Earthquakes are vibrations of the earth caused by the rupture and sudden movement of rocks that have been strained (deformed) beyond their elastic limit. The forces that cause deformation and the build-up of strain energy in the rock are referred to as stresses .
Earthquakes occur along faults . Faults are fractures in the lithosphere where regions of rock move past each other ( displaced ).
Normal faults result from tensional stresses along divergent boundaries .
The hanging wall block moves down relative to the footwall block.
Low Richter magnitudes due to the tendency of rocks to break easily under tensional stress.
Shallow focus (less than 20 km) because the lithosphere is relatively thin along diverging plate boundaries.
Refer to pages 36-37, 41.
Examples - all mid-ocean ridges; Continental Rift Valleys such as the basin and range province of the Western U.S. and the East African Rift Valley
Normal Fault Examples Left: Fault scarp near Hebgen Lake, Montana, after the magnitude 7.1 earthquake of August 18, 1959, shows a displacement of 5.5 to 6.0 m. Right: This section of the normal fault scarp was produced by the earthquake of October 28, 1983, at Borah Peak, Idaho. Left: Dixie Valley-Fairview Peaks, Nevada earthquake December 16, 1954
Normal Fault Example – The Basin and Range of Nevada, Utah, and Adjacent Areas
The Faults of the Basin and Range occupy a spreading zone accompanying the Northwest drag of the Pacific Plate against the North American Plate, which moves slightly south of west.
Refer to page 41.
Lower Right: The Wasatch Front is a high fault scarp east of the Salt Lake basin. There has not been an earthquake of any consequence since the founding of Salt Lake city in 1847. However, the Salt Lake City area should still consider themselves at high risk for major earthquakes.
At subduction boundaries there is a continuum of stress along the subducting plate. Shallow focus earthquakes can be generated near the trench, but focal depths can reach down to 700 km as earthquakes are generated along the subducting plate.
Rocks are strong under compression and can store large amounts of strain energy before they rupture. Therefore, these earthquakes can be very powerful.
1960 Southern Chili = 9.5
1964 Alaska = 9.2
Refer to pages 39-41.
Reverse Fault Example –Subduction Zones Chile 1960
On May 22, 1960 the largest earthquake on record struck the coast of Chile with a Mw of 9.5. The earthquake ruptured along a 1,000 km length of the subduction zone. In Chile, the earthquake and the tsunami that followed took more than 2,000 lives. From Chile the tsunami radiated outward, killing 61 people in Hawaii and 122 in Japan.
Refer to pages 39-41
Case in Point page 55.
Left: Stuck to the subducting plate, the overriding plate gets squeezed. Right: An earthquake along a subduction zone happens when the leading edge of the overriding plate breaks free and springs seaward, raising the sea floor and the water above it. This uplift starts a tsunami.
At collision boundaries two plates of continental lithosphere collide resulting in fold-thrust mountain belts.
Earthquakes occur due to the thrust faulting and range in depth from shallow to about 200 km.
Refer to pages 39-41.
Example: The Himalayas from the collision of India with Asia
Reverse Fault Example – Collision Zones Bam, Iran 2003
Earthquakes in Iran and neighboring regions (e.g., Turkey and Afghanistan) are closely connected to their position within the active Alpine-Himalayan belt near the convergence of the Arabian and Eurasian plate.
Map showing the Alpine-Himalayan Belt. Below: Before and after pictures of the 2,000 yr. old citadel in the city of Bam, the largest mud-brick structure in the world. The Mw 6.5 Bam Earthquake destroyed nearly 80% of the adobe buildings killing more than 26,000 people.
These are earthquakes that occur in the stable portions of continents. Many of them occur as a result of re-activation of ancient faults, although the causes of some intraplate earthquakes are not well understood. Refer to pages 42-44.
Charleston, South Carolina, 1886
Mw = 7.3, Mercalli Intensity of IX
Right: The Charleston Earthquake resulted from movements along a segment of the East Coast Fault system, a series of faults trending Northeast near the boundary between continental crust and Atlantic Oceanic crust. The ancient faults are believed to be associated with the early stages of opening of the Atlantic Ocean 200 mya.
We can use the lag time between the P-waves and S- waves to calculate the distance to an earthquake !
If we do this for a minimum of three different seismic stations , we can precisely locate the epicenter. In the figure, each circle has a radius equal to the distance to the earthquake from three separate seismic stations. All three circles intersect at only one point -- the epicenter!
- M L ; based on the highest amplitude wave measured on a seismogram, corrected for distance from the seismograph to the epicenter
- ranges from 1.0 (smallest) to infinity, but 9.0 is typically the highest possible value for an earthquake.
logarithmic scale: each whole unit on the Richter scale represents a ten-fold increase in wave amplitude (ground shaking) and an ~ thirty fold increase in the energy released.
