Spectral Amplitude Ratios Research Articles

Inferring the Focal Depths of Small Earthquakes in Southern California Using Physics-Based Waveform Features

ABSTRACT Determining the depths of small crustal earthquakes is challenging in many regions of the world, because most seismic networks are too sparse to resolve trade-offs between depth and origin time with conventional arrival-time methods. Precise and accurate depth estimation is important, because it can help seismologists discriminate between earthquakes and explosions, which is relevant to monitoring nuclear test ban treaties and producing earthquake catalogs that are uncontaminated by mining blasts. Here, we examine the depth sensitivity of several physics-based waveform features for ∼8000 earthquakes in southern California that have well-resolved depths from arrival-time inversion. We focus on small earthquakes (2<ML<4) recorded at local distances (<150 km), for which depth estimation is especially challenging. We find that differential magnitudes (Mw/ML–Mc) are positively correlated with focal depth, implying that coda wave excitation decreases with focal depth. We analyze a simple proxy for relative frequency content, Φ≡log10(M0)+3log10(fc), and find that source spectra are preferentially enriched in high frequencies, or “blue-shifted,” as focal depth increases. We also find that two spectral amplitude ratios Rg 0.5–2 Hz/Sg 0.5–8 Hz and Pg/Sg at 3–8 Hz decrease as focal depth increases. Using multilinear regression with these features as predictor variables, we develop models that can explain 11%–59% of the variance in depths within 10 subregions and 25% of the depth variance across southern California as a whole. We suggest that incorporating these features into a machine learning workflow could help resolve focal depths in regions that are poorly instrumented and lack large databases of well-located events. Some of the waveform features we evaluate in this study have previously been used as source discriminants, and our results imply that their effectiveness in discrimination is partially because explosions generally occur at shallower depths than earthquakes.

Testing and Design of Discriminants for Local Seismic Events Recorded during the Redmond Salt Mine Monitoring Experiment

ABSTRACT The Redmond Salt Mine (RSM) Monitoring Experiment in Utah was designed to record seismoacoustic data at distances less than 50 km for algorithm testing and development. During the experiment from October 2017 to July 2019, six broadband seismic stations were operating at a time, with three of them having fixed locations for the duration, whereas the three other stations were moved to different locations every one-and-half to two-and-half months. RSM operations consist of nighttime underground blasting several times per week. The RSM is located in proximity to a belt of active seismicity, allowing direct comparison of natural and anthropogenic sources. Using the recorded data set, we built 1373 events with local magnitude (ML) of −2.4 and lower to 3.3. For 75 blasts (RMEs) from the Redmond Salt Mine and 206 tectonic earthquakes (EQs), both ML and the coda duration magnitude (MC) are well constrained. We used these events to test and design discriminants that separate the RMEs from the EQs and are effective at local distances. The discriminants consist of ML−MC, low-frequency Sg to high-frequency Sg, Pg/Sg phase-amplitude ratios, and Rg/Sg spectral amplitude ratios, as well as different combinations of two or more of these classifiers. The areas under the receiver operating characteristic curves (AUCs) of 0.92–1.0 for ML−MC, low-frequency Sg to high-frequency Sg, and Rg/Sg indicate that these discriminants are very effective. Conversely, the AUC of only 0.57 for Pg/Sg suggests that this discriminant is only slightly better than a random classifier. Among the effective classifiers, Rg/Sg, shows the lowest likelihood of misclassification (4.3%) for the populations. Results of joint discriminant analyses suggest that even the arguably ineffective single classifier, like Pg/Sg in this case, can provide some value when used in combination with others.

