Matsushiro Earthquake Swarm Research Articles

The 2024 Mj 7.6 Noto Peninsula, Japan earthquake caused by the fluid flow in the crust

On January 1, 2024 ​at 16:10:09 JST, an Mj 7.6 earthquake struck the Noto Peninsula in the southern part of the Sea of Japan. This location has been experiencing an earthquake swarm for more than three years. Here, we provide an overview of this earthquake, focusing on the slip distribution of the mainshock and its relationship with the preceding swarm. We also reexamined the source areas of other large earthquakes that occurred around the Sea of Japan in the past and compared them with the Matsushiro earthquake swarm in central Japan from 1964 to 1968. The difference between the Matsushiro earthquake swarm and the Noto earthquake swarm is the surrounding stress field. The Matsushiro earthquake swarm was a strike-slip stress field, so the cracks in the crust were oriented vertically. This allowed fluids seeped from the depths to rise and flow out to the surface. On the other hand, the Noto area was a reverse fault stress field. Therefore, the cracks in the earth's crust were oriented horizontally. Fluids flowing underground in deep areas could not rise and spread over a wide area in the horizontal plane. This may have caused a large amount of fluid to accumulate underground, triggering a large earthquake. Although our proposed mechanism does not take into account other complex geological conditions into consideration, it may provide a simple way to explain why the Noto swarm is followed by a large earthquake while other swarms are not.

Causal mechanisms of seismo-EM phenomena during the 1965\u20131967 Matsushiro earthquake swarm

The 1965–1967 Matsushiro earthquake swarm in central Japan exhibited two unique characteristics. The first was a hydro-mechanical crust rupture resulting from degassing, volume expansion of CO2/water, and a crack opening within the critically stressed crust under a strike-slip stress. The other was, despite the lower total seismic energy, the occurrence of complexed seismo-electromagnetic (seismo-EM) phenomena of the geomagnetic intensity increase, unusual earthquake lights (EQLs) and atmospheric electric field (AEF) variations. Although the basic rupture process of this swarm of earthquakes is reasonably understood in terms of hydro-mechanical crust rupture, the associated seismo-EM processes remain largely unexplained. Here, we describe a series of seismo-EM mechanisms involved in the hydro-mechanical rupture process, as observed by coupling the electric interaction of rock rupture with CO2 gas and the dielectric-barrier discharge of the modelled fields in laboratory experiments. We found that CO2 gases passing through the newly created fracture surface of the rock were electrified to generate pressure-impressed current/electric dipoles, which could induce a magnetic field following Biot-Savart’s law, decrease the atmospheric electric field and generate dielectric-barrier discharge lightning affected by the coupling effect between the seismic and meteorological activities.

Mid-crustal fluid related to the Matsushiro earthquake swarm (1965–1967) in northern Central Japan: Geochemical reproduction

The Matsushiro district in the northern part of Ngano Prefecture, Central Japan, experienced a severe earthquake swarm associated with ground uplift and groundwater outflow during a period from 1965 to 1967. Geochemistry and isotope geochemistry of surface water and groundwater in the swarm area have been investigated. Water samples for the analyses were obtained from saline springs and streams, fresh water from streams and a shallow well, and, from seven wells of up to 600m deep drilled in the Matsushiro fault zone. The results of analyses of water from the deep wells in the fault zone indicate a linear increase in dissolved Na, K, Al, Cl, Br, B and SiO4, and isotopic ratios of δD, δ18O and δ13C. The hydrogen and oxygen isotopic ratios of the well water indicate a mixing between the surface water and isotopically heavy water exemplified by the “andesitic water” of Giggenbach (1992). Assuming that the isotopically heavy water has δD similar to the “andesitic water”, the geochemistry of deep-seated brine is reproduced from the analytical data of well water. The reproduced composition of deep-seated brine is enriched in Na, Ca, Cl and HCO3 and has total dissolved solids comparable to seawater. The water of the reproduced composition is estimated to occupy a deep part (>3km below surface) of the fault zone. Available data show that the geochemical regime in the Matsushiro fault zone has remained unchanged since the swarm event. The reproduced composition of water is concluded to be similar to the brine triggering the swarm of earthquakes.The composition of the reproduced water is not as saline as the deep-seated fluid derived from dehydration of a subducting oceanic plate which results in water having a Cl content about twice that of the seawater. The relatively low-salinity nature comparable to seawater is one of the characteristics of mid-crustal fluid in volcanic fields.

