Water Reverberation Research Articles

Patterns of Causative Faults of Normal Earthquakes in the Fluid‐Rich Outer Rise of Northeastern Japan, Constrained With 3D Teleseismic Waveform Modeling

AbstractAccurate earthquake source parameters are crucial for understanding plate tectonics, yet, it is difficult to determine these parameters precisely for offshore events, especially for outer‐rise earthquakes, as the limited availability of direct P or S wave data sets from land‐based seismic networks and the unsuitability of simplified 1D methods for the complex 3D structures of subducting systems. To overcome these challenges, we employ an efficient hybrid numerical simulation method to model these 3D structural effects on teleseismic P/SH and P‐coda waves and determine the reliable centroid locations and focal mechanisms of outer‐rise normal‐faulting earthquakes in northeastern Japan. Two M6+ events with reliable locations from ocean bottom seismic observations are utilized to calibrate the 3D velocity structure. Our findings indicate that 3D synthetic waveforms are sensitive to both event location, thanks to bathymetry and water reverberation effects, and the shallow portion of the lithospheric structure. With our preferred velocity model, which has Vs ∼16% lower than the global average, event locations are determined with uncertainties of <5 km for horizontal position and <1 km for depth. The refined event locations in a good match between one of the nodal strikes and the high‐resolution bathymetry, enabling the determination of the causative fault plane. Our results reveal that trench‐ward dipping normal faults are more active, with three parallel to the trench as expected, while five are associated with the abyssal hills. The significant velocity reduction in the uppermost lithosphere suggests abundant water migrating through active normal faults, enhancing both mineral alteration and pore density.

Robust shallow water reverberation reduction methods based on low-rank and sparsity decomposition.

Using the characteristics of low rank for reverberation and sparsity for the target echo in multi-ping detection, the low-rank and sparsity decomposition method can effectively reduce reverberation. However, in the case of highly sparse reverberation or a stationary target, the distinctions in the characteristics between the reverberation and target echo become ambiguous. As a result, the reverberation reduction performance is degraded. To guarantee a meaningful decomposition based on the random orthogonal model and random sparsity model, the identifiability condition (IC) for the decomposition was derived from the perspective of the low-rank matrix and sparse matrix, respectively. According to the IC, sparsity compensation for the low-rank matrix was proposed to address the false alarm probability inflation (FAPI) induced by highly sparse reverberation. In addition, increasing the dimension of the sparse matrix was also proposed to manage the detection probability shrinkage caused by a stationary target. The robust reverberation reduction performance was validated via simulations and field experiments. It is demonstrated that FAPI can be eliminated by increasing the sparse coefficient of the low-rank matrix to 0.30 and a stationary target could be detected with a large ping number, i.e., a high dimension, of the sparse matrix.

Absence of Near-Trench Early Triggering during the 2012 Mw 7.2 Indian Ocean Strike-Slip Earthquake: Evidence from One-Day Aftershocks

AbstractDynamic earthquake triggering is a widely accepted mechanism of earthquake interaction, which plays a vital role in seismic hazard estimation, although its efficacy at regional distances is under debate. The 2012 Mw 7.2 Indian Ocean event is one of the first reported events to produce dynamic stress triggering at regional distances using backprojection (BP) techniques. Alternatively, the coherent radiators in BP images can be interpreted as localized water reverberation phases. We present further evidence against near-trench triggering during this event. We collected 24 hr seismic recordings of two nearby stations located near the trench. We adopted a waveform denoising algorithm and detected 125 aftershocks using two regional seismic stations with a minimum magnitude of ML∼2.7 and completeness magnitude of ML∼3.6, whereas none of these aftershocks occurred near the trench. The absence of immediate (within one day) aftershocks near the trench suggest the absence of dynamic triggering during the offshore mainshock.

Seismic description and fluid identification of thin reservoirs in Shengli Chengdao extra-shallow sea oilfield

The meandering channel deposit of the upper member of Neogene Guantao Formation in Shengli Chengdao extra-shallow sea oilfield is characterized by rapid change in sedimentary facies. In addition, affected by surface tides and sea water reverberation, the double sensor seismic data processed by conventional methods has low signal-to-noise ratio and low resolution, and thus cannot meet the needs of seismic description and oil-bearing fluid identification of thin reservoirs less than 10 meters thick in this area. The two-step high resolution frequency bandwidth expanding processing technology was used to improve the signal-to-noise ratio and resolution of the seismic data, as a result, the dominant frequency of the seismic data was enhanced from 30 Hz to 50 Hz, and the sand body thickness resolution was enhanced from 10 m to 6 m. On the basis of fine layer control by seismic data, three types of seismic facies models, floodplain, natural levee and point bar, were defined, and the intelligent horizon-facies controlled recognition technology was worked out, which had a prediction error of reservoir thickness of less than 1.5 m. Clearly, the description accuracy of meandering channel sand bodies has been improved. The probability semi-quantitative oiliness identification method of fluid by prestack multi-parameters has been worked out by integrating Poisson's ratio, fluid factor, product of Lame parameter and density, and other prestack elastic parameters, and the method has a coincidence rate of fluid identification of more than 90%, providing solid technical support for the exploration and development of thin reservoirs in Shengli Chengdao extra-shallow sea oilfield, which is expected to provide reference for the exploration and development of similar oilfields in China.

