Development of a Compact Broadband Ocean-Bottom Seismometer

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<p>Ocean bottom seismometers (OBS) are widely in use since recent past to monitor seismicity of slow earthquakes as well as that of ordinary earthquakes. Seismic velocity structures, especially of S-wave are essential to estimate hypocenters of them with accuracy. Here we focus on spatial and temporal stability of ambient noise horizontal to vertical spectral ratio (H/V) spectra calculated from ocean bottom seismometers, as the first step toward future application of ambient noise H/V to estimate S-wave velocity structure. We aim to use the Nakamura’s method (1989) for ambient noise H/V spectra using a 3-component OBS array in the Japan Trench, to image deep structure above the plate interface near the trench. To achieve the imaging, it is necessary to examine spatial and temporal stability of the derived H/V spectra from these seismometers. First, we split each 24-hours record into 1-hour windows after removing the instrumental response, Then, Fourier amplitude spectra of each component is taken and smoothed using Konno and Ohmachi (1998) method, with applying downsampling, mean and trend removal, and tapering to each window. Finally, a 1-hour H/V spectral ratio is calculated with taking quadratic mean of two horizontal components. However, a total of 21 OBS, 3 broadband and 18 short-period, stations have been used in this study. A daily variation and stability of the H/V spectra are examined along with comparing them spatially from one station to another. Stability of the H/V spectra from OBS is promising for carrying out our future endevour of deeper observation using the ambient noise H/V method.</p>

Ocean-bottom seismometers (OBSs) are equipped with seismic sensors that record acoustic and seismic events at the seafloor, which makes them suitable for investigating tectonic structures capable of generating earthquakes offshore. One critical parameter to obtain accurate earthquake locations is the absolute time of the incoming seismic signals recorded by the OBSs. It is, however, not possible to synchronize the internal clocks of the OBSs with a known reference time, given that GNSS signals are unable to reach the instrument at the sea bottom. To address this issue, here we introduce a new method to synchronize the clocks of large-scale OBS deployments. Our approach relies on the theoretical time-symmetry of time-lapse (averaged) crosscorrelations of ambient seismic noise. Deviations from symmetry are attributed to clock errors. This implies that the recovered clock errors will be obscured by lapse crosscorrelations' deviations from symmetry that are not due to clock errors. Non-uniform surface wave illumination patterns are arguably the most notable source which breaks the time symmetry. Using field data, we demonstrate that the adverse effects of non-uniform illumination patterns on the recovered clock errors can be mitigated by means of a weighted least-squares inversion that is based on station-station distances. In addition, our methodology permits the recovery of timing errors at the time of deployment of the OBSs. This error can be attributed to either: i) a wrong initial time synchronization of the OBS or ii) a timing error induced by changing temperature and pressure conditions while the OBS is sunk to the ocean floor. The methodology is implemented in an open-source Python package named OCloC, and we applied it to the OBS recordings acquired in the context of the IMAGE project in and around Reykjanes, Iceland. As expected, most OBSs suffered from clock drift. Surprisingly, we found incurred timing errors at the time of deployment for most of the OBSs.

Many types of slow earthquakes have been discovered at subduction zones around the world. However, the physical process of these slow earthquakes is not well understood. To monitor offshore slow earthquakes, a marine seismic and geodetic experiment was conducted at the Hikurangi subduction margin from May 2014 to June 2015. During this experiment, a large slow slip event (Mw 6.8) occurred directly beneath the ocean bottom seismometer (OBS) network. In this study, S‐wave splitting and polarization analysis methods, which have been previously used on onshore data to investigate tremor and anisotropy, are applied to continuous OBS waveform data to identify tremors that are too small to detect by the envelope cross correlation method. Continuous tremor activity with stable polarization directions is detected at the end of the 2014 slow slip event and continued for about 2 weeks. The tremors are generated around a southwest bend in the slow slip contours and at the landward edge of a subducted seamount. Our findings corroborate a previous interpretation, based on burst‐type repeating earthquakes and intermittent tremor, that localized slow slip and tremor around the seamount was triggered by fluid migration following the large plate boundary slow slip event and indicate tremor occurred continuously rather than as isolated and sporadic individual events.

