Tsunami field survey of the 1992 Nicaragua earthquake

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95368/1/eost9614.pd

Stress relaxation arrested the mainshock rupture of the 2016 Central Tottori earthquake

地震の破壊はなぜ止まるのか? --2016年鳥取県中部地震の断層サイズを決めたもの--. 京都大学プレスリリース. 2021-08-12.After a large earthquake, many small earthquakes, called aftershocks, ensue. Additional large earthquakes typically do not occur, despite the fact that the large static stress near the edges of the fault is expected to trigger further large earthquakes at these locations. Here we analyse ~10, 000 highly accurate focal mechanism solutions of aftershocks of the 2016 Mw 6.2 Central Tottori earthquake in Japan. We determine the location of the horizontal edges of the mainshock fault relative to the aftershock hypocentres, with an accuracy of approximately 200 m. We find that aftershocks rarely occur near the horizontal edges and extensions of the fault. We propose that the mainshock rupture was arrested within areas characterised by substantial stress relaxation prior to the main earthquake. This stress relaxation along fault edges could explain why mainshocks are rarely followed by further large earthquakes

Development of A New Ocean Bottom Seismometer (Model IV of Kyoto University)

Seventy percent of the surface of the earth is covered by ocean. It is necessary to observe earthquakes in the sea for the study of solid earth. The group of ocean-bottom seismological observation of Kyoto University has developed ocean bottom seismometers (OBS) of three types over the last several years. In the present study, we present our newly developed OBS which deploys free-fall and pop-up methods. This OBS is equipped with a three-component geophone (Mark Product L-22D) and a digital recorder. The recorder digitizes seismic data by a 16 bit analog-to-digital converter and writes the data on a magneto-optical disk with a capacity of 326 megabytes (MB). The electronic circuit boards are all housed in a 17-inch-diameter glass sphere. Thus, we will be able to obtain seismic data of high quality reading the seismographs from this high dynamic range seismometer installed on the sea bottom. The present OBS has an electrical corrosion mechanism permitting the release of the main OBS unit from its anchor at the sea bottom when it is retrieved. Recording tests on land and popping-up tests in the sea were performed, confirming that the present system is reliable enough to record seismic data under long-time submarine deployment

Spatial distribution of atypical aftershocks of the 1995 Hyogo-ken Nanbu earthquake

The 1995 Hyogo-ken Nanbu earthquake (MJMA7.3) occurred on January 17, 1995. To reveal the detailed stress field after the mainshock, we relocate hypocenters of aftershocks (M ≥ 2.3) and determine their focal mechanisms using seismic records obtained by GROUPS-95, a temporary dense seismic network in and around the aftershock region. Along the mainshock rupture zone, some aftershocks are nearly N-S compression or normal fault type events, which is inconsistent with the regional stress field of approximately horizontal E-W compression. We call these aftershocks atypical, defined as events which have focal mechanisms with P-axis directions more than 45°from the regional stress field. The atypical aftershocks amount to about 17% of the total. No temporal variations in aftershock mechanism are found in the analysis period. Their spatial distribution is compared with the slip and stress distribution of the mainshock, as well as the P- and S-wave velocity structure. Most of them are located at the upper boundary between the slipped and unslipped zones of the mainshock. We suggest that the atypical aftershocks are caused by the disturbance of the local tectonic stress field due to the heterogeneous coseismic slip

Ocean Bottom Seismometer Handled by Submersible Vessel and Its Observation Prior to the 1993 Hokkaido Nansei-Oki Earthquake

Nowadays, most ocean bottom seismometers (OBS) are of the free-fall and pop-up type. For the observations, they are dropped from ships and self land on the flat sea floor where generally thick sediments cover basement rocks. However, soft sediment affects and distorts incoming seismic signals. If we could place ocean bottom seismometers on hard rock, it could produce high quality seismograms which contained more information on structure and earthquake sources. For this purpose, a submersible vehicle is necessary as it can carry and place an OBS properly on exposed sea floor hard rock. We designed a new type of OBS which can be handled by a submersible vehicle. This new OBS contains instruments within two aluminum cylinders with syntactic foam blocks outside the cylinders for buoyancy. The submersible vessel, Shinkai-6500, was used to set the OBS to observe seismicity in the Okushiri Ridge Area. It was placed on a relatively flat and hard basement outcrop at a depth of 3338m. The OBS was released to the sea surface through a self pop-up system after two days of observation. During this observation period, we noticed very high micro-earthquake activity around the Okushiri Ridge area where one year later the Hokkaido-Nansei-Oki earthquake of M7.8 took place. Such seismic activity could not have been detected by land-based seismic networks alone

