In the north Baja California and western Sonora region exists an important set of geologic faults, some of which extend southeast through the Gulf of California. These faults, which are part of the main border between the Pacific and North America tectonic plates, generate earthquakes of $Mw$ slightly over 7.0. An example of this is the $Mw$ 7.2 El Mayor–Cucapah earthquake of 4 April 2010. This and other past earthquakes caused severe damage and much concern among the inhabitants at both sides of the Mexico–US border. The Northwest Mexico Seismic Network (RESNOM) is a subnetwork of Centro de Investigación Científica y de Educación Superior de Ensenada, Baja California (CICESE’s) network. RESNOM was designed to collect digital information from local and regional earthquakes that occur in the northern Baja California–western Sonora region. Although the design and installation of the network began in 1977, effective seismic monitoring began in June 1980. Through the years, RESNOM evolved from short‐period to broadband seismometers, accelerographs, and Global Positioning System (GPS) receivers collocated at each site. A description of this subnetwork and of some of the results obtained are presented here.

We performed a seismic noise‐level analysis from records of seismic stations located in northern Baja California, Mexico. We used data from stations of the Northwest Mexico Seismic Network, with the goal to characterize the noise spectrum for each station as a function of time. The seismic stations are located in the sedimentary environment of the Mexicali Valley (MV) and the granitic Peninsular Ranges of Baja California (PRBC). The ambient seismic noise was characterized using the power spectral density (PSD) technique (Peterson, 1993). We found that, at periods between 0.1 and 2 s, the median noise levels (MNLs) of stations in the MV are up to 25 dB higher, and near the new high‐noise model (NHNM), than MNLs of PRBC stations. For periods $>2s$⁠, the MNLs are similar for both regions and are between the NHNM and new low‐noise models. We found differences in the noise levels when seismic sensors (different brands but same bandwidth) were interchanged. For periods $<1s$⁠, the MNLs computed from records of Güralp sensors are $∼20dB$ higher than MNLs from Nanometrics sensors; for periods $>10s$ the MNLs of Nanometrics sensors are $∼20dB$ higher than the MNLs from Güralp sensors. We observed daily variations in short‐period noise, related to human activity, such as higher noise levels for periods $<1s$ at daylight in a station in the city of Mexicali. No influence of variations of the sea level of the Pacific Ocean on the PSD of stations of north Baja California was observed. At least for two sites, in and south of MV, there is a direct relationship among variations of pressure and temperature with seismic noise: high pressure and low temperature are related with high‐noise levels in the 4.0–8.5 s period band and vice versa.

To see any change in seismic velocities that may be associated with an abrupt change in the regional geology (granitic rock in contact with sediments), we conducted a refraction seismic study in the Peninsular Ranges of Baja California, which is in the Mexico–southwestern Laguna Salada (LS) region. We installed 30 three‐component portable seismic stations, supplemented with two permanent six‐component stations of the Northwest Mexico Seismic Network (RESNOM). The stations, spaced $∼6km$ along a refraction profile, recorded two blasts; these were the direct shot located to the south of the city of Ensenada and the reverse shot in the southwestern LS (southwest–northeast direction). Record sections show seismograms with impulsive P arrivals at nearby stations. Rays from the two blasts were modeled (using asymptotic ray theory) to obtain a P‐wave velocity model from 0 to $∼15km$ depth along the refraction profile. Our modeling results are as follows: in the southwestern part of the profile (0–25 km distance), a low‐velocity zone of $∼2km/s$ exists between the depths of 0 and 3.5 km; in Sierra Juárez, the mean P‐wave velocity is $∼5.6km/s$ between the depths of 0 and 5 km; and in southwestern LS, a low‐velocity layer of $∼2.5km/s$ exists between the depths of 0 and $∼3km$⁠. We also modeled a layer of $∼6.5km/s$ between 4 and 12 km in the Ensenada–Ojos Negros region, and between the depths of 4 and 8 km below the southwestern LS. From a profile distance of 0 to 50 km, a velocity zone of $∼6.7km/s$ appears between the depths of 12 and 15 km.

