Analysis Of Seismograms Research Articles

Power spectra of random heterogeneities in the solid earth

Abstract. Recent seismological observations focusing on the collapse of an impulsive wavelet revealed the existence of small-scale random heterogeneities in the earth medium. The radiative transfer theory (RTT) is often used for the study of the propagation and scattering of wavelet intensities, the mean square amplitude envelopes through random media. For the statistical characterization of the power spectral density function (PSDF) of the random fractional fluctuation of velocity inhomogeneities in a 3-D space, we use an isotropic von Kármán-type function characterized by three parameters: the root mean square (RMS) fractional velocity fluctuation, the characteristic length, and the order of the modified Bessel function of the second kind, which leads to the power-law decay of the PSDF at wavenumbers higher than the corner. We compile reported statistical parameters of the lithosphere and the mantle based on various types of measurements for a wide range of wavenumbers: photo-scan data of rock samples; acoustic well-log data; and envelope analyses of cross-hole experiment seismograms, regional seismograms, and teleseismic waves based on the RTT. Reported exponents of wavenumber are distributed between −3 and −4, where many of them are close to −3. Reported RMS fractional fluctuations are on the order of 0.01–0.1 in the crust and the upper mantle. Reported characteristic lengths distribute very widely; however, each one seems to be restricted by the dimension of the measurement system or the sample length. In order to grasp the spectral characteristics, eliminating strong heterogeneity data and the lower mantle data, we have plotted all the reported PSDFs of the crust and the upper mantle against wavenumber for a wide range (10−3–108 km−1). We find that the spectral envelope of those PSDFs is well approximated by the inverse cube of wavenumber. It suggests that the earth-medium randomness has a broad spectrum. In theory, we need to re-examine the applicable range of the Born approximation in the RTT when the wavenumber of a wavelet is much higher than the corner. In observation, we will have to carefully measure the PSDF on both sides of the corner. We may consider the obtained power-law decay spectral envelope as a reference for studying the regional differences. It is interesting to study what kinds of geophysical processes created the observed power-law spectral envelope at different scales and in different geological environments in the solid earth medium.

Iranian regional centroid moment tensor catalog: Solutions for 2012–2017

In 2012, the installation of broadband seismic stations in Iran allowed the beginning of the moment tensor project of Iranian earthquakes by the Iranian Seismological Center (IRSC). In a period of five years, 194 moment tensors with 3.7 ≤ MW ≤ 7.6 were computed based on analysis of seismograms recorded at local and regional distances. Included in the catalog are the moment tensors of 83 events, not modeled by GCMT. A reasonable consistency exists between the mechanisms of common events in the two catalogs. The average moment magnitude difference is 0.1 and the Kagan angle is below 20° for 61% of the earthquakes. The P, B and T axes difference is up to 15°. The moment tensor catalog of Iran shows a decrease of the magnitude of completeness by 0.3 after this study. The number of accepted solutions as a fraction of attempts of the project, with the average of 55%, increased from 40% in 2012 to 68% in 2016, due to the expansions of the seismic networks in Iran. The dataset includes earthquakes from all seismic zones in Iran with the dominance of thrust faulting. The calculated depths in this study are consistent with the subregion seismotectonic conditions of Iran. The information on seven earthquakes with MW 6.0+ and 53 events with 5.0 ≤ Mw < 6.0 is also included, and relation to regional faults and reasons of the large/small damaging effects are discussed. Considering the expansion of the broadband seismic stations in Iran, the future development of IRSC moment tensor project should concentrate more on determining the mechanisms of minor events, especially in the regions with no larger earthquakes.

