Ocean Bottom Seismometer Experiment Research Articles

Machine Learning‐Based New Earthquake Catalog Illuminates On‐Fault and Off‐Fault Seismicity Patterns at the Discovery Transform Fault, East Pacific Rise

AbstractOceanic transform faults connect spreading centers and are imprinted with previous tectonic events. However, their tectonic interactions are not well understood due to limited observations. The Discovery transform fault system at 4°S, East Pacific Rise (EPR), represents a young transform system, offering a unique opportunity to study the interplay between faulting and other tectonic events at an early phases of an oceanic transform system. Discovery regularly hosts M5–6 characteristic earthquakes, and the seafloor north of Discovery includes a 35 km‐long rift zone that records a complex history of rifting, faulting and volcanism, suggesting that the transform faults likely interact with regional tectonic activity. We apply a machine‐learning enabled workflow to locate 21,391 earthquakes recorded during a 1‐year ocean bottom seismometer experiment in 2008. Our results indicate that seismicity on the western Discovery fault is separated into seven patches with distinct aseismic and seismic slip modes. Additionally, we observe a patch of off‐fault seismicity near where seafloor abyssal hills intersect the rift zone. This seismicity may have been caused by varying opening rates as spreading rate decreases from north to south in the rift zone. Our findings suggest that the Discovery system is still evolving, and that system equilibrium has not been reached between rifting and faulting. These results reflect the complex yet rarely observed interactions between fault slip, plate rotation, and rifting which are likely ubiquitous at oceanic transform systems.

Analysis of clock drift based on a three‐dimensional controlled‐source ocean bottom seismometer experiment

AbstractClock drift is common for the autonomous ocean bottom seismometer, and a technique of linear drift correction is usually used to fix this kind of timing error assuming that the internal clock has constant drift rate during deployment. However, non‐linear component of clock drift has also been found recently. In order to test the behaviours of the clock drift for 19 Chinese ocean bottom seismometers from a three‐dimensional controlled‐source seismic survey, we have analysed two types of differences in direct wave travel‐times, including one from all shots to the ocean bottom seismometer onboard and another one from the nearest two shots on different shooting lines to the ocean bottom seismometer on the seafloor. Comparison of the differences in direct wave travel‐times from seismic data with and without clock drift correction shows that linear drift correction can commendably eliminate them for most of the ocean bottom seismometers, but some large residuals remain for part of the ocean bottom seismometers. These residuals are inferred to be possibly related to the non‐linear component of clock drift caused by the temperature shock during deployment. The non‐linear clock drift usually varies from different ocean bottom seismometers and thus difficult to correct completely and resultantly will influence the timing accuracy somewhat. Hence, higher uncertainties are suggested to assign to the picked travel‐times for the ocean bottom seismometer with possibly larger non‐linear clock drift in the further tomographic modelling and the internal clock for this kind of ocean bottom seismometer should be recalibrated prior to the next experiment if which requires higher timing accuracy.

A Novel Method to Estimate Orientations of an Ocean-Bottom Seismometer Array for Accurate Measurement of Waveform Phases and Amplitudes

Abstract The misorientation of three-component seismometers restricts the application of relevant seismic experiments such as ocean-bottom seismometer (OBS) arrays. Previous orientation determination relied on an assumption that the propagation azimuth of seismic waves follows the great-circle path (GCP) azimuth. This assumption may yield systematic errors in the estimated orientation, particularly when the ray paths are bent laterally due to velocity heterogeneity in the Earth. Here, we develop a new method for unbiasedly estimating the horizontal orientations of seismic sensors and apply this method to the Blanco transform fault OBS experiment. We first retrieve the orientations relative to the propagation azimuths from the recorded Rayleigh and P waveforms, and then determine the geographic north orientations by calculating the propagation azimuths via an Eikonal-equation-based phase-tracking method that theoretically accounts for the effect of ray bending. Synthetics test validates that the phase-tracking method can retrieve unbiased propagation azimuths of seismic waves. The final results derived from Rayleigh- and P-wave polarization analyses with the respective phase-tracked propagation azimuths are more consistent and the orientation errors are smaller, indicating the robustness and accuracy of this method. Comparing the orientations from our phase-tracking method to those from the GCP assumption, the deviation can reach up to 8° between these two techniques in the study region. Subsequently, when orientations of the synthetics modeled from three-dimensional elastic waveform simulation are deviated according to the GCP-predicted orientations, we find nonnegligible bias in the phase and amplitude measurements that could reduce the accuracy and resolution of following inversion, which indicates the significance of our phase-tracking method in accurate orientation of OBS arrays as well as inland seismic experiments.

