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Standard spectral ratio (SSR) technique has limited applicability in the estimation of spectral amplification in the vast Indo-Gangetic plains (IGP). Here we take recourse to an alternative approach using the recordings of three largest aftershocks of the 2015 Gorkha earthquake (Mw 7.9). We separately compute geometric mean source spectrum of an event from the recordings at hard sites in India and from the target IGP site. The ratio of the source spectrum from the target IGP site to the geometric mean source spectrum from the hard sites (denoted here, for brevity, as RSS) provides the desired spectral amplification. At four soft IGP sites where a comparison is possible, the spectral amplifications from RSS and SSR methods show a reasonable resemblance but also significant differences, owing to the difference in the definition of the reference spectrum. RSS method may be preferable if the input motion can only be prescribed at a generic hard site. We document amplification at 28 IGP sites using the RSS technique. The fundamental frequency, f0, of the sites increase from 0.12 Hz near the foothills of Himalayas to 2.0 Hz at the southern edge of the basin and the amplification reaches about 10. At several sites, f0 is difficult to select and the amplification of ~5 is broadband in the range 0.12–0.7 Hz. Application of SSR technique to teleseismic S-wave data recorded in the IGP reveals that this approach may be useful in the estimation of amplification at low frequencies (f < 0.5 Hz). [ABSTRACT FROM AUTHOR]

We report on rupture directivity during 8 and 10 May 2014 earthquakes (Mw 6.5, 6.1), which occurred on the Mexican subduction thrust in the Guerrero seismic gap. The two events were located near each other and SE of the area that had ruptured in an Mw 7.2 earthquake on 18 April 2014. Near-source recordings show that the rupture during the 8 May propagated roughly along the strike towards ESE, while the directivity during the 10 May event was in the opposite direction. Analyses of the ground motions produced by the two earthquakes at regional distances (spectral ratios, ratios of peak ground accelerations and velocities, observed ground motion parameters, and radiated seismic energy) exhibit strong azimuthal variation, consistent with the directivity. As expected, the motions were enhanced in the forward direction. Since the attenuation of seismic waves from coastal earthquakes is slower along the inland path as compared to the coastal path, the ground motions during the 8 May earthquake at inland sites in the forward direction were exceptionally large. It follows that the combination of the two effects during future large/great earthquakes in the gap may lead to destructive ground motions at inland sites even at regional distances. The combined effect may have been one of the causes of the unprecedented number of deaths and injuries, and damage in Mexico City during the 1985 Michoacán earthquake (Mw 8.0) which ruptured a nearby subduction interface with a ESE directivity. [ABSTRACT FROM AUTHOR]

The 2017 Guptkashi earthquake occurred in a segment of the Himalayan arc with high potential for a strong earthquake in the near future. In this context, a careful analysis of the earthquake is important as it may shed light on source and ground motion characteristics during future earthquakes. Using the earthquake recording on a single broadband strong-motion seismograph installed at the epicenter, we estimate the earthquake’s location (30.546° N, 79.063° E), depth (H = 19 km), the seismic moment (M0 = 1.12×1017 Nm, Mw5.3), the focal mechanism (φ = 280°, δ = 14°, λ = 84°), the source radius (a = 1.3 km), and the static stress drop (Δσs~22 MPa). The event occurred just above the Main Himalayan Thrust. S-wave spectra of the earthquake at hard sites in the arc are well approximated (assuming ω−2 source model) by attenuation parameters Q(f) = 500f0.9, κ = 0.04 s, and fmax = infinite, and a stress drop of Δσ = 70 MPa. Observed and computed peak ground motions, using stochastic method along with parameters inferred from spectral analysis, agree well with each other. These attenuation parameters are also reasonable for the observed spectra and/or peak ground motion parameters in the arc at distances ≤ 200 km during five other earthquakes in the region (4.6 ≤ Mw ≤ 6.9). The estimated stress drop of the six events ranges from 20 to 120 MPa. Our analysis suggests that attenuation parameters given above may be used for ground motion estimation at hard sites in the Himalayan arc via the stochastic method. [ABSTRACT FROM AUTHOR]

Although it has been suggested in the past that the attenuation of seismic waves along the Pacific coast of Mexico and towards the continent differs, the subject has been controversial and, as yet, unresolved. Here we re-examine this issue using more extensive and higher quality data now available. Our analysis of more than 450 local and regional ( R < 400 km) records from interplate earthquakes (5.0 ≤ Mw≤ 8.0; 10 ≤ H < 30 km) shows that the Fourier spectral amplitudes at inland sites exceed those at coastal ones, especially in the frequency range of 0.3–2 Hz. We find that site or source effects are not the cause of this difference. The observed disparity of amplitudes is in fact a consequence of the efficient propagation of Lg and SmS phases along the convergence direction, as it is confirmed by numerical modelling. The simulations show that the energy radiated by interplate earthquakes is efficiently guided towards the continent by the wedge-shape geometry and the structure of the subduction zone, thus enhancing the inland wavefield beyond 100–150 km. On the contrary, for coastal paths there is a rapid drop of amplitude due to lateral heterogeneity and low crustal thickness. Corresponding to geometrical spreading terms of and for trajectories along the coast and towards the continent, the Q( f) of the S-wave group is approximately given by 175 f0.52 and 211 f0.46, respectively. Separate attenuation functions for the two types of trajectories improve the model currently used in estimating Fourier amplitude spectrum of ground motion, and thus, may be useful in a more realistic evaluation of the seismic hazard in the Mexican forearc region. [ABSTRACT FROM AUTHOR]

