Víctor H. Espíndola’s research while affiliated with National Autonomous University of Mexico and other places

The first recorded moderate-intensity mainshock and largest aftershock in the boundary between the Basin and Range and Sierra Madre Oriental provinces are presented. This seismological evidence demonstrates the presence of active faulting in northeastern Mexico. We estimated seismic source parameters and surface deformation related to a low-moderate seismic sequence that occurred from May 11 to August 2, 2023 (3.4 ≤ Md ≤ 5) in the limits of the San Luis Potosí, Nuevo Leon ´ and Tamaulipas states through waveform inversion of the seismic moment tensor and InSAR analysis. The event of May 16, 2023 (Md 5.0, Mw 4.92) is the biggest earthquake instrumentally recorded in this region. The mainshock and the largest aftershock (June 5, 2023; Md 4.9, Mw 4.86) earthquakes were well-recorded over 100–330 km by broadband stations installed in central and northeastern Mexico. Here, it is suggested that both earthquakes were associated with the rupture of El Corcovado normal fault. We estimated a rupture area of approximately 11 km2 with an average displacement of 7 cm. Fault plane solutions of the mainshock were strike 358◦, dip 39◦, rake − 90◦, and Mo 2.685e+16 Nm. El Corcovado fault is associated with a set of normal faults NNW-SSE trending located along the boundary between the Basin and Range and the Sierra Madre Oriental provinces. Such considerations lead to a review of the neotectonic setting of northeastern Mexico and the associated seismic hazard assessment.

In February 2023, a long seismic sequence began in western Mexico City causing widespread panic and some damage to housing infrastructure. On May 11 and December 14, two Mw3.2 mainshocks occurred at less than 700 m depth. Unprecedented satellite interferograms captured tectonic deformations in the two epicentral zones during the days surrounding the earthquakes. Data analysis revealed extended slip with maximum values around 8 cm on two sub-parallel east-west trending normal faults 800 m apart: namely the Barranca del Muerto (BM) fault to the south and the Mixcoac fault to the north. Detailed microseismicity analysis showed that 95% of the slip on the BM fault was aseismic and initiated at least 6 days before the May 11 earthquake on the main asperity, located 1 km east of the hypocenter and ~1.2 km deep. For the December event on the Mixcoac fault, ~70% of the slip was also aseismic but shallower (mostly above 600 m), which can be partially explained by the induced stresses on that fault due to the May slip on the BM fault. A quantitative geomorphological analysis allowed to establish the structural connection between both buried faults and their geomorphic expression to the west, with surface extensions of ~3.5 and ~4.5 km in the hilly area—where the most intense seismicity concentrates. The spatiotemporal patterns of fast and slow earthquakes suggest that the seismotectonics west of the city comprises two mechanically distinct zones: a stable region prone to aseismic deformation to the east where faults are buried under water-rich sediments, and an unstable region to the west, prone to seismic radiation where faults are expressed geomorphologically. Thus, the seismic swarms in this area appear to result from the regional extensional regime, the stresses induced by slow slip on the eastern fault segments and interaction between these faults.

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.

This study presents seismological evidence that the northern sector of the front of the Salient of Monterrey, Sierra Madre Oriental, in northeastern México, is nowadays under tectonic compressional stress. This is derived through the seismic moment tensor inversion of two earthquakes related to a small shallow seismic sequence of four events that occurred from March 16 to 23, 2022, in southern Monterrey. The main earthquake of March 16 (Mw3.69) was felt in some cities of the Monterrey Metropolitan area. The thrust fault mechanisms of the March 16 and March 20 (Mw3.25) earthquakes had depths of 4 and 7 km, respectively, and could be linked with the reactivation of thrust faults below the boundary between the detachment horizon and the basement, which is correlated with geological evidence. For the March 16 earthquake, the nodal fault plane has the following values: strike = 158°, dip = 66°, and rake = 87°, and for the March 20 earthquake, the values were the following: strike = 146°, dip = 82°, and rake 42. These values coincide with the regional trend of the front of the Salient of Monterrey. For this area, recent seismicity recorded at the front of the fold–thrust belt shows a neotectonic compression process.

