Ocean Bottom Seismograph Data Research Articles

Microseismicity Indicates Atypical Small‐Scale Plate Rotation at the Quebrada Transform Fault System, East Pacific Rise

AbstractClosely spaced, multi‐strand ridge transform faults (RTFs) accommodate relative motions along fast spreading mid‐ocean ridges. However, the relations between RTFs and plate spreading dynamics are poorly understood. The Quebrada system is one of the most unique RTF systems at the East Pacific Rise, consisting of four transform faults connected by three short intra‐transform spreading centers (ITSCs). We use seven‐months of ocean bottom seismograph data to study the Quebrada system, and find abundant earthquakes unevenly distributed among three active faults. We identify two deep, diffuse seismicity clouds at the inside corners of the ITSC‐transform fault intersections, and one seismically active fracture zone. The observations suggest a complex regional plate‐motion pattern, including possible heterogeneous rotations within the Quebrada system. Evolution of multi‐strand RTFs may have resulted from a strong three‐dimensional local thermal and fluid effects, while the RTFs may have also regulated regional tectonics, forming an intricate feedback system.

Evaluations of an ocean bottom electro-magnetometer and preliminary results offshore NE Taiwan

Abstract. The first stage of field experiments involving the design and construction of a low-power consumption ocean bottom electro-magnetometer (OBEM) has been completed, which can be deployed for more than 180 d on the seafloor with a time drift of less than 0.95 ppm. To improve the performance of the OBEM, we rigorously evaluated each of its units, e.g., the data loggers, acoustic parts, internal wirings, and magnetic and electric sensors, to eliminate unwanted events such as unrecovered or incomplete data. The first offshore deployment of the OBEM together with ocean bottom seismographs (OBSs) was performed in NE Taiwan, where the water depth is approximately 1400 m. The total intensity of the magnetic field (TMF) measured by the OBEM varied in the range of 44 100–44 150 nT, which corresponded to the proton magnetometer measurements. The daily variations in the magnetic field were recorded using the two horizontal components of the OBEM magnetic sensor. We found that the inclinations and magnetic data of the OBEM varied with two observed earthquakes when compared to the OBS data. The potential fields of the OBEM were slightly, but not obviously, affected by the earthquakes.

Using Mirror Migration of OBS Data to Image the Deepwater Area of South China Sea

Ocean bottom seismograph (OBS) is widely used in investigating deep crustal structure, which is characterized by a large amount of data information and abundant frequency components because of its multi-component acquisition. OBS is seldom used in deepwater oil and gas exploration and basin research due to the high cost. The complicated seismic wave field is caused by the complex seabed topography, basin and oil and gas structure in deepwater area, which increases the difficulty of image processing. In addition to reflection imaging, we utilize the multiple of OBS data to make accurate imaging and have achieved desirable results in a deep sea area in South China Sea in this paper. Firstly, the original P and Z components of OBS data are processed by wave field separation to obtain the upgoing wave filed and downgoing wave filed. Secondly, its image velocity filed is constructed. Finally, downgoing wave data is used to image (called mirror migration). Compared with conventional migration, the mirror migration can clearly image the seabed and provide better illumination for shallow layer below the seafloor in the case of sparse nodes, which is proved by the migration results of theoretical and real data in this paper.

The identification and application of multiple phases recorded by ocean bottom seismometer

