Body Wave Arrival Times Research Articles

Constraints on Lateral Variations of Martian Crustal Thickness From Seismological and Gravity Field Measurements

AbstractUsing body wave arrival times from 31 seismic events recorded on Mars by the InSight mission, combined with topography and gravity field modeling, we constrained lateral variations of crustal thickness through a Bayesian inversion approach. The parameterization of the seismic structure relies on quantities that influence the thermochemical evolution of Mars, enabling the seismic velocities and densities in the different planetary envelopes to be consistently linked through common physical assumptions. Compared to a 1D structure, models with lateral variations of crustal thickness show two possible interpretations of the thermal evolution of Mars, with either a hot or cold scenario at the present‐day. We found the hot scenario to be more compatible with InSight's radiotracking data and the tidal Love number. We relocated the marsquakes and derived maps of seismicity recorded by InSight, which is mostly located along or North of the boundary between the Northern lowlands and the Southern highlands.

Structure of the Martian Crust Below InSight From Surface Waves and Body Waves Generated by Nearby Meteoroid Impacts

AbstractWe measure group velocity dispersion of surface waves generated by two meteoroid impacts on Mars close to the lander of the InSight mission. This allows us to probe the crustal structure in the first few kilometers beneath the InSight lander. In combination with body wave arrival times from five impact events, we obtain direct seismic constraints on the seismic velocity of the crust in the vicinity of the InSight landing site. We confirm the existence of a uppermost low‐velocity layer with a mean thickness of ∼1.2 km, interpreted as layered volcanic materials, possibly interstratified with sedimentary and altered materials. Our joint inversion of surface and body waves shows a four‐layer model for the Martian crust, compatible with high‐ and low‐frequency P‐to‐S receiver functions estimated in previous studies.

Optimal Network Design for Microseismic Monitoring in Urban Areas - A Case Study in Munich, Germany

Well-designed monitoring networks are crucial for obtaining precise locations, magnitudes and source parameters, both for natural and induced microearthqakes. The performance of a seismic network depends on many factors, including network geometry, signal-to-noise ratio (SNR) at the seismic station, instrumentation and sampling rate. Therefore, designing a high-quality monitoring network in an urban environment is challenging due to the high level of anthropogenic noise and dense building infrastructure, which can impose geometrical limitations and elevated construction costs for sensor siting. To address these challenges, we apply a numerical optimization approach to design a microseismic surveillance network for induced earthquakes in the metropolitan area of Munich (Germany), where several geothermal plants exploit a deep hydrothermal reservoir. First of all, we develop a detailed noise model for the city of Munich, to capture the heterogeneous noise conditions. Then, we calculate the expected location precision for a randomly chosen network geometry from the body-wave amplitudes and travel times of a synthetic earthquake catalog considering the modeled local noise level at each network station. In the next step, to find the optimum network configuration, we use a simulated annealing approach in order to minimize the error ellipsoid volume of the linearized earthquake location problem. The results indicate that a surface station network cannot reach the required location precision (0.5 km in epicentre and 2 km in source depth) and detection capability (magnitude of completeness Mc = 1.0) due to the city´s high seismic noise level. In order to reach this goal, borehole stations need to be added to increase the SNR of the microearthquake recordings, the accuracy of their body-wave arrival times and source parameters. The findings help to better quantify the seismic monitoring requirements for a save operation of deep geothermal projects in urban areas.

