Earthquakes notes

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Earthquakes notes

Notes on earthquakes

Earthquakes are short-lived episodes of ground shaking produced when blocks of Earth suddenly shift. They typically last for a few seconds (small earthquakes) to several minutes (largest earthquakes) and produce several types of seismic waves that propagate through the Earth.

Most earthquakes are caused indirectly by plate tectonics. To understand the origin of earthquakes you need to know the basics of plate tectonics.

Plate Tectonics: Plate tectonics is the theory that the surface layer of the Earth (the lithosphere) is broken into blocks that move horizontally with respect to one another over a soft, pliable region of Earth (the asthenosphere). The lithospheric blocks are called plates. Plates move at 1-10 cm/year (about how fast your fingernails grow) and have moved vast distances in the Earth’s 4.6 billion year history.

Plates can and usually do contain both continental and oceanic material. There are approximately 7 major and 12 minor plates. The boundaries between plates often do not coincide with the boundaries between continents and oceans.

Lithosphere and asthenosphere are both rock, but asthenosphere is so hot it can flow like gooey tar. The lithosphere is relatively cool, rigid, and strong.

Lithosphere consists of a thick slab of mantle overlain by a cap of crust. There are two different kinds of lithosphere: continental and oceanic. Continental lithosphere is thicker and less dense than oceanic lithosphere.

There are three types of boundaries between plates: (1) convergent [plates coming together]; (2) divergent [plates separating] and (3) transform [plates sliding past one-another].

The movement of plates is driven by the sinking of dense oceanic lithosphere at subduction zones. Subduction zones are a type of convergent plate boundary where one side, always an oceanic plate, slides beneath the other side (either oceanic or continental) and into the mantle.

The consumption of lithosphere at subduction zones is balanced by the production of new lithosphere at mid-ocean ridges (MORs), which are divergent plate boundaries that separate oceanic plates. At MORs asthenosphere rises and melts, and creates new oceanic lithosphere.

Almost all earthquakes occur along plate boundaries because plate boundaries are the loci of horizontal forces that push and stretch rocks, causing them to break and produce earthquakes. Earthquakes are produced at all three types of plate boundaries. Locations far from plate boundaries, like Florida, experience few earthquakes.

Rock Deformation: Earthquakes occur when rocks break and the blocks on opposite sides shift. Earthquakes do not occur everywhere in the Earth because rocks do not break everywhere in the Earth.

Breaking is a one form of deformation, but rocks may also deform by changing their shape (like putty) or by flowing (like molasses). These behaviors are called brittle (breaking), ductile (changing shape), and fluid (flowing). Deformation is caused by stress, which is force divided by area: F/A. In the Earth, most stresses arise from the movement of tectonic plates.

A brittle material deforms elastically under small stresses, which means its shape returns to normal when the stress is removed. If the stress exceeds some limit, however, the brittle material breaks. This is called brittle failure. Strong materials fail at higher stresses than weak materials. Rock at the surface of the Earth is an example of a brittle material.

A ductile material deforms permanently under small stresses; the material does not “rebound” when the stress is removed. Play dough is an example of a ductile material.

A fluid material deforms permanently under ANY tiny stress; it flows. The rate of flow depends on the viscosity of the material, or its “stiffness”. Examples of fluids—with increasing viscosity--include water, oil, tar, peanut butter, tar, and asthenospheric rock.

Rocks can be brittle, ductile, or fluid. Earthquakes occur when rocks break, and thus only occur when rocks are brittle.

There are four factors that determine whether a rock is brittle, ductile, or fluid: (1) temperature; (2) pressure; (3) rock type; and (4) strain rate. The most important effect is temperature.

Temperature: Rocks are brittle at low temperatures (i.e., surface of Earth) and become ductile and eventually fluid as temperature is increased. The rock in the asthenosphere is fluid because it is very hot (It is not, however melted).

Pressure: Rocks are brittle at low pressure and become ductile as pressure is increased.

Rock Type: Minerals are natural solid compounds that have: (a) a specified chemical formula; and (b) an orderly arrangement of their atoms. Quartz, diamond, and calcite are examples of minerals. Minerals are aggregates of one or more minerals stuck together. Granite, limestone, and marble are examples of rocks.

