C1 February 1 Earthquakes1 Presentation

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Information about C1 February 1 Earthquakes1 Presentation
Education

Published on March 21, 2008

Author: Tibald

Source: authorstream.com

Earthquakes:  Earthquakes Earthquake Vocabulary; Seismicity (geography of Earthquakes); Describing Earthquakes; Focal mechanisms and seismo-tectonics (how earthquakes relate to tectonic processes); Seismic hazard and risk; risk mitigation Slide2:  Earthquakes A first order manifestation of plate tectonics Slide3:  Seismology is one of many complementary methods to study Earth deformation. Seismology gives us a view over rupture time scales (seconds) to recurrence time scales (years to thousands of years), but other geophysical and geological information is available Slide4:  PLATE KINEMATICS, directions and rates of plate motions Can observe directly Primary constraint on lithospheric processes PLATE DYNAMICS, forces causing plate motions Hard to observe directly Observe indirect effects (seismic velocity, gravity, etc) Studied via models Closely tied to mantle dynamics Kinematics is a primary constraint on models Slide5:  EARTHQUAKE LOCATION Least squares fit to travel times Accuracy of location (truth) depends primarily on velocity model Precision (formal uncertainty) depends primarily on network geometry (close stations & eq within network help) Locations can be accurate but imprecise or precise but inaccurate (line up nicely but displaced from fault) Epicenters (surface positions) better determined than depths or hypocenters (3D positions) because seismometers only on surface Locations are obtained automatically by examining the first portions of seismograms from many stations. The information includes S-P times, and other phases, including depth phases like pP, sS and others Slide6:  The modern global array of seismometers is able to map seismicity quite well. There are narrow plate boundaries, broad plate boundary zones & regions of intraplate deformation. These can be mapped in underwater or remote areas equally well. INTRAPLATE NARROW BOUNDARIES DIFFUSE BOUNDARY ZONES Stein & Wysession, 2003 5.1-4 Large earthquakes are infrequent, but involve large energy release and seismic moment (the quantity of interest in assessing slip):  Large earthquakes are infrequent, but involve large energy release and seismic moment (the quantity of interest in assessing slip) Seismo-Tectonics:  Seismo-Tectonics Slide9:  EARTHQUAKES & TECTONICS Locations map plate boundary zones & regions of intraplate deformation even in underwater or remote areas Focal mechanisms show strain field and imply force orientations Slip & seismic history show deformation rate Depths constrain mechanical structure of lithosphere and define subduction zones, regions of down going material PACIFIC NORTH AMERICA San Andreas Fault, Carrizo Plain 36 mm/yr Focal Mechanisms show the lower half of the ‘focal sphere’, indicating auxiliary planes and Tension and Compression axes:  Focal Mechanisms show the lower half of the ‘focal sphere’, indicating auxiliary planes and Tension and Compression axes Slide11:  Focal Mechanisms Reveal transition of boundary type between Pacific and North American Plate PACIFIC wrt NORTH AMERICA pole CONVERGENCE - ALEUTIAN TRENCH 54 mm/yr EXTENSION - GULF OF CALIFORNIA STRIKE SLIP - SAN ANDREAS Stein & Wysession, 2003 5.2-3 Slide12:  1964 ALASKA EARTHQUAKE Ms 8.4 Mw 9.1 Pacific subduction beneath North America ~ 7 m of slip on 500x300 km2 of Aleutian Trench Second or third largest earthquake recorded to date ~ 130 deaths Catalyzed idea that great thrust fault earthquakes result from slip on subduction zone plate interface TRENCH-NORMAL CONVERGENCE - ALEUTIAN TRENCH 54 mm/yr Slide13:  Map Fault Plane with Aftershocks for large events 26 December 2004 Sumatra-Andaman event (red star shows epicenter) and aftershock locations (yellow dots). Seafloor bathymetry is shown as the background, with lighter colors for shallower regions. Slide14:  Materials at distance on opposite sides of the fault move relative to each other, but friction on the fault "locks" it and prevents slip Eventually