Earthquake Hazard

Earthquakes are defined as strong motion of the planet's surface. This lecture will comprise two sections, one on the physical dynamics underlyng earthquake hazard and one on the social dynamics making people vulnerable to them.

Physical Dynamics

Causes:

• Tectonism:

  • This refers to forces operating within the planet, which tend to increase the contrast in surface elevations. That is, they make high places even higher or low places even lower, increasing the ruggedness of the terrain.

  • These relate to the ongoing generation of thermal energy by radioactive decay in the earth's interior and the transmission towards the surface of this energy.

  • Another source of thermal energy is that generated by the accretion of our planet from dust, rocks, and planetesimals adhering to one another gravitationally (and crashing into one another) from its very beginning about 4.6 billion years ago (Gya) throughout the time of heavy bombardment of the early solar system (until roughly 3.7 Gya). Even now, planets still get hit by stray debris left over in the solar system (e.g., the Chelyabinsk meteor of 2013, Tunguska in 1908, Barringer Meteor Crater in Arizona around 50,000 years ago, the Chicxulub impact that did in (most of) the dinosaurs, the Shoemaker-Levy 9 comet that hit Jupiter in 1994, continuing observations of small impacts by the orbiters around Mars)

  • This endogenic energy triggered melting of the accreted planet very early on, eventually leading to internal differentiation into core, mantle, and crust and ongoing loss of heat to the surface.

  • This energy powers the breakage of the earth's lithosphere (crust plus a bit of the uppermost mantle) into plates of various sizes: major plates, minor plates, platelets, and terranes, which then move atop the soft solid æsthenosphere (upper mantle, with materials capable of flow).

  • Here is a general map of the major and minor plates:
    

    
    

  • The relative motion of adjacent plates creates tectonic boundaries between them, which can be classified as:

    • Divergent (moving apart) or spreading zones or constructive zones (new lithosphere is created here as mantle rocks melt as they rise closer to the surface due to depressurization and that magma is then appended to the edges of the diverging plates).

      • The dominant stress (or force applied to the rock) is, thus, tensional

      • Earthquakes and volcanic activity are common in constructive boundaries, but they're usually not particularly violent

      • Examples include the Mid-Atlantic Ridge and the East African Rift Zone/Red Sea/Dead Sea

    • Convergent (colliding) or destructive zones (lithosphere is destroyed here, that is, subducted or carried down into the æsthenosphere. These are also known as subduction zones.

      • The dominant stress here is compressional

      • Destructive boundaries are the most, well, destructive plate boundaries, with frequent earthquakes, some of them extremely violent, with a mixture of shallow-focus and very deep focus earthquakes (the deep ones coming from the subducted plate)

        • The 1960 Chilean earthquake, with a moment magnitude (Mw) of 9.5 !

        • The 1964 Alaska earthquake, Mw of 9.2 !

        • The 2004 Sumatra earthquake (and tsunami), Mw of 9.1 !

        • The 2011 Japan earthquake, Mw of 9.0

        • The Pacific Northwest (Cascadia Subduction Zone) and the southern coast of Mexico are tectonic boundaries of this type, with the potential for very large earthquakes and tsunami

    • Lateral shear zones, where relative motion is in opposite directions but parallel (generating shear stress), like along our own San Andreas.

      • These zones are called conservative zones because, averaged out over their length, lithosphere is neither created nor destroyed here: It's conserved.

      • Earthquakes here are also quite frequent and there are many major quakes, though generally not quite as crazy-violent as the monsters in destructive zones:

        • The biggest one in California was the Fort Tejón earthquake in 1857, estimated Mw 7.9

        • The 1906 earthquake in San Francisco is estimated at Mw of 7.8

        • The Northridge earthquake in 1994 had a Mw of 6.7

  • There is still a lot of debate over the specific kinds of mechanisms involved and how much of the planet's interior they involve

    • Earlier, it was argued that the motions at the surface reflected convection plumes of mantle material coming up from the mantle-core boundary (but the æsthenosphere in which this convection is supposed to happen is really too thin, averaging 80-200 km thick, for conventional convection cells (like trying to imagine a convection system in a thin puddle).