Some rocks remain brittle over a wider range of temperatures than other rocks. The metamorphic rock quartzite and igneous rock granite are examples of rocks that remain brittle over a wide temperature range. Other rocks, such as the sedimentary rocks shale and rock salt, become ductile at low temperatures.

Strain Rate: Strain rate is the rate of deformation. Rocks are elastic (and thus brittle) at high strain rates, and may be ductile or fluid at very low strain rates. The effect of strain rate can be demonstrated with Silly Putty, which will bounce (elastic) and break at high strain rates (i.e., brittle) show permanent deformation at low strain rates (i.e., ductile), and even flow if enough time is given.

Both pressure and temperature increases toward the center of the Earth. The change in temperature with depth is called the geothermal gradient and is on the order of 20ºC/km near the surface. Both of these effects tend to make rocks less brittle with depth.

Most rocks are brittle above 15-20 km depth and ductile below this depth. This boundary is called the brittle-ductile transition (BDT), and corresponds to a temperature of 300-450ºC.

The vast majority of earthquakes occur above the BDT, because below this depth rocks bend, not break, and thus cannot produce earthquakes.

Deep Earthquakes: Almost all deep earthquakes (greater than BDT depth) occur at subduction zones in an inclined zone called a Wadati-Benioff zone. The Wadati-Benioff zone extends to 670 km depth, where the deepest earthquakes occur.

Deep earthquakes occur in the Wadati-Benioff zone for two reasons: (1) rocks remain cool, below the BDT temperature; and (2) sudden mineral changes induce zones of weakness that allow rocks to suddenly shift, producing earthquakes. The first explanation is responsible for most earthquakes that affect the surface.

The down-going plate in a subduction zone can remain cool because it descends faster than it heats up (heat flow is very slow). This permits temperatures below 400ºC to occur nearly 200 km below the surface, which is below the BDT temperature for many rocks. Below 200 km the rock is too hot to behave in a brittle fashion.

The largest earthquakes occur at subduction zones, and 90% of the total energy released by earthquakes occurs at subduction zones. This is because rocks are strongest in compression and can accumulate the most energy before breaking and shifting. Rocks are weakest in extension, and thus earthquakes at divergent boundaries tend to be small.

Faults: The surface along which rocks slide when they break is called a fault or fault plane. The movement starts at a location called the hypocenter (also called focus) and propagates outward at the speed of sound to form a rupture surface. The epicenter is the location on the Earth’s surface directly above the hypocenter.

Faults accommodate movement of rocks. The movement is typically episodic; the episodes of movement produce earthquakes. Earthquake geologists are interested in faults because faults = earthquakes.

Faults can be recognized by: (a) offsets of horizontal layers; (b) scarps [abrupt changes in slope]; (c) offsets of natural horizontal features like ridges and streams; (d) offsets of man-made features like roads, fences, etc.; (e) juxtaposed rocks on the surface; and (e) earthquakes.

Erosion tends to flatten fault scarps over time, and erase evidence of slow-moving faults.

Blind faults do not break the surface, and can often only be identified by the earthquakes they produce.

Most faults occur in semi-parallel clusters near plate boundaries. There may be one main fault and many secondary or minor faults. Large faults are commonly subdivided into segments, portions that behave similarly and produce earthquakes in a similar way.

Fault planes range from vertical to nearly horizontal, and often get flatter with depth.

Faults are classified as either strike-slip (movement entirely horizontal) or dip-slip (motion in part vertical).

The fault plane for strike-slip faults is usually vertical. These faults can be subdivided into left-lateral and right-lateral strike-slip faults, depending on the direction of motion of the block across the fault from the observer.

Dip-slip faults are further subdivided into normal faults and reverse faults. The direction of movement of a dip-slip fault is given relative to the hanging and foot walls of the fault.

The hanging wall slides down the “ramp” of a normal fault. These faults are caused by extension and may be thought of as gravity faults. Parallel series of normal faults produce alternating uplifted blocks (“horsts”) and downdropped blocks (“grabens”), and are common in area of the Earth that are being stretched, such as the US southwest.

