Ocean Bottom Seismographic Observations Research Articles

Fault geometry of M6-class normal-faulting earthquakes in the outer trench slope of Japan Trench from ocean bottom seismograph observations

Since the 2011 Mw 9.0 Tohoku-oki earthquake, intra-plate normal-faulting earthquakes, including several M7-class earthquakes, have occurred in the outer trench slope area from the trench to the outer rise along the Japan Trench. Concerns regarding large earthquakes and associated tsunamis have also arisen. Based on aftershock distributions, several outer trench slope normal-faulting earthquakes (hereinafter referred to as outer-rise earthquakes) are likely related to the rupture of multiple faults. However, few observations have clearly shown how multiple faults act during outer-rise earthquakes. During the ocean bottom seismograph (OBS) observations in the outer trench slope area of the central Japan Trench from September 2017 to July 2018, three M6-class normal-faulting earthquakes (Mw 6.2 on September 20, Mw 6.2 on October 06, and Mw 6.0 on November 12) occurred around the OBS network. The near-field OBS observations provided detailed information on hypocenter locations and focal mechanisms of the mainshocks and aftershocks, including immediately after the mainshocks. We investigated the fault configurations of normal-faulting earthquakes based on OBS observations. During the September 2017 earthquake, the mainshock ruptured high-angle normal faults with a dip angle of 65°. Off-fault aftershock activities that were not directly related to the mainshock rupture and could be explained by the stress changes caused by the mainshock were confirmed. However, hypocenter distributions and focal mechanisms of the main and aftershocks of the October and November 2017 earthquakes suggest that the mainshock ruptured multiple faults with various dipping directions, angles, and strike orientations. The complicated fault geometry should be considered a possible fault model for large outer-rise earthquakes and related tsunamis.

Fault geometry beneath the western and Central Marmara Sea, Turkey, based on ocean bottom seismographic observations: Implications for future large earthquakes

Beneath the Marmara Sea, Turkey, the Main Marmara Fault (MMF), the offshore part of the North Anatolian Fault (NAF), is a well-known seismic gap for future M > 7 earthquakes. However, its detailed fault geometry and microearthquake activity have been debated for several decades. Using data acquired from long-term ocean bottom seismograph (OBS) observations, we made precise hypocenter estimations based on 3-D Vp and Vs velocity structures and assessed the fault geometry beneath the western and central parts of the MMF. The results indicate a segmentation boundary between the near-vertical western part and the south-dipping eastern part located around 28.10°E. Enriched OBS locations indicate microseismicity along both the inner and outer boundary faults of the Central Basin, especially on the western side. A comparison with previously published results suggests that the seismicity pattern has not changed for at least two years, between 2014 and 2016. Using a combined dataset of this and previous studies, lateral variations in the dip angle along the MMF fault segment from 27.4°E to 28.8°E were investigated. Based on this, we depicted on-fault seismicity along the MMF and defined three inactive areas of microseismicity. Two are located in the western segment, corresponding to the rupture area of the 1912 Ms. 7.4 earthquake, and the other, the largest, is located on the eastern segment. From a comparison of previous seismic and geodetic studies, it is considered that this area is a fully locked zone and has the potential for large earthquakes. Having compared the difference between hypocenter locations determined from OBSs and land-based stations, it is proposed that the epicentral locations of the mainshock and aftershocks of the September 26, 2019, M 5.7 earthquake are located much closer to the MMF than locations reported from only land-based results.

Geometry and segmentation of the North Anatolian Fault beneath the Marmara Sea, Turkey, deduced from long‐term ocean bottom seismographic observations

AbstractBoth the geometry and the depth of the seismogenic zone of the North Anatolian Fault under the Marmara Sea (the Main Marmara Fault (MMF)) are poorly understood, in part because of the fault's undersea location. We recorded 10 months of microseismic data with a dense array of ocean bottom seismographs and then applied double‐difference relocation and 3‐D tomographic modeling to obtain precise hypocenters on the MMF beneath the central and western Marmara Sea. The hypocenters show distinct lateral changes along the MMF: (1) both the upper and lower crust beneath the Western High are seismically active and the maximum focal depth reaches 26 km; (2) seismic events are confined to the upper crust beneath the region extending from the eastern part of the Central Basin to the Kumburgaz Basin; and (3) the magnitude and direction of dip of the main fault change under the Central Basin, where there is also an abrupt change in the depth of the lower limit of the seismogenic zone. We attribute this change to a segment boundary of the MMF. Our data show that the upper limit of the seismogenic zone corresponds to sedimentary basement. We also identified several seismically inactive regions within the upper crust along the MMF; their spatial extent beneath the Kumburgaz Basin is greater than beneath the Western High. From the comparison with seafloor extensometer data, we consider that these regions might indicate zones of strong coupling that are accumulating stress for release during future large earthquakes.

