Microearthquakes in the Guadalajara Metropolitan Zone, Mexico: evidence from buried active faults in Tesistán Valley, Zapopan

Numerous microearthquakes, ML ≤ 3.8, corresponding to background seismicity and swarms were observed from September 3, 2017, to January 1, 2018, mainly in the Tesistán Valley, north of the Guadalajara Metropolitan Zone (GMZ). We located 188 tectonic microearthquakes and identified 11 clusters of similar events from a spatio-temporal analysis and waveform cross-correlations. Our results confirm the presence of continuous seismicity in the GMZ that long went unobserved. Most M L ≥ 2.5 events and some clustered events are located in the northeastern Tesistán, close to the NNE-SSW fault corresponding to the eastern edge of the Zapopan Graben, a structure evidenced by 2015–2016 seismicity. Seismicity recorded during 2020 by a recent local seismic network installed in Zapopan reaffirms that frequent microseismicity is related to active faults that cross the cities of Zapopan and Guadalajara. The microseismicity distribution suggests minor faults with the same orientation and sense of displacement as the main structures bounding the Zapopan Graben, which corresponds to structures known as synthetic faults. This arrangement is common within the Basin and Range tectonic province. The seismicity in the northeast boundary of Jalisco Block is closely related to faults formed by Cenozoic deformation events that might be reactivated due to modern crustal dynamics. Active faults and the possibility of synthetic structures are a hypothesis that necessitates long-term seismic monitoring in order to assess the seismic hazard in the GMZ, which is a crucial factor for urban planning.


3 Background
The Guadalajara Metropolitan Zone (GMZ) and its surroundings have been affected by at least two large earthquakes: The 1567 MW = 7.2 Ameca and the 1875 M ≥ 6? San Cristóbal de la Barranca (Suter 2015(Suter , 2020a, both located in the Tepic-Zacoalco Rift, at the northeast tectonic boundary of the Jalisco Block (Fig. 1a). Moreover, the GMZ has a long history of moderate and small earthquake activity that represents a risk not yet evaluated in this region (e.g., Ordoñez 1912;Suárez et al. 1994;Singh et al. 2017;Marín-Mesa et al. 2019;Núñez-Cornú et al., 2021a).
For instance, in 1912, a moderate swarm caused damage to buildings in the cities of Guadalajara and Zapopan, and according to historical reports, similar seismicity was felt in 1844 (Ordoñez 1912). Although it is presumed that the epicenters of the 1912 swarm were near Guadalajara, there was no evidence of surface rupture that could reveal a faulting structure. Nevertheless, the epicenter was likely close to the Río Grande de Santiago, where the large earthquake of 1875 was also assumed to have occurred (Fig. 1). This earthquake affected the towns of San Cristóbal de la Barranca, Zapopan, and Guadalajara.
In the early 1990s, portable stations recorded five microearthquakes MC ≤ 2.5 during 20 days of monitoring: Three events were on the Zacoalco half-graben, ~ 60 km southwest of Guadalajara, and the other two were closer to the GMZ (Suárez et al. 1994). The focal Plates: RP = Rivera, CP = Cocos, NAP = North-America. The insert map shows the regional location by red dashed lines. Instead, the green shadowed box shows the coverage of Fig. 1b and Fig. 2, where Guadalajara Metropolitan Zone (GMZ) is located. Star symbols indicate the epicenter of the most significant earthquakes in the region with their respective year of occurrence. The stations belonging to the Jalisco Seismic Accelerometric Telemetric Network (RESAJ) are shown as pentagon symbols, and the location of the main volcanoes and calderas in the region as triangle symbols. b Seismicity in the GMZ, during 2015, 2016, and 2020 (Núñez-Cornú et al. 2021a and2021b). The seismic stations belonging to the Zapopan Seismic and Accelerometric Network (RESAZ) are shown as blue triangles, and the nearest station of the RESAJ as the green pentagon. Dashed lines below the seismicity represent the faults interpreted by Núñez-Cornú et al. (2021a) mechanism obtained from temporary seismic networks indicates an N-S normal fault plane solution along the Zacoalco half-graben. Subsequently in a 1997 swarm, 33 microearthquakes with magnitudes between 1.5 and 3.5 were recorded with portable stations over a period of almost one month. This swarm, also located on the Zacoalco half-graben, exhibited focal mechanisms related to normal faults oriented NW-SE (Pacheco et al. 1999), parallel to the regional deformation pattern. In October 2012, the National Seismological Service of Mexico (SSN) reported a MW = 4.4 earthquake, located to the northwest of Chapala Lake, about 30 km south of the GMZ. Its focal mechanism was oriented N-S, with nearly pure normal faulting (Yamamoto et al. 2018).
