A note on parameters T0 and Vs30 for microzonation in southern Ensenada City, Baja California, Mexico
Una nota sobre los parámetros T0 y Vs30 para la microzonificación en la zona sur de la ciudad de Ensenada, Baja California, México
J. Antonio Vidal-Villegas1, *
, Germán Martínez-Romo1, Patricia Andrade-García1
1 Departamento de Sismología, División de Ciencias de la Tierra, Centro de Investigación Científica y de Educación Superior de Ensenada, Baja California. Carretera Ensenada-Tijuana 3918, Zona Playitas, Ensenada, 22860, Baja California, México.
* Corresponding author: (J. A. Vidal-Villegas) This email address is being protected from spambots. You need JavaScript enabled to view it.
How to cite this article:
Vidal-Villegas, J. A., Martínez-Romo, G., & Andrade-García, P. (2026). A note on parameters T0 and Vs30 for microzonation in southern Ensenada city, Baja California, Mexico. Boletín de la Sociedad Geológica Mexicana, 78(2), N261125. https://doi.org/10.18268/ BSGM2026v78n2N261125
Manuscript received: July 27, 2025. Corrected manuscript received: November 16, 2025. Manuscript accepted: November 19, 2025.
ABSTRACT
Seismic noise was collected to determine the natural period (T0) and shear-wave velocity (Vs30) south of Ensenada City (called “Ex Ejido Chapultepec”). This zone has had significant housing development. Short- and intermediate-period seismometers (2s and 10s, respectively) together with 24-bit recorders were used to record ambient noise. The parameter T0 was obtained using the HVSR technique, and Vs30 was obtained using the SPAC technique. Regarding the results, the T0 values range from 0.2 s to 2.4 s, and the velocity profiles exhibit Vs30 values ranging from 148 to 549 m/s. These values align with those of the local geology of the study area and are crucial for habitation design and seismic hazard studies in Ensenada City.
Keywords: microzonation, HVSR and SPAC methods, T0 and Vs30 parameters.
RESUMEN
Para determinar el periodo natural (T0) y la velocidad de corte (Vs30), registramos ruido sísmico al sur de la ciudad de Ensenada, en el Ex Ejido Chapultepec. Esta zona de la ciudad ha experimentado un crecimiento habitacional importante, y se desconocían estos parámetros útiles para efectos de microzonificación. Para la obtención de ruido sísmico se utilizaron sismómetros de periodo corto e intermedio (2 s y 10 s) en combinación con grabadoras de 24 bits de resolución. El valor de T0 se calculó mediante la técnica de cocientes espectrales HVSR (por sus siglas en inglés), y Vs30 se obtuvo mediante el método SPAC (autocorrelación espacial). Los resultados muestran que los valores de T0 varían entre 0.2 y 2.4 s y los de Vs30 entre 148 y 549 m/s. Estos valores están en consonancia con la geología local del área de estudio y resultan importantes para el diseño habitacional y para los estudios de peligro sísmico de la ciudad de Ensenada.
Palabras clave: microzonificación, métodos HVSR y SPAC, parámetros T0 y Vs30.
1. Introduction
Nowadays, it is essential to understand the soil’s physical properties at the sites where buildings or houses are constructed. In addition to geotechnical parameters (such as material type, density, and degree of compaction), engineering seismology provides non-invasive tools to determine other parameters, including the soil’s natural period and the site’s subsurface shear velocity. The last 30 to 40 meters of the subsoil structure, before the free subsurface, produce significant modifications to the seismic signal and are referred to as site effects (Chávez-García, 2007). In areas with seismic activity and human settlements, are necessary microzoning studies. The results of these studies are helpful to engineers in properly designing structures with characteristics that resist the effects of a relevant earthquake. In urban growth, it is essential to conduct microzoning studies to identify suitable zones for construction.