Normal‐faulting earthquakes beneath the outer slope of the Japan Trench after the 2011 Tohoku earthquake: Implications for the stress regime in the incoming Pacific plate

After the 2011 Mw 9.1 Tohoku earthquake, numerous intraplate earthquakes occurred beneath the outer slope of the Japan Trench. Based on ocean bottom seismograph observations, these earthquakes occurred in the oceanic crust and uppermost mantle of the Pacific plate at depths shallower than about 40 km and had normal‐faulting focal mechanisms at all depths. Before the 2011 earthquake, normal‐faulting earthquakes beneath the outer trench slope occurred only at depths shallower than 20 km, whereas those at depths of around 40 km had reverse‐faulting mechanisms. These observations suggest that the stress regime in the Pacific plate was changed by the 2011 earthquake. The tensional stresses that now extend to depths of about 40 km may play an important role not only in the occurrence of large normal‐faulting earthquakes but also in hydration of the uppermost mantle of the incoming Pacific plate prior to the subduction.

Spatial distribution of earthquakes off the east coast of the Kanto region along the Japan Trench deduced from ocean bottom seismographic observations and their relations with the aftershock sequence of the 2011 off the Pacific coast of Tohoku Earthquake

The 2011 Tohoku earthquake hit the forearc region of the Japan Trench on March 11, 2011. The rupture zone seemed to reach off the coast of the Kanto region. We had conducted ocean bottom seismographic observations off the coast of the Kanto in 2008 and estimated 851 hypocenter locations around the south part of the 2011 off the Pacific coast of Tohoku Earthquake by using over 50 ocean bottom seismometers (OBSs) and routine data jointly. The hypocenters distributed some clusters, and we found a few seismic gaps at the boundary zones of the clusters. The most remarkable seismic gap was positioned at the edge of intervened Philippine Sea plate (PHS) between the North American plate (NA) and Pacific plate (PAC). We compare the epicenter distributions with the 2011 aftershock distribution determined by routine data. The aftershocks are segmented spatially and there are some seismic gaps among the segments. The remarkable low places are consistent with the boundary zones of each cluster we estimated from the 2008 data. We infer that those regions have strong heterogeneity resulting from strong deformations caused by various subduction processes, such as intervening PHS between NA and PAC, seamount chains and changes in physical properties.

Aftershock distribution of the 26 December 2004 Sumatra-Andaman earthquake from ocean bottom seismographic observation

AbstractWe deployed an OBS network in February–March 2005 in the rupture area of the Sumatra Andaman earthquake on 26 December 2004. We placed 17 short-term OBSs and two long-term OBSs, and recovered OBSs after observation for 19–22 days. The hypocenter distribution from 10-day data of 17 OBS revealed the detailed structure of aftershock seismicity offshore of Sumatra Island. Aftershock seismicity associated with the subducting slab starts 40 km inward from the Sunda trench axis; it ceases at 50 km depth beneath the Aceh Basin, approximately 240 km inward from the trench axis. Aftershocks in 120–170 km from the trench axis consist of a surface with a dip of 10–12° dominated by a dip-extension type mechanism. Beyond the southwestern edge of the Aceh Basin, the aftershock activity becomes higher, and dominated by dip-slip type earthquakes, with a slightly increased dipping angle of 15–20°. Three along-arc bands of shallow seismicity were identified at 70 km inward from the Sumatra trench, 110 km inward from the trench, and in the south of the Aceh Basin. These locations correspond to steep topographic slopes in the accretionary prism, suggesting the present evolutional activity of the accretionary prism offshore Sumatra Island.