S-wave attenuation of the shallow sediments in the North China basin based on borehole seismograms of local earthquakes

S-wave velocity and attenuation structures of shallow sediments play important roles in accurate prediction of strong ground motion. However, it is more difficult to investigate the attenuation than velocity structures. In this study, we developed a new approach for estimating frequency-dependent S-wave attenuation (⁠|$Q_S^{ - 1}$|⁠) structures of shallow sediments based on multiple time window analysis of borehole seismograms from local earthquakes. Multiple time windows for separating direct and surface-reflected S-waves in local earthquake waveforms at borehole stations are selected with a global optimization scheme. With respect to different time windows, the transfer functions between direct and surface-reflected S-waves are achieved with a weighted averaging scheme, based on which frequency dependent |$Q_S^{ - 1}$| values are obtained. Synthetic tests suggest that the proposed method can restore robust and reliable |${\rm{\ }}Q_S^{ - 1}$| values, especially when the data set of local earthquakes is not abundant. We utilize this method for local earthquake waveforms at 14 borehole seismic stations in the North China basin, and obtain |$Q_S^{ - 1}$| values in 2–10 Hz frequency band, as well as average |${V_P}$|⁠, |${V_S}$| and |${V_P}/{\rm{\ }}{V_S}$| ratio for shallow sediments deep to a few hundred metres. Results suggest that |$Q_S^{ - 1}$| values are to 0.01–0.06, and generally decrease with frequency. The average attenuation structure of shallow sediments within the depth of a few hundred metres beneath 14 borehole stations in the North China basin can be modelled as |$Q_S^{ - 1} = {\rm{\ }}0.056{f^{ - 0.61}}$|⁠. It is generally consistent with the attenuation structure of sedimentary basins in other areas, such as Mississippi Embayment sediments in the United States and Sendai basin in Japan.

Three Component Seismogram Analysis: A Case Study of the M = 7.7 South Java, Indonesia Earthquake on July 17, 2006 and Changes in Coulomb Stress and Seismicity Rate

Three Component Seismogram Analysis: A Case Study of the <i>M</i> = 7.7 South Java, Indonesia Earthquake on July 17, 2006 and Changes in Coulomb Stress and Seismicity Rate

Hovsgol earthquake 5 December 2014, M W = 4.9: seismic and acoustic effects

A moderate shallow earthquake occurred on 5 December 2014 (M W = 4.9) in the north of Lake Hovsgol (northern Mongolia). The infrasonic signal with duration 140 s was recorded for this earthquake by the “Tory” infrasound array (Institute of Solar-Terrestrial Physics of the Siberian Branch of the Russian Academy of Science, Russia). Source parameters of the earthquake (seismic moment, geometrical sizes, displacement amplitudes in the focus) were determined using spectral analysis of direct body P and S waves. The spectral analysis of seismograms and amplitude variations of the surface waves allows to determine the effect of the propagation of the rupture in the earthquake focus, the azimuth of the rupture propagation direction and the velocity of displacement in the earthquake focus. The results of modelling of the surface displacements caused by the Hovsgol earthquake and high effective velocity of propagation of infrasound signal (~ 625 m/s) indicate that its occurrence is not caused by the downward movement of the Earth’s surface in the epicentral region but by the effect of the secondary source. The position of the secondary source of infrasound signal is defined on the northern slopes of the Khamar-Daban ridge according to the data on the azimuth and time of arrival of acoustic wave at the Tory station. The interaction of surface waves with the regional topography is proposed as the most probable mechanism of formation of the infrasound signal.

Moment Tensor Inversion Based on the Principal Component Analysis of Waveforms: Method and Application to Microearthquakes in West Bohemia, Czech Republic

ABSTRACT We develop and test a new hybrid approach of the amplitude and waveform moment tensor inversions, which utilizes the principal component analysis of seismograms. The proposed inversion is less sensitive to noise in data, being thus more accurate and more robust than the amplitude inversion. It also suppresses other unmodeled phenomena, like a directivity of the source, errors caused by local site effects at individual stations, and by time shifts in arrivals of observed and synthetic signals due to an inaccurate velocity model. This inversion is computationally less demanding than the full waveform inversion and thus applicable to large sets of earthquakes. The approach is numerically tested on synthetic data with various levels of noise. The applicability of the inversion is demonstrated on inverting more than 800 microearthquakes that occurred during the 2014 activity in West Bohemia, Czech Republic. The analysis revealed several distinct clusters of moment tensors. Focal mechanisms corresponding to moment tensors of three clusters are left‐lateral strike slips associated with the most active fault in the focal zone. Another cluster is characterized by right‐lateral strike slips associated with the fault conjugate to the main fault. Finally, we identified a cluster with pure reverse focal mechanisms that are anomalous and not expected to occur in the region. These mechanisms were not detected in previous seismic activity, and they have an unfavorable orientation with respect to regional tectonic stress. This might indicate a presence of local stress heterogeneities caused, for example, by an interaction of faults or fault segments in the focal zone.

