Back Azimuth Research Articles

Tectonic Inheritance With Dipping Faults and Deformation Fabric in the Brittle and Ductile Southern California Crust

AbstractPlate motions in Southern California have undergone a transition from compressional and extensional regimes to a dominantly strike‐slip regime in the Miocene. Strike‐slip motion is most easily accommodated on vertical faults, and major transform fault strands in the region are typically mapped as near vertical on the surface. However, some previous work suggests that these faults have a dipping geometry at depth. We analyze receiver function arrivals that vary harmonically with back azimuth at all available broadband stations in the region. The results show a dominant signal from contrasts in dipping foliation as well as dipping isotropic velocity contrasts from all crustal depths, including from the ductile middle to lower crust. We interpret these receiver function observations as a dipping fault‐parallel structural fabric that is pervasive throughout the region. The strike of these structures and fabrics is parallel to that of nearby fault surface traces. We also plot microseismicity on depth profiles perpendicular to major strike‐slip faults and find consistently NE dipping features in seismicity changing from near vertical (80–85°) on the Elsinore Fault in the Peninsular Ranges to 60–65° slightly further inland on the San Jacinto Fault to 50–55° on the San Andreas Fault. Taken together, the dipping features in seismicity and in rock fabric suggest that preexisting fabrics and faults may have acted as strain guides in the modern slip regime, with reactivation and growth of strike‐slip faults along northeast dipping fabrics both above and below the brittle‐ductile transition.

Seismic Response of a Mountain Ridge Prone to Landsliding

ABSTRACTDuring an earthquake, site effects can play an important role in triggering landslides. To document the seismic response of steep hillslopes, we deployed broadband seismometers across a mountain ridge in Taiwan, in an area with a high earthquake-induced landslide hazard. The ridge has a simple, representative shape, and landslides have previously occurred there. Our seismometer array has recorded continuously during more than 1 yr, with both ambient-noise and regional moderate earthquakes as sources. Processing horizontal and vertical signal components, we show that the ridge has a complex response, which we attribute to the combined effects of the subsurface geology and the topographic geometry. Amplification and directionality of ground motion are observed both high and low on the ridge, giving rise to localized, elevated, earthquake-induced landslide hazard. Our database contains earthquakes with mostly similar locations, making it difficult to determine the effect of earthquake back azimuth on the ridge response. A part of the ridge response, possibly due to topographic effects, seems to be explained by a model derived from a frequency scale curvature proxy at low frequency. If correct, this would be a promising first step toward improving local ground-motion estimation in mountain areas. However, the definition of appropriate scaling parameters of site effects based on geophysical measurements, for use in regional and global landslide hazard equations applicable to mountain areas with substantial regolith thickness, remains a significant challenge.

Deep and CNN fusion method for binaural sound source localisation

In binaural sound source localisation, front–back confusion is often the challenging problem when localising sources in the noisy or reverberant environments. Hence, a novel algorithm fusing deep and convolutional neural network (CNN) is proposed to address this issue. First, joint features, which consist of interaural level differences (ILDs) and cross-correlation function (CCF) within a lag range, are extracted from binaural signals. Second, with the extracted CCF–ILD features, CNN is used for the front–back classification task, while deep neural network is used for azimuth classification task. The front–back features extracted by the CNN can be leveraged as additional information for the sound source localisation task. Also, an angle-loss function is designed to avoid the overfitting problem and to improve the generalisation ability of this method in adverse acoustic conditions. Finally, two branches are concatenated and then followed by an output layer, which generates the posterior probability of azimuth angles, and the azimuth corresponding to the maximum posterior probability is chosen as the direction of sound source. Experimental results demonstrate the effectiveness of the authors’ method for front–back decision and azimuth estimation in noisy and reverberant environments.