    • A more recent conception depicts upward moving plumes of hotter material, which push up under the lithosphere, creating a bulge there and, eventually, rock failure and rifting, which allows some of this material to come up to the surface and be added to the edges of the constructive boundary there. This bulge allows diverging plates to slide down a gentle, very long slope away from the rift above the plume, where it eventually encounters other plate boundaries, leading to subduction and the formation of a destructive plate boundary there.

  • These internal motions in the planet and the surface motions of the lithosphere create such tremendous stresses that they produce enormous strain on the lithosphere. Strain is deformation of rock materials in response to stress. Strain can be expressed as:

    • Folding, especially of sedimentary rock beds:

      • Elastic (or reversible strain)

      • Plastic (or irreversible bending that leaves warps in the rock beds even should the stress be removed):

        • Anticlines (rock beds warp upward, convex to the surface)

        • Synclines (rock beds warm downward, concave to the surface)

        • Here's an image from Rutgers University showing both an anticline and a syncline:

          

    • Faulting, which is rock failure, cracking and breaking: The formation of a new fault or failure and motion along an existing one causes an earthquake

      • Fault characteristics provide a framework for classifying relative motion

      • Fault plane: essentially the slice in the earth along which motion takes place

      • Fault strike: the trace of the fault plane's intersection with the surface

      • Fault dip: usually the fault plane dips into the earth at some angle to the surface, rather than being perfectly vertical.

      • Here's a diagram from the webpage of Dr. Malcolm Reeves at the University of Sasketchewan:

        

      • A dip slip features motion along the fault plane that is mostly up and down.

        • The faces of a dipping fault:

          • Foot wall -- the side of a fault lying below the other face, supporting it, like a foot supports your body

          • Hanging wall -- the side of a fault hanging above the footwall

        • Relative motion of the footwall and the hanging wall defines different dip slip fault types:

          • Normal: Extensional -- the hanging wall slides down the face of the footwall

          • Reverse: Compressional -- the hanging wall is pushed up the footwall

          • Thrust: Extreme compressional -- the hanging wall rides well over the footwall at a very oblique angle

            

      • A strike-slip or transcurrent fault features predominantly lateral motion along the fault plane. That is, the motion goes along the fault's trace with the ground surface:

        • Left lateral strikes are those in which, if you were standing on one side of the fault when a quake happens, the landscape on the other side of the fault would shift to your left (you might see offset roads, railroads, streams). In California, the Garlock Fault (southern wall of the Sierra, terminating in the San Andreas near Gorman) is the only major left-lateral fault in California

        • Right lateral faults are those in which the landscape opposite you across the fault will shift to your right -- most strike slips in California are right-lateral, such as the San Andreas, the Hayward, the Sierra Madre, the Newport-Inglewood.

      • Oblique faults are those in which relative motion is a combination of strike slip and dip slip. In actuality, most earthquakes do involve some components of both types of motion.

• More localized causes of quakes:

  • Plate tectonic stresses are not the only source of faulting: There are more localized contributors, too:

    • Volcanism (can generate a strange sort of microseismicity, called harmonic tremors, and swarms of small earthquakes often accompany the movement of magma, or molten mineral matter, and its filling of a magma chamber under a volcano before its eruption)

    • Karst processes (subterranean erosion of limestone country by slightly acidic groundwater can create caverns underground and these sometimes collapse -- think of those sinkholes that occasionally open up in Florida -- creating minor quakes).

    • Subsidence (when the surface sags due to the loss of subterranean mechanical support from a variety of causes, some natural, such as karstic processes underground, and some anthropogenic, including water and petroleum extraction) above)

    • Human activity can generate earthquakes! Here are a few examples:

      • Water and petroleum extraction can cause small quakes as the overlying surface sags and the aquifers or oil traps/reservoirs collapse -- this has often occurred in the southern San Joaquin Valley, for instance, in the context of irrigation and oil extraction.

