Double-frequency Microseism Research Articles

Locating the Source Regions of the Single and Double-Frequency Microseisms to Investigate the Source Effects on HVSR in Site Effect Analysis

Locating the Source Regions of the Single and Double-Frequency Microseisms to Investigate the Source Effects on HVSR in Site Effect Analysis

Multi-instrument observations of microseisms generated by typhoon Kalmaegi (2014) over the Northwestern Pacific

The typhoon-generated microseisms, originating from the complex energy coupling and transferring among the Atmosphere-Ocean-Solid Earth spheres, can be detected remotely by seismometers as the strongest ambient seismic noise. The lack of in situ observations during the passage of typhoons has hampered numerical modeling of wind fields and ocean waves, and limited our understanding of the generation mechanisms of microseisms associated with typhoons. Here we present a comprehensive investigation of microseisms generated by typhoon Kalmaegi (September 2014) based on multiple-instrument constraints from observations including terrestrial and ocean-bottom seismic stations as well as ocean buoys. To understand the generation mechanisms, we apply an improved frequency-domain beamforming method to seismic array data leading to successful location of double-frequency (DF) microseism source regions. For comparison, we calculate the typhoon-induced ocean waves and theoretical source regions of the DF microseisms using the coupled ocean–atmosphere–wave–sediment transport modeling system with validation by ocean buoy observations. Both observations and numerical modeling results reveal two different generation mechanisms for typhoon-induced DF microseisms during the lifespan of Kalmaegi. When over the Philippine Sea, the DF microseisms were generated mainly by opposing ocean waves from two distinct storms. After Kalmaegi entered the South China Sea, the DF microseisms were generated mainly by the fast-moving typhoon with source regions just trailing behind, with the minimum frequencies determined by the typhoon translation speed. DF microseisms generated in coastal source regions were not detected by ocean bottom seismometers, suggesting that DF microseisms might not effectively propagate across the ocean-basin seafloor covered by thick sediments, owing to severe seismic attenuation and spreading losses. This information is crucial for the use of DF microseisms for future tracking and monitoring of typhoons.

A new broad-band ocean bottom seismograph and characteristics of the seismic ambient noise on the South China Sea seafloor based on its recordings

SUMMARY The ocean is the primary source of seismic ambient noise. Therefore, seismic recordings at seafloor stations should reveal noise characteristics more directly than land stations. However, due to a lack of broad-band seismic instrumentation, seafloor noise studies using seafloor stations have been inadequate compared to land-based instrumentation. In this study, we use seismic data collected at the South China Sea (SCS) seafloor by newly developed ocean bottom seismographs (OBSs) to analyze the ambient noise features in this marginal sea. The broad-band OBS, dubbed ‘Pankun’, has unique shielding to isolate its sensor from the influences of bottom currents. A side-by-side land test between the OBS sensor unit and a standalone seismometer showed that the self-noise caused by the gimbal and the pressure case is insignificant. The recordings on the SCS seafloor have distinct noise spectra. The double frequency microseisms (DFMs) have a single instead of double peak like that seen for Pacific stations. The peak appears in a lower period range (1–5 s) than in the global noise model, indicating that the primary source region for the DFM is the SCS itself. The high-frequency content of the DFM is attenuated more as it propagates from its source region (seafloor) to land stations. The single frequency microseism (SFM) peak on the spectrum is weak, reflecting that SFMs, generated in shallow water along the coast, have difficulties propagating back into the deep ocean due to the substantial increase in seafloor depth. A long-period Earth's hum signal is also identifiable on the vertical component at periods greater than 50 s, probably due to the anti-current design of the OBS. Although the seasonal sea state mainly affects the noise level, extreme events such as typhoons can produce short-term abnormally high DFMs in the basin. However, the DFM highs caused by such events exhibit complex patterns, depending on the wind speed, duration, and area covered by the events.

