After the 1995 Hyogoken-Nanbu (Kobe M7.3) earthquake, intensive studies for the active faults in Japan have been carried out especially to the 98 fault zones having potentials of the M7 class or greater earthquakes. And then long-term predictions for these zones have been evaluated on the future large earthquakes. The author briefly reviewed about such items as the active fault maps in urban area on the scale of 1: 25,000 organized by the Geographical Survey Institute, strip maps of the major active faults published from the Geological Survey of Japan, trench excavation surveys, drillings, seismic reflection surveys, segmentation and grouping problems for the long traced active faults, comprehensive atlases and books, long-term evaluation for the major faults (zones), and seismological assessment for the nuclear power plant. Present-day perspectives for the active fault studies in Japan were widely described in this article.
That faults are the origin of earthquakes was not an accepted theory in Japan for several decades prior to 1960, although the progressive accumulation of fault displacement in the Quaternary Time had been demonstrated in seismic areas. The Earthquake Prediction Program proposed by seismologists in 1962 stimulated geologists and geomorphologists to start active fault studies. During the 1960-1970's, the following results were made clear: 1) the distribution of active faults in onshore Japan, shown in 123 sheet maps of 1: 200,000 scale with detailed inventories by the Research Group for the Active Faults of Japan; 2) the extensive occurrence of strike-slip type active faults, almost none of which was known on the Japanese Islands at that time; 3) the existence of Quaternary crustal stress field with east-west compression in most of the Japanese Islands, recognized from the conjugate fault system of the central Japan; 4) the quantitative relation between earthquake magnitude and length of the surface trace of co-seismic fault for the onland Japanese earthquakes, which has been used in Japan to estimate magnitudes of future earthquakes; 5) the very long recurrence intervals of activity of a fault, generally longer than the order of 1000 years. In 1980-1990's, especially after the 1995-Kobe earthquake, excavation studies were performed extensively in more than one hundred active faults in onshore Japan. The active fault data obtained so far made it possible to prepare seismic hazard maps with probabilities of the occurrence of strong seismic motion in a specified period.
The prospectus of the Japanese Society for Active Fault Studies established in September 2007, refers to significance of active fault studies in Japan. Each scientist, engineer and person related to this filed has own views and expectations about future of active fault studies. In this short paper I emphasize my view on the selected fields and their importance for future active fault studies, including comprehensive understanding of active faults, official compilation of detailed active fault maps and inventories, Active Fault Acts for mitigation of seismic hazards, simulation of earthquakes based on active fault data, active fault studies related active folding, submarine active faults and roles of the Japanese Society of Active Fault Studies.
Method and result of offshore active fault survey were reviewed. Acoustic and seismic waves are widely used for offshore topographic and geologic surveys. Higher frequency acoustic waves have high-resolution but attenuate rapidly in water or sediments, thus they are mainly used for seafloor topographic survey or shallow high-resolution seismic profiling survey. Multi-narrow beam sounding provided evolutionary detailed seafloor topographic maps that clearly show fault traces. Lower frequency seismic waves are widely used for survey of deep sea and deep subsurface geology, but their resolution is generally too low to evaluate the activity of faults in late Pleistocene or Holocene time. Multi-channel seismic profiling survey and digital signal processing technology tremendously improved quality of seismic profiles. Offshore active fault maps around Japan were published in 1980's and 1990's based mainly on analyses of single channel seismic profiles. The events of active fault have been identified only in shallow bay areas using high-resolution seismic profiles and sediment cores. In contrast, it is generally difficult to determine events in open sea areas, because of low quality of seismic profiles. Multi-channel seismic profiling system using a high-frequency sound source made it possible to obtain high quality seismic profiles in the open shallow sea area and showed an active fault in the source area of the 2007 Noto-Hanto earthquake. In the deep sea, low-frequency seismic profiling system generally show clear geologic structure including active faults, but it is difficult to determine their activity in the late Pleistocene and Holocene period. Analyses of turbidites and dive surveys using submersibles have been conducted to determine the ancient events of fault activity in the deep-sea area. There is no enough data of offshore active faults, especially in very shallow marine area along coast.