Chapter 18 Tectonic and volcanic deformation at São Miguel Island, Azores, observed by continuous GPS analysis 2008–13

Abstract We use a Global Positioning System (GPS) to unravel the complex geodynamics of the Azores Triple Junction where tectonic and volcanic activities coexist. The temporal analysis of densely distributed continuous GPS data on São Miguel for the period 2008–13 provides an improved understanding of interactions between present-day plate boundary kinematics and volcanic deformation. We find a high-strain-rate (0.28 ppm a –1 ) zone between Congro and Furnas, which accommodates about 50% of the Eurasian–Nubian plate spreading as predicted by the MORVEL plate angular velocity model. The seismic unrest of Fogo–Congro (2011–12) shows a strong similarity with the Matsushiro (Japan) earthquake swarm (1965–66) and the Campi Flegrei (Italy) volcanic unrest (1969–72 and 1982–85), in that an edifice-scale inflation associated with intense high-frequency earthquakes and inflation–deflation reversals coincided with a sharp drop in seismicity. We propose the following hypothesis for the Fogo unrest: (1) the primary inflation source beneath Fogo promotes lateral diffusion of fluids that is selectively guided by existing cracks/fissures formed from regional extension; (2) an influx of fluids increases pressure in cracks/fissures and generates lower-frequency earthquakes; and (3) discharge of fluids causes pressure decrease and dilatancy recovery (i.e. seismic quiescence).

松代群発地震の初期発展過程

Matsushiro earthquake swarm is a most well known and well studied earthquake swarm in the history of seismology. From viewpoints of geophysics, geology and geochemistry, various observations, field surveys and analyses had been made to reveal characteristics of the swarm. On the basis of these studies, several models had been proposed to explain why and how the swarm originated and developed. However, majority of studies had focused on the activity around the climatic stage of the swarm in 1966 and so far, studies on the initial stage of the swarm in mid 1965 had remained few due to lack of sufficient observation data. The exact area where the swarm was born had not been known and the swarm was vaguely believed to originate from the area around Mt. Minakami in former Matsushiro town (now belongs to Nagano city). Considering that the manner of initial development of swarm activity represents an important characteristic of the swarm earthquake, we tried to get more clear view about initial stage of the swarm activity in this study. We re-investigated seismograms obtained by routine observation and studied seismograms of temporal stations for the first time which had not been processed yet. By scanning analogue seismograms, we made a complete data set of S-P times with high precision for each station. Although the data set of S-P times from two stations are not sufficient for conventional hypocenter location, we were able to narrow down a possible source area of the swarm activity under the reasonable assumptions. By considering direction of initial motion of P waves and assuming local velocity model of the upper crust in the region and plausible focal depth of 4.5km of the swarm earthquake, we found that the area of swarm in the very beginning in August, 1965 is located about 4km north-east of the Matsushiro earthquake fault (MEF) in former Wakaho town near its border with former Matsushiro town. Size of the initial swarm area was 3-4km in diameter. While swarm activity in the initial region was gradually decaying in September new swarm activity appeared separately from the initial swarm area around southern and south-west part of the MEF. Activity in the new swarm area had been increasing and it was developed to more intense swarm after October, 1965 when establishment of temporal seismic station network of the Earthquake Research Institute, University of Tokyo enabled detailed hypocenter location. The new swarm area coincides with the area where large amount of ground water moved upward and was released on the ground surface in the climatic stage of the swarm. It was well known that source area of the swarm was split and expanded toward north-east and south-west after March, 1967 in its climatic stage. Present study on the initial development of swarm area suggested that characteristics of the Matsushiro earthquake swarm such as splitting and expansion of its source area toward northeast and southwest were inherent in their early stage of the activity in August and September, 1965.