Absolute Centroid Location of Submarine Earthquakes from 3D Waveform Modeling of Water Reverberations

AbstractOceanic transform faults (OTFs) represent an attractive tectonic environment to investigate how slip is accommodated within the crust. However, as most of these fault systems grow in the deep ocean, where few local seismic observations are available, characterizing their earthquake behavior is complicated and remains a formidable challenge. Here we present a novel approach for retrieving precise centroid locations of submarine earthquakes that is based on the modeling of water phases in teleseismic records. Using a hybrid method for simulating far‐field body waves with 3D source‐side structures, we demonstrate that the scattered energy generated by the continuous bounces of an earthquake's P‐wave trapped in the ocean is modelable and carries information about its source location. As a case study, we use a realistic bathymetry model of the Gofar transform fault on the East Pacific Rise and simulate the seismic wavefield at US Array stations for four of its moderate‐sized (Mw5.0+) earthquakes. Our modeling results show that water phases are sensitive to a ∼5 km change in the earthquake's horizontal location and that a remarkable agreement between observed and synthetic water phases is achieved when the location of an event is close to its true one. We then relocate three of these events by systematically computing their water phases in candidate locations until a satisfactory waveform fit is achieved. This analysis technique paves a new route for studying earthquake source properties in isolated marine environments and serves as a mean to investigate the seismic behavior of OTFs on a global scale.

Estimating Oceanic Crustal Structure from OBS Data Using Teleseismic P Wave Wavefield Continuation

Ocean bottom seismometers (OBS) have been widely used in studies of oceanic crustal structure. However, it is challenging to estimate the oceanic crustal structure using teleseismic P waves recorded at a single OBS station, due to water reverberations and the close arrival time between the Moho-converted Ps phase and its multiples. We propose herein a method (the H–β algorithm) based on wavefield continuation and decomposition, whereby the oceanic crustal thickness and average S wave velocity can be estimated by minimizing the energy of the upgoing S wave within the uppermost mantle. Forward tests indicate that the H–β algorithm can resolve the thickness and S-wave velocity of each crustal layer robustly. We apply this method to an OBS station near Hawaii (station PL11) and obtain the oceanic crustal structure beneath the station. The resolved crustal thickness is consistent with previous studies.

Simplification of roughness bottom backscattering model at small grazing angle in shallow-water

Bottom backscattering due to roughness seafloor is the main source of shallow water reverberation, especially in the waveguide with downward reflection profile or a calm sea-surface. Empirical backscattering models with a simple form has an important limitation to analyzing other characteristics of reverberation except for the intensity characteristics, which originates from optics and describes the relationship between the bottom backscattering strength and scattering grazing angle of plane-wave in half-infinite space. In the shallow water, such a plane-wave backscattering model cannot be used due to frequency dispersion. The model of bottom backscattering based on physical scattering principle is made to relieve such a limitation, but thereby bringing about another restraint by a geoacoustics model. The bottom backscattering model, which is formulated during modeling the full-wave reverberation theory at small grazing angle in range-independent shallow water waveguide, is simplified by combining with bottom reflection coefficient model which is independent of the geoacoustics model. The bottom reflection coefficient model as referred to the proposed phase parameter <i>P</i> in this paper is equivalent to velocity and density of sediment to describe sound field interacted with sea-bottom. Therefore simplification of bottom backscattering model can be handled by the phase parameter without any knowledge of bottom geoacoustic parameters. The angular dependency and intensity dependency of bottom backscattering due to roughness seafloor at small grazing angle are studied more in depth through such a simplified model. Marking 2/<i>P</i> as the cut-off point, the grazing angle is divided into two stages. Near the critical angle, as grazing angle is greater than 2/<i>P</i> and less than critical grazing angle, the angular dependency of bottom backscattering due to roughness seafloor is weighted by phase parameter of bottom reflection coefficient, while the intensity dependency is independent of phase parameter. At each small grazing angle, as grazing angle is less than 2/<i>P</i>, the angular dependency of bottom backscattering due to roughness seafloor is proportional to incident and scattering grazing angle squared and irrespective of phase parameter of bottom reflection coefficient which is like the empirical bottom backscattering model, while the intensity dependency is proportional to the fourth power of phase parameter. So the bottom has different influences on the angular dependency and intensity dependency of bottom backscattering in different stages of grazing angle.

