SUMMARY The rate of earthquakes with magnitudes Mw ≤ 7.5 in the Ometepec segment of the Mexican subduction zone is relatively high as compared to the neighbouring regions of Oaxaca and Guerrero. Although the reason is not well understood, it has been reported that these earthquakes give rise to a large number of aftershocks. Our study of the aftershock sequence of the 2012 Mw7.4 Ometepec thrust earthquake suggests that it is most likely due to two dominant factors: (1) the presence of an anomalously high quantity of overpressured fluids near the plate interface and (2) the roughness of the plate interface. More than 5400 aftershocks were manually detected during the first 10 d following the 2012 earthquake. Locations were obtained for 2419 events (with duration magnitudes Md ≥ 1.5). This is clearly an unusually high number of aftershocks for an earthquake of this magnitude. Furthermore, we generated a more complete catalogue, using an unsupervised fingerprint technique, to detect more smaller events (15 593 within 1 month following the main shock). For this catalogue, a high b-value of 1.50 ± 0.10 suggests the presence of fluid release during the aftershock sequence. A low p-value (0.37 ± 0.12) of the Omori law reveals a slow decaying aftershock sequence. The temporal distribution of aftershocks shows peaks of activity with two dominant periods of 12 and 24 hr that correlate with the Earth tides. To explain these observations, we suggest that the 2012 aftershock sequence is associated with the presence of overpressured fluids and/or a heterogeneous and irregular plate interface related to the subduction of the neighbouring seamounts. High fluid content has independently been inferred by magnetotelluric surveys and deduced from heat-flow measurements in the region. The presence of fluids in the region has also been proposed to explain the occurrence of slow-slip events, low-frequency earthquakes and tectonic tremors.
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.
Abstract The Acapulco earthquake of 2021 broke a segment of the southeast Guerrero seismic gap along the Mexican subduction thrust. The rupture initiated offshore Acapulco (16.770° N, 99.942° W) and propagated down-dip toward northeast. This source directivity is confirmed from both (1) an analysis of local and regional recordings as a function of azimuth and (2) kinematic inversion of near-source, band-pass filtered (0.025–0.5 Hz) displacement seismograms and Global Positioning System static coseismic displacement vectors. The inversion reveals little slip near the hypocenter (<0.5 m) and significant slip distributed over an area of ∼184 km2, with the large slip patches in the northeast part of the fault. The estimated average slip and static stress drop are 260 cm and 18.6 MPa, respectively. Moment rate function reported by National Earthquake Information Center–U.S. Geological Survey from finite-fault modeling is simple, and it resembles other Mexican subduction earthquakes in the 7.0 ≤ M ≤ 7.5 range. Moment rate spectrum is well fit by the Brune ω−2 source model. Radiated seismic energy from teleseismic P waves is 7.5×1014 J, and ER/M0 is 2.1×10−5. Radiated energy enhancement factor—a measure of source complexity—is small, 5.8, similar to other Mexican subduction thrust earthquakes. Seismograms at DeBilt of the 2021 and the 11 May 1962 Acapulco earthquakes show an extraordinary similarity, seldom seen at M 7.0 level. The 2021 earthquake seems a repeat of the 1962 earthquake. The slip deficit since 1962 corresponding to a plate convergence rate of 6.2 cm/yr and perfect coupling is 366 cm. Thus, the seismic slip of 260 cm during that 2021 earthquake suggests a coupling ratio of 0.7, greater than 0.3 and 0.5 reported from geodetic measurements. Large moment release in the southeast seismic gap appears to have a periodicity of ~60 yr. Because 60 yr have elapsed since the last sequence earthquakes (1957 MS 7.5; 1962 MS 7.0 and 6.8), a renewal of large earthquakes in the region may be expected.
A seismic swarm began on August 26, 2020 in the Bransfield Basin (South Shetland Islands-Antarctica).The King George Island is part of the South Shetland microplate situated in a complex tectonic context (to the North, East, West and South the South Shetland -Phoenix subduction zone, the Shackleton and Hero fracture zones, and the Brasfield back-arc basin are located).The Observatorio Geofísico del Uruguay has installed since March 2020 a seismological station (raspberry-shake 3D, AM.R4DE2) at the Artigas Antarctic Scientific Base (BCAA: -62.1802, -58.8853).The station was operating from March 8 to 13 and began recording again on August 8, 2020.The only months that have been able to be reviewed in their entirety have been August 2020 and January 2021.Additionally, due to the existence of some gaps (time without data), not all earthquakes were recorded in the seismograms, this could be the product of instrumental errors or data transmission system lapses through the internet.The seismic records were parameterized by their waveform and S-P arrival times.The seismic source distance was calculated according to the arrival times of the P and S phases using the Bransfield basin velocity model proposed by Robertson et al. (2003) recalculated in Loureiro Olivet et al. (2021 under review) who obtained a value of 6.11 km/s for Vp and 6.55 for the Omori coefficient.The epicentral distance and depth were calculated considering the angle of incidence of the P wave obtained from particle motion analysis and the hypocentral distance.The angle of incidence was calculated using the formula of Wiechert (1907).For the localization we use single station observations which require the calculation of the backazimuth for which the amplitudes of the first P wave were considered.From this seismic swarm, 5652 events have been parameterized to date, of which 815 present ML ≥ 4. From the 5652 events reviewed manually, 757 correspond to August, 3119 to September, 222 to October, 569 to November, 290 to December, 206 to January, 152 to February, 27 to March, 268 to April and 42 to May.From the 815 earthquakes of ML ≥ 4, 49 correspond to August, 217 to September, 78 to October, 218 to November, 47 to December, 65 to January, 33 to February, 8 to March, 85 to April and 15 to May.The average depth obtained is approximately 11 km while the epicentral distance is ca. 25 km.According to the epicentral location, most of the seismic events occur in the upper plate being related to the movement of active normal faults of ENE attitude that limit the Bransfield basin.In addition, epicenters near surface faults with NNW directions have been recorded within King George Island and near the NNW Artigas fault, which limits two blocks of different cortical thickness within the South Shetland microplate.Also, some events have been located near the caldera of the Orca volcano in the Bransfield basin.