Oil the possible relationship between the Matsushiro earthquake swarm and the inactivity of Asama-yama Volcano

During the famous Matsushiro earthquake swarm more t h a n 700 000 (volcanic) earthquakes were observed. At (he same time remarkable contortion of the ground also took place together with other strange geophysical phenomena. In the course of these events, volcano Asama-yama (one of the most active volcanoes of the world) showed a perfect inactivity. This volcano is situated at a distance of about 30 km measured from Matsushiro, that is very near the site of the events. In the paper a causal relationship is suggested between the geophysical phenomena at .Matsushiro and the inactivity of Asama-yama. Two alternative possibilites are t r e a t e d briefly. The hypothetical character of these ideas is strongly emphasized by the author, however both possibilities appear to be physically real and can explain all the important geophysical events observed on the spot.

Status of the TOUGH-FLAC simulator and recent applications related to coupled fluid flow and crustal deformations

This paper presents recent advancement in and applications of TOUGH-FLAC, a simulator for multiphase fluid flow and geomechanics. The TOUGH-FLAC simulator links the TOUGH family multiphase fluid and heat transport codes with the commercial FLAC 3D geomechanical simulator. The most significant new TOUGH-FLAC development in the past few years is a revised architecture, enabling a more rigorous and tight coupling procedure with improved computational efficiency. The applications presented in this paper are related to modeling of crustal deformations caused by deep underground fluid movements and pressure changes as a result of both industrial activities (the In Salah CO 2 Storage Project and the Geysers Geothermal Field) and natural events (the 1960s Matsushiro Earthquake Swarm). Finally, the paper provides some perspectives on the future of TOUGH-FLAC in light of its applicability to practical problems and the need for high-performance computing capabilities for field-scale problems, such as industrial-scale CO 2 storage and enhanced geothermal systems. It is concluded that despite some limitations to fully adapting a commercial code such as FLAC 3D for some specialized research and computational needs, TOUGH-FLAC is likely to remain a pragmatic simulation approach, with an increasing number of users in both academia and industry.

Modeling crustal deformation and rupture processes related to upwelling of deep CO2‐rich fluids during the 1965–1967 Matsushiro earthquake swarm in Japan

In Matsushiro, central Japan, a series of more than 700,000 earthquakes occurred over a 2‐year period (1965–1967) associated with a strike‐slip faulting sequence. This swarm of earthquakes resulted in ground surface deformations, cracking of the topsoil, and enhanced spring outflows with changes in chemical compositions, as well as carbon dioxide (CO2) degassing. Previous investigations of the Matsushiro earthquake swarm have suggested that migration of underground water and/or magma may have had a strong influence on the swarm activity. In this study, employing coupled multiphase flow and geomechanical modeling, we show that observed crustal deformations and seismicity could have been driven by upwelling of deep CO2‐rich fluids around the intersection of two fault zones: the regional east Nagano earthquake fault and the conjugate Matsushiro fault. We show that the observed spatial evolution of seismicity along the two faults and magnitudes surface uplift are convincingly explained by a few megapascals of pressurization from the upwelling fluid within the critically stressed crust, a crust under a strike‐slip stress regime near the frictional strength limit. Our analysis indicates that the most important cause for triggering of seismicity during the Matsushiro swarm was the fluid pressurization with the associated reduction in effective stress and strength in fault segments that were initially near critically stressed for shear failure. Moreover, our analysis indicates that a 2‐order‐of‐magnitude permeability enhancement in ruptured fault segments may be necessary to match the observed time evolution of surface uplift. We conclude that our hydromechanical modeling study of the Matsushiro earthquake swarm shows a clear connection between earthquake rupture, deformation, stress, and permeability changes, as well as large‐scale fluid flow related to degassing of CO2 in the shallow seismogenic crust. Thus our study provides further evidence of the important role of deep fluid sources in earthquake fault dynamics and surface deformations.

