Active fault survey
Activity(for active faults)
Aftershock ==> refer to "Foreshock/Main shock/Aftershock"
Degree of certainty(for active faults)
Differential settlement ==> refer to "Alluvium"
Earthquakes known from historical accounts
Eastern margin of the Sea of Japan(Japan sea) ==> refer to "Plate tectonics "
Epicenter ==> refer to "Hypocenter/Source(Focal)region"
Fault movement ==> refer to "Fault"
Focal region ==> refer to "Hypocenter/Source(Focal)region"
Foreshock ==> refer to "Foreshock/Main shock/Aftershock"
GPS(Global Positioning System)
Great earthquake ==> refer to "Magnitude"
Hypocenter ==> refer to "Hypocenter/Source(Focal)region"
Interplate earthquake ==> refer to "Plate tectonics"
Interval of activity(for active faults)
Intraplate earthquake ==> refer to "Plate tectonics"
Landslide ==> refer to "Sediment disasters"
Low-frequency earthquakes==> refer to "Tsunami/Wave source region(Tsunami)/Low-frequency earthquakes"
Main shock ==> refer to "Foreshock/Main shock/Aftershock"
Mantle ==> refer to "Crust/Mantle"
Seismic ground motion
Shallow inland earthquakes ==> refer to "Active fault"
Slope failure ==> refer to "Sediment disasters"
Soft Ground ==> refer to "Ground"
Source region ==> refer to "Hypocenter/Source(Focal)region"
Subsurface fault ==> refer to "Active fault"
Strong motion ==> refer to "Seismic ground motion"
Topographic survey ==> refer to "Active fault survey"
Trench ==> refer to "Plate tectonics"
Trenching ==> refer to "Active fault survey"
Trough ==> refer to "Plate tectonics"
Tsunami ==> refer to "Tsunami/Wave source region(Tsunami)/Low-frequency earthquakes"
Underground structure survey ==> refer to "Active fault survey"
Wave source region(Tsunami) ==> refer to "Tsunami/Wave source region(Tsunami)/Low-frequency earthquakes"
Weak Ground ==> refer to "Ground"
Active fault to term index
Active faults are those faults that have been active during the most recent geological period and will continue to be active in the future. Active faults are defined in "Active Faults in Japan, Revised Edition," as faults to have been active during the Quaternary Period from about two million years ago to the present. Some researchers in this field, however, define those as faults to have been active during the period from several hundreds thousand years ago to the present.
The existence of active faults is confirmed by traces of repeated slip left in the topography (Fig.2-21) and the stratum. If the slip for the topography and stratum formed earlier is greater than that formed more recently, it is thought to be proof that slip has recurred. The older a fault, the more frequently it will have experienced earthquakes, and the slip will be that much greater. It is also believed that slip will recur in the same way, eventually causing an earthquake.
On extremely active faults, the slip accumulates through repeated activity. This forms the boundary between mountainous areas and other lowlands including basins and plains. Therefore, active faults are frequently discovered near major topographical boundaries such as these. Shallow inland earthquakes of M 7 or more often occur on active faults.
In areas with a thick alluvium, the slip accumulated through repeated fault movement will not necessarily appear on the surface. These are called buried faults.
It is important to understand the nature of each fault to evaluate the potential for earthquakes to occur on shallow land in the future. These characteristics include how active faults are (their activity), and the extent of slip that occurs during an earthquake, the interval of the activity on the active faults, and when the most recent activity occurred. To obtain these information, recent active fault surveys often include trenching (I.e., digging directly into the fault), in addition to studying the topography.
Related terms: Activity (for Active faults), Interval of activity (for Active faults), Degree of certainty (for Active faults), Active fault survey
Active fault survey to term index
Earthquakes on active faults on land generally occur at least every 1,000 years or longer. Therefore, it is difficult to determine the history of active faults from historical accounts alone; data that predates recorded history is required. Surveys on active faults to obtain this data include topographical surveys, trenching, and the studies of underground structure. Generally speaking, topographical studies are conducted first by aerial photography. This verifies the location of the fault and its degree of certainty. Then, a trenching is conducted to determine how earthquakes have occurred on the fault in the past. Finally, the underground structure is studied to determine the shape of the deeper parts of the fault.
In many cases, the shear of the earth and distinctive topography (called "fault displacement topography", Fig.2-21) occurs as a result of repeated fault movement. Topographical survey using aerial photography enables the discovery of the topography created by repeated fault movement using three-dimensional mapping of two aerial photographs. This verifies detailed and subtle protuberances and inconsistencies in the topography.
In addition to aerial photography, field investigations also play an important role. Detailed observation of the shear topography is possible at the site, as is observation of cliffs and other features exposed by the strata and faults creating the topography. To determine the activity of the active fault, it is necessary to know when a stratum or topography slipped by an active fault was created. Samples and other data are then collected.