In this study seismic noise records from the permanent broadband network of the Mexican National Seismological Service (Servicio Sismológico Nacional) and from the temporal broadband networks of MASE and VEOX have been used to compute the Green functions from vertical-vertical component of noise cross correlations between station pairs. MASE (Mesoamerican Seismic Experiment) consisted of one hundred stations deployed along a profile perpendicular to the trench from Acapulco, Guerrero, to Tempoal, Veracruz, Gulf of Mexico. This experiment was carried out from December 2004 to May 2007. Forty six of these stations were relocated along a N-S profile crossing the Tehuantepec Isthmus from the Gulf of Mexico to the Pacific coast. These stations operated from July 2007 to March 2009 and this stage of the experiment was called VEOX (Veracruz-Oaxaca). From the Green functions extracted from the correlation for each pair of stations, Rayleigh wave dispersion curves were computed which represent the average group velocity between station pairs. Furthermore, regional earthquakes recorded by the stations were used in order to compute Rayleigh wave dispersion curves, which represent the average group velocity between epicenter-station pair. This mixed set of group velocity measurements was inverted to obtain tomographic images at discrete periods between 5 and 50 s. Resolution tests show that the regions of best coverage surround both temporal experiments. A good velocity model was obtained in a large area between the two experiments. The images show good correlation at short periods (< 15 s) with the shallowest geological features. Low velocity anomalies are related with sedimentary basins and Cenozoic volcanic areas, whereas high velocity anomalies correlate with the forearc region. A low-velocity anomaly was clearly observed at the north of the Isthmus of Tehuantepec where an active volcanic field is located. At larger periods (> 25 s) a low velocity region coincides with the border of the Trans- Mexican Volcanic Belt, suggesting the presence of a mantle wedge. For these periods, high velocity anomalies are related with the shallow presence of the oceanic mantle in the forearc and the presence a shallower mantle in the backarc, where the continental crust becomes thin. [ABSTRACT FROM AUTHOR]

The empirical Green's function (EGF) technique is applied in the frequency domain to 962 broad-band seismograms (3.3 ≤ MW ≤ 6.1) to determine stress drop and source scaling of the 2009 April L’Aquila earthquakes. The station distance varies in the range 100–250 km from the source. Ground motions of several L’Aquila earthquakes are characterized by large azimuthal variations due to source directivity, even at low magnitudes. Thus, the individual-station stress-drop estimates are significantly biased when source directivity is not taken into account properly. To reduce the bias, we use single-station spectral ratios with pairs of earthquakes showing a similar degree of source directivity. The superiority of constant versus varying stress-drop models is assessed through minimization of misfit in a least-mean-square sense. For this analysis, seismograms of 26 earthquakes occurring within 10 km from the hypocentres of the three strongest shocks are used. We find that a source model where stress drop increases with the earthquake size has the minimum misfit: as compared to the best constant stress-drop model the improvement in the fit is of the order of 40 per cent. We also estimate the stress-drop scaling on a larger data set of 64 earthquakes, all of them having an independent estimate of seismic moment and consistent focal mechanism. An earthquake which shows no directivity is chosen as EGF event. This analysis confirms the former trend and yields individual-event stress drops very close to 10 MPa at magnitudes MW > 4.5 that decrease to 1 MPa, on the average, at the smallest magnitudes. A varying stress-drop scaling of L’Aquila earthquakes is consistent with results from other studies using EGF techniques but contrasts with results of authors that used inversion techniques to separate source from site and propagation effects. We find that there is a systematic difference for small events between the results of the two methods, with lower and less scattered values of stress drop resulting from the EGF approach. [ABSTRACT FROM PUBLISHER]

Abstract: We study the influence of different source characteristics (depth, distance, type and azimuth) on the site effect in Acapulco and the Valley of Mexico. Site amplification was estimated by means of spectral ratios (both horizontal-to-vertical spectral ratio and standard spectral ratio techniques were applied) from earthquake recordings at soft and hard sites. In Acapulco, 125 Mexican earthquakes covering a hypocentral range of 7–290km and a depth range (H) of 3–61km were analyzed in three different groups of hypocentral distances. In the Valley of Mexico, we estimate site effect at five locations using recordings from shallow-coastal interplate (200⩽Δ⩽570km; H⩽35km) and normal-faulting, intermediate-depth inslab (132⩽Δ⩽738km; 32⩽H⩽178km) earthquakes, as well as from teleseismic events. Our results seem to point to negligible dependence of site effects on the source location and type. [Copyright &y& Elsevier]

Examines the dynamic rupture and resultant stress change in normal faulting earthquake in the subducting Cocos plate. Role of consistent waveform inversion on the detection of highly heterogeneous distribution of stress drop; Variation in dynamically generated source time function with location on the fault; Pattern of stress-drop distribution.