A new ocean bottom seismometer for seismicity study, evaluation of seismic hazard, and for collection of refraction data has been designed. The basic system contains a feedback controlled long-period vertical seismometer, a short-period vertical seismometer and a pressure-compensated crystal hydrophone. The long-period seismometer response models a standard 15 second long-period suspension - 100 second galvanometer combination allowing study of long-period phenomena never before recorded on a free-fall/ pop-up ocean bottom seismometer. The entire system is under the control of a cosmos microprocessor. The microprocessor produces modified IRIG time code and such housekeeping functions as delayed turn on; cage, uncage and centering for the long-period suspension; in situ calibration and timed ballast release. The ocean bottom seismometer also includes an acoustic transponder for location, navigation and release.

<p>Seismic instrumentation is critical for instantaneous tsunami early warning systems as well as assessing long-term risk of tsunami activity in areas with high seismic hazard. Ocean Bottom Seismometer (OBS) systems provide real-time data in areas with appropriate infrastructure or batch data from offline temporary autonomous stations.</p><p>OBS systems detect ground motion from seismic waves significantly before detecting any pressure change in the water column from an associated tsunami due to the order of magnitude difference in wave velocity. Güralp’s OBS systems combine seismic and pressure detection in both permanent cabled networks and temporary non-cabled systems utilising near-real-time acoustic transmission. All seismic sensors used in Güralp systems are sensitive to both earthquakes as well as other tsunami-triggering events such as landslides (e.g. Anak Krakatau, 2018) or volcanic eruptions (e.g. Hunga Tonga–Hunga Haʻapai, 2022).</p><p>Cabled systems provide obvious benefits of real-time data, confidence of installation and flexibility to add additional instrumentation without power consideration. For example, Güralp Orcus and Maris cabled OBS systems are both deployed off the western coast of North America monitoring volcanic and tectonically induced earthquakes that have potential to cause tsunamis. Seismometers at these stations coupled with pressure gauges allow for immediate notification of a threat and subsequent refinement of hazard estimates using surrounding assets such as dedicated DART buoys.</p><p>Both Orcus and Maris allow for multiple auxiliary systems to be incorporated into the system while maintaining as well as providing additional installation flexibility for operators. Orcus has facility for both strong & weak motion seismometers in addition to auxiliary sensors while Maris has the unique feature of operating at any angle without the need for a gimbal mechanism, simplifying installation and network design considerations.</p><p>The Güralp Aquarius is the latest generation autonomous OBS for short-to-medium term or rapid response campaigns to monitor areas with increased seismic and tsunami hazard. Aquarius also uses omnidirectional capabilities as well as acoustic communication of seismic data to the surface to improve operator confidence of installation. Acoustic communication also allows for near-real-time communication with land-based warning systems after a significant seismic event in anticipation of a tsunami. This can be verified and communicated after the initial seismic wave using onboard pressure gauges. In areas where surface communication is not required, intelligent battery systems optimise deployment lengths beyond 18 months for maximum data/cost benefit.</p><p>Güralp is also pioneering the use of seismic sensors and auxiliary equipment within Science Monitoring And Reliable Telecommunications (“SMART”) cables which have already been shown to be useful in incorporating pressure gauges to detect tsunami events. These cables utilise regular telecommunication cables making uses of their natural communication and power source qualities to improve sensor network coverage. Güralp is currently manufacturing a demonstration system to be deployed in the Ionian Sea, monitoring seismic and volcanic activity with the aim of indicating practicality and data quality using this installation method.</p>