In recent years, data obtained from ocean bottom seismographs (OBS) have played a major role in studies of the oceanic crust and mantle structure, thus greatly facilitating improved understanding of deep structures in the Earth. In addition to primary, first arrival phases, multiple seismic phases can often be detected in OBS data sections. These phases tend to have high energy and good continuity, but are often rejected as invalid signals in conventional OBS data processing. However, multiple phases can be real reflections of subsurface interfaces, and contain a large amount of important information about subsurface structures. For example, in shallow sediments with poor resolution of first arrival phases, an inversion using multiple phases can improve the resolution of the sediments. Similarly, deep multiple phases can also reflect the media characteristics near the Moho, and improve the imaging precision of crustal structures. By analyzing the measured data from seismic line OBS2013-ZN at stations OBS06 and OBS07, the secondary Pg, PmP and Pn phases are identified behind the first arrival Pg, PmP, and Pn phases, with travel time differences in the 1.53–1.62 s range. Analysis of waveform characteristics reveals that the amplitude of the secondary phases is more prominent, and greater than that of the first arrival phases in some cases. The waveform characteristics of these two phases are very similar in terms of apparent velocity. The overall form of the multiple phases is also approximately the same as that of the first arrivals, with a correlation coefficient of ~0.8. According to the recognition characteristics described, accurate arrival times of the first arrival and secondary phases are detected in the seismic sections of stations OBS06 and OBS07. The theoretical travel time is first calculated from the secondary phases based on the fire time, offset, and reduced time of seismic phases. After repeated analysis and identification, the uncertainty in detection is set to 80–150 ms. A total of 151 secondary phases are detected at OBS06 and 110 at OBS07, and first arrival phases are detected from these stations with an uncertainty of 80–110 ms. To determine the secondary reflecting interface corresponding to the secondary seismic phases detected at OBS06 and OBS07, their propagation paths are analyzed by ray tracing. A new local velocity structure is obtained using travel time inversion based on the original P-wave velocity model by combining the first arrival and secondary phases. The results show that the velocity structure changes in both shallow sediments and mid to deep crust-mantle depths compared with previous results of processing first arrival phases. The fitting results of travel time shows that the secondary phases traversed sedimentary intervals several times and increased the ray-coverage density in the model, providing more constraints on travel-time inversion and improved imaging precision of the sediments and crust. To validate the reliability of the imaging method, the average one-dimensional velocity is calculated for the region directly below OBS06 and OBS07 based on the velocity structure obtained. Next, the single-channel seismic waveforms of OBS06 are synthesized by forward modeling of reflectivity and these are compared with the single-channel waveforms in the actual seismic data. The results show that the first arrival phases of synthetic and actual seismic waveforms are highly consistent in terms of both travel time and form. These findings demonstrate the reliability of the secondary seismic phases detected, and also validate the reliability of the propagation paths of secondary seismic phases. This in turn shows that the velocity structure obtained is credible. This study provides a reference for making full use of wide-angle OBS data and future studies of multiple phases, and has implications for investigating fine details of crustal structures, particularly in terms of improving imaging precision.

Interferometric OBS imaging for wide-angle seismic data

Marine wide-angle seismic data obtained using air guns and ocean-bottom seismographs (OBSs) are effective for determining large-scale subseafloor seismic velocities, but they are ineffective for imaging details of shallow seismic reflection structures because of poor illumination. Surface-related multiple reflections offer the potential to enlarge the OBS data illumination area. We have developed a new seismic imaging method for OBS surveys applying seismic interferometry, a technique that uses surface-related multiples similarly to mirror imaging. Seismic interferometry can use higher order multiple reflections than mirror imaging, which mainly uses first-order multiple reflections. A salient advantage of interferometric OBS imaging over mirror imaging is that it requires only single-component data, whereas mirror imaging requires vertical geophone and hydrophone components to separate upgoing and downgoing wavefields. We applied interferometric OBS imaging to actual 175 km long wide-angle OBS data acquired in the Nankai Trough subduction zone. We obtained clear continuous reflection images in the deep and shallow parts including the seafloor from the OBS data acquired with large spacing. Deconvolution interferometry is more suitable than correlation interferometry to improve spatial resolution because of the effects of spectral division when applied to common receiver gathers. We examined the imaging result dependence on data acquisition and processing parameters considering the data quality and target depth. An air-gun-to-OBS distance of up to 50 km and a record length of 80 s were necessary for better imaging. In addition, our decimation tests confirmed that denser OBS spacing yielded better quality and higher resolution images. Understanding crosstalk effects due to the acquisition setting will be useful to optimize methods for eliminating them. Interferometric OBS imaging merged with conventional primary reflection imaging is a powerful method for revealing crustal structures.

