Japan Meteorological Agency Magnitude Research Articles

Geological features of landslides caused by the 2018 Hokkaido Eastern Iburi Earthquake in Japan

Abstract Before dawn on 6 September 2018, a powerful earthquake with a Japan Meteorological Agency magnitude ( M j ) of 6.7 hit central Hokkaido, causing more than 6000 landslides. As human damage, 36 of the 44 fatalities from the earthquake were from earthslides in Atsuma Town. Most slope movements were shallow earthslides of mantle-bedded tephra and soil layers, but some were deep rockslides involving basement rocks such as shale and mudstone of Miocene. Although the shallow earthslides were easily distinguishable in photos from satellites or airplane, the rockslides were more difficult to identify owing to their vegetation. Based on the quick interpretation of a high-resolution (0.5 m horizontal resolution) shaded relief map made from digital elevation model data by airborne laser survey and supplemental field surveys, we effectively identified 259 rockslides (196 certain ones and 63 possible ones) in Atsuma and Mukawa towns under newly invented identification criteria based on the scarp depth and positional relation between scarp and ridge topography. It was revealed that most rockslides were distributed within 10 km from the epicentre, while earthslides were distributed until 20 km north of the epicentre, and they seemed to be controlled by the thickness of mantle-bedded tephra and the soil layer. We also identified many traces of past rockslides and earthslides. The results show the possibilities for effective measurement of slope by clarifying the landslide distribution both this earthquake and past ones using high-resolution digital elevation model data.

Underestimation of Microearthquake Size by the Magnitude Scale of the Japan Meteorological Agency: Influence on Earthquake Statistics

AbstractMagnitude scales based on the amplitude of seismic waves, including the Japan Meteorological Agency magnitude scale (Mj), are commonly used in routine processes. The moment magnitude scale (Mw), however, is more physics based and is able to evaluate any type and size of earthquake. This paper addresses the relation between Mj and Mw for microearthquakes. The relative moment magnitudes among earthquakes are well constrained by multiple spectral ratio analyses. The results for the events in the Fukushima Hamadori and northern Ibaraki prefecture areas of Japan imply that Mj is significantly and systematically smaller than Mw for microearthquakes. The Mj‐Mw curve has slopes of 1/2 and 1 for small and large values of Mj, respectively; for example, Mj = 1.0 corresponds to Mw = 2.0. A simple numerical simulation implies that this is due to anelastic attenuation and the recording using a finite sampling interval. The underestimation affects earthquake statistics. The completeness magnitude, Mc, for magnitudes lower than which the magnitude‐frequency distribution deviates from the Gutenberg‐Richter law, is effectively lower for Mw than that for Mj, by taking into account the systematic difference between Mj and Mw. The b values of the Gutenberg‐Richter law are larger for Mw than for Mj. As the b values for Mj and Mw are well correlated, qualitative argument using b values is not affected. While the estimated b values for Mj are below 1.5, those for Mw often exceed 1.5. This may affect the physical implication of the seismicity.

A new approach for estimating seismic damage of buried water supply pipelines

SummaryConventional damage prediction methods for lifeline structures are primarily based on peak ground motion measurements. However, line structures such as lifelines suffer damage that is mainly induced by the strain of the ground and therefore are likely to be vulnerable to sharp spatial changes in the ground motion. In this study, we propose a measure for evaluating the damage incurred by underground water supply pipelines based on the spatial gradient of the peak ground velocity (PGV), in an attempt to quantify the effects of the geospatial variabilities in the ground motion on pipeline damage. We investigated the spatial distribution of the damage caused to water pipelines during the Niigata‐ken Chuetsu earthquake on October 10, 2004 (Japan Meteorological Agency magnitude (MJMA) of 6.8) and the Kobe earthquake on January 17, 1995 (MJMA7.3) and compared the surveyed damage with the PGV distribution as well as with the gradients of the PGV calculated around the damage areas. For the Kobe earthquake, we used the PGV distribution obtained by the strong‐motion simulation performed by Matsushima and Kawase . In case of the Chuetsu earthquake, we estimated the ground motion using a broadband‐frequency‐based strong‐ground‐motion simulation method based on a multiasperity source model. In both cases, we calculated the gradients of the PGV along the geographical coordinates, with the amplitude of the PGV gradient vector being employed as the damage estimator. Our results show that the distribution of damage to underground water supply pipelines exhibits a greater correlation with the gradients of the PGV than with the PGV itself. Thus, the gradient of the PGV is a useful index for preparing initial‐screening hazard maps of underground facilities. Copyright © 2017 John Wiley & Sons, Ltd.