strain accumulated is more than the rocks on the fault can withstand, and the fault slips in earthquake Earthquake reflects regional deformation ELASTIC REBOUND OR SEISMIC CYCLE MODEL S&W 4.1-3 Slide15:  1906 SAN FRANCISCO EARTHQUAKE (magnitude 7.8) ~ 4 m of slip on 450 km of San Andreas ~2500 deaths, ~28,000 buildings destroyed (most by fire) The extensive documentation of the event compiled as a scientific report by a California State commission was the start of scientific understanding of the relation of earthquakes & surface faults Boore, 1977 S&W 4.1-2 Slide16:  Over time, slip in earthquakes adds up and reflects the plate motion Offset fence showing 3.5 m of left-lateral strike-slip motion along San Andreas fault in 1906 San Francisco earthquake ~ 35 mm/yr motion between Pacific and North American plates along San Andreas shown by offset streams & GPS Expect earthquakes on average every ~ (3.5 m )/ (35 mm/yr) =100 years But probably nearer 200 yrs because not all motion is on the San Andreas Moreover, it’s not periodic SEISMIC CYCLE AND PLATE MOTION Slide17:  1989 LOMA PRIETA, CALIFORNIA EARTHQUAKE MAGNITUDE 7.1 ON THE SAN ANDREAS Davidson et al Slide18:  Caused some of the highest ground accelerations ever recorded. It illustrates that even a moderate magnitude earthquake can do considerable damage in a populated area. Although the loss of life (58 deaths) was small due to earthquake-resistant construction the $20B damage makes it the most costly earthquake to date in the U.S. Los Angeles Basin Thrust earthquakes indicate shortening 1994 Northridge Ms 6.7 AFTTERSHOCKS S&W 4.5-9 Slide19:  CHALLENGES OF STUDYING EARTHQUAKE CYCLE Cycle lasts hundreds of years, so don’t have observations of it in any one place Combine observations from different places in hope of gaining complete view Unclear how good that view is and how well models represent its complexity. Research integrates various techniques: Most faults are identified from earthquakes on them: seismology is primary tool to study the motion during earthquakes and infer long term motion. Faults labeled ‘inactive’ have seen major events (example: San Fernando, 1971). In a distributed shear zone (like southern California), there are many faults. Also Historical records of earthquakes Field studies of location, geometry, and history of faults Geodetic measurements of deformation before, during, and after earthquakes - Laboratory results on rock fracture Slide20:  EARTHQUAKE RECURRENCE IS HIGHLY VARIABLE Reasons are unclear: randomness, stress effects of other earthquakes on nearby faults… M>7 mean 132 yr s 105 yr Sieh et al., 1989 Extend earthquake history with paleoseismology S&W 1.2-15 Permanent GPS Station Networks provide observation of inter-seismic portions of the cycle:  Permanent GPS Station Networks provide observation of inter-seismic portions of the cycle Slide22:  SAR image of Hayward fault (red line), part of San Andreas fault system, in the Berkeley (east San Francisco Bay) area. Color changes from orange to blue show about 2 cm of gradual movement. This movement is called aseismic creep because the fault moved slowly without generating an earthquake GEODETIC DATA GIVE INSIGHT INTO DEFORMATION BEYOND THAT SHOWN SEISMOLOGICALLY Study aseismic processes Study seismic cycle before, after, and in between earthquakes, whereas we can only study the seismic waves once an earthquake occurs Slide23:  ELASTIC REBOUND MODEL OF STRIKE-SLIP FAULT AT A PLATE BOUNDARY Large earthquakes release all strain accumulated on locked fault between earthquakes Coseismic and interseismic motion sum to plate motion Interseismic strain accumulates near fault Stein & Wysession, 2003 4.5-12 Slide24:  FAR FIELD SLIP RATE D ~ 35 mm/yr Z.