By virtue of dense networks of GPS and highly sensitive seismic stations, occurrences of deep low-frequency tremors/earthquakes have been recognized worldwide in the deep portions of subduction plate boundaries (e.g., Schwartz & Rokosky, 2007). Since these tremors and earthquakes have moment release rate smaller than that of regular earthquakes, the low-frequency tremors (LFTs) /earthquakes are classified into “slow-earthquake” group (Ide et al., 2007). The slow-earthquakes occur sometimes independently and sometimes break into chain-reaction propagating at a speed of about 10 km/day. However, a systematic explanation of the origin of such slow-earthquake migrations in some subduction zones is still lacking. First of this study, we investigate the mechanisms of slow-earthquake migration by comparing observational results and numerical simulation results. Since some LFTs are found to be modulated by Earth tides (e.g., Nakata et al., 2008) and the moment release rate of the slow earthquake group is much smaller than that of regular earthquakes (Ide et al., 2007), they likely have low-stress drop and are sensitive to shear stress perturbations possibly induced by the preseismic slip of nearby megathrust earthquakes. Second of this study, we propose a new method to detect precursory change around the large asperity prior to a megathrust earthquake, focusing on the spatio-temporal change of migration speed and moment release rate for nearby slow earthquakes. Recently, lowfrequency tremors/earthquakes have been also detected in the shallower portion of frictional transition zone on the subduction plate boundaries (Obana & Kodaira, 2009). Third of this study, we also perform another numerical simulation of shallow low-frequency tremors/earthquakes, comparing with the characteristics of the deep low-frequency tremors/earthquakes and discussing the strategy of Dense Oceanfloor Network System for Earthquakes and Tsunamis (DONET) toward an anticipated Tonankai Earthquake.

Tha main goal of CAGE20-5 cruise was to continue with an Ocean Bottom Seismometer (OBS) experiment to address objectives of the SEAMSTRESS project (Tectonic stress effects on Arctic methane seepage[SB1] ). Along the eastern segment of Vestnesa Ridge, the cruise recovered 7 OBS after a one-year deployment and subsequentially redeployed 10 OBS in the larger array on the southern flank of Vestnesa Ridge. The overall aim is to detect local and regional earthquakes that relate to and/or impact local faulting within Vestnesa Ridge and may control fluid flow. Another objective of CAGE20-5 was to collect seismic lines to complete the data base and refine site selections for the IODP985 proposal. We also collected a 3D seismic volume on the elongated depression north of Knipovich Ridge where massive gas hydrates and pore fluid pressure pulses from piezometer data have been observed
 The cruise may be known as: CAGE20_5

Summary A temporary array of seven ocean bottom seismometers (OBS) was deployed offshore the Guerrero subduction zone in Mexico to monitor previously unreported shallow seismicity. These OBS instruments are especially valuable for studying earthquake activity in the Guerrero seismic gap, where a future large event could severely impact densely populated regions of Mexico. This study investigates the shallow seafloor structure, including site effects, shear wave attenuation, and velocity models, using both earthquake data and ambient seismic noise. We employed spectral inversion to estimate the quality factors of shear wave attenuation and site effects. Additionally, we calculated the microtremor horizontal-to-vertical spectral ratio (HVSR) as a proxy for site response and invert it using constraints from hydroacoustic seafloor profiles, parametric sub-bottom profile system (TOPAS), to derive the shallow velocity structure beneath the stations. The inclusion of TOPAS data in the inversion significantly improved convergence, reduced misfit, and resulted in more reliable subsurface models. The HVSR inversions indicate the presence of water-saturated sediments within the upper 250 m, characterized by shear-wave velocities ranging from 55.2 to 1950 m/s and Vp/Vs ratios between 1.80 and 27.84. Strong attenuation effects, typical of marine environments, were observed, with Q(f) values as low as Q = 86f0.62 in the forearc accretionary wedge. Our attenuation estimates are consistent with those found in other offshore subduction zones, contributing to a broader understanding of shallow structures in similar tectonic settings worldwide. We found strong agreement between the estimated site effects and HVSR results, underscoring their close relationship and supporting the reliability of our site response estimates. This is the first study in Mexico to use OBS data to characterize offshore attenuation, site effects, and velocity structure, information that will support future seismological analyses, including earth structure imaging and investigations of both large earthquakes and shallow slow earthquakes in the Guerrero seismic gap.