S-wave velocity structures and Vp/Vs ratios beneath the South Yellow Sea from ocean bottom seismograph data

In 2013, a wide-angle seismic survey (OBS2013) was conducted perpendicular to the south coast of the Shandong Peninsula in China, to investigate the crustal structure of the South Yellow Sea (SYS). Previous interpretation of the crustal structure of SYS did not use S-wave and Vp/Vs ratios as a constraint. In this study, we constructed the converted S-wave velocity model and Vp/Vs ratios of the northern SYS. Many receiver gathers showed good reflected and refracted S-phases, particularly in the Qianliyan Uplift. The S-wave crustal structure and Vp/Vs ratios were obtained based on a previous P-wave model which utilized RayInvr software by adjusting travel times. Results demonstrated that the S-wave velocities increased with depth, as in the P-wave model. This paper provided a lithologic interpretation of the velocity model. In the uppermost layer beneath the water, most Vp/Vs ratios were high (>3). The nappe beneath the northwestern OBS2013 mainly consisted of granite and felsic gneiss. In the North Depression (ND), the Vp/Vs ratios of the marine sedimentary layer displayed the characteristics of carbonate rock. However, in the southeast of the ND, the sediment beneath the continental deposit layer was most likely composed of sandstone. The marine sedimentation in the Central Uplift (CU) mainly probably consisted of carbonate rocks with sandstone. Lack of strata with rich sand in the Permian and Triassic period suggested there was a more drastic uplift in the ND than the one in the CU during the Indo-Chinese epoch.

Seismicity within the Philippine Sea Plate South of the Nankai Trough Axis off the Kii Peninsula: Estimates from Ocean Bottom Seismographic Data in 2013 and 2014

Seismicity within the Philippine Sea Plate South of the Nankai Trough Axis off the Kii Peninsula: Estimates from Ocean Bottom Seismographic Data in 2013 and 2014

Australian national ocean bottom seismograph fleet advances conventional exploration

A fleet of new Australian ocean bottom seismographs (OBSs) have broadband frequency range, and similar instruments are available at only five or six institutions globally. These OBSs are multi-purpose devices able to record passive-source seismic data (earthquakes, ambient noise) as well as active-source (airgun generated) data and, at the same time, to monitor seismic survey noise and whale calls for environmentally responsible exploration. OBS data collected during commercial seismic surveys in Australian waters prove that it is possible to image the velocity distribution of the whole crust and upper mantle from analysis of both reflected and refracted phases generated by an industry-standard broadband airgun array. This means that valuable information on a regional scale can be obtained as a by-product of commercial seismic surveys. Three-component recording capability of OBSs allows analysis of S-waves in addition to the P-waves that are conventionally used in marine reflection surveys.

Offshore seismicity in the western Marmara Sea, Turkey, revealed by ocean bottom observation

The faults’ geometry and their seismic activity beneath the Marmara Sea have been under debate for a couple of decades. We used data recorded by three ocean bottom seismographs (OBSs) over a period of 3 months in 2014 to investigate the relationship of fault geometry to microseismicity under the western Marmara Sea in Turkey. We detected a seismic swarm at 13 to 20 km depth beneath the main Marmara fault (MMF), and the maximum depth of seismogenic zone was 25 km within the OBS observation area. These results provided evidence that the dip of the MMF is almost vertical and that the seismogenic zone in this region extends into the lower crust. Our analysis of past seismicity indicated that the seismic swarm we recorded is the most recent of an episodic series of seismic activity with an average recurrence interval of 2–3 years. The repetitive seismicity indicates that the MMF beneath the western Marmara Sea is coupled and that some of the accumulated strain is released every 2 to 3 years. Our study shows that OBS data can provide useful information about seismicity along the MMF, but more extensive studies using more OBSs deployed over a wider area are needed to fully understand the fault geometry and stick–slip behavior of faults under the Marmara Sea.

