Part 2 - Engineering Characterization of Earthquakes and Seismic Hazard. Earthquake Environment

Transcription

1 Part 2 - Engineering Characterization of Earthquakes and Seismic Hazard Ultimately what we want is a seismic intensity measure that will allow us to quantify effect of an earthquake on a structure. S a S d S d Tgiven Τ Period Τ Period Probability of exceedence U.C. Berkeley Spring 2003 UC Regents 4-1 Earthquake Environment Sources of Earthquakes Meteor Impact Volcanoes Reservoir Induced Well Injection Tectonic Associated with continental drift at edges of continental plates mid-continent mid-ocean ridges U.C. Berkeley Spring 2003 UC Regents

2 Earthquake Occurrence U.C. Berkeley Spring 2003 UC Regents 4-3 Earthquake Fatalities Total: U.C. Berkeley Spring 2003 UC Regents

3 Earthquakes in the US U.C. Berkeley Spring 2003 UC Regents 4-5 US Earthquakes U.C. Berkeley Spring 2003 UC Regents

4 Large Earthquakes in 1900s U.C. Berkeley Spring 2003 UC Regents 4-7 Largest Earthquakes in US U.C. Berkeley Spring 2003 UC Regents

5 Finding Historic Earthquakes and Faults Many www sites have data bases to locate past earthquakes in a region. See nks.html#maps / Event 1819 event U.C. Berkeley Spring 2003 UC Regents 4-9 Finding Faults United States Geologic Survey (quadrangles) California Division of Mines & Geology (Alquist Priolo Special Study Zones) Geologic reconnaisance South San Francisco. San Andreas Fault Berkeley, Hayward Fault U.C. Berkeley Spring 2003 UC Regents

6 Elastic Rebound Theory Fault A B C D Straight Fence Initial Time U.C. Berkeley Spring 2003 UC Regents 4-11 Elastic Rebound Theory Fault A B C D Straight Fence Initial Time U.C. Berkeley Spring 2003 UC Regents

7 Elastic Rebound Theory Fault A Straight Fence B C D Fault Offset 13 ft Initial Time U.C. Berkeley Spring 2003 UC Regents 4-13 Elastic Rebound Theory Fault d time A B C D Straight Fence d Initial Time time Permanent Displacement U.C. Berkeley Spring 2003 UC Regents

8 Faulting Fault length and offset can be significant Event Length Offset 1906 San Francisco 250 miles 21 feet 1959 Chile 600 miles San Fernando 15 miles 3 feet 1857 Fort Tejon feet Epicenter Fault Epicentral distance Vs. distance to fault trace Distance to fault Epicentral Distance Site U.C. Berkeley Spring 2003 UC Regents 4-15 Surface Faulting Strike-slip Dip slip Basic Types of Faulting Normal Reverse Combinations Mid-Plate events -No apparent fault rupture due to depth or cover by alluvial deposit Subduction zones -Common in Japan, Mexico, Pudget Sound, South America U.C. Berkeley Spring 2003 UC Regents

9 Subduction Zone Faulting U.C. Berkeley Spring 2003 UC Regents 4-17 Major Faults in California U.C. Berkeley Spring 2003 UC Regents

10 Faulting in Northern and Southern CA U.C. Berkeley Spring 2003 UC Regents 4-19 Earthquake Shaking in Bay Area 1906 San Francisco 1989 Loma Prieta U.C. Berkeley Spring 2003 UC Regents

11 Terminology A Epicenter (ground surface) Surface trace O Site O B B D Section EE Hypocenter (start of rupture) OA - Epicentral Distance E OB - Hypocentral Distance OC - Distance to surface trace OD - Distance to rupture A E U.C. Berkeley Spring 2003 UC Regents 4-21 Hazard depends on size, location and frequency of occurrence of earthquakes near building site, and characteristics of ground shaking produced. Ground shaking due to a particular event depends on: Source Mechanism Travel Path Geological/Local Soil Effects Soil-Structure Interaction Seismic Hazard Site Region Building Site Mechanism Travel Path U.C. Berkeley Spring 2003 UC Regents

