Aftershock Observations Research Articles

Precise aftershock distribution of the 2019 Yamagata-oki earthquake using newly developed simple anchored-buoy ocean bottom seismometers and land seismic stations

An earthquake with a magnitude of 6.7 occurred in the Japan Sea off Yamagata on June 18, 2019. The mainshock had a source mechanism of reverse-fault type with a compression axis of WNW–ESE direction. Since the source area is positioned in a marine area, seafloor seismic observation is indispensable for obtaining the precise distribution of the aftershocks. The source area has a water depth of less than 100 m, and fishing activity is high. It is difficult to perform aftershock observation using ordinary free-fall pop-up type ocean bottom seismometers (OBSs). We developed a simple anchored-buoy type OBS for shallow water depths and performed the seafloor observation using this. The seafloor seismic unit had three-component seismometers and a hydrophone. Two orthogonal tiltmeters and an azimuth meter monitored the attitude of the package. For seismic observation at shallow water depth, we concluded that an anchored-buoy system would have the advantage of avoiding accidents. Our anchored-buoy OBS was based on a system used in fisheries. We deployed three anchored-buoy OBSs in the source region where the water depth was approximately 80 m on July 5, 2019, and two of the OBSs were recovered on July 13, 2019. Temporary land seismic stations with a three-component seismometer were also installed. The arrival times of P- and S-waves were read from the records of the OBSs and land stations, and we located hypocenters with correction for travel time. A preliminary location was performed using absolute travel time and final hypocenters were obtained using the double-difference method. The aftershocks were distributed at a depth range of 2.5 km to 10 km and along a plane dipping to the southeast. The plane formed by the aftershocks is consistent with the focal mechanism of the mainshock. The activity region of the aftershocks was positioned in the upper part of the upper crust. Focal mechanisms were estimated using the polarity of the first arrivals. Although many aftershocks had a reverse-fault focal mechanism similar to the focal solution of the mainshock, normal-fault type and strike–slip fault type focal mechanisms were also estimated.Graphical

Dynamic rupture simulation of 2018, Hokkaido Eastern Iburi earthquake: role of non-planar geometry

The 2018, Hokkaido Eastern Iburi, Japan, earthquake is an event characterized by complexity of the rupture process and slip pattern, which may involve both reverse and strike-slip motion depending on the locations on the fault surface. We perform dynamic rupture simulations based on simple physical laws, conditions for stressing and fault friction, and the non-planar fault geometry constrained by the aftershock observation. The complex fault geometry is numerically treated by the boundary integral equation method accelerated by the fast domain portioning method. The fault geometry is characterized primarily by the combination of six fault planes. As a result, we are able to explain several observed features of the event, including the spatial variation of the final fault slip and rupture velocity, which are inferred from the kinematic slip inversion. We also succeed in refining the constraint of the regional stress field in the focal area based on the simulation. Our results show that the overall patterns of the complex rupture event can be reproduced by a relatively simple model of the regional stress and the fault friction, if the geometrical complexity of the fault is properly taken into account.

Rapid estimation of the hazard posed by strong aftershocks for Kamchatka and the Kuril Islands

This paper deals with the rapid estimation of the hazard posed by strong aftershocks for Kamchatka and the Kuril Islands based on the 12-hour aftershocks, namely, their rate, time of expectation, and maximum magnitude. The data set consists of main shocks and aftershocks as reported in the Catalog of the Kamchatka Branch of the Geophysical Survey, Russian Academy of Sciences (GS RAS) for 1968–2016. We used both the Gutenberg–Richter relation and the Omori–Utsu law to find that the aftershock rates in two time intervals are connected by a relationship of proportionality, with the constant of proportionality being independent of the lowest magnitude of these earthquakes. With this property in mind, we calculated the constants of proportionality for estimating the rate of aftershocks with a magnitude above threshold values based on the data for the first 12 hours after the main shock. We have derived easily remembered rules for estimating the aftershock rates that can be expected for 5 days and for 1 month with magnitudes above a fixed value based on the 12-hour aftershock observations. We also derived empirical regression relations to estimate the magnitude of the largest aftershock that can be expected to occur during 1 year after the main shock and the time of the last aftershock to occur whose magnitude is 5.2 or greater.

