1 The Seismic Sequences of December 2015 (ML= 4.3) and May 2016 1 (ML = 4.9) in Guadalajara, Jalisco, México 2 3 Francisco Javier Núñez Cornú1, Walter Manuel Rengifo1, Felipe de Jesús Escalona Alcázar 4 2 , Diana Núñez1, Claudia Beatriz Quinteros Cartaya1, Elizabeth Trejo Gómez1, Carlos 5 Suárez Plascencia1 6 7 1 C. A. Centro de Sismología y Volcanología de Occidente (SisVOc). Universidad de 8 Guadalajara, Centro Universitario de la Costa, Avenida Universidad 203, Puerto Vallarta, 9 Jal. México 10 2Unidad Académica Ciencias de la Tierra, Universidad Autónoma de Zacatecas, Calzada de 11 la Universidad 108, Zacatecas, Zac., MÉXICO, CP 98080 12 13 Corresponding Author: 14 Francisco Javier Núñez Cornú, pacornu77@gmail.com, Tel. 51 322 226 2229 15 Centro de Sismología y Volcanología de Occidente, Universidad de Guadalajara 16 Keywords: Crustal seismicity; Seismic hazard; Jalisco Block; Guadalajara, Jal.; Zapopan 17 Graben 18 19 Abstract 20 Historically, the city of Guadalajara has been affected not only by great regional earthquakes 21 (M > 7.0) associated with the subduction process and regional crustal structures but also by 22 local seismic sequences, that caused moderate to severe structural damage to buildings, 23 whose source is not clear. Between December 2015 and May 2016, two seismic sequences 24 occurred, affecting the city of Guadalajara. Both seismic sequences were recorded by the 25 Jalisco Seismic Accelerometric Telemetric Network. The preliminary locations for May 26 2016 sequence estimated by the Antelope automatic system show alignment with an NNE-27 2 SSW trend, west of the city of Guadalajara. The subsequent relocations of theses earthquakes 28 show two N-S alignments at the west of the city of Guadalajara, which agree with December 29 2015 hypocenters. The focal mechanisms analysis of the earthquakes shows that most of 30 them correspond to normal fault mechanisms that are parallel to the hypocentral alignments 31 suggesting the existence of two active faults responsible for the seismic sequences. 32 Furthermore, these structures might constitute a graben, which we refer to as Zapopan 33 Graben. Additionally, we calculated that these faults are 21 and 28 km length, respectively, 34 which indicates that could have the potential to generate shallow earthquakes that reach 35 magnitudes of 6.2 and 6.5, and could cause significant damages in the Guadalajara 36 Metropolitan Zone. 37 38 3 Introduction 39 The Guadalajara Metropolitan Zone (GMZ) includes the towns of Guadalajara, Zapopan, 40 Tlaquepaque, Tonalá, and Tlajomulco is located near the intersection of the Tepic-Zacoalco 41 Rift (TZR), the Colima Rift (CR) and the Chapala - Tula Rift (CTR) (Fig. 1). 42 43 44 45 46 47 48 49 50 51 52 53 Figure 1. Tectonic setting of the region. TMVB: TransMexican Volcanic Belt. 54 55 4 The GMZ and its surroundings rest on two great volcano-sedimentary plains, the Atemajac, 56 and Tesistán (Fig. 2). In this area, the population in 1912 was 210,839 inhabitants (Dirección 57 General de Estadística, 1910). By 1950, it reached 902,987 inhabitants. In 1957, the GMZ 58 began to take shape with the construction of the Industrial Zone in the south of Guadalajara. 59 This fact was a turning point in the urban progression of the city, catapulting the expansion 60 of the urban layout and its population growth. By 1964, the population reached one million 61 inhabitants and an approximate area of 90 km2, with a density of 11,111 people/km2 and by 62 2015, it reached 4.5 million people, an area of approximately 2,900 km2 and a density of 63 1,552 people/km2, representing about 60% of the total population of the Jalisco state. The 64 accelerating increase of the urban layout has been sustained by the development of industry, 65 commercial activities and services, which projects the city in the long term as an essential 66 role of demographic attraction, urbanizing surrounding agricultural lands over the entire 67 northwest sector on the plain of Tesistán (Fig. 2). 68 Currently, GMZ is the second most populated city in Mexico. In this scenario, the La 69 Primavera Caldera is located in the western part of GMZ, within the Zapopan Municipality, 70 and a geothermal project is being developed in this area by the Federal Electricity 71 Commission. 