Double-Frequency Microseisms on the Thick Unconsolidated Sediments in Eastern and Southeastern Coasts of United States: Sources and Applications on Seismic Site Effect Evaluation

Double-Frequency Microseisms on the Thick Unconsolidated Sediments in Eastern and Southeastern Coasts of United States: Sources and Applications on Seismic Site Effect Evaluation

Exploring source regions of single- and double-frequency microseisms recorded in eastern North American margin (ENAM) by cross-correlation

Summary Strongly asymmetric cross-correlation functions (CCF) of ambient noise indicate an inhomogeneous distribution of the noise sources, and can be used to locate these sources. With this premise, a grid-search procedure is applied to explore the frequency dependent source locations of the single- (SF) and double-frequency (DF) microseisms recorded in eastern North American margin (ENAM). The frequency dependent Rayleigh wave group velocities are determined based on CRUST1.0 model and the Preliminary Reference Earth model and then the theoretical travel time differences from each hypothetical source to all station pairs are calculated by the fast marching method. Then a misfit function at each hypothetical source is defined to take the difference between the calculated travel time differences and the time lags observed from CCFs. A factor of reliability is defined considering the Rayleigh wave attenuation, the correlation coefficients between the microseisms and the Wavewatch III hindcasts of ocean wave spectra, and the efficiency of conversion from DF energy in water to seismic Rayleigh waves as a function of water depth and frequency. Together with the correlation analyses of the power spectra densities of the microseisms among stations, the primary source regions of SF and DF microseisms recorded in ENAM are estimated: 1) the sources of SF microseisms are likely distributed in the continental shelf dominantly as well as adjacent deep ocean area in western North Atlantic Ocean, and 2) the long-period and short-period DF microseisms appear generated in deep ocean near the continental slope and in the continental shelf and slope of ENAM, respectively.

Seismological Observations of Ocean Swells Induced by Typhoon Megi Using Dispersive Microseisms Recorded in Coastal Areas

Typhoons in the western Pacific Ocean can generate extensive ocean swells, some of which propagate toward Taiwan, Luzon, and the Ryukyu Islands, impacting the coasts and generating double-frequency (DF) microseisms. The dispersion characteristics of DF microseisms relevant to the propagation of ocean swells were analyzed using the fractional Fourier transform (FrFT) to obtain the propagation distance and track the origins of typhoon-induced swells through seismic observations. For the super typhoon Megi in 2010, the origin of the induced ocean swells was tracked and localized accurately using seismic records from stations in eastern Taiwan. The localized source regions and calculated wave periods of the ocean swells are in good agreement with values predicted by ERA5 reanalysis from the European Centre for Medium-Range Weather Forecasts (ECMWF). However, localized deviations may depend on the effective detection of dispersive DF microseisms, which is tied to both coastline geometry and the geographic locations of seismic stations. This work demonstrates the effectiveness of seismological methods in observing typhoon-induced swells. The dispersion characteristics of DF microseisms recorded by coastal stations could be used as a proxy measure to track and monitor typhoon-induced swells across oceans.

Analysis of seismic noise in the Romanian-Bulgarian cross-border region

We present the first detailed investigation of the background seismic noise recorded in the Romanian-Bulgarian cross-border region over 3 years (2012–2015). We used the power spectral densities probability density functions (PSD PDFs) to study the noise variations in the period domain (0.025–1 s) as well as in the secondary microseism band (2–10 s). Strong diurnal variations and an increase of the noise levels during working days were observed at high frequencies at all stations, thus confirming the anthropic origin of the noise at low periods. The noise variations observed at longer periods (> 1 s) are relatively small among the stations and are related to season changes. The dominant feature in the noise spectra between 2 and 10 s is the double-frequency peak (DFP) whose amplitude increases and changes during winter. For a specific interval, from 25th to 27th of January 2014, when a storm was reported in the Black Sea area, the maximum of the DFP shifted from larger periods (~ 5.5 s) at stations far from the Black Sea towards smaller periods (~ 1.8 s) at stations located on the coastline. The polarization analysis showed that the short period double-frequency microseisms originating from the Black Sea dominate during the winter month. Finally, we showed that site conditions vary due to noise variations related to weather conditions in the Black Sea or to changes in anthropogenic noise sources.