Ocean‐Bottom Seismograph Performance during the Cascadia Initiative

Online Material: Figures showing channel uptime and quality. The Cascadia Initiative (CI) is a community designed and implemented onshore/offshore seismic and geodetic network deployed along the Cascadia subduction zone from northern California to Vancouver Island (Toomey et al. , 2014). The goal of the initiative was to investigate a variety of questions related to megathrust earthquakes (e.g., Atwater et al. , 1995), episodic tremor and slip (e.g., Rogers and Dragert, 2003), volcanic arc structure (e.g., Parsons et al. , 1998), and the overarching interactions between the Juan de Fuca and Gorda plates. The CI operated from 2011 to 2015 and was funded by the National Science Foundation as part of the American Recovery and Reinvestment Act. The funding supported the construction of an amphibious array (AA), which comprised 60 three‐component broadband ocean‐bottom seismographs (OBSs), the deployment and operation of land‐based seismographs as part of the USArray Transportable Array, and the upgrade of Plate Boundary Observatory Global Positioning System (GPS) stations to high‐rate real‐time data. The Woods Hole Oceanographic Institution (WHOI), Scripps Institution of Oceanography (SIO), and the Lamont–Doherty Earth Observatory (LDEO) constructed the OBSs as part of the Ocean Bottom Seismograph Instrument Pool (OBSIP). The Cascadia Initiative Expedition Team (CIET; cascadia.uoregon.edu) deployed the OBSs over four consecutive summers from 2011 to 2014, and each deployment lasted approximately one year. The CIET team deployed almost four times more instrumentation than the average broadband OBSIP experiment, and data recovered from the CI are openly available to the entire scientific community immediately after instrument recovery. In an effort to make the CI data widely available, the Incorporated Research Institutions for Seismology (IRIS) OBSIP Management Office (OMO) performs data quality assessments as part of archiving the data at the IRIS Data Management Center (DMC). In this study, we focus on OBS station performance, data return, and horizontal …

Seismological imaging of ridge–arc interaction beneath the Eastern Lau Spreading Center from OBS ambient noise tomography

The Lau Basin displays large along-strike variations in ridge characters with the changing proximity of the adjacent subduction zone. The mechanism governing these changes is not well understood but one hypotheses relates them to interaction between the arc and back-arc magmatic systems. We present a 3D seismic velocity model of the shallow mantle beneath the Eastern Lau back-arc Spreading Center (ELSC) and the adjacent Tofua volcanic arc obtained from ambient noise tomography of ocean bottom seismograph data. Our seismic images reveal an asymmetric upper mantle low velocity zone (LVZ) beneath the ELSC. Two major trends are present as the ridge-to-arc distance increases: (1) the LVZ becomes increasingly offset from the ridge to the north, where crust is thinner and the ridge less magmatically active; (2) the LVZ becomes increasingly connected to a sub-arc low velocity zone to the south. The separation of the ridge and arc low velocity zones is spatially coincident with the abrupt transition in crustal composition and ridge morphology. Our results present the first mantle imaging confirmation of a direct connection between crustal properties and uppermost mantle processes at ELSC, and support the prediction that as ELSC migrates away from the arc, a changing mantle wedge flow pattern leads to the separation of the arc and ridge melting regions. Slab-derived water is cutoff from the ridge, resulting in abrupt changes in crustal lava composition and crustal porosity. The larger offset between mantle melt supply and the ridge along the northern ELSC may reduce melt extraction efficiency along the ridge, further decreasing the melt budget and leading to the observed flat and faulted ridge morphology, thinner crust and the lack of an axial melt lens.

Removing Infragravity-Wave-Induced Noise from Ocean-Bottom Seismographs (OBS) Data Deployed Offshore of Taiwan

Abstract Vertical ocean‐bottom seismograph (OBS) data at frequencies below 0.05 Hz are contaminated by noise induced by infragravity waves. We constructed the transfer function between pressure and velocity data from OBSs deployed in Taiwan waters to remove the wave pressure‐induced noise from seismic recordings. Data were analyzed from five portable broadband OBSs deployed each for 10 months at water depths from 1740 to 4600 m and from a cabled, shallow‐buried seismograph (EOS1) installed on the seafloor at 300 m depth. Removing long‐period noise from these OBS data improves the identification of teleseismic phases such as P , S , SS , P diff , and PKIKP that are otherwise ambiguous or unidentifiable. For EOS1, infragravity‐wave signals completely mask the P and S waveforms in the 10–50 s period band suitable for centroid moment tensor (CMT) solutions for most of the local events. Application of the transfer functions to predict and remove wave deformation yielded clean prominent P and S waveforms at these periods and aided in the CMT determination for small events jointly with land stations. The relative amplitudes of the wavenumber‐normalized transfer function for some of the OBSs are mostly determined by the thickness of the sediment at the OBS site.

