7.Can we predict a "big earthquake"?
Can seismologists predict earthquakes? The answer depends on the time scale of the prediction. Based on our current knowledge of seismic zone distribution and seismic frequency, we can make long-term predictions (on time scales from decades to hundreds of years). For example, we can say with certainty that an earthquake will shake Istanbul in the next century, but not in northern Canada. However, seismologists are unable to make accurate short-term predictions (by hour to year time scale). For example, we cannot say that there will be an earthquake in San Francisco in 40 days. However, new technology allows seismologists to warn people seconds to minutes before the earthquake waves hit. In this section, we will explore the scientific basis of long-term prediction and early-seismic early warning systems.
7.1 Long-term prediction
Long-term prediction estimates the probability or possibility of an earthquake occurring within a specific time. For example, a seismologist might say, “The probability of a major earthquake next time. Only 20% of the state in 50 years.” This sentence suggests a one-fifth chance of an earthquake happening in 50 years. Seismologists call research that leads to long-term predictions called earthquake risk assessments. Urban planners use such an assessment to design building codes for a region: requiring stronger building codes makes sense for areas with greater earthquake risk, as buildings are more likely to be quake during their useful life. The basic premise of earthquake risk assessment is that earthquakes may occur in areas where multiple earthquakes have occurred in the past. Therefore, earthquake zones (regions with frequent earthquakes) are areas with high earthquake risks. This does not mean that catastrophic earthquakes don't happen far away from the seismic zone (they can and do), but the probability of earthquakes occurring in these places will be smaller within any given time window.
To provide a more specific indicator of the possibility of earthquakes, seismologists attempt to specify a time interval for repeated occurrences for earthquakes of a given size in a region, i.e. the average time between consecutive events. Since the concept of recurrence intervals can be confusing, seismologists sometimes specify annual probability, i.e. the possibility of an earthquake in a certain year:
Annual probability = 1/recurrence interval (annual probability=1/recurrence interval)
For example, if the recurrence interval of Mw7 earthquake in a certain area is 100 years, then the annual probability of such an earthquake is 1%. Note that as pressure accumulates over time on the fault, elastic rebound theory suggests that the probability of earthquakes occurring each year may gradually increase over time.
In order to determine the recurrence of a major earthquake in a certain earthquake zone, seismologists must determine the time of previous earthquakes in that earthquake zone. For those where historical records are not long enough, researchers look for evidence of large earthquakes preserved in geological records. For example, where sedimentary layers accumulate on faults in basins, researchers may dig a trench to search for records of buried volcanic sand or fault stratigraphy in the formation. Each such strata records the time of the earthquake, and its age can be determined by yearning the C isotope of the plant fragments. Seismologists count the number of years between consecutive earthquake events, and then calculate the average to obtain the reproduction interval.
This plot shows the underground sedimentary layers near the active fault, buried sand volcanoes and broken rock layers (denoted by numbers) representing the time of earthquakes in the past
7.2 Earthquake early warning system
Short-term prediction that the earthquake will occur on a specific date or within a time window from a certain day to a certain year, which is false. In fact, such predictions may never be reliable. However, the concept of short-term prediction should not be confused with the concept of earthquake early warning systems, which are based on real signals and have the potential to save lives. The working principle of the early warning system of
is as follows: When an earthquake occurs, seismic waves generated by the earthquake begin to pass through the earth. Seismometers placed between the epicenter and the city will detect seismic waves before they reach the city. Once the seismic detector detects an earthquake, the transmitter will send a signal to the control center, and the control center will automatically broadcast an emergency signal to the city.These signals travel at the speed of light and arrive in the city a few seconds to one minute before the seismic waves occur. The arrival of the signal can trigger the automatic shutdown of natural gas pipelines, trains, nuclear reactors and electrical wires. The signal can also trigger an alarm, triggering radio, television and mobile networks to remind people to take precautions.