The Characteristics of Microseisms in South China Sea: Results From a Combined Data Set of OBSs, Broadband Land Seismic Stations, and a Global Wave Height Model

AbstractIt has long been recognized that ocean gravity waves can generate microseisms through their interactions and coupling with the solid earth. Their generation mechanisms, wave types, and propagation have been studied and debated intensively. In this study, we focus on the characteristics of microseisms in South China Sea (SCS) and compare them with globally observed microseisms. We use data from six ocean bottom seismometers, 10 publicly available broadband land seismometers near SCS, and a reanalyzed global wave height model from 1 May 2012 to 20 August 2012 (UTC), the time span of ocean bottom seismometers' data. We apply three techniques including power spectral density, correlation, and frequency dependent polarization analysis. Our results show that (1) microseisms observed in SCS are dominated by sources from adjacent oceans, instead of the common global sources from the Pacific, Atlantic, or Indian Ocean; (2) the split of double‐frequency microseisms (DFMs) is observed in SCS, especially on land stations: the sources are located near the central sea basin for long‐period DFMs (0.1–0.25 Hz) and are local near stations for short‐period DFMs (0.25–0.5 Hz); (3) typhoons both strengthen microseisms and affect the source locations of microseisms with the biggest influence on the short‐period DFMs; and (4) microseisms in or near SCS are a mixture of Love and Rayleigh waves and the relative dominance of Love and Rayleigh waves changes with locations.

Seismic Remote Sensing of Super Typhoon Lupit (2009) with Seismological Array Observation in NE China

The p-wave double-frequency (DF) microseisms generated by super typhoon Lupit (14–26 October 2009) over the western Pacific Ocean were detected by an on-land seismological array deployed in Northeastern China. We applied a frequency-domain beamforming method to investigate their source regions. Comparing with the best-track data and satellite observations, the located source regions of the p-wave DF microseisms, which corresponded to the strongest ocean wave–wave interactions, were found to be comparable to the typhoon centers in the microseismic frequency band of ~0.18–0.21 Hz. The p-wave DF microseisms were probably excited by the nonlinear interaction of ocean waves generated by the typhoon at different times, in good agreement with the Longuet–Higgins theory for the generation of DF microseisms. The localization deviation, which was ~120 km for typhoon Lupit in this study, might depend on the speed and direction of typhoon movement, the geometry of the seismological array, and the heterogeneity of the solid Earth structure. The p-wave DF microseisms generated in coastal source regions were also observed in the beamformer outputs, but with relatively lower dominant frequency band of ~0.14–0.16 Hz. These observations show that the p-wave DF microseisms generated near typhoon centers could be used as a seismic remote sensing proxy to locate and track typhoons over the oceans from under water in a near-real-time and continuous manner.

Double‐Frequency Microseisms in Ambient Noise Recorded in Mississippi

Abstract This research presents the power spectral density (PSD) of double‐frequency (DF) microseisms of 13 continuous single‐point long‐term ambient noise recordings (LTRs) at five inland and two coastal locations selected in Mississippi and Louisiana states and 234 single‐point short‐term ambient noise recordings (STRs) in northern Mississippi. By correlating PSD of LTRs with the simultaneous ocean data (including significant ocean wave frequency, significant ocean wave height, ocean wind speed, and above‐ocean atmosphere pressure) of the Atlantic Ocean and Gulf of Mexico, as well as local atmosphere pressure and wind speed, DF microseisms observed in northern Mississippi were shown to be shaped by a combined impact of both wave climates of the Atlantic Ocean and Gulf of Mexico. Particle motion analysis and calculation of vibration angle defined separately for LTRs and STRs strengthen this conclusion. The DF and PSD level at DF of STR plots in the horizontal direction versus unconsolidated sediment thickness show significant differences from the plots in the vertical direction, which indicates that the shear‐wave resonance in thick sediments modifies the DF microseisms more obviously in the horizontal than in the vertical direction.

