Xyoli Pérez-Campos

Subducting slab ultra-slow velocity layer coincident with silent earthquakes in southern Mexico.

Seismic mapping suggests that silent earthquakes may be related to an ultralow velocity layer on top of a subducting slab. Hot Silent Quakes Subduction zones tend to produce the largest and potentially most destructive earthquakes. Recent observations show that some deformation in several subduction zones seems to be occurring through small or “silent” quakes. The origin of these silent quakes, and their effect on the seismic hazard, is uncertain. Song et al. (p. 502) use a specific seismic signal to map out thin regions with low seismic velocities on the subduction zone beneath southern Mexico. The regions seem to occur at depths below the seismogenic zone where temperatures are higher. These high temperatures and the silent quakes may reflect the release and episodic trapping of fluids from metamorphic reactions. Great earthquakes have repeatedly occurred on the plate interface in a few shallow-dipping subduction zones where the subducting and overriding plates are strongly locked. Silent earthquakes (or slow slip events) were recently discovered at the down-dip extension of the locked zone and interact with the earthquake cycle. Here, we show that locally observed converted SP arrivals and teleseismic underside reflections that sample the top of the subducting plate in southern Mexico reveal that the ultra-slow velocity layer (USL) varies spatially (3 to 5 kilometers, with an S-wave velocity of ~2.0 to 2.7 kilometers per second). Most slow slip patches coincide with the presence of the USL, and they are bounded by the absence of the USL. The extent of the USL delineates the zone of transitional frictional behavior.

Nonvolcanic tremor observed in the Mexican subduction zone

Nonvolcanic tremor (NVT) activity is revealed as episodes of higher spectral amplitude at 1–8 Hz in daily spectrograms from the continuous seismological records in Guerrero, Mexico. The analyzed data cover a period of 2001–2007 when in 2001–2002 a large slow slip event (SSE) had occurred in the Guerrero-Oaxaca region, and then a new large SSE occurred in 2006. The tremor burst is dominated by S-waves. More than 100 strong NVT bursts were recorded in the narrow band of ~40 × 150 km^2 to the south of Iguala City and parallel to the coastline. Depths of NVT hypocenters are mostly scattered in the continental crust between 5 and 40 km depth. Tremor activity is higher during the 2001–2002 and 2006 SSE compared with that for the “quiet” period of 2003–2005. While resistivity pattern in Guerrero does not correlate directly with the NVT distribution, gravity and magnetic anomaly modeling favors a hypothesis that the NVT is apparently related to the dehydration and serpentinization processes.

An apparent mechanism dependence of radiated seismic energy

We develop an extension to the method of Boatwright and Choy [1986] for determining the radiated seismic energy Es that accounts for factors that bias the estimate. We apply our technique to 204 events worldwide during the period 1992–1999 and find that the apparent stress is on average largest for strike-slip events (0.70 MPa), while for reverse and normal events it is significantly smaller (0.15 and 0.25 MPa, respectively). These results support the mechanism dependence of Es reported by Choy and Boatwright [1995], although we find that once likely sources of bias are accounted for, the mechanism dependence is not as strong as found previously. The source of the mechanism dependence is unclear, but one possibility is that it reflects a mechanism-dependent difference in the stress drop. This hypothesis is suggested by the scaling of slip with width in large strike-slip earthquakes and makes two predictions, which could be used to test it. The first is that the discrepancy should disappear for the very largest dip-slip earthquakes as the length of the fault greatly exceeds the downdip extent. The second is that the discrepancy ought to disappear for smaller earthquakes. The first can not yet be tested due to a lack of recent, very large dip-slip earthquakes. The second is supported by the lack of mechanism dependence to Es for smaller earthquakes. An alternative hypothesis is that the apparent mechanism dependence could result if faults are opaque during rupture, blocking seismic radiation across them [Brune, 1996]. This could cause radiated seismic energy to be trapped preferentially in the crust near the source volume for dipping faults. There remains, however, a large discrepancy between estimates of Es obtained from teleseismic versus regional data. This discrepancy indicates a problem with teleseismic and/or regional estimates of the seismic energy and must be resolved before a definite conclusion can be drawn.