VS30 Characterization of Texas, Oklahoma, and Kansas Using the P-Wave Seismogram Method

The P-wave seismogram method is used to develop estimates of the time averaged shear wave velocity of the upper 30 m ( V S30) at 251 seismic stations in Texas, Oklahoma, and Kansas. Geologic conditions at the sites are documented using large-scale geologic maps. The V S30 values from the P-wave seismogram method agree well with the limited in situ measurements across the study area and correlate well with the mapped geologic units. Compared with the V S30 proxy values assigned to the stations by the Next Generation Attenuation–East (NGA-East) project, the P-wave seismogram method generally produces larger V S30 estimates. These differences are likely due to the fact that very few V S measurements in Texas, Oklahoma, and Kansas were available for use in the development of the NGA-East proxies. Analysis of the P-wave seismogram V S30 values indicates that, in this geographic area, incorporating rock type along with geologic age better distinguishes the average V S30 of these materials than geologic age alone.

Automatic event detection and picking of P, S seismic phases for earthquake early warning and application for the 2008 Wenchuan earthquake

We proposed a set of high-precision combination algorithms to detect and pick seismic phases for the earthquake early warning (EEW). First by the polarisation analysis of the three-component seismograms, we developed two filters to separate P and S waves for each record through a sliding time window. Then based on the short term average/long term average characteristic function on the polarised traces, an amplification coefficient, in terms of δ, was introduced to be multiplied by the original ratio to sensitively reflect the changes of signals’ amplitude and frequency, where a better detection of the phase arrival was achieved. Next according to the preliminary detections, we used the Akaike information criteria function combined with the higher order statistics to refine the signal and lock on the arrival time with a higher degree of accuracy. We tested our techniques to the main-shock and aftershocks of the Ms 8.0 Wenchuan earthquake, where hundreds of three-component acceleration records over magnitudes of Ms 5.0 were treated. In comparison to the analyst picks, the results of the proposed detection algorithms were shown to perform well and can be applied for the early warning of impending earthquakes occurred with diverse focal mechanisms, complicated propagation process and site effects.

A3-11 2014年Kelud火山噴火時の広帯域地震計記録解析(地殻変動と火山の地震,口頭発表)

A3-11 2014年Kelud火山噴火時の広帯域地震計記録解析(地殻変動と火山の地震,口頭発表)

UP-22 2014年御嶽山噴火に伴う地震活動および地震波の解析結果序報(緊急学術セッション(ポスター))

UP-22 2014年御嶽山噴火に伴う地震活動および地震波の解析結果序報(緊急学術セッション(ポスター))

Using Dynamic Fourier Analysis to Discriminate Between Seismic Signals from Natural Earthquakes and Mining Explosions

&lt;abstract&gt; &lt;p&gt;A sequence of intraplate earthquakes occurred in Arizona at the same location where mining explosions were carried out in previous years. The explosions and some of the earthquakes generated very similar seismic signals. In this study Dynamic Fourier Analysis is used for discriminating signals originating from natural earthquakes and mining explosions. Frequency analysis of seismograms recorded at regional distances shows that compared with the mining explosions the earthquake signals have larger amplitudes in the frequency interval ~ 6 to 8 Hz and significantly smaller amplitudes in the frequency interval ~ 2 to 4 Hz. This type of analysis permits identifying characteristics in the seismograms frequency yielding to detect potentially risky seismic events.&lt;/p&gt; &lt;/abstract&gt;

Estimation Algorithm for Low Frequency Seismic Motions Based on Extended Semblance Function

Abstract Monitoring low frequency and long period ground motions is very important for earthquake detection and alarm systems, and signal processing techniques help to investigate the mechanism of deep ground motions and earthquake occurrence by detecting effective seismic information from the seismograms. In this paper a frequency domain algorithm is presented to estimate the travel time difference of seismic waves with multi-path interferences caused by the refraction effect. The spectra of the seismograms are calculated through the fast algorithm, and the multi-path parameters as well as the difference of travel time between a reference position and the seismographic stations are given by an optimization algorithm in the frequency domain. The new approach can work under conditions of refraction interference, and improve the estimation performance by using an extended semblance function. Its effectiveness is demonstrated through some numerical examples, and it is shown that the proposed algorithm is applicable to the analysis of low frequency seismograms.