The fine structure of double‐frequency microseisms recorded by seismometers in North America

AbstractWe performed a frequency‐dependent polarization analysis on ambient seismic energy recorded by 1768 USArray Transportable Array (TA) seismometers for the time period of 1 April 2004 through 31 October 2014. The seismic energy has strong seasonal variations in power and polarization at essentially all stations; however, the annual variation is much smaller. One year of data is sufficient to determine the average properties of the ambient seismic wavefield at a particular site. The average power and dominant period in the double‐frequency (DF) microseism band, defined here as periods of 2–10 s, vary significantly and coherently across North America. Proximity to a coastline generally leads to increased DF microseism amplitude, but site geology is much more important, with sedimentary basins having especially large DF amplitudes. The western U.S. as a whole has longer dominant DF periods than the central and eastern U.S., with the southeastern U.S. having the shortest dominant DF periods. Power spectral density estimates at many TA stations show a splitting of the DF microseism peak into two distinct subpeaks. This has been observed previously in data recorded by ocean bottom seismometers, with the shorter‐period DF peak attributed to the local sea and the longer‐period DF peak attributed to more distant, coastally generated microseisms. In the case of the land‐based TA data analyzed here, the DF splitting arises from simultaneous microseism generation at various source areas (Pacific Ocean, Atlantic Ocean, and Gulf of Mexico) with distinct, preferred excitation frequencies. DF microseism source properties derived from global models of ocean wave interaction support this interpretation.

Microseisms from huge Indian Ocean storms in May 2007

Ocean waves are acknowledged to be the cause of microseisms ubiquitous in seismograms. The excitation and source locations of microseisms, however, remain enigmatic. In this study, the characteristics of microseisms generated by extraordinary storms in the Indian Ocean in the middle of May, 2007 were investigated. Spectral analysis showed that two huge ocean-swell systems, one that began around May 9 and the other around May 14, generated strong microseisms: single-frequency (SF) microseisms ranging from 0.04–0.06 Hz and double-frequency (DF) microseisms ranging from 0.08–0.12 Hz. The dispersion of ocean waves generated a progressive frequency shift of microseismic peaks. A comparison among microseisms at deep-ocean islands and coastal seismic stations indicated that most of the dispersed DF microseisms followed SF microseisms generated in coastal areas. However, a dispersed DF microseism can also occur with no preceding excitation of an SF microseism; this has been observed at deep-ocean islands and coastal stations simultaneously, and implies that some of the DF microseisms recorded at inland stations may have been generated in the deep ocean. Frequency-wavenumber analysis of the DF band indicated the presence of non-dispersed body-wave microseisms when the swells were in the middle of the ocean. However, microseisms showing a progressive frequency shift of spectral intensity propagated dominantly as surface-waves and were observed when the swells were close to the coast and when a favorable condition for wave-wave interaction reached in the deep ocean.

Passive-source ocean bottom seismograph (OBS) array experiment in South China Sea and data quality analyses

Long-term passive source ocean bottom seismograph (OBS) observatory is challenging due to various technical difficulties. In order to gain experience in this field, and to reveal the lithospheric structure beneath the extinct ridge in the central South China Sea (SCS), we carried out a passive source OBS array experiment, which includes 18 OBSs, in the deep portion of SCS. Here we present the instrumentation, the OBS deployment and recovery of this experiment, and more importantly, the data quality evaluated by a number of approaches. Through processing and inspecting waveforms from global, regional and local earthquakes, we find that most of recovered OBSs have good data quality with discernible main phases. The ambient noise analyses of OBS recordings show that their noise is higher than the global average, and the horizontal component is noisier than the vertical, indicating current impacts on horizontal components are more severe. In the period range of 5–10 s, there is a noise notch for the SCS OBSs, and noise levels of horizontal components are comparable to the vertical. This feature, which is not seen at OBS stations in open ocean, suggests the distant sources for double frequency microseism in this marginal sea are not significant. In addition, we successfully determined the orientations for 7 OBSs by investigating their Rayleigh wave polarizations; and we demonstrated the dispersion feature of Rayleigh waves through the frequency-time analysis. Finally, we summarized lessons learned from this experiment regarding the passive source OBS investigations in SCS.