From July 2020 to January 2021, Núñez-Cornú et al. (2021b) reported 19 seismic events ML = 2.4 located in the northeastern Zapopan Graben and further NE, along its eastern fault. These events were recorded by five recently installed stations belonging to the Zapopan Seismic and Accelerometric Network (RESAZ; Núñez et al. 2020), together with the closest stations from RESAJ (Fig. 1b). So far, in this region, seismicity has remained low in magnitude (M ≤ 4;SSN,2021;Núñez-Cornú et al. 2021b), but regional seismic networks have been barely detecting microearthquakes.
Most of the seismic sequences and swarms in this region have not been well recorded due to a lack of enough near-source stations. Also, fault characterizations in the GMZ based on surface geology have been limited due to the sedimentary infill and the growth in the urbanized zones that limit observation capabilities.
The hitherto reported seismicity is just a small subset of the actual number of earthquakes that have occurred in this region, and considering that the GMZ is one of the most significant economic and urban cities in Mexico hosting five densely populated municipalities with over four million people, and it is necessary to recognize the seismicity in GMZ as a potential hazard.
This work analyzes the most significant number of microearthquakes in GMZ that have been located so far, leveraging recordings from a dense temporary monitoring network during the last four months of 2017. With our observations, we examine whether there is a consistency with previously reported seismicity and refine the interpretation of the local active structures, such as the Zapopan Graben. This work illuminates the importance of the seismic hazard as a potential risk not previously addressed.

Tectonic setting
The tectonic context of Mexico's central-western and the southwestern coast ( Fig. 1a) is related to subduction along the Middle America Trench. The northern segment of the trench arises from convergence between the Rivera plate and the North American plate, where the Rivera plate subducts with a velocity rate that decreases northward from ~ 4.3 cm/year to ~ 1.9 cm/year (Kostoglodov and Bandy 1995;DeMets and Traylen 2000;Suárez et al. 2013). At the southeast of the Rivera plate, the Middle America Trench continues with the subduction of the Cocos plate, and both oblique subductions have led to a stress imbalance that splintered the North American plate. Therefore, a lithospheric section, known as the Jalisco Block, is rifting away from the North American plate with a slow relative SW slow motion (2 mm/year).
The Jalisco Block has oblique convergence in the west that generates compressive stresses, meanwhile giving rise to Colima and Tepic-Zacoalco rifts as tectonic boundaries in the continental part (Luhr et al. 1985;Allan 1986;Allan et al. 1991;Bandy and Pardo 1994;Ferrari and Rosas-Elguera 2000;Selvans et al. 2011). It has been proposed that these rifting zones follow ancient structures partly reactivated since the Pliocene (Rosas-Elguera et al. 1996). The Jalisco Block is situated within the western part of a continental magmatic arc, from Late Miocene to Recent, known as the Trans-Mexican Volcanic Belt (Demant 1982), where the tectonic and volcanic activity have given rise to stratovolcanoes, calderas, monogenetic volcanic fields, and basins (Moore et al. 1994;Rosas-Elguera et al. 1997;Ferrari et al. 2012). Seismicity and faulting indicate that deformation of the Trans-Mexican Volcanic Belt is mainly extensional, whose effects are more noticeable in the western part.