Acceleration response of part-screw pile composite foundation under long-term train load excitation

This paper investigates the vibration response of a new part-screw pile composite foundation under moving train loads via a model test method. Based on the acceleration signals of the pile and the surrounding soil, a non-linear fitting method is used to establish the relationship between the dynamic attenuation coefficient and the depth of the foundation. Next, the effect of spatial location and train speed on the pile-soil dynamic interaction is investigated by introducing the pile-soil spectral amplitude ratio δ p − s . In addition, the dynamic susceptibility of the part-screw pile to damage under dynamic train loads is revealed with a wavelet packet energy distribution index E DIS . Afterwards, the cumulative deformation evolution characteristics of screw pile composite foundations under long-term train loading were revealed using the frequency domain integration method. The results of this work reveal that the faster the train runs, the larger the pile-soil spectrum amplitude ratio and the more prominent the ‘dynamic concentration effect’ of the part-screw pile. The change in cross-section of the part-screw pile results in a sudden change in the vibration energy transmitted from the top of the pile, which makes the variable section of the part-screw pile more likely to cause dynamic damage or even deformation. The cumulative deformation evolution process of soil between part-screw piles under long-term cyclic train loading can be divided into three stages: slow change period, fluctuation rising period and stable deformation period. The research results have important reference significance for the optimization design of high-speed railway part-screw pile composite foundation and the planning of railway running speed.

Investigating the Role of Fluids in the Source Parameters of the 2013–2014 Mw 5 Matese Seismic Sequence, Southern Italy

Abstract We investigate the variability of Brune stress drop (Δσ), apparent stress (τa), and Savage–Wood radiation efficiency (ηsw=τa/Δσ), in the 2013–2014 Mw 5.0 earthquake sequence that struck the Matese area in the southern Apennines range of Italy. The sequence is clustered in a relatively small crustal volume in the 13–22 km depth range, which is greater than that of background seismicity and normal-faulting sequences that occurred under the range axis, usually located in the first 15 km of the crust. We find high Savage–Wood radiation efficiency values for most of the analyzed earthquakes located in a narrow crustal volume, with values ranging from well above the self-similarity value to very high values as high as 0.55. In addition, a large variability in radiation efficiency (up to 90%) is observed for two similar magnitude events at different depths. Previous studies reported seismic evidence of fluid involvement in the nucleation process of the Matese earthquakes. By integrating our results with crustal geophysical data published recently, we propose that most of the earthquakes characterized by high values of ηsw are nucleated within high pore pressure zones located in the crystalline midcrust of Adria. We reckon that high pore pressure fluids of deep origin played a role in the rupture process and were responsible for the mixed shear-tensile sources inferred from the analysis of the S-wave/P-wave spectral amplitude ratio for most of 2013–2014 earthquakes.

Combination of optical images and SAR images for detecting landslide scars, using a classification and regression tree

ABSTRACT Landslides are some of the most destructive and recurrent natural hazards worldwide. Landslides are triggered by natural phenomena such as extreme rainfall and earthquakes, causing human and economic losses. A rapid response to landslide events is necessary to assess damage mitigation and save lives and property. This study developed a landslide detection model using differential spectral indices and amplitude ratio changes with a classification and regression tree (CART), aiming to detect landslide scars after the occurrence of these events in Asian regions for testing different environment condition. The multi-temporal SAR and optical stack images were pre-processed to reduce speckle noise, seasonal noise, and atmospheric noise. This study explored change detection approaches with a minimum threshold of amplitude ratio change (Aratio), using Sentinel−1 images and the relative difference in the normalized difference vegetation index (rdNDVI), differential bare soil index (dBSI), and differential brightness index (dBI) was obtained using Sentinel−2 images. The accuracy of the model was examined by F1-scores. The accuracy of the model for landslide detection was considered moderately good to excellent. As a result of the landslide detection model, amplitude ratio change detection improved the model as revealed by the F1-scores. Moreover, this study found that differential spectral indices could be used to classify the types of landslides (deep-seated and shallow landslides) according to the level of surface changes and texture of the collapsed material after landslide events.

Relationship between SDOF-Input-Energy and Fourier Amplitude Spectral Amplification Ratios

ABSTRACT This article presents the theoretical relationship between the site amplification ratio of the input energy spectrum for a single-degree-of-freedom system (SDOF) and that of the Fourier amplitude spectrum (FAS). The relationship indicates that the input energy spectral ratio is the result of smoothing the square of the Fourier amplitude spectral ratio. The spectral window for smoothing is determined by the bedrock-motion FAS and the oscillator transfer function for the relative velocity. The characteristics of the input energy spectral ratio were derived based on the idea of smoothing and confirmed based on real seismic records. The findings of this study contribute to a better understanding of site effects on the input energy spectrum.