A comparison of frequencies of large earthquakes between the observed and expected on the major active faults in Japan may suggest some underestimation of the occurrence rate of large events by the long-term earthquake forecasts. The main cause was thought to be the forecasts often anticipate a single large earthquake on a long fault zone, without assuming smaller earthquakes on segmented zones. However, the principal factor of the likely underestimation is that those events without enough evidence on fault zones cannot be evaluated by a trench excavation survey. Historical records show poor or no seismic surface rupture associated with a large earthquake with magnitude around 7 on some of the major fault zones. Recent improvements on interpreting aerial photographs have brought a detailed pattern of active faults and fault segments. Together with information on underground structures obtained from geological and gravity anomaly maps, the detailed feature of active faults may enable us to decipher behavioral fault segments without solely relying on paleoseismic records. Except for volcanic areas a short active fault on the surface should have a longer subsurface fault length within the earth's crust. The magnitude of earthquakes on such a fault is 6.9 or larger on JMA scale. This inference is based on reports on surface ruptures associated with large shallow crustal earthquakes in Japan. Only the 1945 Mikawa earthquake with magnitude 6.8 appears to be an exception to that no prominent surface rupture appears associated with earthquakes with magnitude 6.8 or smaller.
Severe damage from the 1995 Hyogoken-Nanbu earthquake gave us a lesson that a cooperation of research of active fault and strong ground motion is very important for the prediction of strong ground motion from inland shallow earthquake. That is an ace of the scientific world for the mitigation of earthquake damage. In this paper we take up two subjects which are important to put the strong motion prediction to the practical use. One is to estimate the magnitude of a future big earthquake from the active fault data and the other is to estimate the upper level of strong ground motions caused by blind faults. Expectations for active fault research to realize a useful strong motion prediction are described.
This article presents the current state of numerical simulation of faulting. There are several physical models of faulting such as continuum modeling or granule material modeling. The resulting mathematical problems are different, and sometimes do not have a unique solution even though the problems are posed for physical phenomena of faulting processes. This is a primary reason of the difficulty of the faulting simulation that must solve such ill-posed mathematical problems. Summarizing the physical models, this article explains several representative works for the faulting simulations. The limitation of the current faulting simulation is discussed, and researches which contribute to improve the faulting simulation are pointed out.
The trace of the surface rupture that appeared in the 1999 ChiChi Earthquake, Taiwan, closely followed the frontal slope of the north-south-trending mountain range. Some foundations for major roads running along the fault have suffered serious damages. When embedded foundations bearing superstructures are forcibly shaken, their top ends can be cracked due to large inertia forces from their superstructures, and with these plastic hinges formed, the entire structure will then experience large soil deformations. The sequence of failures will be thus a key to estimate performances of these structures near active faults. Rehabilitation issues often attract less attention than those in immediate aftermaths of earthquakes, and have never given prominent coverage by news media. However, both, the 1999 ChiChi earthquake, Taiwan, and the 2005 Kashmir earthquake, Pakistan, formed a great number of debris sources along their activated faults. Heavy rains in the monsoon of 2006 that followed the Kashmir earthquake were responsible for raising river beds by several meters. To cope with all these problems, much is expected on our shoulders.
We first briefly review the Japanese seismic building code, and show that the current code does not include explicitly counter measures against active faults. In addition, it does not include the information about the active faults and the near-fault strong ground motions in its hazard map and response spectra. Next, we explain the near-fault strong ground motions, such as the directivity pulses and the fling step. We also show various examples of building damage caused by the 1995 Kobe earthquake and the 1999 Chi-Chi, Taiwan, earthquakes. The directivity pulses overturned a lot of old wooden houses in the direction normal to the active faults during the Kobe earthquake, whereas the fling step and ground failure due to the surface faulting tilted many buildings on the fault during the Chi-Chi earthquake. Finally, we discuss about various technologies to cope with those the near-fault strong ground motions and the ground failure, and emphasize the importance of the collaborations among geologists, seismologists, and engineers for mitigation of building damage.
Japan entered a longstanding depopulation process in 2006. The total population after 50 years is expected to decrease to about 70% of the current one. In this situation, it is important to avoid social impact due to disasters by guiding population from vulnerable area to safer area. This research focused the risk of active faults among various kinds of natural hazards and studied on the effect of land use control by active fault zoning in Japan. First, the meaning of land use control along active faults in the society whose population started to decrease was discussed. If the residences in the seismic vulnerable area are relocated to the safer area by the land use control plan, these vacant lands could be effectively used for disaster-prevention facilities. So far, Japan has not adopted the earthquake fault zoning act due to several reasons, while it has been carried out for 30 years in U. S.. However, considering that more lands will become vacant due to depopulation in the future, the possibility of introducing fault zoning act will increase and the discussion on active fault zoning will become more meaningful. Then, the distribution of population and buildings in the neighborhood of the active faults was analyzed based on GIS databases of the active faults, population and building stocks. The fault zones were hypothetically set along the active faults. In case of the fault zone whose width was decided to be 0.4km referring to the fault zoning act in U. S., the population inside the fault zone was 2.89 million and it corresponded to 2.3% of the total population in Japan. Half of the population living along the faults was located in Kinki area and the effect of land use control was different according to the region. The population inside the fault zone was increased in proportion to the width of the zone. Finally, the possibility of active fault zoning in Japan was discussed. Based on the obtained results, we think that it is meaningful and possible to adopt active fault zoning in Japan for earthquake damage reduction. However, the appropriate width of fault zones should be discussed considering all the factors such as lessons learnt from past earthquake damage, uncertainty of the traces of both strike-slip and dip-slip fault, social impact of the zones.
