Pn Waves Research Articles

A Pn-wave spectral inversion technique based on trust region reflective algorithm

This paper suggests utilizing the trust region reflective (TRR) algorithm to address the inverse problem related to seismic spectrum analysis of Pn waves. By applying this approach, it becomes feasible to simultaneously deduce the seismic moment, corner frequency, tomographic model of Pn-wave attenuation, and site responses while incorporating appropriate constraints based on prior knowledge to ensure solution accuracy. The efficacy of this method has been validated using synthetic data. Specifically, the recorded Pn waves from four underground explosions in North Korea were employed to evaluate the performance of the proposed technique. The derived seismic moment and corner frequency for these events were found to align with the characteristics of the seismic data, earthquake magnitude calculations, and empirical data. The resulting tomographic model of Pn-wave attenuation revealed a spatial distribution of high and low attenuation, indicating significant lithospheric heterogeneity in northeastern China. Areas with low Q0 and low η, such as the Xialiaohe Basin and southwestern Changbai Mountain, suggest a high-temperature environment in the upper mantle cap layer or the presence of molten substances. Conversely, regions with high Q0 and high η, like the northern Changbai Mountain and the vicinity of the eastern Tanlu Fault Zone, characterized by high Pn-wave velocity and thick crust, indicate minimal modification or destruction in the lithosphere. Furthermore, site responses were determined for 75 seismic stations in northeastern China, with their characteristics preliminarily analyzed and interpreted in conjunction with geological context. The iterative process based on the TRR algorithm for Pn-wave spectral inversion proposed in this study demonstrates robust convergence and enhances the fitting accuracy of the observed spectrum. The inversion outcomes from this spectral technique yielded synthetic Pn-wave spectra that closely matched the observed spectra of the four underground explosions in North Korea across various frequency bands for over 90 % of the stations.

A Pn magnitude scale mb(Pn) for earthquakes along the equatorial Mid-Atlantic Ridge

SUMMARY We developed a short-period Pn magnitude scale mb(Pn) for earthquakes along the equatorial Mid-Atlantic Ridge. Due to low signal-to-noise ratios, teleseismic body wave magnitude and long-period surface wave magnitude cannot be confidently determined for small earthquakes of mb < 4. Local magnitude scales are also not useful for these events because the oceanic environment does not allow the propagation of crustal phases. However, regional high-frequency Pn waves from these small- to moderate-size (mb 3–6) earthquakes are well recorded in the equatorial Atlantic region and can be used to assign magnitudes. We measured over 2041 Pn peak amplitudes on vertical records from about 21 stations in northeastern Brazil and 11 stations in western Africa in the distance range of 700–3700 km. We analysed data from 189 events from the global centroid moment tensor catalogue to tie our mb(Pn) scale to Mw so that seismic moments can be readily estimated. Pn arrivals show apparent group velocity between 7.9 km s−1 at short ranges (∼1000 km) and up to 9.1 km s−1 at 3500 km. The measured peak amplitudes have a frequency between 0.8 and 3 Hz at 1000–1800 km, but at greater distances, 1800–3700 km, they show a remarkably consistent frequency of about 0.8 Hz. The peak amplitude attenuates at a higher rate at short distances (∼0.65 magnitude units between 700 and 2000 km) but attenuates at a lower rate at long distances (∼0.35 magnitude units between 2000 and 3700 km). The low rate of amplitude decay with distance and nearly constant frequency content of the peak amplitudes suggest that Pn waves propagate efficiently in the lower part of the upper mantle in the equatorial Atlantic Ocean basins. These are important attributes of oceanic Pn waves that can be used to assign magnitude for small- to moderate-size earthquakes in the equatorial mid-Atlantic region. The estimated station corrections correlate well with upper mantle low-velocity anomalies, especially in Brazil.