The hanging wall is shoved up the ramp of a reverse fault. These faults are caused by compression. Large, regional, low-angle reverse faults are called thrust faults. The fault plane on thrust faults may even be horizontal, and there may be many kilometers of displacement.

Waves: Earthquakes produce various types of seismic waves as the rocks vibrate and shake.

Waves are characterized by a wavelength and amplitude. Wavelength is also inversely related to frequency, which is defined as number of cycles per second (Hz), and period which is defined as the time between successive crests or troughs to pass.

Seismic energy from an earthquake radiates out in all directions away from the hypocenter and propagates through the Earth. The path of the energy can be traced with ray fronts, likes ripples on a pond, or ray paths, which are perpendicular to the ray.

Earthquake waves are recorded by a seismograph or seismometer, which produces a recording of the Earth’s vibrations called a seismogram. Seismographs work by dampening the motion of a recorder (a pen) with a weight, so that the ground moves relative to the pen during shaking.

Earthquakes produce both body waves, which travel through the Earth, and surface waves which only travel on its surface. Body waves travel faster than surface waves.

There are two kinds of body waves: P waves and S waves. P waves are the fastest waves, and propagate as a compression and dilation of the Earth.

S waves are slower, and propagate as shearing motion perpendicular to the direction of the wave. S waves cannot travel through fluids, like the outer core, because the fluids are not elastic to shear forces.

Surface waves are the slowest waves and include Rayleigh and Love waves.

Rayleigh waves resemble water waves, while Love waves cause the surface to move perpendicular to the motion of the wave.

Large earthquakes can also excite the entire Earth to vibrate. These free body oscillations can cause the surface of the Earth to swell and contract as much as several cm.

All seismic waves increase in velocity toward the center of the Earth as far as the outer core, which is molten. The increase in velocity with depth causes body waves to curve as they enter the Earth, and eventually re-emerge on the surface. The bending of waves as they pass from one medium to another is called refraction.

Because waves travel with velocity P > S > surface they arrive at distant stations at different times. The time lag between the S and P wave arrivals is proportional to the distance from the epicenter. Distances inferred from 3 stations can be used to triangulate the location of the epicenter.

Large earthquakes are sometimes preceded by smaller foreshocks and are always succeeded by aftershocks. Aftershocks are due to the readjustment of the blocks of earth following a large shift, occur on or near the same fault, and decrease in frequency with time after the mainshock.

Magnitude: The magnitude of an earthquake is a measure of its size, and indirectly the amount of energy it releases. Magnitudes are expressed with a logarithmic scale that ranges from near M=0, for the smallest earthquakes that are barely felt, to about M=9 for the largest earthquakes.

The most famous magnitude scale is the Richter scale, ML, though geologists rarely use it anymore. It is based on the maximum amplitude observed on a seismogram, which is then adjusted for: (a) instrument effects; and (b) attenuation, which is the tendency for the amplitude of waves to decrease with distance from the source.

All magnitude scales are logarithmic in the amplitude of shaking, which means that an increase on one magnitude unit corresponds to an increase in amplitude by 10×.

Magnitude scales are also logarithmic in the energy released by an earthquake, except that an increase in one magnitude unit corresponds to an increase in energy of 32×. An increase in two magnitude units corresponds to an increase in energy of 1,000×.

Earthquake waves are a complex mixture of frequencies. Large earthquakes tend to produce energy with a larger component of low-frequency (long-wavelength) waves, and the shaking produced by these waves lasts longer.

Amplitudes of shaking max-out at about M=6.5, a phenomenon called saturation. Saturation occurs partly because the largest earthquakes produce a longer period of shaking, not necessarily more intense shaking.

The most common magnitude scale used today for moderate and large earthquakes it the moment magnitude scale, MW. MW is directly related to the energy released by an earthquake and is computed from all the waves, including the very long-period waves and free body oscillations. MW does not saturate. MW is related to the seismic moment, which is the product of the area of rupture, the rock strength, and the amount of slip.