      • Fracking entails the use of special liquids injected into hydrocarbon-bearing deep rock layers under extreme pressure to induce wide networks of microcracks and propping them open with sand or other particulates in the fracking fluid. This enables the movement of natural gas or petroleum, where it can be collected from an extraction well. There is microseismicity involved in the production of these cracks, but there is also evidence that the alteration of local stress fields by fracking or by deep-well disposal of fracking fluids has generated earthquakes up to 3.6 Mw. A worrying new finding notes that fracking pressures transmit stresses to nearby faults. These may be placed closer to a critical limit. A distant earthquake sends mixes of seismic waves through the body and surface of the earth, and these may trigger critically loaded faults near a fracking field, producing earthquakes that may exceed 5.0 M_w.

        Ellsworth, William L. 2013. Injection-induced earthquakes. Science 341, 1225942 (12 July): 142-149. doi: 10.1126/science.1225942.

      • Explosions, as in war, nuclear tests, industrial accidents, or in the induced explosive collapse of large buildings: the planes striking the World Trade Centers buildings 1 and 2 were recorded as 0.7 and 0.9 Mw, and their collapses produced earthquakes measured at 2.1 and 2.3 Mw, while the collapse of Building 7 produced an earthquake measured at 0.6 Mw

      • Humans can cause earthquakes through another mechanism, too: dam loading.

        • There was a famous earthquake in on August 1, 1975, up in Butte County (6.1 Mw, 5.7 Ml) that followed the completion of the Oroville Dam in 1968 and its filling by 1970.

        • There was a 6.5 Mw earthquake in 1967 in Maharashtra, India, the Koynanagar Earthquake, associated with the filling of the Koyna Dam, and this one killed at least 180 people

        • The 7.9 Mw Sichuan Earthquake of May 12, 2008, which killed over 69,000 people (with another 18,000 unaccounted for and probably dead), has been linked to the filling of the Zipingpu Dam in 2006 (despite the warnings of the Chinese Earthquake Bureau). The earthquake damaged 2,380 dams in China (including the Zipingpu Dam), so this may cascade into more tragedy later.

Ground motion generated by an earthquake fans out in waves. These have describable attributes:

• Wavelength: the distance from one wave crest (or wave trough) to the next, usually measured in kilometers

• Frequency: how many wave crests or troughs pass a given point in a fixed time interval, generally less than 20 Hz or cycles per second (though they can get up to 250 Hz)

  • Humans, on average, can hear sounds in the 20 Hz to 20,000 Hz range, so we generally don't hear quakes (if they do get into our hearing range, they sound like a rumbling or roaring sound), but some other species can hear them

  • For example, pigeons can hear down as far as 0.05 Hz, but only as high as 6,000 Hz, so they can hear earthquake waves and even the sound of ocean surf from as far away as Kansas! In fact, there's some evidence they use infrasound as part of their navigation systems!

  • Dogs can hear in a range from 40 Hz up to 60,000 Hz and, so, like us, they cannot hear earthquake waves.

• Period: the inverse of frequency, or 1/f, representing the time it takes for one complete wave cycle to pass a given point, for earthquakes usually somewhere between 0.1 to 100 seconds

• Amplitude: is the degree of displacement from neutral, so the height of a wave crest or the depth of a wave trough:

  

• Attenuation with distance: earthquakes are dispersive phenomena. The intensity of ground motion drops off with distance as wave energy is dispersed over an ever enlarging sphere or, at the surface, in an ever-widening (and increasingly distorted) circle

• Velocity:

• generally about 2-8 km/second near the surface and faster deeper down

• faster with increasing density

• faster with increasing incompressibility (bulk modulus or resistance to strain under compressional stress)

• faster with increasing rigidity (shear modulus)

• faster with increasing uniformity of material (reduces reflection/refraction effects internally)

• Acceleration: earthquake waves change velocity as they move into different media, as described above. Acceleration affects other properties, such as frequency and amplitude.

• As seismic waves slow down, their amplitude increases, which intensifies ground shaking.