Determination of Earthquake Depths Using Data of Cross-Sectional and Areal Deep Seismic Studies in Siberia

Abstract ––Information on the distribution of earthquake hypocenters for many seismically active zones of Siberia remains insufficient, which is associated with sparse networks of seismological observations. The paper presents the results of determining the earthquake depths in several seismogenic areas in the Altai-Sayan region, Cisbaikalia, Transbaikalia, and Yakutia using the travel time of longitudinal refracted waves from the Mohorovičič discontinuity (Pn waves) from earthquakes and the recently obtained information about the deep structure of these regions. The depth determination algorithm is tested using data from aftershocks of large earthquakes occurring in Tuva in 2011 and 2012 (ML = 6.7 and 6.8), recorded both by the regional seismological network and by a local group of seismic stations. Different methods are applied to reveal that some aftershock depths have a close match, including those for the main shocks – the Tuva-1 and Tuva-2 earthquakes. Another good match of earthquake depths is obtained using Рn waves with materials from regional and detailed studies of the Baikal branch of the Geophysical Survey of the Russian Academy of Sciences in the Muyakan activation site in the Baikal rift zone. The resulting data complement the information on the hypocenter of the main shock and confirm the change of the Muyakan activation cluster from large depths to shallow ones since the onset of activation in 2014. New information on the earthquake depths using Рn waves is obtained in Yakutia along the border of the largest Eurasian and Okhotsk tectonic plates. It is revealed that they decrease to 6–12 km as compared to higher depths of 20–30 km in adjacent areas. The resulting series of new information on the distribution of earthquake hypocenters using Рn waves is extremely important mostly because it indicates the possibility of identifying and redefining earthquake hypocenters using previous seismological observations in seismically active zones of Siberia.

Uppermost mantle structure and dynamics of the Tanlu fault zone: New insights from Pn anisotropic tomography

Whether the segmentation of the Tanlu fault zone and mechanism of strong crustal earthquakes are related to the uppermost-mantle structure is still unclear. Here a new high-resolution uppermost-mantle velocity and anisotropy model of the Tanlu fault zone and surrounding areas is determined by using 13,045 Pn arrival-time data. These data are handpicked from seismograms recorded at 120 newly deployed TanluArray portable seismic stations and 337 Chinese provincial seismic stations. The pattern of Pn velocity and anisotropy agrees well with the surface geological and tectonic features. Obvious low-velocity (low-V) anomalies are observed in tectonically active areas, such as the Taihang mountain orogenic belt, the eastern Dabie mountain orogenic belt, and the Sulu fold belt, whereas areas with stable tectonics, such as the North China Basin and the South Yellow Sea Basin, exhibit high-velocity (high-V) anomalies. The fast propagation directions (FPDs) of Pn waves in the study area exhibit a complex distribution, but the FPDs show a rotating feature around the margin of the South Yellow Sea Basin. The block feature of the high-V anomalies beneath the North China Basin and segmentation of the low-V anomalies along the Tanlu fault zone may be related to lithospheric delamination and thermal erosion caused by hot and wet upwelling flows in the big mantle wedge. The 1668 Tancheng earthquake (M8.5) occurred near a low-V anomaly, and recent moderate and strong earthquakes (M > 4.0) took place along the boundaries of the velocity anomalies. These results offer new perspectives on the mantle dynamics and mechanism of strong earthquakes in and around the Tanlu fault zone.

Olivine Fabrics in the Oceanic Lithosphere Constrained by Pn Azimuthal Anisotropy

AbstractWe constrain olivine fabrics in the oceanic lithosphere using active and passive seismic observations of Pn azimuthal anisotropy. We first analyze active‐source data derived from a broadband ocean bottom seismometer array that was deployed in the Northwestern Pacific. We infer the azimuthal dependence of the Pn velocity, including the 2ϕ and 4ϕ terms of the sinusoidal functions, where ϕ is the back azimuth. We observe a skewed azimuthal dependence of the Pn velocity, with a large peak‐to‐peak amplitude of about 12%. Similar features are observed using an independent data set of teleseismic Pn waves. We constrain the direction of the crystallographic axes of olivine to explain the observed azimuthal dependence and identify A‐type olivine with a slightly dipping a‐axis and slightly tilting b‐axis being compatible with our observations. In contrast, we find that D‐type olivine with any direction of crystallographic axes cannot explain our observations. Secondary deformation and recrystallization in the older Pacific may be responsible for this strong and complex seismic anisotropy in the study region.