Ensenada, one of the main cities in Northern Baja California (NBC), Mexico, is experiencing steady population growth and development of urban infrastructure. Therefore, it is essential to know the soil’s characteristics in areas where settlements are located. Engineering seismology methods allow us to determine this. For instance, using ambient noise provides a means for studying the subsoil. Aki (1957) proposed the Spatial Autocorrelation (SPAC) method to determine the dispersion curve of Rayleigh waves. Moreover, Nakamura (1989) proposed the HVSR (H/V Spectral Ratio) method to determine the natural period (T0) of the surface layer. The use of both methods (SPAC and HVSR) allows the generation of seismic zoning maps. Concerning Ensenada, a microzoning study has determined the dominant period (Ibarra-Torúa, 2004). However, this study focused on the west–central part of the city (Figure 1); the southern part also required attention.
The present study aimed to perform seismic microzoning in the southern part of the city of Ensenada (Ex Ejido Chapultepec and its neighborhoods). Using HVSR and SPAC methods, the natural period of soil (T0) and Vs30 were determined.
2. Area of study
The area of study, located south of Ensenada city, is delimited to the north by the Ejido Chapultepec and to the south by the Maneadero Valley (Figure 1). It comprises about thirteen settlements. The Ex Ejido Chapultepec and the beginning of the Maneadero valley connect with the center of Ensenada city through the highway Ensenada‑Maneadero. In 2010, the National Institute of Statistics and Geography reported a population of 7055 economically active individuals (as of June 26, 2013). Due to this growth, with houses typically composed of one or two levels, it was necessary to conduct the present study.
3. Tectonics and seismicity
The active faults nearest to the study area are: i) the San Miguel-Vallecitos-Calabazas fault system, located approximately 50 km north‑northeast of the city of Ensenada; ii) Tres Hermanos, 25 km northeast of the city of Ensenada; iii) Descanso-Estero, which shows microseismicity and is the boundary between the city of Ensenada and Maneadero Valley (Figures 2A and 2B). Offshore of Ensenada City are the Coronado Banks fault, considered the extension to the northwest of the Agua Blanca fault, and to the west, the San Diego and San Clemente faults. Regarding historical seismicity, one fault has generated significant earthquakes: the San Miguel fault, which has produced six earthquakes with magnitudes ranging from 6.0 to 6.8, including two in 1954 (Leeds, 1979) and four in 1956 (Shor and Roberts, 1958). The current activity of this fault demonstrates its ability to generate strong earthquakes that could affect Ensenada City due to its proximity. Another important tectonic feature of the NBC is the Agua Blanca fault, located south of Ensenada City. Although there are no records of significant seismic activity during the initial recording period in northern Baja California, Mexico, and southern California, USA, shortduration swarms of low-magnitude earthquakes have predominated, eventually followed by events with magnitudes around 4 (Munguía and Vidal, 1991). At NBC, continuous seismic monitoring began in 1976 and continues to this day (VidalVillegas et al., 2018). In addition, the faults near the coast and in urban areas play a significant role in future seismic hazard analysis (Figures 2A and 2B). The San Miguel-Vallecitos-Calabazas fault system and Tres Hermanas fault generate a high level of seismic activity. The earthquake magnitude of this activity can reach 4.5 (Figure 2A). As noted, Ensenada City is near active faults that generate microseismicity and moderate seismicity. However, in the past, relevant earthquakes have occurred (Mag. > 6), so Ensenada City could be affected if a major earthquake were to take place.
4. Methodology
4.1. SPAC METHOD
This technique aims to estimate the shallow subsurface structure from seismic noise. The noise must be recorded simultaneously at several stations, forming an instrumental array. Based on this information and following the process described by Aki (1957), the Modified SPAC method for circular arrays (explained and extended by Shabani et al., 2010) is used to obtain a surface-wave scattering curve, which is then used to determine a surface-velocity structure. The SPAC method is based on computing the spatial autocorrelation function:
Where rjc is the distance between the sensor j and the central sensor c, Өjc is the azimuth, k is the spatial wave-number of the frequency f, and Φ is the propagation direction of the wavefront recorded by the array. Aki (1957) referred to the azimuthal average of the autocorrelation function as autocorrelation coefficients for the same radial distance.