Efficient 3-D frequency-domain mono-parameter full-waveform inversion of ocean-bottom cable data: application to Valhall in the visco-acoustic vertical transverse isotropic approximation

Computationally efficient 3-D frequency-domain full waveform inversion (FWI) is applied to ocean-bottom cable data from the Valhall oil field in the visco-acoustic vertical transverse isotropic (VTI) approximation. Frequency-domain seismic modelling is performed with a parallel sparse direct solver on a limited number of computer nodes. A multiscale imaging is performed by successive inversions of single frequencies in the 3.5–10 Hz frequency band. The vertical wave speed is updated during FWI while density, quality factor QP and anisotropic Thomsen's parameters δ and ϵ are kept fixed to their initial values. The final FWI model shows the resolution improvement that was achieved compared to the initial model that was built by reflection traveltime tomography. This FWI model shows a glacial channel system at 175 m depth, the footprint of drifting icebergs on the palaeo-seafloor at 500 m depth, a detailed view of a gas cloud at 1 km depth and the base cretaceous reflector at 3.5 km depth. The relevance of the FWI model is assessed by frequency-domain and time-domain seismic modelling and source wavelet estimation. The agreement between the modelled and recorded data in the frequency domain is excellent up to 10 Hz although amplitudes of modelled wavefields propagating across the gas cloud are overestimated. This might highlight the footprint of attenuation, whose absorption effects are underestimated by the homogeneous background QP model (QP = 200). The match between recorded and modelled time-domain seismograms suggests that the inversion was not significantly hampered by cycle skipping. However, late arrivals in the synthetic seismograms, computed without attenuation and with a source wavelet estimated from short-offset early arrivals, arrive 40 ms earlier than the recorded seismograms. This might result from dispersion effects related to attenuation. The repeatability of the source wavelets inferred from data that are weighted by a linear gain with offset is dramatically improved when they are estimated in the FWI model rather than in the smooth initial model. The two source wavelets, estimated in the FWI model from data with and without offset gain, show a 40 ms time-shift, which is consistent with the previous analysis of the time-domain seismograms. The computational efficiency of our frequency-domain approach is assessed against a recent time-domain FWI case study performed in a similar geological environment. This analysis highlights the efficiency of the frequency-domain approach to process a large number of sources and receivers with limited computational resources, thanks to the efficiency of the substitution step performed by the direct solver. This efficiency can be further improved by using a block-low rank version of the multifrontal solver and by exploiting the sparsity of the source vectors during the substitution step. Future work will aim to update attenuation and density at the same time of the vertical wave speed.

Very low frequency earthquakes off the Pacific coast of Tohoku, Japan

AbstractWe found very low frequency earthquakes (VLFEs) at a shallow subduction zone close to the Japan Trench off the Pacific coast of Tohoku, Japan. Centroid moment tensor solutions of VLFEs showed reverse fault mechanisms with a compression axis in the east‐west direction. A cross‐correlation analysis of seismograms with template events between 2005 and 2013 revealed three major VLFE clusters and their temporal evolution. A VLFE cluster in the central off‐Tohoku region located in the large slip area of the 2011 Tohoku earthquake was detectable only before the Tohoku earthquake. However, VLFEs in the northern and southern off‐Tohoku regions at the rim of the large slip area were activated after the Tohoku earthquake. The change in the activity may reflect the stress redistribution by the coseismic and/or afterslip processes of the Tohoku earthquake.