The methods used to determine when a stratum or topographical feature was created include dating of the radioactive carbon within the organic material in the stratum, as well as analyzing the volcanic ash covering the topographical features. Volcanic ash is a good indicator for determining age since ash from a large volcanic eruption accumulates in an extremely short time.
The most direct way to survey the activity history of an active fault is to dig a trench horizontally across the fault and observe the stratum (see Fig.2-24).
For some excavations, this determines the slip of the exposed fault and information relating to the age of the stratum. Determining the long-term history of the activity period and interval and slip will provide data for forecasting the magnitude and timing of earthquakes.
[Underground structure survey]
Trenchings usually elicit information on the stratum to a depth of several meters. Various underground structure prospecting techniques are employed to determine the shape and location of deeper underground faults. Seismic exploration and boring studies are the most frequently used. Underground structure surveys are used to estimate the amount and location of stratum nonconformity, even for active faults that cannot be recognized at the surface. This underground structural prospecting enables us to gain information regarding faults that cannot be obtained through trenchings. Such information includes the structure of areas deep underground, and faults that lie under the sea floor, riverbeds, and other locations.
Related terms: Active fault, Activity (for Active faults), Interval of activity (for Active faults), Degree of certainty (for Active faults), Earthquakes known from historical accounts
Activity(for active faults) to term index
Activity for active faults is a quantitative expression of fault movement. This is manifested by the average speed (called the "mean slip rate") of the slip accumulated over a long period by an active fault.
The formula for determining the activity is as follows. T is the time elapsed from the formation of the topographical feature or stratum until the present. D is the slip for the topographical feature or stratum resulting from the recurrence of seismic activity on the active fault. Thus, the average speed of slip on the active fault, when S is the mean slip rate, is S = D/T.
A simple and frequently used means to represent activity is to rank the average speed of slip for an active fault from A-C.
Active faults with an A-class activity have an average slip of one meter or more, but less than 10 m every 1,000 years.
Active faults with a B-class activity have an average slip of 10 cm or more, but less than one meter every 1,000 years.
Active faults with a C-class activity have an average slip of one centimeter or more, but less than 10 cm every 1,000 years.
There are no known faults on land in Japan that average more than 10 m of slip every 1,000 years. In Japan, there are about 100 active faults with an A-class activity, about 750 active faults with a B-class activity, and about 450 active faults with a C-class activity.
The active faults with a C-class activity have a slow average slip speed. Therefore, the accumulated slip remaining in the topography is often not apparent due to later erosion, and there could be many more active faults that exist with a C-class activity.
Related terms: Active fault, Interval of activity (for Active faults), Degree of certainty (for Active faults), Active fault survey
Alluvium to term index
Alluvium is a stratum formed through sedimentation in rivers and seas. In other words, they are the most recently formed strata. They consist primarily of unconsolidated mud, sand, and rock, and form the basis for lowlands (alluvial plains).
These beds are formed as shown in Appended figure 1-1. The surface of the ocean was at its lowest level about 18,000 years ago. At that time, erosion by rivers formed deep valleys. Later, as the sea level rose these valleys were buried with deposits. This deposits constitutes an alluvium.
Alluvium is generally weaker than the older stratum that exists below it (i.e., the basement), and are highly susceptible to earthquakes. In thick alluvium (30 m or deeper), the seismic ground motion during an earthquake is easily amplified, and geological hazard readily occurs through the differential settlement of structures and liquefaction.
Alluvial plains cover only about 13% of the land area of Japan, but the country's most important cities are concentrated in these plains. Therefore, measures to deal with soft ground are the primary challenge in national efforts to prevent earthquake disaster.
Related terms: Ground, Liquefaction, Seismic ground motion
Crust/Mantle to term index
The earth's interior structure is similar to that of an egg. The exterior portion of the globe that corresponds to the shell of an egg is called the "crust," the part that corresponds to the egg white is called the "mantle," and the part that corresponds to the yolk is called the "core."
The crust is about 30-40 km thick on land, and about 50-60 km thick in the Himalayas and other mountainous regions. The crust is about 30 km thick in Japan. The thickness of the crust in the oceans is usually less than 10 km. Given that the earth has a radius about 6,370 km, the crust represents an extremely thin surface covering no matter where it is measured.
The mantle extends from the lower part of the crust to the upper part of the core at a depth of 2,900 km. It accounts for about 83% of the earth's volume. The mantle is not uniform, but comprises three layers. The layer closest to the crust is called the "upper mantle."
The crust is more fragile than the mantle. Thus, earthquakes in shallow locations on land occur in the crust. The plate in the term "plate tectonics" refers to the crust and part of the upper mantle. These are few tens kilometers thick. (See the section on plate tectonics.)
Related terms: Plate tectonics
Crustal movement to term index
Various forces are applied to the crust that comprises the surface of the earth, resulting in various movements. In this report, the movement that causes deformation in the surface of the earth is called "crustal movement." There are various types of crustal movement. These range from those in which plates move over long periods of geological time, with upheavals of mountains and subsidence of plains, to deformations during earthquakes that occur over short periods of time. Thus, these movements differ in terms of time and scale.