Where C is the azimuthal average of the autocorrelation function; f is the frequency vector at which coherence is calculated; J0 is the Bessel function of the first kind and zero-order; v(f) is the phase velocity of the Rayleigh wave; r is the radius that forms the array; Ө is the azimuth; k is the wave number.
The coefficient can also be determined directly in the frequency domain using the fast Fourier transform with the following expression:
Where Re[arg] is the real part of the complex argument, Scx is the cross-power spectrum between ground motions at the two observation points, Sc is the power spectrum at the center of the array, Sx is the power spectrum at the point at distance r in the direction x, w is the angular frequency, and Ө is the azimuthal average of the autocorrelation function.
4.2. HVSR TECHNIQUE
This technique, proposed by Nakamura (1989), utilizes seismic noise as data and aims to eliminate the effects of the source and the path, thereby isolating the so-called site effect. This technique assumes the presence of a layer of sediment on the rock basement. Calculating the spectral ratio between the horizontal and vertical components (HVSR) allows us to identify the preferential vibration frequency of the sediment layer. The computation of HVSR is as follows:
Where SHS= Fourier amplitude spectrum of the horizontal component and SVS= Fourier amplitude spectrum of the vertical component. The theoretical basis of this technique is unclear; however, experimental studies show that, for soft soils with an impedance contrast, the method yields a clear peak that correlates with the fundamental soil period.
5. Data
5.1. SPAC METHOD AND GEOMETRY OF ARRAYS
Seismic noise was collected using eight SARA triaxial seismographs (f0= 2 Hz, 100 samples per second). For each site, three to five records of seismic noise were collected, each lasting 10 minutes. The selected sites were chosen for their accessibility and surface area, as the experiment required approximately 400 m2. Finally, based on the geological map from Gastil et al. (1975), an effort was made to obtain at least one record for each soil type.
The arrangement of the sensors is justified by the desired vertical resolution. Sensors with a larger separation will detect longer wavelengths than those of sensors with a smaller separation. Shorter wavelengths will show layer information at shallower-depths, while longer wavelengths will show deeper strata. Table 1 indicates the type of arrangement used, and Figure 3 shows the location of the sites where seismic noise was collected (to apply SPAC and HVSR techniques).
5.1. HVSR TECHNIQUE
To compute the fundamental period, in addition to the sites listed in Table 1, we included 10 additional sites to improve coverage south of Ensenada City. The noise recordings were collected using a triaxial PMD intermediate-period seismometer (1/10 s to Nyquist frequency). The record duration was 10 minutes, and the sampling rate was 100 samples per second. The equipment’s resolution is 24-bit. Figure 3 also shows the proposed sites for recording seismic noise. To compute the HVSR, 40-s segments per component were used, and the Fourier transform was computed. The horizontal component was obtained by means of the root mean square of the N-S and E-W components. Next, the HVSR was calculated for each segment, and the mean HVSR curve was obtained by averaging the HVSRs of all segments.
6. Results
6.1. VS30
The procedure for obtaining the Rayleigh waveform dispersion curve, for example, is described for the CONALEP site. The same calculation was performed for the additional 10 sites (Table 1). For the CONALEP site, with a circular array deployed, the most stationary time windows were selected. The normalized autocorrelation function was obtained after performing the two previous processes (window selection and array geometry). The autocorrelation function provides information on the characteristic frequencies of the Rayleigh wave. If this function does not exhibit well-defined highs and lows, the signal under study lacks periodicity, making it unsuitable for application of the SPAC method. Figure 4A shows the autocorrelation functions for the different distances found in the circular array of the CONALEP site.
Substituting the autocorrelation coefficients C(f) and solving for the phase velocity v(f) of the argument of the Bessel function gives the Rayleigh wave-dispersion curve for the different modes of vibration contained in the seismic noise (Figure 4B).
The experimental dispersion curve, the input to the inversion process, was selected based on the first decrease of the correlation coefficients (zero crossing). This process, performed using the Dinver module of the Geopsy package (Wathelet et al., 2020), enables us to generate theoretical dispersion-curves models. Each theoretical model was compared to the experimental model using the standard deviation criterion. Each model generated by Dinver has a small standard deviation relative to the previous one. The inversion was based on minimizing the misfit between the observed and computed values. A subsoil model generated from each dispersion curve is a function of the shear-wave velocity (Figure 5A for the CONALEP site). Figures 5B, 5C, and 5D illustrate the shear-wave velocity models for the Macroplaza, Campo de Béisbol, and Tecnológico sites, respectively.