• Seismic waves slow down in poorly consolidated alluvial (river) and marine deposits, which very commonly cover California valleys and basins to great depth.

Earthquake waves occur in a variety of types, two basic types and then important variations on those two types:

• Body waves are the waves that propagate outward in all directions from the focus of an earthquake in an ever-widening, if irregular sphere, through the body of the earth. These occur in two very distinct subtypes:

  • Primary or comPressional, often referred to simply as P waves.

    • In P wave motion, a given rock particle will move back and forth longitudinally (in the same direction the wave itself is propagating).

    • The motion involves repeated compression and rarefaction, sort of like how a Slinky moves.

    • Here is a very nice animation of P wave motion and its effect on a rock particle: http://www.geo.mtu.edu/UPSeis/images/P- wave_animation.gif

    • P waves travel the fastest of all, attaining speeds of 5-8 km/sec near the surface

  • Secondary or Shear, often called S waves

    • In S wave motion, a given rock particle will move perpendicular to the direction the wave itself is moving, rather like the wave in a kid's skipping rope.

    • This is the classic wave form used in textbooks to illustrate such concepts as crests and troughs, amplitude, period, and frequency

    • Here is an animation of S wave motion and its effects: http://www.geo.mtu.edu/UPSeis/images/S- wave_animation.gif

    • S waves move more slowly than P waves, generally just under 60% of the speed of a P wave.

    • That is why they are called secondary waves: They arrive at a seismic station after the primary wave.

    • As seismic waves accelerate and decelerate as they move through different earth materials, the ratio between P waves and S waves is maintained: They speed up and slow down together.

    • This relationship is the basis of figuring out how far an earthquake's focus is from a seismic station: The wider the interval between the arrival of the P waves and the S waves, the farther the focus of the quake is. I have a lab for you to illustrate this (http://www.csulb.edu/~rodrigue/geog558/labs/epicenter.html).

• Surface waves are the waves that move along the surface of the earth or, sometimes, along the surfaces of discontinuities within the crust below ground. In other words, these waves are more confined in their motion, compared with body waves, and this can produce a concentration of violent shaking at the surface. These waves can produce the greatest amplitudes in the seismograph traces. Here are the two most common ones:

  • Love waves, like S waves, produce motion at right angles to the direction of wave propagation, but, unlike the S wave, this motion can occur in only one perpendicular plane (that of the ground surface) and move outward along the surface (not through the body of the earth or the intersection of the body and the surface).

    • These are the fastest of the surface waves

    • They tend to develop best in layered materials, like sedimentary strata

    • They produce some of the worst shaking, lateral or horizontal shaking

    • Here is an animation: http://www.geo.mtu.edu/UPSeis/images/Love_animation.gif

  • Rayleigh waves feature rotary motion, much like a sea wave, kind of a queasy seasick motion.

    • A rock particle rotates in a circular path, the upper part of the cycle moving toward the earthquake source area, the lower part away from the source

    • They tend to develop best in homogeneous surface materials, such as an igneous or metamorphic block.

    • Here is an animation: http://www.geo.mtu.edu/UPSeis/images/Rayleigh_animation.gif

Origin points for earthquakes

• Hypocenter or focus is the actual point or area on a fault plane where the earth materials failed or ruptured. This is typically underground, usually within about 25 km of the surface but, in some very rare instances in destructive zones, as far down as 700 km.

• Epicenter is that point on the surface directly above the focus

• Here is a diagram of the relationship between epicenter and focus, courtesy of the U.S. Geological Survey:

  

• So, how is the epicenter determined? This is a question of interest to emergency managers, since the worst damage in a quake will be at or fairly near the epicenter

  • Epicenter determination is, fundamentally, a problem of triangulation, based on the different velocities of the different wave types

  • Primary waves travel fastest, usually 5-8 km/second (but they can run anywhere from 1-14 km/sec.), while secondary waves run about 58% of their velocity, usually somewhere between 2-5 km/sec (can be from 1-8 km/sec.). Love waves are next in velocity, usually around 2-6 km/sec.), while Rayleigh waves are the slowest, running somewhere between 1-5 km/sec.