Comparison of Observed Ground‐Motion Attenuation for the 16 April 2016Mw 7.8 Ecuador Megathrust Earthquake and Its Two Largest Aftershocks with Existing Ground‐Motion Prediction Equations

A megathrust subduction earthquake (Mw7.8) struck the coast of Ecuador on April 16th, 2016 at 23h58 UTC. This earthquake is one of the best-recorded megathrust events up to date. Besides the mainshock, two large aftershocks have been recorded on May 18th, 2016, at 7h57 (Mw 6.7) and 16h46 (Mw6.9). These data make a significant contribution for understanding the attenuation of ground motions in Ecuador. Peak ground accelerations and spectral accelerations are compared with four ground-motion prediction equations developed for interface earthquakes, the global Abrahamson et al. (2016) model, the Japanese equations Zhao et al. (2006) and Ghofrani and Atkinson (2014), and one Chilean equation Montalva et al. (2016). The four tested GMPEs are providing rather close predictions for the mainshock at distances up to 200km. However, our results show that high-frequency attenuation is greater for backarc sites, thus Zetal2016 and Metal2016, which are not taking into account this difference, are not considered further. Residual analyses show that G&A14 and Aetal2016 are well predicting the attenuation of ground motions for the mainshock. Comparisons of aftershock observations with Aetal2016 predictions indicate that the GMPE provide reasonable fit to the attenuation rates observed. The event terms of the Mw6.7 and Mw6.9 events are positive but within the expected scatter from worldwide similar earthquakes. The intra-event standard deviations are higher than the intraevent variability of the model, which is partly related to the poorly constrained VS30 proxys.

2016年熊本地震で被災した文化財建造物での余震観測と地震動の評価

2016年熊本地震で被災した文化財建造物での余震観測と地震動の評価

Retraction: Evaluation of ground motions at damaged bridge sites along Kyushu Expressway during the 2016 Kumamoto earthquakes based on the temporary aftershock observations

Retraction: Evaluation of ground motions at damaged bridge sites along Kyushu Expressway during the 2016 Kumamoto earthquakes based on the temporary aftershock observations

Retraction: Evaluation of ground motions at damaged bridge sites in Nishihara Village and Minamiaso Village during the 2016 Kumamoto Earthquake based on the temporary aftershock observations

Retraction: Evaluation of ground motions at damaged bridge sites in Nishihara Village and Minamiaso Village during the 2016 Kumamoto Earthquake based on the temporary aftershock observations

常時微動観測と臨時余震観測による福島県浪江町市街地の地盤震動特性評価

2011年東北地方太平洋沖地震の本震では、福島県浪江町の市街地において地震動による建物被害が発生した。周辺の市町村と比較して建物被害が顕著であるのみならず、市街地においても地区毎の被害率に差が見られた。本研究では、被害率の差の原因を調べるため、地区毎の表層地盤構造の違いを明らかにすることを目的とした常時微動観測、および臨時余震観測を実施した。微動H/Vスペクトルによる卓越周波数の分布、および微動アレー観測により求められる表層のS波速度モデルによると、市街地では概ね一様な表層地盤が示唆される。また、臨時余震観測の記録から地区毎の震動特性の差を比較したところ、PGV比およびスペクトル比において、差がばらつきの範囲内であった。震動特性の差の小さいサイト間を評価する場合にはばらつきの影響を考慮する必要があり、本研究のように狭い範囲を対象とした評価では特にその問題が顕在化するため、ばらつきの程度に応じて評価の対象とする空間解像度を定めることが重要である。

Changes in the stress field after the 2008 M7.2 Iwate‐Miyagi Nairiku earthquake in northeastern Japan

AbstractIn order to investigate changes in the stress field following the 2008 M7.2 Iwate‐Miyagi Nairiku earthquake in NE Japan, we determined many focal mechanisms within the focal area, using data from both the dense aftershock observation network deployed just after the main shock and other temporary and routinely operated stations in the surrounding area. Applying stress tensor inversions to these focal mechanism data, we estimated the detailed spatial distribution of the principal stress orientations in this event's source area. Estimated orientations of the maximum principal stress (σ1) axes differ significantly before and after the main shock. Before the main shock, the σ1 axis is oriented WNW‐ESE across the entire studied area, which is parallel to the plate convergence direction. After the main shock, however, the orientation of the σ1 axis changed significantly and showed remarkable spatial variation. In the northern and southern parts of the aftershock area, located relatively far from the main shock large slip area, the σ1 axes are oriented WNW‐ESE, similar to those before the main shock. In contrast, to the east and west of the central part of the aftershock area, the σ1 axes are oriented N‐S and NE‐SW, respectively. This characteristic spatial pattern of σ1 axis orientations is approximately consistent with that of static stress change associated with the main shock rupture, which strongly suggests that the spatially heterogeneous stress after the main shock is caused by such static stress change. The differential stress magnitude before the main shock is estimated to be less than 10–30 MPa, suggesting that the main shock faults are weak.