Ground Motion Amplification at Natural Rock Arches in the Colorado Plateau

Abstract Thousands of rock arches are situated within the central Colorado Plateau—a region experiencing small- to moderate-magnitude contemporary seismicity. Recent anthropogenic activity has substantially increased the seismicity rate in some areas, raising questions about the potential for vibration damage of natural arches, many of which have high cultural value. However, predictions of the vibration response and potential for damage at a given site are limited by a lack of data describing spectral amplification of ground motion on these landforms. We analyzed 13 sandstone arches in Utah, computing site-to-reference spectral amplitude ratios from continuous ambient seismic data, and compared these to spectral ratios during earthquakes and teleseismic activity. We found peak ground velocities on arches at their dominant natural modes (in the range of 2–20 Hz) are ∼20–180 times the velocity on adjacent bedrock, due to amplification arising from slender geometry and low modal damping (0.8%–2.7%). Ambient spectral ratios are generally 1.2–2.0 times the coseismic spectral ratios. Because arches experience highly amplified ground motion, the range of earthquakes considered potentially damaging may need to be revised to include lower-magnitude events. Our results have implications for conservation management of these and other culturally valuable landforms.

Adaptive singular value decomposition for bearing fault diagnosis under strong noise interference

Singular value decomposition (SVD) is an effective tool for analyzing the signals from mechanical systems and for fault diagnosis, which is a non-parametric signal analysis method free from phase shift and waveform distortion. In SVD, the embedding dimension of the Hankel matrix is an important parameter that directly influences the effectiveness of the SVD. However, the embedding dimension is usually determined by experience, which is quite subjective and limits the applicability of SVD. As such, a novel SVD method, named adaptive SVD (ASVD), is proposed in this paper. In ASVD, novel criteria are defined to obtain the specific embedding dimensions for different mechanical signals by means of numerical simulation. A novel phenomenon, that the singular value pairs change periodically with the step size of half-cycle sampling points, is found and it can be used to calculate specific embedding dimension instead of selecting it from a range using experience. Meanwhile, the envelope spectral amplitude ratio index is developed for addressing the issue of excessive decomposition in classic SVD. Lastly, an ASVD-based bearing fault diagnosis method is proposed to adaptively select useful sub-signals and to detect faults. Both simulated signal and experiment signals, collected from different bearing test rigs are used to verify the effectiveness of the proposed method. The results show that the proposed method has a satisfactory ability to eliminate interference noise and detect bearing fault.

Yield Estimation and Event Discrimination of the 4 August 2020 Beirut Chemical Explosion

AbstractWe investigate the 4 August 2020 Beirut accidental chemical explosion based on regional seismic data recorded at both on- and off-shore stations. The Lg-wave body-wave magnitude is mb(Lg)=3.30±0.46 for the Beirut explosion. The explosive yield obtained using an empirical magnitude-yield relation based on a fully buried explosion source model is only 0.112 kt. Alternatively, the yield estimated using an empirical relation between the yield and crater size is 1.22 kt, with the uncertainties between 0.48 and 2.3 kt. The latter is closer to reality. The P/S spectral amplitude ratios, including Pg/Lg, Pn/Lg, and Pn/Sn, are calculated for the Beirut explosion and nearby natural earthquakes. We find the P/S spectral ratios are effective in discriminating the explosion from earthquakes in the Northwestern Arabia plate. By comparing the spectral ratios of large open-pit explosions, including the Beirut, Xiangshui, and Tianjin explosions, with those from historical nuclear explosions, buried small chemical explosions, and natural earthquakes, we further investigate the detailed differences of network-averaged P/S spectral ratios between different source types.

