GEOL 240 Post-Midterm II

wood-frame buildings

amazingly eq-resistant design. strong shear resistant walls & roof - nail plywood to skeleton adds shear resistance, must move as a block - everything ties together in 3 dimensions - not eq-resistant if built on cripple walls designed to provide a crawl space. held up by 2x4 wood blocks. sheet with plywood to make shear resistant - need to build on concrete pads and drill into and tie wood to it

SF-Oakland bay bridge

steel double decker, reinforced into steel concrete. floor of bay is mud- gold from the Sierra navade was plaster gold, which is deposited in gravels. mine in gravels used hydraulic mining, funneling a stream from the mountains, resulting in mud filled bay. - east half of the bay bridge sat on wooden poles

ding bat apartment buildings

"soft" 1st floor buildings car port on the 1st floor, held up by steel pipes LA mandated these buildings need to add shear resistance (solid walls) so can move as a unit when shaking In the 1994 Mw 6.7 Northridge eq approximately 200 of these were either destroyed or damaged so severely that they were torn down - need new steel frame tied into the wooden frame of building

1987 Whittier Narrows Earthquake

- Mw 6.0 - Puente Hills Blind Thrust Fault - 50 sq. km rupture - average slip of 20 cm

sand volcanoes

- formed by pressurized deposit fluid and sand escaping to surface through low pressured cracks - water in btwn sand grains liquify - seen in the 1979 Mw 6.5 Imperial Valley eq

steel-framed buildings

- high-rises in DTLA - main source of failure is the beam-to-column connection: must stay at 90º when the ground moves. must be the strongest part of building. steel moment resisting frame: columns carry vertical loads- columns tilted more 10º are unable to resist gravity Bending of columns and beams act to keep columns vertical Welded beam/column connections are critical Column-to-beam connections must be stronger than the columns themselves assumed that weld strength is greater than the steel's plastic yield strength -need ductile deformation in the columns to sway

earth-triggered landslides

- mostly triggered by eqs - shaking a landform like a cliff makes it easy to trigger landslife - ex) 2008 Mw 7.9 Wenchuan- landslide created a damn. as the water built up behind it, dam could be overtopped, causing a flood downstream. luckily this was mitigated

LA's liquefaction hazard zones

- only elevation changes in the basin are man-made - flat - under the SW part of LA is the Compton blind thrust fault. if this ruptures, the LA river will flow backwards. - the last big flood, the basin was a shallow, muddy lake--> basin is all weak, unconsolidated sediment - basin covered with river flood plains - pumping groundwater out has mitigated liquefaction hazard

magnitude scales

-assign a single value to an eq -open scale: no minimum or maximum -magnitude depends on the amount of area the fault plane slips. the more the fault plane slips, the greater the magnitude. -magnitude is exponential, so highly unlikely to exceed a Mw 9.8 because rupture will run out of the fault plane area -minimum magnitude can reach a negative, smallest is Mw -4. the slippage begins at a single spot (hypocenter), goes 0 km/hr before slip, quickly accelerates to km/sec slippage. acceleration phase of up to thousands km/sec, generating seismic waves -concept of magnitude comes from astronomy (star brightness, which has a huge scale range and why magnitude is logarithmic)

retrofit brick buildings

-most worrisome area is San Bernardino: located in btwn 2 faults and has barely any retrofitted brick buildings - focus on bolting the interior frame - rod is threaded through brick wall into wooden framework - multiple strategies 1) run an anchor bolt from outside to inside with diagonal braces connecting parapet to the roof 2) vertical steel beams anchored to brick wall to resist collapse 3) bolts go through the brick wall from the exterior & are anchored to the floor beam on the interior

Why are megathrust eqs so big?

-on a strike-slip, the down-dip width is only 15 km -subduction megathrusts are colder, shallower, and lower -geothermal gradient: for every km length, megathrust is 10 or 15x the width of strike slip

how to retrofit steel-framed buildings?

1) build an exoskeleton (steel diagonal moment resisting frames) 2) add flanges & bolts to column-to-beam connections, but need to pull building apart, so worry about brittle welds.