-K. Shen S&W 4.5-13 Slide25:  PACIFIC-NORTH AMERICA PLATE BOUNDARY ZONE: PLATE MOTION & ELASTIC STRAIN ~ 50 mm/yr plate motion spread over ~ 1000 km ~ 35 mm/yr elastic strain accumulation from locked San Andreas in region ~ 100 km wide Locked strain will be released in earthquakes Since last earthquake in 1857 ~ 5 m slip accumulated Elastic strain Broad PBZ Stein & Sella 2002 Earthquake Hazard and Risk:  Earthquake Hazard and Risk Slide27:  Hazard is the natural occurrence of earthquakes and resulting ground motion and other effects. Risk is the danger the hazard poses to life and property. Hazard is a property of Earth, risk is a consequence of human actions. Areas of high hazard can have low risk because few people live there, and areas of modest hazard can have high risk due to large populations and poor construction. Earthquake risks can be reduced by human actions, whereas hazards cannot – example of contrasting consequences of the same hazard: Bam, Iran earthquake: M 6.5 30,000 deaths San Simeon, Ca earthquake: M6.5 2 deaths NATURAL DISASTERS: HAZARDS AND RISKS Slide28:  In general, the most destructive earthquakes occur where large populations live near plate boundaries. The highest property losses occur in developed nations where more property is at risk, whereas fatalities are highest in developing nations. Estimates are that the 1990 Northern Iran shock killed 40,000 people, and that the 1988 Spitak (Armenia) earthquake killed 25,000. Deaths can largely be attributed to poor quality construction. Even in Japan, where modern construction practices reduce earthquake damage, the 1995 Kobe earthquake caused more than 5,000 deaths and $100 billion damage. On average during the past century earthquakes have caused about 11,500 deaths per year. Unusually high casualties in certain years (1970’s in China ~250,000; 2004 Sumatra –Andaman ~300,000) The earthquake risk in the United States is much less than in many other countries because large earthquakes are relatively rare in most of the U.S. and because of earthquake-resistant construction. ‘Earthquakes don’t kill people - Buildings kill people’ EARTHQUAKE HAZARD & RISK Tsunamis:  Tsunamis Slide30:  Sumatra-Andaman 2004 INTERSEISMIC: India subducts beneath Burma at about 20 mm/yr Fault interface is locked EARTHQUAKE (COSEISMIC): Fault interface slips, overriding plate rebounds, releasing accumulated motion and generating tsunami HOW OFTEN: Fault slipped ~ 10 m --> 10000 mm / 20 mm/yr = 500 yr Longer if some slip is aseismic Faults aren’t exactly periodic, likely because chaotic nature of rupture controls when large earthquakes occur Stein & Wysession, 2003 4.5-14 INDIA BURMA Tsunami generated SUMATRA TRENCH Tsunami Generation - Sea Floor Displacements Vertical and Horizontal (sloping sea floor) create displacement through the entire water column, creating a ‘shallow water wave’ whose wavelengths greatly exceed the water depth:  Tsunami Generation - Sea Floor Displacements Vertical and Horizontal (sloping sea floor) create displacement through the entire water column, creating a ‘shallow water wave’ whose wavelengths greatly exceed the water depth Cascadia and Alaskan subduction zones are the two main regions near US that are candidates for mega-thrust events, with historical documentation of tsunami generation:  Cascadia and Alaskan subduction zones are the two main regions near US that are candidates for mega-thrust events, with historical documentation of tsunami generation Simulation of tsunami from 1700 Cascadia earthquake. Waves are ‘shallow water’ speed = sqrt(g x waterdepth) waterdepth=4000 m  200 m/sec, allowing hours of warning for sufficiently distant sources. This tsunami was known in Japan, but not connected with Cascadia until recently. Other recent tsunami fatalities in US – 1960 Chile (Hilo, Hawaii) and 1964 Alaska (Crescent City, California):  Simulation of tsunami from 1700 Cascadia earthquake. Waves are ‘shallow water’ speed = sqrt(g x waterdepth) waterdepth=4000 m  200 m/sec, allowing hours of warning for sufficiently distant sources. This tsunami was known in Japan, but not connected with Cascadia until recently. Other recent tsunami fatalities in US – 1960 Chile (Hilo, Hawaii) and 1964 Alaska (Crescent City, California) Examples of Earthquake Hazard and Risk:  Examples of Earthquake Hazard and Risk Slide35:  1989 LOMA PRIETA, CALIFORNIA EARTHQUAKE The two-level Nimitz freeway collapsed along a 1.5 km section in Oakland, crushing cars Freeway had been scheduled for retrofit to improve earthquake resistance. Retrofits include wrapping columns to prevent ‘explosion’ of unconfined concrete Slide36:  1989 LOMA PRIETA, CALIFORNIA EARTHQUAKE Houses collapsed in the Marina district of San Francisco, and fires were ignited. The Marina District was filled-in SF Bay. Shaking amplified by low shear strength of landfill Stein & Wysession 2003 2.4-10 (USGS) Slide37:  1971 Ms 6.6 SAN FERNANDO EARTHQUAKE 1.4 m slip on 20x14 km2 fault Thrust faulting from compression across Los Angeles Basin Fault had not been previously recognized as active. 