The seismicity and structure of Izu-Bonin arc mantle wedge at 31°N revealed by ocean bottom seismographic observation

The Izu-Bonin arc system is the subduction zone forming the plate boundary between the downgoing Pacific plate and the overriding Philippine Sea plate. Seismicity along the Izu-Bonin subduction zone is very different in character from other western Pacific subduction zones. Few large earthquakes have occurred at shallow depths (0–100 km), but many large earthquakes have occurred at greater depths (>400 km). Other unique characteristics of this subduction zone include the existence of serpentine seamounts exposed along the forearc slope and the presence of low-velocity (<7.3 km/s) material between the two plates in a zone extending from the forearc seamounts to the mantle wedge. To investigate these unique characteristics, we carried out an ocean-bottom seismic experiment in 1999 to estimate the hypocenter distribution and the structure of the mantle wedge simultaneously by performing 3D event locations employed within different velocity models. We obtained the following results: (1) No earthquake occurred on the upper surface of the subducting plate and some were located as far as 20 km away from the upper surface. Most events occurred within the mantle of the subducting slab. (2) There were no earthquakes in the mantle wedge above the subducting slab. (3) The low mantle velocity area in the mantle wedge terminates ∼140 km west of the trench axis. (4) The subducting slab has a dip of 55° to the west. From these results we suggest that the low-velocity material between the plates is chrysotile, a low-temperature, low-strength, low friction phase of serpentine, which may act as a lubricant on the plate boundary. The western boundary of the low mantle velocity region in the mantle wedge, which is about 140 km west of the trench axis and at a depth of about 20 km, coincides with the temperature-controlled transition from chrysotile to antigorite (the high-temperature phase of serpentine) along the plate boundary. Our results suggest that chrysotile may migrate upward and eastward along the plate boundary, while antigorite may move downward with the subducting slab.

Microseismicity around rupture area of the 1944 Tonankai earthquake from ocean bottom seismograph observations

Along the Nankai trough, southwestern Japan, the Philippine Sea plate (PSP) is subducting beneath the Eurasian plate, and large interplate earthquakes have occurred repeatedly with a recurrence interval of about 100–200 years. The most recent large thrust event in the eastern Nankai trough off Kii Peninsula was the 1944 Tonankai earthquake. In this region, current seismicity is very low and hypocenters are not determined accurately by the land seismic network. We conducted microseismicity observations around the rupture area of the 1944 Tonankai earthquake using ocean bottom seismographs (OBSs). Hypocenters were determined using a 2-D seismic velocity structure model based on an airgun-OBS seismic survey. Results obtained show that the seismicity was relatively active near the trough axis. These earthquakes may relate to deformation of the subducting Philippine Sea plate. On the other hand, microseismicity in the rupture area of the 1944 Tonankai earthquake was very low. This low-level seismic activity in the co-seismic rupture area of the 1944 Tonankai earthquake likely relates to a single large asperity off Kii Peninsula.

Microseismicity around rupture area of the 1944 Tonankai earthquake from ocean bottom seismograph observations

Along the Nankai trough, southwestern Japan, the Philippine Sea plate (PSP) is subducting beneath the Eurasian plate, and large interplate earthquakes have occurred repeatedly with a recurrence interval of about 100–200 years. The most recent large thrust event in the eastern Nankai trough off Kii Peninsula was the 1944 Tonankai earthquake. In this region, current seismicity is very low and hypocenters are not determined accurately by the land seismic network. We conducted microseismicity observations around the rupture area of the 1944 Tonankai earthquake using ocean bottom seismographs (OBSs). Hypocenters were determined using a 2-D seismic velocity structure model based on an airgun-OBS seismic survey. Results obtained show that the seismicity was relatively active near the trough axis. These earthquakes may relate to deformation of the subducting Philippine Sea plate. On the other hand, microseismicity in the rupture area of the 1944 Tonankai earthquake was very low. This low-level seismic activity in the co-seismic rupture area of the 1944 Tonankai earthquake likely relates to a single large asperity off Kii Peninsula.

Seismicity and crustal structure related to the Miyake‐jima volcanic activity in 2000

The largest earthquake swarm ever recorded in Japan began off the islands of Miyake‐jima, Kozu‐shima and Nii‐jima, and was linked to volcanic activity on Miyake‐jima, at the northern end of the volcanic front of the Izu‐Ogasawara (Bonin) arc. Ocean bottom seismographic observations made directly above the swarm region gave clear indication of a magmatic intrusion. The hypocenter distribution shows a vertical sheet, 2 km wide and at a depth of 12–20 km, that broadens markedly at depths up to 12 km. The P wavespeed structure obtained from a seismic refraction survey shows no significant difference in the characteristics of the shallow crust between the strong swarm region and inactive region. However, a low wavespeed region was detected 10–15 km northwest of Miyake‐jima.