    • P > S > L > R

    • No matter the material, relationships among wave types are mostly constant

      • Vp/Vs = \/3/1 = 1.73 and Vs/VP = 0.58

      • Vr = 0.92Vs and Vs = 1.09Vr

      • Vr = 1.28Vl and Vl = 0.78Vr

  • The difference in arrival times implies distance. Usually, the interval between the easily recognized P wave arrival and the S wave arrival is converted into an estimated distance from the focus.

  • One station only creates a radius with that distance, but you don't know in which direction the focus lies

  • Two stations create intersecting radii = 2 points, but that still doesn't constrain the location of the focus

  • Three or more stations allow triangulation

  • Epicenter is probabilistic, actually: +/- 10-20 km, assuming the focus is shallow (the deeper the focus, the more the straight-line distance to the focus (hypotenuse) will diverge from the distance to the epicenter (think of the focus-epicenter-station travel as like that of the adjacent and opposite sides of a triangle, instead of the shorter hypotenuse)

  • It takes a long time to nail it down as inputs from seismic stations all around the world are integrated (and that can be unfortunate as scientists try to deal with epicenter-happy reporters)

Measurement of earthquake "size"

• There are two basic measures of an earthquake's size: magnitude and intensity, and these are quite different

• Magnitude is a measure of energy release, energy to do an earthquake's "work" (in the physical science sense)

  • Magnitude is a direct function of the area of the fault plane that ruptures -- the larger that failing area, the more area there is to emit waves. Measuring it, however, has been less than a direct art.

  • Charles Richter (1958) came up with a measure that exploited the known characteristics of the then most common seismograph in use, the Wood-Anderson model

    • His system is based on the largest trace on that seismograph, measured in mm, which is then calibrated to show what that trace would have looked like, had the seismograph been exactly 100 km from the epicenter

    • His famous score represents the common logarithm of the horizontal amplitude of the largest trace on that seismograph (interpolated for 100 km).

    • As such, each step in his scale represents 10 times the amplitude of the next smaller number, so an 8 represents a trace 10,000 times that of a 4.

    • That represents an energy release is 31.6 times greater for each unit gain on the Richter scale (31.6 times as much, say, TNT)

    • Here is a short lab that expresses Richter's system through use of an elegant little graph called a "nomogram": (http://www.csulb.edu/~rodrigue/geog558/labs/nomogram.html).

    • The Richter scale has a number of problems:

      • It is specifically calibrated around a seismograph that has been superseded by electronic systems

      • It relies strictly on surface waves alone, whichever produces the biggest displacement of the seismograph pen, rather than allowing input from all the different kinds of waves emitted by an earthquake

      • It capped out somewhere around 8, given that an earthquake bigger than that would top the Wood-Anderson's pen and drum system's ability to record an extremely violent trace (so, news stories about an earthquake that's 9 or 10 on the "Richter" scale are completely out of it).

  • So, today, there are several magnitude representation systems, and the Richter is sometimes used in local situations with quakes that fall in the 1-8 range (though the upper part of the range becomes more and more inaccurate, due to limitations in the trace-recording system). When it's reported, it's called local-magnitude or M_l.

  • The most commonly used and internationally consistent measure of magnitude these days is moment-magnitude or M_w.

  • This represents an earthquake's magnitude as a function of:

    • the surface area along the fault plane that was displaced

    • the average length of movement along the fault plane

    • the average rigidities of materials

    • the numerical system is very similar to Richter, pretty much interchangeably in the middle of the range, but moment-magnitude can accurately represent the really big quakes, those above 8.

  • However it's measured, magnitude and frequency have a distinctive relationship, of the semi log-linear variety

  • Very roughly:

    • log N = a + bM

    • where:

    • N = number of quakes at a given M (magnitude) in a given time and area

    • M = target magnitude

    • a and b constants to be calculated (for the time and area), basically the way you do a simple linear regression (for those of you who have taken a class in statistics)

  • In English, this means, the bigger they are, the rarer they are

  • that said, however, those above 6.1 release about 90% of seismic energy

• Intensity is the second major approach to representing an earthquake's size. Intensity is based on human-reported effects and impacts.