Because the tsunami is so dangerous, it is predicted that its arrival can save thousands of lives. At the Tsunami Warning Center in Hawaii, observers track earthquakes near the Pacific Ocean and use data transmitted by tidal gauges, buoys and subsea pressure gauges to determine whether a particular earthquake triggered a tsunami. If observers detect tsunamis, they will issue emergency warnings to authorities near the Pacific Ocean.
floats can detect tsunamis on the high seas, so that people on land can get warnings
8. Prevent earthquake damage and casualties
Damage and death caused by earthquakes of a certain scale depends on many factors, including: the distance between the epicenter and the center of the population; the depth of the focal point; the architectural style of the epicenter; the steepness of the slope, whether the fracture displaces the seabed; whether the affected area is close to the ocean; whether the building foundation is on solid bedrock or on fragile materials; in the event of an earthquake, people are outdoors or indoors; whether the government can quickly provide emergency services. To minimize earthquake disasters, one can work hard to build stronger buildings and choose safer locations to build.
8.1 earthquake project
The death toll of magnitude 6.8 earthquake that occurred in Armenia in 1988 was 400 times that of magnitude 6.7 earthquake that occurred in California San Fernando in 1971. The comparison of deaths reflects the differences in architectural styles and quality in these two regions, as well as the characteristics of the ground floor of the building. Armenia's reinforced concrete slab buildings and masonry houses are prone to collapse, while California's structures are basically built in accordance with building codes that require structures to withstand stresses caused by earthquakes. Most buildings are curved and twisted, but they don't collapse and crush people to death.
community can mitigate or reduce the consequences of earthquakes by taking reasonable precautions. Earthquake engineering (designing earthquake-resistant buildings) can help save lives and property. In areas where major earthquakes are prone to occur, buildings and bridges should be constructed with certain elasticity so that ground vibrations will not crack them. Additionally, the support should be strong enough to withstand the weight of the floors that are not static but may drop after a rebound. In some cases, simple changes in architectural practice can make the building stronger. For example, wrapping the cables on the support column of the bridge can increase its strength many times, bolting the bridge span to the top of the support column can prevent the bridge span from rebounding, bolting the building to the foundation, adding diagonal support to the frame, avoiding excessive twisting and shearing.
If these structures are designed to withstand vibrations, damage to the structure can be avoided
In areas with high earthquake risks, certain types of buildings should be avoided. For example, concrete blocks, unreinforced concrete and unreinforced brick houses will crack and collapse in wooden structures, steel beams or reinforced concrete buildings while remaining unchanged. A traditional heavy and crisp tile roof may break and bury residents inside, while a lightweight metal sheet roof does not. Through seismic reinforcement, that is, the process of strengthening the existing structure, it can make inappropriate structures safer.
8.2 Earthquake Area Division
In seismic active areas, urban planners can reduce disasters through earthquake zoning. Zoning depends on an assessment of land stability and do not build buildings on top or bottom of steep cliffs covered by soft soil or potentially liquefied wet sand and where landslides or dam breakage may cause flooding. Zoning should prohibit the construction of critical buildings (schools, hospitals, fire stations, communication centers, power plants) on active faults because fault sliding can damage and destroy buildings.
8.3 Prevent casualties
Even carefully prepared buildings and planning cannot avoid all earthquake casualties. Therefore, communities in earthquake zones should develop emergency plans for disaster response.They should develop strategies to provide personnel, equipment and supplies for rescue and recovery. Individuals living in high-risk areas of earthquakes should also assume the responsibility of protecting themselves and their homes from earthquake damage. Simple precautions include bolting or strapping the bookshelf and water heater to the wall, installing locking bolts on the cabinets, knowing how to cut off gas and electricity, and where to find the family. Schools, factories and offices should hold earthquake preparation exercises, and individuals should know where to seek protection from falling objects. As long as the lithosphere plate continues to move, the earthquake will continue to shake us. But we can reduce the chance of damage or injury by being prepared.