Temporal Variation and Frequency Dependence of Seismic Ambient Noise on Mars From Polarization Analysis

AbstractWe applied a polarization analysis of InSight seismic data to estimate the temporal variation and frequency dependence of the Martian ambient noise field. Low‐frequency (<1 Hz) P waves show a diurnal variation in their dominant back azimuths that are apparently related to wind and the direction of sunlight in a distant area. Low‐frequency Rayleigh waves (0.25–1 Hz) show diurnal variations and a dominant back azimuth related to the wind direction in a nearby area. Low‐frequency signals that are derived mainly from wind may be sensitive to subsurface structure deeper than the lithological boundary derived from an autocorrelation analysis. On the other hand, dominant back azimuths of high‐frequency (>1 Hz) waves point toward the InSight lander, especially in daytime, indicating that wind‐induced lander noise is dominant at high frequencies. These results point to the presence of several ambient noise sources as well as geologic structure at the landing site.

Rockfall localization from seismic polarization considering multiple triaxial geophones and frequency bands

Boulder/rock mass movements generate ground vibrations that can be recorded by geophone networks. Generally, there are two methods applied to rockfall trajectory reconstruction or rockfall seismic localization. One method uses seismic wave arrival times and is achieved by minimizing the differences in signal arrival times between multiple stations by grid map searching. The other method uses seismic polarization and is achieved by calculating event-source back azimuths from the seismic polarizations of rockfall signals. In this study, we proposed the use of an overdetermined matrix for joint localization based on the polarization method. The overdetermined matrix considers the contributions of all geophones in the network, and at each geophone is assigned a different weight according to the recorded signal qualities and the reliability of the calibrated back azimuths. This method shows a great advantage relative to the case in which only two sensors are employed. Besides, we suggested three marker parameters for proper frequency band selection in back azimuth calculations: energy, rectilinearity, and a special permanent frequency band (SPF). We found that the back azimuths calculated with energy and an SPF are generally close to the real back azimuths measured in the field, while the SPF is limited by seismic attenuation due to a long-distance propagation. The localization results of rockfalls were validated by using field camera videos and in situ calibrations. Three typical cases and 43 artificially released rockfalls are presented in this paper. The proposed method provides an interesting way to locate rockfall events and track rockfall trajectories and avoids the difficulties of obtaining accurate arrival times, as required by the arrival times method.

Seismic Azimuthal Anisotropy for the Southeastern Tibetan Plateau Extracted by Wave Gradiometry Analysis

AbstractWave Gradiometry (WG) is a dense array data processing method used to extract seismic velocity and other properties of the earth. In this study, we propose using WG for a plane wave field to measure azimuthal anisotropy by fitting the phase velocities from different back azimuths. We applied this new method to the Temporary Western Sichuan Array and obtained the isotropic and azimuthal anisotropy for period bands centered at T = 20, 40, and 60 s in the southeast Tibetan Plateau. Our results show that the fast orientations of anisotropy are well consistent with the strikes of the fault system and orogenic belt. The weak anisotropic magnitude in the northern part of the south Chuandian subblock and a consistence of fast orientation in boundary fault zone (BFZ) at different periods may be attributed to anisotropic relics associated with the Emeishan Large Igneous Province. Comparing our results with those of previous studies, we found that our anisotropic results are consistent with anisotropic structures measured by the Pms phase in the plateau area, while a systematical angular difference exists in the BFZ, Sichuan Basin, and south Chuandian subblock where high Rayleigh wave phase velocity was observed. The Lijiang‐Xiaojin fault and Longmenshan fault appear to form a deep‐rooted strong boundary at the lithospheric scale, as they serve as a boundary for anisotropic structures, isotropic velocity, and the other three WG parameters.

