In the long-term evaluation for the major active fault zones, there are many cases where mean recurrence interval and the time of the latest event are evaluated with a range of value. In the preparation of the seismic hazard map, the models are built using the earthquake occurrence probability based on the median values of the respective ranges of the recurrence interval and the time of the latest event, hereinafter referred to as "average case".
To prevent from an underestimating the occurrence probability for each earthquakes, the smallest value from mean recurrence intervals and the oldest time of the latest event is applied to the calculation so that the occurrence probability might be the highest value. The case is called "Maximum case".
Rocks increase their consolidation and grow harder as they go deeper beneath the ground surface. The hardness of a rock and the S-wave velocity are correlated. The S-wave velocity of rock increases with progression of hardness. As a term used in engineering fields such as architecture and civil engineering, engineering bedrock refers to bedrock as the base of a high-rise building. While S-wave velocity varies with kinds of structure and bedrock conditions, it is approximately 300-700 m/s or more in general cases. In the "Probabilistic Seismic Hazard Maps (PSHM)", it is defined as bedrock with S-wave velocity of 400m/s. Meanwhile in the detailed method, hard bedrock with S-wave velocity of approximately 300 - 700 m/s or more is called engineering bedrock.
The exceedance probability refers to the probability that shakes will exceed a certain level of intensity at a point for a certain time period (over the next 30 or 50 years, in this guidebook).For example, the "Map of ground motions of JMA Seismic intensity for a 3% exceedance probability occurring within 30 years from the present" means the probability that each point is affected by shakes exceeding its JMA seismic intensity shown on the map is 3% within 30 years from the present.
The Active fault is a fault that has seismic activity in the geologically recent period and that is estimated to become active in the future. In "Active Faults in Japan 'Revised Edition'" (compiled by the Research Group for Active Faults of Japan) an active fault is defined as a fault that displaced the earth materials repeatedly in the Quaternary period (the period from about 2 million years ago to the present day).
JMA Seismic intensity refers to the scale that the Japan Meteorological Agency specifies in classifying seismic ground motions. JMA seismic intensity scale runs from zero to seven. Because seismic intensities 5 and 6 involve a wide degree of damage, each of them is divided into "Lower" and "Upper." Accordingly, the JMA Seismic intensity is currently expanded to a total 10 scales.
These are the fault zones selected by the Headquarters for Earthquake Research Promotion (HERP) for research preferentially. Among active faults in the whole of Japan, highly active fault zones that are considered to cause earthquakes that are very influential socially and economically are selected for the list of the major active fault zones.
The HERP classifies the major active fault zones into the group of "Earthquakes with Specified Seismic Source Faults."
Japan is an earthquake-prone country according to the world standard because the plate on the ocean side (oceanic plate) subducts into the plate on the land side (continental plate) beneath the Japanese Archipelago. In summary, the Pacific plate and the Philippine Sea plate on the ocean side sink into the Eurasian plate on the land side; therefore, there is complicated behavior under Japan that causes various types of earthquakes. Of these earthquakes, interplate earthquakes and earthquakes within the subducting plate are called subduction-zone earthquakes.
The Headquarters for Earthquake Research Promotion evaluates the scale and probability of future earthquakes based on the results from research and studies of the past earthquakes in the major active fault zones and subduction-zones, and publishes the evaluation results. This kind of evaluation is called long-term evaluation because it handles the next several decades. The "PSHM" are prepared based on these evaluation results. In the evaluation of characteristic earthquakes occurring in the major active fault zones, the probability of occurrence has uncertainty because of a scarcity of information on mean recurrence interval and the latest event.
In the long-term evaluation, recurrence interval of the target earthquakes are evaluated by the time of the latest events obtained from historical documents, the geochronological information obtained from the geological survey and the average slip rate (geological strain rate).
In the long-term evaluation, the latest occurrence time of target earthquakes is evaluated by the time of the latest events obtained from the historical documents and the geochronological information obtained from the geological survey. It is usually expressed as "X years ago".
The conventional method is a method to calculate the intensity of seismic ground motion on engineering bedrock (Vs=400m/s) based on attenuation relation for the ground motion estimated empirically. It is a simple method compared with the detailed method, so that it is called "Conventional Method". For calculation of the intensity of seismic ground motion on ground surface, site amplification factor from "Engineering bedrock" to ground surface is used.
The detailed method is a method to calculate the intensity of seismic ground motion on "Engineering bedrock in the detailed method (Vs is approximately 350-700m/s or above)" based on the earthquake source model and the three dimensional structure model. The detailed data such as the fault figure with asperity, slip of the fault model, rupture starting point and site amplification factor are necessary. The representative method is the hybrid method to compose the Stochastic Green's function method and the three-dimensional finite difference method.
Ground motion decreases with increasing hypocentral distance. The relation quantitatively shows the characteristics that the amplitude of ground motion attenuates depending on distance. "Attenuation relation for the ground motion" is empirically estimated with the past seismogram.
In the detailed method, bedrock with S-wave velocity of approximately 300-700m/s or above is called engineering bedrock. In determining the engineering bedrock, subsurface model reflecting boring data of the region is used. Also termed the "Engineering bedrock in the detailed method" to be distinguished from "Engineering bedrock".
