Large Epicentral Distances Research Articles

Effects of the seismic attenuation pattern at the bend of the Southeastern Carpathians on the peak ground motions from local crustal earthquakes

The effects of the seismic attenuation pattern observed at the bend of the Southeastern Carpathians on the peak ground motions (PGMs) produced by local crustal earthquakes are analyzed using theoretical experiments. The synthetic seismograms – ground velocity time histories, vertical component – are calculated by the multimodal summation method in layered inelastic media. The theoretical waveforms evidence that the lateral variations of the attenuation structure may result in higher peak amplitudes at larger epicentral distances, in other words, the wave attenuation may dominate the wave spreading in the study area, within epicentral distances up to a few tens of km. The result is in good agreement with the observations, and emphasize the substantial contribution of the crustal attenuation to the pattern of ground motions caused by crustal, as well as intermediate-depth earthquakes of Vrancea.

Seismic T Phases in the Western-Central Mediterranean: Source of Seismic Hazard?

Abstract The Algerian offshore earthquake of 18 March 2021, Mw 6.0, was felt by people in various Italian regions, also at large epicentral distance. This unusual human perception far from the source prompted us to analyze the waveforms recorded by land seismic stations installed along the Iberian, French, and Italian coasts. On some seismograms of the selected network, prominent T phases are detected. T waves can travel in the SOund Fixing And Ranging (SOFAR) channel over great distances (thousands of kilometers) with little loss in signal strength and be recorded by near-coastal seismometers after the P (primary) and S (secondary) phases (hence T or tertiary phases). To explain the subjective perception of ground shaking with quantities that are measured on the seismogram, we estimated the empirical macroseismic intensities for both body and T phases and we calculated the body-wave seismic attenuation. The P-wave anelastic attenuation analysis shows two main wave propagation patterns that reflect lithosphere heterogeneity of the Algerian, Liguro-Provençal, and Tyrrhenian basins. We find that in some cases, in particular along the Italian and French coasts, the largest ground shaking is caused by the T phase. Our observations confirm that the central-western Mediterranean Sea is a favorable site for T-wave propagation and suggest that the T phases should be taken into account in ground-shaking hazard assessment for the central-western Mediterranean.

On the measurement ofSdiff splitting caused by lowermost mantle anisotropy

SUMMARYSeismic anisotropy has been detected at many depths of the Earth, including its upper layers, the lowermost mantle and the inner core. While upper mantle seismic anisotropy is relatively straightforward to resolve, lowermost mantle anisotropy has proven to be more complicated to measure. Due to their long, horizontal ray paths along the core–mantle boundary (CMB), S waves diffracted along the CMB (Sdiff) are potentially strongly influenced by lowermost mantle anisotropy. Sdiff waves can be recorded over a large epicentral distance range and thus sample the lowermost mantle everywhere around the globe. Sdiff therefore represents a promising phase for studying lowermost mantle anisotropy; however, previous studies have pointed out some difficulties with the interpretation of differential SHdiff–SVdiff traveltimes in terms of seismic anisotropy. Here, we provide a new, comprehensive assessment of the usability of Sdiff waves to infer lowermost mantle anisotropy. Using both axisymmetric and fully 3-D global wavefield simulations, we show that there are cases in which Sdiff can reliably detect and characterize deep mantle anisotropy when measuring traditional splitting parameters (as opposed to differential traveltimes). First, we analyze isotropic effects on Sdiff polarizations, including the influence of realistic velocity structure (such as 3-D velocity heterogeneity and ultra-low velocity zones), the character of the lowermost mantle velocity gradient, mantle attenuation structure, and Earth’s Coriolis force. Secondly, we evaluate effects of seismic anisotropy in both the upper and the lowermost mantle on SHdiff waves. In particular, we investigate how SHdiff waves are split by seismic anisotropy in the upper mantle near the source and how this anisotropic signature propagates to the receiver for a variety of lowermost mantle models. We demonstrate that, in particular and predictable cases, anisotropy leads to Sdiff splitting that can be clearly distinguished from other waveform effects. These results enable us to lay out a strategy for the analysis of Sdiff splitting due to anisotropy at the base of the mantle, which includes steps to help avoid potential pitfalls, with attention paid to the initial polarization of Sdiff and the influence of source-side anisotropy. We demonstrate our Sdiff splitting method using three earthquakes that occurred beneath the Celebes Sea, measured at many transportable array stations at a suitable epicentral distance. We resolve consistent and well-constrained Sdiff splitting parameters due to lowermost mantle anisotropy beneath the northeastern Pacific Ocean.