Результаты первичной обработки данных инфразвуковой группы на острове Валаам

The infrasound array VALS developed in Kola Branch GS RAS has been installed in June 2016 on the Valaam Island in addition to the continuously operating seismic station VALR. The array consists of 3 spaced low-frequency microphones. The data with a sampling rate of 100 Hz is stored continuously at the acquisition computer; the timing is carried out using GPS. In addition to the acquisition system, an infrasound signal detector is installed on the computer. It works in near real-time mode and enables us to find signals and compute their back azimuths. At the end of 2018, a new version of the detector was developed at the Kola Branch GS RAS. The detector began to work much faster, which enabled us to carry out data processing for 2.5 years in two frequency ranges in a short time. The main task of the array is acoustic monitoring, the detection of infrasound events, the determination of their parameters, and the selection of events of natural origin. The data are also used (in combination with the VALR seismic station data) to locate near seismic events, especially weak ones. The analysis of the obtained data revealed the prevailing directions to the signal sources. The change of directions to sources in time was investigated, seasonal features were revealed. Acoustic events were detected in the frequency bands 1–5 Hz and 10–20 Hz, and a significant difference was found in the azimuthal distribution of events for these ranges. A joint analysis of acoustic and seismic data showed that the part of events with both acoustic and seismic components is low – it is almost completely exhausted by career explosions. It was also noted that in addition to explosions in nearby quarries (Kuznechnoye, Pitkäranta) located at a distance of 50–60 km, according to acoustic data, events corresponding to explosions at quarries located at a distance of 100 km or more were repeatedly identified.

Bayesian Selection of Atmospheric Profiles from an Ensemble Data Assimilation System using Infrasonic Observations of May 2016 Mount Etna Eruptions

AbstractObserved infrasound arrivals are considered as a way to provide additional knowledge of the middle atmosphere for numerical weather prediction (NWP) models. To do so, a Bayesian approach, based on the discrepancies between predicted and observed infrasound arrival characteristics, is adopted for selecting the most likely atmospheric states, in terms of temperature and wind velocity profiles, among an ensemble of NWP model analyses. The predicted characteristics, in terms of trace velocity and the back azimuth angle, are computed using a three‐dimensional ray‐tracing model and atmospheric profiles extracted once a day from ensemble of analyses produced by the Météo‐France global NWP model. The performance of the method is demonstrated using a set of thousands of volcanic eruptions that correspond to an upsurge in volcanic activity of Mount Etna (Sicily) during May 2016. It is shown that the Bayesian approach allows the identification of the most likely members and hence provides additional knowledge on the atmosphere truly probed by the infrasound. When the atmospheric temperature and wind profiles are reasonably meeting the effective sound speed assumption requirements, the members retrieved are similar in terms of effective sound speed but not in terms of wind and temperature. For improving both temperature and wind velocity profile selection, additional infrasound arrival characteristics are then required (e.g., travel time, amplitude, etc.). The confidence on the retrieved profiles is also sensitive to the time of validity of the meteorological analyses and on the definition of the surface conditions involved in the estimation of the predicted trace velocities.

Characterizing Thunder‐Induced Ground Motions Using Fiber‐Optic Distributed Acoustic Sensing Array

AbstractWe report for the first time on a distributed acoustic sensing (DAS) array using preexisting underground fiber optics beneath the Penn State campus for detecting and characterizing thunder‐induced ground motions. During a half‐hour interval from 03:20–03:50 UTC on 15 April 2019 in State College, PA, we identify 18 thunder‐induced seismic events in the DAS array data. The high‐fidelity DAS data show that the thunder‐induced seismics are very broadband, with their peak frequency ranging from 20 to 130 Hz. We use arrival times of the 18 events to estimate the phase velocity of the near surface, the back azimuth, and location of thunder‐seismic sources that are verified with lightning locations from the National Lightning Detection Network. Furthermore, the dense DAS data enable us to simulate thunder‐seismic wave propagation and full waveform synthetics and further locate the thunder‐seismic source by time‐reversal migration. Interestingly, we found that thunder‐seismic power recorded by DAS is positively correlated with National Lightning Detection Network lightning current power. These findings suggest that fiber‐optic DAS observations may offer a new avenue of studying thunder‐induced seismics, characterizing the near‐surface velocity structure, and probing the thunder‐ground coupling process.

