Component Seismograms Research Articles

Curated Regional Earthquake Waveforms (CREW) Dataset

We have assembled CREW, the Curated Regional Earthquake Waveforms Dataset, which is a dataset of earthquake arrivals recorded at local and regional distances. CREW was assembled from millions of waveforms with quality control through semi-supervised learning. CREW includes 2.3 million waveforms that have global coverage. Each waveform consists of a 5 minute three component seismogram with labels for both a P and S arrival. CREW provides a high quality labelled waveform data set that can be used to develop and test machine learning models for the analysis of earthquakes recorded at regional distances.

Local magnitude determination for Turkey from two independent seismic networks and a consistent M4 + catalogue

SUMMARY Turkish seismicity is routinely recorded by two broad-band seismic networks: the Turkish National Seismic Network (AFAD ‘TU’ network) and the Bogazici University Kandilli Observatory And Earthquake Research Institute (KOERI ‘KO’ network). Each of them has over 200 seismic stations distributed across the country. We obtain a new uniform and consistent local magnitude (Ml) scale for Turkey using data from both networks, as well as the M4 + catalogue with Ml values that results from applying said scale to the events that occurred between 2007 and 2016 within the country. We measure the half peak-to-peak amplitude of the vertical component seismograms and estimate the attenuation parameters through an iterative procedure that fits the logarithm of the amplitude as a function of the hypocentral distance, the local magnitude, the station corrections and a constant accounting for possible systematic differences between amplitudes measured on horizontal seismographs and those measured on vertical seismographs. We additionally scale Ml to the moment magnitude measured by an independent source, obtaining a magnitude relationship accounting for attenuation adjustments, as well as differences between maximum horizontal and vertical amplitudes. The estimated Ml formula shows that station corrections from both networks regionally agree, and that non-zero corrections are centred on zero, within −0.48 and 0.46 magnitude units (corresponding to the 2.5 and the 97.5 quantiles). The new M4 + catalogue with consistent Ml values for AFAD and KOERI shows magnitude residuals that cannot be explained by a Gaussian distribution. We also show a very good agreement between our new Ml value and the original Ml values of AFAD and KOERI, with median differences below 0.1 magnitude units.

On the effects of wrongly aligned seismogram components for shear wave splitting analysis

Seismic anisotropy inside the Earth’s interior, especially in the upper and lowermost mantle, is commonly studied by measuring shear wave splitting. This is mostly done by the determination of the splitting parameters fast polarization direction and delay time as well as the splitting intensity. The applied techniques highly rely on the correct temporal alignment of the single component traces (vertical, North, East referred to as Z, N, E components) of an earthquake relative to each other. Mixing wrongly aligned recording components would result in misleading and wrong data representations, including the particle motions in both the ZNE and the ray (LQT) coordinate systems and waveforms in the LQT coordinate system. The main pitfall in this context is that start and end times of the single traces in general differ due to data storage details. Unfortunately, the code of the widely used MATLAB based shear wave splitting software package SplitLab contains an error which can cause a wrong relative temporal alignment of the input seismograms in some cases. This effect distorts under certain conditions splitting signals or simulates non-existing ones. We show examples and offer a remedy.

Estimation of source parameters and scaling relations for local earthquakes of Lohit Valley in Arunachal Himalaya, Northeast India

We estimate the source parameters (stress drop, corner frequency, source radius and scalar seismic moment) for local earthquakes that occurred during 2007–2008 in the Lohit Valley, Arunachal Himalaya in northeast India. The source parameters of 33 selected earthquakes (2.0 ≤ ML ≤ 4.4) are estimated by spectral analysis of P‐waves of the vertical component. The results are authenticated further by spectral analysis of S‐waves of horizontal component seismograms. Both P‐ and S‐wave spectral analyses show comparable results. The earthquakes' seismic moment varies from 2.66 × 1012 to 1.16 × 1015 N.m. The estimates of stress drop and source radius range from ~0.145 to 31.85 bar and ~242 to 647 m, respectively. The majority of the earthquakes show a low‐stress drop (≤10 bar). We interpret that the low‐stress drop is plausibly due to the presence of creeping patches in the thrust sheets in the study area within the upper ~10 km depth, which is not able to withstand much stress and release it in the forms of micro‐earthquakes. The scaling relations have been developed between important source parameters (ML, Mw, M0, ∆σ, and fc), which can serve as useful inputs for the assessment of earthquake hazards in the region.