MagInfoNet: Magnitude Estimation Using Seismic Information Augmentation and Graph Transformer

AbstractIn this study, we propose a reliable data‐driven tool, MagInfoNet, to enhance the accuracy of magnitude estimation. Its architecture was assembled using the Pre‐Inform and Mag‐Pred modules to replace and update the key functions of traditional seismic analysis workflows. The Pre‐Inform module with the residual network was used for data pretreatment by combining the intrinsic characteristics of seismic signals with the potential features of the arrival and travel times. Meanwhile, using a graph transformer with an improved cyclic graph, the Mag‐Pred module was used to calculate magnitudes by the preprocessed information and the autocorrelation of seismic time series. Training and testing data were randomly selected from the Stanford Earthquake Data Set. The results show that the estimation accuracy, generalization, and robustness of the proposed MagInfoNet are better than those of three machine learning models. Besides, MagInfoNet can perform better for those samples with larger epicentral distances, enhancing the monitoring capacity of existing system for earthquake events in remote areas. Finally, we discuss the interpretability of the explainable MagInfoNet to verify the role of advanced neural network modules.

A New Mechanism for Earthquake‐Enhanced Permeability

AbstractEarthquakes may enhance the permeability of Earth's crust, but the mechanisms for such changes are not entirely clear. Recent studies have revealed a distinct coseismic response at large epicentral distances to great earthquakes and at close distances to major earthquakes , where both the horizontal and vertical permeabilities increased by orders of magnitude and the increases stayed at the co‐seismic level for many years, which cannot be explained by the existing models. To explain such prolong increases of the vertical permeability I propose a new mechanism, supported by laboratory and field observations of earthquake‐triggered hydrogeologic processes, that great earthquakes may pressurize and liquefy groundwater systems and inject the liquefied sands into fractures to keep them open. The proposed mechanism is testable and implies prolonged upward migration of hot fluids from deep basins, which may be an important process for subsurface transport of heat and solutes and may also be useful to understand the safety of groundwater and the security of underground repositories.

Constraining Europa's ice shell thickness with fundamental mode surface wave dispersion

Determining the thickness of Europa's outer ice shell is a key factor for understanding Europa's internal dynamics, evolution, and potential habitability. As such, one of the primary goals of any future lander mission to Europa would be to constrain the thickness of the ice shell and confirm the presence of a global subsurface ocean. Tidally induced ice fracturing events provide a natural source of seismic energy to illuminate the subsurface, thus a seismic instrument onboard a future lander mission could provide a promising means to probe ice shell thickness. A variety of seismic techniques could be used to constrain Europa's interior structure and dynamics, including body wave, surface wave, and normal mode seismology. Here, we use numerical simulations of seismic wave propagation on Europa in order to investigate the potential of using long period dispersion measurements of Rayleigh and flexural waves to constrain the ice shell thickness. Since the sensitivity kernels of group velocity dispersion measurements depend strongly on the structure of the ice shell, inverting for ice shell thickness is a non-linear problem.To address this, we use either a grid search or Markov chain Monte Carlo inversion approach, and test the method on a variety of plausible models of Europa's interior. Additionally, we demonstrate our approach in a “blind” inversion using the 1 week long synthetic catalogs of Europa's seismicity from Panning et al. (2018). We find that under most scenarios, group velocity dispersion measurements between periods of 25–250 s can constrain Europa's ice shell thickness to within several km uncertainty, although the method becomes increasingly inaccurate for thicker ice shells and at large epicentral distances. Our results, which suggest that surface waves from naturally occurring ice fracturing events on Europa can be used to help determine ice shell thickness, may help set instrument requirements for spaceflight capable seismometers aimed at exploring icy ocean worlds.

