below the North American Plate Historic Examples of deep slab earthquakes: Olympia M SeaTac M Nisqually M 6.8

2 Isoseismic maps show intensity 1949 Olympia earthquake 1965 Seatac earthquake 1949 Tacoma Narrows landslide Damage to a masonry building from the 1965 earthquake Damage to road due to liquefaction during the 1965 earthquake (Olympia) Nisqually earthquake, 2001 Epicenter What the shaking was like? why the funny distributions?? Map is based on acceleration data (28% g), Summary: Deep earthquakes in the subducting slab (aka intraplate) Where? at depths of km in the Juan de Fuca subducting plate Why? Phase changes in the subducting slab and tension How strong? M (Moderate strength) How frequent? one every years on average Shaking effects? seconds Area affected? Localized in area above subducting slab (i.e. Puget Sound region) Potential damage? Significant to extensive

3 Subduction zone earthquakes Where? Along the interface between the Juan de Fuca and North American Plates Why? Strain builds on locked portion of fault then it fails and slips violently! Locking and Bulging GPS: ½ in/year What can happen when the subduction zone breaks? Tsunami!!! uplift subsidence Rebound uplift subsidence Tahoma Historic Examples of subduction zone megathrust quakes: Alaska, 1964 M 9.2 Chile, 1960 M 9.5 and 2004 in Sumatra!!!! And deposition by tsunami Coastal evidence of subduction zone megathrust earthquakes subsidence and tsunami deposits Top: forest killed by inundation of seawater after tectonic subsidence during 1964 Alaska megathrust earthquake (Mg. 9.2) Bottom: forest killed by inundation of seawater after tectonic subsidence 300 years ago on the Copalis river SW Washington coast!! ~ Mg. 9+???

4 How frequent? One every 600 years on avr Last one 300 years ago Paleo-seismological evidence (carbon-dating on tree stumps in submerged areas) Historic record of tsunami in Japan (January 26, 1700 quake) Stumps of trees killed on Jan. 26, 1700 AD (at low tide along the west shore of Willapa Bay) How strong can subduction zone quakes be? Historical record of similar earthquakes (i.e. Chile and Alaska), Tsunami wave heights (13 ft. Japan) Evidence of earthquake and vertical displacement along the entire PNW coast from the prehistoric events suggest events of M These are the BIG ones!!! Summary: Subduction Zone megathrust earthquakes Where?: at the interface of the Juan de Fuca and North American plate Why?: locking-bulging, slipping-rebounding mechanism How frequent?: every ~600 years How strong?: 8-9+M (This is the BIG one!) Area potentially affected? the entire PNW Shaking effects? 1-5 minutes Potential damage? extensive to catastrophic Shallow Crustal Earthquakes Where? Faults in the North American Plate There are many local faults in the Puget Sound region (more than this!)

5 An Example: The Seattle Fault LIDAR image showing surface evidence of the Seattle Fault (Bainbridge Island) New ideas about the Seattle Fault Toe Jam fault Bench uplifted ~20 ft ~ 900 AD The Old Model The New Model Depth in miles to the main faults Folds M6.4 Seattle Seattle fault M6.7? M7.2 Slip at depth Shallow slip, multiple sources Tacoma fault

6 BB, Bellingham basin; EB, Everett basin; SB, Seattle basin; TB, Tacoma basin; DMF, Devils Mountain fault; UPF, Utsalady Point fault; SWIF, southern Whidbey Island fault; SF, Seattle fault; TF, Tacoma fault; OF, Olympia fault; V, Vancouver; B, Bellingham; E, Everett; S, Seattle; T, Tacoma; O, Olympia; WI, Whidbey Island; KA, Kingston arch; SU, Seattle uplift. Figure 1 (A) Kinematic model of Cascadia forearc, simplified from Wells and others (Wells and others, 1998) and Wells and Simpson (2001). Northward migration of the Oregon Coast Range squeezes western Washington against North America, producing faults and earthquakes in the Puget Lowland. (B) Generallized map of the Puget Lowland and surrounding regions. Redrawn from Brocher and others (2001). Northward compression causes contraction: rocks move up or down on faults GPS velocities Annual strain from GPS McCaffrey, Qamar, King, Wells, et al, s GJI, 2007 Velocities rotate clockwise (block rotation) decrease away from coast (compression above SZ) Blue are campaign sites, red are continuous sites (PBO) Many institutions: RPI, UW, GSC, USGS, OSU, NGS, PANGA, PBO Summary of New Model of Seattle and Tacoma shallow crustal faults Seattle & Tacoma faults in part are blind thrust faults Tacoma Fault is an important, seismogenic fault!! Seattle and Tacoma Faults are connected and could rupture simultaneously What would happen in a moderate Seattle Fault Earthquake? A magnitude 6.7 earthquake on the Seattle Fault will cause $33 billion in damage and economic loss kill more than 1,600 people injure 24,000 cripple the region's transportation and utilities infrastructure and damage more than 180,000 residential, commercial, and industrial buildings Lines of evidence for an earthquake along the Seattle fault 1100 ka Uplifted wave-cut platforms Massive landslides (earthquake-triggered) Tsunami deposits and submerged areas

7 Restoration Point, Bainbridge Island The date of shells found on the bench is ~ 1100 years! Location of prehistoric landslides in the Puget Sound region Landslides causing drowned forests have been found in Lake Washington and Lake Sammamish A landslide (slump block) was found under water A view of the drowned forest in Lake Sammamish Location of tsunami deposits in submerged areas The radiocarbon date on the trees is ~ 1100 years!

8 West Point, Discovery Park (Seattle): Offset rock layers along the Seattle Fault; exposure in Vasa Park, Bellevue. 1992, Jacoby, Williams, Buckley Cross correlated trees from Lake WA and West Point Tsunami-laid sand was found here The radiocarbon date on tree logs in tsunami-laid sand is ~ 1100 years.! Lake Washington (landslide) West Point (tsunami) Lake WA = solid Comparing tree rings from Lake Washington and West Point subfossil trees Correlating geologic events (with or without calendric ages) Red and maroon dots are trees in landslide deposits or lakes dammed by landslides; Yellow dots also drowned trees Subfossil trees = preserved remnants of pas ecological communities, i.e. time capsules! landslide deposit Day Lake, a landslidedammed lake Cline et al, 1980, Jr. Wild. Mgmt. Spider Lake, SE Olympic Mountains drought

9 USGS geologist Bob Schuster at the Church Mountain debris avalanche deposit, Glacier WA How strong was the last earthquake on the Seattle fault? Based on vertical displacement measurements probably around M 7-8 Summary Shallow Crustal Earthquakes Where? Shallow in the North American plate (includes the Puget Sound region) Why? regional compressional stress applied on pre-existing crustal faults How strong? M 7-8 range How frequent? Uncertain. Damage? localized but extensive Summary 3 Sources of Earthquakes Each with its own characteristics and dangers Crustal Earthquakes used to be thought of as minor, and are increasingly perceived as very dangerous If you live here for long, you will be in a damaging earthquake!