Helicopter Crash on the Spitsbergen Archipelago: Infrasound and Seismic Signals Decryption

A Convers Avia Airline Mi-8 helicopter crashed near the Russian settlement of Barentsburg on October 26, 2017. The moment of impact was recorded by the Barentsburg seismo-infrasound array 2.5 km from the crash site. The seismo-infrasound array consists of a micro-aperture infrasound group, including three low-frequency microphones and a broadband seismic station located at the same site. A detailed analysis of the obtained records of the seismometer and the low-frequency microphones is presented here. Two types of signals determined by the helicopter fall are detected in seismic channels: a wave caused by the water impact of the helicopter and propagating through the earth, and an air wave that was also generated by the water impact but propagating through the atmosphere. The second type of wave was also recorded by low-frequency microphones from the infrasonic microgroup. The procedure for determining the source coordinate by the complex analysis of acoustic and seismic signals is described. The difference in arrival times of seismic and acoustic waves is used to determine the distance to the source, and the back azimuth is calculated using the differences in the arrival times of the acoustic wave to the spaced microphones of the infrasound microarray. The records of the acoustic signals associated with the operation of the helicopter rotor before the crash and at the moment of water impact are examined. A detailed analysis of the frequency and amplitude composition of the received seismic and infrasound signals has allowed us to not only determine the exact place and time of the helicopter crash, but also to estimate the expected trajectory of its movement before the fall, as well as to recreate some details of the accident.

Terrestrial single-station analog for constraining the martian core and deep interior: Implications for InSight

We used a terrestrial single-station seismometer to quantify the uncertainty of InSight (INterior explorations using Seismic Investigations, Geodesy and Heat Transport) data for determining Martian core size. To mimic Martian seismicity, we formed a catalog using 917 terrestrial earthquakes, from which we randomly selected events. We stacked ScS amplitudes on modeled arrival times and searched for where ScS produced coherent seismic amplitudes. A core detection was defined by a coherent peak with small offset between predicted and user-selected arrival times. Iterating the detection algorithm with varying signal-to-noise (SNR) ranges and quantity of events determined the selection frequency of each model and quantified core depth uncertainty. Increasing the quantity of events reduced core depth uncertainty while increasing the recovery rate, while increasing event SNR had little effect. Including ScS2 multiples increased the recovery rate and reduced core depth uncertainty when we used low quantities of events. The most-frequent core depths varied by back azimuth, suggesting our method is sensitive to the presence of mantle heterogeneities. When we added 1° in source distance errors, core depth uncertainty increased by up to 11 km and recovery rates decreased by <5%. Altering epicentral distances by 25% added ~35 km of uncertainty and reduced recovery rates to <50% in some cases. From these experiments, we estimate that if InSight can detect five events with high location precision (<10% epicentral distance errors), that there is at least an 88% chance of core depth recovery using ScS alone with uncertainty in core depth approaching 18 km and decreasing as more events are located. Plain language summaryWe used a seismometer on Earth to study how well the NASA mission, InSight (INterior explorations using Seismic Investigations, Geodesy and Heat Transport) could detect the Martian core. We created a catalog of earthquakes from which we randomly selected events to mimic the seismic events (marsquakes) InSight is expected to detect. Several models of Earth's interior were tested to see which best predicted the arrival of seismic waves reflecting off the core. We found utilizing more events allowed us to determine the core size better than using a limited number of events. Using an additional seismic phase also helped us recover a more accurate core depth if there were not a lot of events. We were also able to find variations in the mantle by using events within certain location ranges. Since the InSight team may not be able to locate marsquakes as accurately as we locate earthquakes, we changed the location of the earthquakes. When changes in location were small, we were still able to determine the size of the core. Larger changes in location prevented us from finding an accurate core depth. If InSight detects at least five events with high location accuracy, we will be able to determine the Martian core size.