72 The GMZ has a high seismic hazard according to historical reports of both large and medium 73 earthquakes, and local seismic swarms (Waitz and Urbina, 1919; Arreola-Ochoa, 2015). For 74 this reason, it is essential to evaluate and identify the seismogenic structures that exist in the 75 area to assess the seismic hazard that these structures can present both to the population of 76 the GMZ and the infrastructure, including the facilities of the Geothermal Plant at Cerritos 77 Colorados inside the La Primavera Caldera. 78 5 79 80 81 82 83 84 85 86 87 88 Figure 2. Preliminary Antelope locations and Hypo71 relocations for December 2015 and May 2016 89 seismic swarms. Different epicentral locations reported from different sources for May 2016 Main 90 Earthquake. Tesistán (red line) and Atemajac (green line) structures limits shown over the 91 Guadalajara Metropolitan Zone (GMZ) (Map data: Google Earth: Image @2020 CNES/Airbus, 92 @2020 Maxar Technologies).93 94 In December 2015, a seismic sequence began with an ML= 4.2 earthquake; this seismic 95 sequence was studied by Marín-Mesa et al. (2019). The second sequence began on May 11, 96 2016, with an earthquake ML = 4.9 that took place at the GMZ. Different locations for the 97 hypocenter were reported by different agencies: the hypocenter reported by USGS is 98 Guadalajara Metropolitan Zone December 2015 and May 2016 Seismic Sequences Legend Antelope preliminar location 1-Epicenter cGMT 2-Epicenter MainEq Hypo71 3-Epicenter Singh et al (2017) 4-Epicenter SSN 5-Epicenter USGS 6-Epicenter Yamamoto et al(2018) Epicenters December 2015 Epicenters Hypo71 40 kmImage Landsat / CopernicusImage Landsat / Copernicus 1 23 4 5 6 6 20.751ºN, 103º.499ºW, depth 10 km, mb = 4.5. gCMT reports 20.88°N, 103.51°W, 99 depth:17.4 km, Mw = 4.9; Normal Fault, Fault plane: strike 23°, dip=46°, slip-87°. The 100 Servicio Sismológico Nacional (SSN) reports 20.8135ºN, 103.518ºW depth = 8 km, M = 4.8 101 (Fig.2). Here, we analyze the data for this earthquake and its aftershocks registered by a local 102 seismic network. 103 104 Tectonic Setting 105 The study area is located on the Jalisco Block (JB) in western Mexico (Fig. 1). It is close to 106 the intersection between the Late Oligocene-Early Miocene Sierra Madre Occidental (SMO) 107 silicic province and the Late Miocene to recent Trans Mexican Volcanic Belt (TMVB) 108 basaltic to andesitic arc magmatism (Fig. 1). The tectonic boundary of the Jalisco Block to 109 the North is the TZR (Bandy et al., 1995; Ferrari and Rosas-Elguera, 1999). The TZR formed 110 at the end of the Miocene, and it is oriented NW-SE with a length of 250 km and a maximum 111 width of 50 km (Frey et al., 2007). The eastern boundary is an N-S structure of 190 km length 112 and 30 to 60 km width, known as the CR (Allan, 1986). Towards the south, the boundary is 113 the Middle American Trench (MAT) along which the Rivera and Cocos plates are subducted 114 beneath the North American plate. 115 The detachment of the JB from North American plate began in the Early Pliocene (Luhr et 116 al., 1985) and is a response to the rifting of the Gulf of California and subsequent extension 117 (Luhr et al., 1985; Allan et al., 1991). The movement along the TZR and CTR has been 118 considered to be either NW-SE (Luhr et al., 1985; Allan, 1986; Bourgois and Michaud, 1991) 119 or SW-NE (Ferrari et al., 1994; Selvans et al., 2011). DeMets and Stein (1990) and Rosas-120 7 Elguera et al. (1996) support that the movement of the JB to the NW could be due to the 121 oblique subduction of the Rivera and Cocos plates respect to North America (Núñez-Cornú 122 et al., 2002), combined with plate rearrangement due to the separation of the Baja California 123 peninsula from mainland Mexico (Luhr et al., 1985). The subduction velocity varies from 124 1.2 to 2.3 cm/year according to distance to the pole (Minster and Jordan, 1978). Deformation 125 within the JB is extensional (Kostoglodov and Bandy, 1995) though maybe insignificant 126 (Duque-Trujillo et al., 2014). 