5 main building types

1. Unreinforced masonry (mud, brick, and stone) 2. Non-ductile steel-reinforced concrete 3. Ductile steel-reinforced concrete 4. Steel-framed 5. Wood-framed

1868 Mw 7 Hayward Earthquake

1st great SF eq. concentration of damage on filled land. built many buildings on creek beds, knowing the buildings wouldn't fare well in an eq.

20 vs. 60 story steel frame building

20 story building would fare worse in an earthquake because taller buildings are designed to a higher standard due to wind shear.

LA Basin

30,000 ft deep, filled with sand & gravel, which amplifies the intensity & duration of shaking. deepest point is where the 105 & 710 cross in SE LA - a rupture in the Coachella valley on SAF will travel into the LA basin in huge ground motions, and energy will get trapped in the basin

base isolation

A mechanism to isolate a structure from earthquake shaking in the ground. isolators are large rubber blocks laminated with steel and pure lead columns inside. allows the building to move as one. the lead melts during the shaking, allowing movement without structural damage.

John Hall's design of a 20-story steel MRF (moment-resisting frame) building

Cal Tech professor. chose a medium shaking site while another engineer picked the lowest shaking site. showed that his building would be okay. in his computer simulation, when the ground moved, allowed some of the welds to break, causing a cascade failure showed that good welds start to deform and building tilts and fails. when welds are allowed to break, get a cascade failure at a lower shear.

pre-2005, total energy released on earth...

Chile released 1/4 total energy (since 1900) Alaska Mw 9.4 released 20% but Mw<6 don't release much energy, so it shows the exponentiality of moment magnitude

current building codes

Current building codes are mostly prescriptive rules based on the building type and seismic zone. Codes have been developed by fixing deficiencies from past earthquakes (e.g., 1933 (brick), 1940(non-ductile concrete good), 1971 (non-ductile concrete bad), 1994 problem with welds in steel buildings). If you've got a good building code, who needs a seismologist? Largemagnitudeearthquakes(i.e.infrequent) may be the primary threat to our societies (e.g., Mw>7 Puente Hills Thrust) - codes are meant to prevent building collapse & save lives

Earthquake occurence in the SF Bay Area

Do increases in small-moderate earthquakes presage a bigger event? in the early 1800s, only a few eqs, but later in the 1800s, eqs increased in frequency and magnitude--> culminated in the 1906 Mw 7.8 SF eq. no large eqs until the 1960s and 1989 Mw 6.9 eq. eqs build up & shut down.

source directivity of source effects

Doppler Shift: change in frequency. as high frequency energy comes toward you, it decreases in frequency as it passes. sound source moving at 80% wave velocity- as it approaches, the sound waves going out in the propagating direction, piling waves together. in the other direction, energy spreads out. in an eq, source of seismic energy is the propagating front of the rupture. propagating pulse along fault and pulse follows behind it. -only part of the fault is slipping at any one time -rupture front propagates at ~80% of the shear wave velocity. shear waves at the front pile up, get bigger as they move along--> causing Source Directivity (piling up S wave velocity hitting simultaneously) ex) 1992 Landers Mw 7.3 rupture in the Mojave- a site was right on the fault, felt big directivity. at every time stamp, radiated S-wave energy, but the rupture front went almost as fast as the S-waves. in the opposite direction, energy spreads out & waves hit at different time. all the energy went north in this eq. ex.2) Kobe eq- strike slip, directivity pulse went to the North & South ex.3) 1906 SF- rupture began at 8-10 km depth. SAF offshore of SF, which saved downtown from destruction. needed enough propagation direction, but nucleated close to SF as possible, didn't propagate far enough to have a directivity pulse. as S wave passes, ground moves side to side, even moving several meters.

1995 Kobe Japan Earthquake

Mj (Japanese magnitude)- standard before moment magnitude. estimate Mj is 7.2, but Mw 7.9 every news outlet reported 7.2

Richter scale

Ml- local magnitude. Charles Richter developed the 1st practical scale. he installed a seismometer network in SoCal in the 1920s. -S waves= largest wave phase, used to develop the scale. -S waves are most abundant at high frequencies at 1 second periods. -Wood-Anderson seismometer: measures horizontal ground motion. -Richter borrowed magnitude concept from astronomy at CalTech -for every magnitude unit, amplitude of the largest S wave increased by a measure of 10. -magnitude 0 is the smallest S wave he could accurately record -flaw: small eqs generate high frequency energy, but huge eqs generate low frequency energy. 1 second periods only useful for small eqs. the Richter scale underestimate eqs as they increased.