Preliminary strong ground motion simulation at seismic stations within nuclear power plant sites in South Korea by a scenario earthquake on the causative fault of 2016 Gyeongju earthquake

Stochastic and an empirical Green's function (EGF) methods are preliminarily applied to simulate strong ground motions (SGMs) at seismic stations within nuclear power plant (NPP) sites in South Korea by an assumed large earthquake with MW6.5 (scenario earthquake) on the causative fault of the 2016 Gyeongju earthquake with MW5.5 (mainshock). In the stochastic method, a ratio of spectral amplitudes of observed and simulated waveforms for the mainshock is assumed to be an adjustment factor. In the EGF method, SGMs by the mainshock are simulated assuming SGMs by the 2016 Gyeongju earthquake with MW5.0 (foreshock) as the EGF. To simulate SGMs by the scenario earthquake, a ratio of fault length to width is assumed to be 2:1 in the stochastic method, and SGMs by the mainshock are assumed to be EGF in the EGF method. The results are similar based on a bias of the simulated response spectra by the two methods, and the simulated response spectra by the two methods exceeded commonly standard design response spectra anchored at 0.3 g of NPP sites slightly at a frequency band above 4 Hz, but considerable attention to interpretation is required since it is an indirect comparison.

Spectral amplification of ground motion linked to resonance of large-scale mountain landforms

Amplification of seismic energy in steep topography plays an important role controlling the location of earthquake-induced landslides. Alpine mountains represent extreme topography, therefore large amplification may be anticipated, however suitable data needed to probe the limits of topographic effects in these demanding locations are rare. Here we present new ambient vibration data from seismic stations on the summit and ridge of one of the tallest freestanding mountains in the Swiss Alps – the Matterhorn – comparing these to a nearby local reference. Results show elevated spectral power at mountain stations between 0.4 and 1 Hz, and directional site-to-reference spectral amplitude ratios up to 14, which we attribute in part to topographic resonance. We used ambient vibration modal analysis and numerical eigenfrequency modeling to identify the fundamental mode of the Matterhorn at 0.42 Hz, as well as evidence for a second, mutually-perpendicular mode at a similar frequency. We identified high modal damping ratios of ∼20% for these modes, which we ascribe to radiative energy loss. A short campaign measurement at another mountain of comparable shape but smaller scale showed similar modal properties with a higher fundamental frequency of 1.8 Hz and peak spectral ratios of 6. Tracking of resonant frequencies over one year at the Matterhorn revealed no measurable seasonal variations related to near-surface environmental changes (e.g. temperature, ice). Our results demonstrate large spectral amplifications linked to resonance of high-relief mountain landforms, which is likely to be a widespread effect making such areas more prone to co-seismic rock damage and landslides.

Identification of infrasonic and seismic components of tremors in single-station records: application to the 2013 and 2018 events at Ioto Island, Japan

Infrasonic stations are sparse at many volcanoes, especially those on remote islands and those with less frequent eruptions. When only a single infrasound station is available, the seismic–infrasonic cross-correlation method has been used to extract infrasound from wind noise. However, it does not work with intense seismicity and sometimes mistakes ground-to-atmosphere signals as infrasound. This paper proposes a complementary method to identify the seismic component and the infrasonic component using a single microphone and a seismometer. We applied the method to estimate the surface activity on Ioto Island. We focused on volcanic tremors during the phreatic eruption on April 11, 2013, and during an unconfirmed event on September 12, 2018. We used the spectral amplitude ratios of the vertical ground motion to the pressure oscillation and compared those for the tremors with those for known signals generated by volcano-tectonic earthquakes and airplanes flying over the station. We were able to identify the infrasound component in the part of the seismic tremor with the 2013 eruption. On the other hand, the tremor with the unconfirmed 2018 event was accompanied by no apparent infrasound. We interpreted the results that the infrasound with the 2013 event was excited by the vent opening or the ejection of ballistic rocks, and the 2018 event was not an explosive eruption either on the ground or in the shallow water. If there was any gas (and ash) emission, it might have occurred gently undersea. As the method uses the relative values of on-site records instead of the absolute values, it is available even if the instrument sensitivity and the station site effects are poorly calibrated.