The seismic ground shaking is produced not only on rupture starting point (hypocenter) but also by continuous fracture of rock mass with a planar spreading. The whole of fractured domain including the hypocenter is called "Seismic source fault".
The surface trace of the fault is an appearance position where a plane of seismic source fault is extended to ground surface. (In either case of the vertical seismic source fault and the seismic source fault with dip.)
Averaged Hazard Map shows the expected intensities of earthquake ground motions with long-term return period such as 500-100,000 years. The map is created to represent an effect of earthquake that has relatively low probability but could cause strong motion. All of earthquakes in the map are evaluated as Poisson process in seismic activity model. For example, a map of "Return Period of 1,000-year", which is equivalent to "3% Probability of Exceedance in 30 Years" in PSHM shows distribution of intensities that will possibly experience once in 1,000 years. The number of earthquakes involved varies strongly depending on the return period. Effects of earthquakes with lower frequency are covered as the return period increases.
Earthquakes around Japan can be cataloged by the return periods as:
1,000-year: Major subduction-zone earthquakes
10,000-year: Almost all of subduction-zone earthquakes and earthquakes on major active fault zones
100,000-year: Almost all of earthquakes including those of without specified source faults
Even if each of occurrence frequency is once in 100,000 years, an earthquake will occur once in 10 years on anywhere around Japan if there are 10,000 such earthquakes. Following facts may be helpful to understand the long-term return period. The number of destructive earthquakes occurred in Japan (death toll over 50) in the past 200 years is known as 23 times and its occurrence frequency is about once in 10 years. It corresponds to each of occurrence frequency is once in 100,000 years.
The probability that ground shaking exceeds a certain level of intensity at a site for a certain period (within next 30 or 50 years) is called "exceedance probability".The Probabilistic Seismic Hazard Maps (PSHM) can either show a distribution of ground motion intensity or exceedance probability by fixing one of them in turn. For example, "Seismic intensity of PSHM, for a 3% exceedance probability within 30 years" means the ground shaking will occur with 3% probability within 30 years.There are two type PSHM of maps: "Average case" and "Maximum case", results from computing probability of occurrence respectively, by using the mean values of average EQ activity interval and the latest event, and by using the smallest value of the mean EQ recurrence interval and the oldest event.
The earthquakes are grouped into two categories in terms of its properties to make the seismic hazard information usable well. PSHM by Earthquake Category indicates probability distribution on the given groups in quartiles(*) to show its relative influence in whole of Japan. (*)Four colors are allocated to represent the four equivalent-quantities of datasets where the whole meshes are sorted based on its exceedance probability with a descending order.
The earthquakes are grouped into three categories in terms of its properties to make the seismic hazard information usable well. PSHM by Earthquake Category indicates probability distribution on the given groups in quartiles(*) to show its relative influence in whole of Japan. (*)Four colors are allocated to represent the four equivalent-quantities of datasets where the whole meshes are sorted based on its exceedance probability with a descending order.
The PSHM Contribution Factor maps display distributions of the most contributive earthquake category, which are based on the exceedance probabilities of each category. The Contribution Factor at each site varies dependent on the ground motion level.
Maps of Conditional Probability of Exceedance (CPE) show the exceeding probabilistic map that ground shaking exceed a certain level of seismic intensity when scenario earthquakes occur. Maps of the expected seismic intensity show the distributions of the average value of seismic intensity when scenario earthquakes occur.
This map offers the geomorphologic classification map in a standard area mesh in whole of Japan, approximately 1km and 250m square. Classifying the geomorphologic attribute, as the higher spatial resolution about 250m square, has improved in the 2009 (and later) version map.
This map shows amplification factor map obtained from the 30m average S-wave velocity(AVS30), which has improved from the 2009 version. The amplification factor means amplified ratio calculated from the engineering bedrock (Vs=400m/s) up to ground surface.
The map shows the 3D deep subsurface structure model down to the engineering bedrock, and to the seismic bedrock, in order to carry out the strong motion simulation. The maps show distribution of elevation and depth of the upper boundary of the each layer by color gradation.
Maps of the PEX (population exposure to seismic intensity) show distribution of population exposed to a certain level of seismic intensity during a scenario earthquake. The maps displayed by "Average" are the PEX maps obtained from an expected IJMA, which shown at the "CPE" tab. The maps displayed by "CASE N" are the PEX maps obtained from an IJMA of the CASE N, which shown at the "SESM" tab. The populations of night time is based on "2005 Population Census". The populations of day time is based on "2006 Population Census, Establishment and Enterprise Census".
Based on the long-term earthquake evaluations for subduction-zone earthquakes that consider variety of seismic source area, Earthquake Category I (Subduction-zone earthquakes with specified seismic source faults) and Earthquake Category II (Subduction-zone earthquakes without specified seismic source faults) are unified to a category "Subduction-zone Earthquakes", and Category III is renamed to a category "Shallow Earthquakes in land area and in sea area" from 2020 version. Please see the following documents (in Japanese, from reports "National Seismic Hazard Maps for Japan" by HERP) for the details of the categories for each map version.
The earthquakes are grouped into three categories in terms of its properties to make the seismic hazard information usable well. Please see the following documents (in Japanese, from reports "National Seismic Hazard Maps for Japan" by HERP) for the details of the categories for each map version.