Refer to page 48-50.
Above: The magnitude of the earthquake can be estimated using an earthquake nomograph, on which a straight line is plotted between the P-S time (distance) and the maximum wave amplitude. This line intersects the central line at the approximate magnitude of the earthquake.
The Local Magnitude Scale developed by Richter was strictly valid only for certain frequency and distance ranges. Therefore new magnitude scales were developed all calibrated to Richter's original method . These include body-wave magnitude, M B , and surface-wave magnitude, M S , each valid for a particular frequency range.
Because of the limitations of all three magnitude scales, a new scale, known as moment magnitude, or M W , was developed. M W is a measure of the seismic moment , or total energy expended during an earthquake. M W depends on the rock strength, area of rock broken, and amount of offset across the fault.
based on people’s reported perceptions of shaking (subjective), and the type and extent of damage produced (objective).
ranges from I (not felt by people) to XII (catastrophic destruction)
Refer to page 47.
An example of the Modified Mercalli scale follows in the next slide.
Modified Mercalli Scale Example: The 1994 Northridge, California Earthquake Right: A map of Modified Mercalli intensities for the 1994 Northridge, California Earthquake. Left: TriNet ShakeMap showing the distribution of shaking during the 1994 Northridge Earthquake.
Ground acceleration is the rate of increase in velocity, or the strength of the shaking. During an earthquake, the ground accelerates from being stationary to a maximum velocity before slowing and reversing its movement.
Acceleration is normally designated as some proportion of the acceleration due to gravity (g). 1.0 g is the acceleration felt by a freely falling body.
The amount of displacement during fault movement and the length of surface rupture are generally proportional to the magnitude of the accompanying earthquake.
Refer to page 50.
Above: Graph of the relationship between maximum displacement and earthquake magnitude on all types of faults. Left: Graph of the relationship between surface rupture length and earthquake magnitude on all types of faults
Distance from the Epicenter Seismic waves attenuate with distance. Refer to page 52.
Ground rupture and surface faulting occur due to horizontal or vertical displacement of faults that break the surface. Damage can result from the ground shifting upward (called uplift ) or downward (called subsidence ).
Surface Faulting and Ground Rupture Land Uplift and Subsidence Above: The photo shows a fault scarp -- a cliff created by movement along a fault. This scarp formed during the 1992 Landers, CA, quake. Left: The 1999 Taiwan earthquake caused ground movement over a fault rupture of 50 miles. The photo shows a fresh scarp cutting across the running track at a local high school.
Liquefaction is a quicksand like condition that occurs in water-saturated soil and rock. The shaking of earthquake waves causes the soil or rock to turn into a weak, fluid-like mass . Structures built on areas that liquefy may fall over or sink. Refer to pages 52-53.
Above: Buildings in Niigata, Japan, fell over when the sediments below them liquefied during the 1964 earthquake The figure below shows how liquefaction can occur. Shaking of water-saturated soils causes the particles to settle, driving the water out from between the particles and forcing it upward, thus liquefying the areas above.
Most damage and collapse of structures occurs due to sideways movement of the ground from earthquake waves. This process is called horizontal ground acceleration , or base shear . Refer to pages 75-79.
Base shear causes the building to deform from a rectangle into a parallelogram.
The most deadly type of failure from base shear is “story-shift”, in which the sideways acceleration causes floors to shift resulting in the collapse of either individual floors or the whole building -- a situation called pancaking . Few or no occupants survive such collapses.
Mitigation of Base Shear Refer to pages 79-81.
Shear walls : diagonal braces or plywood sheeting built into the walls, to keep the building from deforming during base shear.
Bolting to the foundation , so the building does not slide off .
Base isolation : putting the building on large rubber pads, rolling wheels, or slippery Teflon plates. This allows the ground to move under the building, thus isolating the building somewhat from ground motion.
Basement of Veterans Administration Medical Center, Long Beach, CA
The building practices described on the last few slides make a huge difference in quake survival! The bar graph shows us the connection between year of construction and amount of damage to buildings during the 1995 Kobe, Japan, earthquake. More recent buildings were built to stricter codes, and thus fared much better during the quake.
In addition to buildings, highway overpasses, bridges, and multi-decked freeways also suffer major damage from base shear, most commonly due to the failure of the concrete supporting columns. Refer to pages 75-79.
Left and Right: The photo shows the collapse of freeways from the 1994 Northridge, CA, quake. Left and Right: Collapse of expressways from the 1995 Kobe, Japan, quake.
Buildings have natural vibration frequencies in the same range as earthquake waves. Shaking of the building will be amplified when the frequency of the building is close to the frequency of the waves produced by the earthquake. Refer to pages 80-81, 83.
