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A fully probabilistic seismic hazard model with a single domain and sufficiently accurate resolution level for national analyses has been developed, for the Caribbean and Central America, to support the design of parametric earthquake policies offered by the Caribbean Catastrophe Risk Financing Facility to sovereign countries. This model provides updated earthquake hazard and risk information for 34 countries in the region, allowing to obtain detailed seismic hazard results at any location within the area of analysis. Besides a detailed zonation of subduction and crustal sources, updated seismicity and tectonic data have been utilized. Different seismicity, geometric, and rupture models have been adopted using state-of-the-art methodologies and tools. The main output of this model is a stochastic event-set, which is the hazard representation utilized not only to compute a long-term overview of the seismic hazard in the region but also to probabilistically estimate earthquake modeled losses at a national level that are used as triggers for underpinning parametric earthquake insurance. Although not of direct interest for parametric insurance purposes, this model also yields hazard maps and uniform hazard spectra for different return periods. For the operational purposes of the parametric insurance coverage, a postevent tool was developed to calculate, in near-real time, the ground-motion intensities associated with any earthquake within the region under study. This is done automatically by choosing a rupture plane from the moment tensor solution based on predefined rules while maintaining full consistency with all the assumptions made in the probabilistic seismic hazard analysis. The predicted ground motions in the region and the vulnerability functions developed for the exposed assets are then used to estimate whether the economic losses for each affected country exceed the trigger value stated in the country-specific insurance policy conditions and whether a payout should be issued, which is typically disbursed within 14 days. [ABSTRACT FROM AUTHOR]

Dos tipos de sismos intraplaca en la placa de Cocos que subduce debajo de la placa Norte America ocurren en Guerrero, México, y áreas adyacentes: (A) inversos de gran echado y (B) de fallamiento normal. Los de tipo A se localizan a ~10-35 km de la costa, a una profundidad de ~35 km, y revelan compresión en la dirección del echado de la placa, probablemente causada por su desdoblamiento. Los de tipo B son ligeramente más profundos que los del tipo A cuando ocurren cerca de la costa, pero si ocurren más adentro del continente, donde la placa se vuelve horizontal, alcanzan profundidades de 40-50 km. Estos eventos revelan extensión en la placa subducida orientada en la dirección de su echado. El análisis de los sismos del 27 de abril y del 22 de mayo de 2009 revela que se trata de eventos intraplaca en la placa de Cocos subducida del tipo A y B, respectivamente. Los espectros de fuente obtenidos a partir de datos locales y regionales dan una caída de esfuerzos de Brune, Δσ, de ~49 y 34 MPa, respectivamente, un poco mayores que la mediana de Δσ de 30 MPa reportada previamente para sismos intraplaca mexicanos. Nuestras estimaciones de energía radiada, ER, son 3.55x1013 J y 2.29x1013 J, lo que arroja un cociente ER/M0 de 5.63x10-5 y 6.54x10-5, y un esfuerzo aparente, σa, de 3.9 MPa y 4.6 MPa, respectivamente (correspondientes al M0 reportado en el catálogo Global de CMT), valores razonables para sismos intraplaca. Las aceleraciones máximas del terreno (PGA), como una función de la distancia están en buen acuerdo con relaciones de atenuación previamente obtenidas para sismos intraplaca mexicanos. Los movimientos del terreno en el Valle de México generados por sismos intraplaca son más ricos en altas frecuencias en comparación con aquéllos de eventos interplaca, especialmente en la zona dura. Esto refleja tanto una naturaleza más energética de las fuentes intraplaca como una distancia más cercana a la fuente de muchos de estos sismos. Los resultados obtenidos en este estudio nos dan confianza en nuestro conocimiento de la naturaleza de las fuentes intraplaca y nuestra habilidad de estimar movimientos fuertes para eventos futuros. doi: https://doi.org/10.22201/igeof.00167169p.2010.49.3.111