The number of Vs-velocity layers ranged from 2 (CONALEP site) to 5 (Terreno Víctor site). Based on these velocity profiles, like those shown in Figure 5, the next step was to calculate the average Vs30. The following equation allowed us to compute it (Gutíerrez-Astete and Escalona‑Medina, 2016):
Where Vs30 is the average shear velocity, n is the number of layers at 30 m before the free surface, h is the thickness of layer i, and Vsi is the shear velocity at layer i.
Figure 6A shows a map of the calculated average Vs30 for the 11 sites (Table 1) distributed across the study area (Ex Ejido Chapultepec and its surroundings). Concerning the geology, the area under study is part of the Todos Santos Coastal Plain (Gastil et al., 1975), with a maximum elevation of 100 m above mean sea level. Quaternary fluvial deposits, characterized by outcrops of extrusive igneous rocks (andesitic and volcanoclastic), line the eastern border (Figure 6B). To the south, there are well-developed deposits of alluvium (deposits of gravels, sands, and little-consolidated clays), and to the west there is one beach strip of fine sand (Allen et al., 1960; Pérez-Flores et al., 2004). To the north-northeast, the basement outcrops, and towards the southwest, it extends to depths of 600 m (Pérez-Flores et al., 2004).
6.2. ISOPERIODS
The dominant period for each site, estimated using the Nakamura technique, enabled us to extend the Ensenada City isoperiods map (calculated by Ibarra-Torúa, 2004) southward. Figure 7 shows the HVSR computed for six sites, along with the corresponding natural frequency. The periods calculated in the present study were consistent with those in the west-central area (Figure 8). The dominant periods show a correlation between the local geology and the Vs30. For instance, high Vs30 values correspond to short periods and compact soils. The range of periods obtained is from 0.2 s (rock) to 2.4 s (loose sand).
7. Conclusions
Results of subsoil exploration (at a depht of around 40 m) in the southern part of the city of Ensenada, using the SPAC technique, were presented. Correlations between station pairs up to 100 m enabled us to estimate the shear-wave velocity. It was possible to calculate rock depth at one site, whereas at the others, the shallow sediment layer was assessed. The number of layers derived from the modeling generally varied from 2 to 5. In the area of study, three predominant geological units are identified: igneous (fractured), alluvium, and littoral. On average, the estimated Vs30 values for each geological unit were 442-549 m/s, 248-386 m/s, and 148-200 m/s, respectively. These values, for instance, are crucial in future seismic hazard studies in Ensenada City.
Regarding the natural periods, a correlation is observed between the local geology and Vs30 (at higher Vs30, the period is shorter, corresponding to compact soils). The range of periods obtained was 0.2 s (rock)—2.4 s (loose sand). The integration of these periods with those previously reported for the west and central parts of Ensenada City is essential for civil engineering purposes.
Contributions of authors
(1) Conceptualization: JAVV; (2) Data analysis or acquisition: GMR; (4) Drafting of the original manuscript: GMR, PAG; (5) Drafting of the revised and edited manuscript: JAVV; (6) Graphic design: PAG; (8) Interpretation: JAVV, GMR.
Funding
GMR and PAG received a scholarship from the former National Council of Science and Technology of México (CONACYT). A CICESE internal research project provided funds for the fieldwork.
Acknowledgments
To J. G. Acosta-Chang, L. Mendoza-Garcilazo, and J. Gómez-Valdéz for fruitful discussions in the development of this work. Thanks to the personnel who have made the Northwestern Mexico Seismic Network operational. The comments of two anonymous reviewers and Raúl Castro, guest editor, enabled us to refine and improve the manuscript.
Conflicts of interest
The authors have no conflict of interest.
Handling editor
Raúl Castro Escamilla.
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