If an earthquake comes, hide under a solid table near the wall
9. Earthquake research inside the earth
Seismic detector was invented decades ago, by measuring the mass and shape of the earth, researchers Reenwei Earth consists of three concentric layers of different density: the crust, the mantle, and the core. To deepen this basic understanding, define the specific depth of the boundaries between layers and describe the characteristics of layers, researchers need a tool that allows them to "see" the inside of the earth. Research on seismic waves provides such a tool. By measuring the speed at which seismic waves propagate, seismologists have been able to provide more accurate images of the earth's interior.
simplified images of the earth's interior, first proposed in the 19th century. The earth has crust, mantle and core
9.1 velocity control and the bending of seismic waves
The ability and propagation speed of seismic waves in materials depends on several characteristics of the material: density, stiffness and compression. Therefore, the travel time of a set of waves refers to the time from their epicenter to the seismometer. Research on seismic waves reveals the fact that
seismic waves move at different speeds in different types of rocks. For example, the propagation speed of longitudinal waves in peridotite is 8 kilometers per second, while the sandstone is only 3.5 kilometers per second. Therefore, when a wave enters another, it accelerates or slows down.
seismic waves move further in peridotite than in sandstone
P wave propagates slowly in liquids than in solids with the same composition. Therefore, longitudinal waves propagate slowly in magma than in solid rocks and in molten ferroalloys than in solid ferroalloys.
longitudinal wave propagates faster in solid ferroalloys than in liquids with the same composition (such as molten ferroalloys)
Both P and S waves can pass through solids, but only P waves can pass through liquid
If a seismic wave propagates in two different materials at different speeds, it will bounce and bend at the boundaries of the two materials. This bounce phenomenon is called reflection, and bending phenomenon is called refraction. The angle of the reflected wave reflected from the boundary is always the same as the angle at which the incident wave hits the boundary. However, the angle at which the refractive wave bends at the boundary depends not only on the comparison of wave velocities in the material up and down the boundary, but also on the angle between the wave and the boundary. As waves propagate from high-speed medium into low-speed medium, they bend downward and away from the boundary. In contrast, the wave bends upwards toward the boundary.
Refraction and reflection of seismic waves
9.2 Discovery of the crust-mantle boundary
In 1909, Croatian seismologist Andrija Mohorovičić pointed out that the average P wave velocity of the seismometer reaching within 200 km of the epicenter is 6 kilometers per second, and the average P wave velocity of the seismometer reaching 200 km of the epicenter is 8 kilometers per second. To explain this observation, he believed that P waves reaching nearby seismometers travel along a shallow path, which makes them completely within the crust and propagate relatively slowly, while P waves reaching distant seismometers pass through the mantle, and they propagate more quickly in the mantle. The wave refraction he realized was passing through the crust-mantle boundary.
From this observation Mohorovičić calculates the crust-mantle boundary, which he believes is at a depth of 35 to 40 kilometers in the land crust. Later studies showed that the depth of the crust-mantle boundary below the continent was between 25 and 70 kilometers, and the crust-mantle boundary of the oceanic crust was between 7 and 10 kilometers. In honor of Mohorovičić, the crust-mantle border is now known as the Moho face.
Discovery of Moho surface
Seismologists have determined that seismic waves propagate at different depths of the mantle at different speeds. Specifically, the earthquake velocity is lower than the overlying part of the mantle at a depth of about 100 to 200 kilometers below the seabed. This 100-200 km deep strata is now called the Low Speed Band (LVZ). Researchers believe that LVZ corresponds to a layer where mantle rock undergoes a slight partial melting. Because seismic waves travel slower in liquid than in solids, even a little melt can slow down the speed of seismic waves. Simply put, LVZ distinguishes between the bottom of the lithosphere and the top of the asthroid under the oceanic plate. Seismologists did not find well-developed LVZ under the mainland.
is below about 200 kilometers, and the seismic wave velocity in the mantle under the continent and oceans increases with the increase of depth. Seismologists explain that increasing velocity with depth means that the compressibility of mantle peridotite gradually decreases, and the rigidity increases, and the density increases with depth.