Nonseismic Signals in the Ocean: Indicators of Deep Sea and Seafloor Processes on Ocean‐Bottom Seismometer Data

AbstractOcean‐bottom seismometers (OBSs) commonly record short‐duration events (SDEs) that could be described by all of these characteristics: (i) duration &lt;1 s, (ii) one single‐wave train with no identified P nor S wave arrivals, and (iii) a dominant frequency usually between 4 and 30 Hz. In many areas, SDEs have been associated with gas‐ or fluid‐related processes near cold seeps or hydrothermal vents, although fish bumps, instrumental, or current‐generated noise have been proposed as possible sources. In order to address some remaining issues, this study presents results from in situ and laboratory experiments combined with observations from two contrasting areas, the Sea of Marmara (Turkey) and the Chilean subduction zone. The in situ experiment was conducted at the European Multidisciplinary Seafloor and water column Observatory‐Molène submarine observatory (near Brest, France) and consisted in continuously monitoring two OBSs with a camera. The images revealed that no fish regularly bumped into the instruments. Laboratory experiments aimed at reproducing SDEs' waveforms by injecting air or water in a tank filled by sand and seawater and monitored with an OBS. Injecting air in the sediments produced waveforms very similar to the observed SDEs, while injecting air in the water column did not, constraining the source of SDEs in the seafloor sediments. Finally, the systematic analysis of two real data sets revealed that it is possible to discriminate gas‐related SDEs from biological or sea state‐related noise from simple source parameters, such as the temporal mode of occurrence, the back azimuth, and the dominant frequency.

SKS Splitting Beneath Mount St. Helens: Constraints on Subslab Mantle Entrainment

AbstractObservations of seismic anisotropy can provide direct constraints on the character of mantle flow in subduction zones, critical for our broader understanding of subduction dynamics. Here we present over 750 new SKS splitting measurements in the vicinity of Mount St. Helens in the Cascadia subduction zone using a combination of stations from the iMUSH broadband array and Cascades Volcano Observatory network. This provides the highest density of splitting measurements yet available in Cascadia, acting as a focused “telescope” for seismic anisotropy in the subduction zone. We retrieve spatially consistent splitting parameters (mean fast direction Φ: 74°, mean delay time ∂t: 1.0 s) with the azimuthal occurrence of nulls in agreement with the fast direction of splitting. When averaged across the array, a 90° periodicity in splitting parameters as a function of back azimuth is revealed, which has not been recovered previously with single‐station observations. The periodicity is characterized by a sawtooth pattern in Φ with a clearly defined 45° trend. We present new equations that reproduce this behavior based upon known systematic errors when calculating shear wave splitting from data with realistic seismic noise. The corrected results suggest a single layer of anisotropy with an ENE‐WSW fast axis parallel to the motion of the subducting Juan de Fuca plate; in agreement with predictions for entrained subslab mantle flow. The splitting pattern is consistent with that seen throughout Cascadia, suggesting that entrainment of the underlying asthenosphere with the subducting slab is coherent and widespread.

Borehole Microseismic Imaging of Hydraulic Fracturing: A Pilot Study on a Coal Bed Methane Reservoir in Indonesia

Over the last decade, microseismic monitoring has emerged as a considerable and capable technology for imaging stimulated hydraulic fractures in the development of unconventional hydrocarbon resources. In this study, pilot hydraulic-fracturing treatments were operated at a coal-bed methane (CBM) field in Indonesia to stimulate the flow and increase the reservoir's permeability while the monitoring system was set in a single near-vertical borehole. Locating event sources accurately is fundamental to investigating the induced fractures, but the geometry of a single downhole array is a challenging data processing task, especially to remove ambiguity of the source locations. The locating procedure was reviewed in 3 main steps: (i) accurate picking of P- and S-wave phases; (ii) inclusion of P-wave particle motion to estimate the back azimuth; (iii) guided inversion for hypocenter determination. Furthermore, the seismic-source moment magnitudes were calculated by employing Brune's model. Reliable solutions of locations were obtained as shown statistically by uncertainty ellipsoids and a small misfit. Based on our results, both induced and triggered seismicity could be observed during the treatments and therefore conducting intensive monitoring is important. The triggered seismicity is an undesired activity so disaster precautions need to be taken, in particular for preventing reactivation of pre-existing faults.