1999 Izmit, Turkey earthquake

Mo 7.5 American & European news outlets said this was a Ms 7.8, but Turkey still used Md (duration magnitude). Md 6.7 by Turkey people cry government coverup, showing the importance of different scales

1933 Long Beach Earthquake

Most important eq, but only Mw 6.4 - ruptured small part of the Newport-Inglewood system - brick building collapsed - schools originally mandated to be brick because concerned about fires - CA legislature passed the Field Act: mandate schools to be built with eq-resistant design. 1st building code to mandate eq-resistance. - bricks have no shear resistance, so easily broken

1971 San Fernando Earthquake

Mw 6.7

1994 Northridge Earthquake

Mw 6.7 400 sq. km rupture avg displacement of 50 cm

1975 Haicheng Earthquake

Mw 7.3 -abundant foreshocks -China was closed off to the west -had the Chinese involved in eq science by looking at animal behavior before an eq. the West disproved of this -however, this was the only eq predicted in advance by the government- people moved out of houses to sleep outside & evacuated, so had a low death toll -region had dozens of little eqs, but was historically aseismic -led to complacency, so in 1976 a Mw 7.8 strike-slip eq occurred without warning. the main shock was felt first -250k death toll, but the West estimates 750k -showed that the majority of eqs don't have detectable foreshocks

1992 Landers Earthquake

Mw 7.3 surprising little damage to the town south of the rupture. news outlets showed one damaged building. journalists are prone to exaggerating eqs.

1857 Fort Tejon Earthquake

Mw 7.9 350 km long 5,000 sq. km of fault ruptured. -both fort tejon and the 1906 SF eqs broke ~5,000 sq. km of fault

1906 San Francisco Earthquake

Mw 7.9 450 km long 6,750 sq. km rupture

1985 Mexico City Earthquake

Mw 8 liquefaction due to fine grain sand. with extreme shaking the gravel liquefied subduction megathrust energy propagated out rupture was not even in Mexico City city is built on an old filled in lake bed, shallow water table building need to be anchored down to solid layer below the liquefiable layer - Cortez saw a Lake Tenochtitlan and saw an Aztec empire on a fill island. - mexico city was built on a bowl-shaped sedimentary basin of mud & clay from previous marshy land. - the entire lake was filled in over centuries - seismic energy 300 km got trapped in the basin, waves pile up bringing slugs of energy. - dominant frequency was 2-4 sec seismic energy. - for every 10 stories, add a 2nd, in terms of harmonic frequency - Mexico City is rich in 2 sec energy- most damage to 10-20 story buildings. - buildings need to be anchored down to solid layer below the liquefiable layer

1887 Assam Earthquake

Mw 8.9, along the Himalayan subduction megathrust - geologist Oldham put 2 m tall stone blacks as survey markers, the block shot upwards and translated 6.5 ft away, indicating strong vertical motion - as a rupture propagates along a thrust, stress holds it together. as stress reach this, stress almost reaches 0. as the rupture passes, stress builds back up & locks - rupture reaches surface causing strong vertical motion= fault flip - hanging wall tip flips in the air

196o Chile Earthquake

Mw 9.5, biggest of the big during the seismometer era (since 1900) -broke 1,000 km of subduction megathrust -tsunami hit Hilo, HI -down-dip width of 150 km--> 1,000x150=150,000 sq. km of fault broken

SoCal earthquakes April-June 1992

NE of the SAF is the main shock - had little earthquakes as foreshocks -foreshock: April Mw 6.1 -June 1992: Mw 7.3 Landers earthquake -aftershock: Big bear Mw 6.3, within 1 rupture radius. considered an aftershock even though it was on a different fault, "triggered" eq

unreinforced masonry

Ordinary masonry walls are not reinforced, so they have no resistance to lateral movement. - held together by gravity - performs poorly in eqs - Jericho: earliest place where these buildings built. located adjacent to the Dead Sea fault- left-lateral strike slip - city of Troy: 8 different cities built atop rubble of previous cities. when the Greeks besieged Troy, eq knocked down the walls allowing them to take over. - Unreinforced masonry Stanford buildings destroyed during 1906 Mw=7.9 San Francisco earthquake, San Andreas fault

Modified Mercalli Intensity Scale

a closed, 12 point scale developed to evaluate earthquake intensity based on the amount of damage to various structures, proximity to epicenter, and type of ground.

