Earthquake hazards. Earthquakes account for more than 50% of the deadliest natural disasters.

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1 Earthquake hazards Earthquakes account for more than 50% of the deadliest natural disasters. Caused more than 800,000 deaths in the last decade alone. Abbott (2009)

2 Assessing earthquake hazards surface rupture and ground shaking hazards developing earthquake rupture forecasts (ERF) ground shaking prediction - attenuation relations - physics-based simulations fault and hazard maps

3 Types of earthquake hazards surface faulting GROUND SHAKING liquefaction landslides flooding TSUNAMIS 1994 Northridge (M 6.7) earthquake 1994 Chronmo Sohn/Sohn/Photo Researchers, Inc

4 Surface faulting A direct manifestation of fault slip at the Earth s surface 1992 Landers (M 7.3) earthquake, California 1994 Northridge (M 6.7) earthquake Relation between earthquake magnitude & fault slip log(ad) = (0.69Mw) 4.8 AD = average displacement (m); Mw = moment magnitude Wells & Coppersmith (1994)

5 Strike-slip fault surface ruptures 1992 Landers (M7.3), CA, earthquake Surface ruptures are relatively discrete and linear, and involve predominantly lateral motions. Yeats et al., 1997

6 Normal fault surface ruptures Normal fault ruptures tend to form straight, segmented fault traces with significant vertical offsets. Wang and Deng, 1988; Yeats et al., 1997

7 Thrust and reverse fault scarps Ostler fault, New Zealand Thrust and reverse fault ruptures tend to form complex, curved and segmented fault traces. Yeats et al., 1997 Surface rupture of the Chenglupu fault 2008 Wenchuan, China (M 7.9) earthquake Most standard (catalog) earthquake location techniques (Flinn, 1967; Buland, 1976) are based on: Automated P-S arrivals picks 1-D global or regional velocity models least squares minimization of arrival-location misfits 1999 Chi-Chi (M=7.6), Taiwan earthquake Li et al., 2010

8 Response of the built environment to surface faulting 2008 Wenchuan, China (M 7.9) earthquake Xu et al., 2009 Level of destruction along fault trace can be extreme building thrown off foundations and completely toppled. Damage is, however, often very localized. Yeats et al., 1997

9 Response of the built environment to surface faulting 1971 San Fernando, CA (M 6.6) earthquake A series of hospitals and schools located along the fault trace were heavily damaged, leading to 65 deaths. Yeats et al., 1997

10 1972 Alquist-Priolo Act, CA Purpose and application of chapter (a)...the Legislature declares that this chapter is intended to provide policies and criteria to assist cities, counties, and state agencies in the exercise of their responsibility to prohibit the location of developments and structures for human occupancy across the trace of active faults... requires notification to buyers of existing structures What defines an active fault trace? - Generally its evidence of Holocene ( 10,000 yrs) or Quaternary (< 2.5 million years) surface rupture. Yeats et al., 1997

11 Types of earthquake hazards surface faulting GROUND SHAKING liquefaction landslides flooding TSUNAMIS 1994 Northridge (M 6.7) earthquake 1994 Chronmo Sohn/Sohn/Photo Researchers, Inc

12 Characteristics of earthquake ground shaking generally greatest near earthquake source (diminishes as a function of distance due to attenuation) 1994 Northridge (M6.7) Earthquake, CA maximum shaking, and its duration, scale roughly to magnitude - shaking may locally exceed 1g & 1m/s - lasts a few seconds to several minutes 1994 Northridge (M 6.7) earthquake

13 Characteristics of earthquake ground shaking Intense ground shaking is also localized by: basin amplification & focusing resonance (of basins & buildings ) 1994 Northridge (M6.7) Earthquake, CA 1994 Northridge (M 6.7) earthquake

14 Basin amplification & wave focusing Soft sediment Hard rock

15 Basin amplification & wave focusing Earthquake waves are amplified in deep basins with slow velocity (soft) sediments.

16 Resonance Tendency of systems to vibrate (or oscillate) at large amplitude at certain frequencies. Small periodic forces (earthquake waves) can produce very large oscillations. Both basins and buildings can resonate from earthquake waves, producing larger building vibrations that often exceed the failure limit.

17 LA basin earthquake simulation

18 1985 Mexico (M 8.1) earthquake: An example of the effects of basin amplification and resonance Abbott (2009)

19 Amplification of seismic waves in buildings

20 In the absence of viable earthquake prediction, how do we prepare for earthquakes? Stochastic methods (Statistical approach that forecasts future earthquakes based on the distribution of past earthquakes). Global earthquake occurrence Aftershock forecasting The Gutenberg-Richter magnitude-frequency scaling relationship gives the number of earthquakes per year of magnitude M W or greater, N(M W ). Many more small than large earthquakes

21 At regional scales, however, dangerous faults are often not illuminated by high levels of background seismicity

22 2010 Maule, Chile (M 8.8) earthquake 2010 earthquake occurred in a seismic gap, along the northern limit of the 1960 M9.5 earthquake Seismic hazard map based on past earthquake occurrence USGS % probability of exceeding the contoured shaking level