has a depth of between 410 kilometers and 660 kilometers, and the seismic wave velocity in the mantle increases in steps. A major growth stage occurs at a depth of 660 kilometers. Experiments show that this discontinuity of seismic velocity occurs deep in the formation, and pressure re-arranges the atoms in the mineral into denser minerals of the same components, a phenomenon called phase transition. The discontinuity of seismic velocity is the basis for subdividing the mantle into the upper mantle (660 km) and the lower mantle (660 km). The lowest part of the upper mantle, between 410 and 660 kilometers, is called the transition zone.
Because the physical properties of the mantle change with depth, the velocity of longitudinal waves in the mantle changes with depth
9.3 The structure of the core
In the 20th century, researchers installed seismic detectors in many observatory stations around the world, hoping to record seismic waves generated by large earthquakes anywhere on the earth. In 1914, one of the researchers found that according to the results measured along the Earth's surface curve from the epicenter, the P wave of a certain earthquake could not reach the seismometer within the range of 103 degrees and 143 degrees. This area is now called the P-wave Shaded Area. The existence of the P-wave shadowed area means that there is a major boundary deep in the earth, where the seismic wave speed suddenly drops and refracts downward. Seismologists calculated that the depth of this boundary was 2,900 kilometers based on the size of the shadow area, which they believed was the core-mantle boundary.
Seismologists also found that S waves cannot reach seismometer observation stations between 103 degrees and 180 degrees (S wave shadowed area), which means that transverse waves cannot pass through the core. The transverse wave cannot pass through the liquid, and therefore the fact that the transverse wave cannot pass through the core means that the core, or at least part of the core, is composed of liquid.
At first, seismologists believed that the entire core might be a liquid ferroalloy. But in 1936, Danish seismologist Inge Lehmann discovered that when P waves pass through the core, they reflect on the boundary of the core. She believes that the earth core includes two parts: an outer core composed of liquid ferroalloy and an inner core composed of solid ferroalloy. The depth of the inner and outer nuclear boundary is ultimately determined by measuring the time when seismic waves pass through the earth, reflect at the inner and outer nuclear boundary and return to the surface. These measurements indicate that the depth of the boundary is about 5155 kilometers.
Shaded Area and Core Discovery: P wave cannot reach P wave shadow area; S wave will not reach S wave shadow area; seismic wave reflects at the kernel-kernel boundary
9.4 Modern image of the earth
After hard work, seismologists compiled data on the propagation time of seismic waves to form a velocity-depth curve chart, which shows the average depth and average change amount of sudden changes in the seismic wave velocity. The depth of major changes corresponds to the main and sub-layers below the center of the earth. The curves of the velocities of
P waves and S waves vary with the increase of Earth's depth, the velocity of S waves in the outer core is not shown in the figure because the S wave cannot pass through molten iron (liquid)
More detailed studies in recent years have shown that the onion-like Earth stratification model we currently describe is too simplified.Using a technique called seismic tomography, seismologists can generate three-dimensional images of the earth's internals with changes in earthquake velocity, just like a doctor does a three-dimensional CT (computed tomography) scan a human body. Tomography allows seismologists to identify areas in the mantle where seismic waves propagate faster or slower than expected, and these studies have led to the recognition that at a particular depth, the velocity of seismic waves varies significantly with location. Lower velocities may represent warmer mantle matter, while higher velocities may represent colder mantle matter, because when rocks become hotter and softer, it can deliver seismic energy slowly. The emergence of warm and cold zones is the result of mantle convection. The interior of the earth is indeed a vibrant place!
3D tomography image of the earth, areas with lower earthquake velocities may be relatively warmer than their surroundings, while areas with higher velocities may be relatively cold
Concept image of the interior of the earth: the hotter mantle material rises, and the colder mantle material sinks. Plates form on the surface and then sink into the mantle