Application of Horizontal-to-Vertical Spectral Ratios of Earthquake Ground Motions to Identify Subsurface Structures at and around the K-NET Site in Tohoku, Japan

Abstract We propose an optimal way to use horizontal‐to‐vertical spectral ratios (HVRs) for subsurface structure exploration, based on the diffuse field concept (Kawase et al. , 2011; Sanchez‐Sesma et al. , 2011). This approach is applicable to both earthquake and microtremor ground motions. We show here analyses of the observed ground‐motion data at and around a K‐NET station in Miyagi Prefecture, Japan, where very large peak horizontal ground acceleration was observed during the earthquake of 11 March 2011 off the Pacific coast of Tohoku, Japan. We compare HVRs of the strong motions for the mainshock and the largest peak acceleration aftershock with those averaged over tens of weak motions to observe soil nonlinearity effects on the HVRs. Then, we determine detailed velocity profiles from the HVRs at the K‐NET Tsukidate station and the temporary aftershock observation sites. We find that HVRs can be explained quite well by the identified velocity profiles at all the target sites. The observed peak at 9 Hz for the averaged weak‐motion data originates in the topmost layers lying over the engineering bedrock.

New constraints from seismology and geodesy on the Mw = 6.4 2008 Movri (Greece) earthquake: evidence for a growing strike-slip fault system

The 2008 Mw = 6.4 Movri earthquake ruptured a NNE right lateral strike-slip fault about 30 km south of the city of Patras. Although some strike-slip activity on minor faults was known, there was no tectonic evidence of large scale NS striking fault and such a large event was not anticipated. Following the event, a network of six stations was installed for 4 months in the epicentral area in order to monitor aftershocks and in particular the northern part of the rupture area closest to the city of Patras. We combine these new aftershock observations with GPS measurements of an already existing geodetic network in the area performed just after the earthquake, as well as with SAR interferograms, together with already published source studies, in order to refine already proposed models of this event. The combined data set allows defining much more accurately the lateral and vertical limits of the rupture. Its length inferred from geodesy is ∼15 km and its modelled upper edge ∼17 km. The seismic moment then constrains the lower edge to coincide, within a few kilometres, with the Moho interface. The absence of seismicity in the shallow crust above the co-seismic fault is interpreted as a result of the decoupling effect of possible presence of salt layers above the rupture area, near 14 to 16 km in depth, which favours our interpretation of an immature strike-slip fault system, compatible with the absence of surface ruptures. The immature character of this large crustal fault is further suggested by the high variability of focal mechanisms and of fault geometries deduced from aftershock clusters, in the strike direction. Its geometry and mechanism is consistent with the crustal shear, striking NNE, revealed by GPS in this region. This shear and faulting activity might be generated by the differential slip rate on the subduction interface, 50 km to the south, leading to a north-northeastward propagating strike-slip fault zone. The wide extension of the aftershock distribution forming a NNE alignment, beyond the rupture area towards the north, suggests a localization process of the shear strain, which could be the preliminary stage of fault propagation further to the NNE. An alternative speculative model for this regional stress could be the existence of a well-developed NNE striking shear zone within the uppermost mantle, marking at depth the southward propagation of the northern branch of the North Anatolian fault. Both models may not be exclusive of each other, and in fact their sources may be mechanically interdependent.

Pore pressure distribution in the focal region of the 2008 M7.2 Iwate-Miyagi Nairiku earthquake

The pore fluid pressure distribution in the focal region of the 2008 Iwate-Miyagi Nairiku earthquake was investigated through an analysis of the diversity of focal mechanisms. We inverted stress orientations and focal mechanisms directly from P-wave polarity data obtained from a dense aftershock observation network and other temporarily and routinely operated stations. The estimated stress orientation is consistent with that typical of NE Japan. Specifically, the σ 1 axis is oriented WNW-ESE nearly parallel to plate convergence, and the σ 3 axis is nearly vertical, consistent with a reverse-faulting stress regime, with some exceptions in the central part of the aftershock area. We obtained 2,497 well-determined focal mechanisms whose average number of P-wave polarity data is more than 70. The spatial distribution of pore fluid pressure was estimated by using the obtained orientations of the principal stresses and earthquake faults. The pore pressure ratio for each earthquake fault was calculated under the assumption that reduced frictional strength was caused by pore fluid pressure. The results showed that the diversity of the focal mechanisms here obtained requires high pore fluid pressures at many of the earthquake faults in the focal region. The spatial pattern of pore pressure ratio shows that areas of higher pore pressure ratio are distributed around the large coseismic slip area near Kurikoma volcano. Immediately beneath these high pore pressure areas, there exists a distinct seismic low-velocity zone that continues down to the mantle wedge below. These observations suggest that crustal fluids supplied from the mantle wedge have contributed to producing high pore pressures and to lowering the frictional strengths of those faults. Crustal fluids may also have contributed to generating the mainshock rupture.