Characteristics of the surface ruptures associated with the 2016 Kumamoto earthquake sequence, central Kyushu, Japan

The 2016 Kumamoto earthquake sequence started with a M J (Japan Meteorological Agency magnitude) 6.5 event on April 14, and culminated in a M J 7.3 event on April 16. Associated with the sequence, approximately 34-km-long surface ruptures appeared along the eastern part of the Futagawa fault zone and the northernmost part of the Hinagu fault zone. We carried out an urgent field investigation soon after the earthquake to map the extent and displacement of surface ruptures with the following results. (1) The rupture zone generally consisted of a series of left-stepping en echelon arrays of discontinuous fault traces of various lengths. (2) Slip exceeding 100 cm occurred on previously unrecognized fault traces in the alluvial lowland of the Kiyama plain and on the western rim of the Aso volcano caldera. (3) Large slip with maximum dextral slip of 220 cm was measured throughout the central section of the rupture zone along the Futagawa segment, and the slip gradually decreased bilaterally on the adjoining northeastern and southwestern sections. (4) The surface rupture mostly occurred along fault traces mapped in previous active fault investigations. (5) Most of the surface ruptures were produced by the mainshock, and significant postseismic slip occurred after the mainshock.

Hidden aftershocks of the 2011Mw9.0 Tohoku, Japan earthquake imaged with the backprojection method

AbstractThe first 25 h of the aftershock sequence following the 11 March 2011Mw9.0 Tohoku, Japan earthquake is investigated using a backprojection method. In total, 600 aftershocks are imaged during this time period. These aftershocks are distributed over a 500 by 300 km area and include many events in the outer rise. The backprojection events are compared with the Japan Meteorological Agency (JMA) catalogue, which is composed of earthquakes recorded by local seismic networks in Japan. Surprisingly, half of the backprojection events are not found in the JMA catalogue. These events cluster near the Japan Trench and in the outer rise and fill in gaps in the spatial distribution of the early aftershock sequence where large main shock slip is thought to have occurred. These results show that the JMA magnitude of completeness is very high near the trench following the 2011 Tohoku main shock, and earthquakes as large as magnitude 6.8 went undetected by local seismic networks.

How precisely can we anticipate seismic intensities? A study of uncertainty of anticipated seismic intensities for the Earthquake Early Warning method in Japan

The precise calculation of anticipated seismic intensity is an important component of Earthquake Early Warning (EEW) procedures. The EEW method adopted by the Japan Meteorological Agency (JMA) uses event magnitude, hypocentral distance, and site amplification factor for this calculation, in which the site amplification factor is represented by a single scalar without consideration of spectrum contents. Even when two earthquakes occur at the same location with the same magnitude, their observed distributions of seismic intensity are not always the same. And even at adjacent measurement stations, the interstation difference in seismic intensity of one earthquake is not always the same as that of another earthquake. To evaluate these expected uncertainties in the current JMA EEW method, we analyzed the distribution of recorded seismic intensities from adjacent earthquakes and also compared the intensities at adjacent observation sites. The uncertainties are 0.29 JMA intensity units when the JMA magnitude is used as an index of source factor and 0.22 when the average of the observed seismic intensities is used. The uncertainties are 0.21 when site amplification factor is represented by single scalar value. These results may indicate the intrinsic precision limits of anticipated seismic intensities in the current JMA EEW method.

Emergency Response to Sediment-Related Disasters Caused by Large Earthquakes in Japan - the Case of the Iwate-Miyagi Nairiku Earthquake in 2008 -

Learning the lessons to be taught by large earthquakes of the past is one key to solving the problems of sediment-related disasters of the future, including slope failures, deep-seated landslides, and landslide dam (natural barriers formed by landslides). Our case subject is the Iwate-Miyagi Nairiku Earthquake in 2008 and the emergency response to disasters of Japan’s central government and other organizations. The earthquake occurred on 14th June 2008 and had a JMA (Japan Meteorological Agency) magnitude of 7.2 and a maximum seismic intensity of 6 upper on the JMA seismic intensity scale. The hypocenter in a mountainous area underlain by thick volcanic ejecta triggered over 3,000 slope failures, deep-seated landslides, and debris flows. The earthquake created 15 landslide dams which were expected to cause serious damage downstream if dams collapsed. Emergency measures taken included channel excavation and pumping of landslide dams. Moreover, emergency checking of potential danger sites immediately after the earthquake found 20 sites requiring emergency measures. The relationship between seismic intensity and sites of slope failure and deep-seated landslide showed that seismic intensity exceeding 5 upper caused such disasters and required emergency checking.