Focal mechanisms and stress field in the Nobi fault area, central Japan

In this study, we obtained 728 focal mechanisms of small earthquakes with depths shallower than 20 km that occurred from May 2009 to May 2013 in the Nobi fault area in central Japan. The averages of the azimuths of the P- and T-axes were N97 degrees +/- 23 degrees E and N6 degrees +/- 32 degrees E, and the averages of the dips of the P- and T-axes were 11 degrees +/- 10 degrees and 32 degrees +/- 25 degrees, respectively. These variations in the P- and T-axes come from variation of the focal mechanisms; both strike-slip and reverse fault earthquakes were observed in the study area. A stress tensor inversion method was applied to the focal mechanisms, and we obtained and characterized the spatial pattern of the tectonic stress. We found that the maximum principal stress (sigma(1)) is oriented E-W over almost the entire study area. The stress ratio R, which is defined as R = (sigma(1) - sigma(2))/(sigma(1) - sigma(3)), ranges from 0.65 to 0.98, and the average R over the entire study area is 0.82. The average stress ratio is close to unity, indicating sigma(2) approximate to sigma(3), and thus the dominant stress in this region is a uniaxial compression in the direction of sigma(1). The direction of the sigma(1)-axis fluctuates locally at the southeastern end of the seismic fault ruptured by the 1891 Nobi earthquake. This fluctuation is limited to within a very narrow zone across the seismic fault in the upper crust shallower than approximately 10 km, suggesting that most of the deviatoric stress at the southeastern end of the seismic fault ruptured by the 1891 Nobi earthquake was not released

Focal mechanisms and stress field in the Nobi fault area, central Japan Seismology

In this study, we obtained 728 focal mechanisms of small earthquakes with depths shallower than 20 km that occurred from May 2009 to May 2013 in the Nobi fault area in central Japan. The averages of the azimuths of the P- and T-axes were N97° ± 23° E and N6° ± 32° E, and the averages of the dips of the P- and T-axes were 11° ± 10° and 32° ± 25°, respectively. These variations in the P- and T-axes come from variation of the focal mechanisms; both strike-slip and reverse fault earthquakes were observed in the study area. A stress tensor inversion method was applied to the focal mechanisms, and we obtained and characterized the spatial pattern of the tectonic stress. We found that the maximum principal stress (σ 1) is oriented E-W over almost the entire study area. The stress ratio R, which is defined as R = (σ 1 - σ 2)/(σ 1 - σ 3), ranges from 0.65 to 0.98, and the average R over the entire study area is 0.82. The average stress ratio is close to unity, indicating σ 2 ≈ σ 3, and thus the dominant stress in this region is a uniaxial compression in the direction of σ 1. The direction of the σ 1-axis fluctuates locally at the southeastern end of the seismic fault ruptured by the 1891 Nobi earthquake. This fluctuation is limited to within a very narrow zone across the seismic fault in the upper crust shallower than approximately 10 km, suggesting that most of the deviatoric stress at the southeastern end of the seismic fault ruptured by the 1891 Nobi earthquake was not released