Discrimination Between Small Earthquakes and Quarry Blasts in Egypt Using Spectral Source Characteristics

Source parameters calculated from displacement spectra of both P and S waves are used to discriminate between earthquakes and quarry blasts in three regions of Egypt during the 2009–2015 period. We use vertical component seismograms from 440 earthquakes and 450 quarry explosions with MD 1.5 to 3.3 to calculate source parameters, including scalar moments and corner frequencies. The Mo(P,S) vs. fc(P,S) and P- to S-wave corner frequency fc(P)/fc(S) ratios are used to distinguish quarry blasts from earthquakes. A comparison of Mo(P,S) vs. fc(P,S) for both earthquakes and explosions in Egypt demonstrates that explosions had significantly lower corner frequencies than earthquakes, particularly for S-wave displacement spectra. In contrast to the Northern and Central regions, the Southern Egyptian region provides a perfect separation of corner frequencies of earthquakes and explosions for both P- and S-waves. The empirically derived average ratio of fc(P)/fc(S) for earthquakes is 1.28, 1.26 and 1.26 in the Northern, Central and Southern Egyptian regions, respectively. For explosions, average fc(P)/fc(S) ratios are 1.89, 1.86 and 2.0 in the three Egyptian regions, respectively. According to these findings, the average ratio of fc(P) to fc(S) for explosions is higher than those for earthquakes, implying that the differences in ratios enhance the ability of the fc(P) vs. fc(S) approach to discriminate between earthquakes and explosions. Based on the average fc(P)/fc(S) ratios vs. Mw in the whole of Egypt, the observed fc(P)/fc(S) discrimination threshold value for separating quarry explosions from earthquakes is 1.51–1.52.

Determination of Shear Wave Splitting Parameters in 2018 Lombok Earthquake Using Rotation Correlation Method: Preliminary Result

Shear-wave splitting (SWS), or the propagation of two independent shear waves, can be used as an indicator of seismic anisotropy. In this study, we utilize this concept using aftershock data of the 2018 Lombok earthquake which had been acquired in period of August 4 – September 9, 2018. The goal of this research is to better understand the crack distribution related to the rupture zone of the 2018 Lombok earthquake. After applying instrument correction to the data, the waveform data were then windowed in each P and S arrival time. To determine the SWS parameters, we performed rotation in each horizontal seismogram components. The horizontal components were rotated from azimuth 0° to 180° with an increment of 1°. Cross-correlation coefficient (CCC) was determined for each rotation angle. The polarization direction and the SWS delay time were chosen from the parameters shown in the highest value of CCC.

Production of a homogeneous seismic catalog based on machine learning for northeast Egypt

Abstract This research presents a new approach which addresses the conversion of earthquake magnitude as a supervised machine-learning problem through a multistage approach. First, the moment magnitude (M w) calculations were extended to lower magnitude earthquakes using the spectral P-wave analyses of the vertical component seismograms to improve the scaling relation of M w and the local magnitude (M L) of 138 earthquakes in northeastern Egypt. Second, using unsupervised clustering and regression analysis, we applied the k-means clustering technique to subdivide the mapped area into multiple seismic activity zones. This clustering phase created five spatially close seismic areas for training regression algorithms. Supervised regression analysis of each seismic area was simpler and more accurate. Conversion relations between M w and M L were calculated by linear regression, general orthogonal regression (GOR), and random sample consensus (RANSAC) regression techniques. RANSAC and GOR produced better results than linear regression, which provides evidence for the effects of outliers on regression accuracy. Moreover, the overall multistage hybrid approach produced substantial improvements in the measured-predicted dataset residuals when individual seismic zones rather than all datasets were considered. In 90% of the analyzed cases, M w values could be regarded as M L values within 0.2 magnitude units. Moreover, predicted magnitude conversion relations in the current study corresponded well to magnitude conversion relations in other seismogenic areas of Egypt.