\u201cN\u201d-shaped Y/X coda spectral ratio observed for in-line-type OBS networks; S-net and ETMC: interpretation based on natural vibration of pressure vessel

We have identified “N”-shaped Y/X amplitude spectral ratios in S-coda records from a significant number of OBSs (ocean bottom seismometers) belonging to in-line-type ocean bottom networks of S-net and ETMC deployed around the Japan Trench and Sagami Trough, respectively. The “N”-shape reflects a sharp peak and notch at approximately 5–13 Hz and 10–23 Hz, respectively. This shape does not characterize OBSs belonging to node-type ocean bottom network of DONET deployed near the Nankai Trough. For S-net stations, the “N”-shape is not clearly formed for stations installed within grooves dug in the seafloor. We interpret the “N”-shaped Y/X amplitude spectral ratio is caused by the natural vibrations of a cylindrical pressure vessel that is placed sideways (long-axis lies in the horizontal plane) on the seafloor. The notch and peak frequencies in the Y/X amplitude spectral ratio likely correspond to natural frequencies of longitudinal (X-direction) and torsional and/or bending (Y-direction) vibrations, respectively. These natural vibrations are not observed for buried OBSs or those installed within grooves in the seafloor probably because they are better coupled to the seafloor. We propose a simple model to evaluate the extent to which the peak and notch have formed, which depends on the natural frequencies and coupling of the pressure vessel. We suggest users of in-line-type OBSs carefully examine if there are different responses between the X and Y components when frequencies about > 3 Hz are used. When installing OBS networks in the future, installing OBSs and cables within grooves dug in the seafloor or by burial will be effective in suppressing such natural vibrations.

Classification of Local Seismic Events in the Utah Region: A Comparison of Amplitude Ratio Methods with a Spectrogram‐Based Machine Learning Approach

AbstractThe capability to discriminate low‐magnitude earthquakes from low‐yield anthropogenic sources, both detectable only at local distances, is of increasing interest to the event monitoring community. We used a dataset of seismic events in Utah recorded during a 14‐day period (1–14 January 2011) by the University of Utah Seismic Stations network to perform a comparative study of event classification at local scale using amplitude ratio (AR) methods and a machine learning (ML) approach. The event catalog consists of 7377 events with magnitudes MC ranging from −2 and lower up to 5.8. Events were subdivided into six populations based on location and source type: tectonic earthquakes (TEs), mining‐induced events (MIEs), and mining blasts from four known mines (WMB, SMB, LMB, and CQB). The AR approach jointly exploits Pg‐to‐Sg phase ARs and Rg‐to‐Sg spectral ARs in multivariate quadratic discriminant functions and was able to classify 370 events with high signal quality from the three groups with sufficient size (TE, MIE, and SMB). For that subset of the events, the method achieved success rates between about 80% and 90%. The ML approach used trained convolutional neural network (CNN) models to classify the populations. The CNN approach was able to classify the subset of events with accuracies between about 91% and 98%. Because the neural network approach does not have a minimum signal quality requirement, we applied it to the entire event catalog, including the abundant extremely low-magnitude events, and achieved accuracies of about 94%–100%. We compare the AR and ML methodologies using a broad set of criteria and conclude that a major advantage to ML methods is their robustness to low signal‐to‐noise ratio data, allowing them to classify significantly smaller events.

An Examination of Amplification and Attenuation Effects in the Atlantic and Gulf Coastal Plain Using Spectral Ratios

AbstractThe Atlantic and Gulf Coastal Plain in the southern and southeastern United States contains extensive Cretaceous and Cenozoic sedimentary sequences of variable thickness. We investigated the difference in response of sites in the Coastal Plain relative to sites outside that region using Fourier spectral ratios from 17 regional earthquakes occurring in 2010–2018 recorded by the Earthscope transportable array and other stations. We used mean coda and Lg spectra for sites outside the Coastal Plain as a reference. We found that Coastal Plain sites experience amplification of low‐frequency ground motions and attenuation at high‐frequencies relative to average site conditions outside the Coastal Plain. The spectral ratios at high frequencies gave estimates of the difference between kappa at Coastal Plain sites and the reference condition. Differential kappa values determined from the coda are correlated with the thickness of the sediment section and agree with previous estimates determined from Lg waves. Averaged estimates of kappa reach ∼120 ms at Gulf coast stations overlying ∼12 km of sediments. Relations between Lg spectral ratio amplitudes versus sediment thickness in successive frequency bins exhibit consistent patterns, which were modeled using piecewise linear functions at frequencies ranging from 0.1 to 2.8 Hz. For sediment thickness greater than ∼0.5 km, the spectral amplitude ratio at frequencies higher than approximately ∼3 Hz is controlled by the value of kappa. The peak frequency and maximum relative amplification at frequencies less than ∼1.0 Hz depend on sediment thickness. At 0.1 Hz, the mean Fourier amplitude ratio (Coastal Plain/reference) is about 2.7 for sediment of 12 km thickness. Analysis of residuals between observed and predicted ground motions suggests that incorporating the amplification and attenuation as functions of sediment thickness may improve ground‐motion prediction models for the Coastal Plain region.