Seismic signature detection during the 2018 Anak Krakatau flank collapse and tsunami using seismic amplitudes from regional-scale monitoring

On 22 December 2018, the major flank collapse of Anak Krakatau volcano generated a tsunami that struck the surrounding coasts of Java and Sumatra islands in Indonesia without warning. It was later suggested that the corresponding seismic event lacked the body-wave arrivals typical of tectonic earthquakes, causing difficulties for the automated detection system to recognize the event. We explore the possibility of detecting the seismic signature of such events without relying on the arrival times of body waves, by measuring seismic amplitudes in a regional seismic network at the expected arrival times from a fixed, potential source and comparing them to the theoretical attenuation of surface waves. We propose a fast detection method and evaluate the method using seismograms recorded during the flank collapse and tsunami episode as well as several known tectonic earthquakes. Detailed examinations of the detection results confirm the seismic signatures of the flank collapse and teleseismic events as suggested by previous studies. We also find more seismic signatures suggesting the occurrence of two possible smaller collapse events and variations in the eruptive activity related to the major flank collapse, as well as body and surface wave signals from two teleseismic earthquakes that were present during this episode. Finally, we construct a timeline of events during this devastating episode, combining our results with previous studies as well as insights from weather radar observations. With the ability to detect and discriminate various types of seismic events from each other, the detection method can be useful in assisting the existing monitoring and early warning systems in detecting major volcano-related tsunamigenic events.Graphical

Seismogenic Structure of the 2014 M6.5 Ludian Earthquake From Three‐Dimensional Joint Inversion of Magnetotelluric Data and Seismic Arrival Times

AbstractOn 3 August 2014, a destructive earthquake with a magnitude of 6.5 occurred in the Ludian region in Yunan, China, causing heavy casualties. However, the seismogenic structure of the Ludian earthquake is still unclear. To address this issue, we have developed a three‐dimensional joint inversion algorithm for magnetotelluric data and seismic body wave arrival times based on the cross gradient structural similarity constraint, which utilizes the complementary advantages of the two different datasets. We use arrival times of P‐ and S‐waves from 6,369 local earthquakes recorded by 30 seismic stations and magnetotelluric impedances from 127 MT stations to perform the joint inversion to construct a combined 3‐D P‐wave velocity (Vp), S‐wave velocity (Vs), and resistivity models and the locations of relocated aftershocks in the source region of the Ludian earthquake. Our models show significant low Vp/Vs ratios and low resistivity in the upper crust of the main shock zone, and the relocated aftershocks form an L‐shaped zone. Based on petrophysical studies and analysis of the cross‐plots of different physical properties, we infer the existence of cracks/fractures and fluids as well as high quartz contents in the source region. These fluids may play a key role in promoting the Ludian earthquake by reducing the effective normal stress of the fault zone, and the existence of high quartz contents further makes the crustal rocks in the source region more brittle. Our study provides a new evidence how the properties of the crust in the focal area have some control on the genesis of moderate to large earthquakes.

Joint Inversion of Body Wave Arrival Times, Surface Wave Dispersion Data and Receiver Functions: Method and Application to South China

AbstractWe have developed a new joint inversion method that incorporates body wave arrival times, surface wave dispersion and receiver functions to simultaneously update earthquake locations and constrain three‐dimensional P‐wave (Vp) and S‐wave velocity (Vs) models. Due to complementary sensitivities of the three types of data, the proposed joint inversion algorithm can reduce the intrinsic non‐uniqueness of inversions using fewer types of data and better determine smooth velocity variations and velocity discontinuities. Synthetic tests demonstrate the advantages of this new joint inversion algorithm in resolving velocity structures, especially in constraining velocity gradients across the Moho interface. We have applied the proposed joint inversion algorithm to image the lithosphere velocity structure of south China. The inverted Vp and Vs models fit body wave arrival times, surface wave dispersion and receiver functions well. We further analyzed the distribution of Vs gradients across the Moho interface in detail, which helps us better understand tectonics in south China.