Time Correction of Ocean-Bottom Seismometers Using Improved Ambient Noise Cross Correlation of Multicomponents and Dual-Frequency Bands

Abstract An effective approach was developed for identifying and correcting ocean-bottom seismometer (OBS) time errors through improving ambient noise cross-correlation function (NCCF) analysis and combination with other methods. Significant improvements were illustrated through analyzing data from a passive-source seismic experiment in the southwestern sub-basin of the South China Sea. A novel method was first developed that can effectively identify errors in the sampling frequency of the OBS instruments. The traditional NCCF method was then expanded by increasing the analyzed data spectrum from a single-frequency band to dual-frequency band pairs, thus doubling the number of available data points and substantially improving the time correction quality. For data with relatively low signal-to-noise ratios, the average time errors were reduced from the original average values of 60–80 ms by the conventional methods to <40 ms using the improved approaches. The new multistep procedure developed in this study has general applicability to analysis of other OBS experiments. The demonstrated significant improvements in the data quality are critical for advancing seismic tomography and other modern marine geophysical studies that require high accuracy in the OBS data.

Sub-surface magma movement inferred from low-frequency seismic events in the off-Nicobar region, Andaman Sea

Monitoring volcanic activity along the submarine volcanoes that are usually induced by subsurface magmatism is a challenge. We present fresh set of Ocean Bottom Seismometer (OBS) data that shows geophysical evidence indicative of subsurface magmatism along the submarine volcanoes in the off Nicobar region, Andaman Sea. In this region, we observed for the first time, hybrid very long-period earthquakes documented by passive OBS experiment. These events were initiated by high-frequency (5–10 Hz) with a clear onset of P-phase followed by low-frequency (0.01–0.5 Hz) oscillations in the range of 300–600 s with a prominent high-frequency (10–40 Hz) hydro-acoustic phase. A total of 141 high-frequency events were detected on 21st and 22nd March 2014 out of which 71 were of low-frequency oscillations. These events are distributed in the northwest–southeast direction along the submarine volcanic arc and Seulimeum strand of Great Sumatra fault. Off Nicobar region has been witnessing frequent earthquake swarms since 26th December 2004 tsunamigenic Sumatra earthquake. These swarms occurred in January 2005, March and October 2014, November 2015 and March 2019. The occurrence of low-frequency earthquakes and prominent hydro-acoustic phase are suggestive of sub-surface tectonic and magmatic influence. We propose that upward movement of magma pulses from deeper magma reservoir to the shallow magma chamber activated the strike-slip movement of sliver faults and induced earthquake swarms in the off Nicobar region.

Ocean‐Bottom Seismometer Instrument Orientations via Automated Rayleigh‐Wave Arrival‐Angle Measurements

After more than 10 years of the U.S. ocean‐bottom seismometer (OBS) Instrument Pool operations, there is still need for a consistent and accurate procedure to determine the orientation of the horizontal seismometer components of passive‐source free‐fall broadband OBSs with respect to geographic north. We present a new Python‐based, automated, and high‐accuracy algorithm to obtain this information during postprocessing of the data. As with some previous methods, our new method Doran–Laske‐Orientation‐Python (DLOPy) is based on measuring intermediate‐period surface‐wave arrival angles from teleseismic earthquakes. A crucial new aspect of DLOPy is the consultation of modern global dispersion maps when setting up the analysis window. We repeat measurements at several frequencies to lower biases from wave propagation in laterally heterogeneous structure. We include measurements from the first minor and major great‐circle arcs to further lower biases caused by uneven geographical data coverage. We demonstrate the high accuracy of our technique through benchmark tests against a well‐established “hands‐on” but slow technique using data from instruments of the Global Seismographic Network for which orientations are well documented. We present results for all Cascadia Initiative deployments, along with a number of other OBS experiments. Compared to other widely used automated codes, DLOPy requires fewer events to achieve the same or better accuracy. This advantage may be greatly beneficial for OBS deployments that last as short as a few months. Our computer code is available for download. It requires minimal user input and is optimized to work with data disseminated through the Incorporated Research Institutions for Seismology Data Management Center. [Electronic Supplement:][1] Tables containing station parameters, including location and depth, as well as calculated orientation for the H1 component of the broadband sensors. [1]: http://www.bssaonline.org/lookup/suppl/doi:10.1785/0120160165/-/DC1

The velocity structure of a fossil spreading centre in the Southwest Sub‐basin, South China Sea

AbstractWe present results from an ocean bottom seismometer experiment surveying the fossil spreading centre in the Southwest Sub‐basin of the South China Sea. The detailed velocity model shows that oceanic layer 2 exhibits across‐axis variations in thickness and velocity, whereas oceanic layer 3 displays a variation in crustal thickness. A low‐angle (24°) SE‐dipping oceanic detachment fault is proposed to explain the anomalous structure on the NW side of the spreading centre, which exhibits uplifting of the upper mantle beneath a thinned oceanic crust. The inferred oceanic detachment fault was at its initial stage, localized within the basaltic crust and did not exhume lower crust. We suggest that the low‐velocity (7.6–7.9 km/s) body located within the upper mantle beneath the footwall of the detachment fault is caused by both mantle serpentinization and partial melting. The difference in crustal thickness in the Southwest Sub‐basin indicates that the magma supply varied in time and space during or even after the seafloor spreading. Compared with other fossil spreading ridges, the fossil spreading centre of the northeast Southwest Sub‐basin studied here represents a third type of fossil spreading ridge, characterized by a reduced melt supply at the waning stage of spreading and a strong post‐spreading magmatism. Copyright © 2016 John Wiley & Sons, Ltd.