  • the Mercalli Modified Intensity scale (MMI) is the intensity scale used with earthquakes.

  • It was built by Giuseppe Mercalli on an older intensity scale and then modified by several others, including Wood of seismograph fame and Richter, going through a number of name changes for each edition. It is now simply called the Modified Mercalli Intensity Scale.

  • It is based on the ensemble of observed effects, which are categorized into twelve classes, each represented by a Roman numeral (to avoid confusion with the Arabic numeral scale used for magnitude). The MMI numerals are roughly related to magnitude:

    • 8-8.9 ~ XI-XII

    • 7-7.9 ~ IX-X

    • 6-6.9 ~ VII-VIII

    • 5-5.9 ~ VI-VII

    • 4-4.9 ~ IV-V

    • 3-3.9 ~ II-III

    • 1-2.9 ~ I

  • The U.S. Geological Survey has a nice summary of the MMI at https://earthquake.usgs.gov/learn/topics/mercalli.php

  • One nice feature of the MMI is that it can be used to characterize spatial variations in earthquake effects during a single event (magnitude simply represents the energy release in an earthquake and a single number is eventually assigned to that event).

  • These spatial variations in intensity can be mapped, using "isoseisms," or lines on a map enclosing a given intensity, which gets us geographers pretty excited!

  • Try your hand at creating an isoseismal map. Here is the link: http://www.csulb.edu/~rodrigue/geog458558/labs/isoseism.html.

  • The USGS now has a phone-in or online system for collecting people's impressions of an event in a systematic fashion, instantaneously building isoseismal maps on the fly as people call in/fill out online forms. The web site is searchable as "Did you feel it?" and here's the direct link: https://earthquake.usgs.gov/data/dyfi/

Social dynamics

Earthquake impacts are generally transmitted to humans through architecture failures of one kind or another.

• Dams crack and fail under seismic stresses, with lethal consequences downstream (earlier, I mentioned the damage done to the Zipingpu Dam near the epicenter of the 2008 Sichuan Earthquake -- and to more than 2,000 others, creating an enduring hazard in China should another earthquake (or even unusual flooding) happen before they can be repaired).

• Bridges and roads fail, posing great danger to those on them or under them (bridges)

• Lifelines of other types fail, such as water mains (with firefighting consequences), electrical grids, gas lines, and sewer lines, posing all sorts of cascading secondary effects

• Buildings fail, crushing their occupants or trapping them

• Materials in buildings fly around and can create blunt force trauma

There are a number of things that can be done to mitigate these architectural failures and thereby increase the length of time the structures can stand and the probabilities that their occupants or users can be either gotten out of harm's way or survive the temblor in them

• This is the task of engineering geology (which makes seismic geological information available to the engineering and architectural professions), seismic engineering, and planning (which can limit construction by seismic hazard zones and can implement and enforce building codes appropriate for the seismic situation and the local culture).

• The idea is to replace aseismic construction with earthquake resistant design.

Ground shaking is made up of mixtures of waves with different periods and frequencies, as discussed above, which vary from periods of seconds to periods so very short that they set up an audible hum or roar. Similarly, amplitude varies, as does specific direction of motion.

• As the quake ground motion persists, buildings accumulate damage from compression, tension, and shear stresses, which progressively weaken them, sometimes to the point of failure.

• Duration is critical -- great quakes may not necessarily generate higher amplitude motion -- earth materials saturate in terms of the amount of motion they can transmit -- the energy is released instead in duration of shaking -- and a building that can "take" 20 seconds of intense shaking may not be able to withstand 2 minutes of it (a sobering thought: The Alaska earthquake generated intense shaking for 4 minutes).

Both structures and soil and rock formations have a fundamental period, consisting of a wavelength at which they tend to oscillate more strongly (like a bell oscillates at a constant tone when hit by a clapper).