Joint inversion of body wave arrival times, surface wave dispersion data and receiver functions: Method and Application to south China

Joint inversion of body wave arrival times, surface wave dispersion data and receiver functions: Method and Application to south China

Joint inversion of body wave arrival times, surface wave dispersion data and receiver functions: Method and Application to south China

Joint inversion of body wave arrival times, surface wave dispersion data and receiver functions: Method and Application to south China

Joint inversion of body wave arrival times, surface wave dispersion data and receiver functions: Method and Application to south China

Joint inversion of body wave arrival times, surface wave dispersion data and receiver functions: Method and Application to south China

USTClitho2.0: Updated Unified Seismic Tomography Models for Continental China Lithosphere from Joint Inversion of Body-Wave Arrival Times and Surface-Wave Dispersion Data

Abstract Xin et al. (2019) presented 3D seismic velocity models (VP and VS) of crust and uppermost mantle of continental China using seismic body-wave travel-time tomography, which are referred to as Unified Seismic Tomography Models for Continental China Lithosphere 1.0 (USTClitho1.0). Compared with previous models of continental China, the VP and VS models of USTClitho1.0 have the highest spatial resolution of 0.5°–1.0° in the horizontal direction and are useful for better understanding the complex tectonics of continental China. Although USTClitho1.0 is implicitly constrained by surface-wave data by using the VS model from surface-wave tomography and the converted VP model as initial models for body-wave travel-time tomography, the predicted surface-wave dispersion curves from USTClitho1.0 do not fit the observed data well. Here, we present updated 3D VP and VS models of the continental China lithosphere (USTClitho2.0) by joint inversion of body-wave arrival times and surface-wave dispersion data. Compared with the previous joint inversion scheme of Zhang et al. (2014), similar to Fang et al. (2016), it is further improved by including the sensitivity of surface-wave dispersion data to VP in the new joint inversion system. As a result, the shallow VP structure is also better imaged. In addition, the new joint inversion scheme considers the large topography variations between the eastern and western parts of China. Thus, USTClitho2.0 better resolves the upper-crustal structure of the Tibetan plateau. Compared with USTClitho1.0, USTClitho2.0 fits both body-wave arrival times and surface-wave dispersion data. Thus, the new velocity models are more accurate and can serve as a better reference model for regional-scale tomography and geodynamic studies in continental China.

Double Differencing by Demeaning: Applications to Hypocenter Location and Wavespeed Tomography

ABSTRACTDouble differencing of body-wave arrival times has proved to be a useful technique for increasing the resolution of earthquake locations and elastic wavespeed images, primarily because (1) differences in arrival times often can be determined with much greater precision than absolute onset times and (2) differencing reduces the effects of unknown, unmodeled, or otherwise unconstrained variables on the arrival times, at least to the extent that those effects are common to the observations in question. A disadvantage of double differencing is that the system of linearized equations that must be iteratively solved generally is much larger than the undifferenced set of equations, in terms of both the number of rows and the number of nonzero elements. In this article, a procedure based on demeaning subsets of the system of equations for hypocenters and wavespeeds that preserves the advantages of double differencing is described; it is significantly more efficient for both wavespeed-only tomography and joint hypocenter location-wavespeed tomography. Tests suggest that such demeaning is more efficient than double differencing for hypocenter location as well, despite double-differencing kernels having fewer nonzeros. When these subsets of the demeaned system are appropriately scaled and simplified estimates of observational uncertainty are used, the least-squares estimate of the perturbations to hypocenters and wavespeeds from demeaning are identical to those obtained by double differencing. This equivalence breaks down in the case of general, observation-specific weighting, but tests suggest that the resulting differences in least-squares estimates are likely to be inconsequential. Hence, demeaning offers clear advantages in efficiency and tractability over double differencing, particularly for wavespeed tomography.

LORS2 Software Package for Monitoring the Seismicity of the Khibiny Massif: Construction Principles and Basic Algorithms

The paper describes a software package developed for automated monitoring of seismic activity of the Khibiny mountain massif. The general structure of the software package is described, and the applied methods and algorithms are considered in detail. In particular, the algorithm for locating the source of a seismic event via grid searches, which are specified as sets of circles and are categorized during execution of the algorithm, is described in detail. The proposed location methods are combined into several packages—scenarios to optimize the time spent by a geophysicist in routine interactive processing of a large dataset. Because the described software package was created to process data from seismic networks located directly in the mining area, much attention is paid to increasing the reliability of detecting target signals in a high-noise environment and the accuracy in determining the arrival times of body waves. To solve these problems, the system implements a new approach to locating close events, based on the idea of a holistic perception of a signal record, which is very close to the concept of human perception of objects. The algorithms and methods for detecting and locating seismic event generation sites implemented in the software package can significantly increase the reliability of the solutions obtained and increase the accuracy and efficiency of the final interactive processing of seismic signal records.