DAMAGE TO A SHIELD TUNNEL CAUSED BY THE CHUETSU OKI EARTHQUAKE

A strong earthquake with JMA (Japan Meteorological Agency) magnitude of 6.8 occurred with its epicenter located in Chuetsu-oki in Niigata Prefecture, on July 16 2007. This earthquake caused extensive damage, completely destroying about 1,000 homes, triggering 105 landslide disasters, and damaging 1,090 agricultural facilities and peripheral facilities of a nuclear power plant. The damage included a shield tunnel used as a drainage canal: damage unprecedented in Japan. This paper reports the results of a detailed survey of this damage, and describes dynamic response analysis carried out using near-source seismic waves. The cause of damage is considered to be the topographical effects of the shape of the mountain above the underground shield tunnel. In the future, it will be necessary to perform seismic design carefully in areas with similar topographical conditions.

RELATIONSHIPS AMONG DIFFERENT MAGNITUDES FOR INLAND EARTHQUAKES CLASSIFIED BY FAULT TYPE AND THEIR APPLICATION TO STRONG MOTION PREDICTION

We examined how the relationships systematically change among the moment magnitude MW, the Japan Meteorological Agency magnitude MJ, and the short-period magnitude m for inland earthquakes classified by fault type. The examination was extended to the relationships among the magnitudes for inter-plate earthquakes and to those for slab earthquakes. It was found out that MJ was about 0.5 larger than MW in inland earthquakes caused by strike-slip faults, MJ was about 0.2 larger than MW in inland earthquakes caused by dip-slip faults, m was about 0.2 smaller than MW in inland earthquakes caused by strike-slip faults, and m was about 0.1 larger than MW in inland earthquakes caused by dip-slip faults. We applied these relationships to the procedure for evaluating fault parameters, and showed two examples of strong motions predicted for strike-slip and dip-slip faults 40 km long.

Structural Heterogeneity in Northeast Japan and Its Implications for the Genesis of the 2004 and 2007 Niigata Earthquakes

Abstract We utilized 188,766 P -wave and 179,328 S -wave high-quality arrival-time data from 14,666 local earthquakes to determine the three-dimensional (3D) seismic velocity ( V P , V S ), V P / V S ratio, crack density ( e ), and bulk velocity ( V ϕ ) structures in and around the source areas of the 2004 and 2007 Niigata earthquakes (Japan Meteorological Agency magnitude [ M J ] 6.8) in Japan in order to understand the generation mechanisms of the Niigata mainshocks. The aftershock hypocenters were relocated by applying the double-difference location method simultaneously with the tomographic inversion, indicating a complicated fault system that includes not only two parallel dominant faults dipping to the northeast but also some conjugate faults crossing the Niigata mainshock hypocenters. In general, high velocity ( V P , V S ) with low e at 11 km depth is visible in the southeastern part of this area in contrast to anomalous low velocity with high crack density in the northwestern part along the Niigata-Kobe tectonic zone (NKTZ). These characteristics of the velocity and crack density are consistent with the spatial distribution of active faults along the NKTZ. The surface geological features that indurated pre-Neogene rock outcrops were observed in the southeastern area, whereas deep sedimentary basins were observed on the opposite side. Low V P and low V S zones with high V P / V S anomalies were imaged in and/or under the source areas, showing agreement with those revealed by previous studies. All the features of the velocity and V P / V S structures, together with the images of the crack density and bulk velocity, suggest the presence of fluids under the source areas. Such fluids might have weakened the mechanical strength of the fault zone of the source areas, thus triggering the 2004 and 2007 Niigata earthquakes. Our present study, together with other recent tomographic images, reveals that fluids penetrated into the source rocks from the lower crust and uppermost mantle as a result of dehydration of the subducting Pacific slab and thus may play a key role in the genesis of these large crustal earthquakes.