Lifelines such as electric power and communication systems, gas supply system, water and wastewater systems, highways and railways are important utilities and transportation systems for recent urban cities. The focus of this report is on the past earthquake damage to lifeline, the characteristics of lifeline damage and the issues associated with the disaster mitigation. Outlines of the active fault-induced damage, seismic ground motion, damage to infrastructures were described. Although the steady progress has been made to develop the disaster mitigation program for lifelines, new strategies for coping with catastrophic losses should be developed. The necessity of lifeline risk management is also mentioned in this report.
Earthquake insurance systems for residences and household goods were established in 1966, with the Niigata Earthquake in 1964 as the turning point. Losses to be covered in earthquake insurance are losses arising concerning the object insured due to fire, destruction, burial or flood directly or indirectly caused by earthquake, volcanic eruption or tsunami, and, moreover, the degree of loss is total loss, half loss or partial loss. The government undertakes reinsurance contracts of earthquake insurance. Therefore earthquake insurance that is operated jointly by the non-life insurance companies and the government is the insurance to have high publicity. on the tax system, deduction for non-life insurance premiums was modified, and deduction for earthquake insurance premiums was newly introduced in January 2007. There are two types of earthquake insurance in Japan--one for residences and the other for offices and factories, etc. --and this paper deals with the former. Premium rates for earthquake insurance are stipulated as“ standard rates” in the Law Concerning Non-Life Insurance Rating Organizations (Law No.193 of 1948). The standard rates are calculated by non-life insurance rating organizations, filed with the Financial Services Agency under the Rating Organization Law and determined through such procedures as assessment. A premium rate for earthquake insurance is composed of a risk premium rate and a loading rate. Risk premium rates are to be appropriated to the insurance claims to be paid arising from earthquake disasters, etc., which will occur in the future. Loading rates are to be appropriated to the expenses of operating the earthquake insurance, and the paperwork cost of insurance policies, adjustment costs at the time of payment of insurance claims, and the commissions to be paid to agencies. Even in Japan, which is recognized as the most earthquake prone country in the world, the number of occurrence of earthquake disasters is very small, compared with other disasters. Therefore, the“ law of large numbers”, the basics of rating method are not applicable to earthquake risks. So, in earthquake insurance rating, the risk premium rate of earthquake insurance is calculated with estimation of earthquake loss. In earthquake loss estimation, the data regarding earthquake sources used in probabilistic seismic hazard map made by the Headquarters for Earthquake Research Promotion were used from the revision in 2007. In this paper, we explain the summary of the current earthquake insurance system and earthquake insurance rate at first. Then we report it about statistics of earthquake insurance, difficulties of making seismic risk insurable, rating method, and estimation of earthquake loss for rating. Finally we report it about expectation for the future active fault research from the viewpoint of rate calculation of earthquake insurance.
“ Active fault” is a technical term used in the field of geosciences, but it has become familiar among the Japanese society since the January 17,1995 Kobe earthquake, which attacked the southern part of Hyogo Prefecture, resulting in a terrible disaster, because the seismogenetic fault for that earthquake just appeared on the already mapped active fault. The decade after the Kobe earthquake can be defined as“ the time when active fault has commonly has been accepted and its significance has been generally understood”. One of important concept is that the active faults have been repeatedly occurred on the same fault traces, and then those traces will be active in future. Thus we need to know the nature, magnitude and frequency of faulting for the understanding of future earthquakes as the fundamental data to reduce the possible disaster. In this paper I review the active fault research introduced by mass media and mention what kind of information on active faults are necessary for confirming the safety of critical facilities or safe town planning. In 2007, Japanese Society of Active Fault Studies was established, as the first academic society of active fault studies in the world. I expect that this society can contribute for disaster mitigation, urban design, social infrastructures, and safety for nuclear power plant, based on accurate scientific data.