Hydrogeochemical Modeling for Natural Analogue Study of CO2 Leakage due to Matsushiro Earthquake Swarm

Abstract For natural analogue study of CO2 leakage, the Matsushiro field in central Japan was selected to investigate hydrogeochemical behavior due to CO2 leakage from a fault zone during the period of the Matsushiro Earthquake Swarm between 1965 and 1967. One of the main objectives of the modeling study is to understand effects of hydrogeological characteristics of shallow aquifers and seal mechanism due to mineral precipitation in a fault zone. Reactive geochemical transport simulations were carried out using TOUGHREACT. Based on the historical data including discharge rate, chemical composition of water and seismic activity, we developed a numerical model to reproduce the geochemical behavior during and after the swarm earthquakes. A two-dimensional model (2000 m×1800 m) for the fault zone using the Multiple Interacting Continua (MINC) method was set up to evaluate geochemical effects during degassing and mixing of brine upwelling along the fault zone. The simulation results can capture the observed trend at the Matsushiro field. Supercritical and gaseous CO2 occur in the deep and the shallow fracture zones, respectively. During the earthquake swarm, CO2 saturated brine moves up along the fracture zone and CO2 degassing occurs in the shallow zone. Subsequently, water pH increases and a small amount of calcite precipitates. Lateral groundwater flow in shallow aquifers, which transports the CO2 away from the fault zone, makes CO2 surface discharge small. At the same time, calcite dissolves after the swarm due to lateral fresh water recharge. The result is consistent with the fact that no calcite observed in the core from the 80 m borehole drilled during the course of this study, indicating that CO2 discharge is likely affected by lateral shallow groundwater flow. Measurements of surface CO2 flux and soil gas δ 13 C indicated that little seepage of CO2 occurs in the area of the fan deposits at present, 40 years after the earthquake swarm. Using the same model, long term leakage simulations (if supercritical CO2 leaked along fault zone from the storage reservoir) were carried out to test whether CO2 surface leakage occurs or not. Two simulations were performed, with CO2 upflow rates of 50,000 and 100,000 kg/year in an area of 25 m×25 m. CO2 surface leakage does not occur in the first case, while CO2 leakage occurs in the second case. CO2 surface leakage is reduced because CO2 dissolves in groundwater flowing laterally in shallow aquifer. These simulations indicate that it is important to understand hydrogeological characteristics of aquifers underlain by reservoir as well as cap rocks.

ＣＯ２地中貯留のナチュラルアナログ―松代での地化学調査―

Environmental assessments and safety control during and after CO2 injection are essential for CO2 geological storage, and we are required to evaluate long-term environmental changes and safety. However, long-term changes are difficult to detect directly because the leakage of CO2 is expected to be small and the evaluation is, sometimes, requested to cover more than 1,000 years. To solve this problem, a natural-analogue study, which inquires into environmental changes at present through a comparison with past geological phenomena, is one possible approach. When the Matsushiro earthquake swarm began in 1965, a large volume of subsurface water accompanied by CO2 gas was discharged along fracture zones. A natural-analogue study on the CO2 discharge during the earthquake swarm should be helpful to create a scenario of leakage and a guideline for the safety of CO2 geological storage. Surveys of the CO2 content in soil gas and CO2 flux emissions from the surface were carried out with carbon isotope ratio measurements to understand the current state at Matsushiro, and to make a conceptual model for environmental assessments and safety control. From geological and geophysical points of view, it is said that deep water gushing out from the surface caused the swarm of earthquakes. As this deep water is still gushing out, we planned to measure CO2 concentrations in soil gas and CO2 flux to examine present CO2 activities at Matsushiro. Because CO2 in the soil is also produced by activities of microbes, however, we decided to measure the isotope ratio of the carbon to distinguish CO2 in deep groundwater origin from that produced by microbes. We selected five survey lines and three survey areas based on previous geochemical measurements and fissure distribution during the earthquake swarm, and measured CO2 concentration in soil, CO2 flux, and isotope ratio. Although there were survey points on the thick fan deposit where CO2 concentration in the soil and CO2 flux were high, the isotope ratio indicated that the carbon is produced by the activity of microbes. On the other hand, the isotope ratio of the samples collected from the thin fan deposit area shows deep subsurface water as the origin. An investigation well was drilled into the basement. Subsurface water samples were collected near the bottom of the well in the igneous rock formation. Geochemical analyses and carbon isotope ratio measurements show higher concentrations of chloride and abiogenic CO2, indicating that groundwater of a deep origin with CO2 is still rising. We are now making a conceptual model of hydrogeological history at the next step. This natural analogue study of CO2 seepage could indicate the importance of understanding shallow hydrogeological characteristics in a CO2 storage field.