The detection and analysis of offshore seismic processes worldwide often require the use of ocean-bottom seismometers (OBSs). However, most OBS deployments are done with stand-alone stations, with data recovery delayed by months. On the other hand, electrically cabled OBS, which allows for real-time monitoring, remains exceptional due to the high cost of manufacturing, installation, and maintenance. Here, we present a new perspective for cabled array of OBSs, using purely optical seismometers, plugged at the end of long fiber-optic cables, aimed at reducing their cost for observatories requesting real-time data. The optical seismometer was developed in the last decade by the École Supérieure d’Électronique de l’Ouest, based on the Fabry–Perot interferometer, tracking at high resolution the displacement of the mobile mass of a mechanical geophone (no electronics nor feedback). A prototype was successfully installed at the top of La Soufrière volcano of Guadeloupe in 2019. We replicated this sensor and installed it 5 km offshore Les Saintes islands, at 43 m depth (Guadeloupe, Lesser Antilles) to characterize the swarm-type activity persistent after the 2004 M 6.3 earthquake (Interreg Caraïbe PREST project). The installation cruise, FIBROSAINTES, was supported by the Flotte Océanographique Française. A plow designed by GEOAZUR carried the cable and was pulled on the seafloor by the vessel ANTEA. The landing cable was connected to the interrogator, with a real-time telemetry to the Institut du Physique du Globe de Paris/Observatoire Volcanologique et Sismologique de Guadeloupe. The OBS has been qualified with local land-based velocity broad band stations. The analysis of local earthquake swarms suggests transient creep on the major normal faults. This successful installation opens promising perspectives for real-time monitoring in on-land or offshore sites, presenting harsh environmental conditions, in which commercial, electrical seismic sensors are difficult and/or costly to install and maintain.

We have developed a broadband ocean bottom seismometer (BBOBS) and its new generation (BBOBS-NX) with the penetrator sensor system since 1999. With them, we performed many practical observations to create a new research category of ocean bottom broadband seismology. As the next step in seafloor geophysical observation, the BBOBS and the BBOBS-NX can be a breakthrough in realizing a geodetic observation network on the seafloor. Although vertical displacement observation by the absolute pressure gauge has been widely conducted in recent years, other geodetic observations are rarely performed. A few trials to measure the seafloor tilt were performed, but those looked inadequate for practical observations. Note that the broadband sensor in our BBOBSs has a mass position signal output, which can be used to measure the tilt change. As the horizontal component noise level of the BBOBS-NX is good at a long period range, we expected it to be adequate for the tilt measurement. At the first evaluation, we performed a comparison with a water-tube tiltmeter. The result was comparable with a resolution of better than 1 µ radian. A practical observation at the south of Boso Peninsula (KAP3 site) was conducted as thein-situstudy from April, 2013. In January, 2014, a slow slip event (SSE) occurred near this site. The tilt data were processed by removing steps, mechanical relaxation, and tides. The results show a clear peak started from late December 2013. Two more 2 year-long tilt observations began in 2015: one was at the KAP3 site and another was off the Miyagi Prefecture at the slope to the Japan Trench. The latter was recovered in 2017 with about 1.5 years of data, which indicate a large continuous tilt up to several tens of µ radian. This amount of tilt can be explained by a similar already estimated SSE. Mobile tilt measurement at the seafloor can be a powerful tool to study SSEs, as they can be located above the source area and also possible to build an observation array for a practical study because of its low cost and ease of deployment compared with a seafloor borehole site.

<p>The Guerrero seismic gap, at the Mexican subduction zone, has been a region of great seismological interest because of the absence of a large earthquake in more than 110 years. If an earthquake were to rupture the entire Guerrero seismic gap the resulting earthquake could be disastrous to major Mexican cities. Additionally, the Guerrero subduction zone has plenty of slow earthquake activity with large slow slip events and tectonic tremors, located at the deep plate interface. To obtain a new and unique observation point of seismicity in the Guerrero seismic gap and continue evaluating its seismic risk, we deployed an array of ocean bottom seismometers (OBS) offshore the Guerrero seismic gap. We were able to detect and locate shallow tremors near the trench and deduce that a portion of the shallow plate interface undergoes stable slip. We used data from the OBS to analyse the new catalogue of shallow tremors and describe their source. Focal mechanisms of shallow tremors were estimated using S wave polarisation. We found that slip azimuth tends to follow the subduction plate motion, suggesting that tremors rupture at the plate interface. We also estimated shallow tremor radiated seismic energy. We found a heterogeneous energy release of shallow tremors along strike. Our observations of a heterogeneous shallow tremor energy release can be explained with the different mechanical properties, inside and outside the Guerrero seismic gap, and help to characterise the seismogenic zone at the shallow plate interface.</p>

