El sitio arqueológico de Monte Albán, Oaxaca, testimonio de la grandeza de la civilización zapoteca, sufrió daños significativos durante los sismos del 30 de septiembre de 1999 (Mw 7.5, R = 130 km), cerca de Puerto Escondido, y del 8 de septiembre de 2017 (Mw 8.2, R = 383 km), frente a las costas de Chiapas. Dado que los grandes sismos de subducción a lo largo de la costa del Pacífico de Oaxaca y los eventos intraslab en la placa de Cocos subducida bajo el continente son relativamente frecuentes, se deduce que la zona arqueológica ha sido sometida a movimientos fuertes del suelo en numerosas ocasiones. Para reforzar adecuadamente el monumento ante futuros eventos, es deseable conocer el movimiento del suelo en Monte Albán durante grandes sismos pasados, especialmente los de 1999 y 2017. Desafortunadamente no existen registros in situ de estos eventos. Sin embargo, existe una estación acelerográfica, OXLC, a 7 km del sitio, que ha registrado seis sismos de magnitud importante desde 1999. Si se conoce la amplificación espectral del movimiento del suelo en Monte Albán con respecto a OXLC, entonces los registros en OXLC pueden utilizarse para estimar el movimiento del suelo en Monte Albán mediante la aplicación de la teoría de vibraciones aleatorias. Una red de movimientos fuertes de 12 canales operó de forma intermitente entre 2004 y 2019 en Monte Albán. Los sensores fueron instalados en la base, a media altura y en la cima de la Plataforma Norte. Muchos sismos moderados fueron registrados simultáneamente por la red y la estación OXLC. Los cocientes espectrales de los movimientos en diferentes niveles de la Plataforma Norte con respecto a los registros de OXLC proporcionan las amplificaciones deseadas en el rango lineal. Usamos estos cocientes espectrales para estimar los movimientos del suelo en Monte Albán durante los seis sismos mencionados anteriormente. Las aceleraciones horizontales máximas estimadas del suelo, (PGA)H, en la base de la plataforma durante los eventos de 1999 y 2017 son de 137 y 129 cm/s2, respectivamente. La (PGA)H estimada durante el sismo de Huatulco del 23 de junio de 2020 (Mw, R = 154 km) es aproximadamente la misma que durante el evento de 1999; de hecho, los valores de velocidad máxima (PGV) y desplazamiento máximo (PGD) son mayores. Sin embargo, no hay reportes de daños durante 2020. Una posible explicación es que los esfuerzos de recuperación y conservación posteriores al sismo de 2017 fortalecieron adecuadamente las estructuras vulnerables, evitando daños durante 2020. Los daños en Monte Albán parecen ocurrir si la (PGA)H excede aproximadamente los 120 cm/s2 en la base; tal movimiento tiene un periodo de retorno estimado de unos 20 años. Una estimación aproximada de la (PGA)H en la base durante el sismo del 15 de enero de 1931 (M 7.8), que devastó la ciudad y el estado de Oaxaca, es de 480 cm/s2; se espera que este tipo de movimiento se exceda una vez cada 100 años. Los reportes de daños a Monte Albán en 1931 son escasos porque gran parte del monumento no era más que escombros. La extrapolación del limitado periodo de observación, que por lo tanto debe tomarse con cautela, sugiere que una aceleración de 1 g en la base podría excederse una vez cada 1000 años. Esto podría haber ocurrido durante el gran sismo interplaca de Oaxaca en 1787.