Vector intensity properties from direct and reverberant field from a mid-frequency sonar in shallow water

The Intensity Vector Autonomous Recorder (IVAR) is a benthic lander system that coherently measures 3-axis acoustic particle velocity and pressure (combined sensor). Results using IVAR in a mid-frequency propagation and reverberation study conducted near Geoje Island, Republic of Korea, are presented. The measurements were made within shallow semi-circular bay (depth ~25 m) with bay opening to deeper waters, and with directional wave measurements made within the bay waters. This presentation will focus on measurements made at a fixed range (100–1000 m, depending on test) from the Autonomous Reverberation Measurement System (ARMS), a benthic lander with a source and receive array mounted on a rotation stage in order to collect reverberation data as a function of bearing angle. Several vector and scaler metrics emerge based on different combinations of second-order acoustic fields are discussed, such as rate of energy transport and active and reactive intensity that lend insight into the process of shallow water reverberation. For example, in one continuous measurement made over a 12 hour period, there is high correlation between wave slope and horizontal beam width as determined with horizontal active intensity. This and other effects related to variability in shallow water reverberation will be discussed.

Coherent Seismic Arrivals in the P Wave Coda of the 2012 Mw 7.2 Sumatra Earthquake: Water Reverberations or an Early Aftershock?

AbstractTeleseismic records of the 2012 Mw 7.2 Sumatra earthquake contain prominent phases in the P wave train, arriving about 50 to 100 s after the direct P arrival. Azimuthal variations in these arrivals, together with back‐projection analysis, led Fan and Shearer (, https://doi.org/10.1002/2016GL067785) to conclude that they originated from early aftershock(s), located ∼150 km northeast of the mainshock and landward of the trench. However, recently, Yue et al. (, https://doi.org/10.1002/2017GL073254) argued that the anomalous arrivals are more likely water reverberations from the mainshock, based mostly on empirical Green's function analysis of a M6 earthquake near the mainshock and a water phase synthetic test. Here we present detailed back‐projection and waveform analyses of three M6 earthquakes within 100 km of the Mw 7.2 earthquake, including the empirical Green's function event analyzed in Yue et al. (, https://doi.org/10.1002/2017GL073254). In addition, we examine the waveforms of three M5.5 reverse‐faulting earthquakes close to the inferred early aftershock location in Fan and Shearer (, https://doi.org/10.1002/2016GL067785). These results suggest that the reverberatory character of the anomalous arrivals in the mainshock coda is consistent with water reverberations, but the origin of this energy is more likely an early aftershock rather than delayed and displaced water reverberations from the mainshock.

Localized water reverberation phases and its impact on backprojection images

AbstractCoherent radiators imaged by backprojections (BP) are commonly interpreted as part of the rupture process. Nevertheless, artifacts introduced by structure related phases are rarely discriminated from the rupture process. In this study, we use a calibration event to discriminate between rupture and structure effects. We reexamine the waveforms and BP images of the 2012 Mw 7.2 Indian Ocean earthquake and a calibration event (Mw 6.2). The P wave codas of both events present similar shape with characteristic period of approximately 10 s, which are backprojected as coherent radiators near the trench. S wave BP does not image energy radiation near the trench. We interpret those coda waves as localized water reverberation phases excited near the trench. We perform a 2‐D waveform modeling using realistic bathymetry model and find that the steep near‐trench bathymetry traps the acoustic water waves forming localized reverberation phases. These waves can be imaged as coherent near‐trench radiators with similar features as that in the observations. We present a set of methodologies to discriminate between the rupture and propagation effects in BP images, which can serve as a criterion of subevent identification.

Coherence in shallow water reverberation

Coherent structure of reverberation intensity versus time and depth is observed during TREX13, and the structure resembles that of sound propagation versus range and depth. The experiment was conducted in shallow water approximately 20 m depth with both source and vertical receiving arrays moored, so the ping-to-ping incoherence in reverberation is believed to be caused by time-dependence of water column properties, especially rough sea surfaces. To understand the observed coherence, simulation of reverberation field is performed, where reverberation pressure, rather intensity, is calculated so phase coherence is included. Possibility of inferring sound propagation information from measured reverberation is discussed. [Work supported by the US Office of Naval Research.]