A Generalized H‐κ Method With Harmonic Corrections on Ps and Its Crustal Multiples in Receiver Functions

AbstractThe H‐κ method (Zhu &amp; Kanamori, 2000, https://doi.org/10.1029/1999JB900322) has been widely used to estimate the crustal thickness (H) and the ratio of P to S velocities (VP/VS ratio, κ) with receiver functions. However, in regions where the crustal structure is complicated, the method may produce biased results, arising particularly from dipping Moho and/or crustal anisotropy. H‐κ stacking in case of azimuthal or radial anisotropy with flat Moho has been proposed but not for cases with plunging anisotropy and dipping Moho. Here we propose a generalized H‐κ method called H‐κ‐c, which corrects for these effects first before stacking. We consider rather general cases, including plunging anisotropy and dipping interfaces of multiple layers, and use harmonic functions to correct for arrival time variations of Ps and its crustal multiples with back azimuth (θ). Systematic synthetic tests show that the arrival time variations can be well fitted by cosθ and cos2θ functions even for very complex crustal structures. Correcting for the back azimuthal variations significantly enhances H‐κ stacking. We verify the feasibility of the H‐κ‐c method by applying it to 40 permanent stations in various geological setting across the Mainland China. The results show clear improvement after the harmonic corrections, with clearer multiples and stronger stacking energy, as well as more reliable H‐κ values. Large differences in H (up to 5.0 km) and κ (up to 0.09) between the new and traditional methods occur mostly in mountainous regions, where the crustal structure tends to be more complex. We caution in particular about systematic bias when the traditional method is used in the presence of dipping interfaces. The modified method is simple and applicable anywhere in the world.

Evidence for a large strike-slip component during the 1960 Chilean earthquake

SUMMARYThe strainmeter record observed at Isabella (ISA), California, for the 1960 Chilean earthquake (Mw = 9.5) is one of the most important historical records in seismology because it was one of the three records that provided the opportunity for the first definitive observations of free oscillations of the Earth. Because of the orientation of the strainmeter rod with respect to the back azimuth to Chile, the ISA strainmeter is relatively insensitive to G (Love) waves and higher order (order ≥ 6) toroidal modes, yet long-period G waves and toroidal modes were recorded with large amplitude on this record. This observation cannot be explained with the conventional low-angle thrust mechanism typical of great subduction-zone earthquakes and requires an oblique mechanism with half strike-slip and half thrust. The strain record at Ogdenburg, New Jersey, the Press–Ewing seismograms at Berkeley, California, and the ultra-long period displacement record at Pasadena, California, also support the oblique mechanism. We tested the performance of the ISA strainmeter using other events including the 1964 Alaskan earthquake and found no instrumental problems. Thus, the ISA observation of large G/R and toroidal/spheroidal ratios most likely reflects the real characteristics of the 1960 Chilean earthquake, rather than an observational artefact. The interpretation of the large strike-slip component is not unique, but it may represent release of the strike-slip strain that has accumulated along the plate boundary as a result of oblique convergence at the Nazca–South American plate boundary. The slip direction of the 2010 Chilean (Maule) earthquake ( Mw = 8.8) is rotated by about 10° clockwise from the plate convergence direction suggesting that right-lateral strain comparable to that of an Mw = 8.3 earthquake remained unreleased and accumulates near the plate boundary. One possible scenario is that the strike-slip strain accumulated over several great earthquakes like the 2010 Maule earthquake was released during the 1960 Chilean earthquake. If this is the case, we cannot always expect a similar behaviour for all the great earthquakes occurring in the same subduction zone and such variability needs to be considered in long-term hazard assessment of subduction-zone earthquakes.