MSS/1: Single‐Station and Single‐Event Marsquake Inversion

AbstractSEIS, the seismometer of the InSight mission, which landed on Mars on 26 November 2018, is monitoring the seismic activity of the planet. The goal of the Mars Structure Service (MSS) is to provide, as a mission product, the first average 1‐D velocity model of Mars from the recorded InSight data. Prior to the mission, methodologies have been developed and tested to allow the location of the seismic events and estimation of the radial structure, using surface waves and body waves arrival times, and receiver functions. The paper describes these validation tests and compares the performance of the different algorithms to constrain the velocity model below the InSight station and estimate the 1‐D average model over the great circle path between source and receiver. These tests were performed in the frame of a blind test, during which synthetic data were inverted. In order to propagate the data uncertainties on the output model distribution, Bayesian inversion techniques are mainly used. The limitations and strengths of the methods are assessed. The results show the potential of the MSS approach to retrieve the structure of the crust and underlying mantle. However, at this time, large quakes with clear surface waves have not yet been recorded by SEIS, which makes the estimation of the 1‐D average seismic velocity model challenging. Additional locatable events, especially at large epicentral distances, and development of new techniques to fully investigate the data, will ultimately provide more constraints on the crust and mantle of Mars.

An array-assisted deep learning approach to seismic phase-picking

Rapid increase of seismic data poses challenges to seismic data processing, in which phase-picking is a crucial procedure and accordingly attracts attention from seismologists. Among the breakthroughs in deep learning methods for seismic data processing, seismologists have especially proposed many seismic phase-picking methods based on deep learning, which have shown much better accuracy than traditional automatic phase-picking methods. However, further improvements are needed to meet the requirements for processing continuous seismic records. Here we propose a new approach combining seismic array scheme and U-Net, an end-to-end neural network developed in image segmentation. Previous studies have shown that U-Net can classify waveforms and determine the onset of seismic phases accurately with the help of contracting and expanding paths. Based on U-Net, we first design a deep learning phase picker (PP) model with a long (40 s) time window. To avoid difficulties in time window selection and interference between P- and S-phase identifications, the model involves two networks with the same structure, which pick the P and S phases separately. To improve model training, we employ data transformation to expand the sample set, add typical noises to the sample set for better resemblance of real data, and slightly increase the weight of the seismic signal containing seismic records for a better balance between precision and recall. We also adopt a more effective array-assisted phase picker (APP) model which correlates the picking results from different stations to minimize misidentification. Using data from Hi-net and Alibaba Aftershock AI Capture Contest, we design training and test sets of different sizes to examine the performance and generalization ability of the model. Results show that the APP can effectively pick the body-wave phases on continuous records with a good generalization ability. The relationship between the model performance and training set is such that the model performance is directly related to the data quality rather than the size of the training set, and there is no serious overfitting even for a small training set. Statistics further show that the model has no special requirements for the locations of time windows and can handle seismic records with large epicentral distances. By comparison with other models (model with short time windows or the one of simultaneous picking of P- and S-phases), we find that APP can successfully process event-crowded waveform data and effectively reduce the influence of noise. Finally, we apply the trained model to a seismic network in the Inner Mongolia. Results show that the number of earthquakes detected by the APP is 30 times the manually-detected one, and that 98.1% of the events in the catalog are detected by the APP. The successful application to a seismic network different from the training set suggests that the APP is robust and has strong generalization ability.