Crustal Structure of the Chuan‐Dian Block Revealed by Deep Seismic Sounding and its Implications for the Outward Expansion of the East Tibetan Plateau

AbstractThe Chuan‐Dian Block (CDB) is located in the southeastern margin of the Tibetan Plateau, with a complex geological structure and active regional faults. The present tectonic condition with strong crustal deformation is closely related to the ongoing collision of the India and Eurasia plates since 65 Ma. The study of the crustal structure of this area is key to revealing the evolution and deep geodynamics of the lateral collision zone of the Tibetan Plateau. Deep seismic sounding is the most efficient method with which to unravel the velocity structure of the whole crust. Since the 1980s, 19 deep seismic sounding profiles have been captured within the CDB area. In this study, we systematically integrate the research results of the 19 profiles in this area, then image the 3D crustal velocity, by sampling with a 5 km spacing and 2D/3D Kriging interpolation. The results show the following. (1) The Moho depth in the study area deepens from 30 km in the south to 66 km in the north, whereas there is no apparent variation from west to east. The Pn wave velocity is higher in stable tectonic units, such as 7.95 km/s in the Lanping‐Simao block and 7.94 km/s in the western margin of the Yangtze block, than in active or mobile tectonic units, such as 7.81 km/s in the Baoshan block, 7.72 km/s in the Tengchong block and 7.82 km/s in the Zhongdian block. (2) The crustal nature of the Tengchong block, the northern Lanping‐Simao block and the Zhongdian block reflects a type of orogenic belt, having relatively strong tectonic activities, whereas the crustal nature of the central Lanping‐Simao block and the western margin of the Yangtze block represents a type of platform. The different features of the upper‐middle crust velocity, Moho depth and Pn wave velocity to both sides of the Red River fault zone and the Xianshuihe fault zone, reflect that they are clearly ultra‐crustal. (3) Based on the distribution of the low velocity zones in the crust, the crustal material of the Tibetan Plateau is flowing in a NW–SE direction to the north of 26°N and to the west of 101°E, then diverting to flowing eastwards to the east of 101°E.

MAPPING OF S-WAVES ATTENUATION FIELD AT EAST KAZAKHSTAN AND WEST ALTAY REGIONS BY DATA OF MKAR SEISMIC STATION

We are analyzing recordings of local earthquakes in the regions of Eastern Kazakhstan and Western Altay, obtained by stations MAKZ and MKAR. We consider correlation dependence of maximum amplitudes ratio for Sn and Pn waves (Sn/Pn parameter) on distance. On the base of this dependence mapping of S wave attenuation field in the lithosphere of this region is carried out. A special attention is paid to high attenuation zones, comparison with seismicity is carried out. It is established, that the highest attenuation corresponds to the rupture zone of large Chu earthquake of 2003 (Mw=7.3). Besides, an anomaly of high attenuation is found in the area of Semipalatinsk test site (STS) and to the south of it. We discuss reasons of high shear wave attenuation in rupture zones of large earthquakes, and also within some sections of the STS.

Identification of and Favorable Conditions for a New Long-Offset Seismic Phase from Offshore–Onshore Seismic Surveys

Abstract Offshore–onshore seismic survey is one of the main methods to study crustal structures in offshore–onshore transitional zones. At present, the seismic waves commonly used in imaging are the crustal refraction (Pg), the crustal reflection from the Moho (PmP), and the upper-mantle refraction (Pn) waves. The propagation distances of Pg and PmP are commonly less than 210 km, and Pn propagates with an apparent velocity of ∼8 km/s. In 2015, two offshore–onshore wide-angle seismic lines with a length of ∼350 km were acquired in the Pearl River Estuary. In addition to Pg, PmP, and Pn, a new seismic phase was observed, which has a long propagation distance (offset of ∼170–290 km), low apparent velocity (∼5.85 km/s), and low frequency (∼4–7 Hz). Similar seismic phases have been widely reported in previous offshore–onshore and reservoir seismic surveys, but the understanding of these phases is still limited. Herein, we used both raytracing and waveform modeling methods to identify the new seismic phase as the secondary Pg phase, which reflects from the surface (named Pg2Pg). We also discuss favorable conditions for Pg2Pg, including (1) a thin sedimentary layer with low velocity at the surface in which the reflection of Pg occurs, which can reduce the incidence angles and hence increase the energy of the reflected waves; (2) a sedimentary basement dipping toward the sea at the positions of the air gun shots, which focuses seismic waves; (3) relatively smooth interfaces of the medium, which can reduce the scattering of Pg2Pg; and (4) air guns that can excite low-frequency signals, which can reduce the attenuation of seismic waves. Checkerboard tests and practical applications show that Pg2Pg can significantly improve upper-crustal resolution, especially for onshore areas. Our research promotes the data mining of offshore–onshore seismic surveys, which is important for obtaining more detailed crustal structures.