Aftershock distribution of the 1994 Sanriku‐oki earthquake (Mw 7.7) revealed by ocean bottom seismographic observation

The 1994 Sanriku‐oki earthquake (Mw 7.7) ruptured the entire seismogenic zone of the Pacific plate subduction beneath northeastern Japan arc. We deployed 18 ocean bottom seismographs and recorded its aftershocks, which span the whole seismogenic zone, with unprecedented accuracy. We calculate hypocenters and mechanisms to improve our understanding of the plate subduction geometry and plate coupling. The rupture of the mainshock initiated at the updip end of the seismogenic zone defined as the trenchward limit of the aftershock and background seismicity. The rupture continued to slip along a shallow dipping (&lt;10°) interface for ∼60 km, and then slipped across an asperity where the dip angle starts to increase at ∼143°E. Here aftershocks are few, suggesting complete release of strain. The rupture continued another 100 km or so to the downdip end beneath the coastline, where the plate dip angle reaches 30° to smoothly connect to the dip angle of the Wadati‐Benioff zone. Coseismic slips are larger westward of 143°E, where the mantle wedge comes into contact with the subducting plate. The stress redistribution due to the mainshock seems to have caused aftershocks with normal faultings to occur offset above and below the plate boundary.

海底地震観測によるトカラ海峡-奄美大島北部海域の地震活動

海底地震観測によるトカラ海峡-奄美大島北部海域の地震活動

海底地震観測による Tj&amp;ouml;rnes Fracture Zone の地震活動

海底地震観測による Tj&amp;ouml;rnes Fracture Zone の地震活動

Earthquake distribution in the subduction zone off eastern Hokkaido, Japan, deduced from ocean-bottom seismographic and land observations

SUMMARY In 1986, observations of microearthquakes were conducted with the use of five ocean-bottom seismometers (OBSs) on the continental slope east of the Kushiro Submarine Canyon (KSC) off Hokkaido Island, Japan. The observation area is located about 50-100 km landward of the southern Kuril Trench beneath which the Pacific Plate is subducted toward Hokkaido Island. The OBS data were analysed together with land data on Hokkaido Island to determine the precise earthquake distribution and the geometry of the subducted plate. The OBS data revealed for the first time the seaward seismicity east of KSC. High and shallow seismicity is concentrated in an area between the coast line of Hokkaido Island and the 2000 m isobath. A region between the 2000 and 4000 m isobaths corresponds to the source area of an interplate earthquake occurring in 1973 (the Nemuro Hanto-Oki earthquake, M= 7.4). The OBS array detected more than 10 events in this region. Most of them, however, are classified as earthquakes occurring within the subducted plate, and the remarkable aftershock activity of the 1973 event is not recognized. The seismic activity is extremely low between the 4000 and 6000 m isobaths as is commonly seen in subduction zones. The seismicity is also very low beneath the trench, in contrast with the adjacent trench areas. In a vertical section taken perpendicular to the trench axis, most of the located events form a seismic plane dipping landward. The dip-angle of the plane is 20d-25d beneath the continental slope, while 35d-40d under Hokkaido Island. The change in dip angle occurs 150 km landward of the trench axis. This seismic plane is located 30-40 km below the plate boundary defined from the aftershock distribution of the Nemuro Hanto-Oki event. The seismicity on the plate boundary is correlated with the geometry of the subducted plate. The high activity occurs in a restricted region where the subducted plate shows a remarkable bend. The seismic activity obtained is different from those in the adjacent regions along the southern Kuril Trench. In particular, the regional difference in seismicity is remarkable between the western and eastern sides of KSC. Our result suggests that the subduction zone along the Kuril Trench is divided into several tectonic blocks and KSC is one of their boundaries.

海底地震観測による1987年6月の福島沖の地震活動

海底地震観測による1987年6月の福島沖の地震活動

Spatial distribution of earthquakes off Sanriku, northeastern Japan, in 1989 determined by ocean-bottom and land-based observation.

In October-November, 1989, five M≥6 earthquakes, including an M7.1 earthquake, sequentially took place off Sanriku, northeastern Japan, and the earthquake activity became very high in this region. Since the seismically active zone was far from the land observation network, it was difficult to obtain accurate hypocenter locations. We carried out ocean bottom seismographic observation in order to locate the hypocenters precisely and investigate characteristics of the activity. Combining the OBS and land station data for hypocenter determination, we obtained two main conclusions. (1) Almost all epicenters of the present activity distribute landward from the 4 km isobath. Some earthquake clusters were observed including an M5.6 earthquake and its aftershocks. (2) Focal depths of the earthquakes are shallower than 20 km and the activity is interpreted to take place around the boundary between the subducting Pacific plate and the landward plate. Taking account of the mechanisms of the larger earthquakes in this active period, this activity is a thrust fault type and is different from the 1933 Sanriku earthquake which was a normal fault type and might have broken the entire oceanic lithosphere.