S wave velocity structure below central Mexico using high‐resolution surface wave tomography

Shear wave velocity of the crust below central Mexico is estimated using surface wave dispersion measurements from regional earthquakes recorded on a dense, 500 km long linear seismic network. Vertical components of regional records from 90 well-located earthquakes were used to compute Rayleigh-wave group-velocity dispersion curves. A tomographic inversion, with high resolution in a zone close to the array, obtained for periods between 5 and 50 s reveals significant differences relative to a reference model, especially at larger periods (>30 s). A 2-D S wave velocity model is obtained from the inversion of local dispersion curves that were reconstructed from the tomographic solutions. The results show large differences, especially in the lower crust, among back-arc, volcanic arc, and fore-arc regions; they also show a well-resolved low-velocity zone just below the active part of the Trans Mexican Volcanic Belt (TMVB) suggesting the presence of a mantle wedge. Low densities in the back arc, inferred from the low shear wave velocities, can provide isostatic support for the TMVB.

A Source Study of the Bhuj, India, Earthquake of 26 January 2001 (Mw 7.6)

We study the source time function (STF) and radiated seismic energy (E_R) of the M_w 7.6 Bhuj earthquake using the empirical Greens function (EGF) technique. Our estimations of the STF and E_R are based on teleseismic P waves and regional seismograms, respectively. We find that the STFs as a function of azimuth have a similar shape and nearly constant duration of 18 sec. This suggests that the rupture propagation was essentially radial. The STFs show a sharp rise in the first 6 sec. The E_R estimated from the EGF technique is 2.1 × 10^(23) erg. We find that E_Rs computed from integration of corrected velocity-squared spectra of teleseismic P waves and regional seismograms are in excellent agreement with the ER obtained from the EGF technique. Since the seismic moment, M_0, is 3.4 × 10^(27) dyne cm, we obtain E_R/M_0 = 6.2 × 10^(-5). The radiation efficiency, η_R, during the Bhuj earthquake was low, about 0.23. The sharp rise of the STFs and η_R = 0.23 can be explained by Sato and Hirasawas (1973) quasi-dynamic, circular source model with an effective stress of ∼ 300 bar and the ratio of rupture to shear-wave velocity, V_R/β, of ∼ 0.5. The corresponding estimate of slip velocity at the center of the fault is 156 cm/sec. V_R/β ∼ 0.5 is in reasonable agreement with the duration of the STF and the reported dimension of the aftershocks, as well as with the results of inversion of teleseismic body waves. The observations may also be explained by a frictional sliding model, with gradual frictional stress drop and significant dissipation of energy on the fault plane. This model requires an average dynamic stress drop of about 120 bar and V_R/β ∼ 0.7 to explain both the rapid rise in the first 6 sec of the STFs and, along with a static stress drop of 200 bar, the observed E_R/M_0. High static stress drop is a common feature of most crustal earthquakes in stable continental regions. An examination of the available data, however, does not suggest that most of them also have relatively low radiation efficiency.

Reconciling Teleseismic and Regional Estimates of Seismic Energy

Estimates of the radiated seismic energy based on teleseismic and regional data often differ by up to an order of magnitude, with a tendency for regional estimates to be larger than teleseismic estimates for the same event. In this study we compare the velocity spectrum determined from teleseismic data after correction for radiation pattern and propagation effects, with the velocity spectrum determined from regional data, after the corresponding corrections, for nine earthquakes in the Middle America subduction zone of Mexico. This comparison of the corrected spectra is used to identify and reduce the sources of the regional versus teleseismic energy discrepancy, which is about an order of magnitude for these events. We find that the teleseismic attenuation operator needs to be calibrated. In our case, for the tectonic environment of the Mexican subduction zone, we need a teleseismic attenuation operator that is stronger at high frequencies than the global average. A larger factor, however, is the correction needed to account for site amplification. This correction has an impact on both regional and teleseismic data, but it has a larger influence on the regional estimates because the angle of incidence for teleseismic waves is steep and the stations are located on more competent rock. By modifying the teleseismic attenuation operator and applying site corrections based on a generic site model, we essentially eliminate the order-of-magnitude discrepancy between teleseismic and regional estimates of the radiated seismic energy for these events.