Introduction to the special section for the 2005 West Off Fukuoka Prefecture Earthquake

On 20 March 2005, the 2005 West Off Fukuoka Prefecture earthquake with Japan Meteorological Agency magnitude (MJMA) of 7.0 occurred in the offshore region of Fukuoka Prefecture, in northern Kyushu, Japan. This earthquake caused more than 1,000 casualties and damaged many buildings in the surrounding area, especially on Genkai Island as well as in and around Fukuoka City. Because the 2005 West Off Fukuoka Prefecture earthquake was one of the largest intraplate earthquakes in recorder history occurring ages at the junction of the Southwestern Japan Arc and the Ryukyu Arc, intensive seismological and geodetic studies were carried out in its aftermath. Ten papers from these studies were published as the special section for the 2005 West Off Fukuoka Prefecture earthquake (1) of Earth, Planets and Space, Vol. 58, No.1, 2006. In addition, eight papers are published in the special section (2) of this issue. These studies have revealed important characteristics of the earthquake as summarized in the following paragraphs. The mainshock and aftershocks were mainly aligned NW-SE in an approximately 25 km long trend, and the hypocenters of the aftershock region were distributed on a nearly vertical plane at depths of 2–16 km. The mainshock was located near the central part of the aftershock region, with a depth of 9.5 km. These findings suggest that the strike and dip of the fault plane of the earthquake are approximately N60◦W and 90◦, respectively, and the length and width of the fault are about 25 km and 14 km, respectively. The largest aftershock of MJMA 5.8 occurred near the SE edge of the main aftershock region, and the aftershock region extended about 5 km in the SE direction based on the secondary aftershock activity, which had a different strike angle from that of most of the aftershocks. Following the most intense aftershock activity, there was no distinct enlargement of the aftershock region, and the aftershock activity gradually declined (Shimizu et al., 2006). The fault structure has been discussed in detail on the basis of precise analyses of aftershock distribution based on onshore and offshore seismic observations made with densely located ocean bottom seismometers (Uehira et al., 2006). The fault plane is bent near both the NW and SE

Magnitude and location of historical earthquakes in Japan and implications for the 1855 Ansei Edo earthquake

Japan Meteorological Agency (JMA) intensity assignments IJMA are used to derive intensity attenuation models suitable for estimating the location and an intensity magnitude Mjma for historical earthquakes in Japan. The intensity for shallow crustal earthquakes on Honshu is equal to −1.89 + 1.42MJMA − 0.00887Δh − 1.66logΔh, where MJMA is the JMA magnitude, Δh = (Δ2 + h2)1/2, and Δ and h are epicentral distance and focal depth (km), respectively. Four earthquakes located near the Japan Trench were used to develop a subducting plate intensity attenuation model where intensity is equal to −8.33 + 2.19MJMA −0.00550Δh − 1.14 log Δh. The IJMA assignments for the MJMA7.9 great 1923 Kanto earthquake on the Philippine Sea–Eurasian plate interface are consistent with the subducting plate model; Using the subducting plate model and 226 IJMA IV–VI assignments, the location of the intensity center is 25 km north of the epicenter, Mjma is 7.7, and MJMA is 7.3–8.0 at the 1σ confidence level. Intensity assignments and reported aftershock activity for the enigmatic 11 November 1855 Ansei Edo earthquake are consistent with an MJMA 7.2 Philippine Sea–Eurasian interplate source or Philippine Sea intraslab source at about 30 km depth. If the 1855 earthquake was a Philippine Sea–Eurasian interplate event, the intensity center was adjacent to and downdip of the rupture area of the great 1923 Kanto earthquake, suggesting that the 1855 and 1923 events ruptured adjoining sections of the Philippine Sea–Eurasian plate interface.

Three-Dimensional Finite-Difference Waveform Modeling of Strong Motions Observed in the Sendai Basin, Japan

We perform three-dimensional (3D) finite-difference (FD) waveform modeling of strong motions in the frequency range 0.2 to 1.67 Hz observed in the Sendai basin, Japan, during the Japan Meteorological Agency magnitude ( M J) 5.0 1998 Miyagiken-Nanbu earthquake. In a previous, we estimated S -wave velocity structures above the pre-Tertiary bedrock at six sites in the Sendai basin based on array records of microtremors. To interpolate these velocity structures in space we conduct single-station microtremor measurements at a total of 61 locations and estimate the S -wave velocity structure at each site by modeling the horizontal-to-vertical spectral ratio of fundamental mode Rayleigh waves. An initial 3D model of the basin is constructed using the velocity structures estimated from both array and single-station microtremor measurements, along with other information such as surface geology. This model encompasses a region 33 km long, 30 km wide, and 19 km deep. The final model is obtained through a trial-and-error process by fitting 3D FD synthetic waveforms to the bandpass-filtered (0.2 Hz to 1.67 Hz) displacement records 12 stations for the 1998 Miyagiken-Nanbu earthquake. We compute the synthetics using a fourth-order staggered-grid 3D FD method with variable grid spacing. As the 3D model is modified, the source parameters (strike, dip, rake, and seismic moment) are estimated by a grid search method using 1D site-specific models derived from the modified 3D model. The observed waveforms are reproduced well at most stations by the final 3D basin model. This agreement suggests the validity of the final 3D basin model for theoretical strong-motion prediction of large earthquakes in the frequency range from 0.2 to 1.67 Hz. Manuscript received 7 June 2000.