Aftershocks of the December 7, 2012 intraplate doublet near the Japan Trench axis

On December 7, 2012, a pair of large Mw 7.2 intraplate earthquakes occurred near the Japan Trench axis off Miyagi, northeast Japan. This doublet consisted of a deep reverse-faulting event followed by a shallow normal-faulting event. Aftershock observations using conventional and newly developed ultra-deep ocean bottom seismographs in the trench axis area showed that the shallow normal-faulting event occurred in the subducting Pacific plate just landward of the trench axis. The shallow normal-faulting aftershock activity indicated that in-plate tension in the incoming/subducting Pacific plate extends to a depth of at least 30 km, which is deeper than before the 2011 Tohoku-Oki earthquake, whereas in-plate compression occurs at depths of more than 50 km. Hence, we concluded that the neutral plane of the in-plate stress is located between depths of 30 and 50 km near the trench axis.

Distribution and migration of aftershocks of the 2010 Mw 7.4 Ogasawara Islands intraplate normal-faulting earthquake related to a fracture zone in the Pacific plate

We describe the aftershocks of a Mw 7.4 intraplate normal-faulting earthquake that occurred 150 km east Ogasawara (Bonin) Islands, Japan, on 21 December 2010. It occurred beneath the outer trench slope of the Izu-Ogasawara trench, where the Pacific plate subducts beneath the Philippine Sea plate. Aftershock observations using ocean bottom seismographs (OBSs) began soon after the earthquake and multichannel seismic reflection surveys were conducted across the aftershock area. Aftershocks were distributed in a NW-SE belt 140 km long, oblique to the N-S trench axis. They formed three subparallel lineations along a fracture zone in the Pacific plate. The OBS observations combined with data from stations on Chichi-jima and Haha-jima Islands revealed a migration of the aftershock activity. The first hour, which likely outlines the main shock rupture, was limited to an 80 km long area in the central part of the subsequent aftershock area. The first hour activity occurred mainly around, and appears to have been influenced by, nearby large seamounts and oceanic plateau, such as the Ogasawara Plateau and the Uyeda Ridge. Over the following days, the aftershocks expanded beyond or into these seamounts and plateau. The aftershock distribution and migration suggest that crustal heterogeneities related to a fracture zone and large seamounts and oceanic plateau in the incoming Pacific plate affected the rupture of the main shock. Such preexisting structures may influence intraplate normal-faulting earthquakes in other regions of plate flexure prior to subduction.

The good, the bad and the ugly - lessons from and methodologies for extracting shear-wave velocity profiles from microtremor array measurements in urban Newcastle, NSW

We report on our experience gained in applying the multiple-mode spatially-averaged coherency method (MMSPAC) at 25 sites in Newcastle (NSW) for the purpose of determining shear-wave velocity profiles as part of an earthquake hazard study. The MMSPAC technique is logistically viable for use in urban and suburban areas, both on grass sports fields and parks, and on footpaths and roads. A set of seven recording systems designed for earthquake after-shock observations plus a team of three personnel is sufficient to survey three sites per day. The uncertainties relating to local noise sources from adjacent road traffic or from service pipes contribute to loss of low-frequency SPAC data in a way which is difficult to predict in survey design. Coherencies between individual pairs of sensors should be studied as a quality-control measure with a view to excluding noise-affected sensors prior to interpretation; useful data can still be obtained at a sites one sensor is excluded. The combined use of both SPAC data and horizontal:vertical spectral ratio (HVSR) data in inversion and interpretation is necessary in order to make effective use of low frequency data (typically 0.5 to 2 Hz at these sites) and thus resolve shear-wave velocities in bedrock below 20 to 50 m of unconsolidated sediments.