Comparative study of tectonic tremor locations: Characterization of slow earthquakes in Guerrero, Mexico

Deep tectonic tremor in Guerrero, Mexico, has been observed using dense temporal seismic networks (i.e., the Meso-American Subduction Experiment and Guerrero Gap Experiment (G-GAP) arrays) during two different time periods. We apply a set of seismic waveform analysis methods to these data sets to constrain the locations of tremors and determine the associated moment tensors. First we detect and locate the tremors. Next, very low frequency (VLF) signals are identified by stacking waveform data during tremor bursts, and their moment tensors are determined. Finally, to better investigate the link between tremors and VLF earthquakes, we detect VLF events using a matched filtering algorithm to search continuous seismic records. None of the 11 VLF events detected by this method occurred in the absence of tremor bursts suggesting they are indeed part of the same phenomena. Unlike previous investigations, our results for the G-GAP period reveal that downdip tremor activity (i.e., in the so-called “sweet spot”) is segmented into two patches separated by 40 km in the along-trench direction, indicating possible variations in the geometry of the plate interface and/or slab effective pressure. Moment tensors of VLF signals are consistent with shear slip on the near-horizontal plate interface, but source depths are about 5 km deeper than the established plate interface. The slip directions of the VLF events are slightly (~10°) counterclockwise of the plate convergence direction, indicating that strain energy promoting left-lateral strike-slip motion may accumulate in the continental crust during the interseismic period.

Intraslab versus Interplate Earthquakes as Recorded in Mexico City: Implications for Seismic Hazard

We study the relative importance of interplate and intraslab earthquakes in the seismic hazard of Mexico City by analyzing accelerograms recorded at the hill-zone site of CU (1964–2012) and the lake-bed site of SCT (1985–2012). Amax exceeded 6 gal during 20 earthquakes at CU during this period. Of these, eight were intraslab events so that the exceedance rate of Amax ≥ 6 gal from both types of earthquakes is roughly about the same. The estimated return period of Amax of 30 gal from the two types of earthquakes is ∼100 yrs. If we consider high-frequency (2.5–8.5 Hz) acceleration (AmaxHF) at CU, then the top 7 out of the 20 events are all intraslab earthquakes. Even at the lake-bed site of SCT, the AmaxHF values are, generally, associated with intraslab earthquakes. It follows that the risk from both types of earthquakes to low-rise construction in the city needs careful assessment.

Bend Faulting at the Edge of a Flat Slab: The 2017 Mw7.1 Puebla-Morelos, Mexico Earthquake

We present results of a slip model from joint inversion of strong motion and static GPS data for the Mw7.1 Puebla-Morelos earthquake. We find that the earthquake nucleates at the bottom of the oceanic crust or within the oceanic mantle with most of the moment release occurring within the oceanic mantle. Given its location at the edge of the flat slab the earthquake is likely the result of bending stresses occurring at the transition from flat slab subduction to steeply dipping subduction. While the event strikes obliquely to the slab we find a good agreement between the seafloor fabric offshore the source region and the strike of the earthquake. We argue that the event likely reactivated a fault first created during seafloor formation. We hypothesize that large bending related events at the edge of the flat slab are more likely in areas of low misalignment between the seafloor fabric and the slab strike where reactivation of preexisting structures is favored. This hypothesis predicts decreased likelihood of bending related events northwest of the 2017 source region but also suggests that they should be more likely southeast of the 2017 source region.