127 Along the northern JB boundary, in the TZR region, the deformation started at the end of the 128 Miocene and continues to present. During this time, the volcanism varies from subduction-129 related to intraplate (Righter and Carmichael, 1992; Rosas-Elguera et al., 1996). The major 130 stratovolcanoes such as Sangangüey, Ceboruco, Tequila, among others, formed during the 131 initial stages of the TMVB formation (Ferrari and Rosas-Elguera, 1999). At the edge of the 132 TZR, the faults constituted during the Pliocene acted as channels to the magma migration 133 and favored the formation of monogenetic volcanoes; most of them align with the major NW-134 SE and N-S faults, suggesting a structural control (Ferrari and Rosas-Elguera, 1999; Rossotti 135 et al., 2002). Within the TZR, the La Primavera Caldera is at the end of a long-lived silicic 136 upper crustal chamber that is at the cooling stage; however, it has enough heat to develop a 137 geothermal system (Maciel-Flores and Rosas-Elguera, 1992). 138 139 Historical Seismicity 140 The Jalisco region has experienced numerous destructive earthquakes of great magnitude 141 with epicenters along the coast and inland. The historical macroseismic data for the region 142 8 date back to 1544 (Núñez-Cornú, 2011), 143 reported in the past 474 years (Núñez-Cornú et al., 2018) (Fig. 3). Recently, Suter (2018, 144 2019a, 2019b, 2020a, and 2020b) has studied and reviewed some of the historical 145 earthquakes in Jalisco that have caused significant damage. Suter (2019a) studied and 146 concluded that 1563, May 27, MI = 8.0, earthquake took place offshore Puerto de Navidad 147 (now named as Barra de Navidad), and the estimated rupture area can be compared to the 148 1932 and 1995 earthquakes. The 1567 Ameca, Jalisco Earthquake, December 28, Mw = 7.2, 149 was also studied by Suter (2015, 2019a, 2020b). The earthquake doublet of October 22-23, 150 1749 at the northern Colima Graben (1749a and 1749b, Fig. 3) was revised by Suter (2019b). 151 The November 6, 1774, MI = 6.0 Bolaños Graben Earthquake (Fig 3) is described by Suter 152 (2020a). 153 Suter (2018) analyzed the macroseismic data of October 2, 1847, Jalisco Earthquake, and 154 concluded that there were two earthquakes the same day. The first one, a subduction type 155 earthquake, took place at 07:30 am offshore Tecomán, Colima with an estimated magnitude 156 of Mw = 7.4 (1847a-Fig. 3). The second one, a shallow intraplate type earthquake with an 157 estimated magnitude MI = 5.7 (1847b-Fig. 3), took place at 09:30 am and affected the western 158 part of the CTR, destroying the city of Ocotlán and other towns nearby. Last century six big 159 earthquakes took place: 1932, June 03 M = 8.2; 1932, June 18, M = 7.8; 1934, November 160 30, M = 7.2; 1941, April 15, M = 7.9; 1948, December 3, M = 7.0; 1995, October 9. At date, 161 27 big destructive earthquakes occurred in Jalisco have been identified (Fig. 3), and most of 162 them have been associated with the subduction process and/or active crustal faults. There 163 also occurred earthquakes with magnitude greater than 6.0 and less than 7.0 in the region. 164 9 For the period 1918 to 1973, Singh et al. (1984) reported 17 earthquakes; while for the period 165 1973 2020, the SSN reports ten earthquakes in this region. 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 Figure 3. Seismic stations (inverted green triangles) that recorded the May 11-13, 2016 seismic 181 sequence. Historical destructive earthquakes (red circles) near or within the Jalisco Block after 182 Núñez-Cornú et al. (2018) and Suter (2015, 2018, 2019a, 2019b, 2020a, 2020b). 183 10 184 Moreover, destructive seismic swarms that occurred in the inland region of Jalisco have also 185 been reported. The first record of seismic swarms in the GMZ was reported by the 186 Commission on the earthquakes of February 1875 (Matute, 1875), who referred to the events 187 between 1685 and 1687. The seismic sequence of 1749 - 1750 was reported by Martínez 188 Gracida, (1886), and Orozco y Berra, (1887). Waitz and Urbina (1919) list the occurrence of 189 other series of events at GMZ in the following periods: between 1770 and 1771 reported by 190 Martínez-Gracida (1886) and Orozco and Berra (1887); in 1783 in the zone of Ahuacatlán 191 (Fig. 3), by Martínez-Gracida, (1886); in 1806 from March 25 to June at GMZ and in 1844 192 from March 27 to May 27, which alarmed the population of Guadalajara and its surroundings. 