High‐Precision Relocation With the Burial Depths of the North Korean Underground Nuclear Explosions by Combining Pn and Pg Differential Traveltimes

AbstractIn seismic nuclear monitoring, an accurate source depth is an important prerequisite for reliably estimating the explosive yield. Relative location methods are often used for this purpose. However, the conventional method based on regional Pn waves usually fails to give satisfactory constraints on the source depth, mainly due to the strong trade‐off between the burial depth and origin time. This study explores a high‐precision relative location method to simultaneously determine the relative epicenter and source elevation by using differential traveltimes from both downward‐takeoff Pn waves and horizontal‐takeoff Pg waves. The properties of both Pg and Pn waves, including the consistency between waveforms, the reliability of differential traveltime measurements, and their sensitivities to epicenter and depth variations, are investigated. By jointly applying both types of waveform data, the proposed method significantly enhances the constraint on the source depth variation. This method is applied to regional seismic data collected from China, South Korea, and Japan to determine the relative epicenters, origin times and relative burial depths of six North Korean underground nuclear explosions. The source depths are then used to provide burial‐depth corrections to estimate the explosive yields. The depths of the six North Korean nuclear explosions detonated on October 2006, May 2009, February 2013, January 2016, September 2016, and September 2017 are determined to be 330, 540, 506, 468, 521, and 570 m, respectively, and their yields after burial depth corrections are 1.6, 5.7, 13.4, 12.6, 21.7, and 225.7 kt, respectively.

The Characteristics of the Short-Period S-wave Attenuation Field in the Lithosphere of Turkmenistan and Northeastern Iran and their Relation to Seismicity

The characteristics of the short-period S-wave attenuation field in the lithosphere of Turkmenistan and northeastern Iran are considered. The data obtained by the Karatau station at distances of up to 1750 km were used. The ratios of the maximum amplitudes of Pn and Sn waves (Sn/Pn parameter) were analyzed. The correlation dependence of this parameter on distance has been obtained. These data are compared with data for the regions of Caucasus, Pamir, and the Baikal Rift Zone. The West Turkmenian depression experiences significantly lower attenuation; the Turan Plate shows an intermediate level and the Alborz Range has a higher level. It is shown that attenuation in the source zones decreases considerably with time after large earthquakes. A zone of higher attenuation has been identified in the eastern Alborz Range; it is probably related to preparation of a large earthquake. The connection of heterogeneities in the attenuation field with deep-seated fluids migration is discussed.

Local and regional P-wave spectral attenuation model for Iran.

A robust frequency-dependent local and regional P-wave attenuation model is estimated for continental paths in the Iranian Plateau. In order to calculate the average attenuation parameters, 46 337 vertical-component waveforms related to 9267 earthquakes, which are recorded at the Iranian Seismological Center (IRSC) stations, have been selected in the distance range 10-1000km. The majority of the event's magnitudes are less than 4.5. This collection of records provides high spatial ray path coverage. Results indicate that the shape of attenuation P-wave curve versus distance is not uniform and has three distinct sections with hinges at 90 and 175km. A trilinear model for attenuation of P-wave amplitude in the frequency range 1-10Hz is proposed in this study. Fourier spectral amplitudes are found to decay as R- 1.2 (where R is hypocentral distance), corresponding to geometric spreading within 90km from the source. There is a section from 90 to 175km, where the attenuation is described as R 0.8, and the attenuation is described well beyond 175km by R- 1.3. Moreover, the average quality factor for Pg and Pn waves (QPg and QPn ), related to anelastic attenuation is obtained as Qpg = (54.2±2.6)f (1.0096±0.07)and Qpn = (306.8±7.4)f(0.51±0.05).There is a good agreement between the results of the model and observations. Also, the attenuation model shows compatibility with the recent regional studies. From the results it turns out that the amplitude of P waves attenuates more rapidly in comparison with the global models in local distances.