The 2015 Mw 8.3 Illapel, Chile, Earthquake: Direction‐Reversed Along‐Dip Rupture with Localized Water Reverberation

National Science Foundation of China 41374040 41090294 Hellman Fellowship University of California, Los Angeles (UCLA) Faculty Research Grant National Aeronautics and Space Administration (NASA) NAS7-03001 JPL Award 1468977

A fluid‐rich layer along the Nankai trough megathrust fault off the Kii Peninsula inferred from receiver function inversion

AbstractInvestigation of fluid distribution along megathrust faults is an important issue, since the fluid affects frictional properties and thus slip behavior on faults. Scattered teleseismic phases, or receiver functions (RFs), have made significant contributions to understanding the fluid content of subducting plates beneath the onshore regions but have been rarely applied in offshore settings. In this study, we conducted receiver function inversion analysis to investigate detailed seismic properties near the megathrust fault using ocean bottom seismometers deployed off the Kii Peninsula, southwest Japan. RFs were calculated at high frequencies (up to 4 Hz), removing the effect of water reverberations from vertical component records. Our inversion was performed in two steps: first, we modeled sediment layer by a simple stacking method and then solved for deeper structure by a waveform inversion. The results indicate the presence of a thin low‐velocity zone (LVZ) of a thickness of 0.2–1.2 km with a S wave velocity of 0.7–2.4 km/s along the plate interface. We interpret this LVZ as thin fluid‐rich sediment layer between the overriding and subducting plates that acts as a pathway of fluid migration.

Investigation of Sea Surface Effect on Shallow Water Reverberation by Coupled Mode Method

The effects of rough sea surface on the long-range bottom reverberation in shallow seas are studied by the coupled mode reverberation theory. The scattering effect caused by irregular rough sea surface is described by couple coefficients. The decaying rules of long-range bottom reverberation level are simulated at different sea states, and the rough sea surface effect on the coherence of distant bottom reverberation is also discussed. It is indicated that irregular upper boundary has changed the propagation effect of the shallow water waveguide, and bottom reverberation, which is dominated among other kinds of reverberation in shallow water, is affected by the sea surface scattering as the increasing sea state. Compared with other literatures, the emphasis of this paper is to present the mechanism of rough sea surface scattering by describing the transfer of energy between different modes, and the details of energy transitions between different modes which are caused by sea surface scattering are presented for different sea states. With the increasing sea state, stronger mode coupling caused by surface scattering would affect the intensity and its space coherence of bottom reverberation obviously.

A new time-domain model for bottom backscatter

Bottom backscatter is important for a number of underwater applications: it is a source of noise in target detection and a source of information for sediment classification and geoacoustic inversion. Bottom backscatter measures sound scattered from roughness and volume heterogeneity in different sediment layers. While current models can successfully handle scattering from such sediment layers, they cannot correctly predict the scattered intensity as a function of time. A new model for bottom backscatter is introduced which provides an exact solution for scattered intensity as a function of time under first order perturbation theory. Examples of backscatter from various sediment layers will be shown in contrast to previous models. Issues such as the “spherical wave effect” will be addressed, and applications to model shallow water reverberation will also be discussed.

Hydrous state of the subducting Philippine Sea plate inferred from receiver function image using onshore and offshore data

AbstractExploring the hydrous state of subducting oceanic crust is intriguing because it is considered to affect the strength of megathrust faults that cause various types of earthquakes; however, its state beneath offshore regions remains unclear. In this study, we investigated fluid contents along the subducting Philippine Sea plate around the Kii Peninsula by receiver function (RF) analysis using data from both on‐land stations and ocean bottom seismometers (OBSs). The vertical component of OBS records contains dominant water reverberations, and thus, conventional methods fail to estimate RFs correctly. We therefore developed a method to calculate RFs that removes such reverberations. The RFs calculated by our method showed considerable improvement for later phase identification, compared with those obtained using a conventional method. Resultant RF amplitudes suggest the existence of low‐velocity zones directly beneath the plate interface of both onshore and offshore regions. We interpreted this as evidence of hydrous oceanic crust, which extends from 5 km to 35 km depth to the plate interface. Reduction of RF amplitudes beneath the Kii Peninsula suggests that dehydration of the oceanic crust increases the seismic velocity, and the accompanying densification makes the plate interface permeable. This permeable plate interface may characterize the location of non‐volcanic tremors. This contrasts with long‐term slow slip events because it is believed that they occur along the sealed plate interface. Comparison between the plate geometry and local earthquakes reveals the paucity of earthquakes in the oceanic crust below a certain depth, which provides further insight into the dehydration process in the oceanic crust.