El sismo intraslab de 2017 (Mw7.1) ha sido uno de los temblores más destructivos en la historia de la Ciudad de México. Diversas medidas del movimiento del terreno revelan que el evento de 2017 fue inusualmente energético en CU, el sitio de referencia en la zona de lomas de la ciudad y, por ende, en todo el Valle de México en el intervalo de frecuencias entre 0.4 a 1 Hz. Esto se puede observar, por ejemplo, en el espectro de Fourier de las aceleraciones (FAS), la velocidad máxima del suelo (PGV) y el espectro de respuesta de pseudoaceleración (Sa), con un 5% de amortiguamiento, para periodos estructurales de 1 ≤ T ≤ 1.8 s (0.55 ≤ f ≤ 1 Hz). Sin embargo, la causa de las grandes aceleraciones en CU sigue sin resolverse. La cuestión merece un análisis cuidadoso de todos los datos disponibles. El 7 de diciembre de 2023, ocurrió un sismo intraslab de magnitud Mw 5.8 en la proximidad al evento de 2017. Analizamos por separado, los registros del sismo de 2017 y por otro lado, de manera conjunta, los registros del sismo de 2023 y del 2017 con el fin de aislar la causa de la radiación anómala de energía sísmica en altas frecuencias. En este contexto, utilizamos los registros del sismo de 2023 como funciones empíricas de Green (EGFs). Los valores sintetizados de Sa para un sismo de Mw 7.1, usando los registros de 2023 (Mw 5.8) como EGFs y asumiendo la misma caída de esfuerzos, Δσ, de 3 MPa para ambos eventos, son significativamente menores que los observados, independientemente del azimut. Encontramos que la fuente fue inusualmente energética en todos los azimuts, y que el efecto de la directividad de la ruptura en el comportamiento de las grandes aceleraciones fue relativamente pequeño. La posibilidad de que el comportamiento anómalo de las aceleraciones en la ciudad en 2017 se debiera a una dirección particular del campo de ondas incidente en la estructura 3D del Valle de México puede descartarse, ya que los registros de aceleración del sismo de 2023, que ocurrió casi en la misma ubicación que el sismo de 2017, no muestran nada anómalo. La Sa simulada utilizando los registros del evento de 2023 como EGFs sugiere que un hipotético sismo intraslab de Mw 7.1, con características de fuente similares al evento de 2023, a una distancia de aproximadamente 130 km de CU, debería causar poco o ningún daño en la ciudad. En otras palabras, si la fuente del sismo de 2017 hubiera sido una versión ampliada del evento de

The seismic behaviour of the near-trench plate interface of the Guerrero seismic gap and other segments of the Mexican subduction zone is likely to play a critical role in the seismic and tsunami hazard of the region. In this context, a detailed study of the near-trench 2002 April 18 Mw 6.7 earthquake that occurred about 55 km off the coast of Guerrero and generated a small tsunami attains particular importance. From an analysis of the teleseismic P waves and S waves, local recordings and aftershock distribution, we find that the rupture most likely began at a subducted seamount, propagated unilaterally towards NW, parallel to the trench for similar to 54-58 km and a duration of similar to 68-70 s. The moment rate function is highly rugged, with two dominant pulses separated by about 50 s. Although relatively small in magnitude, the earthquake has all the characteristics of a tsunami earthquake: the slip occurs very close to the trench, the rupture speed is slow (similar to 1 km s-1), the high-frequency radiation is deficient, and, in common with tsunami earthquakes, the moment-scaled radiated energy is low (ER/M0 = 1.45 x 10-6). We confirm that the duration of the event (similar to 70 s) is extraordinarily long compared to that expected from scaling relations (similar to 12.8 s), consistent with it being the most anomalous of all the events studied in the last 40 yr. Our results support a conditionally stable upper 15 km of the plate interface in the region reported from recent offshore seismic observations.

Repeating large earthquakes (M >= 7), waveforms for which are nearly identical, have been identified only on the Mexican subduction thrust near Acapulco. These earthquakes occurred on 1962 (Ms 7.0) and 2021 (Ms 7.0, Mw 7.0). Here, we report on two more sequences of three repeating large earthquakes each in eastern and western Oaxaca, Mexico. The repeating earthquakes in eastern Oaxaca occurred on 23 March 1928 (Ms 7.5), 1965 (Ms 7.6, Mw 7.5), and 2020 (Ms 7.4, Mw 7.4), and in western Oaxaca on 4 August 1928 (Ms 7.4), 1968 (Ms 7.2, Mw 7.3), and 2018 (Ms 7.2, Mw 7.2). Galitzin seismograms of the earthquakes in each sequence at DeBilt, The Netherlands or at Strasbourg, France are strikingly similar for at least 2600 s after the P-wave arrival. Similarity of waveforms of earthquakes in each sequence and tests with seismograms of events locations for which are accurately known suggest that their source areas were less than 10-20 km of each other. Moment-rate functions of these events are remarkably simple. We also document quasi-repeating earthquakes in central Oaxaca on 17 June 1928 (Ms 7.6) and 29 November 1978 (Ms 7.6, Mw 7.6). Such events have similar locations with large overlap in primary slip but are not identical. Recently, Michoacan-Colima earthquakes of 1973 (Ms 7.5, Mw 7.6) and 2022 (Ms 7.6, Mw 7.6) were reported as quasi-repeaters. Repeating or quasi-repeating large earthquakes imply that they are known for all the other events in the sequence if we know the location and gross source parameters of one of them. This permits the estimation of recurrence periods and the delineation of seismic gaps with greater confidence. Repeating and quasi-repeating large earthquakes in Oaxaca, an unique observation, shed new light on seismic hazard of the region, provide further support for the characteristic earthquake model, and reveal remarkably persistent behavior of ruptures through multiple earthquake cycles.