Are deep‐ocean‐generated surface‐wave microseisms observed on land?

AbstractRecent studies attribute land double‐frequency (DF) microseism observations to deep water generation. Here we show that near‐coastal generation is generally the dominant source region. This determination is based on observations at land and ocean seismic stations, buoys, gravity‐wave hindcasts, and on beamforming results from continental seismic arrays. Interactions between opposing ocean wave components generate a pressure excitation pulse at twice the ocean wave frequency that excites pseudo‐Rayleigh (pRg) wave DF microseisms. pRg generated in shallow coastal waters have most of their energy in the solid Earth (“elastic” pRg) and are observed by land‐based and seafloor seismometers as DF microseisms. pRg generated in the deep ocean have most of their energy in the ocean (“acoustic” pRg) and are continuously observed on the ocean bottom, but acoustic pRg does not efficiently transition onto continents. High‐amplitude DF signals over the [0.2, 0.3] Hz band observed on the deep seafloor are uncorrelated with continental observations and are not clearly detectable at individual continental stations or by land seismic‐array beamforming. Below 0.2 Hz, modeling and some observations suggest that some deep water‐generated elastic pRg energy can reach continental stations, providing that losses from scattering and transition across the continental‐shelf boundary to the shore are not substantial. However, most observations indicate that generally little deep‐ocean‐generated DF microseism energy reaches continental stations. Effectively, DF land observations are dominated by near‐coastal wave activity.

A low-frequency passive seismic array experiment over an onshore oil field in Abu Dhabi, United Arab Emirates

A low-frequency passive seismic experiment using an array of 49 3C broadband seismometers was conducted over an onshore oil field in the emirate of Abu Dhabi in the United Arab Emirates. The aim of the experiment was to understand the characteristics and origins of the microseism (0.15–0.4 Hz) and microtremor (about 1–6 Hz) signals recorded, the latter having been reported as being a hydrocarbon indicator above several reservoirs in the region. The recorded array data were analyzed for their polarization, apparent velocities, and wavefront azimuths using various techniques, including spectral and time-frequency analyses, particle motion, H/V spectral ratios, and high-resolution frequency-wavenumber (f-k) analyses. In the frequency range of 0.15–0.4 Hz, the dominant feature observed consisted of double-frequency microseisms peaks generated by the nonlinear interactions of ocean waves with the shoreline along the coasts of the Arabian Sea and the Arabian Gulf. The f-k analyses confirmed that microtremor events in the frequency range of 2–3 Hz have an azimuth pointing toward a major oil pipeline and oil plant facilities to the west–southwest of the study area, as well as a motorway to the southeast. This would indicate that the microtremor events are probably caused by local sources, including the continuous movement of oil through the pipeline, the noise from oil plant facilities, as well as nearby traffic noise. This interpretation was confirmed by the polarization analysis performed on the data. The data also indicated that no clear correlation exists between the microtremor signal and local meteorological conditions. Although some body waves with an infinite apparent velocity generated by earthquakes were recorded, no other body waves that could have possibly been generated by hydrocarbon reservoirs were observed using the analyses techniques used in this study. Therefore, our results indicated that for the site under investigation, the microseism and the microtremor signals detected could not be related to the presence of hydrocarbon accumulations in the subsurface, but instead they may be attributed to local anthropogenic sources.

Seismic ambient noise around the South China Sea: seasonal and spatial variations, and implications for its climate and surface circulation

With its strong seasonal variation in wave climate and various bathymetric features due to the complex tectonics, the South China Sea (SCS) provides a natural laboratory to study the microseism. We collected data from seismic stations around the SCS and calculated their noise spectra, through which seasonal and spatial variations of microseism, as well as the general feature of seismic ambient noise in this marginal sea were revealed. Microseism seasonal variations in general reflect influences of the East Asian monsoon in winter and the Indian monsoon in summer, respectively. The two microseism components, the single frequency microseism (SFM) and the double frequency microseism (DFM), show striking alternating variation patterns both seasonally and spatially. These variation patterns, along with the bathymetric feature near the stations, indicate SFM and DFM are generated through different physical mechanisms. More interestingly, seasonal and spatial variations of DFM appear to be consistent with the basin-scale surface circulation model of the SCS, in which the upper SCS experiences cyclonic in winter and anti-cyclonic in summer. These consistencies provide observational evidence for the hypothesis that the cyclonic depression is a favorable condition to generate DFM.