A novel time-domain polarization filter based on a correlation matrix analysis

Polarization filters (PFs) are widely used for denoising seismic data. These filters are applied in the fields of seismology, microseismic monitoring, vertical seismic profiling, and subsurface imaging. They are primarily useful to suppress ground roll in seismic reflection data and enhance P- and S-wave arrivals. Traditional implementations of PFs involved the analysis of the covariance matrix or the singular value decomposition (SVD) of a 3C seismogram matrix. The linear polarization level, known as rectilinearity, is expressed as a function of the covariance matrix eigenvalues or by the data matrix singular values. Wavefield records that are linearly polarized are amplified, whereas others are attenuated. Besides the described implementations, we have developed a new time-domain PF based on the analysis of the seismic data correlation matrix. The principal idea is to extend the notion of the correlation coefficient in 3D space. This new filter avoids the need for diagonalization of the covariance matrix or SVD of data matrix, which are often time-consuming. The new implementation facilitates the choice of the rectilinearity threshold: We determine that linear polarization in three dimensions can be represented as three classic 2D correlations. A good linear polarization is detected when a high linear correlation between the three seismogram components is mutually observed. The tuning parameters of the new filter are the length of the time window, the filter order, and the rectilinearity threshold. Tests using synthetic seismograms indicate that optimal results are reached with a filter order that spans between 2 and 4, a rectilinearity threshold between 0.3 and 0.7, and a window length that is equivalent to one to three times the period of the signal wavelet. Compared with covariance-based filters, the new filter can enhance the signal-to-noise ratio by 6–20 dB and reduces computational costs by 25%.

The Most Parsimonious Ultralow‐Velocity Zone Distribution From Highly Anomalous SPdKS Waveforms

AbstractThe locations of ultralow‐velocity zones (ULVZs) at the core‐mantle boundary (CMB) have been linked to a variety of features including hot spot volcanoes and large low‐velocity province (LLVP) boundaries, yet only a small portion of the CMB region has been probed for ULVZ existence. Here we present a new map of lower mantle heterogeneity locations using a global collection of highly anomalous SPdKS recordings based on a dataset of more than 58,000 radial component seismograms, which sample 56.9% of the CMB by surface area. The inference of heterogeneity location using the SPdKS seismic phase is challenging due to source‐versus receiver‐side ambiguity. Due to this ambiguity, we conducted an inversion using the principle of parsimony. The inversion is conducted using a genetic algorithm which is repeated several thousand times in order to construct heterogeneity probability maps. This analysis reveals that at probabilities 0.5, 0.25, and 0.125 up to 1.3%, 8.2%, or 19.7% of the CMB may contain ULVZ‐like heterogeneities. These heterogeneities exist in all lower mantle settings, including both high‐ and low‐velocity regions. Additionally, we present evidence that the Samoan ULVZ may be twice as large as previously estimated, and also present evidence for the existence of additional mega‐sized ULVZs, such as a newly discovered ULVZ located to the east of the Philippines. We provide new evidence for the ULVZ east of the Philippines through an analysis of ScP records.

Towards a homogeneous moment magnitude determination for earthquakes in South Africa: Reduction of associated uncertainties

New MW–ML magnitude relations valid for the periods January 1971–March 1997, April 1997–September 2012 and October 2012–present are presented for South Africa. Three MW–ML relations were derived for the respective periods by applying a general orthogonal regression using 85 mining-related and tectonic events. Moment magnitudes (MW) were calculated using spectral analysis of the vertical component seismograms. A Brune model for source radiation was assumed. Attenuation functions previously derived from local seismic events were used. For each event, the final MW was computed by taking the average of the values determined at four or more stations. These MW values were used to compute new MW –ML regression equations for three previous ML relations that had been used to compile the South African earthquake catalogue. The new MW–ML relations were then compared with the MW determined from the Harvard Centroid Moment Tensor (CMT) project. The MW computed using the relation derived for the most recent period (October 2012–present) produces MW values that are similar to those reported by the Harvard CMT solutions. The relations can be used in the homogenization of the South African earthquake catalogue. Application of the new relations for the three periods will reduce uncertainty in the homogenized catalogues, which are important when calculating seismic hazard parameters (i.e. when estimating activity rates, b-values and Mmax).