On 11 May 2023 a local earthquake in Mexico City was felt very strongly in Mixcoac, San Angel, and Coyoacán. The event was part of a seismic sequence that had begun about 6 months earlier. Peak Ground Acceleration (PGA) at the closest station (distance ~ 1 km), located in the hill zone, was ~ 0.18 g. Although the response spectrum at short periods at this station exceeded the design spectrum specified in the Mexico City´s Building Code, no structural damage was reported. Moment tensor inversion of bandpass filtered (0.08 – 0.24 Hz) displacement records yields M0 = 6.8x1013 N-m (Mw 3.2), H = 0.7 km, and the likely fault plane characterized by φ = 2700, δ = 760,On 11 May 2023 a local earthquake in Mexico City was felt very strongly in Mixcoac, San Angel, and Coyoacán. The event was part of a seismic sequence that had begun about 6 months earlier. Peak Ground Acceleration (PGA) at the closest station (distance ~ 1 km), located in the hill zone, was ~ 0.18 g. Although the response spectrum at short periods at this station exceeded the design spectrum specified in the Mexico City´s Building Code, no structural damage was reported. Moment tensor inversion of bandpass filtered (0.08 – 0.24 Hz) displacement records yields M0 = 6.8×1013 N-m (Mw 3.2), H = 0.7 km, and the likely fault plane characterized by φ = 2700, δ = 760, λ = -750. These source characteristics are very similar to those estimated for the 17 July 2019 earthquake which occurred during a swarm-like seismic activity about 5 km to the north. Spectral analysis of recordings at 19 sites in the hill zone, 14 in the transition zone, and 41 in the lake-bed zone reveals great variability of the ground motion within each of the zones. Estimated stress drop, Δσ, is 0.5 MPa. A large disparity is found between the observed source spectrum and theoretical source spectrum; their ratio provides an estimation of the amplification of seismic waves as they travel through the layers of decreasing velocity at shallower depth. We denote this ratio as the site effect. Predicted PGA and PGV for an Mw 3.2 earthquake, computed using stochastic technique (Boore 1983, 2003), assuming a Brune ω-2 source, Δσ = 0.5 MPa and including the site effect, are in reasonable agreement with the observations. Expected PGA and PGV at the epicenter of a postulated Mw 5 earthquake are 0.6 g and 60 cm/s at a generic hill-zone site; the expected values are twice as large in the lake-bed zone. These predictions should, however, be taken with caution as they are based on several approximations.

We study two moderate earthquakes that occurred offshore the State of Veracruz, in the southwestern Gulf of Mexico, on 29 October 2009 (Mw 5.7) and 4 August 2021 (Mw 4.8). The former was located near the town of Alvarado and latter near the city of Veracruz. The events were well recorded by accelerographs and seismographs at local and regional distances. W-phase regional centroid moment tensor inversion reveals that they had reverse-faulting mechanism, similar to several other earthquakes in the southwestern Gulf of Mexico. Of the seven focal mechanisms now available along the southwestern margin, two are strike slip and the rest are of thrust type, suggesting a heterogeneous stress regime. We take advantage of local and regional recordings produced by these two earthquakes to study the characteristics of the ground motion. Source spectra computed at each station separately (without correcting for the site effect), assuming a reasonable geometrical spreading and Q = 141f 0.63, show remarkably high variability due to difference in path and local site effects. The geometric mean apparent source spectrum (source spectrum including site effects) of both earthquakes may be modeled by an ω2 -Brune source model with a stress drop, Δσ, of 40 MPa. These source spectra, along with the application of stochastic method, yield peak ground acceleration (PGA) and velocity (PGV) as a function of distance in general agreement with the observations. Of greater practical importance is the ground motion at sedimentary sites in the city of Veracruz and at the Laguna Verde Nuclear Power Plant (LVNPP) site, especially from a postulated Mw 6.5 earthquake which is a reasonable scenario event for the region. For the city of Veracruz and LVNPP we estimate site effect with respect to the ω2 -Brune source with Δσ of 2 MPa. There is some support for this Δσ. We apply both stochastic and empirical Green’s functions (EGF) techniques in the estimation of the ground motion. The recording of the 2021 earthquake is taken as the EGF, and Δσ of the EGF and the target event are assumed to be the same and equal to 2 MPa. The predicted PGA and PGV at sedimentary sites in the city of Veracruz and LVNPP above the hypocenter (depth = 20 km) from the postulated Mw 6.5 earthquake are 0.2 g and 10 cm/s and 0.18 g and 3 cm/s, respectively. These results are preliminary as they are based on several assumptions.