How moderate sea states can generate loud seismic noise in the deep ocean

The location of oceanic sources of the micrometric ground displacement recorded at land stations in the 0.1–0.3 Hz frequency band (“double frequency microseisms”) is still poorly known. Here we use one particularly strong noise event in the Pacific to show that small swells from two distant storms can be a strong deep‐water source of seismic noise, dominating temporarily the signals recorded at coastal seismic stations. Our interpretation is based on the analysis of noise polarization recorded all around the source, and the good fit achieved for this event and year‐round between observed and modeled seismic data. The model further suggests that this is a typical source of these infrequent loud noise bursts, which supports previous inconclusive evidences of the importance of such sources. This new knowledge based on both modeling and observations will expand today's limits on the use of noise for climate studies and seismic imaging.

The Origin of Double-Frequency Microseism and Its Seasonal Variability at King Sejong Station, Antarctica

Korea Polar Research Institute has been operating a broadband seismic station (KSJ1) at the King George Island (KGI), Antarctica, since 2001. Examining ambient seismic noise levels using power spectral analysis for the period of 2006–2008 at the KSJ1, we observed a seasonal pattern at a 4–10 s period. The amplitude of double-frequency (DF) microseism reaches a peak in May. Correlation of the DF energy and its predominant period with significant ocean-wave height and peak wave period models from the WAVEWATCH III and polarization analysis consistently indicate that ocean swell in the Drake Passage is a possible source to excite the DF microseism at the KGI. We also found that the temporal variation of DF amplitude is coincident with the seasonal change of ocean-acoustic ambient noise level around the KGI, which implies that incorporating long-term seismic and hydroacoustic noise information might give us an opportunity to figure out the characteristics of local climate variation near the Antarctic Peninsula.

Seismic monitoring of western Pacific typhoons

Typhoons inflict large damage to societies, but are usually difficult to monitor in close proximity in real-time without expensive instruments. Here we study the possibility of using seismic waveforms on the seafloor and on land to monitor the turning of a far away or approaching typhoon. Up to 67% of the typhoons making landfall in Taiwan come from the eastern shore, so that we deployed broadband ocean-bottom seismometers (OBSs) offshore eastern Taiwan in 2006 to study ground motion in close proximity to a typhoon. Typhoons generate ocean waves, which generate pressure signals in the water column before being transmitted to the seafloor as seismic waves and recorded by the OBSs. The ground motions on the seafloor correlate with locally increased (ocean) wave heights and wave periods, suggesting that the ground motions are mostly induced by in situ or nearby pressure fields, as shown by coherence function analyses. When a typhoon turns and changes wave-wave interaction near the source region, a new set of en echelon patterns develops which can be observed by OBSs and land stations. Similar features occur when a typhoon crosses a landmass and re-enters the ocean. The energy level ratio between the single-frequency and double-frequency microseisms also changes abruptly when the typhoon turns. These features can potentially help near real-time early warning with little cost to complement other conventional typhoon early warning methods.

Shallow-water seismoacoustic noise generated by tropical storms Ernesto and Florence

Land-based seismic observations of double frequency (DF) microseisms generated during tropical storms Ernesto and Florence are dominated by signals in the 0.15-0.5 Hz band. In contrast, data from sea floor hydrophones in shallow water (70 m depth, 130 km off the New Jersey coast) show dominant signals in the ocean gravity-wave frequency band, 0.02-0.18 Hz, and low amplitudes from 0.18 to 0.3 Hz, suggesting significant opposing wave components necessary for DF microseism generation were negligible at the site. Florence produced large waves over deep water while Ernesto only generated waves in coastal regions, yet both storms produced similar spectra. This suggests near-coastal shallow water as the dominant region for observed microseism generation.