Three-dimensional high-resolution velocity structure imaging and seismicity study of the Tangshan fault

Based on the 4415 earthquakes body wave arrival times recorded by the 47 fixed and 33 temporary stations in Tangshan and its surrounding areas, the Tangshan fault’s area focal parameters and three-dimensional fine velocity structure model were inversed by using the consistency-constrained double-difference tomography method. The results revealed that the relocated earthquakes were roughly vertical to the fault’s strike, with the maximum depth exceeding 25 km. The Tangshan fault’s area Vp, Vs and Vp/Vs values demonstrate obvious inhomogeneity, and high speed bodies prone to the brittle fault are dominant on both sides of the fault in the upper crust. Small low velocity bodies appeared initially at the Ms7.8 Tangshan earthquake source in the 14–16 km range of the middle crust, followed by a gradual expansion of the low velocity area with an increase in depth. Variation in the velocity structure below the Tangshan earthquake source shows that the high temperature mantle source invading the crust has weakened the upper crust’s high-speed area, leading to stress accumulation in the local area, which is an important factor for the occurrence of earthquakes in the Tangshan region. The relationship between the regional seismic structure and Tangshan seismic sequence, causing migration of surrounding blocks and seismic activity in recent years was analyzed based on the Tangshan fault’s 3D highresolution velocity structure imaging. It is considered that the Tangshan area unique velocity structure not only plays a pivotal role in promoting horizontal tectonic stress during seismic activity, but is also the root cause of the aftershock evolution process and the fault’s morphological characteristics.

3D Seismic Velocity Models for Alaska from Joint Tomographic Inversion of Body-Wave and Surface-Wave Data

AbstractWe present two new seismic velocity models for Alaska from joint inversions of body-wave and ambient-noise-derived surface-wave data, using two different methods. Our work takes advantage of data from many recent temporary seismic networks, including the Incorporated Research Institutions for Seismology Alaska Transportable Array, Southern Alaska Lithosphere and Mantle Observation Network, and onshore stations of the Alaska Amphibious Community Seismic Experiment. The first model primarily covers south-central Alaska and uses body-wave arrival times with Rayleigh-wave group-velocity maps accounting for their period-dependent lateral sensitivity. The second model results from direct inversion of body-wave arrival times and surface-wave phase travel times, and covers the entire state of Alaska. The two models provide 3D compressional- (VP) and shear-wave velocity (VS) information at depths ∼0–100 km. There are many similarities as well as differences between the two models. The first model provides a clear image of the high-velocity subducting plate and the low-velocity mantle wedge, in terms of the seismic velocities and the VP/VS ratio. The statewide model provides clearer images of many features such as sedimentary basins, a high-velocity anomaly in the mantle wedge under the Denali volcanic gap, low VP in the lower crust under Brooks Range, and low velocities at the eastern edge of Yakutat terrane under the Wrangell volcanic field. From simultaneously relocated earthquakes, we also find that the depth to the subducting Pacific plate beneath southern Alaska appears to be deeper than previous models.

Imaging the subsurface using induced seismicity and ambient noise: 3-D tomographic Monte Carlo joint inversion of earthquake body wave traveltimes and surface wave dispersion