Spectral ratio analyses of explosion earthquakes at Sakurajima Volcano, Japan

It's quite important to clarify what physical parameters control the magnitude of volcanic explosion to understand magma processes in the conduit. Analyses of explosion earthquakes, which are associated with vulcanian eruptions, enable us to quantitatively evaluate dynamic process of the explosive eruptions. In the present study, we analyze hundreds of explosion earthquakes recorded at Sakurajima volcano, Japan, during 2012–2013. Applying a spectral ratio method that can retrieve the source spectral information without being disturbed by heterogeneous structure to three-component seismograms recorded at three stations operated by Japan Meteorological Agency, we examine scaling relations in explosion earthquakes. To obtain an averaged feature, we group the observed explosion earthquakes into five classes (I, II, III, IV, V) according to their maximum amplitudes and calculate the spectral ratios between the five classes. Because vulcanian eruptions continue to effuse volcanic ash for several to tens of minutes, spectral amplitude ratios are calculated by shifting a time window of 10 s for five successive 10s-long time windows from the onset to coda wave. The results show that the spectral amplitude ratios are characterized by the low and high corner frequencies: about 2 Hz and 3.5 Hz, respectively. With increasing the magnitude of explosion earthquakes, amplitude ratios at low and high frequency ranges, which are 0.5–2 Hz and 3.5–10 Hz, respectively, increase, while the corner frequencies do not significantly change. Since the explosion earthquakes at Sakurajima volcano occur at a same crater without changing the vent radius, the observed spectral amplitude ratio change may be attributed to pressure changes in the conduit. Another finding is that the amplitudes of spectral ratio gradually decrease with lapse time keeping the corner frequencies. Such a lapse time dependence of spectral ratios is completely different from those of tectonic earthquakes in which direct and coda waves show a same spectral amplitude ratio. We infer that the early part of the seismic waves of explosion earthquakes is generated by an initial rapid pressure release in the conduit while coda waves are by successive oscillations of the magma system during continuous ash emission.

Analysis of local site conditions through geophysical parameters at a city under earthquake threat: Çanakkale, NW Turkey

Çanakkale is the second province, after Istanbul, that has territories in both Asia and Europe. The city, also known as Dardanelles, is located on the Gallipoli peninsula in the northwest of Turkey, and the Biga peninsula which is an extension of Anatolia at the west. The region is tectonically quite active and it has high earthquake generation potential. The city has experienced a significant earthquake (Mw = 6.8) once again that occurred on May 24, 2014. In addition to these challenges, remarkable part of the city's territory and also potential new settlement areas are located on thick alluvium. Thus, mentioned-above disadvantages clearly increase the importance of this study, which focuses on determining the areas having different earthquake hazard potentials with regard to local site conditions by performing seismic risk assessments, as well as providing a basis for the preparation of settlement suitability maps on different scales that will lead knowledge for zoning plans. To that end, an integrated geophysical and geotechnical study was performed in a detailed manner. Multi-channel analysis of surface waves (MASW) and microtremor (MT) measurements were carried out at properly distributed 110 stations representing the whole survey area. The geotechnical study, planned to contribute geophysical interpretation, involved the drilling of 110 boreholes that are close to geophysical measurement stations. In addition to examining Vs30 variations based on the MASW, the ground dominant vibration period maps were also produced through MT time series analysis. In particular, the mean Vs velocity (Vs30) up to the first 30 m and the ground oscillation period change based on spectral amplitude ratios (HVSR) yielded substantial information that helped generate the seismic microzonation maps and also make the ground classification. These outputs contributed to exhibit the risk zones in the coastal city that has a dense and narrow settlement plan.