Crustal movements over a wide area were detected in the past by measuring the plane position (through triangulation survey, etc.) and the height (through leveling surveys). A determination of the height of the sea surface enables the measurement of upheavals and subsidence of the coast over the long term. In this report, we include diagrams of the extension and contraction of the earth's crust determined by analyzing the former survey results by region. Recently, global positioning system (GPS) have been effectively used to continuously observe crustal movements over a wide area. In addition, detailed observation of changes in the extension, contraction, and incline of the earth's surface can be made using underground tunnels.
Related terms: GPS
Degree of certainty(for active faults) to term index
The existence of active faults is often confirmed by specialists from evidence discovered in aerial photographs. There are several ways in which active faults appear, depending on the characteristics of the active fault itself and the topography of the location. Some active faults cannot be identified by topography alone. In these cases, the degree of certainty expresses the extent to which faults actually seem to be active. The degree of certainty for active faults is classified as follows:
Degree of certainty I: The location of the fault and the direction of the slip are apparent. It is certain from topographical characteristics that it is an active fault.
Degree of certainty II: The location of the fault and the direction of the slip can be hypothesized. There is insufficient data to place this in the I category; however, lack of positive evidence indicates the topography and that the extremely new stratum is repeatedly slipping.
Degree of certainty III: It is possible that this is an active fault, but the direction of slip is not apparent. Doubts remain that the fault could have been formed by other causes, such as erosion from rivers or the sea.
In this report, active faults with degrees of certainty of I and II are shown in the maps.
Related terms: Active fault, Activity (for Active faults), Interval of activity (for Active faults), Active fault survey
Destructive earthquake to term index
This report uses the term "destructive earthquake" to refer to earthquake that cause some sort of damage, regardless of the type or extent of the damage. Most destructive earthquakes are M 6.5 or larger, but earthquakes of M 5 or so can be destructive if the focal depth is very shallow or the ground near the epicenter is weak. Also, a seismic intensity of at least 5 lower in JMA scale is usually recorded in locations where damage occurs.
Related terms: Seismic intensity, Magnitude, Hypocenter, Epicenter
Earthquake swarm to term index
Groups of earthquakes that occur frequently in a concentrated area without clearly defined foreshocks, main shock, or aftershocks are called "earthquake swarms." In the (foreshock,) main shock, and aftershock sequence of seismic activity, the frequency of aftershocks usually decline in a regular manner over time. In case of earthquake swarms, however, there is a repeated ebb and flow, until they eventually diminish.
Individual earthquakes that are a part of earthquake swarm activity seldom reach M 6, but some earthquakes in the M 5 range can cause damage locally. One of the most well-known examples of earthquake swarms are those that occurred from 1965 over a several-year period in Matsushiro, Nagano Prefecture. During this time, there were more than 60,000 felt earthquakes. The largest of the earthquakes in this swarm was M 5.4. Recently, there have been frequent earthquake swarms off the east coast of the Izu Peninsula. Many earthquake swarms occur near volcanoes, but any relation they might have with volcanoes is still unclear.
Earthquake swarms have been placed in the figures in this report that show primary destructive earthquakes, even if they cause no damage.
Related terms: Foreshock, Main shock, Aftershock
Earthquakes known from historical accounts to term index
Past earthquakes that occurred before the start of earthquake observation using modern equipment are known primarily through historical accounts. Accordingly, in this report these are sometimes referred to as "historical earthquakes."
The quality and quantity of historical accounts differ by era and location, thus there is no national homogeneity for earthquake knowledge. For example, there are a wealth of historical accounts in the Kinki region for earthquakes in the capital, and many earthquakes are known. Fewer earthquakes are known in regions with fewer historical accounts, however. This does not necessarily mean that fewer earthquakes occurred in these regions. Also some earthquakes are not known because of lost records, while some earthquakes are known but did not actually occur because of mistaken records.
Faults, traces of liquefaction, and other evidence of earthquakes discovered from trenchings and the excavation of historical ruins provide clues for discovering earthquakes that occurred when historical accounts were not being kept. These earthquakes, and those that are known from historical accounts, are termed "old earthquakes." The hypocenter and source region of earthquakes known through historical accounts has been estimated based on the nature of destruction and tsunami produced. Accordingly, there is a great deal of variation in their characteristics. The magnitude of old earthquakes is estimated from the extent of the damage caused.
Modern earthquake observation using equipment began in Japan in 1885, and only 100 years have elapsed since then. The recurrence interval for earthquakes is more than 100 years, even for earthquakes with short recurrence intervals, such as interplate earthquakes. This interval can be more than 1,000 years for earthquakes that occur on active faults on land. Therefore, earthquake information from historical accounts is extremely important to understand the characteristics of seismic activity in this country.
In this report, the notations of the destructive earthquakes in the figures are changed before and after 1885.
Related terms: Active fault survey, Liquefaction
Fault to term index
A stratum which is originally continuous sometimes shows offset with a plane as a boundary. This type of structure is termed a "fault," and the boundary plane is termed a "fault plane."