Three‐dimensional geometry of axial magma chamber roof and faults at Lucky Strike volcano on the Mid‐Atlantic Ridge

AbstractWe present results from three‐dimensional (3‐D) processing of seismic reflection data, acquired in June 2005 over the Lucky Strike volcano on the Mid‐Atlantic Ridge as a part of the Seismic Study for Monitoring of the Mid‐Atlantic Ridge survey. We use a 3‐D tomographic velocity model derived from a coincident ocean bottom seismometer experiment to depth convert the poststack time‐migrated seismic volume and provide 3‐D geometry of the axial magma chamber roof, fault reflectors, and layer 2A gradient marker. We also generate a high‐resolution bathymetric map using the seismic reflection data. The magma chamber roof is imaged at 3.4 ± 0.4 km depth beneath the volcano, and major faults are imaged with dips ranging between 33° and 50°. The magma chamber roof geometry is consistent with a focused melt supply at the segment center and steep across‐axis thermal gradients as indicated by the proximity between the magma chamber and nearby faults. Fault scarps on the seafloor and fault dip at depth indicate that tectonic extension accounts for at least 10% of the total plate separation. Shallow dipping reflectors imaged in the upper crust beneath the volcano flanks are interpreted as buried lava flow surfaces.

Unusual signals recorded by ocean bottom seismometers in the flooded caldera of Deception Island volcano: volcanic gases or biological activity?

AbstractAn ocean bottom seismometer (OBS) network was deployed for 1 month at Deception Island volcano, Antarctica, in early 2005. Although only two volcano-tectonic and three long-period events were observed, the three OBSs located > 2 km apart inside the caldera detected over 3900 events that could not be attributed to known volcanic or hydrothermal sources. These events are found on one instrument at a time and occur in three types. Type 1 events resemble impulsive signals from biological organisms while type 2 and type 3 events resemble long-period seismicity. The largest number of events was observed in a region of volcanic resurgence and hydrothermal venting. All three types occur together suggesting a common cause and they show evidence for a diurnal distribution. The events are most likely to be due to aquatic animals striking the sensors, but a geological source is also possible. In the first case, these signals indicate the presence of a biological community confined to the caldera. In the second case, they imply widespread hydrothermal activity in Port Foster. Future OBS experiments should bury the seismometers, include a hydrophone, deploy instruments side-by-side, or include a video camera to distinguish between biological and geological events.

Efficient and accurate modeling of ocean bottom seismometer data using reciprocity

Seismic experiments in which the number of sources is considerably larger than the number of receivers occur regularly. An important example is the collection of crustal scale seismic data using ocean bottom seismometers and marine sources. We describe a method to accurately and efficiently compute synthetic seismograms for such experiments by using finite differences and reciprocity. We show numerically how to decompose an explosive source into its equivalent body force components using the staggered-grid finite-difference technique with a fourth-order approximation for the spatial derivative and a second-order approximation for the temporal derivative. This decomposition results in a source configuration where the equivalent body forces are defined in 12 points around the point where the ex-plosive source is applied. We then use the derived equivalent body forces for the explosive source and seismic reciprocity theorems to convert the common receiver gather to a common shot gather. The method is tested on a structurally complex elastic model of the crust and the results show that it is accurate within floating point precision. The synthetic data are compared to data from a real ocean bottom seismometer experiment conducted across a continent-ocean transition zone. A good fit in terms of traveltime is observed for many of the prominent seismic phases. The amplitude fit of these arrivals is not always as good as the traveltime fit. This indicates that full-waveform modeling of such data can provide useful information about the subsurface that cannot be obtained from traveltime modeling. If enough data are available, the modeling method can be used in full-waveform inversion.