ABSTRACTA 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.

El sismo de Michoacán-Colima el 19 de septiembre de 2022 (Ms 7.6, Mw 7.6) rompió el límite NW de la interface entre las placas de Cocos y norteamericana, causando daño severo a muchas poblados y ciudades en los estados de Michoacán y Colima. El daño fue además agravado por una réplica de magnitud importante (Mw 6.7) el 22 de septiembre. El sismo principal inició debajo de la costa a una distancia hipocentral de 22 km de la estación sísmica de Maruata (MMIG) donde las aceleraciones y velocidades máximas registradas, PGA y PGV, fueron de 1g y 28 cm/s, respectivamente. El epicentro de la réplica más grande se localizó a ~30 km al SE del sismo principal. El modelado de falla finita del sismo principal presentado por el Servicio Geológico de los Estados Unidos (USGS), revela una propagación de la ruptura a lo largo del rumbo de la falla hacia la dirección NW con una caída de esfuerzos estáticos Δσs, of 3.7 MPa. Nuestra estimación de energía radiada, ER, es 3.44x1015J, de tal manera que ER /M0 es de 1.27 × 10−5 valor similar al calculado para otros grandes sismos de subducción cuyas área de ruptura no se extienden hacia la trinchera. El área que contiene las réplicas del sismo principal de 2022 se traslapa con el área de réplicas del sismo del 30 de enero de 1973 (Mw7.6). Los sismográmas Galitzin de los dos sismos registrados en la estación DeBilt (DBN) localizada en los Países Bajos son razonablemente similares de tal manera que pueden ser clasificados como eventos quasi-repetidos. Por otro lado, el sismograma DBN del sismo del 15 de abril de 1941 (MS 7.7), cuya localización no se conoce bien del todo, aunque se sabe que ocurre en la misma región, difiere sustancialmente de los sismogramas de 1972 y 2022, sugiriendo que el primero rompió un área diferente de la del sismo de 1941. Un análisis extensivo de registros regionales exhibe el efecto de directividad observada en los datos de movimientos fuertes y en los cocientes de aceleraciones del sismo principal y de las aceleraciones de la réplica mayor. La directividad explica la dependencia azimutal observada en los cocientes de PGA y PGV, los cocientes espectrales, la distribución de PGA y la respuesta espectral a 2s Sa (T = 2 s). Debido a la directividad, los valores de PGA, PGV y Sa (T = 2 s) en el Valle de México durante el sismo principal y la réplica mayor fueron muy similares a pesar de la diferencia en magnitud de 0.9. En CU (el sitio de roca firme de referencia en la Ciudad de México), PGA y PGV durante ambos eventos fueron de ~ 6 cm/s2 and 2 cm/s, respectivamente, valores más bajos que los esperados para el sismo principal y más altos que los esperados para la réplica mayor.

We use accelerograms, seismograms, and data from sparse continuous Global Positioning System (GPS) and campaign-mode GPS stations, deployed along the Pacific coast of Mexico, to study scaling of horizontal peak ground displacement (PGD) with seismic moment (M-0) in the epicentral zone above the Mexican subduction thrust. The thrust interface is located at a depth of similar to 25 km below the coast. We select recordings with (S-P) time <= 5.9 s (R <= 46 km) and reduce the amplitudes to (S-P) time of 3.2 s (R = 25 km). The dataset consists of 58 events and covers a M-0 range of 10(13)-10(21) N.m. We find that the double integration of accelerograms, using piecewise linear detrending schemes, leads to sufficiently accurate estimation of PGD to study the scaling relation. The sparse data for great earthquakes are complemented with theoretical static displacement computed using the model of Okada (1992). For earthquakes with M-0 <= 1.26x10(18) N.m (M-w <= 6.0) the point-source, far-field approximation holds, and the PGD data follows theoretically expected M-0(2/3) scaling. For great earthquakes (M-0 > 1.26x10(21) N.m; M-w > 8.0), static offset (which is approximately equivalent to PGD) scales as M-0(1/3). About twothirds of the observed PGD data fall within a factor of 0.67 and 1.5 of the relation given above. The relationship may be useful in earthquake engineering as well as for rapid estimation of magnitude for early tsunami alert.