関東地震のマグニチュード

Magnitudes for the 1923 Kanto earthquake and its major aftershocks were determined in JMA (Japan Meteorological Agency) scale. The original definition of the JMA magnitude is a magnitude that is calculated by the Tsuboi's formula from the maximum amplitudes in horizontal components of seismograms obtained by the regional observation network of JMA. The used seismograms were recorded by standard seismographs, which were the displacement type with the natural period of about 5s and damping ratio of about 8. However, the seismometers had not been yet standardized before 1925 and various types had been used whose instrumental responses were quite different from those of the standard seismographs. The purpose of the present study was that the JMA magnitudes of the 1923 Kanto earthquake and its major 3 aftershocks were determined in consideration of the difference of the instrumental responses. Fortunately, unsaturated seismograms by the Imamura's type strong motion seismographs (displacement type) have been preserved at 7 stations of JMA. The natural period and damping ratio of each seismograph have been evaluated from the free oscillation records preserved at each station. The records for the main shock and aftershocks were digitized and corrected in the instrumental responses to calculate the seismograms with the instrumental response of the standard seismograph of JMA. After that, the maximum amplitudes were measured on the corrected records and the magnitude was determined for each earthquake following the definition of the JMA magnitude. The determined JMA magnitude was 8.1±0.2 for the main shock. All the results were consistent within the difference of 0.2 with the customary results, which were determined from the uncorrected amplitude and seismic intensity data. The standard deviations were smaller than 0.2 for all the events, which shows higher reliability of the present results, comparing with the past ones.

Fault Length and Direction of Rupture Propagation for the 1993 Kushiro-Oki Earthquake as Derived from Strong Motion Duration.

The Kushiro-Oki earthquake occurred off Kushiro, Hokkaido, Japan, on January 15, 1993. The epicentral coordinates given by the Japan Meteorological Agency (JMA) are 42°51'N and 144°23'E. The focal depth is 107 km and the JMA magnitude is 7.8. According to Kasahara (1993), this earthquake took place in the Pacific plate descending beneath Hokkaido. The double-planed deep seismic zone appears in this region and the present earthquake is located on the lower seismic plane. A cluster of aftershocks was observed in an area of about 40 •~ 40 km2 and the epicenter of the main shock is located near the southeast corner of the aftershock area. Kasahara (1993) named this the Kushiro-Oki cluster. Another cluster of aftershocks was observed in a small area which is located at about 50 km east of the epicenter of the main shock. This one is called the Akkeshi-Oki cluster by Kasahara (1993) and the activity is not so energetic as that of the Kushiro-Oki cluster. There is a seismic gap in space between the two clusters. The hypocenters of the Kushiro-Oki cluster formed an almost horizontal plane at a depth of about 100 km and the hypocenter of the main shock is also located on it. The main shock is composed of the main event and the pre-event that occurred 6 s before the main event. A focal mechanism solution has been obtained for the pre-event

日本列島およびその周辺地域に起こる浅発地震のマグニチュードと地震モーメントの関係

The relation between seismic moment M0 and magnitude M of JMA (Japan Meteorological Agency) is examined for the earthquakes of M>5 in and around Japan. The data for the events in subduction zones including the earthquakes along eastern margin of the Japan sea are approximately explained by the relation aslogM0=1.5M+16.2(1)This equation was proposed by SATO (1989) based on the similarity relation of source parameters. In consideration of the difference between M and surface-wave magnitude Ms, the equation (1) can be modified so as to describe more detailed trend of the data distribution of M0 and M. On the other hand, the data for inland events which occurred in the upper crust cannot be explained by the equation (1). Another relation is available for these events as follows:logM0=1.17M+17.72(2)Two reasons are possible for the difference of the M0-M relations between the events in the subduction zone and in the inland. One is that the stress drop of the inland events is larger than that of the subduction events. The other is that the inland events show large excitation of surface-waves in the medium periods, because the focal depths of them are shallower on the average, comparing with those of the events in the subduction zone. The JMA magnitude is determined from the maximum amplitudes of seismic waves in the medium period range, which are usually given not by S-waves but by surface-waves for large events. This fact supports the latter reason.