We estimated Moho depths from data recorded by permanent and temporary broadband seismic stations deployed in northern Baja California, Mexico, using the receiver function technique. This region is composed of two subregions of contrasting geological and topographical characteristics: the Peninsular Ranges of Baja California (PRBC), a batholith with high elevations (up to 2,600 m); and the Mexicali Valley (MV) region, a sedimentary environment close to sea level. Crustal thickness derived from the P‐to‐S converted phases at 29 seismic stations were analyzed in three profiles: Two that cross the two subregions, in ∼W‐E direction, and the third one that runs over the PRBC in a N‐S direction. For the PRBC, Moho depths vary from 35 to 45 km, from 33°N to 32°N; and from 30 to 46 km depth from 32°N to 30.5°N. From a profile that crosses the subregions in the W‐E direction; Moho depths vary from 45 to ∼34 km under western and eastern PRBC, respectively; with an abrupt change of depth under the Main Gulf Escarpment (30°), from ∼32 to 30 km; and depths of 17–20 km under the MV. Moho depths of the profile in an ∼ W‐E direction at ∼31.5°N are from ∼30 to 40 km, under topography that increases from 0 to 2,600 m; and became shallower (16 km depth) as the profile reaches the Gulf of California. These results show that deeper Moho is related to higher elevations with an abrupt change under the Main Gulf Escarpment, except for western PRBC where the Moho depth is not simply reflecting isostatic compensation.

We conducted experimental work to explain the large peak ground accelerations observed at the Cerro Prieto volcano in Mexicali Valley, Mexico. Using ambient noise and earthquake data, we compared horizontal‐to‐vertical spectral ratios (HVSRs) computed for sites on the volcano against those calculated for locations outside it. High‐HVSR values (∼11 at ∼2 Hz) were obtained on the top of the volcano at 183 m of altitude, decreasing for sites located at lower elevations. We calculated a median HVSR of ∼1 at 2 Hz from HVSRs computed for nine sites located along an N18°E transect and at an average elevation of ∼25 m. The earlier comparison suggests a relative amplification on the volcano. In addition, we calculated HVSRs from accelerograms generated by 62 earthquakes (⁠ $2.6≤ML≤5.4$⁠; $4.6≤Mw≤7.2$⁠) recorded at four locations: two on the volcano (at 194 and 110 m of elevation) and two outside it. These last two sites, located up to 6 km away in a north‐northwest and south‐southwest direction relative to the volcano, are at an average altitude of 22 m. For the four locations, we also computed the HVSRs from ambient noise data. Although the HVSR results derived from both types of data are slightly different, we also found high HVSRs for the two sites on the volcano and low HVSRs for the two sites outside it, corroborating the relative amplification on the volcano. Using the 1D wave propagation modeling, based on the stiffness matrix method, we modeled the experimental HVSRs to analyze the local site effects. Therefore, we propose that the ground‐motion amplification at the Cerro Prieto volcano may be due to a combination of its topography and shallow site effects.

Mexicali, capital of Baja California, has 1,049,792 inhabitants and lies in a high-seismic-hazard zone in northwestern Mexico, according to CENAPRED, the MDOC-CFE-2015 seismic regionalization, and the ASCE 7-22 “Hazard Toolkit”. This study develops a probabilistic seismic hazard map to estimate peak ground accelerations with a 2% probability of exceedance in 50 years, using the OpenQuake platform. The study area coincides with the 2025 urban development plan polygon for the central population area defined by the Municipal Institute for Research and Urban Planning of Mexicali. The Imperial and Cerro Prieto faults, the Pescaderos–Indiviso fault system, and the Laguna Salada fault were modeled as seismic sources. Four PEER-NGA ground motion prediction equations and regional geophysical and geotechnical data were employed to characterize shear-wave velocity (Vs30). Design response spectra were generated for each grid point for the 21 periods specified in ASCE 7-22. A representative Vs30 of 236 m/s was obtained, and the a, b, and Mc parameters were derived for the seismic catalog. Resulting peak ground accelerations range from 0.842 g to 1.221 g, with a maximum spectral pseudo-acceleration of 2.23 g at 0.30 s.