• Very crudely, the fundamental period in seconds is roughly the number of stories divided by 10-ish (from 3-20), so a 20 story building has a fundamental period of ~2 seconds, and a 5 story building has one of about 0.5 seconds

• You get the potential for severe damage when a quake goes on long enough for the building to begin resonating at its fundamental period, which then amplifies the ground motion by the addition of this resonant motion to the ground motion itself as the wave crests and troughs from the quake and from the resonant motion line up.

• Another situation is when there is a conflict in fundamental periods between a building and the rock or soil formation on which it rests or between two adjacent buildings or different parts of a complex structure.

  • You get wave interference, which increases amplitude at one point in the structure and cancels out amplitudes elsewhere in the structure, creating terrific shear and torsion stresses.

  • Tall buildings have longer fundamental periods and so do worse on soft ground that also has a long fundamental period.

  • Low buildings with shorter fundamental periods do worse on firmer ground that also has a short fundamental period.

  • Critical points in buildings are the joints between horizontal and vertical elements of the load-bearing structure or, more generally, between sections with different aspect ratios. This was a factor in the partial collapse of the Olive View Hospital during the Sylmar earthquake of 1971, when three of the stairwells fell away: Aerial view of damaged Olive View Hospital (1971)

Performance of building types:

• Adobe and mortared stone buildings are very common in much of the more arid parts of the developing world due to their cheapness and ease of construction.

  • They have almost zero resistance to horizontal motion and fail as an unsorted heap of rubble, crushing their occupants.

  • Coming up with cheap, easy folk architecture is one of the great and so far unsuccessful missions of development and disaster planning in the world today.

  • There is some work being done on making adobes more earthquake resistant

• Wood-frame buildings can do very well in quakes due to their flexibility, IF they are well tied together and anchored to the foundations and not top-heavy. One-story residential structures can perform quite well, but multi-story light wood structures are more susceptible to lateral forces and do particularly poorly if they feature a soft first story (as in under-building parking). Here is a current review of wood frame structures.

  • In Kobe, Japan, post-WWII housing was wood-frame with flimsy walls and heavy traditional tile rooves and they failed, resulting in over 6,000 deaths.

  • In the Northridge earthquake exactly a year earlier, wood-frame single-family homes, especially those younger than ~1950, performed very well, with only three deaths caused by the collapse of a single-family home.

• Mobile homes can perform well in quakes due to their lightness and flexibility, but only if they are braced to a resilient foundation, not the jackstand ("mobile" homes) system. If unbraced, they come off their supports, severing gas lines, so secondary fire hazard is very high. Coming off their supports will almost certainly cause water heaters to detach from their water and gas lines, causing crush dangers, as well as water and/or fire hazards. As a result of the failure to brace mobile homes to a foundation or even a good bracing system, they do not live up to their potential and wind up much more susceptible to earthquake damage than wood frame single family dwellings

• Steel-frame construction (beam and joist) can sway too much in tall buildings, but at least the flexibility of steel beams allows them to be overstressed and severely deformed without losing all strength and failing completely (extreme plastic deformation). The Northridge earthquake drew attention to a key weakness in such structures, however, which is the welded connections among horizontal beams and vertical columns. These sheared and cracked in the Northridge earthquake, most famously in the Getty Museum then under construction. Effective standards for mitigating this weakness in new construction have been developed and best-practice joint inspection and retrofitting techniques are being developed for pre-Northridge welded steeel moment frame structures. A brief outline of these developments is available from the Structural Engineers Association of California (SEAOC).

• Reïnforced concrete structures can do well, too, if the rebar in columns is very well tied in (otherwise the rebar breaks out of columns and these may then collapse, which played a rôle in the failure of the Oliver View Hospital in the Sylmar Earthquake). SEAOC has a brief overview of these issues. Ironically, there is a harmful interaction between the iron in the steel rebar and concrete, which ultimately damages the concrete and compromises the seismic resistance of aging re*iuml;nforced concrete structures.

  • As the rectangular structure becomes a parallelogram due to the motion of a quake, the fill material will sometimes crack and pop out or just crack in an X pattern (diagonal or cross-cracking).