Misfit functionals in Laplace‐Fourier domain waveform inversion, with application to wide‐angle ocean bottom seismograph data

ABSTRACTIn seismic waveform inversion, non‐linearity and non‐uniqueness require appropriate strategies. We formulate four types of L2 normed misfit functionals for Laplace‐Fourier domain waveform inversion: i) subtraction of complex‐valued observed data from complex‐valued predicted data (the ‘conventional phase‐amplitude’ residual), ii) a ‘conventional phase‐only’ residual in which amplitude variations are normalized, iii) a ‘logarithmic phase‐amplitude’ residual and finally iv) a ‘logarithmic phase‐only’ residual in which the only imaginary part of the logarithmic residual is used. We evaluate these misfit functionals by using a wide‐angle field Ocean Bottom Seismograph (OBS) data set with a maximum offset of 55 km. The conventional phase‐amplitude approach is restricted in illumination and delineates only shallow velocity structures. In contrast, the other three misfit functionals retrieve detailed velocity structures with clear lithological boundaries down to the deeper part of the model. We also test the performance of additional phase‐amplitude inversions starting from the logarithmic phase‐only inversion result. The resulting velocity updates are prominent only in the high‐wavenumber components, sharpening the lithological boundaries. We argue that the discrepancies in the behaviours of the misfit functionals are primarily caused by the sensitivities of the model gradient to strong amplitude variations in the data. As the observed data amplitudes are dominated by the near‐offset traces, the conventional phase‐amplitude inversion primarily updates the shallow structures as a result. In contrast, the other three misfit functionals eliminate the strong dependence on amplitude variation naturally and enhance the depth of illumination. We further suggest that the phase‐only inversions are sufficient to obtain robust and reliable velocity structures and the amplitude information is of secondary importance in constraining subsurface velocity models.

Structural heterogeneities around the megathrust zone of the 2011 Tohoku earthquake from tomographic inversion of onshore and offshore seismic observations

We performed a three-dimensional seismic tomography around the coseismic slip area of the 2011 Tohoku earthquake. By combining data from the aftershock period collected by ocean bottom seismographs (OBSs), OBS data from previous studies off the Miyagi coast, and land seismic data, we modeled the detailed seismic velocity structure along the plate boundary from the Japan Trench to near the coastline. Our results indicate that VP, VS, and VP/VS along the plate boundary change drastically about 60 km landward from the trench axis. Trenchward of this boundary, velocities are consistent with a fluid-rich environment (low VP and VS and high VP/VS) associated with the occurrence of sediment compaction and opal-to-quartz metamorphism. This area also corresponds to the contact between the slab and upper crust or between the slab and the sediment layer. A comparison of our results with numerical simulations and geological studies suggests that thermal pressurization might have occurred near the trench axis during the 2011 Tohoku earthquake. To the west of this boundary, where VP, VS, and VP/VS have moderate values, our model showed small-scale heterogeneous structures in the subducted oceanic crust near the hypocenter of the Tohoku earthquake with regions of low and high VP/VS on the updip and downdip sides of the hypocenter, respectively. We interpret the former as corresponding to the strong coupling patch and the latter as the localized fluid-rich zone. Small-scale heterogeneities thus may affect the nucleation and rupture processes of large earthquakes.