Preface to the Focus Section on Geophysical Networks and Related Developments in Latin America

Latin American countries share similar history and culture, and complicated tectonic settings. Subduction of the Rivera, Cocos, and Nazca plates had produced 23 M ≥ 8:0 subduction and intraplate earthquakes since 1900 when seismic instrumentation took off. This seismicity includes the 22 May 1960 Mw 9.5 Valdivia, Chile, earthquake, the largest ever recorded in the world; and the most recent intraplate Mw 8.2 earthquake in the Gulf of Tehuantepec, in southern Mexico. Subduction is also present to the east of the Caribbean plate with earthquakes of M ≥ 7:0, including some historical megathrust earthquakes (Robson, 1964). In addition, 18.6% of the large deep earthquakes (M ≥ 6:0) in the world have occurred in South America, including two of the largest (31 July 1970 Mw 8.0 in Colombia and 9 June 1994 Mw 8.2 in Bolivia). Seismic hazards in the region are also associated with transform faults earthquakes (M ≥ 7:0) that take place at the boundaries between the North American plate and the Pacific and the Caribbean plates; as well as the South American plate and the Caribbean and Scotia plates. Other significant earthquakes have occurred in the region, such as the Nicaragua tsunami earthquake (2 September 1992 Mw 7.7) and the destructive Haiti earthquake (12 January 2010Mw 7.0). In fact, in almost all countries in the region, there is at least one earthquake that has had a significant impact on the society, infrastructure, and economy. Geophysical networks are the basis for observation of Earth processes. In consonance with development in the rest of the world, seismological networks in Latin America were first established in the early 1900s. The following noncomprehensive list of stations reveals the rapid development of observation in the Latin American region. According to Udías (2015), the first seismographs (seismoscopes) were locally built and installed by G. Heredia in Mexico at the end of the nineteenth century. An independent seismological station, composed by two Bosch-Omori seismographs, was installed in 1906 in Havana, Cuba. A three-component Vicentini seismograph was installed in 1907 at the Astronomical Observatory in La Plata, Argentina. After the 1906 Central Chile earthquake, F. Montessus de Ballore developed a countrywide network consisting of a central station with Bosch-Omori, Wiechert, and Stiattesi components located in Santiago, and four horizontal Wiechert instruments of 200 kg spaced at about 800 km, complemented by 29 Agamennone seismoscopes. The efforts continued through the following decades with stations installed at Tacubaya, Mexico; La Paz, Bolivia; Huancayo, Peru; and Bogota, Colombia, among others, further propelled by the International Geophysical Year (1958–1959) and the deployment of the World Wide Standardized Network, from the mid-1960s on. One of the objectives of this focus section of SRL is to give network operators an opportunity to present information about the state of these local, regional, and national networks. From north to south, for Mexico, this focus section includes a description of the national Mexican broadband network (Pérez-Campos et al., 2018), and four regional seismic networks (Castro et al., 2018; Córdoba-Montiel et al., 2018; Quintanar et al., 2018; Vidal-Villegas et al., 2018). It also introduces a regional strong-motion network (Núñez-Cornú et al., 2018), the Global Positioning System (GPS) TLALOCnet initiative (Cabral-Cano et al., 2018), and a dedicated network for seismic early warning (Suárez et al., 2018). For Central America, Linkimer et al. (2018) describe the national seismic network of Costa Rica, and Strauch et al. (2018) summarize the Nicaraguan network and the effort of Central America for earthquake monitoring and tsunami early warning. For the Caribbean, Bent et al. (2018) report the current efforts in Haiti for real-time earthquake monitoring and Sardiña et al. (2018a,b) summarize the capabilities and performance of the PacificTsunamiWarning Center for Puerto Rico, the Virgin Islands, and the Caribbean. For South America, Alvarado et al. (2018) introduce the Ecuadorian seismic, volcanic, and geodetic networks. Vargas et al. (2018) and Mora-Páez et al. (2018) present the geophysical and geodetic infrastructure, respectively, in Colombia. Bianchi et al. (2018) describe the Brazilian national network, Sánchez Bettucci et al. (2018) describe the Uruguayan national network, and Barrientos and National Seismological Center (CSN) Team (2018) present the Chilean network. Furthermore, Piñón et al. (2018) describe the Argentine continuous satellite monitoring network. Moreover, the focus section includes the description of products of these regional and national networks. In particular, Salazar et al. (2018) introduce a comprehensive strong-motion database for El Salvador, which constitutes a major source of information for seismic hazard evaluation for that country. For near-real-time products, Folesky et al. (2018) obtain rupture direction from local data collected by the Integrated Plate