Was the 1952 Tokachi‐oki earthquake (Mw= 8.1) a typical underthrust earthquake?: Plate interface reflectivity measurement by an air gun‐‐ocean bottom seismometer experiment in the Kuril Trench

The Kuril Trench subduction zone is one of the most seismogenic regions, where underthrust earthquakes with M> 8 recur along the trench. The seismic gap between the source areas of the 1973 Nemuro‐oki and 2003 Tokachi‐oki earthquakes, which are typical underthrust earthquakes faulting with rupture velocities of ∼3 km/s, has been ruptured by the 1952 Tokachi‐oki earthquake. The seismic gap has also slipped incidental to neighboring asperities. The difference in slip pattern on the plate interface generally appears as a spatial difference in seismic structure on the plate interface, such as a reflectivity of the plate interface. We estimated the crustal velocity structure and analyzed the reflectivity of the plate interface to investigate the physical properties of the plate interface by performing an air gun–ocean bottom seismometer experiment on the along‐trench profile across the seismic gap. Strong reflections from the plate interface were observed in the 1952 Tokachi‐oki source area including the seismic gap, rather than in the 1973 Nemuro‐oki source area. The strong reflectivity of the plate interface in such the seismic gap with an incidental slip suggests that a slip pattern in the corresponding seismic gap would be conditionally stable. The coupling condition in the source areas of the eastern part of the source area of the 1952 earthquake is different from that in source areas of typical underthrust earthquakes, such as the 2003 Tokachi‐oki and 1973 Nemuro‐oki earthquakes. Our results suggest that the 1952 Tokachi‐oki earthquake was a complex earthquake with the characteristic of a tsunami earthquake.

Spatial variations in the frequency-magnitude distribution of earthquakes in the southwestern Okinawa Trough

AbstractThe relations between the frequency of occurrence and the magnitude of earthquakes are established in the southern Okinawa Trough for 2823 relocated earthquakes recorded during a passive ocean bottom seismometer experiment. Three highb-values areas are identified: (1) for an area offshore of the Ilan Plain, south of the andesitic Kueishantao Island from a depth of 50 km to the surface, thereby confirming the subduction component of the island andesites; (2) for a body lying along the 123.3°E meridian at depths ranging from 0 to 50 km that may reflect the high temperature inflow rising up from a slab tear; (3) for a third cylindrical body about 15 km in diameter beneath the Cross Backarc Volcanic Trail, at depths ranging from 0 to 15 km. This anomaly might be related to the presence of a magma chamber at the base of the crust already evidenced by tomographic and geochemical results. The highb-values are generally linked to magmatic and geothermal activities, although most of the seismicity is linked to normal faulting processes in the southern Okinawa Trough.

Two-dimensional elastic anisotropic/AVO modelling for the identification of BSRs in marine sediments using multicomponent receivers

In seismic sections, the presence of a gas hydrate stability zone (GHSZ) is often marked by a bottom-simulating reflector, which has a negative polarity with respect to the seafloor. The present study reveals the response of seismic wave characteristics and amplitude versus offset (AVO) effects of large offset compressional (P) and converted (PS) waves for a GHSZ/free-gas configuration, using a two-dimensional elastic anisotropic modelling technique. The modelling results would provide ‘a priori’ information, which allows unique determination of parameters of seismic models for the design of ocean bottom seismometer experiments over continental margins for the purpose of gas hydrate exploration. The AVO analyses on long offset P and PS waves based on synthetic data yield a typical gas hydrate/free-gas response, as the reflectivity increases with incidence angle.

Deep structure of the southern Kerguelen Plateau (southern Indian Ocean) from ocean bottom seismometer wide‐angle seismic data

Wide‐angle seismic data collected during the Kerguelen ocean bottom seismometer experiment provide the first images of the deep structure of the southern Kerguelen Plateau and support a new interpretation of the origin of the plateau. Velocity models based on travel time inversions and reflectivity synthetic seismograms show a 22‐km‐thick crust composed of ∼1.6 km of sedimentary cover, ∼5.3 km of upper crust, ∼11.0 km of lower crust, and a 4‐ to 6‐km‐thick reflective zone immediately above Moho. Velocities in the upper crust (from 3.8–4.5 km/s at top to 6.0–6.5 km/s at bottom) are consistent with the basaltic nature of this layer, the top of which was sampled during the Ocean Drilling Program. Velocities in the lower crust increase continuously from 6.60 km/s at the top to 6.90 km/s at 19.5 km depth. The reflective zone at the base of the crust identified by wide‐angle reflections is observed only along the NNW‐SSE direction. It consists of alternating high‐ and low‐velocity layers with an average velocity of 6.70 km/s in the NNW‐SSE direction and ≥6.90 km/s in the perpendicular direction. Strong azimuthal anisotropy is also observed in the upper mantle with velocities of 8.60 and 8.00 km/s, in the NNW‐SSE and E‐W directions, respectively. The absence of high velocities at the base of the crust that characterizes many large‐volume mafic provinces, the reflective lower crust, and anisotropy in upper mantle suggest that the southern Kerguelen Plateau represents a stretched continental fragment overlain by basaltic flows isolated from the Antarctic margin during the early opening of the Indian Ocean.