At the northwestern part of the Trans-Mexican Volcanic Belt is the Sierra Madre Occidental; it is one of the largest ignimbrite provinces in the World (McDowell and Keizer, 1977;McDowell and Clabaugh, 1979;Bryan and Ferrari, 2013), whose silicic volcanism was developed during the Late Eocene to Early Miocene. To the east of the Sierra Madre Occidental and possibly in parts of the Trans-Mexican Volcanic Belt, the Oligocene to Recent Basin and Range extensional deformation formed normal faults with preferred trends varying from NNW to NNE (Aranda-Gómez et al. 2000;Aranda-Gómez and McDowell, 2010).
Some faults of the south Sierra Madre Occidental are assumed to be responsible for the large 1875 earthquake (M ≥ 6?), likely located near the town of San Cristóbal de la Barranca, ~ 45 km northwest of the GMZ ( Fig. 1; Suter 2020a, b). Studies by Rossotti et al. (2002) inferred a fault structure along a segment of the Río Grande de Santiago, likely covered by fluvial deposits, which could be related to both the 1875 earthquake and the 1912 swarm.
On the other hand, the GMZ is about 40 km north of the triple rift junction between Tepic-Zacoalco, Chapala, and Colima rifts (Fig. 1a). In the northwest of the GMZ, the Tepic-Zacoalco rift is formed by normal faults oriented WNW-ESE (Ferrari and Rosas-Elguera 2000;Ferrari et al. 2012), where the Plan de Barrancas-Santa Rosa-Cinco Minas graben define the contact between the Jalisco Block and the Sierra Madre Occidental (Fig. 1b). In the southwest of the GMZ, the Ameca, San Marcos, and Zacoalco faults define the southern edge of the Tepic-Zacoalco rift, where the most significant earthquake (M W = 7.2) occurred near the GMZ (Fig. 1a), presumably generated by the Ameca-San Marcos normal fault system in 1567, near the triple rift junction (Suter 2015;Suter 2020a, b).
In contrast with the trending of the Tepic-Zacoalco rift and the nearly N-S trending of the Colima rift, the other branch of the triple rift junction corresponds to the Chapala rift, which is oriented E-W. Geological studies and seismicity suggest that the Chapala rift still has active deformation (Garduño-Monroy et al. 1993;Suter 2018) and has structures capable of generating M > 5 earthquakes, such as the 1847 earthquake that occurred ~ 50 km southeast of Guadalajara (Suter 2018).
Despite significant fault systems cross-cutting the northeastern part of the Jalisco Block, only a few local faults have been identified within the GMZ. For instance, recent seismicity suggests that two active quasi-parallel normal faults cross the GMZ, about 10 km distant from one another and oriented N-S and NNE-SSW; these have been interpreted as the Zapopan Graben (Figs. 1b and 2; Núñez-Cornú et al. 2021b). Also, in Zapopan, specifically in the Tesistán Valley, earth fissures have appeared through volcanic sedimentary sequences after heavy rainfall seasons, with alignments parallel to the regional deformation pattern. This fissuring at the land surface suggests structural control of the basement, in which likely there are buried tectonic faults responding to stresses induced by water seepage that exploits fractures associated with the tectonic structures (Suárez-Plascencia et al. 2005).

Data
A local temporary seismic network was installed in La Primavera Caldera and its surroundings ( Fig. 2), from September 2017. It was operative for almost eight months, until April 2018. This monitoring was carried out within the framework of the P24-CeMIE-Geo project: "Passive seismic and magnetotelluric exploration of the Ceboruco and La Primavera volcanic fields," which was deployed to record magmatically related seismicity that could be useful to identify potential geothermal zones (Solorio et al., 2018). This seismic network, which we will refer to as the PR-TSN, consisted of 25 stations; each of which included an Obsidian (KMI) with Lennartz Le3D sensor, with an eigenfrequency of 1 Hz, configurated with a sampling of 100 Hz.
In addition to seismic events related to La Primavera Caldera, the PR-TSN stations also recorded numerous tectonic microearthquakes, which allowed us to study the GMZ from a seismotectonic perspective. For this application we used the dataset of 19 seismic stations recorded between September 3, 2017, and January 1, 2018.