Event couple spectral ratio <i>Q</i> method for earthquake clusters: application to northwest Bohemia

Abstract. We develop an amplitude spectral ratio method for event couples from clustered earthquakes to estimate seismic wave attenuation (Q−1) in the source volume. The method allows to study attenuation within the source region of earthquake swarms or aftershocks at depth, independent of wave path and attenuation between source region and surface station. We exploit the high-frequency slope of phase spectra using multitaper spectral estimates. The method is tested using simulated full wave-field seismograms affected by recorded noise and finite source rupture. The synthetic tests verify the approach and show that solutions are independent of focal mechanisms but also show that seismic noise may broaden the scatter of results. We apply the event couple spectral ratio method to northwest Bohemia, Czech Republic, a region characterized by the persistent occurrence of earthquake swarms in a confined source region at mid-crustal depth. Our method indicates a strong anomaly of high attenuation in the source region of the swarm with an averaged attenuation factor of Qp<100. The application to S phases fails due to scattered P-phase energy interfering with S phases. The Qp anomaly supports the common hypothesis of highly fractured and fluid saturated rocks in the source region of the swarms in northwest Bohemia. However, high temperatures in a small volume around the swarms cannot be excluded to explain our observations.

Frequency‐Dependent Attenuation of P and S Waves in Southern California

AbstractAccurate models of crustal attenuation structure are important for simulating seismic wavefields at high frequencies (f > 1 Hz). In this study, we collected P and S waveforms from 160 regional earthquakes (3.3 ≤ M ≤ 5.7) recorded at 218 broadband stations of Southern California Seismic Network and measured spectral amplitudes of P and S waves in the 1‐ to 10‐Hz band using integrals over wavelet transforms. We accounted for source structure and geometrical spreading by referencing the spectral amplitudes to values computed from 1‐D synthetic seismograms. We inverted the spectral amplitude ratios for a 1‐D, frequency‐dependent crustal attenuation model. When plotted against wavenumber, the depth averages of both QP and QS are described by the same function, Q(k) = (240 ± 20) k0.40 ± 0.05 (1 km−1 ≤ k ≤ 17 km−1). Our results are consistent with attenuation dominated by first‐order Born scattering from an isotropic, self‐affine distribution of elastic heterogeneities with an outer scale of ~10 km, a root‐mean‐square relative amplitude of ~8% and a fractal dimension of ~3.8. The inversions accounted for frequency‐dependent variations in the source spectra and site response. The source spectra roll off at an average rate of about 2 for both wave types. The station residuals, which reflect unresolved lateral structure, show stronger attenuation in the Los Angeles region and weaker attenuation in the batholithic terrains to the north and south, as well as in the Proterozoic terrains on the eastern side of the study region.

LOCATION AND IDENTIFICATION OF SEISMIC EVENTS AROUND NORTH KOREAN NUCLEAR TEST SITE FOLLOWING THE 3 SEPTEMBER 2017 UNDERGROUND NUCLEAR TEST

At least thirteen small seismic events that occurred close to and following the North Korean underground nuclear test (UNT) on 3 September 2017 were detected and located by using seismic waveform data from regional stations. Other than the first event, widely reported to be a cavity collapse, the magnitudes of these shocks range from 2.3 to 3.6. Based on their timing, and proximity to the UNTs, these shocks may be considered as aftershocks. They are located within 1 km of each other with small relative location errors, approximately 50 m. We assess a method to classify each of these small events separately based on their spectral amplitude ratios derived from regional P- and S-waves. We examine additional possible small seismic events around the North Korean test site by using seismic data from stations in South Korea, Russia, and northeastern China including IMS seismic arrays, GSN stations, and national network stations. A linear discriminant function based on Mahalanobis methods applied to P/S spectral ratios does a better job of screening events than does a simple average of such ratios.