Accurate focal depth determination of oceanic earthquakes using water-column reverberation and some implications for the shrinking plate hypothesis

Investigation of oceanic earthquakes is useful for constraining the lateral and depth variations of the stress and strain-rate fields in oceanic lithosphere, and the thickness of the seismogenic layer as a function of lithosphere age, thereby providing us with critical insight into thermal and dynamic processes associated with the cooling and evolution of oceanic lithosphere. With the goal of estimating hypocentral depths more accurately, we observe clear water reverberations after the direct P wave on teleseismic records of oceanic earthquakes and develop a technique to estimate earthquake depths by using these reverberations. The Z–H grid search method allows the simultaneous determination of the sea floor depth (H) and earthquake depth (Z) with an uncertainty less than 1 km, which compares favorably with alternative approaches. We apply this method to two closely located earthquakes beneath the eastern Pacific. These earthquakes occurred in ∼25 Ma-old lithosphere and were previously estimated to have similar depths of ∼10–12 km. We find that the two events actually occurred at dissimilar depths of 2.5 km and 16.8 km beneath the seafloor, respectively, within the oceanic crust and lithospheric mantle. The shallow and deep events are determined to be a thrust and normal earthquake, respectively, indicating that the stress field within the oceanic lithosphere changes from horizontal deviatoric compression to horizontal deviatoric tension as depth increases, which is consistent with the prediction of lithospheric cooling models. Furthermore, we show that the P-axis of the newly investigated thrust-faulting earthquake is perpendicular to that of the previously studied thrust event, consistent with the predictions of the shrinking-plate hypothesis.

A modeling approach to scattering and reverberation in shallow water

A physics-based modeling approach is described that allows prediction of reverberation in complex shallow water environments. An integral expression is presented for the backscatter intensity with a factorized integrand comprised of two kernels, the two-way propagator and the scattering kernel. The propagator is defined by Green’s function, describes the local intensity, and can be calculated using available models, such as PE, normal modes, or ray approximations. An expression for the scattering kernel is obtained using a unified approach to volume and roughness scattering [Ivakin, JASA Feb. 1998], and represents a sum of volume and roughness scattering coefficients. They are specified for sea-water column and rough heterogeneous seabed with continuous spatial fluctuations of compressibility and density, and/or discrete randomly distributed targets, such as bubbles, fish, shells, and others. Results of numerical simulations for shallow water reverberation time/range series, based on a PE propagation model, are presented, and potential contributions of different mechanisms of scattering are compared and discussed. The approach is applied to consider reverberation in a complex shelly sand/mud environment, such as one at the Target and Reverberation Experiment 2013 (TREX), and results in model/data comparisons based on analysis of TREX acoustic scattering data and environmental ground truth measurements. [Work supported by ONR-OA.]

Propagation measurement using source tow and moored vertical line arrays during TREX13

The objective of TREX13 (Target and Reverberation EXperiment 2013) is to investigate shallow water reverberation by concurrently measuring propagation, local backscatter, and reverberation, as well as sufficient environmental parameters needed to achieve unambiguous model/data comparison. During TREX13 the Marine Physical Laboratory (MPL) conducted propagation and forward scatter measurements. The MPL effort during TREX13 included deploying three, 32-element (0.2 m element spacing), vertical line arrays along the Main Reverberation Track at a bearing of ~128° and ranges ~2.4 km, ~4.2 km, and ~0.5 km from the R/V Sharp, where reverberation measurements were being made. In addition, MPL carried out repeated source tows in the band of 2–9 kHz along the Main Reverberation Track, using tonal and LFM waveforms. The experimental procedure is described and the resulting source-tow data is examined in the context of Transmission Loss and its implications for reverberation.

SONAR Equation perspective on TREX13 measurements

Modeling shallow water reverberation is a problem that can be approximated as two-way propagation (including multiple forward scatter) and a single backward scatter. This can be effectively expressed in terms of the SONAR equation: RL = SL-2 x TL + SS, where RL is reverberation level, SL is the source level, TL is the one way transmission loss, and SS is the integrated scattering strength. In order to understand the reverberation problem at the basic research level, both propagation and scattering physics need to be properly addressed. A major goal of TREX13 (Target and Reverberation EXperiment 2013) is to quantitatively investigate reverberation with sufficient environmental measurement to support full modeling of reverberation data. Along a particular reverberation track at the TREX13 site, TL and direct-path backscatter were separately measured. Environmental data were extensively collected along this track. This talk will bring together all the components of the SONAR equation measured separately at the TREX13 site to provide an assessment of the reverberation process along with environmental factors impacting each of the components.