Hypocenter location
Since we focused on tectonic seismicity, we have excluded events related to volcanic sources for our analyses. Moreover, because our analysis was targeting very local earthquakes, we included only those events whose arrival time difference between S-and P-phases is less than ~ 5 s.
We first identified seismic events in the recordings by automatically picking phase arrival times, using Antelope™ software (BRTT 2020) through a short-term average/ long-term average signal energy algorithm. Then, we reviewed the continuous records and manually picked those arrivals that could not be automatically recognized due to noise or other signals. Hypocenters were estimated through the Hypo71 program (Lee and Lahr 1975), using a local P-wave velocity model of the Jalisco Block (Fig. 3a) based on the previous studies (Núñez-Cornú et al., 2002;Rutz-López and Núñez-Cornú, 2004).
More than 200 events that occurred during four months were identified. Most of the locations that we considered acceptable had uncertainties of less than 2.5 km (ERH and ERZ), and RMS < 0.2 s (Fig. 3); low-quality event locations were discarded from the analyses. Our final catalog of 188 hypocenters is shown in Fig. 4. A majority of these reside at depths < 20 km and correspond to low magnitude earthquakes, M L ≤ 3.8, chiefly located in the Zapopan area (northern GMZ), specifically in the Tesistán Valley. Hypocentral distribution is shown in Fig. 4c and d, which correspond to cross sections, parallel and perpendicular to the general trend of the seismicity (Fig. 4a).

The largest earthquakes
During the first four months of recovering seismic data by PR-TSN, only the two largest events, the M L = 3.8 (September 3, 2017) and M L = 3.5 (September 29, 2017), were reported by seismological agencies (e.g., SSN, 2021;ISC, 2021). Particularly, the M L = 3.8 earthquake was also recorded by several stations from RESAJ, so we were able to capitalize on both PR-TSN and RESAJ datasets to estimate location for this event.
We obtained similar locations for both M L = 3.8 and 3.5 earthquakes (Fig. 5), but the locations reported in the catalogs are 10 km westward of our estimated hypocenters, and the M L = 3.5 event is 30 km southward (see Table 1). Because our analysis used more and nearer stations, we opted to work with our hypocentral estimates, which are in concordance with most seismicity that appears to occur in the northeast of Zapopan.
We used also the P-wave polarities of the M L = 3.8 event from the PR-TSN and RESAJ waveforms, and estimated its first-motion focal mechanism using the MEC93 code (Núñez-Cornú and Sánchez-Mora 1999). Despite not having optimal azimuthal coverage, the distribution of the polarities shows low scattering and allow to get tight solutions. One of the focal mechanisms that we could consider as plausible solution is a reverse faulting with a N20ºE nodal plane parallel to the eastern fault of the Zapopan Graben. This type of focal mechanism was also observed by Núñez-Cornú et al. (2021a) and interpreted as part of a wrench structure or due to its association to rightlateral movement along the northern edge of the Tepic-Zacoalco rift (Nieto-Obregón et al. 1985;Rodríguez-Castañeda and Rodríguez-Torres 1992;Núñez-Cornú et al. 2021a).

Spatio-temporal distribution of seismicity
A significant volume of microearthquakes stands out in Tesistán Valley, Zapopan, which corresponds to a microearthquake swarm and background seismicity that, at first glance, seems to have a poorly defined spatial distribution. The dense cloud of seismicity covers a region in excess of 300 km 2 (Fig. 4), consisting mainly of very shallow events (≤ 5 km) with some toward the northeast of Zapopan exhibiting depths approaching 17 km. A few isolated and dispersed microearthquakes in the region were deeper than 20 km. In Fig. 6, we show a cumulative event count curve driven largely by microearthquake occurrence, which is characterized by sporadic changes in the seismicity rate. This behavior is typical in swarms (Mogi 1963), unlike behavior typically observed in a mainshock-aftershock sequence, whose cumulative function would be smoother. On September 26, the first seismicity increase was observed, during which the largest events were those of M L = 3.1 and M L = 3.5, on September 27 and 29, respectively. The M L = 3.5 event appears to have marked the final stage of this activity rise. Hereafter, a series of very shallow microearthquakes occurred during October and November. From September to November, most of the events were concentrated at < 10 km depth, and from November, seismicity was spatially scattered, reaching greater depths. From December 26 to 29, the more intense activity led to multiple M L ≤ 2.5 events that reached maximum depths of ~ 17 km (Figs. 4 and 6).