Characteristics of the Short-Period S-Wave Attenuation Field in the Source Zone of the Strongest Tohoku Earthquake of March 11, 2011 (Mw = 9.0)

The characteristics of the short-period S-wave attenuation field in the source zone of the strongest Tohoku earthquake of March 11, 2011 in the northeast of Japan are considered (Mw = 9.0). The records of shallow local earthquakes obtained by the MAJO station at distances of 250 to 700 km are processed. A method based on the analysis of the ratio of the maximum amplitudes of the Sn and Pn waves (the Sn/Pn parameter) is used. The source zone is divided into four regions bounded by coordinates 36°–37°, 37°–38°, 38°–39°, and 39°–40°18′ N; and 140°30ʹ–145° E. It is established that all the regions contain segments of a rapid decrease in the Sn/Pn values, which are followed by segments of their sharp growth at small epicentral distances in all the areas. Another segment of rapid decrease in the Sn/Pn parameter is identified in all regions at relatively large distances. It is assumed that the first segments of rapid decrease in the Sn/Pn values are related to the gradual sinking of the S-wave rays in the mantle wedge. In this case, the minimum values of the parameter correspond to rays partly moving along the foot of this wedge. Such an effect is explained by the fact that the biggest content of the deep-seated fluids ascending due to the dehydration of the oceanic crust rocks correspond to the bottom of the mantle wedge. The segments of a sharp increase in the Sn/Pn values are likely to correspond to the propagation of rays within the upper part of the plate characterized by very weak attenuation. The second segments of the rapid decrease in the Sn/Pn parameter are related to the ray penetration into the waveguide formed in the bottom part of the plate formed as a result of the dehydration of the mantle rocks. The mean values Sn/Pn(Δ) in the four areas are much lower than in the source zone of the strongest Maule earthquake of February 27, 2010 (Chile, Mw = 8.8). This effect agrees with the earlier assumption about the larger fluid content in the subduction zones in the west of the Pacific Ocean compared to the east. Furthermore, this allows us to explain the features of the aftershock processes in these two enormous regions of the Pacific Ring.

Pn wave velocity and anisotropy underneath the central segment of the North-South Seismic Belt in China

We present a Pn wave velocity and anisotropy model of the central segment of the North-South Seismic Belt in China, where there are numerous stable basins and active faults, making this segment attractive for extensive studies. The model was obtained by a tomographic analysis of 49,973 Pn wave phase readings collected by the China Earthquake Networks Center and temporary stations in Yunnan and Sichuan. The tomographic velocity model shows that the average Pn wave velocity is 8.06 km/s; prominent high-velocity (high-V) anomalies are visible under the Sichuan Basin, the Zoige Basin and the Ordos block, which clearly outline their tectonic margins. A pronounced low-velocity (low-V) zone is observed from the Songpan-Ganzi block to the Chuan-Dian and Daliangshan blocks, suggesting the presence of hot material upwelling. The station delay data show a gradual variation from negative to positive values, possibly reflecting a crustal thickness variation from the southwest to the northeast of the study area. A correlation between the Pn wave anisotropy and the distribution of velocity anomalies is observed: anisotropy is relatively weaker in the high-V anomaly zones beneath stable basins, while it is stronger in the low-V anomaly zones and the high-to-low-V anomaly transition zones. The high-resolution velocity and anisotropy tomographic model that we obtained could also provide a better understanding of the study area seismicity, since the occurrence of strong earthquakes seems to be related to the presence and strength of lateral heterogeneities at the uppermost mantle level.

Upper mantle anisotropy and tectonic deformation beneath the Ordos Block in the western North China Craton: Constraints from wide-angle seismic data

The Ordos Block is located in the northern part of the North China Craton and its interior is highly stable. A series of fault basins and faults has developed around the periphery, and seismic activity is frequent. To research the dynamic mechanisms that caused the special geomorphology and geological features of the Ordos Block further, this study processed data from two nearly orthogonal profiles laid out in this area and obtained the P-wave velocity structure characteristics of the area. The results showed that the crust–mantle velocity structure, interface depth, and the relationship with the peripheral regions obtained from the two profiles are rather complex: the internal crust structure of the Ordos Block is relatively simple, the low-velocity layer is not developed, the interface does not undulate much, and the crust thickness of the Ordos Block is about 42.0 km. These results show that the Ordos Block is part of a sinking residual feature of the ancient stable continental lithosphere. The crust–mantle structure of the block and its peripheral coupled region shows complex crustal structure features. The velocity structure is abnormally disordered and the interface shows declining or depressed anomalies, revealing the background characteristics of the active structure of the block and the periphery. Another major feature is that the velocity of the Pn wave at the uppermost mantle near the intersection of the two profiles has directional and anisotropic features, i.e., the value of the velocity in the east–west direction is significantly greater than that in the north–south direction and the difference is 0.25 km/s. The reason for this phenomenon may be that since the Mesozoic Era the Ordos Block has been mainly subject to east–west compressive stress, forming a series of north–south structural deformations.