Characteristics of hydroseismograms in Jingle well, China

Hydroseismograms reflect the oscillations of well water levels caused by seismic waves. In this study, the relationships between the hydroseismogram oscillation amplitude and three factors (seismic magnitude, epicentral distance, and focal depth) in Jingle well, China were analyzed. The results revealed that hydroseismogram amplitude increases exponentially with seismic magnitude as Hmag=7.848×10-8×e2.1690×m+0.047(m>Mw5.8) (m – seismic magnitude), decreases exponentially with epicentral distance as Hdis=-2.1108×10-5×e2.3864×γ+0.011 (γ – epicentral distance), and decreases exponentially with focal depth as Hdep=-1.6438×10-3×e0.0254×d-0.203 (d is focal depth). However, these relationships become uncertain when the epicentral distance exceeds 4000 km and are not applicable to nuclear explosions. Based upon the single well–aquifer model constructed for Jingle well, the amplification (μ) of the water level oscillation by ground vibrations μ=AbAa=R2-r2r2 depends on the frequency of the water-level oscillations. For relatively high-frequency oscillations (caused by earthquakes with relatively small epicentral distances), the amplification will be small, while for low-frequency oscillations (caused by earthquakes with relatively large epicentral distances), the amplification will be large. Using the S-transform, the dominant hydroseismogram period increases with the epicentral distance as τ=0.5826×γ+12.51, which means that the amplification (μ) increases with the epicentral distance. Of course, the amplitude of ground oscillations will be greatly attenuated with increasing epicentral distance. Due to the anisotropy of the earth and seismic wave superposition, the relationships between the hydroseismogram amplitude and the seismic magnitude, epicentral distance, and focal depth become blurred or even uncertain with increasing epicentral distance and focal depth, leading to poor goodness-of-fit between the dominant hydroseismogram period and epicentral distance. The quantitative relationships between the hydroseismogram amplitude and the seismic magnitude, epicentral distance, and focal depth, which determine the ratio between the amplitudes of water-level oscillations and ground vibrations and the relationship between groundwater oscillations and seismic wave transmission, are proposed for the first time in the present work. These results will improve understanding of hydrogeological conditions and could also be important in identifying reliable earthquake precursors.

Enhancing Tsunami Warning UsingPWave Coda

AbstractMost large tsunamis are generated by earthquakes on offshore plate boundary megathrusts. The primary factors influencing tsunami excitation are the seismic moment, faulting geometry, and depth of the faulting. Efforts to provide rapid tsunami warning have emphasized seismic and geodetic methods for quickly determining the event size and faulting geometry. It remains difficult to evaluate the updip extent of rupture, which has significant impact on tsunami excitation. TeleseismicPwaves can constrain this issue; slip under deep water generates strongpwPwater reverberations that persist as ringingPcodaafter the directPphases from the faulting have arrived. Event‐averagedPcoda/Pamplitude measures at large epicentral distances (>80°), tuned to the dominant periods of deep waterpwP(~12–15 s), correlate well with independent models of whether slip extends to near the trench or not. Data at closer ranges (30° to 80°) reduce the time lag needed for inferring the updip extent of rupture to <15 min. Arrival ofPPandPPPphases contaminates closer distancePcodameasures, but this can be suppressed by azimuthal or distance binning of the measures. Narrowband spectral ratio measures and differential magnitude measures ofPcodaand directP(mB) perform comparably to broader band root‐mean‐square (RMS) measures.Pcoda/Plevels for large nonmegathrust events are also documented. Rapid measurement ofPcoda/Pmetrics after a large earthquake can supplement quick moment tensor determinations to enhance tsunami warnings; observation of largePcodalevels indicates that shallow submarine rupture occurred and larger than typical tsunami (for givenMW) can be expected.

Shallow crustal imaging using distant, high-magnitude earthquakes

SUMMARYGlobal phases, viz. seismic phases that travel through the Earth’s core, can be used to locally image the crust by means of seismic interferometry. This method is known as Global Phase Seismic Interferometry (GloPSI). Traditionally, GloPSI retrieves low-frequency information (up to 1 Hz). Recent studies, however, suggest that there is high-frequency signal present in the coda of strong, distant earthquakes. This research quantifies the potential of these high-frequency signals, by analysing recordings of a multitude of high-magnitude earthquakes (≥6.4 Mw) and their coda on a selection of permanent USArray stations. Nearly half of the P, PKP and PKIKP phases are recorded with a signal-to-noise ratio of at least 5 dB at 3 Hz. To assess the viability of using the high-frequency signal, the second half of the paper highlights two case studies. First, a known sedimentary structure is imaged in Malargüe, Argentina. Secondly, the method is used to reveal the structure of the Midcontinent Rift below the SPREE array in Minnesota, USA. Both studies demonstrate that structural information of the shallow crust (≤5 km) below the arrays can be retrieved. In particular, the interpreted thickness of the sedimentary layer below the Malargüe array is in agreement with earlier studies in the same area. Being able to use global phases and direct P-phases with large epicentral distances (>80°) to recover the Earth’s sedimentary structure suggests that GloPSI can be applied in an industrial context.