Seismic imaging and velocity structure around the JFAST drill site in the Japan Trench: low Vp, high Vp/Vs in the transparent frontal prism

Seismic image and velocity models were obtained from a newly conducted seismic survey around the Integrated Ocean Drilling Program (IODP) Japan Trench Fast Drilling Project (JFAST) drill site in the Japan Trench. Pre-stack depth migration (PSDM) analysis was applied to the multichannel seismic reflection data to produce an accurate depth seismic profile together with a P wave velocity model along a line that crosses the JFAST site location. The seismic profile images the subduction zone at a regional scale. The frontal prism where the drill site is located corresponds to a typically seismically transparent (or chaotic) zone with several landward-dipping semi-continuous reflections. The boundary between the Cretaceous backstop and the frontal prism is marked by a prominent landward-dipping reflection. The P wave velocity model derived from the PSDM analysis shows low velocity in the frontal prism and velocity reversal across the backstop interface. The PSDM velocity model around the drill site is similar to the P wave velocity model calculated from the ocean bottom seismograph (OBS) data and agrees with the P wave velocities measured from the core experiments. The average V p/V s in the hanging wall sediments around the drill site, as derived from OBS data, is significantly larger than that obtained from core sample measurements.

PS-wave processing and S-wave velocity inversion of OBS data from Northern South China Sea

Abstract It is very important to convert seismic data from the time domain to the depth domain so we can explain the geological information more obviously. This paper provides a method for time to depth conversion. PS-wave data from an Ocean bottom seismograph (OBS) survey of the Northern South China Sea was processed through the separation of PP and PS wavefields and the rotation of horizontal components. An inverse modeling of travel times was performed for the determination of the S-wave velocity (Vs). The migration section of the single channel seismic data was used to define the model horizons and to help control their geometry. Wide angle hydrophone data of OBS were used to determine the P-wave travel times and wide angle horizontal component data of OBS were used to determine the PS-wave travel times. The chosen travel times from various shots were inverted for P- and S-wave interval velocities using RayInvr , which calculates theoretical travel times via ray tracing. Damped least squares optimization is performed to fine-tune the fits between observed and calculated travel times. The achieved trends of the P-wave velocity (Vp) and Vs curves are similar and the velocity increases in the layer where gas hydrates are present.

On acoustic waveform tomography of wide-angle OBS data--strategies for pre-conditioning and inversion

We successfully apply the acoustic Laplace–Fourier waveform tomography method to delineate P-wave velocity structures of the mega-splay fault system in the central part of the seismogenic Nankai subduction zone offshore Japan, using densely sampled wide-angle ocean bottom seismograph (OBS) data originally acquired in 2004. Our success is due to new and carefully designed data preconditioning and inversion strategies to mitigate (i) the well-known non-linearity of waveform inversion, (ii) the challenges arising from crustal-scale survey designs (e.g. undersampling of the OBSs), and (iii) modelling errors due to the use of the acoustic assumption. We identify a sixfold set of key components that together lead to the success of the high-resolution waveform tomography image: (i) Availability of low-frequency components (starting at 2.25 Hz) reducing the non-linearity, and access to large offset data (up to 55 km) increasing the depth of illumination and the recovery of low wavenumber components. (ii) A highly accurate traveltime tomography result (with an rms error of approximately 60 ms) that further mitigates the non-linearity. (iii) A hierarchical inversion approach in which phase spectra are inverted first to reduce artefacts from the acoustic assumption, and amplitude information is only incorporated in the final stages. (iv) A Laplace–Fourier domain approach that facilitates a multiscale approach to mitigate non-linearity by restricting the inversion to the low frequency components and early arrivals first, and sequentially including higher frequencies and later arrivals. (v) A pre-conditioning strategy for eliminating undesirable high wavenumber components from the the gradient. (vi) A strategy for source estimation that reduce the influence of the instrumental design. In the OBS case study used for illustration purposes, Laplace–Fourier waveform tomography retrieves velocity anomalies as small as 700 m (horizontally) and 350 m (vertically) above the top of the Philippine Sea Plate. The resulting velocity structures include low-velocity zones and thrust structures which have not been previously identified clearly. The velocity models are validated by scrutiny of synthetic and observed waveforms, by evaluating the coherency of source estimates, and by comparison with 3-D pre-stack migrated (PreSDM) images. Chequerboard tests and point-scatter tests demonstrate both the reliability and the limitations of the acoustic implementation.