Earthquakes are phenomena (fault movement) in which the rock mass rapidly slips on both sides of the fault plane. Generally, this slip starts at a point and spreads to the surrounding area along the fault plane. This rapid fault motion releases the energy of the strain that has accumulated underground. The fault that is the source of the earthquake cannot frequently be seen at the earth's surface. With large earthquakes greater than M7 or larger that occur at shallow depths on land, part of the underground fault usually appears on the earth's surface, and creates slip there.
Faults are classified as either "dip-slip faults" or "strike-slip faults" depending on the direction of the slip motion. Also, dip-slip faults are classified as either "normal faults" or "reverse faults," while strike-slip faults are classified as "right-lateral strike-slip faults" or "left-lateral strike-slip faults." (See appended figure 1-2.) Actual faults are seldom as pure as those shown in the figures, and dip-slip and strike-slip faults are frequently mixed.
The fault type is closely related to the forces working at the site. Generally, reverse faults and strike-slip faults occur in at locations with a horizontal compression force. Normal faults often occur where a force pulls horizontally. The area near Japan is subject to compression from plate motion, so reversed faults and strike-slip faults are often seen. On the Beppu-Shimabara rift valley, however, normal faults are observed.
Related terms: Hypocenter/Source region, Active fault
Foreshock/Main shock/Aftershock to term index
When a big earthquake occurs, other smaller earthquakes frequently occur at the same location. The initial earthquake is called the "main shock," and the smaller ones that occur later are called "aftershocks." The number of aftershocks increases immediately after the main shock, but empirical observation shows the number of aftershocks decrease over time in a regular manner. The phase "the aftershocks gradually decreased" occasionally used in this report is based on this empirical observation. The magnitude of the aftershocks is smaller, in many cases, by about one or more in magnitude scale than the main shock, but these aftershocks can cause damage when they follow large earthquakes. The largest of the aftershocks is called the "largest aftershock," and the area in which the aftershocks are distributed is called the "aftershock area." Most aftershocks occur in the main shock's source region, and the distribution of aftershocks that occur immediately after the main shock -- from several hours to one day later -- clearly show the main shock's source region.
The figures to show trends in aftershock frequency for the destructive earthquakes in each region show aftershock frequency as observed at certain observation station or the earthquake frequency as observed by the Japan Meteorological Agency's seismographic network. When observed by this network, the earthquakes felt at one or more observation station are called "felt aftershocks."
Earthquakes sometimes occur in an area that will become the source region of the main shock before the main shock occurs. These are called "foreshocks." In most cases, these foreshocks are small in size and infrequent, but they can cause damage if a sufficient number occur. Most foreshocks also occur immediately before or a few days before the main shock, but some occur a month or more before the main shock. It is difficult, however, to determine whether an earthquake is a foreshock prior to the main shock occurrence.
As previously noted, earthquakes consisting of only main shocks and aftershocks are known as "main shock-aftershock" types. When foreshocks occur, this is described as "foreshock-main shock-aftershock" seismic activity.
Also, groups of earthquakes without clearly defined foreshocks, main shocks, or aftershocks that occur frequently in a concentrated area are called "earthquake swarms." (Refer to the section on earthquake swarms.)
Related terms: Source region, Earthquake swarm
GPS(Global Positioning System) to term index
GPS(global positioning system) is a system for determining position using satellites. The system was developed in the U.S. for determining the positions of aircraft and ships. There are 24 GPS satellites orbiting the earth at a height of 20,000 km. Radio waves are received simultaneously from four or more of these satellites, and the position of reception is determined based on the position of each satellite and the time of reception. This system is also used for car navigation systems. A method has been developed for determining relative positions by comparing reception data from multiple points, enabling the measurement of distance with extremely high accuracy. This accuracy ranges from 1/1,000,000 to more than 1/10,000,000 -- an error of one millimeter to one centimeter per 10 km. This method can be used for the highly accurate, continuous detection of crustal deformation over a wide area. This has replaced the previous method of measurement, which required considerable time and effort.
In Japan, the Geographical Survey Institute and other agencies have established about 1,000 points nationwide for continuous observation (electronic standard points) by the end of March 1997 to continuously and efficiently observe crustal deformation over a wide area.
Related terms: Crustal movement
Ground to term index
This word refers to the foundation of civil engineering structures, and the surface layer of the planet on which construction occurs. Depending on its hardness, ground is classified as consolidated ground (rock mass), semi-consolidated ground, or unconsolidated ground (soil). Embankment areas and reclaimed land are referred to as "artificial ground."
Ground that is comprised of very soft clay or loose sand is called "soft ground," and lacks the strength to support buildings as the foundation of structures. Such damage as sliding collapse and ground subsidence readily occurs. Liquefaction and differential settlement occur easily during earthquakes.
The primary constituent elements of soft ground in Japan are the most recent strata of alluvium, which are distributed in coastal plains, and the shallow valleys that carve out low marshy land, flats, and hilly areas behind rivers. Ground created by landfill is also soft ground.