Study of Geo-Electric Data Collected by the Joint EMSEV-Bishkek RS-RAS Cooperation: Possible Earthquake Precursors

By employing the cross-correlogram method, in geo-electric data from the area of Kyrgyzstan for the period 30 June 2014–10 June 2015, we identified Anomalous Telluric Currents (ATC). From a total of 32 ATC after taking into consideration the electric current source properties, we found that three of them are possible Seismic Electric Signal (SES) activities. These three SES activities are likely to be linked with three local seismic events. Finally, by studying the corresponding recordings when a DC alternating source injects current into the Earth, we found that the subsurface resistivity seems to be reduced before one of these three earthquakes, but a similar analysis for the other two cannot be done due to their large epicentral distance and the lack of data.

Performance of High‐Rate GPS Waveforms at Long Periods: Moment Tensor Inversion of the 2003 M w 8.3 Tokachi‐Oki Earthquake

Abstract We present a comparison of long‐period (30 T M w 8.3) with both very broadband seismograms and synthetic waveforms. We show that GPS provides valuable waveform data between 40 and 160 s periods, especially in the near field of the earthquake where seismograms are saturated. There are no long‐period broadband seismic data available in the near field of large earthquakes (∼100 km for earthquakes M w >7 and ∼500 km for earthquakes M w >8). The comparison with synthetic seismic displacement waveforms shows that the GPS waveforms are performing well at periods of 40–80 s at epicentral distances between 150 and 600 km and at periods of 80–160 s at epicentral distances between 150 and 400 km. At longer periods and larger epicentral distances, the amplitudes do not always exceed the noise level (∼2–3 mm), but we still observe at selected GPS sites that the phase and the amplitude of the surface waves are of good quality, up to ∼1200 km. We conclude that GPS waveforms in the near field of a large event can be used for seismological applications. In this article, we recover the focal mechanism of the Tokachi‐Oki event by inverting the GPS data recorded within 300 km of the epicenter.

Possible Mechanisms for Generation of Anomalously High PGA During the 2011 Tohoku Earthquake

Mechanisms are suggested that could explain anomalously high PGAs (peak ground accelerations) exceeding 1 g recorded during the 2011 Tohoku earthquake (M w = 9.0). In my previous research, I studied soil behavior during the Tohoku earthquake based on KiK-net vertical array records and revealed its ‘atypical’ pattern: instead of being reduced in the near-source zones as usually observed during strong earthquakes, shear moduli in soil layers increased, indicating soil hardening, and reached their maxima at the moments of the highest intensity of strong motion, then reduced. We could explain this assuming that the soils experienced some additional compression. The observed changes in the shapes of acceleration time histories with distance from the source, such as a decrease of the duration and an increase of the intensity of strong motion, indicate phenomena similar to overlapping of seismic waves and a shock wave generation, which led to the compression of soils. The phenomena reach their maximum in the vicinity of stations FKSH10, TCGH16, and IBRH11, where the highest PGAs were recorded; at larger epicentral distances, PGAs sharply fall. Thus, the occurrence of anomalously high PGAs on the surface can result from the combination of the overlapping of seismic waves at the bottoms of soil layers and their increased amplification by the pre-compressed soils.

