Geotechnical Extreme Events Reconnaissance Report: The Performance of Structures in Densely Urbanized Areas Affected by Surface Fault Rupture During the August 24, 2014 M6 South Napa Earthquake, California, USA. Julien Cohen-Waeber University of California Berkeley jwaeber@berkeley.edu AGU Poster: S33F-4922 Co-Authors: Jonathan Bray, Robert Lanzafame, Nicholas Sitar. Complete Report Online Visit: http://www.geerassociation.org AGU2014 Press Conference, Tuesday December 16, 2014
Surface Fault Rupture in Urban Environments South Napa Earthquake: 1st occurrence of surface fault rupture in N. California since the 1906 San Andreas event. 1st occurrence in a dense urban environment in N. California. What should we expect of the next big event? What does this event tell us? Are we prepared? Left House: only hairline crack at corner of garage. Right House: shifted 6 cm off foundation resulting in significant damage [NSF-GEER; N 38.3038 W 122.3430; 08/25/14 14:02] [NSF-GEER; N 38.3038 W 122.3430; 08/25/14 13:46]
Surface Fault Rupture Hazard Current standard of practice for mitigation is avoidance where possible. Present legislature: California Alquist-Priolo Earthquake Fault Studies Act (1972)* Requires 15 m. set back from active faults for human occupancy structures Unless proven otherwise Surface faulting is affected by: 1. Fault Characteristics 2. Overlying Soil 3. Foundation & structure Realistically: Effects of surface fault rupture can be acceptable or unacceptable [Bray et al. (1994), Earthquake Fault Rupture Propagation Through Soil] (* Alquist-Priolo is presently under review by Technical Advisory Committee)
South Napa Earthquake Surface Fault Rupture Detailed mapping of surface fault rupture through 27 single/two story wood frame structures Comprehensive record of seismic damage (measured and photographed) Qualitative assessment and comparison of structural performance and failure modes 5 km. Napa Surface Fault Rupture Trace N Satellite Images: [Google Earth 9/12/14] [NSF-GEER Brown s Valley Recon. Teams; 8/25/14 8/28/14]
Performance of Structures on Spread Foundations Structures experienced significant through-going surface rupture. Shallow foundations allowed for a relative decoupling from the ground surface. Structures experienced angular distortion from a lack of foundation rigidity. Surface rupture along tilted detached garage [NSF-GEER; N 38.3052 W 122.3369; 08/28/14 ] Cracked strip footing and tilted cripple stud wall E W W E
Rupture between piers under neighboring property Performance of Structures on Pier and Grade Beam Foundations Deep, stiff foundation forced rupture to edge or structural weakness No accommodation for differential displacement (anchored). Structures sheared along rupture trace. Door cracks Detached roof beam [NSF-GEER; N 38.3135 W 122.3431; 08/28/14 ]
South Napa Earthquake Damage Summary Cracked garage slab 1. Significant structural damage was limited to the rupture trace. 2. Light, wood frame structures performed well. 3. New construction and seismic retrofits performed well. 4. Structures on shallow spread footings were relatively decoupled from the ground but experienced angular distortion. [NSF-GEER; N 38.3024 W 122.3436; 08/27/14 11:33] 11 cm off foundation 5. Structures on pier and grade beam foundations were anchored to the ground and sheared with rupture. Damage Summary (Number of Properties Observed) Damage Type Strip Footings (19) [NSF-GEER; N 38.3027 W 122.3436; 08/25/14 10:11] Pier and Grade Beam (8) Foundation Damage 6 6 Cracks in Concrete 9 4 Disp (structure/structure/gro 16 6 und) Disp (structure/foundation) 4 1
KEY OPPORTUNITY TO ADVANCE UNDERSTANDING What should we expect of the next big event? Expect more of the same modes of failure and structural performance to varying degrees. What does this event tell us and are we prepared? While the current system works, and Alquist-Priolo and the state of practice serve their purpose, Ground movement due to surface fault rupture can be analyzed and mitigated as for other ground movement hazards (landslides, subsidence, lateral spreading) Safe design in hazardous fault zones is possible with the proper considerations (Bray, 2009) : 1. Land Use Planning 2. Eng. Geology (Site characterization for non-arbitrary setbacks). 3. Geotechnical Eng. (Ductile earth fills, soil reinforcement, slip layers). 4. Structural Eng. (Strong ductile foundations, avoid piles/piers, flexible structures and isolation joints). See GEER website for GEER-CGS-PEER-USGS Report Ver. 2.0 < http://www.geerassociation.org/ >