MEMS Accelerometer Mini-Array (MAMA): A Low-Cost Implementation for Earthquake Early Warning Enhancement

Earthquake Early Warning Systems (EEWS) are often challenged when the earthquakes occur outside the seismic network or where the station density is sparse. In these situations, poor locations and large alert delays are more common because of the limited azimuthal coverage and the time required for the wavefield to reach the minimum number of seismic stations to issue an alert. Seismic arrays can be used to derive the directivity of the wavefield and obtain better location. However, they are uncommon because of the prohibitive cost of the sensors. Here, we propose the development of an array-based approach using mini-arrays of low-cost Microelectromechanical Systems (MEMS) accelerometers and show how they can be used to improve EEWS. In this paper, we demonstrate this approach using data from two MEMS Accelerometer Mini-Arrays (MAMA) deployed at University of California Berkeley and Humboldt State University. We use a new low-cost ( &lt;U.S. $150) Data Acquisition Unit and solve for the back azimuth of seven events with magnitudes ranging from Mw 2.7 to 5.1 at distances of 5 km to 106 km.

Detection Method for Pairs of P and S Waves of Deep Low‐Frequency Earthquakes Using a 3‐D Array in the Tokai Area of the Nankai Subduction and Its Application to Hypocenter Determination

AbstractWe report on a novel method that uses a three‐dimensional (3‐D) array to detect P and S wave pairs of the deep low‐frequency earthquakes (LFEs) occurring along the Nankai subduction zone. In an effort to determine LFE hypocenters precisely we attempt to find their P and S pairs and obtain S‐P times using a 3‐D array (6 km in the horizontal direction) made up 14 seismic stations with deep (600 m at the deepest) borehole seismographs in the Tokai area. During operation we observed remarkable LFE activities on 10–30 November 2010. Using these data, we succeeded in distinguishing between P and S phases in complex LFE seismic waves using an effect of velocity filtering in semblance analysis via the 3‐D array. We also obtained high‐quality propagation parameters (back azimuths and incidence angles) and S‐P times for the LFEs. Referring to these identified P and S phases, after manually reading the arrival times at the 3‐D array and regular network, we could precisely relocate the hypocenters of 15 LFEs. These hypocenters were distributed in a depth range from 26 to 34 km, and approximately followed the plate interface. A comparison of the LFE hypocenter distribution to seismic tomography indicates that the LFEs are distributed in the oceanic crust and mantle wedge with relatively low velocities under an undrained cover with relatively high velocities following the land Moho (land crust bottom). We believe that our detection method will be useful in the automated detection and preliminary hypocenter determination of LFEs.

Accurate determination of P-wave back azimuth and slowness parameters by sparsity constrained seismic array analysis

Accurate determination of P-wave back azimuth and slowness parameters by sparsity constrained seismic array analysis

Infrasound Signal Detection and Back Azimuth Estimation Using Ground‐Coupled Airwaves on a Seismo‐Acoustic Sensor Pair

AbstractWe present a new infrasonic signal detection and back azimuth determination technique that requires just one microphone and one three‐component seismometer. Ground‐coupled airwaves (GCAs) occur when an incident atmospheric acoustic wave impinges on the ground surface and is partially transmitted as a seismic wave. GCAs are commonly detected hundreds of kilometers away on seismic networks and are observed to have retrograde particle motion. Horizontally propagating acoustic waves and GCAs have previously been observed on collocated infrasound and seismic sensor pairs as coherent with a 90° phase difference. If the sensors are spatially separated, an additional propagation‐induced phase shift is present. The additional phase shift depends on the direction from which the acoustic wave arrives, as each back azimuth has a different apparent distance between the sensors. We use the additional phase shift, the coherence, and the characteristic particle motion on the three‐component seismometer to determine GCA arrivals and their unique back azimuth. We test this technique with synthetic seismo‐acoustic data generated by a coupled Earth‐atmosphere 3‐D finite difference code, as well as three seismo‐acoustic data sets from Mount St. Helens, Mount Cleveland, and Mount Pagan volcanoes. Results from our technique compare favorably with traditional infrasound array processing and provide robust GCA detection and back azimuth determination. Assuming adequate station spacing and sampling, our technique provides a new and robust method to detect infrasonic signals and determine their back azimuth, and may be of practical benefit where resources are limited and large sensor networks or arrays are not feasible.