SUMMARYSeismic body wave traveltime tomography and surface wave dispersion tomography have been used widely to characterize earthquakes and to study the subsurface structure of the Earth. Since these types of problem are often significantly non-linear and have non-unique solutions, Markov chain Monte Carlo methods have been used to find probabilistic solutions. Body and surface wave data are usually inverted separately to produce independent velocity models. However, body wave tomography is generally sensitive to structure around the subvolume in which earthquakes occur and produces limited resolution in the shallower Earth, whereas surface wave tomography is often sensitive to shallower structure. To better estimate subsurface properties, we therefore jointly invert for the seismic velocity structure and earthquake locations using body and surface wave data simultaneously. We apply the new joint inversion method to a mining site in the United Kingdom at which induced seismicity occurred and was recorded on a small local network of stations, and where ambient noise recordings are available from the same stations. The ambient noise is processed to obtain inter-receiver surface wave dispersion measurements which are inverted jointly with body wave arrival times from local earthquakes. The results show that by using both types of data, the earthquake source parameters and the velocity structure can be better constrained than in independent inversions. To further understand and interpret the results, we conduct synthetic tests to compare the results from body wave inversion and joint inversion. The results show that trade-offs between source parameters and velocities appear to bias results if only body wave data are used, but this issue is largely resolved by using the joint inversion method. Thus the use of ambient seismic noise and our fully non-linear inversion provides a valuable, improved method to image the subsurface velocity and seismicity.

Seismic structure beneath the Reykjanes Peninsula, southwest Iceland, inferred from array-derived Rayleigh wave dispersion

The aim is to obtain a site-specific S-wave structure beneath the Reykjanes Peninsula, southwest Iceland. Nine broadband stations of the Reykjanet network are used to find Rayleigh-wave phase velocity dispersion in a relatively wide range of periods (from 3 to 50 s). The records analyzed were made in the years 2013 to 2015 and concern fourteen selected earthquakes whose epicentral distances range from tens of kilometers to almost ten thousand kilometers. Our approach to retrieving Rayleigh phase velocity dispersion involves two partly independent methods allowing for array apertures larger than a wavelength: 1) the zero-crossing point method, and 2) the phase-plane method. The two methods used here work with seismograms decomposed into quasi-harmonic components and implicitly assume a single plane wave propagation. The good match between the dispersion curves obtained by means of the two methods indicates that the assumption has been reasonably fulfilled. The Rayleigh wave phase velocity dispersion data are inverted into a horizontally layered isotropic S-wave velocity model of the Earth's crust and uppermost mantle by a modified method of the single-parameter variation. At shallow depths, the derived model is rather similar to some previous models that were derived predominantly from the arrival times of body waves. At depths exceeding about 20 km, the dispersion data require low S-wave velocities, indicating a noticeable low-velocity zone.

Focal Depths of Earthquakes in the Crimea–Black Sea Region

The problem of determining focal depths of earthquakes in the Crimea–Black Sea region is considered. Based on the results of interannual studies, it is found that the focal depths of Crimean earthquakes are mainly crustal, with maximum values of up to 60 km. Some recent publications, however, have described deep-focus earthquakes with depths of up to 300 km which were “revealed” in the Crimean region. In this respect, there arose the need to study such a large difference in estimated focal depths. Convincing examples show that the sensational “revelation” of deep earthquakes in Crimea was caused by incorrect processing of the experimental data, in particular, due to (1) a sharp distortion in the recorded arrival times of body waves, (2) exclusion of data from stations nearest to a source, (3) unreasonable arbitrary selection of data from seismic stations, and (4) dropping of data from the worldwide seismological network, including those on deep seismic phases. Thus, the conclusions about the presence of deep mantle earthquakes in Crimea are erroneous. We have redetermined the parameters of hypocenters and verified that the focal depths of earthquakes in the Crimea–Black Sea region are no more than 60 km. Based on these data, we analyze the features of the spatial distribution of focal depths to show that earthquake sources are grouped along conduits that dip southeastward, from the continental part of Crimea toward the Black Sea Basin, in the case of grouping of sources in the Alushta–Yalta and Sevastopol areas. The seismic focal layer of the Kerch–Anapa area dips northeastward, from the Black Sea beneath the North Caucasus.