Six-degree-of-freedom near-source seismic motions II: examples of real seismogram analysis and S-wave velocity retrieval

Near-source records obtained by the mechanical seismic sensor Rotaphone are presented. The Rotaphone can measure six components of seismic movements, three translational and three rotational. The apparent S-wave phase velocity is determined and the possibility to obtain the wavepath S-wave velocity directly under the receiver is discussed. Rotation-to-translation ratios (RTRs) characterize the strength of rotations compared to translations. The Rotaphone records of local microearthquakes were obtained in various European seismoactive regions over the last few years. Three case studies, analyzed in detail, include various geological structures and seismograms recorded at various epicentral distances from 0.7 to 14.9 km. Also, the source depth varies from 4.8 to 10.4 km. The first case is an event from the West Bohemia intraplate seismic swarm region. The seismogram was recorded only 0.7 km from the epicenter. This case shows the complexity of rotation-to-translational relations near the epicenter. The second case is from the Corinthian Gulf active-rift region. The study confirms the expectation of the theory concerning rotations connected with the direct S wave; however, difficulties follow from a very complex 3D geological structure in the vicinity of the station, complicated by a distinctive topography with steep slopes of the hills. The third example is from South Iceland, near the active Katla volcano. The data in this case satisfy the rotation-to-translation relations very well, which is probably caused by the relatively simple geological setting and appropriate source-to-receiver configuration. The RTRs are computed for all three cases, and their frequency dependence is discussed.

Identification of T-Waves in the Alboran Sea

Analyses of seismograms from ~1,100 north-Moroccan earthquakes recorded at stations of the Red Sismica de Andalucia (Southern Spain) reveal the systematic presence of late phases embedded in the earthquake codas. These phases have distinctive frequency contents, similar to the P and S spectra and quite different to the frequency contents of the earthquake codas. They are best detected at near-shore stations. Their amplitudes decay significantly with distance to the shoreline. The delays with respect to the P-wave onsets of the preceding earthquakes are consistently around 85 s. Late phases are only detected for earthquakes located in a small region of about 100 × 60 km centered at 35.4°N, 4.0°W near the northern coast of Morocco. Several hypotheses could, in principle, explain the presence of these late phases in the seismograms, for example, the occurrence of low-energy aftershocks, efficient wave reflections, or Rayleigh waves generated along the source-station paths. However, we conclude that the most-likely origin of these phases corresponds to the incidence of T-waves (generated by conversion from elastic to acoustic energy in the north-Moroccan coast) in the southern coast of the Iberian Peninsula. T-waves are thought to be generated by energy trapping in low-velocity channels along long oceanic paths; in this case, we demonstrate that they can be produced in much shorter paths as well. Although T-waves have been already documented in other areas of the Mediterranean Sea, this is the first time that they have been identified in the Alboran Sea.

Characteristics of bottom-simulating reflectors for Hydrate-filled fractured sediments in Krishna–Godavari basin, eastern Indian margin

The bottom-simulating reflector (BSR) is weak and patchy on the seismic section in the Krishna–Godavari basin, eastern Indian margin, where massive gas-hydrates have been recovered at Site 10 of the Indian National Gas-hydrates Program Expedition 01 (NGHP-01). The depth of the BSR near this site is around 160 m below the sea floor (mbsf). The average reflection coefficient from the BSR is −0.06, significantly smaller than the common global values of −0.1 to −0.2. The BSR shows a strong lateral variation in amplitudes along the seismic line due to the presence of faults. The methane solubility is modeled using a theoretical model of the gas-hydrates system, and methane concentrations from the pressure core show that the distribution of free gas below BSR is not uniform. A combination of synthetic seismogram analysis and rock physics modeling leads to the conclusion that weak and patchy BSRs are primarily caused by lateral discontinuities induced by the gas-filled fractures below BSRs. The free gas zone is thin and it shows segmented characteristics on the seismic section and acoustic impedance profile that we inverted. Fault zones increase the permeability and therefore trap gas in associated fractures that can scatter seismic energy and create low velocity zones.