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).

A reliable estimation of seismic hazard-facing Mexico City from local earthquakes has suffered from poor seismic instrumentation, complex crustal structure, large and variable site amplification, and lack of knowledge of recurrence period of earthquakes on the mapped faults. Owing to recent improvement in local seismic networks, an earthquake swarm activity, which occurred in June-August 2019, was well recorded. The largest event of the sequence, an M-w 3.2 earthquake, caused panic in the city and produced peak ground acceleration (PGA) exceeding 0.3g at the closest station (MHVM) about 1 km away. An analysis of the event shows that it had normal-faulting focal mechanism, consistent with northeast-southwest-oriented mapped faults in the region. It was located at a depth of similar to 1 km and had a low stress drop (similar to 0.1 MPa). We find that the high PGA for this low stress-drop event resulted from high-frequency amplification at MHVM (about factor of similar to 6 around 13 Hz), likely due to topographic site effects, superimposed on a pervasive broadband amplification of seismic waves at hill-zone sites in the Valley of Mexico (up to similar to 10 in the frequency band of 0.2-10 Hz). Simulation of ground motion for a scenario M-w 5.0 earthquake, using an empirical Green's function technique, reveals that such an event may give rise to significant seismic intensities in the lake-bed zone of Mexico City. The results emphasize the need to re-evaluate the seismic hazard to Mexico City from local crustal earthquakes in the Valley of Mexico.

El patrón y nivel de daño en la región de México Central durante el sismo de septiembre de 2017 Mw7.1 en Morelos-Puebla son diferentes a los observados durante el sismo de Tehuacán (Mw6.9) en 1999 a pesar de que ambos sismos intraplaca tienen magnitudes similares y profundidades focales comparables 57 km y 60 km, respectivamente. El sismo de 2017 causo claramente mas daño en la región de México Central. Los epicentros de ambos eventos están separados 127 km. Mediante el análisis de los registros sísmicos de México Central encontramos que el área expuesta a diferentes niveles de aceleración máxima del suelo y velocidad máxima del suelo es comparable para los dos eventos. Por ejemplo, el área expuesta a aceleraciones máximas del suelo mayores a 150 cm/s? es de 12,700 km? para el sismo de 1999 y 15400 km? para el sismo de 2017. La forma de los contornos de intensidades y localización epicentral sugiere una ruptura bilateral para el evento de 2017 y una ruptura con directividad hacia el norte para el sismo de 1999. Los cocientes espectrales para los dos eventos revelaron una fuente más energética hacia el norte para el sismo de 1999 que para el sismo de 2017 lo cual es consistente con resultados reportados previamente de directividad en la ruptura. Se concluye que la distinta localización de los dos eventos junto con la diferente distribución de las poblaciones, monumentos históricos y el incremento de población desde 1999 fueron las principales causas de la diferencia de los daños entre los dos eventos.

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.

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 M-w 7.5 Ometepec-Pinotepa Nacional, the 2014 M-w 7.2 Papanoa, and the 2018 M-w 7.2 Pinotepa Nacional earthquakes, as well as of their M >= 4.0 aftershocks to estimate their seismic moment M-0 and radiated seismic energy E-S. 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) over tilde) = log(E-S/M-0), 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) over tilde) (-4.64 + 0.25 and -4.62 + 0.25), whereas the other one shows low values of log((e) over tilde) (-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) over tilde) is spatially heterogeneous along the trench, suggesting variations on the shear strength and coupling at the interface.

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.

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, M w 5.3), the focal mechanism (φ = 280°, δ = 14°, λ = 84°), the source radius (a = 1.3 km), and the static stress drop (Δσ s ~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 ≤ M w  ≤ 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.