火山噴火と大地震の関係 : 日本とインドネシアの比較研究

近年、地震活動、火山噴火活動が活発化の傾向にあり、火山帯においてはそれらは相互相関していると考えられる。日本とインドネシアはたびたび地震や火山の影響を被ってきたが、その発生パターンには両国間に差異がある。時系列解析の結果、インドネシアでは地震は強い非定常性を示すのに対し、日本では季節性の変動を示す。時系列解析モデルARIMA(3, 1, 2) とARIMA(2, 0, 1)x(1, 0, 1) を各々インドネシアと日本における地震予測に適用した。 火山噴火と大地震には時間と距離に依存した関係がある。インドネシアに本両国において震央位置が迅速に得られ火山活動が十分モニターされるならば、この時間と距離の相関解析は地震の事前予測に役立つものであることが示唆される。Now a days eruptive activity and seismicity are increasing significantly, and in the volcanic belt, they are thought to be inter correlated. Japan and Indonesia are frequently affected by earthquakes and eruptions but occurring patterns are not similar in both the countries. The results of time series analysis indicate that the earthquakes follow strong non-stationary property for Indonesia but on the other hand, Japan has seasonal effect. By applying some time series tools ARIMA(3, 1, 2) and ARIMA(2, 0, 1)x(1, 0, 1) models has proposed for forecasting the earthquakes of Indonesia and Japan, respectively. The criterion for occurrence of a large earthquake based on timing of volcanic eruptions has a time-distance relationship. The results of the analysis strongly suggest that time-distance relations may help to predict an earthquake before it strikes if the epicentral location can be identified in advance and if the activity of the volcanoes is well monitored for both Japan and Indonesia.近年、地震活動、火山噴火活動が活発化の傾向にあり、火山帯においてはそれらは相互相関していると考えられる。日本とインドネシアはたびたび地震や火山の影響を被ってきたが、その発生パターンには両国間に差異がある。時系列解析の結果、インドネシアでは地震は強い非定常性を示すのに対し、日本では季節性の変動を示す。時系列解析モデルARIMA(3,1,2) とARIMA(2,0,1)x(1,0,1) を各々インドネシアと日本における地震予測に適用した。 火山噴火と大地震には時間と距離に依存した関係がある。インドネシアに本両国において震央位置が迅速に得られ火山活動が十分モニターされるならば、この時間と距離の相関解析は地震の事前予測に役立つものであることが示唆される。Now a days eruptive activity and seismicity are increasing significantly, and in the volcanic belt, they are thought to be inter correlated. Japan and Indonesia are frequently affected by earthquakes and eruptions but occurring patterns are not similar in both the countries. The results of time series analysis indicate that the earthquakes follow strong non-stationary property for Indonesia but on the other hand, Japan has seasonal effect. By applying some time series tools ARIMA(3,1,2) and ARIMA(2,0,1)x(1,0,1) models has proposed for forecasting the earthquakes of Indonesia and Japan, respectively. The criterion for occurrence of a large earthquake based on timing of volcanic eruptions has a time-distance relationship. The results of the analysis strongly suggest that time-distance relations may help to predict an earthquake before it strikes if the epicentral location can be identified in advance and if the activity of the volcanoes is well monitored for both Japan and Indonesia

Report on DELP 1986 Cruises in the Northwestern Pacific : Part III: Seismic Structure Revealed by Explosion Experiments

Several interesting features of the crust to upper mantle structure beneath the Northwest Pacific Basin were revealed by a seismic explosion experiment conducted during the DELP cruises in 1986. A large lateral heterogeneity was found both in the configuration of the crust/mantle structure and in the sub-Moho lithosphere. The data from the long range explosion experiment along the WSW-ENE survey line do not support the speculation of large scale anisotropy in this region. The lateral heterogeneities in the crust/mantle structure are indicated by variations of the velocity gradients within oceanic layer 3 and in the velocity transitions from layer 3 to the upper mantle. Wide-angle reflections from inside the sub-Moho lithosphere are clearly recorded with a good signal to noise ratio by small explosions of only 20kg at distances as far as 450km. However, the appearance of these phases differs from place to place. In some places, they are not observed. Lateral heterogeneity of this reflective layer seems to be related to such tectonic features as the fracture zone as well as Nakwe deepsea channel.1986年度夏に実施されたDELPによる北西太平洋海域の発破を利用した地震波構造探査によって,この海域の海底地殻下部から上部マントルの地震波速度構造に関して,いくつかの特徴的な解析結果が得られたので報告する.海洋地殻内部及びモホへの漸移形態のみならず,モホ以深に於ける速度構造にも地域的に非一様性がみられる. WSW-ENE方向の長距離発破測線から得られた記録からは,上部マントルにおける,大規模な速度異方性の存在を支持するような結果は見られなかった.モホ・マントルに関わる地域的非一様性は,海洋地殻第3層に於ける速度の勾配と,モホ面に於ける速度の跳びによって特徴づけられる.モホ以深のリソスフェア内部の何等かの層構造によると思われる広角反射波が,小薬量の発破に依っても450km先まで記録されている.この際だった反射面の存在様式は地震波速度構造の大規模な地域的非一様性と,地磁気によって示されているような破砕帯の存在に深く関係していると考えられる