Adjacent buildings of different heights will sway at different frequencies, and therefore can collide during an earthquake.
Aftershocks are earthquakes that follow the largest shock of an earthquake sequence. The main earthquake changes the stress pattern in areas around the epicenter, and the crust must adjust to these changes.
Aftershocks can continue over a period of weeks, months, or years. In general, the larger the main shock, the larger and more numerous the aftershocks, and the longer they will continue.
Fires commonly break out during quakes due to ruptured gas lines or downed electrical lines. Impassible roads and ruptured water mains compound the problem. In some urban quakes, fires have caused more damage than the ground shaking itself . Case in Point page 88.
Photos show an uncontrolled fire in San Francisco after the 1989 Loma Prieta quake.
Some of the Most Catastrophic Earthquakes in Terms of Casualties
The largest earthquakes do not necessarily kill the most people. Most of the high death counts come from countries notable for poor building construction or unsuitable building sites. Refer to pages 82-83.
Ground deformation: Measurements taken in the vicinity of active faults sometimes show that prior to an earthquake the ground is uplifted or tilts due to the strain building on the fault.
Foreshocks: Small earthquakes that precede a large quake by a few seconds to a few weeks. The pattern and intensity of foreshocks usually increase in magnitude and may cluster or migrate down a fault to the place where the main shock will eventually occur.
Water Level in Wells: As rocks become strained in the vicinity of a fault, new fractures may form causing a change in the path of groundwater subsequently causing changes in the water levels in wells.
Emission of Radon Gas - Radon is an inert gas that is produced in the radioactive decay of uranium. Radon is inert and remains in rock until some event forces it out. Deformation resulting from strain may form fracture in the rock which could serve as pathways for the Radon to escape into groundwater resulting in background radioactivity in wells.
The only successful prediction of a major earthquake was the 1975 magnitude 7.3 Haicheng Earthquake in northeastern China. The prediction was based on
bulging of the ground surface near the fault,
changes in groundwater levels,
and strange animal behavior.
Based on these observations, over 1 million people were ordered to evacuate and remain outside in the winter cold. The earthquake collapsed more than 90% of the houses and, were it not for the prediction, would have killed hundreds of thousands of people.
No reliable short-term precursors have been found. Therefore research today focuses on longer-term warnings or forecasts . In this approach, geologists attempt to identify regions where large earthquakes are likely to occur within the next several years or decades. While this does not provide short-term warnings, it is useful for long-range planning for building codes and emergency response services.
A few faults show a sequential migration of earthquakes along the fault with time believed to be the result of progressive earthquake failure . For faults that follow this pattern it is believed that one earthquake triggers the next. Refer to pages 67-68, Case in Point page 87.
Since 1939, there have been seven earthquakes measuring over magnitude 7.0 along the North Anatolian Fault in Turkey, each occurring at a point progressively further west. By analyzing the stresses caused along the fault by each earthquake, they were able to forecast the 1999 Izmut earthquake. It is thought that an earthquake will soon strike further west along the fault, possibly in the heavily populated city of Istanbul.
A seismic gaps is a section of an active fault that has not had a recent earthquake. The theory here is that if a portion of a fault has been “locked” for some time (i.e. has not had an earthquake in a long time), then strain may have built up to especially high levels there, and a large quake may occur in the near future.
Recurrence interval is a statistical estimate of the expected time interval between an event of a given magnitude. It is a statistical probability!
The theory is that faults should behave in the future like they have behaved in the past, producing a characteristic number of quakes of particular sizes over a given time interval . Refer to pages 63-64, 68-70, Case in Point page 88.
Earthquake Regularity For nearly 150 years, the San Andreas Fault moved at regular intervals near Parkfield generating earthquakes of ~ magnitude 6 at intervals averaging about 22 years. This series prompted seismologist to predict that there was approximately a 95% chance that the next M6 earthquake would occur between 1985 and 1993. So the USGS installed a wide variety of expensive instruments around the Parkfield area. There, however was no M6 earthquake in this region until September 28, 2004.
Earthquake Regularity This map, based on the history of earthquakes on particular faults in the San Francisco region, shows the predicted probabilities of one or more magnitude 6.7 or greater quakes occurring on these faults by 2032. Refer to pages 70-73.
Paleoseismology is the study of the long-term geologic history of faults to determine their earthquake history and possible future activity. The methodology assumes that the earthquakes produced by the fault have a characteristic recurrence interval . Refer to page 65-66.
Paleoseismology Older sedimentary layers are offset more because they have experienced a series of fault movements. Radiocarbon dating of organic layers broken along the fault can reveal the maximum dates of fault movements.