During early September some stations were still being installed; hence, the number of microearthquakes that occurred in those days is likely underrepresented. Even so, we could address the M L = 3.8 September 3 event, which was the largest during our We assume the catalog of microearthquakes during those first days of September is incomplete, although we note an overall low level of microearthquake activity coincident with the ML 3.1 event on November 30 when our recording capability was better.
On the other hand, we can observe that only ten events were above 2.5 M L , and most of the epicenters are located in the eastern of Zapopan (Fig. 6b), which could be preliminarily interpreted as some fault structure crossing Tesistán Valley, or even this seismicity might be associated to the Zapopan Graben.

Clustering from waveform correlations
A high cross-correlation coefficient among waveforms is a helpful tool for identifying fault structures because it implies that events with nearly identical waveforms, recorded at the same station, were generated by the same seismogenic structure or the same fault asperity zone (Geller and Muller 1980;Tsujiura 1983;Nishigami 1987;Aster and Scott 1993).  Since microearthquakes with highly similar waveforms are widely observed in swarms, we attempt to identify similar events in the seismic readings through waveform cross-correlation (CC) analysis and, thus, to find more temporal and spatial patterns that evidence the buried faults at the Tesistán Valley.
The smaller the correlation time window length, the more likely that CC results in large values; hence we avoid overestimating the waveform similarity by selecting a window length of 2.8 times the difference between the S-and P-arrival time (Baisch et al. 2008), which is ~ 5-7 s for local events analyzed in this work. These time windows were long enough to include P-and S-phases.
We filtered the waveforms using a band-pass from 2 to 15 Hz for correlation processing. We selected a correlation coefficient threshold of 0.75, chosen by inspection to find a reasonable trade-off between similarity, the quantity of clustered events, and epicentral closeness (Cattaneo et al. 1999). This approach was helpful for our dataset; nevertheless, it is advisable to use an alternative exploring method in other cases when the analysis includes a more considerable number of events from several years of data (e.g., Ferretti et al. 2005).
Clusters of more than two high-correlated earthquakes are multiplets, referred to by Tsujiura (1983) as an "earthquake family." In a dendrogram-based scheme, some entities may be clustered highly with more than one family but as the clustering progresses, these families may become joined through their common member links. (Nishigami 1987;Cattaneo et al. 1999).
We built a CC Matrix to identify similar earthquakes and used hierarchical agglomerative algorithms (see dendrogram in Appendix; Johnson 1967;Eads 2007). In Figs. 7,8,and 9, we show the analysis for one station located in the north-center of the network array (P11; see Fig. 2). A total of 58 microearthquakes were classified into 11 clusters.
Two doublets that occurred on September 3 and December 28 were identified as Cluster 01 and Cluster 10, respectively (Figs. 7 and 9). The Cluster 01 contains the M L = 3.8 and the M L = 2.3 events, whereas Cluster 11 corresponds to smaller events with M L = 1.9 and M L = 1.8. The other clusters are multiplets that consist of up to 17 events, and, in each one, we observe similar microearthquakes separated in time by hours to months.
Microearthquakes during September are distributed in Cluster 02 and Cluster 03. In Cluster 02, however, some earthquakes from September have been clustered with others that occurred almost one month later (Fig. 7). The waveform similarity shows that the increasing seismicity rate of a few days of September is not an isolated activity but a repetitive seismic activity related to the same fault or asperity whose ongoing slip behavior is not temporally constrained.
