Mark Petersen, Arthur Frankel, Steve Harmsen , and Gavin Hayes

1. Key scientific issues for Pacific NW region in the next version of the U.S. National Seismic Hazard Maps Mark Petersen, Arthur Frankel, Steve Harmsen, and Gavin Hayes U.S. Geological Survey Golden, CO and Seattle, WA

2. Scientific research and information Earthquake sources Wave propagation (linear and nonlinear) Fault interaction, triggering, Episodic Tremor and Slip Earthquake monitoring, Ground-motion studies Crustal deformation studies Paleoseismology, LIDAR, geologic mapping Potential field mapping Seismic reflection, refraction, surface wave, tomography, geotechnical, borehole studies Products National seismic hazard maps Urban seismic hazard maps Site-specific PSHA Scenario ground motion maps Shakemaps Loss estimation (e.g. PAGER) Earthquake forecasts Synthetic seismograms Liquefaction, landslide, and surface rupture hazard maps Applications Seismic provisions in building codes Seismic design Seismic retrofit Emergency preparedness and management Early warning Land-use planning Earthquake insurance

3. M 7.6 Probabilistic Seismic Hazard MethodologyExample a b Earthquake Sources a Ground motion Hazard curve r1 d1 d4 annual probability of exceeding pga peak ground acceleration r2 d2 M7.6 d3 high seismicity zone San Andreas fault 0.25g 0.5g distance peak ground acceleration (pga) r3 Specify recurrence rates of earthquakes for each source that can affect site of Interest Time independent or time dependent Attenuation relations tell you median ground motions that each potential earthquake will produce at site, and variability Hazard curve: describes probability of having ground motions ≥ a certain intensity

4. From National Seismic Hazard Maps to Building Codes USGS develops national seismic hazard maps with input from external community Building Seismic Safety Council develops design procedure to apply to hazard maps, published in 2009 NEHRP Provisions (FEMA) International Code Council adopts design procedures for 2012 IBC and IRC (also adopted in ASCE 7 2010) • Early 2012 - Pacific NW Workshop • Advisory Committee • Special workshops (e.g., turbidite data – 2010 and depth of seisogenic zone - 2011) Merging of UBC, SBC and BOCA codes into IBC Engineers choose design criteria (1997 and 2009)

5. Earthquake Sources Where do earthquakes occur and how often? Earthquake catalogs (instrumental and historical) Geologic mapping, fault slip rates EQ chronologies from paleoseismology potential fields, seismic reflection/refraction Source models for the National Seismic Hazard Maps Photo from Nelson et al. (2003) From McCrory (2004)) Cedar trees killed by subsidence from the 1700 Cascadiasubductionzone earthquake. Crustal velocity measurements (GPS) Core samples from turbidites in abyssal plain Photo from Brian Atwater From Zeng and Shen From C. Goldfinger

6. Earthquake Ground motions Subduction interface ground motions Western Pacific GSN Strong Ground motion BC Hydro developing new equations (forearc and backarc relations)

7. National Seismic Hazard Mapping Project involved in hazard analysis in many coastal areas of the U.S. and world American Samoa Alaska Hawaii Conterminous U.S. Alaska

8. CascadiaSubduction Zone:What are the gaps in knowledge?What are the critical research issues? • Fault geometry and Seismogenic depths • Earthquake sizes and recurrence rates • Ground motions

9. Crustal Sources Olympia fault (no recurrence info) Tacoma fault (event 1100 years ago) Other? South Whidbey Is. fault Seattle fault Tacoma fault Olympia fault

10. Subduction Zone Source Geometry and Seismogenic Depth • Location of the subducting slab (seismic reflection data, low level seismicity e.g., McCrory et al.) • Shallow dip – few large earthquakes • Determine location of coseismic rupture extent (e.g., episodic tremor and slip (ETS) events dissipate 80-100% stress accumulation below 25 km (Chapman and Melborne, 2009), thermal models of 350° isotherm (Fleuck et al., 1997); geodetic models (McCaffrey et al, 2007), Chile earthquake extends down 55 km (Hayes, 2009).