Active tectonics in the Gulf of California and seismicity (M > 3.0) for the period 2002–2014

We present a catalog of accurate epicenter coordinates of earthquakes located in the Gulf of California (GoC) in the period 2002–2014 that permits us to analyze the seismotectonics and to estimate the depth of the seismogenic zone of this region. For the period April 2002 to December 2014 we use body-wave arrival times from regional stations of the Broadband Seismological Network of the GoC (RESBAN) operated by CICESE to improve hypocenter locations reported by global catalogs. For the northern region of the GoC (30°N–32°N) we added relocated events from the 2011-Hauksson-Yang-Shearer, Waveform Relocated Earthquake Catalog for Southern California (Hauksson et al., 2012; Lin et al., 2007). From October 2005 to October 2006 we incorporated hypcenters located by Sumy et al. (2013) in the southern GoC combining an array of ocean-bottom seismographs, of the SCOOBA experiment, with onshore stations of the NARS-Baja array. This well constrained catalog of seismicity highlights zones of active tectonics and seismic deformation within the North America-Pacific plate boundary. We estimate that the minimum magnitude of completeness of this catalog is Mc=3.3±0.1 and the b=0.92±0.04 value of the Gutenberg-Richter relation. We find that most earthquakes in the southern GoC are generated by transform faults and this region is more active than the central GoC region. However, the northern region, where most deformation is generated by oblique faults is as active as the southern region. We used the ISC catalog to evaluate the size distribution of seismicity of these regions, and the b value of the Gutenberg-Richter relation and found that b is slightly lower in the central GoC (b=0.86±0.02) compared to the northern (b=1.14±0.04) and the southern (b=1.11±0.04) regions. We observed seismicity that occurs in the Stable Central Peninsular Province, despite the fact that significant active deformation has not been identified in this region.

3D vP and vS models of southeastern margin of the Tibetan plateau from joint inversion of body-wave arrival times and surface-wave dispersion data

A new 3D velocity model of the crust and upper mantle in the southeastern (SE) margin of the Tibetan plateau was obtained by joint inversion of body- and surface-wave data. For the body-wave data, we used 7190 events recorded by 102 stations in the SE margin of the Tibetan plateau. The surface-wave data consist of Rayleigh wave phase velocity dispersion curves obtained from ambient noise cross-correlation analysis recorded by a dense array in the SE margin of the Tibetan plateau. The joint inversion clearly improves the vS model because it is constrained by both data types. The results show that at around 10 km depth there are two low-velocity anomalies embedded within three high-velocity bodies along the Longmenshan fault system. These high-velocity bodies correspond well with the Precambrian massifs, and the two located to the northeast of 2013 MS 7.0 Lushan earthquake are associated with high fault slip areas during the 2008 Wenchuan earthquake. The aftershock gap between 2013 Lushan earthquake and 2008 Wenchuan earthquake is associated with low-velocity anomalies, which also acts as a barrier zone for ruptures of two earthquakes. Generally large earthquakes (M ≥ 5) in the region occurring from 2008 to 2015 are located around the high-velocity zones, indicating that they may act as asperities for these large earthquakes. Joint inversion results also clearly show that there exist low-velocity or weak zones in the mid-lower crust, which are not evenly distributed beneath the SE margin of Tibetan plateau.

The influence of one-dimensional velocity sections on determining the key parameters of the earthquake sources in Azerbaijan

The paper addresses the construction of one-dimensional (1D) velocity models in the seismogenic regions of Azerbaijan taken individually and the analysis of implications of these models for estimating the key parameters of earthquake sources in Azerbaijan. We considered and analyzed the seismological data from the local earthquakes, the arrival times of the P-, P-g, Pn-, S-, Sg-, and Sn-waves recorded by the network of telemetry stations during the period from 2005 to 2014 with ml ≥ 2.5. For constructing the models, we used the VELEST program which calculates 1D velocity models from travel times of seismic waves. As a result, the 1D models were built for ten regions of Azerbaijan; the key parameters of the hypocenters of the earthquakes were recalculated; and the corrections to the body-wave arrival times at the observation stations were obtained, which increased the accuracy of locating the hypocenter of earthquakes.