Many events occurred in December make up Cluster 08, Cluster 09, Cluster 10, and Cluster 11. Indeed, Cluster 09 is the largest cluster obtained in our analysis, consisting of 17 events that occurred on December 26 and 27 (Fig. 9). Some other microearthquakes during October and November are in Cluster 05, three similar events that occurred on November 15 are in Cluster 07, and events from October to early December are in Cluster 06 (Fig. 8).

Results and interpretation
From the epicenter distribution of the events clustered by waveform similarity, we infer that these microearthquakes were generated by asperities rupturing along a common seismogenic source, crossing the Tesistán Valley. Location variation of similar waveform microearthquakes might be explained by multiple shocks of adjacent fault patches that were successively triggered, under a dominant focal mechanism, although minor picking inconsistency among the repeating waveforms could also be a contributing factor (e.g., Rowe et al. 2002Rowe et al. , 2004. We highlighted the epicenters of the located events belonging to the identified clusters in Fig. 10a. Several clustered events in the western Zapopan support the hypothesis of a NE-SW trending fault that extends just over 15 km depth (Fig. 10b), which constitutes the eastern fault of the Zapopan Graben (Núñez-Cornú et al. 2021a). Maximum seismogenic depth is consistent with the hypocenters observed in seismic sequences analyzed in previous studies (Singh et al. 2017;Yamamoto et al. 2018;Marín-Mesa et al. 2019;Núñez-Cornú et al. 2021a). We show in Fig. 10c, d the seismogenic depths, better defined by the overview of 2015, 2016, 2017, and 2020 events. The seismicity distribution exhibits two linear trends quasi-parallel to the faults bounding the Zapopan Graben, which are ~ 8 km long and reach ~ 10 km depth.
In general, the seismicity in this area is related to graben structures. In the north of the Trans-Mexican Volcanic Belt, particularly in the Basin and Range province, the grabens have a step-like array (Aranda-Gómez 1989;Aranda-Gómez et al. 2000). Nevertheless, some reverse focal mechanisms in this area were also reported by previous works (Núñez-Cornú et al. 2021a). This mechanism could be associated with wrench faults, which could be generated in regions with multiple deformation stages and be associated with strike-slip Fig. 7 Clusters 01, 02, 03, and 04. For every cluster, the overlapped seismograms (plot at the top) illustrate the relative size of the earthquakes in the respective cluster. The signals correspond to the vertical component of the seismograms from the station P11 (see Fig. 2). The Cross-correlation matrix is reduced to each particular cluster Fig. 8 Clusters 05, 06, 07, and 08. Same as previous figure faults (e.g., Harding 1990;Sefunc and Efeoglu 2005;Chen 2020). In the study area, there is the possibility of having Basin and Range deformation oriented nearly N-S preferred trend (Henry and Aranda-Gómez, 1992;Aranda-Gómez et al. 2000;Suter 2020b). The lateral component along the Tepic-Zacoalco rift has been subject to discussion since it could be toward the NW (e.g., Luhr et al. 1985;Allan 1986;DeMets and Stein 1990;Carrillo-de la Cruz et al. 2016) or the SW (Ferrari et al. 1994;Urrutia-Fucugauchi 2006).
On the other hand, the buried domino-like basement structure and its influence on the ground fissures formation and distribution have been reported in valleys with groundwater overexploitation (e.g., Brunori et al. 2015;Castellazzi et al. 2016). The sedimentary infill of the valleys obscures any surface expression of the faults; moreover, many of them are not currently active. However, their effects on ground failure allow us to define the fault Fig. 9 Clusters 09, 10, and 11. Same as previous figure distribution in depth. Several geophysical studies have assessed the relationship between ground deformation and urban fracture distribution (e.g., Aranda-Gómez 1989;Ochoa-González et al. 2018;Muro-Ortega et al. 2022;among others). In the study area, the subsurface structure has been reported with ground fractures oriented from N-S to N20°W (Suárez-Plascencia et al. 2005), parallel to the Zapopan Graben. We have inferred two synthetic faults inside the Zapopan Graben (Fig. 11); that is, faults with the same trend and sense of movement as those faults bordering the graben. Also, these synthetic faults suggest a structural control of the basement faults in the location of the ground fractures. Since the study area is at the edge between the Trans-Mexican Volcanic Belt and the Basin and Range province, the faults may extend into this zone and undergo a tectonic reactivation process within the Jalisco Block. These structures would thus accommodate the main NW-SE movement along the Tepic-Zacoalco rift and, therefore, the low magnitude events could be related to the slow motion along the rift (Rosas-Elguera et al. 1996;Selvans et al. 2011).