A Point-process Analysis of the Matsushiro Earthquake Swarm Sequence: The Effect of Water on Earthquake Occurrence

Temporal characteristics of the famous Matsushiro earthquake swarm were investigated quantitatively using point-process analysis. Analysis of the earthquake occurrence rate revealed not only the precise and interesting process of the swarm, but also the relation between pore water pressure and the strength of the epidemic effect, and the modified Omori-type temporal decay of earthquake activity. The occurrence rate function λ(t) for this swarm is represented well as where f(t) represents the contribution of the swarm driver, which was the erupting water from the deep in this case, and the second term represents an epidemic effect of the modified Omori type. Based on changes in the form of f(t), this two-year long swarm was divided into six periods and one short transitional epoch. The form of f(t) in each period revealed the detail of the water erupting process. In the final stage, f (t) decayed according to the modified Omori-formula form, while it decayed exponentially in the brief respite of the water eruption in the fourth period. When an exponential decay of swarm activity is observed, we have to be cautious of a sudden restart of the violent activity. The epidemic effect is stronger when the pressure of the pore water is higher. Even when the pressure is not high, the p value in the epidemic effect is small, when there is plenty of pore water. However, the epidemic effect produced about a quarter of the earthquakes even though there was not much pore water in the rocks.

The late Professor Takahiro Hagiwara: His career with earthquake prediction

Takahiro Hagiwara, Professor Emeritus of the University of Tokyo, was born in 1908, and passed away in 1999. His name is inseparably tied with earthquake prediction, especially as the founder of the earthquake prediction program of Japan, and as a distinguished leader of earthquake prediction research in the world. This short article describes the career of Prof. Hagiwara focusing on his contribution to earthquake prediction research. I also sketch his activities in the development of instruments, and the multi-disciplinary observation of the Matsushiro earthquake swarm to show the starting point of his scientific strategy: good observation.

同位体比から見た松代群発地震地域の深部流体の起源

同位体比から見た松代群発地震地域の深部流体の起源

松代群発地震地域に湧出する深層地下水

松代群発地震地域に湧出する深層地下水

Topographic anisotropy of slickenside from deep borehole sample in an earthquake swarm region and its generation mechanism : Y. Kuwahara, N. Oyagi & H. Takahashi, Journal of Physics of the Earth, 41(2), 1993, pp 75–85