Towards upgraded capability in offshore seismic studies with the new National Australian Pool of Ocean Bottom Seismographs

In 2013, Australia, for the first time in its history, will obtain a national pool of ocean-bottom seismographs (OBSs) suitable for multi-scale experiments at sea and for onshore-offshore combined observations. Twenty broadband OBS instruments will be purchased for short- and long-term deployment (up to 12 months) to a maximum depth of 6 km. The instruments will be made available to Australian researchers through ANSIR, with only the costs of mobilisation and deployment to be met. It is anticipated that the OBS facility will greatly enhance the research capabilities of Australian scientists in the area of Earth imaging, offshore exploration, and natural hazard assessment. OBS experiments in Australia have been limited so far, with the only data set collected by Geoscience Australia in 1995–96 on a number of coincident reflection/refraction seismic transects across the northwestern Australian Margin. The main findings from that experiment will be reviewed in the context of recent OBS data processing and acquisition advances. The scope of the experiments with the new national Australian pool of OBSs will be presented, as well as practicalities and logistics of the OBS experiments.

Seismicity near the hypocenter of the 2011 off the Pacific coast of Tohoku earthquake deduced by using ocean bottom seismographic data

We relocated hypocenters of the foreshock, mainshock, and aftershocks of the 2011 off the Pacific coast of Tohoku earthquake (M 9.0) in the middle part of the Japan Trench where the earthquake rupture initiated. Ocean Bottom Seismographs (OBSs), deployed in the area, recorded the earthquakes and these data provide improved images of the hypocenter distribution. The mainshock hypocenter was relocated slightly westward from that reported by Japan Meteorological Agency (JMA), placing it near the intersection between the plate boundary and the Moho of the overriding plate. The foreshock seismicity mainly occurred on the trenchward side of the mainshock hypocenter, where the Pacific slab contacts the island arc crust. The foreshocks were initially activated at the up-dip limit of the seismogenic zone ~30 km trenchward of the largest foreshock (M 7.3, two days before the mainshock). After the M-7.3 earthquake, intense interplate seismicity, accompanied by epicenters migrating toward the mainshock hypocenter, was observed. The focal depth distribution changed significantly in response to the M-9 mainshock. Earthquakes along the plate boundary were almost non-existent in the area of huge coseismic slip, whereas earthquakes off the boundary increased in numbers in both the upper and the lower plates.

Seismicity of the Atlantis Massif detachment fault, 30°N at the Mid‐Atlantic Ridge

At the oceanic core complex that forms the Atlantis Massif at 30°N on the Mid‐Atlantic Ridge, slip along the detachment fault for the last 1.5–2 Ma has brought lower crust and mantle rocks to the seafloor. Hydroacoustic data collected between 1999 and 2003 suggest that seismicity occurred near the top of the Massif, mostly on the southeastern section, while detected seismicity along the adjacent ridge axis was sparse. In 2005, five short‐period ocean bottom seismographs (OBS) were deployed on and around the Massif as a pilot experiment to help constrain the distribution of seismicity in this region. Analysis of six months of OBS data indicates that, in contrast to the results of the earlier hydroacoustic study, the vast majority of the seismicity is located within the axial valley. During the OBS deployment, and within the array, seismicity was primarily composed of a relatively constant background rate and two large aftershock sequences that included 5 teleseismic events with magnitudes between 4.0 and 4.5. The aftershock sequences were located on the western side of the axial valley adjacent to the Atlantis Massif and close to the ridge‐transform intersection. They follow Omori's law, and constitute more than half of the detected earthquakes. The OBS data also indicate a low but persistent level of seismicity associated with active faulting within the Atlantis Massif in the same region as the hydroacoustically detected seismicity. Within the Massif, the data indicate a north‐south striking normal fault, and a left‐lateral, strike‐slip fault near a prominent, transform‐parallel, north‐facing scarp. Both features could be explained by changes in the stress field at the inside corner associated with weak coupling on the Atlantis transform. Alternatively, the normal faulting within the Massif might indicate deformation of the detachment surface as it rolls over to near horizontal from an initial dip of about 60° beneath the axis, and the strike‐slip events may indicate transform‐parallel movement on adjacent detachment surfaces.