Seismic Analysis of the Accidental WWII Bomb Explosion in Euskirchen, Germany, on 3 January 2014

Besides the well‐known forensics in law enforcement, forensic techniques are used in many scientific fields, including archaeology, biology, geology, and seismology. In seismology, the discrimination between earthquakes and chemical and nuclear explosions is by far the most important and extensively studied area (e.g., Thirlaway, 1973; Stump et al. , 1999, 2002; Douglas, 2007; Bowers and Selby, 2009). However, over the past decades, very diverse sources of seismic energy have been studied, and often astonishing source process details could be revealed by the analysis of seismograms. The sources include explosions, both intentional (e.g., Willmore, 1947, 1949; Holzer et al. , 1996; Koper et al. , 1999) and accidental (e.g., Koper et al. , 2003; Hinzen, 2007), sinking of ships (e.g., Koper et al. , 1999; Koper et al. , 2001; Mucciarelli, 2012), aircraft crashes (e.g., Kim et al. , 2001; McCormack, 2003), meteor approaches and sonic booms (e.g., Kanamori et al. , 1992; Qamar, 1995; Edwards et al. , 2008; Incorporated Research Institutions for Seismology, 2013; Heimann et al. , 2013; Roelofse and Saunders, 2013), and lightning strikes (Hinzen, 2012). On 3 January 2014, the air‐pressure wave of an explosion was widely felt in the eastern part of the Lower Rhine embayment, Germany. At 13:30 local time, the author noticed the wave and sound arriving at the seismic station in Bensberg (BNS) and immediately checked the data of the 40‐station local network of Cologne University. Using the arrivals of the air blast at 330 m/s for five stations and a standard velocity model for explosions, the event was located east of Euskirchen, a city of 55,000 inhabitants at the southern limit of the Lower Rhine embayment. During the data processing, police from the city of Bonn asked about the cause of the boom, which had …

Complex shallow mantle beneath the Dharwar Craton inferred from Rayleigh wave inversion

The 3-D shear velocity structure beneath South India's Dharwar Craton determined from fundamental mode Rayleigh waves phase velocities reveals the existence of anomalously high velocity materials in the depth range of 50–100 km. Tomographic analysis of seismograms recorded on a network of 35 broad-band seismographs shows the uppermost mantle shear wave speeds to be as high as 4.9 km s–1 in the northwestern Dharwar Craton, decreasing both towards the south and the east. Below ∼100 km, the shear wave speed beneath the Dharwar Craton is close to the global average shear wave speed at these depths. Limitations of usable Rayleigh phase periods, however, have restricted the analysis to depths of 120 km, precluding the delineation of the lithosphere–asthenosphere boundary in this region. However, pressure–temperature analysis of xenoliths in the region suggests a lithospheric thickness of at least ∼185 km during the mid-Proterozoic period. The investigations were motivated by a search for seismic indicators in the shallow mantle beneath the distinctly different parts of the Dharwar Craton otherwise distinguished by their lithologies, ages and crustal structure. Since the ages of cratonic crust and of the associated mantle lithosphere around the globe have been found to be broadly similar and their compositions bimodal in time, any distinguishing features of the various parts of the Dharwar shallow mantle could thus shed light on the craton formation process responsible for stabilizing the craton during the Meso- and Neo-Archean.

Stress drop in the sources of intermediate-magnitude earthquakes in northern Tien Shan

The paper is devoted to estimating the dynamical parameters of 14 earthquakes with intermediate magnitudes (energy class 11 to 14), which occurred in the Northern Tien Shan. For obtaining the estimates of these parameters, including the stress drop, which could be then applied in crustal stress reconstruction by the technique suggested by Yu.L. Rebetsky (Schmidt Institute of Physics of the Earth, Russian Academy of Sciences), we have improved the algorithms and programs for calculating the spectra of the seismograms. The updated products allow for the site responses and spectral transformations during the propagation of seismic waves through the medium (the effect of finite Q-factor). By applying the new approach to the analysis of seismograms recorded by the seismic KNET network, we calculated the radii of the sources (Brune radius), scalar seismic moment, and stress drop (release) for the studied 14 earthquakes. The analysis revealed a scatter in the source radii and stress drop even among the earthquakes that have almost identical energy classes. The stress drop by different earthquakes ranges from one to 75 bar. We have also determined the focal mechanisms and stress regime of the Earth’s crust. It is worth noting that during the considered period, strong seismic events with energy class above 14 were absent within the segment covered by the KNET stations.