Hatussas

ancient Turkey, near the north Anatolian fault - ancient Hittite capitol - Hittite god of earthquakes: stands on a sword & will protect people by shoving sword into the ground - holes drilled into stones- oldest evidence of eq-resistant shear, filled with bronze rods

Sacramento-San Joaquin Delta

area where the sediment from rivers are dumped--> water saturated fine grain mud. islands lower than river level and protected by mud levies. the islands get lower and lower because the soil erodes. if the delta is hit by an eq, levies will fail. islands are being developed as more people move in. rupture on the SAF from the N to S will ruin the delta

Downdip fault width vs. fault dip angle (and thermal regime)

as temperatures increase into the earth, at 15 km depth, 400 or 450º C, rocks are too hot to break, so they flow. max depth of eqs outside of subduction zones ~15 km

1692 Port Royal, Jamaica earthquake

big left-lateral strike-slip right through jamaica - city had 4 forts, a cathedral, and a well laid out street plan - Port Royal built on a sandpit a couple feet above sea level. had many heavy stone buildings. - fort & church foundations were found 10 ft below sea level. compared to a pre-eq map, these buildings are still in the same place due to no lateral motion, and went straight down - liquefied sand came to the surface in big quantities as forts sunk

non-ductile steel-reinforced concrete

brittle material during shaking - most of the buildings damaged in the 1999 Izmit eq, lost their first floors - fails in 2 ways 1) inadequate beam confinement: rebar that's wrapped around the vertical rebar within columns to prevent columns from exploding outward during vertical loading. if there's not enough horizontal rebar, then the vertical rebar will break. 2) weak column-to-beam connections: if these break, the building will pancake. weak connections have too little steel and no integrated 3-D network. connection must be 90º - in the cypress structure, confinement failed & had weak column-to-beam ratio -Non-ductile (aka "brittle") columns have inadequate strength in the confinement reinforcing bars wrapped around the vertical reinforcing bar inside columns - these buildings are where most deaths occurred in the 1971 mW 6.7 San Fernando eq--> caused building codes to stiffen - 50k people killed in the 1988 Mw 6.9 Spitak, Armenia eq due to heavy concrete slabs & steel reinforced floor beams. floor beams fell as the building flexed. - vast majority of LA's non high rise buildings

2019 July 4-5 earthquakes

broke an entire system of left-lateral faults - Mw 6.4 on July 4 was the main shock for a day, but later was the foreshock to the Mw 7.1 on July 5

frequency content of source effects

buildings have a natural harmony frequency. big buildings resonate at lower frequencies. small buildings resonate at higher frequencies. good buildings flex with shaking and respond to long period low energy frequency. bridges & dams also respond to low frequency energy. during the 2011 Mw 9.1 Tohoku, Japan earthquake, the tall, well engineered buildings responded well to long period waves

SF business district

built engineered seawall that must be replaces "leaning tower of SF"- Millennium tower. building started tilting because wasn't dug deep enough to reach the bedrock. also built past the original shorelines, so is located on some bay fill.

how to keep columns from exploding?

columns used to be a rectangular shape -retrofitted bridge columns are now wrapped by thick steel sheath. leave existing piers in place and clamp shut with steel. - can build 3-D tightly tied steel bar mesh, tie into the columns, then fill in as a solid shear wall - X bracing

Omori's Law

decay of aftershocks with time (best understood aspect of seismology) -exponential decay= 1/time