Many more small earthquakes occur than large. This relationship is quantified in the Gutenberg-Richter law which plots the magnitude of earthquakes vs. the logarithm of the RI or frequency, and allows precise measurements on small earthquakes to be used to predict the RI of rare large earthquakes.

Intensity: The intensity of an earthquake is a measure of the damage it produces, and is quantified in the Modified Mercalli Intensity Scale. Intensity can be correlated with ground accelerations, which can exceed 1.0 g for the strongest earthquakes (causing objects to fly into the air).

Intensity depends on: (a) degree of attenuation; (b) the type of seismic wave; (c) local effects which amplify shaking; and (d) design of buildings, etc.

Attenuation depends on the rigidity of the Earth in the vicinity of the earthquake. Tectonically active areas near plate boundaries, like the western USA, have warmer crust that rapidly attenuates earthquake waves. Tectonically inactive areas like the eastern USA have colder, more rigid crust that allows seismic waves to travel with less attenuation. Thus earthquakes in the eastern USA have a wider area of potential damage than earthquakes in the west.

P waves rarely cause damage. Close to the epicenter S waves are the most damaging, while farther away surface waves cause the most damage.

Amplification of seismic waves occurs in soft sediments due to: (a) wave slowing; (b) focusing; and (c) reflection, which ‘traps’ seismic energy within a basin. Areas of artificial fill are especially vulnerable to amplification.

Liquefaction occurs when water-saturated sediments temporarily lose their strength during earthquake shaking, and structures above them sink.

Liquefaction can be understood with the effective stress equation, à 0 and the entire load to be borne by P.

Resonance occurs when earthquake waves coincide with the natural frequency of buildings, causing them to sway out of control. The natural frequency of buildings decreases with their height approximately with the equation: Fnat = 0.1 × Number of stories.

Earthquakes also cause damage by secondary effects, such as fires and landslides.

The 1968 Mexico City earthquake caused over 10,000 deaths and $5,000,000,000 in damage even though Mexico City was 350 km from the epicenter. Intensity was especially severe because: (a) low-frequency surface waves did not attenuate significantly en route to Mexico City (attenuation is inversely proportional to frequency); (b) amplitudes were amplified in the soft sediment in the Mexico City valley; (c) the same water-logged sediments often underwent liquefaction; and (d) many buildings 8-18 stories tall resonated with the 1-2 Hz waves.

There are two opposing viewpoints about earthquake prediction.

Globally, earthquakes occur in clusters where forces are concentrated. This leads to the model that earthquakes are most likely where earthquakes have occurred before.

On the other hand, the seismic gap theory predicts that earthquakes are most likely where they have not occurred recently. The seismic gap theory is based on the premise that stress accumulates continually due to plate motions, and is released episodically along faults as earthquakes. Earthquakes release stress buildup, which must re-accumulate before an earthquake will occur again.

Seismic gaps commonly occupy segments of faults, portions that behave similarly seismically. A long fault like the San Andreas can be divided into several segments.

Some segments do not produce earthquake, but creep. Creep is the slow, almost continuous movement along a fault.

Some earthquakes increase the level of stress on adjacent segments of a fault, increasing the likelihood of earthquakes there. This has been demonstrated for the North Anatolian Fault in Turkey, where seismicity has swept from east to west as stress is transferred.

Most deadly earthquakes have occurred and continue to occur in poor countries, especially China and in the Middle East, where unreinforced masonry structures collapse. Most buildings built to modern codes remain standing despite significant damage.

The first protection against earthquake risk is zoning, which restricts the types and density of structures in vulnerable areas.

Framed structures fail during earthquakes because they flex at their joints, and can be stiffened with the addition of shear walls. Shear walls are reinforced panels designed to resist strain which occurs during shaking and swaying.

A second approach to building earthquake resistant buildings is base isolation, in which structures are set atop flexible mounts which do not transfer ground shaking to the structure.

Japan, Taiwan, Mexico, and Turkey have earthquake early warning systems which use the arrival of the P wave to provide several seconds of warning to trains and other vulnerable facilities prior to the arrival of the damaging S wave. Sophisticated signal processing is needed to estimate the magnitude and distance to epicenter from characteristics of the P-wave alone.

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