Related terms: Alluvium, Liquefaction
Hypocenter/Source(Focal)region to term index
Earthquakes are the result of the destruction of underground rock mass. Generally, both sides of rock along a plane (fault plane) will rapidly slip. This slip begins from a certain point, and spreads along that plane to the surrounding areas. The spot where the slip started is called the "hypocenter." The entire area where the slip occurs is called the "source region" or "focal region." The hypocenter or the focal region shown in the figures on primary destructive earthquakes represent a projection onto the earth's surface. The location on the earth directly above the hypocenter is called the "epicenter." (See appended figure 1-3. )
Thus, an earthquake does not occur at a given point, but over a wide planar surface. The greater the magnitude, the larger the source region becomes. For example, in a great earthquake of M 8 or so, the width of the source region will be dozens of kilometers, and the length will be more than one hundred of kilometers. In an earthquake of M 4, on the other hand, the length and the width will be one kilometer.
The hypocenter can immediately be determined by observing the seismic waves, but some time is required to estimate the source region. The source region is estimated by the distribution of the aftershocks, the wave source region of the tsunami, and the fault that appears on the earth's surface.
The source region in the figures in this report generally show the fault model for inland earthquakes and the wave source region of tsunami for earthquakes at sea. The source region for inland earthquakes is often nearly vertical. Therefore, the focal region appears smaller than it actually is when projected on the earth's surface. In contrast, the tsunami's source region shows the area where the tsunami occurred; thus, it tends to be larger than projection of the source region on the earth's surface.
Related terms: Fault, Magnitude
Interval of activity(for active faults) to term index
The interval of activity for active faults is that period of time in which there is repeated activity on a fault or part of fault zone. In other words, it is the recurrence interval of earthquakes. Roughly speaking, the average activity interval for the active faults of A-class activity is from 1,000 to several thousand years. This interval is about 10,000 years for those of B-class activity.
Related terms: Active fault, Activity (for Active faults), Degree of certainty (for Active faults), Active fault survey
Liquefaction to term index
When strong ground motion reaches loosely accumulated sandy ground, a phenomenon occurs in which the stratum itself becomes liquefied and loses the strength to support buildings.
In addition to strong ground motion, other conditions required for liquefaction to occur are a stratum with large water content and a loose accumulation of sand. Sites where these conditions exist and which have the marked potential for liquefaction to occur are those with sandy ground and a high groundwater level. Examples include reclaimed land, land that was originally a riverbed but has since been filled in, and lowlands between sandy hills and reefs.
When liquefaction occurs, the grains of sand float in the groundwater, and the water and sand spouts (sand blow phenomenon; Fig.2-25). The earth loses its capacity to support buildings (Fig.6-28). Objects with a high specific gravity (such as buildings and bridges) sink, while objects with a low specific gravity (such as underground piping and manhole covers) float by buoyancy (floating phenomenon; Fig.3-11). After the water is drained, the sand becomes more compact and the ground returns to its original state, or a less fluid state, regaining its capacity to support objects (See appended Figure 1-4).
Other phenomena also occur, including extensive lateral slip movement of the liquefied stratum, the collapse of embankments and landslides, and the bulging or subducting of bulkheads. Therefore, the damage caused by liquefaction on land can occur on plains where population is concentrated. Such damage disrupts roads and basic infrastructure, and has a serious impact on people's lives.
Also, strata of liquefied sand and traces of sand boiling (which broke through the strata above) have been discovered at sites where historical ruins are being excavated and studied. (See appended Figure 1-5) Frequently, the age of each stratum is carefully determined at the excavation sites, providing valuable clues for researching earthquakes of the past through comparison with records of earthquakes in historical accounts.
Related terms: Alluvium, Ground, Seismic ground motion, Sediment disasters, Earthquakes known from historical accounts
Magnitude to term index
While seismic intensity measures the strength of the seismic ground motion, magnitude is the scale of the source of the earth's shaking. Therefore, the seismic intensity of one earthquake will vary with location, but the magnitude will be the same. The expression that "magnitude represents the scale of an earthquake" is also used in television and newspaper reports.
Seismic intensity can be measured directly, but magnitude cannot. It is estimated from the amplitude of the shaking in each area. The larger the magnitude, the larger the scale of the earthquake. For each increase of one full unit of magnitude, the earthquake energy increases about 30 times.
Magnitude is often expressed as "M." For example, in this report, you will see phase such as "the 1995 Southern Hyogo Prefecture Earthquake (M7.2)."
Earthquakes of M 8 or greater are called "great earthquakes," while those of M 7 or greater are called "large earthquakes." Damage of some kind usually results when an earthquake of this size occurs. Damage sometimes occurs for earthquakes of less than M7, depending on the location of the event.