Surface Wave Magnitude Scale

developed by Richter & Guttenberg after shortcomings of the Richter scale. Ms- measured in 20-40 second periods, for low frequency surface waves -de facto measurement for big subduction zone eqs -flaw: even 40 second periods, Ms still couldn't measure Ms 8+ eqs, which can be released in 100 sec periods at very low frequencies

rupture propagation direction of source effects

during the 1906 SF eq, Santa Rosa was hit the hardest. the directivity pulse didn't develop until the front went north. - rupture of the SAF from N to S will ruin the Sacramento-San Joaquin delta region - direction of rupture controls the intensity of ground motion

foreshock

earthquake of smaller magnitude occurring before the main shock, which is the largest earthquake in a sequence. -only way a foreshock is legitimate is if sometime soon (within minutes, months, or years), another eq occurs.

aftershock

earthquakes with smaller magnitudes than the main shock in the same place -Aftershock sequences for large earthquakes can last decades before rate of aftershocks finally decays back down to pre-mainshock levels -largest aftershock is about 1 magnitude unit smaller than the main. -10 aftershocks 2 magnitudes smaller than the main. -100 aftershocks 3 units smaller than the main

source effects

effects directly related to seismic rupture. 1) location/proximity 2) size/moment-magnitude 3) duration 4) frequency content 5) source directivity

1964 Mw 9.2 Alaska earthquake

example of duration source effect 600 km long rupture on a subduction megathrust. created a tsunami- part of the land was uplifted by 11 meters, which also convinced people of plate tectonics. - at 3 km/sec, the rupture front took 200 sec (3.5 miles) to travel to the opposite end. - large megathrusts have longer shaking duration - if a building is weakly constructed, longer duration will increase damage.

differences in size/magnitude matter

example of magnitude source effect - 1906 SF Mw 7.8 eq had more intense shaking in a more localized area. while the Mw 7.1 on the Hayward fault had a larger felt area but smaller magnitude/shaking

2011 Mw 6.3 Christchurch, NZ

example of proximity source effect -technically an aftershock of the 2010 Mw 7.1 Darfield eq, which occurred underneath farm fields. - directly underneath northern part of the city- enormous damage. 2 large buildings collapsed. lost 25% of the population. - Darfield released 16x more energy, but barely any damage. - Christchurch was damaging because of the urban proximity. - Large areas of the city experienced spectacularly voluminous liquefaction. - created a red zone that is too vulnerable to damage

SF marina district

filled in lagoon with rubble hauled from the 1906 earthquake.

El Centro Record

first on-scale recording of near-source strong ground motions in a large earthquake - 1940 Mw 7 Imperial Valley eq - original seismograms had pendulum swinging aggressively, but need seismogram to record strong ground motion using stiff springs- 1st seismogram where full wave motions close to the eq recorded on scale. can actually quantify shaking, previously was qualitative - seismogram located next to the imperial fault in El Centro - measured velocity in cm/sec- ground velocity is a more important parameter to buildings than acceleration - max velocity is 35 cm/sec on Imperial Valley eq - recorded 0.3 g (horizontal acceleration, 1/3 force of gravity) - engineering community in disbelief because the seismometer malfunctioned because it was assumed the ground can't move that fast - max v in 1992 Landers was 150 cm/sec, 1994 Northridge almost hit 2 m/sec - this eq was the building design standard, but failed to realize there are larger eqs - offset orange trees

site effects

how geology underneath the site affects the strength of ground motions. - bedrock shakes the least - sediment from old cliffs & mountains is the 2nd strongest - 1989 World Series, Oakland A's vs. SF Giants: Mw 6.9 Loma Prieta eq. downtown Santa Cruz heavily damaged. when the S waves hit, people got thrown off their feet. brick buildings collapses. people standing on bedrock didn't experience intense shaking. DT Santa Cruz was built on an old river flood plain - strong ground motions in the Silicon Valley due to the sediment. - shaking in basin can be 11x stronger than bedrock where the epicenter is. - sediment also increases shaking duration, 4x longer relative to bedrock. - in Loma Prieta, SF marina district heavily damaged due to landfill made of dredged up bay mud. the original shoreline was more inland but the city sold lots underwater so coast could move outwards. - Emoryville during Loma Prieta: only 6 buildings collapsed. - part of Cypress highway structure collapsed because it was built on bay fill - seismograms on bedrock show that S waves' amplitudes are small, but 15x higher in soft mud. - sand & gravel S waves are intermediate - slower the seismic wave speed, weaker the sediment