General characteristics of seismic energy release of thrust earthquakes in Mexico have been reviewed in the past; however, a detailed analysis can contribute to a better understanding of the mechanisms that control its distribution along the Guerrero, Mexico, subduction zone. To address it, we obtain the source spectra of the 2012 Mw 7.5 Ometepec‐Pinotepa Nacional, the 2014 Mw 7.2 Papanoa, and the 2018 Mw 7.2 Pinotepa Nacional earthquakes, as well as of their M≥4.0 aftershocks to estimate their seismic moment M0 and radiated seismic energy ES⁠. The first and the last sequences occurred at the southern border of the Guerrero seismic gap, a region where no significant earthquake (⁠M>7.0⁠) has occurred at least in the last century; whereas the second sequence was located at the northern edge of the same seismic gap. The mean value of the log of radiated seismic energy scaled with the seismic moment, log(e)=log(ES/M0)⁠, for this set of earthquakes is -5.05±0.25⁠. We classify the analyzed events into four regions, two in the southern edge of the gap and two in the northern one. At both ends, there is one region that shows regular values of log(e) (⁠-4.64±0.25 and -4.62±0.25⁠), whereas the other one shows low values of log(e) (⁠-5.40±0.25 and -5.55±0.25⁠) that could be related to a possible slow‐rupture behavior. These last regions are identified near the trench at southern Guerrero coast and immediately outside the northern end of the seismic gap. The distribution of log(e) is spatially heterogeneous along the trench, suggesting variations on the shear strength and coupling at the interface. [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]

Se resume la teoría y la metodología con las cuales se pueden estimar parámetros focales de un temblor a partir de señales sísmicas. Se promedian 11 espectros de onda P (usando registros de corto y largos periodos por cada estación) y se combina con 11 espectros de onda G2 para definir el espectro promedio de la fuente entre periodos de 200 seg y 0.5 seg del temblor de Orizaba. El momento sísmico de este temblor de (Mb≈6.7) se estima como 4.8 10₂₆ dina cm. La frecuencia de la esquina es 0.1 cps, a partir del cual se estima el radio de la fuente como 13 km con una caída de esfuerzo de 95 bar.

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]

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]

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]

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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]

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]

We developed equations to predict pseudoacceleration response spectra (5% damping), peak ground acceleration, and peak ground velocity at free-field rock sites for intermediate-depth, normal-faulting inslab earthquakes of Central Mexico. The data set comprises 16 earthquakes (5.2 ≤ Mw ≤ 7.4; 35 ≤ H ≤ 138 km) recorded at local and regional distances (R ≤ 400 km). It represents a homogeneous catalog with respect to tectonic regime, fault mechanism, and soil class. Our results show larger amplitudes in the epicentral area from inslab events than from interplate events, a consequence of higher stress drops during the former type of earthquakes. Peak ground accelerations from moderate to large (Mw >6.0) inslab events significantly exceed those from interplate events with similar magnitude, reaching almost three times for the largest events. The ground motion due to inslab events, however, decays faster than for the thrust events. Our results are in reasonable agreement with other studies based on Japanese and the worldwide data. [ABSTRACT FROM AUTHOR]

We analyze 17 intermediate-depth, normal-faulting, inslab earthquakes of Mexico (4.1 ≤ Mw ≤ 7.4; 35 km ≤ H ≤ 118 km), recorded on hard sites at local and regional distances (R ≤ 600 km), to study spectral attenuation of seismic waves, quality factor Q, source spectra, and Brune stress drop. Assuming 1/R geometrical spreading, the quality factor is given by Q(f) = 251f0.58. Although there is considerable uncertainty in Q due to the trade-off between geometrical spreading and Q, this uncertainty does not influence strongly the estimation of source spectra and stress drops. We find that source spectra of nine events (4.1 ≤ Mw ≤ 6.4) follow the ω2 model, while those of the other eight (5.8 ≤ Mw ≤ 7.4) significantly deviate from it. Interpreting the high-frequency level of the source spectra with the ω2 model yields a nearly constant stress drop, Δσ, with a median value of 304 bars. This is more than 4 times greater than the corresponding value for interplate earthquakes in central Mexico. The observed source acceleration spectra, S(f), is, however, better fit by an empirical source spectrum characterized by two comer frequencies, fa and fb, such that S(f) = f2M0/[{1 + (f/fa)2} ⋅ {1 + (f/fb)2}]½, where fa = 4.962 x 1010M00.454, fb = 4.804 x 105/M00.213, and M0 is in dyne centimeters. This empirical source spectrum may be useful in predicting ground-motion parameters using... [ABSTRACT FROM AUTHOR]

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.