A slickenside was discovered from a boring core sample 1, 880m deep in the Matsushiro earthquake swarm region in Nagano Prefecture, Japan. The slickenside has been considered to be created by earthquake faulting during the earthquake swarm period, because it accompanied fresh rock powder produced by earthquake faulting. The power spectral density of the topography of the slickenside was measured with a stylus profilometer over the wavelength range from 10-4 to 10-2m. Each profile of the surface has a red noise power spectrum over the wavelength range studied, with the power falling off on average between 2 and 3 orders of magnitude per decade decrease in wavelength. Meanwhile, the topography of the slickenside is found to show anisotropy in the spectral amplitude; the amplitude perpendicular to the slip direction is two to four times as large as that parallel to the slip direction. A generation mechanism for such topographic anisotropy is numerically modeled from a viewpoint of the fracture mechanics. It is supposed that the crack tip stress distribution in the plane parallel to the slip direction is represented by a model of a plane strain shear crack (Mode II crack) and that the stress distribution in the plane perpendicular to the slip direction is represented by an antiplane strain shear crack (Mode III crack). The stress field around the crack tip in the Mode III case shows a broader distribution with respect to the azimuth than that in the Mode II case. This difference of a stress field at the crack tip between the Mode II and the Mode III is a possible cause of the topographic anisotropy.

Topographic Anisotropy of Slickenside from Deep Borehole Sample in an Earthquake Swarm Region and Its Generation Mechanism.

A slickenside was discovered from a boring core sample 1, 880m deep in the Matsushiro earthquake swarm region in Nagano Prefecture, Japan. The slickenside has been considered to be created by earthquake faulting during the earthquake swarm period, because it accompanied fresh rock powder produced by earthquake faulting. The power spectral density of the topography of the slickenside was measured with a stylus profilometer over the wavelength range from 10-4 to 10-2m. Each profile of the surface has a red noise power spectrum over the wavelength range studied, with the power falling off on average between 2 and 3 orders of magnitude per decade decrease in wavelength. Meanwhile, the topography of the slickenside is found to show anisotropy in the spectral amplitude; the amplitude perpendicular to the slip direction is two to four times as large as that parallel to the slip direction. A generation mechanism for such topographic anisotropy is numerically modeled from a viewpoint of the fracture mechanics. It is supposed that the crack tip stress distribution in the plane parallel to the slip direction is represented by a model of a plane strain shear crack (Mode II crack) and that the stress distribution in the plane perpendicular to the slip direction is represented by an antiplane strain shear crack (Mode III crack). The stress field around the crack tip in the Mode III case shows a broader distribution with respect to the azimuth than that in the Mode II case. This difference of a stress field at the crack tip between the Mode II and the Mode III is a possible cause of the topographic anisotropy.

The mechanism of the occurrence of the Matsushiro earthquake swarm in central Japan and its relation to the 1964 Niigata earthquake

The frequency curve of the Matsushiro (central Japan) earthquake swarm which began in August 1964 is analyzed from the point of view that earthquakes are fracturing events in the earth's crust. This analysis is based on fracture experiments in the laboratory and estimations of the temporal variation in stress in the focal region of the earthquake swarm. The change in the estimated stress is attributed to pore-water pressure in the earth's crust, which was caused by the upward intrusion of ground water from depth. Just after the 3rd peak of seismic activity, the estimated stress (the pore pressure) decreased markedly. This occurred simultaneously with the outflow of an enormous volume of ground water from a wide area around the focal region of the earthquake swarm. This result supports the water eruption model which was first proposed by Nakamura (1971). A hypothesis is proposed that the Matsushiro earthquake swarm was triggered by the 1964 Niigata earthquake of M = 7.5. This earthquake occurred in an adjacent region along the same active tectonic belt one year before the Matsushiro earthquake swarm. This is strongly supported by the space-time distribution of earthquakes which have occurred in the Niigata-Matsushiro region.