23 Numerous aftershocks fill in seismic gap USGS 2010

24 Thus, our challenge is to identify the inventory of active fault zones in an area and their slip rates in order to assess earthquake potential (magnitudes and repeat times). We do this using many of the techniques and approaches described in this course: geologic and seismologic studies of fault activity, geometries, and styles paleoseismology fault system models informed by geodetic observations of interseismic deformation Raymond fault, Los Angeles, CA slip rate (mm/yr) Meade and Hager (2007)

25 Deterministic hazard assessment (Empirical and/or physics based approaches that attempt to forecast future earthquake occurrence based on knowledge about fault systems and earthquake phenomena. Many earthquake forecasts couple deterministic and stochastic components). What we want to know where? how large? how often? ERF earthquake rupture forecast how will the hazardous ground shaking be distributed? - attenuation relationships - strong ground shaking simulations

26 ERF Maps and Community Fault Model (CFM) define the inventory of earthquake sources in a given region Raymond fault, Los Angeles, CA Plesch et al., (2007)

27 Earthquake magnitude scales closely with rupture area, or fault size Empirical relationships among rupture area, magnitude, & coseismic slip are typically used to estimate earthquake characteristics. Mw = (log A) (A= rupture area) Note: relations are usually specific to a fault type. Perhaps the greatest challenge is making informed assessments of the sizes of fault patches that can rupture in individual earthquakes: fault geometry and segmentation empirical studies of rupture patterns dynamic rupture modeling Shaw & Suppe (1996); Wells & Coppersmith (1992)

28 ERF - Estimating earthquake recurrence Average slip rates determined from geologic and/or geodetic analyses Most ERF s use characteristic rupture concepts, with average repeat time, or recurrence interval (RI), for sceanrio earthquakes given by: RI AD / S AD = average coseismic displacement (mm) (get from Magnitude estimate) S = slip rate (mm/y) Some advanced ERF s use multiple rupture scenarios with assigned weighting, and consider dates of most recent earthquake on individual faults. Shaw & Suppe (1996); Wells & Coppersmith (1992)

29 Forecasts of earthquakes. San Andreas fault system event forecast until 2032 (USGS) Based on knowledge of fault geometry, slip rates, and paleoearthquake histories, one can establish a probabilistic forecast of earthquake occurrence. Probabilities reflect uncertainties about fault parameters and paleoearthquake ages, and variability in paleoearthquake recurrence intervals.

30 How do we translate this knowledge about earthquake occurrence into forecasts of expected ground shaking? Attenuation relationship An empirically derived relationship between ground motions and various properties of an earthquake and earth structure.

31 This approach tries to capture the basic characteristics of earthquake ground shaking generally greatest near earthquake source (diminishes as a function of distance due to attenuation) maximum shaking, and its duration, scale roughly to magnitude - shaking may locally exceed 1g & 1m/s - lasts a few seconds to several minutes 1994 Northridge (M6.7) Earthquake, CA Intense ground shaking is also localized by: basin amplification & focusing resonance (of basins & buildings ) 1994 Northridge (M 6.7) earthquake

32 Using attenuations relationships ERF s and these relationships are the basis for maps of probabilities of hazardous ground shaking. Probability of a site experiencing ground shaking of a specified level over a given time period. Earthquake location & magnitude Sediment velocities Basin depth

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34 SHA considers: ERF - Earthquake Rupture Forecast Intensity Measure Relationship (IMR) - Attenuation Relationship Loss estimates given predicted ground shaking (HAZUS - MH) IMT: Intensity Measure Type (i.e., ground acceleration) IML: Intensity Measure Level (e.g., 1 G) IMR: Intensity Measure Relationship (probability that an IMT will exceed an IML at a site given a specific rupture). Field et al., 2003

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39 What are the limitations of this general approach: Updated infrequently (aside from aftershock forecasts); doesn t consider impacts of recent events. Only provides key intensity measures (e.g., PVA), rather than full ground shaking records (shaking pattern over minutes ). The latter are proving very important in terms of building response. Not physics based generally cannot forecast hazardous phenomena that have not been previously documented and incorporated in empirical relations. - e.g., rupture directivity, rupture path, 3D seismic wave propagation phenomena

40 State-of-the-art: Physics-based earthquake simulations Numerical simulation code Realistic structural models + FD, FE, or Spectral element methods are used to solve the equations of motion (Navier equations) in linear elastic materials. High-performance computing 3D Velocity Models describe elastic waves peed structures (Vp, Vs, density). Grids (FE) or meshes (FE, SEM) are parameterized with these models and used in wave propagation solvers. + = Harvard Odyssey cluster 512 dual quad core harpertowns (2.33GHz), 6TB DRAM, Cisco Infiniband interconnect

41 Building response to ground shaking Krishnan, 2007

42 Resources to learn more about hazards assessment Uniform California Earthquake Rupture Forecast (UCERF) Ned Field, Puente Hills Fault Study

43 Resources to learn more about seismic wave propagation methods