  • X-shaped bracing can improve support.

• NOTE: All SEAOC links are now 404 or paywalled, unfortunately

Building interiors are critical to survivability, too, not just the structural elements.

• Loose materials become projectiles in quakes, flying from one room into another. This is why standing in a doorway is not necessarily a good idea in a quake unless you're in an otherwise unreïnforced structure. The idea that you should run for a doorway is based on pictures of collapsed adobes in California, where the whole structure fell down except the wood-framed doorways, leading to what's essentially an urban legend in American culture about what to do in a quake.

• There are many non-structural mitigations to improve survivability inside a building, such as strapping water heaters, installing quake valves on gas lines, removing heavy pictures and tchotchkes from the head of the bed, installing baffled picture hooks to keep pictures attached to the wall, and placing the bed along the interior walls, keeping water by the bed in case you are trapped for a while.

The idea in earthquake resistant construction is not necessarily to prevent failure, for that is impossible in the largest quakes, but to allow the building to deform or fail in a manner that protects occupants and puts them in predictable crawl spaces that professional rescuers know to look for (e.g., wedges of open space adjacent to interior bearing walls).

There are tremendous inequities in the allocation of people among structures with different levels of earthquake resistance, inequities that arise both from political-economic forces and from cultural quirks.

• In the US, economic forces tend to allocate poorer people to older buildings and increase their population density in those structures.

  • Anyone who's ever been poor, including college students, knows the drill: You swing rent by going in with roommates.

  • Poor people often have a lot of roommates.

• Older buildings may not have been built to any code whatsoever or, more commonly, to codes now superseded by subsequent failures and learning in the engineering profession

• Even in newer buildings, poorer people are in cheaper buildings, and there is a greater chance that corners were cut in construction and inspection

• Also, poverty correlates with other axes of vulnerability: demographic attributes, notably ethnicity/race, age, and disability, creating the possibility that a major earthquake will be a major class quake or race quake, too

In a quake, we've already seen how media bias can exacerbate the already uneven vulnerability, particularly in the response and reconstruction phases

Recovery will be delayed in poorer and minority communities even without biases in media coverage, worsening vulnerability

• Fewer resources

  • Personal assets

  • Insurance (though sometimes the very well-off are also a bit sketchy on the insurance concept)

• Know-how in dealing with a bureaucracy (though sometimes the litigiousness of the better-off can also lead to delays in their own recovery, as happened in the Northridge earthquake, when people with little equity in condominiums in the very upscale Sherman Oaks area "walked," leaving the other homeowners in the association to pay for the repairs to the whole complex, which set off lawsuits, which delayed repairs)

• Immigration status concerns -- if you're undocumented before the quake, you'll probably fall between the seismic and social cracks afterwards with no recourse.

First-responders, emergency managers, and planners can't do much about the large social, political, and economic forces that produce different and unfair vulnerabilities, but they can be aware of the situation and do a few things to promote equity in response and recovery:

• Assume media are biased and sensation-seeking because of the nature of their business model (though many reporters want to do a good job and are concerned about the pressures on the news function exerted by that business model and changes in the media landscape).

• Do independent canvassing and needs assessment in areas you know are likely to be undercovered.

• Before disaster, learn about the groups likely to be overlooked by the media, identify community leaders, and build up informal ties through public education (perhaps someone in your agency who functions as a public information officer?)

• Before disaster, learn about facilities in areas likely to be underserved, which could satisfy the criteria for establishment of a Disaster Assistance Center/Local Assistance Center/Disaster Recovery Center in the event of a sudden-onset disaster. One of the consequences of media bias is a delay in the establishment of these one-stop facilities in poorer and minority communities. Scoping out possibly suitable locations, being aware of media undercoverage, and establishing ties in the community could soften this particular inequity in response and recovery.

• During an earthquake, feed information to your media contacts about what you're seeing in all communities, including poorer and minority ones, so that they might go to areas you point out to do their job, because they heard it directly from you.

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Last revision: 10/26/23