For the earthquakes covered by this report, we use the M value listed in the Chronological Scientific Table (Rika Nenpyou). For those earthquakes not in the Chronological Scientific Table, we used " Materials for a Comprehensive Listing of Destructive Earthquakes in Japan [Revised and Enlarged Edition] " (if they occurred before 1884), Utsu's table for those that occurred from 1885-1925, and Japan Meteorological Agency data for those that occurred after 1926.
Related terms: Seismic intensity
[Relationship between magnitude and seismic intensity]
The relationship between magnitude and seismic intensity resembles that of the wattage of a light bulb and the brightness above a desk (See appended Figure 1-6). Even though the light emitted by the bulb is the same, the brightness above the desk differs depending on the location of the desk. The seismic intensity of the same earthquake will differ depending on the distance and direction from the site (focal region) where the earthquake occurred. For example, the 1995 Southern Hyogo Prefecture Earthquake (M 7.2) had a seismic intensity of 7 in JMA scale in the so-called "earthquake-disaster belt" near the focal region. The intensity decreased as the distance from the focal region increased, however. The intensity was 6 at the Kobe Marine Meteorological Observatory and the Sumoto Weather Station, and 5 at Kyoto, Hikone, and Toyooka. In Tokyo, far away from the focal region, the intensity declined still further to 1.
Even if the locations of the light bulb and the desk do not change, the brightness over the desk will change with the wattage of the light bulb. In the same way, the seismic intensity for earthquakes occurring in the same location will differ with the magnitude.
Generally speaking, the larger the magnitude and the closer to the earthquake's location (focal region), the greater will be the seismic intensity. The seismic intensity will decline in locations farther from the focal region, even when the magnitude is great. Also, the seismic intensity depends on the depth at which an earthquake occurs, the type of fault slip, the way the seismic wave is transmitted, and the conditions of the ground. Therefore, it will not decline uniformly with the distance from the focal region.
Plate tectonics to term index
Plate tectonics is the theory of the movement of dozens of bedrock (plates) of several tens kilometers thick that cover the surface of the earth and exert various distorting forces on the earth. The deformations that occur at plate boundaries cause various geological phenomena, such as earthquakes and volcanoes. Plate tectonics is the integrated theory of these phenomena, and was developed in the latter half of the 1960s.
These plates move in different directions at a speed of several centimeters per year. At their boundaries, these plates can move in divergent directions, converging each other, or passing each other (Fig.2-16). At the boundaries where the plates move away from each other, mountains are formed on the ocean floor, such as the mid-Atlantic Ridge and the eastern Pacific Rise. Large rifts are formed in the middle of these ridge or rise. When plates converge each other, the plates collide forming mountains. One side subducts under the other, forming rows of ocean trenches and arc shapes. These are called "island arcs." Examples include the Aleutians, the Chishima archipelago, the Japanese archipelago, and the Nansei Islands. At the boundaries where the plates slide by each other, strike-slip faults are formed, called "transform faults." Therefore, the plate boundaries are the site of various types of deformation, and earthquakes and volcanoes occur along these plate boundaries. Areas except the plate boundaries are stable, forming continents and large stretches of ocean floor that are stable and have little deformation.
The Japanese archipelago is an area in which several plates converge each other. There are at least three plates: the Pacific Plate, the Philippine Sea Plate, and a land plate (Fig.2-17). The Pacific Plate approaches the Japanese archipelago at about 8 cm per year from a roughly east-southeast direction. It subducts under the land plate at the Chishima and the Japan Trenches, and subducts under the Philippine Sea Plate at the Izu-Ogasawara Trench. The Philippine Sea Plate approaches the archipelago from the southeast at a speed of roughly 3-7 cm per year, and subducts under the land plate from the Sagami Trough and the Suruga Trough to the Nankai Trough and the Nansei Islands Trench.
Large earthquakes occur near the ocean trenches and troughs where the plates subduct. These large earthquakes are generated by fault movement in which the edge of the land plate is dragged along as the Pacific Plate subducts, and the land plate finally rebounds up when the tension has reached its limit (Fig.2-19). This type of earthquake is called an "interplate earthquake" or a "plate boundary earthquake." When the fault length reaches 100 km or longer, the earthquake's magnitude climbs to roughly 8 or larger.
Extensive fracture may occur within the plate subducting from ocean trench. Sometimes earthquakes occur within the subducted plate which occasionally include deep earthquakes. These are called "earthquakes within subducting plates." Also, the force exerted on the surrounding area by the subducting of the plate causes earthquakes within a plate on land. These are earthquakes in shallow locations a slight distance from the plate boundary.
Some have advanced the theory that a plate boundary exists along the eastern margin of the Japan Sea (Fig.2-1). Major earthquakes have occurred here in recent years along a north-south direction. Also, the land plate (East China Sea) is dragged in the direction perpendicular to the row of the Nansei Islands on the northwest side, forming the somewhat shallow Okinawa Trough (Fig.2-7). Several earthquakes have occurred here.
Tectonics is the study of the movement that creates underground structures and related to the cause of earthquakes or volcanic activity.