Preliminary empirical scaling of pseudo relative velocity spectra in Serbia from the Vrancea earthquakes

First frequency-dependent empirical scaling equations of pseudo-relative velocity spectral amplitudes (PSV) of strong earthquake ground motions in the former Yugoslavia were introduced in the mid-1990s by Lee and Trifunac (1990) [15]. This followed the development of the Fourier spectral amplitudes (FS) scaling equations by Lee and Trifunac (1993) [17] in terms of earthquake source parameters, and the region-specific frequency dependent attenuation function given by Lee and Trifunac (1992) [16]. More recently, a new frequency-dependent attenuation function was developed for central and eastern Serbia for earthquakes of intermediate and large magnitudes and for large epicentral distances—exceeding 300km—suggested by Lee et al. (2016) [19] that occur in the Vrancea source region in Romania. In this paper we use this frequency-dependent attenuation function to develop empirical scaling equations for PSV spectral amplitudes in Serbia. These scaling equations will form a basis for macro- and micro-zoning earthquake hazard studies in Serbia.

ULF/ELF Atmospheric Radiation in Possible Association to the 2011 Tohoku Earthquake as Observed in China

Recordings of a search-coil magnetometer located in Yongsheng in the Yunnan province of China have been used to find atmospheric ULF/ELF precursors to the 2011 Tohoku earthquake (EQ) with M=9 which occurred on 11 March 2011 with a large epicentral distance of about 4100 km. The combined characteristics of the horizontal magnetic fields, the ratio of tangential and radial field components in the numerator and root mean square of ellipticity in the denominator, were used to detect seismo-related ULF/ELF signals. This value is found to increase before the EQ, and then to decrease during a few weeks, with a reliable maximum on 8 March (3 days before the EQ). We also analyzed azimuth distributions of pulsed radiation in the frequency range 2-5 Hz during the previous week before the main shock. This radiation reached a maximal value on 10 March - one day before the EQ date, and the azimuth of its source is estimated to coincide with the position of the main shock. The results are further compared with the ULF/ELF precursors of the same EQ observed at 3 stations in Japan by Ohta et al. (2013).

Three‐dimensional seismic velocity structure of Mauna Loa and Kilauea volcanoes in Hawaii from local seismic tomography

AbstractWe present a new three‐dimensional seismic velocity model of the crustal and upper mantle structure for Mauna Loa and Kilauea volcanoes in Hawaii. Our model is derived from the first‐arrival times of the compressional and shear waves from about 53,000 events on and near the Island of Hawaii between 1992 and 2009 recorded by the Hawaiian Volcano Observatory stations. The Vp model generally agrees with previous studies, showing high‐velocity anomalies near the calderas and rift zones and low‐velocity anomalies in the fault systems. The most significant difference from previous models is in Vp/Vs structure. The high‐Vp and high‐Vp/Vs anomalies below Mauna Loa caldera are interpreted as mafic magmatic cumulates. The observed low‐Vp and high‐Vp/Vs bodies in the Kaoiki seismic zone between 5 and 15 km depth are attributed to the underlying volcaniclastic sediments. The high‐Vp and moderate‐ to low‐Vp/Vs anomalies beneath Kilauea caldera can be explained by a combination of different mafic compositions, likely to be olivine‐rich gabbro and dunite. The systematically low‐Vp and low‐Vp/Vs bodies in the southeast flank of Kilauea may be caused by the presence of volatiles. Another difference between this study and previous ones is the improved Vp model resolution in deeper layers, owing to the inclusion of events with large epicentral distances. The new velocity model is used to relocate the seismicity of Mauna Loa and Kilauea for improved absolute locations and ultimately to develop a high‐precision earthquake catalog using waveform cross‐correlation data.

Strong fast long-period waves in the Efpalio 2010 earthquake records: explanation in terms of leaking modes

The January 18, 2010, shallow earthquake in the Corinth Gulf, Greece (Mw 5.3) generated unusually strong long-period waves (periods 4–8 s) between the P and S wave arrival. These periods, being significantly longer than the source duration, indicated a structural effect. The waves were observed in epicentral distances 40–250 km and were significant on radial and vertical component. None of existing velocity models of the studied region provided explanation of the waves. By inverting complete waveforms, we obtained an 1-D crustal model explaining the observation. The most significant feature of the best-fitting model (as well as the whole suite of models almost equally well fitting the waveforms) is a strong velocity step at depth about 4 km. In the obtained velocity model, the fast long-period wave was modeled by modal summation and identified as a superposition of several leaking modes. In this sense, the wave is qualitatively similar to P long or Pnl waves, which however are usually reported in larger epicentral distances. The main innovation of this paper is emphasis to smaller epicentral distances. We studied properties of the wave using synthetic seismograms. The wave has a normal dispersion. Azimuthal and distance dependence of the wave partially explains its presence at 46 stations of 70 examined. Depth dependence shows that the studied earthquake was very efficient in the excitation of these waves just due to its shallow centroid depth (4.5 km).