Determination of high-frequency attenuation characteristic of coda waves in the central region of Nepal Himalaya

The high-frequency coda wave attenuation in the central region of Nepal, in and around the Kathmandu valley, is estimated using vertical component seismograms of local earthquakes recorded at 16 different seismic stations of NAMASTE array. The estimated result is expressed in terms of Qc, quality factor (inverse of coda wave attenuation). The value of coda quality factor (Qc) is estimated at eight central frequencies of 1.5, 3.0, 6.0, 9.0, 12.0, 15.0, 18.0, and 21.0 Hz through four different coda window length from 20 to 50 s at 10 s interval by using the single backscattering model. The value of coda Qc obtained from this study, shows a clear dependence on a frequency according to the power relation, Qc (f)= Q0 f n, where Q0 is Qc at 1 Hz, and f is frequency and n represents the degree of frequency dependence. The mean value of Qc of 16 different seismic stations was obtained as (110 ± 10.6) f 1.03±0.03 at 30 s coda window length, which represents the high attenuation characteristics of the study area, and attenuation decreases with increasing central frequency. Qo increases from 73.1 ± 10.1 to 156.1 ± 13.6 and n decreases from 1.12 ± 1.05 to 0.92±0.03 when the coda window length increases from 20 to 50 s. It is concluded that the study area is tectonically very active, highly heterogeneous, and heterogeneity decrease with depth. The coda Q obtained in this study is compatible with the result obtained in the region having a similar tectonic setting.

Identifikasi Diskontinuitas Seismik Mantel di Barat Laut Pulau Sumatera Menggunakan Data Prekursor SS

410km and 660km discontinuities are very clear and very easily identified discontinuities other than the Moho layer. This research utilizes SS precursor data, bouncepoints in the northern part of Sumatra. The data used is the depth of the epicenter <70 km, earthquake magnitude 5.5 and the distance between the epicenter and earthquake recording station more than 100 0 . This study is a preliminary study to determine changes in the depth of discontinuity in the study area. The SS phase is very well observed in the transversal component seismogram which is the result of the rotation of two horizontal components NS and EW, to obtain a good seismogram a 0.03Hz low pass filter is performed. In this study used 38 data transversal component seismograms, from 76 horizontal component seismograms. The most important thing in this study is the determination of the SS phase used as a reference (point 0), the SS phase is determined using the AK135 table guide, then the SS precursors are determined which can be seen at 450 seconds, 300 seconds, 90 seconds and 50 seconds before the SS . SS prekursors that are very clear at 450 seconds, are strongly suspected as a 660 km discontinuity. SS prekursors can be seen clearly after the stacking process.

Upper crustal Poisson’s ratio and coda-wave attenuation beneath Eastern Anatolia

The attenuation of seismic waves reflects the elastic nature of the media within which the waves propagate. In this study, we calculate the Coda-Q (Qc), frequency dependence (η), Vp/Vs and Poisson’s (υ) ratios by using 2621 vertical component seismograms generated by 987 earthquakes recorded by 13 seismic stations in Eastern Anatolia, and creat a 2-D seismic tomographic Qc model for the region. The obtained model provides significant information for exploring the boundaries of adjacent tectonic units within the upper crust and interpreting their dynamic characteristics. The 2-D Qc model and the other parameters are consistent with the seismotectonic features of Eastern Anatolia. Highly heterogeneous Qc values are observed in the study area dividing it into north-south directed bands of low and high attenuation. The highest η values were obtained beneath the northwestern and eastern parts of the study region. Clear, high and low υ values are obtained in the western and eastern parts of the study area, respectively. The spatial variations in the measured parameters are consistent with many geophysical observations including low Pn velocities, efficient Sn blockage, high heat flow, and widespread volcanism. Different upper crustal thicknesses and inhomogeneous stress distribution along the East and North Anatolian Fault Zones may also contribute to the observed heterogeneities.