12 Regional Seismicity Estimated based on: Tectonic Features (Potential estimated from local faults - Length, annual movement, etc.) Historic Events (frequency of occurrence, gaps) Statistical analysis of similar regions Not a good site Map Frequency Site Magnitude Region x% probability in y years U.C. Berkeley Spring 2003 UC Regents 4-23 Generic Seismic Source Regions Basin & Range Tectonic Province Cascadia Subduction Wasatch Quebec New England San Andreas New Madrid Region Charleston, SC U.C. Berkeley Spring 2003 UC Regents

13 Motion Depends on Source Mechanism Source Effects Type of fault: Strike slip or thrust (relative horizontal and vertical movement) Rupture process (continuous and regular vs. multiple and irregular) Directivity effects (Doppler effects) Near-source phenomenon -- fling Higher frequency Lower frequency waves RUPTURE SLOWER THAN SOUND A B Direction of Rupture C Fault Offset NEAR FAULT MOTION DIFFERENT D U.C. Berkeley Spring 2003 UC Regents 4-25 A Rupture B d C d Near Source Effects Fault Offset t Following Rupture D t Acceleration Velocity Displ. t rupture Fault parallel displacement offset often corresponds to velocity and acceleration pulses with duration t rupture U.C. Berkeley Spring 2003 UC Regents

14 Fault Perpendicular Motions Acceleration High Shear Stresses at Tip of Rupture Velocity Displ. t rupture U.C. Berkeley Spring 2003 UC Regents 4-27 Travel Path Effects P S Surface Wave Propagation PWaves S-Waves Surface Waves U.C. Berkeley Spring 2003 UC Regents

15 Reflection Dispersion Attenuation Focusing Travel Path Effects - 2 Site 1 Fault Site 2 Site 3 Ground Acceleration, g Site 4 Travel Paths Intensity reduces with distance Longer predominant period of motion at distance Longer apparent duration at distance a g Time Distance, km Time Time U.C. Berkeley Spring 2003 UC Regents 4-29 Local Soil Conditions Soil acts like dynamic oscillator S a Surface Soil Rock Softer, deeper or weaker soil will have longer predominant frequency content S a Rock Firm T Soft T U.C. Berkeley Spring 2003 UC Regents

16 Travel Paths: WUS vs CEUS MMI VII: Considerable damage to poorly built structures Mountains and Irregular Geology Mountains and complex geology Flat and uniform geology U.C. Berkeley Spring 2003 UC Regents 4-31 US Seismic Hazard Firm Soil U.C. Berkeley Spring 2003 UC Regents

17 Soil-Structure Interaction Combination of: Inertial Effects -Massof structure influences overall response of soil structure system (Stiff and heavy buildings on soft soil) Kinematic Effects -Flexibility of soil influences dynamic response of structure (tall, light buildings on soft soil) Effective damping (yielding of soil, radiation, uplift, etc.) See Section 5.8 FEMA 368 Structure Soil Structure U.C. Berkeley Spring 2003 UC Regents 4-33 Qualitative Comments on Ground Motions Four General Types of Motions: Single shock - small magnitude, close event Moderately long, irregular motion - moderate to large magnitude event recorded on firm ground at moderate distance from fault. Long motion with pronounced predominant period - soft,deep soil Large pulse (often most easily seen in velocity or displacement) - moderate to large event recorded at close distance accel. accel. accel. accel. T s time time time time U.C. Berkeley Spring 2003 UC Regents

18 32%g Some common records Short effective duration 1940 Imperial Valley Earthquake Vertical motion: Less intense Higher frequency U.C. Berkeley Spring 2003 UC Regents Chile Earthquake Rock Site Soil Site 60 sec 120 sec U.C. Berkeley Spring 2003 UC Regents

19 Near-Fault CA Records 80%g Vertical > horizontal U.C. Berkeley Spring 2003 UC Regents 4-37 Comments Motions in different regions can differ substantially. Motions can vary regionally (fault mechanism and travel path) and by soil condition for the same event. Motions recorded at sites in very close proximity can be quite different. Ground motions have three (six) components, not one. Components in different directions will differ (vertical has much higher frequency content, two horizontal components differ). How can we characterize earthquakes for design? Do we design differently for different types of ground motions? U.C. Berkeley Spring 2003 UC Regents