Failure mechanisms of Ana Slide from geotechnical evidence, Eivissa Channel, Western Mediterranean Sea

This work deals with the failure mechanisms of Ana Slide in the Eivissa Channel, in between the Iberian Peninsula and the Balearic Islands, under the effects of gas charging and seismic loading. In situ geotechnical tests and sediment cores obtained at the eastern Balearic slope of the Eivissa Channel suggest that the basal failure surface (BFS) developed as a result of subtle contrasting hydro-mechanical properties at the boundary between a fine-grained unit (U6) overlying a methane-charged relatively coarser unit (U7). Past methane seepage is inferred from seismic reflection profiles and high magnetic susceptibility values in sediments from the slide headwall area. Past methane charging is also supported by further seismic reflection data and isotopic analyses of benthic foraminifera published separately. The possibility of failure for different critical failure surfaces has been investigated by using the SAMU-3D slope stability model software taking into account the role of free methane in the development of the landslide. Failure would occur after SAMU-3D if the undrained shear strength of units U6 and U7 is strongly degraded (i.e. 95%). Wheeler's theory suggests that a 9% free gas saturation would be required to reduce the undrained shear strength by 95%. However, the theory of the undrained equilibrium behaviour of gassy sediments for this methane concentration shows that the excess fluid pressure generated by gas exsolution, estimated at 12% of the effective stress, is not high enough to bring the slope to fail. This led us to consider seismic loading as an additional potential failure mechanism despite the lack of historical data (including instrumental records) on seismicity in the Balearic Islands, therefore assuming that the historical period is not necessarily representative of seismic activity further back in time (i.e. when Ana Slide occurred ~61.5ka ago). Considering current slope conditions, the most critical failure surface obtained by SAMU-3D relates to peak ground accelerations (PGA) of 0.24g, which relates to magnitude moment Mw=5 at epicentral distances of 1km, and 7≥Mw≥5 at epicentral distances ≤15km to Ana Slide. However, no active faults have been identified at so short distance from Ana Slide. Only when shear strength is degraded due to the presence of free methane in units U6 and U7 is considered, the most critical failure surface obtained by SAMU-3D fits with lower magnitude and larger epicentral distances. Consequently, the most plausible hypothesis to explain the occurrence of Ana Slide is the combination of free gas and seismic loading.

Optimisation of seismic network design: Application to a geophysical international lunar network

Fundamental scientific questions concerning the internal structure and dynamics of the Moon, and their implications on the Earth–Moon System, are driving the deployment of a new broadband seismological network on the surface of the Moon. Informations about lunar seismicity and seismic subsurface models from the Apollo missions are used as a priori information in this study to optimise the geometry of future lunar seismic networks in order to best resolve the seismic interior structure of the Moon. Deep moonquake events and simulated meteoroid impacts are the assumed seismic sources. Synthetic P and S wave arrivals computed in a radial seismic model of the Moon are the assumed seismic data. The linearised estimates of resolution and covariance of radial seismic velocity perturbations can be computed for a particular seismic network geometry. The non-linear inverse problem relating the seismic station positions to the linearised estimates of covariance and resolution of radial seismic velocity perturbations is written and solved by the Neighbourhood Algorithm. This optimisation study favours near side seismic station positions at southern latitudes in order to constrain the deep mantle structure from deep moonquake data at large epicentral distances. The addition of a far side station allows to divide by two the size of the error bar on the seismic velocity model. The monitoring of lunar impact flashes from the Earth allows to improve the radial seismic model in the top of the mantle by adding much more meteor impact data at short epicentral distances due to the high accuracy of the space/time location of these seismic sources. Such meteor impact detections may be necessary to investigate the 3D structure of the lunar crust.