地震発生における間げき流体圧の役割

The essential role of interstitial fluid pressure (pore pressure) in earthquake occurrence is strongly suggested by the observational fact that seismic activity was induced by water injection into a deep well and water impounding of an artificial reservoir. Elaborate studies related to water injection revealed following important characteristics of the induced seismicity : (1) Earthquakes are triggered to occur when the interstitial fluid pressure of the basement rock exceeds some threshold level ; (2) Focal mechanisms of induced earthquakes are in good harmony with the regional stress field ; and (3) Seismic activity propagates as far as several kilometers from the injection well. Based on those observational facts, we conclude that the injected water raising the interstitial fluid pressure releases the tectonic stress naturally accumulated. The effect of pressure change on the fracture strength of rock S is formulated in terms of the effective stress hypothesis asS=τ0+ (σn-P0) tanφwhere σn and P0 are normal stress and interstitial fluid pressure, and τ0 and φ are the coefficient of cohesive strength and internal friction angle, respectively. This criterion of rock fracture may also be applied to account for the reservoir induced seismicity. For the case of reservoir induced seismicity, however, the loading effect of water mass should be taken into consideration since some of the artificial reservoirs exhibit induced aseismicity.The effect of interstitial fluid pressure seems to be essential even for occurrence of natural earthquakes. Detailed studies on the Matsushiro earthquake swarm revealed that the earthquake swarm was brought about by the “water eruption”; high pressure water supplied at the depth beneath Matsushiro erupted to the ground surface accompanying a large number of small earthquakes which were generated by the increase of interstitial fluid pressure. Remarkable upheaval of the focal area, gushing out of large amount of water, and other associated phenomena are consistently interpreted by the water eruption model. It will be of great value for deeply understanding the nature of swarm activity to investigate whether such a mechanism is common or not to other earthquake swarms.The dilatancy-diffusion model of earthquake occurrence is briefly discussed from the viewpoint of the effect of interstitial fluid pressure. The model is a fascinating product of the effective stress hypothesis and results of laboratory experiments of rock fracture. It, however, is still dubious if the results obtained in the laboratory can directly be applied to the earthquake phenomenon occurring in the natural environment. We, therefore, emphasize the especial importance of advanced studies on induced earthquakes which occur under the half-natural and half-controlled circumstances.

Approximate method to estimate the short-period strong motion spectra on bedrock

abstractThe methodology to estimate the strong motion Fourier amplitude spectra in a short-period range (T ≦ 1 to 2 sec) on a bedrock level is discussed in this paper. The basic idea is that the synthetic strong motion Fourier spectrum F˜A(ω) calculated from smoothed rupture velocity model (Savage, 1972) is approximately similar to that of low-pass-filtered strong earthquake ground motion at a site in a period range T ≧ 1 to 2 sec: F˜A(ω)=B˜(ω)·A(ω). B˜(ω) is an observed Fourier spectrum on a bedrock level and A(ω) is a low-pass filter. As a low-pass filter, the following relation,A ( T ) = · a · T n a T n + 1 , ( T = 2 π / ω ) ,is assumed. In order to estimate the characteristic coefficients {n} and {a}, the Tokachi-Oki earthquake (1968), the Parkfield earthquake (1966), and the Matsushiro earthquake swarm (1966) were analyzed. The results obtained indicate that: (1) the coefficient {n} is nearly two for three earthquakes, and {a} is nearly one for the Tokachi-Oki earthquake, eight for the Parkfield earthquake, and four for the Matsushiro earthquake swarm, respectively; (2) the coefficient {a} is related with stress drop Δσ as (a = 0.07.Δσ). Using this relationship between {a} and Δσ, the coefficients {a} of past large earthquakes were estimated.The Fourier amplitude spectra on a bedrock level are also estimated using an inverse filtering method of A ( T ) = a T 2 a T 2 + 1 .

A statistical model for prediction of quasirealistic strong ground motion.

The total power, the duration, and the starting time of band-pass filtered accelerograms are chosen as model parameters to express amplitude variations of ground acceleration in both the time and frequency domains. In order to estimate model parameters of certain earthquakes and site conditions, a probabilistic source model is used and accelerograms recorded during the Matsushiro earthquake swarm are analyzed statistically. The duration and the starting time of the strong ground motion are found to have close relations to the amplification characteristics of surface layers at the sites. As an example, model parameters are estimated by assuming the source parameters and the site conditions of the Miyagi-Oki earthquake of June 12, 1978. Wave-forms, running spectra, and velocity response spectra of synthetic accelerograms resemble those of observed accelerograms.