SUMMARYShallow very low frequency earthquakes (sVLFEs) have occurred recurrently at the shallow plate interface of the Hyuga-nada region of the western Nankai subduction zone, Japan. Although the locations of sVLFE epicentres have been determined using land-based seismic records with moderate accuracy, it is necessary to determine their locations more precisely to explore the relationship between sVLFEs and other shallow slow earthquakes and examine the structural factors that may control sVLFE activity. Here, we identified sVLFE epicentres using seismic records obtained from temporarily deployed ocean bottom seismometers (OBSs) in the Hyuga-nada region. Seismic observations involved the deployment of 5–13 OBSs for approximately 1 yr, with deployments conducted three times during 2014–2016 each time with changing OBS numbers and array distribution. As a result, one sVLFE episode, containing successive Rayleigh wave pulses with slow velocities due to marine sediments, could be detected at a frequency band of 0.1–0.15 Hz per observation, resulting in a total of three episodes. Rayleigh wave amplitudes of ordinary earthquakes in the continuous records were suppressed using earthquake catalogues. We estimated the dispersion curve for the Rayleigh wave group velocity for each array, which represented the averaged group velocity within the array, using coda interferometry, and applied an envelope correlation method (ECM) using the group velocities to continuous records. These processing provided sVLFE epicentres with horizontal distance errors of <5 km. Our results showed that sVLFEs depths, which were inferred from the contour line of the top of the Phillipine Sea Plate, had increased from <10 km to 10–15 km in the region of the subducted Kyusyu-Palau Ridge (KPR). It was also apparent that migration of sVLFE epicentres occurred in 2015 from a depth of 15 km to shallower depths along the northern margin of the subducted KPR. These results identified the subducted KPR as a structural factor controlling the excitation conditions of sVLFE activities.

Reliable interpretation of seismic data in ocean bottom surveys or crust and upper mantle investigations requires precise knowledge of the position of the seismic sensors. Special problems arise when operating with free‐falling ocean bottom seismometers (OBS). Since the ship's position and its towed signal sources (air gun, uniboom) or explosives are fairly well known based on high‐precision navigation systems, one can determine the geophone positions indirectly from their response to the source excitation. Here, a newly developed source parameter estimation code [A. B. Baggeroer, W. A. Kuperman, and H. Schmidt, J. Acoust. Soc. Am. Suppl. 1 79, S56 (1986)] is first used to determine the performance bounds for a localization based on seismic interface waves. Then, an actual shear speed profile is determined by inversion of a new data set obtained using explosive sources, and a maximum‐likelihood estimation of the geophone position is performed. By comparing the results with the actual experimental geometry, it is demonstrated that in spite of the fairly long wavelength of the interface waves, a reasonable resolution can be achieved by such a technique.

Constraining upward mantle flow is essential for understanding global mantle dynamics and linking Earth's interior with surface processes. The ERC-funded UPFLOW (Upward Mantle Flow from Novel Seismic Observations) project addresses the limited understanding of mantle upwellings connecting the deep mantle to the surface by utilising advanced seismic imaging methods and conducting extensive data collection.Between June 2021 and September 2022, UPFLOW deployed 50 and successfully recovered 49 ocean bottom seismometers (OBSs) across a ~1,000×2,000 km² area in the Azores-Madeira-Canaries region, with an average station spacing of ~150-200 km. This multinational collaboration involved institutions from Portugal (IPMA, IDL, Univ. of Lisbon, ISEL), Ireland (DIAS), the UK (UCL), Spain (ROA), and Germany (Potsdam University, GFZ, GEOMAR, AWI). The deployment utilized three OBS frame designs equipped with three-component wideband seismic sensors and hydrophones. The dataset exhibits high-quality recordings, including teleseismic, local seismic events, and non-seismic signals (e.g., whales, ships, and the Tonga eruption), with significant noise reduction observed in vertical component long-period data (T > ~30 s).Initial tomographic results feature a preliminary P-wave model derived from ~8,000 multi-frequency (T ~2.7-30 s) body-wave travel time cross-correlation measurements and over 120 teleseismic events. Integrating UPFLOW's OBS data with global seismic datasets from temporary and permanent stations expands the dataset to approximately 600,000 multifrequency measurements. This comprehensive approach enables the construction of a global P-wave model with enhanced resolution throughout the entire mantle beneath the Azores-Madeira-Canaries region. We compare our new model with existing global tomography models and discuss its geodynamical implications in terms of mantle upwelling processes and their surface expressions.