Michoacán-Colima earthquake of 19 September 2022 (Ms 7.6, Mw 7.6) ruptured the NW end of the Cocos-North American plate interface, causing severe damage to many towns and cities in the states of Michoacán and Colima. The damage was further exacerbated by a major aftershock (Mw 6.7) on 22 September. The mainshock initiated below the coast at a hypocentral distance of 22 km from the seismic station of Maruata (MMIG) where peak ground acceleration and velocity, PGA and PGV, of ~ 1 g and 28 cm/s were recorded. The epicenter of the major aftershock was located ~ 30 km SE of the mainshock. Finite fault modeling of the mainshock by the U.S. Geological Survey reveals a rupture propagation along the strike towards the NW and yields a static stress drop, Δσs, of 3.7 MPa. Our estimated radiated energy, ER, is 3.44x1015J, so that ER /M0 is 1.27 × 10−5 similar to other large Mexican thrust earthquakes whose rupture areas do not extend to the trench. Aftershocks of the 2022 mainshock overlap that of the Colima earthquake of 30 January 1973 (Mw7.6). Galitzin seismograms of the two earthquakes at DeBilt (DBN), The Netherlands, are reasonably similar so that they may be classified as quasi-repeated events. On the other hand, the DBN seismogram of the earthquake of 15 April 1941 (MS 7.7), whose location is poorly known but occurred in the same region, differs greatly from those of the 1973 and 2022 earthquakes, suggesting a different source area for the 1941 event. An analysis of the extensive regional recordings exhibits the effect of the directivity on the ground motion and on the ratio of ground motion during the mainshock to the major aftershock. The directivity explains the observed azimuthal dependence of PGA and PGV ratios, spectral ratios, and PGA and response spectra at 2s, Sa (T = 2 s). Because of the directivity, PGA, PGV, and Sa (T = 2 s) in the Valley of Mexico during the mainshock and the major aftershock were about the same in spite of the magnitude difference of 0.9. At CU (the reference, hard site in Mexico City), PGA and PGV during both events were ~ 6 cm/s2 and 2 cm/s, respectively, lower than expected for the mainshock and higher than expected for the aftershock.

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.

The subduction zone in southern Colombia and northern Ecuador has been the site of significant events in the area that broke the great 1906 earthquake in Ecuador. The 2007 earthquake on Gorgona Island is located within this area, presenting a normal-type focal mechanism and a moment magnitude Mw 6.8. This event is due to the compressive stresses exerted on the continental plate of South America by the Nazca oceanic plate. We performed a kinematic inversion source process of the Gorgona earthquake. Our results show the main slip patch at the hypocenter and a secondary slip release deeper at SW on the fault plane, which agrees with the location of aftershocks; the source time function obtained reflects these energy releases with a total duration of 15 s. We also analyzed the stress state field in this region using the focal mechanism of its early aftershock sequence, obtaining an extensive regime with an azimuth Shmin of 108° and NW-SE direction. The average interseismic coupling in this convergent margin zone is not so high as in the southern areas. Nevertheless, observations of this kind of seismicity and GPS studies in the upper plate of a subduction zone are essential in understanding earthquake cycles and, perhaps, in anticipating slip distributions in future subduction events.

The world experienced the beginning of the COVID-19 pandemic by the end of 2019 to the beginning of 2020. Governments implemented strategies to contain it, most based on lockdowns. Mexico was no exception. The lockdown was initiated in March 2020, and with it, a reduction in the seismic noise level was witnessed by the seismic stations of the national and Valley of Mexico networks. Stations located in municipalities with more than 50 000 people usually experience larger seismic noise levels at frequencies between 1 and 5 Hz, associated with human activity. The largest noise levels are recorded in Mexico City, which has the largest population in the country. The largest drop was observed in Hermosillo, Sonora; however, it was also the city with the fastest return to activities, which seems to correlate with a quick increase in confirmed COVID-19 cases. Mexico initiated a traffic-light system to modulate the re-opening of economic activities for each state. Therefore, since 1 June, noise levels have generally reflected the colour of the state traffic light. Furthermore, the reduction in the noise level at seismic stations has allowed identification of smaller earthquakes without signal processing. Also, people in cities have perceived smaller or more distant quakes.

The world experienced the beginning of the COVID-19 pandemic by the end of 2019, beginning of 2020. Governments implemented strategies to contain it, most based on lockdowns. Mexico was not the exception. The lockdown was initiated in March 2020 and with it, a reduction on the seismic noise level was witnessed by the seismic stations of the national and the Valley of Mexico networks. Stations located in municipalities with more than 50,000 people usually experience larger seismic noise levels at frequencies between 1 to 5 Hz, associated with human activity. The largest noise levels are recorded in Mexico City, with the largest population in the country. The largest drop was observed in Hermosillo, Sonora, however, it was also the city with the fastest return to activities, which seems to correlate with a quick increase in confirmed COVID-19 cases. Mexico initiated a traffic-light system to modulate the re-opening of economic activities for each state. Therefore, since 1 June, noise levels reflect, in general, the colour of the state traffic light. Furthermore, the reduction in the noise level at seismic stations has allowed identification of smaller earthquakes without signal processing. Also, people in cities have perceived smaller or distant quakes.