Crustal structure of western Tibet revealed by Lg attenuation tomography

The lateral Lg wave attenuation in western Tibet and surrounding regions is studied. A total of 706 Two-station method (TSM) and 533 Reverse two-station method (RTSM) pairs are generated from 282 regional earthquakes. The earthquakes are recorded at 48 seismic stations operated across western Kunlun and western Tibet regions. The Lg phases are extracted from the vertical component of seismograms with magnitude greater than 4.6 mb and focal depth less than 50 km to measure the TSM and RTSM Lg Q at 1 Hz (Q0). The LgQ0 tomography map is generated with grid-size 0.5∘×0.5∘ using the least-squares orthogonal factor decomposition (LSQR) algorithm. Lateral variation of Lg attenuation is found to exist across the region. The results provide new insights into the attenuation properties of the crust and its linkage with regional geodynamics. The low LgQ0<50 values are observed in TSM image along the Kunlun Suture, Tinashuihai terrane and in some pockets of north western side of the Lhasa terrane. The high LgQ0>200 is revealed in the central part of Lhasa terrane in western Tibet from both TSM and RTSM images. The high LgQ0 values complemented with the high Pn velocities beneath the central part of Lhasa terrane reveal the trace of underthrusting Indian lithosphere beneath the western Tibet.

Multiyear Shallow Conduit Changes Observed With Lava Lake Eruption Seismograms at Erebus Volcano, Antarctica

AbstractWe analyze near‐repeating broadband seismograms from nearly 3,000 eruptions (2003–2011) from the Erebus volcano lava lake to investigate temporal changes in the shallow eruptive dynamics of an open conduit volcano. Cross‐correlation analysis reveals progressive variable time lags between correlation‐aligned short period (SP) and very long period (VLP) seismogram components ranging over approximately ±1 s and evolving over weeks to months. Lava lake eruptions both excite an SP explosion seismic signal and gravitationally unload the conduit. After a delay of several seconds, this unloading excites a posteruptive, minute‐long VLP seismic signal that persist for several minutes until the lava lake is refilled. VLP‐SP lag variations are consistent across multiple seismic stations and are independent of eruption size, spectral characteristics, eruption frequency, and lava lake morphology changes. Lag changes are interpreted in terms of variable communication time between eruptions and the subsequent elastic and gravitational response of a tomographically imaged near‐summit magma storage and VLP source region several hundred meters below the lava lake. Tomographic and VLP moment tensor studies, combined with modeling, suggest that the elastodynamic communication time between the distinct SP and VLP source regions is mediated by conduit‐guided Stoneley waves within the uppermost magma‐filled conduit system that are sensitive to small shallow conduit geometry changes. Conduit geometry changes may be driven by internal or external processes, including conduit wall melting and refreezing, repeated eruptive slug erosion, and deformation from inner crater inflation or collapse.

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

Estimation of Coda Wave Attenuation in Northern Morocco

We studied the attenuation of coda waves and its frequency and lapse-time dependence in northern Morocco. We analysed coda waves of 66 earthquakes recorded in this region during 2008 for four lapse time windows of length 30, 40, 50, and 60 s, and at five frequency bands with central frequency in the range of 0.75–12 Hz. We determined the frequency dependent Q c relation for the horizontal (NS and EW) and vertical (Z) component seismograms. We analyzed three-component broadband seismograms of 66 local earthquakes for determining coda-Q based on the single back-scattering model. The Q c values show strong frequency dependence in 1.5–12 Hz that is related to high degree of heterogeneity of the medium. The lapse time dependence of Q c shows that Q 0 (Q c at 1 Hz) significantly increases with lapse time that is related to the depth dependence of attenuation and hence of the level of heterogeneity of the medium. The average frequency-dependent Q c(f) values are $$Q_{\text{c}} = (143.75 \pm 1.09)f^{(0.864 \pm 0.006)},$$ $$Q_{\text{c}} = (149.12 \pm 1.08)f^{(0.85 \pm 0.005)}$$ and $$Q_{\text{c}} = (140.42 \pm 1.81)f^{(0.902 \pm 0.004)}$$ for the vertical, north–south and east–west components of motion, respectively. The frequency-dependent Q c(f) relations are useful for evaluating source parameters (Singh et al. 2001), which are the key inputs for seismic hazard assessment of the region.