Ground motion estimation at the farthest liquefaction site during the 2011 off the pacific coast of Tohoku earthquake

Serious damage due to soil liquefaction occurred in a wide area of the Tohoku and the Kanto regions in Japan during 2011 off the Pacific coast of Tohoku Earthquake (MW 9.0). The farthest liquefied site from the epicenter of the 2011 main shock is Ikenouchi, Minamiboso City, whose epicentral distance is approximately 440km. Evaluation of strong ground motions at Ikenouchi is very important to understand why liquefaction was observed in such a far site. For this purpose, in-situ geotechnical investigations including aftershock observation were carried out at Ikenouchi. Then, the ground motions at Ikenouchi during the 2011 main shock was estimated based on the site-effect substitution method considering the microtremor H/V spectral ratios and the aftershock observation results. Finally, 3-D effective stress analyses were carried out using a FEM code ‘FLIP’ to understand how site-specific characteristics of strong ground motions affected the behavior of the ground including liquefaction. A striking feature of the estimated waveforms at the target site is that the waveforms include a much larger number of cycles compared to the records at permanent observation stations around the target site. Such a site-specific feature of strong ground motions was one of the main causes of the occurrence of liquefaction 440km away from the epicenter. The result suggests the importance of taking into account not only soil properties but also site-specific characteristics of strong ground motions for a rational assessment of liquefaction for future large earthquakes.

2011 年東北地方太平洋沖地震で被災した盛土での余震観測と地震動の評価

2011 年東北地方太平洋沖地震で被災した盛土での余震観測と地震動の評価

余震観測記録に基づく2011 年東北地方太平洋沖地震におけるゴム系支承の被災橋梁での地震動の評価

2011年東北地方太平洋沖地震では、北方より(a)仙台北部道路利府高架橋、(b)仙台東部道路東部高架橋、(c)国道6号線日立バイパス旭高架橋、(d)東水戸道路新那珂川大橋において、積層ゴム系支承の損傷を伴う地震動による深刻な被害が発生した。今後の橋梁の耐震性評価のためにも、これらの被災地点での地震動を推定することは非常に重要である。そこで本稿では、被災橋梁の極近傍において余震観測を実施し、得られた記録などに基づいて、本震時における被災橋梁地点(全4地点)での地震動を評価した結果について報告する。

Estimation of S-Wave Velocity Profiles and Site Amplification Around the K-NET Tsukidate Station, Miyagi Prefecture, with Reference to Large PGA During the 2011 off Pacific Coast of Tohoku Earthquake, Japan

In this study we investigated site amplifications in shallow soil around Tsukidate station (MYG004) of KNET in Miyagi prefecture with a seismic intensity of 7 during the 2011 off the Pacific Coast of Tohoku Earthquake from aftershock observation and microtremor explorations. We conducted aftershock observations by installing 8 temporary stations. We estimated site amplifications of S-wave from the observed data. We found that the predominant period of amplification at a site near MYG004 was 0.2 seconds. The amplification factor at a site on cut ground with a distance of 20 m from MYG004 was significantly different. Furthermore, the amplifications were also different with changing predominant periods from 0.1 to 0.5 seconds in an area of 1 km from MYG004. Shallow Swave velocity profiles were deduced from microtremor array explorations at the stations. Results indicated that shallow low-velocity layers with a thickness of less than 30 meters are responsible for variation of the amplifications. Predominant periods in calculated 1D amplifications were similar to those observed in the aftershock observations. The observed amplification factor at the site nearMYG004, however, was two times larger than the calculated one, suggesting the inappropriateness of the 1D assumption at the site possibly due to the effects of a cliff near MYG004. We also found from horizontal-vertical ratios of microtremors that the site effects on the cliff were also different at frequency higher than 10 Hz within 30 meters around MYG004. This also suggests lateral variation in the site effects at the high frequency near MYG004.

Precise aftershock distribution of the 2011 off the Pacific coast of Tohoku Earthquake revealed by an ocean-bottom seismometer network

The 2011 off the Pacific coast of Tohoku Earthquake occurred at the plate boundary between the Pacific plate and the landward plate on March 11, 2011, and had a magnitude of 9. Many aftershocks occurred following the mainshock. Obtaining a precise aftershock distribution is important for understanding the mechanism of earthquake generation. In order to study the aftershock activity of this event, we carried out extensive sea-floor aftershock observations using more than 100 ocean-bottom seismometers just after the mainshock. A precise aftershock distribution for approximately three months over the whole source area was obtained from the observations. The aftershocks form a plane dipping landward over the whole area, nevertheless the epicenter distribution is not uniform. Comparing seismic velocity structures, there is no aftershock along the plate boundary where a large slip during the mainshock is estimated. Activity of aftershocks in the landward plate in the source region was high and normal fault-type, and strike-slip-type, mechanisms are dominant. Within the subducting oceanic plate, most earthquakes have also a normal fault-type, or strike-slip-type, mechanism. The stress fields in and around the source region change as a result of the mainshock.