1994 Mw 6.7 Northridge eq

implications for steel-framed buildings eq was a blind thrust & had a strong up dip directivity pulse to the N & W. - must cover steel frame after welding to prevent rusting - after this eq, found cracks in welds, so were wrong to assume welds are stronger than steel - eq broke through 4 in. thick steel plates

how to retrofit non ductile steel reinforced concrete?

need 4x as much confinement, focusing on the column-to-beam connection

ductile vs non ductile

non ductile is brittle, wrapped at 90º and easily breakable ductile adds denser steel, heavier confinement, and wrapped endpoints

2020 Mw 7 Izmir-Samos earthquake

normal fault off the coast of mainland turkey, aegean sea region is riddled with normal faults, pulling apart occurred on the northern boundary of Samos island -Islands here are uplifted footwalls of a normal fault -long vectors in the Aegean- pulling out faster to the west -northern end of African plate is the last part of region's subduction zone -oceanic lithosphere is sinking into the aegean, pulling turkey

path effects

pathway from vary from rupture to site ex) 2011 Mw 5.8 in Virginia- felt in a huge area due to the cold, old lithosphere that allows waves to propagate more. -huge felt intensity area from the Mw 7.4 New Madrid eq due to the water saturated sediment amplifying the shaking. - 2004 Mw 6.6 in CA has more intense shaking but much smaller felt intensity area. - San Gabriel mountains & depressions filled with sand & gravel at its front act as a propagating force - as Mw 7.8 eq comes through the San Bernardino area into LA, energy splits off, loses directivity pulse, energy runs along the sedimentary basin - path controlled by sedimentary basins at the San Gabriel mountain's front

natural resonant frequency

relation btwn how tall a building is and the frequency of sway. - high frequency, low amplitude waves have a <1 sec period - mid-size buildings have a resonant frequency of 1 sec - tall buildings have >2 sec period, damaged in large amplitude, low frequency quake

Fault Fling

seen in the 1971 Mw 6.7 San Fernando eq - fault dipping North, back down under the hills - road uplifted by 1 m, causing a fault scarp - newly paved road was cracked at the base of scarp, but still continuous - researchers dug trenches along the surface rupture- found that the ground went up by 1.5 m then collapsed - 2 ruptured planes found due to grass underneath - road is not offset - towards the back of the road, road overlapped itself by 1.5 m - motion where the road flips up in the air while the ground slips, come down on top of scarp

in order to have source directivity...

the direction the fault slips must be parallel to the propagating rupture fault ex) vertical strike-slip SAF slips horizontally & parallel to the fault. propagation direction heading north. but not all strike-slips will generate a directivity pulse- must be big & long enough ex) in a thrust fault, will have a directivity pulse if right above/parallel to the hypocenter. the 1999 Chi Chi Mw 7.6 eq in Taiwan had a 90 km N to S rupture, 20 km deep, but no directivity pulse. PHT generates a directivity pulse. 1971 Mw 6.7 San Fernando eq occurred on the N- dipping Sierra Madre fault whereas the 1994 Mw 6.7 Northridge eq occurred on a S-dipping blind thrust fault to the west. directivity pulse hit Santa Clarita valley

Moment Magnitude Scale (Mw)

w= work based on seismic moment. frequency content across a whole spectrum of frequency an eq releases. Mw 9 eqs still release high frequency, but in smaller time periods. seismic moment= Mo= μ x A xD A= area, D= displacement, μ= rigidity -Instead of increasing by a factor of 10, increases by a factor of 32. -Mw 8 releases 32x the energy of a Mw 7 -Mw 8 releases 1,000x energy of a Mw 6 -Mw 7.4 releases 4x the energy of a Mw 7 -Mw 7.8 releases 16x the energy of a Mw 7

ridge-top shattering (ridge rents)

when a seismic wave enters a Ridgecrest, gets trapped inside the crest, causing intense ground motions along the ridge crusts. ridges are made of bedrock

Liquefaction

when the ground shakes, water-logged sediment acts like a fluid. the shaking increases water pressure in pore spaces btwn sand grains - fine grain sand is the most liquefiable - port facilities and areas along river, ocean, and lake are vulnerable