11. Using Slab1.0 to Infer Seismogenic Width Seismogenic width is measured between inferred shallow and deep limits, along the 3D SZ geometry (rather than assuming linear geometries from gCMT dips, which show ~10o bias toward steeper dips, thus causing an underestimation of Sw). Slide from Gavin Hayes

12. Improvements Offered From Regional Data Adding double-difference relocated EQ data set from Fuis et al., 2008. Slide from Gavin Hayes

13. New Research on CascadiaSubduction Source Recurrence Chris Goldfinger (SSA 2010): 19 events define full ruptures indicating a northern Cascadia margin Holocene recurrence rate of ~500 years; 41 events define a Holocene recurrence for the southern Cascadia margin of ~240 years, currently we apply a 500 year recurrence for full ruptures and additional smaller ruptures. We may want to separate the northern and southern and also to compare off-shore and onshore recurrence.

15. Research Priorities determined at Workshop • Looking at more onshore sites for evidence of M8’s • More core locations for turbidites (Hydrate Ridge to Rogue) • Tracking turbidites with chirp data • Alternative correlation possibilities • How much ground motion does it take to trigger turbidites? Can M7’s do it? • More research into segmentation using uplift and GPS data

16. Issues for the 2013 National Seismic Hazard Maps in Pacific NW • Cascadiasubduction zone source model • Recurrence for southern segments? Northern segments? • Clustered behavior? • Geometry (seismicity)? Location of seismogenic layer? • Cascadiasubduction zone ground motion models • New data (Chile, Japan, Indonesia) • Other published relations (Kanno et al., Gregor et al.) • Adjustments for longer distances to current equations • Crustal faults • Yakima fold and thrust belt (published slip rates?) • Olympia (evidence of recurrence?) • Tacoma (relationship to Seattle fault?) • Little Salmon (does this rupture simultaneously with Cascadia) • Geodetic models • Update earthquake catalog

17. Conclusions: Questions we need to answer for updating the hazard maps • What do historic large earthquakes teach us about future earthquakes (sizes, locations)? Do these earthquake rupture in one event or clustered? Can we segment faults? Are segment boundaries persistent? What sizes of earthquakes can we expect on a structure? • What is the ground shaking from interface, inslab, deep, and outer rise earthquakes? • Why do these types of earthquakes occur in some areas and not others? (e.g., absence of deep earthquakes beneath Oregon, outer rise events?) • Can we determine seismogenic sections of subduction zones using ETS events, heat flow, seismicity? • Can Accretionary Wedge Sources rupture in large earthquakes? (M 7.6 - New Hebrides in wedge) • Can we quantify strike-slip faults offshore for use in PSHA (e.g., S. CA – San Clemente)

18. Wish List; Improving Slab1.0 i) More active seismic lines which image shallow SZ geometry. ii) Point measurements of SZ depth - e.g. Receiver Functions. iii) Regional earthquake catalogs; preferably relocated. iv) Independent measurements of SZ coupling - e.g. models derived from GPS data. Slide from Gavin Hayes

19. Using Slab1.0 to Infer Moment-Release Rate Analyzing patterns in moment release can tell is vital information about earthquake cycles. Do background rates vary leading up to, or following mega-thrust earthquakes? What do areas of low moment release mean? High hazard, or low? Do moment release rates correlate with oceanic plate structure, upper plate structure, etc? What causes such along-strike variability? Freymueller, et al. 2008 Amlia Fracture Zone Slide from Gavin Hayes

20. Conclusions • New recurrence rates on Cascadia – how many additional M 8 earthquakes should we include in the model? • Potential time-dependent (renewal and cluster models) branches in logic tree • Geodetic block models and strain-rate models in addition to geologic models. • New ground motion prediction equations on subduction zone and crustal faults

21. Logic tree applied to National Seismic Hazard Maps 2008