A Shallow Seismic Velocity Model for the Groningen Area in the Netherlands

AbstractThe province of Groningen in the Netherlands holds one of the world's largest natural gas fields, and it has been an important source of energy for Western Europe for many decades. The seismicity in recent years called for a better understanding of the local subsurface, and therefore a dense network of 70 boreholes was installed in early 2015. Each borehole is equipped with four geophones and a surface accelerometer. In this study, data from this network are used to determine the shallow velocity structure that is important for the quantification of the seismic hazard and accurate source localizations. Compressional and shear wave velocity profiles with uncertainties are derived for each of the 200 m deep boreholes using passive seismic interferometry applied to local event data. The resulting seismic velocity distributions are presented as contour maps for 50 m depth intervals. The maps show strong lateral variations, where areas of low VP/VS ratio correspond to regions of sedimentary infill. The shear wave velocities were derived using the transverse component seismograms. Because the sensor orientations of the borehole geophones were unknown, they had to be determined first. This was done using a novel method based on cross correlations between the geophones and their colocated surface accelerometer. In addition, by extensive cross‐correlation analysis over the network, several installation inconsistencies were identified and resolved.

Analysis of intermediate period correlations of coda from deep earthquakes

We aim at assessing quantitatively the nature of the signals that appear in coda wave correlations at periods >20 s. These signals contain transient constituents with arrival times corresponding to deep seismic phases. These (body-wave) constituents can be used for imaging. To evaluate this approach, we calculate the autocorrelations of the vertical component seismograms for the Mw 8.4 sea of Okhotsk earthquake at 400 stations in the Eastern US, using data from 1 h before to 50 h after the earthquake. By using array analysis and modes identification, we discover the dominant role played by high quality factor normal modes in the emergence of strong coherent phases as ScS-like, and P'P'df-like. We then make use of geometrical quantization to derive the constituent rays associated with particular modes, and gain insights about the ballistic reverberation of the rays that contributes to the emergence of body waves. Our study indicates that the signals measured in the spatially averaged autocorrelations have a physical significance, but a direct interpretation of ScS-like and P'P'df-like is not trivial. Indeed, even a single simple measurement of long period late coda in a limited period band could provide valuable information on the deep structure by using the temporal information of its autocorrelation, a procedure that could be also useful for planetary exploration.

Using direct and coda wave envelopes to resolve the scattering and intrinsic attenuation structure of Southern California

AbstractCharacterizing scattering and absorbing properties and the power spectrum of crustal heterogeneity is a fundamental problem for informing strong ground motion estimates at high frequencies, where scattering and attenuation effects are critical. We perform a comprehensive study of local earthquake coda waves in Southern California to constrain scattering and intrinsic attenuation structure. We analyze data from 1195 spatially distributed earthquakes from 1981 to 2013 at source depths of 10 to 15 km and epicentral distances from 0 to 250 km with magnitudes larger than 1.8. We stack envelope functions from 28,127 vertical component and 27,521 transverse component seismograms, filtered from 2 to 4 Hz. We model these observations using a particle‐based Monte Carlo algorithm that includes intrinsic attenuation as well as both P and S wave scattering and both single and multiple scattering events. We find that spatially averaged coda wave behavior for Southern California can be explained only with models containing an increase in scattering strength and intrinsic attenuation within the uppermost crust, i.e., they are poorly fit with half‐space models of constant scattering strength. A reasonable fit to our data is obtained with a two‐layer model, composed of a shallow crustal layer with strong wide‐angle scattering and high P and S intrinsic attenuation and a deeper layer with weaker scattering and lower intrinsic attenuation (top 5.5 km: αQI=250, βQI=125, heterogeneity correlation length a = 50 m, fractional velocity heterogeneity ε = 0.4; lower crust: αQI=900, βQI=400, a = 2 km, ε = 0.05).