Accumulations of melt at the base of young oceanic crust

As hot, solid rock ascends through the mantle, the pressure around it decreases while its temperature decreases approximately adiabatically. During the ascent, the melting temperature decreases faster than the adiabat temperature, and if the initial temperature of the rock is high enough, the rock will begin to melt at the depth where the adiabat crosses the melting-temperature curve. Models of upwelling beneath mid-ocean ridges predict significant melt production over a zone several tens of kilometres wider than the ridge-axis region suggesting that some of the ascending melt must arrive away from the ridge axis. Here I report ocean-bottom seismograph observations of an uncommon and previously unreported marine seismic phase, which provide strong evidence that melt accumulates in sills at the base of young oceanic crust. The phase results from the conversion of compressional (P) waves to shear (S) waves on interaction with the crust-mantle boundary (the Moho). This phase is rare and very difficult to model, but when visible, it is strikingly prominent and usually appears at unexpectedly great source-to-receiver ranges. The observations imply the presence of bodies of extremely low rigidity, presumably melt sills, at the Moho close to the receiver. Melt may be fairly mobile at the base of the crust, allowing ridges to accumulate melt through a lateral plumbing system at the locally uplifted boundary between the crust and the underlying thermally expanded mantle. Escape of this melt to the sea floor may be responsible for the formation of off-axis seamounts.

Spatial distribution of microearthquakes beneath the Japan Trench from ocean bottom seismographic observations

The area of the Japan Trench, about 40°N off the Tôhoku District of north-eastern Honshu, Japan, was for the first time extensively covered by an ocean bottom seismograph (OBS) array. Seven OBSs were available for analyses of natural earthquakes and the observation period lasted two weeks during 1980 July and August. During this period, each OBS registered between 300 and 600 earthquakes. About 100 events were simultaneously recorded by at least three OBSs. Hypocentres were calculated by using a generalized inversion technique. Sixty-five events, mostly with magnitudes (M) from 1.5 to 3.0, which were not located by a land network, were determined to be within or near the area covered by the OBS array. The OBS array had nearly uniform detectability for earthquakes with M over 2.0 within the latitude range of 39.5° — 41.5°N and the longitude range of 143° — 146.5°E. Within this area there is a highly active region of microseismicity beneath the seaward wall of the Japan Trench. The seaward activity is concentrated within 100 km of the trench axis and is low beneath the Northwest Pacific Basin. A seismicity gap was found just westward of the trench axis, beneath the landward wall of the trench at a water depth of between 6000 and 4000 m. Seismic activity continues landward from the region west of the gap. The OBS array enabled us to determine the focal depth of microearthquake activity with better resolution than was previously possible. The results show that the microearthquake activity is concentrated within the upper part of the oceanic lithosphere. On the seaward side, most events occur at a depth of less than 30 km beneath the trench wall, while the activity on the landward side starts to deepen westward exactly at the trench axis. This paper reports the first direct observations to clarify the spatial distribution of microearthquakes associated with the bending and/or subducting of the oceanic plate just beneath the Japan Trench axis.

Some ocean-bottom seismograph observations on the Reykjanes Ridge at 59 N

Epicentres of microearthquakes detected by two ocean-bottom seismographs deployed on the Reykjanes Ridge at latitude 59° N can be interpreted as trending between 010° and 020° E of N, parallel to detailed morphological features seen in the area, rather than the overall trend of the ridge (036° E of N). Travel times from an airgun source to the seismographs indicate much slower P wave velocities for the top 4 km at the ridge crest compared with that 5–10 km off axis.

Ocean-bottom seismograph observations on the Mid-Atlantic Ridge near 45 N -- further results

Summary. Analysis of data from events well recorded by ocean-bottom seismographs on the Mid-Atlantic Ridge near 45° N, indicates that most activity is centrally placed beneath the median valley floor. The results suggest a lithospheric thickness of 8 km under the median valley floor. The spatial and temporal behaviour of the largest swarm observed is indicative of active magma intrusion.