These are long and narrow topographical features deep in the sea floor. Those of which slope of both sides are rather steep and located at depths of 6,000 meters or more are called "trenches," while those that are wider and in shallower locations are called "troughs."
Trenches and troughs are usually found in a zone where a plate is subducting, and are formed along mountains and island arcs. The topological characteristics of troughs are not as pronounced as those of trenches, but they are basically identical to trenches in structure and origin. The dragging of continental crust, has led to the formation of some trenches such as the Okinawa Trough.
Related terms: Crust / Mantle, Crustal movement
Seismic gap to term index
The study of interplate earthquakes that recur along plate boundaries over a long time shows that source regions are arrayed in rows without gaps. Therefore it is possible that earthquakes will occur in areas where no earthquakes of this type have occurred yet. These are called "seismic gaps."
Related terms: Source region
Seismic ground motion to term index
Seismic ground motion is the motion of the earth's surface caused by an earthquake. There are several characteristic features of seismic ground motion: one is for large or small motion, another is for long or short period, and yet another is for long or short duration. There are differences depending on the characteristics, but generally speaking, stronger seismic ground motion results in the collapsed houses, liquefaction of the ground, and earth avalanches and landslides.
Seismic ground motion is generally stronger the closer it is to the focal region. Even at some distance from the focal region, however, strong seismic ground motion can be felt if the ground is soft. The strength of seismic ground motion is also influenced by the direction of fault movement, the slip, as well as by the underground structure. For example, there was strong localized seismic ground motion in the 1995 Southern Hyogo Prefecture Earthquake (Hyogo-ken Nanbu Earthquake) due to the progressive direction of fault motion as well as the underground structure near Kobe. This is thought to be one of the factors of the extensive destruction. (Refer to 2-5 (1) in the text.)
Intense seismic ground motion is also called "strong ground motion," and seismic intensity scale is one of the scale to measure the strength of seismic ground motion.
Related terms: Seismic intensity, Ground, Liquefaction, Sediment disasters, Fault movement
Seismic intensity to term index
Seismic intensity is the value expressing the degree of ground motion on the earth's surface caused by an earthquake. The strength of the shaking of the earth's surface is closely related to the damage caused by the earthquake. Therefore, an understanding of seismic intensity is vital to earthquake disaster prevention. In Japan, seismic intensity meters are used to determine seismic intensity, and results are reported immediately after the earthquake by the Japan Meteorology Agency (JMA). JMA uses 10 grades to represent seismic intensity. From the weakest to the strongest, they are: 0, 1, 2, 3, 4, 5 lower, 5 upper, 6 lower, 6 upper, and 7. JMA also has a table that describes the phenomena and damage that will occur in an area when a specific seismic intensity is observed (Reference 1).
Observation of seismic intensity has been conducted since the 1880s in Japan. Before the seismic intensity meter was introduced in 1991, assessments of seismic intensity were based on the strength of the shaking felt and the degree of the damage. The seismic intensity scale has been revised repeatedly, and the scale currently in use was introduced in October 1996. Before that, a seismic intensity scale with eight levels (from 0 to 7) was used (Reference 2). For earthquakes occurred before the start of seismic intensity observation, it is estimated based on the degree of the damage sustained.
The seismic intensity meter is a device that represents the shaking of the earth's surface in quantitative terms (Reference 3). This meter gives intensity that roughly correspond to the seismic intensities that were previously determined without instruments. The intensity determined by calculation is called the "instrumental seismic intensity," and fractional values are rounded down and called "seismic intensity." Because there is a wide range of shaking and phenomena for seismic intensities of 5 and 6, levels of 5 lower, 5 upper, 6 lower, and 6 upper have been established. The acceleration, period and time of continuation of the shaking of the earth have a complex relationship in measuring seismic intensity. The acceleration for the lower limit of instrumental seismic intensity 6.5 for seismic intensity 7 is about 1,900 Gal for 0.1 second period, and about 430 Gal for 2.0 second period. (These are composition of three components when the time of duration is sufficiently long.)
Notes: The descriptions in this report of earthquakes before September 1996 are based on the seismic intensity scales of that time (Reference 2).
Related terms: Magnitude
Reference 1: (Explanatory chart for JMA seismic intensity scale)
Reference 2: (JMA seismic intensity scale )
Reference 3: (JMA notification defining seismic intensity scale)
Seismic waves to term index
During earthquakes, vibration is created in the earth from the slipping motion of the rock mass. This vibration is transmitted as waves to the surrounding area, and are known as "seismic waves." When these waves reach the surface of the earth and cause it to shake, this shaking is perceptible as seismic ground motion.
Seismic waves are divided into body waves, which are transmitted in the earth, and surface waves, which are transmitted along the surface of the earth. Body waves are further divided into P waves and S waves. P waves are longitudinal waves (waves that vibrate in the same direction as the progression of the wave), and dilatation and compression are transmitted. Sound waves are an example of longitudinal waves. S waves are transverse waves (waves that vibrate perpendicularly to the direction of the progression of the wave), and torsion is transmitted. Surface waves are transmitted on the surface, such as the ripples that result when a stone is dropped into a pond.