65 deaths, many in 2 Federal VA Hospitals that were not required to conform to CA codes. This event prompted improvements in building code & hazard mapping Seismic Risk Maps Mainly Reflect Historical Seismicity:  Seismic Risk Maps Mainly Reflect Historical Seismicity Slip deficit on the southern SAF ~20 mm/year x years :  Slip deficit on the southern SAF ~20 mm/year x years 1857 M 7.9 ~1690 M 7.7 Major Earthquakes on the San Andreas Fault, 1690-present 1906 M 7.8 220±13 yrs Slide: Courtesy Kim Olsen TERRA SHAKE Numerically Simulated Ruptures Along the Southern San Andreas Fault:  TERRA SHAKE Numerically Simulated Ruptures Along the Southern San Andreas Fault North-West to South-East Rupture results in much lower damage in the heavily populated LA Basin:  North-West to South-East Rupture results in much lower damage in the heavily populated LA Basin 1999 Izmit Turkey Earthquake:  1999 Izmit Turkey Earthquake The August 1999 event in Turkey filled in a gap following a series of previous events along the North Anatolian Fault, similar in style and size to the San Andreas Seismic Gaps Useful For Long-Term Prediction:  Seismic Gaps Useful For Long-Term Prediction Long Term Prediction:  Long Term Prediction Ismet Turkey:  Ismet Turkey Parkfield, California:  Parkfield, California Between “creeping” and locked sections of the SAFZ The Parkfield Prediction:  The Parkfield Prediction A regular cycle of M6 events led Bakun and Lindh (1985) to forecast another M 6 earthquake would rupture the same segment of the San Andreas Fault at Parkfield within 5 years of 1988. This "Long-Term Prediction" was evaluated and endorsed by the National Earthquake Prediction Evaluation Council in 1985, and the State of California was notified by the USGS that there was a high probability of about M 6 earthquake in the Parkfield region in the 1985-1993 interval. The year 1993 came and went without an earthquake, and thus the temporal element of this long-term prediction clearly failed Slide50:  The 2004 Parkfield Earthquake was about 15 years later than expected Slide51:  Many instruments (strain meters, GPS, seismometers) waiting since 1985 for next M=6, which finally came in 2004. This is also the location of SAFOD San Andreas Fault Observatory At Depth (SAFOD):  San Andreas Fault Observatory At Depth (SAFOD) Liquefaction of Soils upper ~20 m of saturated unconsolidated granular soils, when shaken, lose support of grains, but fluids cannot escape – shear modulus goes to zero.:  Liquefaction of Soils upper ~20 m of saturated unconsolidated granular soils, when shaken, lose support of grains, but fluids cannot escape – shear modulus goes to zero. Sand Boils (above), and intact building collapse (left) are evidence of liquifaction Reducing Risk:  Reducing Risk Tsunami Warning Systems (Pacific Ocean System is in place, none in the Indian Ocean) Geologic and seismic mapping of low shear strength materials (larger amplitude shaking) Map granular saturated materials (candidates for liquefaction) Earthquake Insurance (pooled risk) Reinforced masonry, proper shear walls, high quality concrete, confining system for concrete pillars, proper attachment of building ornaments Reducing Risk:  Reducing Risk Tie-downs for building fittings (duct work, pipes, water heaters, heavy furniture…) Resistant infrastructure (in-ground pipes, above ground wires) Structural systems with replaceable connectors Demolition and rebuild of public structures (practiced in California) Retro-fit of buildings (bolted steel connectors, interior steel frames…) Uniform building codes Continued study of seismicity, fault-fault interaction….

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