The behavior of slip close to the trench during earthquakes is not well understood, and observations of large earthquakes breaking the near trench fault surface are rare. The 1995 Mw 8.0 Jalisco earthquake seems to have broken the near‐trench area, as evidenced by large Ms‐Mw disparity, small high‐frequency radiated energy compared to total energy, and low Er/M0 ratios, in addition to several finite slip models showing large slip near the trench. However, slip models obtained using campaign Global Positioning System data suggest slip near shore. In this study we try to answer whether this event was a near‐trench event or not, by inverting teleseismic P, S, Rayleigh, and Love waves, as well as campaign Global Positioning System static offsets, either separately or jointly, to obtain the slip distribution on the fault as a function of time. We find two possible end‐member scenarios consistent with observed data: (1) coseismic slip distributed between coast and trench and no (or very little) postseismic slip and (2) coseismic slip principally near the trench with large (up to 1.8 m) aseismic slip occurring in the first 5–10 days after the earthquake, with a total moment corresponding to 16% of that of the event. We are unable to distinguish between these two end‐member scenarios by tsunami modeling and finally are neither able to conclude or exclude that the event was a typical near trench event. Key Points: The event nucleated at ~20‐km depth and broke a >150‐km‐long segment of the Mesoamerican Subduction Zone, rupturing toward the NWThere may have been a slip between the trench and coast as suggested by joint inversion of teleseismic records and near‐field static offsetsAnother possibility is that there was coseismic slip near the trench and postseismic slip near the coast [ABSTRACT FROM AUTHOR]

Dispersion analysis of Rayleigh waves is performed to assess the velocity of complex structures such as sedimentary basins. At short periods several modes of the Rayleigh waves are often exited. To perform a reliable inversion of the velocity structure an identification of these modes is thus required. We propose a novel method to identify the modes of surface waves. We use the spectral ratio of the ground velocity for the horizontal components over the vertical component (H/V) measured on seismic coda. We then compare the observed values with the theoretical H/V ratio for velocity models deduced from surface wave dispersion when assuming a particular mode. We first invert the Rayleigh wave measurements retrieved from ambient noise cross-correlation with the assumptions that (1) the fundamental mode and (2) the first overtone are excited. Then we use these different velocity models to predict theoretical spectral ratios of the ground velocity for the horizontal components over the vertical component (H/V). These H/V ratios are computed under the hypothesis of equipartition of a diffuse field in a layered medium. Finally we discriminate between fundamental and higher modes by comparing the theoretical H/V ratio with the H/V ratio measured on seismic coda. In an application, we reconstruct Rayleigh waves from cross-correlations of ambient seismic noise recorded at seven broad-band stations in the Valley of Mexico. For paths within the soft quaternary sediments basin, the maximum energy is observed at velocities higher than expected for the fundamental mode. We identify that the dominant mode is the first higher mode, which suggests the importance of higher modes as the main vectors of energy in such complex structures.EN [ABSTRACT FROM AUTHOR]

The project calls for the deployment of 12 to 15 instruments along the Guerrero gap with anaverage spacing of 20 to 25 km. The stations communicate data in real time via variouscommunications means. The purpose of the network is to collect information aboutground displacement and acceleration along the coast of the Guerrero gap in realtime with a sampling rate of at least 100 samples per second of acceleration and 2Hz of displacement data. It is expected to have an average error in displacementof ±1.5 cm. The geodetic data will be also corrected using the recorded strongmotion data based on a Kalman algorithm that estimates additional corrections in apredictive manner. The seismic instruments are high-resolution, forced balancedaccelerographs. The objective of the project is to maintain a constant surveillance of thesubduction zone along the Guerrero gap. Stations are deployed from Petatlán to theboundary between the states of Guerrero and Oaxaca at as regular spacing as allowedby site availability and communication means. One of the missions of the arraywill be to estimate the moment magnitude immediately after the occurrence of alarge or great earthquake. Also, it is expected that the seismogeodetic network willprovide information about the degree of coupling and the evolution of the seismicand geodetic behavior of the Guerrero gap. Together with the data of the MexicanSeismological Service (SSN) and the Seismic Early Warning System (SASMEX),the information gleaned from the array will help guide immediate civil protectionmeasures. There are now a large number of seismic and geodetic stations in this sameregion: under water, on the coast and farther inland. The project also considerscomplementing the real time geodetic information with routine InSAR analyses. [ABSTRACT FROM AUTHOR]