When we feel the shaking of an earthquake, we first feel rattling and the small vertical undulations of the P waves. This is followed by the swaying horizontal movement of the S waves. P waves travel about 1.7 times faster than S waves. The distance to the hypocenter can be calculated from the difference in the arrival time of P and S waves, also called the "duration of preliminary tremor." Accordingly, they can be used to determine the hypocenter.
Related terms: Seismic ground motion, Hypocenter
Sediment disasters to term index (slope failure, debris flow, landslide)
Seismic ground motion causes sand and stone movement as well as landslide, which can damage buildings and cause injuries and death.
Ordinarily, the slope failure caused by rainfall most often occurs on concave sloping surfaces that contain a thick accumulation of surface material and an accumulation of a large volume of water from the surrounding area (a large water-accumulation area). Also, slope failure is frequently caused by seismic ground motion, as shaking is easily concentrated in an convex sloping surface. On some occasions, an earthquake will cause the collapse of a mountain itself. Well-known examples are the collapse of Mt. Ontake in the 1984 Western Nagano Prefecture Earthquake (M 6.8), and the collapse of Unzen-mayuyama in the 1792 earthquake (M 6.4) in the Shimabara Peninsula. The sloping sections of reclaimed land frequently collapse, too, but the amplification of seismic ground motion occurs more easily on embankments.
Damage is caused when the side of a mountain collapses, and a large debris flow combines with the accumulated material in a valley and water to flow downstream. Also, when the slope failure and debris flow occurs, it blocks rivers, and causes secondary disaster by collapsing embankments. The 1847 Zenkoji Earthquake (M 7.4) formed a lake by damming up the Sai River. The surrounding area was flooded, and the edge of the lake later collapsed, causing extensive destruction in the area downstream.
A landslide is a phenomenon in which earth on a gentle slope slowly falls over a wide area. These are sometimes triggered by seismic ground motion. In the 1995 Southern Hyogo Prefecture Earthquake, there was localized damage in the hilly area of Kobe due to the cracking that accompanies landslides.
Slope failure and landslides are caused by seismic ground motion or rainfall. Underlying these events, however, are such natural factors as regional geology, topography, and groundwater. Also, the slope failure and landslide can be caused by aftershocks following a main shock and rainfall. Therefore, it is necessary to be cautious after a main shock.
Related terms: Seismic ground motion
Tsunami/Wave source region(Tsunami)/Low-frequency earthquakes to term index
Tsunami are waves created on the surface of the ocean by rapid distortion of the topography of the sea floor. The period of tsunami is usually 10-20 minutes, which is longer than that of waves caused by wind ordinarily seen at the shore. Therefore they frequently appear to be an abnormal high or low tide rather than the waves ordinarily seen at the shore. As we will explain later, they move at high speed. Tsunami primarily originate in the upheavals or subsidence of the sea floor caused by shallow earthquakes under the sea floor. In rare instances, they are caused by volcanic eruptions, landslides at the sea floor, and avalanches near the coast. Also, the area where tsunami occur - the area where the upheaval or subsidence occurs on the sea floor to cause the tsunami - is called the tsunami's wave source region. This wave source region tends to be larger than the projection of the underground focal region of the earthquake.
The height of tsunami is rather small in the open sea, but becomes larger when it approaches the shallow coast. It is further amplified by the bottom topography or the shape of coastal line. The tsunami disaster caused by the 1896 and 1933 earthquakes in the open sea off the coast of Sanriku (called Meiji Sanriku Earthquake Tsunami and the Sanriku Earthquake Tsunami) is well known. The tsunami height reached 20 meters in some locations. Sometimes, a tsunami generated by large earthquakes overseas (such as the tsunami from the 1960 earthquake in Chile) can cause damage in Japan.
The velocity of tsunami propagation increases as the water becomes deeper. For example, tsunami in the open sea 4,000 meters deep can move at 200 meters per second. In contrast, the propagation may slow about 10 meters per second in shallower areas closer to the coast, but this is still much faster than people can run.
"Tsunami earthquakes" sometimes simply refer to earthquakes that accompany tsunami. The term is used most often, however, for earthquakes whose faults slip much more slowly than normal and whose tsunami may be quite large even if there is minimal perceptible shaking. In this report, the term "tsunami earthquakes" ("slow earthquakes" or "low-frequency earthquakes") is used to describe the latter type of earthquake. A well-known example of a tsunami earthquake was the 1896 earthquake that caused the Meiji Sanriku Earthquake Tsunami.
Related terms: Source region, Fault
Appended table 2-2 EXPLANATORY TABLE ON JAPAN METEOROLOGICAL AGENCY (JMA) SEISMIC INTENSITY SCALE
Appended table 2-3 JMA seismic intensity scale (1949) and reference items (1978)
Appended table 2-4 Notification No. 4 of the Japan Meteorological Agency