Conclusions
The seismicity observed in the GMZ has demonstrated that this region tends to generate moderate earthquakes, swarms, and constant microearthquakes attributed to local faults activity by reactivating ancient faults between the dominant rift structures.
Most microearthquakes occurred in 2017 correspond to background seismicity (apparently random events or not clustered), with low magnitude M L < 2.5, which look diffusely distributed in time and space and probably are results of minor subsidiaries faults, mainly in Tesistán Valley. The 2.5 ≤ M L ≤ 3.8 were the largest events of the 2017, most of which occurred in the east of Zapopan, close to the fault trace corresponding to the Zapopan Graben interpreted from seismicity observations in 2015 and 2016 (Núñez-Cornú et al. 2021a).
Microearthquake swarms and similar events observed in only four months of 2017 probably are more frequent than has been observed over the years. Since the regional seismic networks record mostly large or moderate earthquakes, the small ones are generally under reported. These small earthquake swarms illuminate the local tectonic activity particularly. However, the possibility of more significant earthquakes could be somewhat related to major seismogenic structures of the tectonic boundary in this region. So far, only M L ≤ 4 earthquakes in Zapopan, mainly in Tesistán Valley, have been reported (e.g., SSN, 2021;Núñez-Cornú et al. 2021b).
According to the seismic sequences observed during several years, the city of Zapopan is the most seismic active area of the GMZ. From distribution analysis of epicenters and clustering, we infer the existence of synthetic faults to the Zapopan Graben. Our results indicate that the Zapopan Graben has step-like synthetic faults, similar to those of the Basin and Range tectonic province north of the Trans-Mexican Volcanic Belt (Aranda-Gómez et al. 2000). The synthetic faults together with those that form the graben, both are active and in slow motion.
Due to the tectonic complexity of the study area, a reverse focal mechanism for the M L = 3.8 earthquake is plausible. Although the tectonic regime has mainly an extensional setting, the reverse mechanism has been observed in particular cases in previous studies of this region (Marín-Mesa et al. 2019;Núñez-Cornú et al. 2021a). In that time, there was not Fig. 11 Schematic block diagram (not so scale) with the faults bounding the Zapopan Graben (black lines) and the synthetic faults (red lines) with orientation and sense of movement subparallel to the graben limits permanent seismic local network in the study area that led to well-constrained focal mechanism estimates. Our observation somehow could contribute to characterizing the active structures within the Tesistán Valley.
On the other hand, identification and characterization of buried fractures are an important research topic. Irrespective of the tectonic regime, the fracture distribution related to the small seismogenic faults inferred from the microseismicity may lead to insights into overexploited aquifers (Galloway et al. 2008;Hernández-Marín et al. 2017;Muro-Ortega et al. 2022).
The clear presence of microearthquake activity on structures beneath and in the vicinity of the GMZ demonstrates the incompleteness of the seismic catalog in this area. Longterm monitoring is critical to establish a better understanding of overall seismic exposure and to better assess the earthquake risk for infrastructure and the large population in this area. This study documents the level of small earthquake activity which has been previously unreported due to a paucity of nearby, long-term monitoring networks, and demonstrates the importance of rectifying this shortcoming in public threat characterization Appendix See Fig. 12.   Fig. 12 a Cross-correlation matrix by waveform similarity. We show only those earthquakes correlated with CC ≥ 0.75 with at least one other earthquake in this plot. b Dendrogram of nine hierarchical clusters of earthquakes with similar waveform (CC ≥ 0.75). The Number ID refers to every earthquake in the dataset