Constraints on crust-mantle transition zone with Pn waveforms: A case study of eastern China and southern Korean Peninsula

The seismic velocity structure of the crust-mantle transition zone (CMTZ) provides key constraints on crustal dynamics, and can be studied with Pn wave which propagates horizontally along the Moho discontinuity. In this study, we first explore an effect of velocity-gradient structures in the CMTZ through synthetic Pn waveforms, and demonstrate that the shape of Pn changes from step-like to pulse-like when the CMTZ becomes from sharp to a gradient velocity structure, respectively. We then use Pn waveforms of the regional earthquakes (the July 2012 Mw 4.8 earthquake in east China and the September 2016 Mw 5.1 earthquake in South Korea) to examine the CMTZ structures to the west and east of the Yellow Sea and thus to constrain tectonic affinity between the eastern China and southern Korean Peninsula. The Pn waveforms from the Mw 4.8 event in east China show that the Moho in southern Yangtze Craton is sharp whereas there may be a gradient structure in CMTZ with a 6–10 km thickness in northern Yangtze Craton. The Pn waveforms from the Mw 5.1 event in South Korea show that the CMTZ in South Korea is also sharp, similar to the southern Yangtze Craton. Observed difference in CMTZ structures suggest that Sulu Orogenic Belt may extend along the north of the Gyeonggi Massif in Korea.

Temporal Variations of the S-Wave Attenuation Field in the Area of the Lop Nor Nuclear Test Site

The characteristics of the shear wave attenuation field in the area of the Lop Nor nuclear test site, China, are considered. Methods are used that analyze the relative level of Sn and Pn waves (parameter Sn/Pn) and the slope of P-coda envelopes. An Sn wave is formed by shear waves reflected from multiple boundaries in the upper mantle, while P-coda is formed by S–P type conversion in the lithosphere of the source region. Twenty-six records of underground nuclear explosions (UNEs) and earthquakes obtained by station BRVK at epicentral distances of ∼1800–2000 km have been processed. It is found that from the late 1960s until the mid-1990s, the mean Sn/Pn values in the records of UNEs have substantially decreased and the slope of the P-coda envelope has increased in the area of the test site. These effects indicate an increase in S-wave attenuation in the lithosphere of the source region. A decrease in Sn/Pn with time in earthquake records for the test site region has also been observed after the termination of UNE series. Similar results were obtained earlier for the areas of the Semipalatinsk and Nevada nuclear test sites. The revealed effects are supposedly caused by deep fluid migration as a result of intensive long-term anthropogenic impact.

Regional Pn Body‐Wave Magnitude Scale mb(Pn) for Earthquakes Along the Northern Mid‐Atlantic Ridge

AbstractWe obtained a robust regional body wave magnitude scale mb(Pn) for earthquakes along the Northern mid‐Atlantic Ridge by using Pn wave recorded at continental stations. The new magnitude scale is mb(Pn) = log10 A [nm] − 1.86 log10 (100/Δ) [km] + C + 1.62, where A is the zero‐to‐peak amplitude of Pn wave on the simulated vertical Wood‐Anderson seismogram in nanometers, Δ is the epicentral distance in kilometers, 1.86 represents amplitude attenuation, C is the station correction, and 1.62 anchors the mb(Pn) to the moment magnitude (Mw), plus amplitude loss in the two crustal legs. By using moment magnitude as reference, we obtained the event magnitude adjustments (EMAs), which represent the difference between the known long‐period Mw and the short‐period mb(Pn). EMAs correlate with the source property, where ridge events show strong long‐period waves but poor high‐frequency signals that lead to mb(Pn) < Mw and positive EMA of +0.08 ± 0.26 magnitude units (m.u.), whereas transform fault earthquakes show relatively strong high‐frequency waves that lead to mb(Pn) > Mw and negative adjustment of −0.20 ± 0.24 m.u. Almost all normal faulting events are along spreading ridges, whereas strike‐slip events are along the fracture zones and transform faults; hence, we developed source‐specific magnitude adjustments (SSMAs), which range from +0.27 m.u. for earthquakes along the ridges, to −0.26 m.u. for earthquakes along the transform faults. Using SSMA, we obtained a regional Pn magnitude that is consistent with Mw and mb(P). The regression relationship is Mw = 0.91 mb(Pn)[SSMA] + 0.46 with a standard deviation of ±0.15 m.u.

High S-wave attenuation anomalies and ringlike seismogenic structures in the lithosphere beneath Altai: Possible precursors of large earthquakes

This paper addresses inhomogeneities in the short-period S-wave attenuation field in the lithosphere beneath Altai. A technique based on the analysis of the amplitude ratios of Sn and Pn waves is used. High S-wave attenuation areas are identified in the West Altai, which are related to the source zones of recent large earthquakes, viz., the 1990 Zaisan earthquake and the 2003 Chuya earthquake. Associated with the Chuya earthquake, a large ringlike seismogenic structure had been formed since 1976. It is also found that ringlike seismogenic structures are confined to high S-wave attenuation areas unrelated to large historical earthquakes. It is supposed that processes paving the way for strong earthquakes are taking place in these areas. The magnitudes of probable earthquakes are estimated using the earlier derived correlation dependences of the sizes of ringlike seismogenic structures and the threshold values of magnitudes on the energy of principal earthquakes with prevailing focal mechanisms taken into consideration. The sources of some earthquakes are likely to occur near to the planned gas pipeline route from Western Siberia to China, which should be taken into account. The relationship of anomalies in the S-wave attenuation field and the ringlike seismogenic structures to a high content of deep-seated fluids in the lithosphere is discussed.

Uppermost mantle structure of the Australian continent from Pn traveltime tomography

AbstractPast studies of the seismic structure of the Australian continent have dominantly exploited surface wave tomography for the mantle, with seismic refraction, receiver functions, and ambient noise used for crustal structure. The 3‐D structure has been summarized in the Australian Seismological Reference Model (AuSREM), for which the zone immediately below the crust is the least well characterized. Pn traveltime tomography provides a way of improving structural information on the uppermost mantle across the continent. We have exploited waveforms from larger events across Australia recorded at both permanent and portable stations since 1993, supplemented by bulletin arrival times. To compensate for the large velocity contrasts, with much faster mantle wave speeds in the center and west, all events were relocated using the AuSREM model. After relocation, consistent patterns of traveltime residuals are obtained. We extract Pn as the first arrival in the distance range 1.8° to 15°. We use the FMTOMO approach to invert the travel‐time residuals to generate a P wave speed structure with a resolution of 3°×3°. There is strong heterogeneity in Pn wave speed in the uppermost mantle across the continent. The fastest Pn wave speed of 8.36 km/s beneath the Precambrian cratons of western and central Australia is 3.99% faster than the global ak135 model, and the slowest Pn wave speed 7.66 km/s on the eastern margin is 4.74% slower. The slowest velocities in the uppermost mantle are found along the eastern margin of the Australian continent beneath the Phanerozoic orogenic belts, with links to Neogene volcanism.

Pn wave geometrical spreading and attenuation in Northeast China and the Korean Peninsula constrained by observations from North Korean nuclear explosions

AbstractWe investigate the geometric spreading and attenuation of seismic Pn waves in Northeast China and the Korean Peninsula. A high‐quality broadband Pn wave data set generated by North Korean nuclear tests is used to constrain the parameters of a frequency‐dependent log‐quadratic geometric spreading function and a power law Pn Q model. The geometric spreading function and apparent Pn wave Q are obtained for Northeast China and the Korean Peninsula between 2.0 and 10.0 Hz. Using the two‐station amplitude ratios of the Pn spectra and correcting them with the known spreading function, we remove the contributions of the source and crust from the apparent Pn Q and retrieve the P wave attenuation information along the pure upper mantle path. We then use both Pn amplitudes and amplitude ratios in a tomographic approach to obtain the upper mantle P wave attenuation in the studied area. The Pn wave spectra observed in China are compared with those recorded in Japan, and the result reveals that the high‐frequency Pn signal across the oceanic path attenuated faster compared with those through the continental path.