193 In 1912 from May 8 to September 10 (Ordoñez, 1912), a seismic swarm affected the 194 municipalities of Guadalajara and Zapopan, alarming the population. Arreola-Ochoa (2015) 195 described this episode "Because of the earthquakes, many families fled the city." Headlines 196 of the newspapers give us an idea of the environment that was serious for those days in the 197 city: "Three-quarters of its inhabitants migrate from Guadalajara, Guadalajara continues to 198 shake." Waitz and Urbina (1919) reported that in May of 1912, 54 events occurred, 199 classifying them into seven strong, ten somewhat strong and 37 light and very light. In the 200 first week of June of that year, 11 events were reported, two somewhat strong and nine very 201 light to light. The tectonic stresses in the area are complex since the three rifts (TZR, CR, 202 CTR) converge; however, it is not known which is the possible seismic source of these 203 seismic sequences. In March 1989, an earthquake swarm took place in the zone of Ameca 204 Amatlán de Cañas (Fig. 3) (Núñez-Cornú et al., 2002). Two seismic sequences occurred from 205 December 15, 2015, until December 20 and during May 11 and May 13, 2016, but only the 206 11 mainshocks were felt like a jolt. The instrumental near field data of this seismicity are the 207 first to be available for the GMZ region. 208 Data and Methods 209 The Jalisco Seismic Accelerometric Telemetric Network (RESAJ) was designed to research 210 and analyze the potential for destructive earthquakes within the Jalisco region (Núñez- Cornú 211 et al., 2018). Each RESAJ station has a 24 bit A/D Quanterra Q330 6Ch or Q330S 6ch DAS 212 digitizer, a Lennartz LE 3D (1 Hz) seismometer, and a Kinemetrics triaxial accelerometer 213 model FBA ES-T. Data are transmitted in real-time to the Central Lab at Centro de 214 Sismología y Volcanología de Occidente (SisVOc) in Puerto Vallarta. Data acquisition was 215 carried out using the Antelope system (Lindquist et al., 2007). 216 The first sequence took place between December 5 20, was studied by Marín-Mesa et al. 217 (2019), who reported seven earthquakes located in the GMZ, at North of the city of Zapopan, 218 with an NNE-SSW alignment direction along 7.8 km, depths < 20 km, and ML < 3.0. The 219 composite focal mechanism calculated from three earthquakes indicated normal fault on a 220 plane in agreement with the epicentral alignment. 221 The second seismic sequence began on May 11, 2016, at 22:35 GMT with an earthquake (ML 222 = 4.9), finishing by May 14 at 23:00 GMT. For this seismic sequence, 17 RESAJ stations 223 were operating and recording the data (Fig. 3); 3 more stations (RMMA, GUMA, NAMA) 224 from a temporary seismic network, located at Mascota, Jalisco area, also recorded this 225 activity. Five of the RESAJ stations are located less than 100 km from the epicentral zone. 226 The Antelope system detected and located 40 earthquakes in the GMZ area, using the IASP91 227 velocity model, with magnitudes between 1.8 and 4.9. These preliminary locations of the 228 12 seismic sequence suggest an alignment NNE-SSW of the earthquakes across the western 229 urban zone of GMZ (Fig. 2). 230 After reviewing the preliminary locations, the first step was to identify all the local 231 earthquakes in the helicorders records at La Primavera Caldera station (CPRJ), the closest 232 station to Guadalajara, to count the total number of events of the seismic sequence. The 11-233 14 May seismicity records were extracted from the Antelope database in mini seed format. 234 This database was converted to a Seisan format (Haskov and Ottomeller, 1999) for the 235 waveform analysis. The arrival times and polarities of the P waves were read from the vertical 236 components, and S waves were read from the horizontal components; error in phase readings 237 is less than 0.1 s. A total of 115 earthquakes were identified, from which we chose 54 events 238 that were recorded by more than five RESAJ stations and had at least 5 P and 2 S wave arrival 239 readings. 240 241 These earthquakes were relocated with the Hypo71 program (Lee and Lahr, 1972) using the 242 velocity model (Fig. 4) proposed by Núñez-Cornú et al. (2002). Eight different depths were 243 used as the initial solution to obtain the best solution. Those with the lowest location errors 244 were selected. The media error values solutions obtained were: Root Mean Square (RMS): 245 0.49s; the standard error of the epicenter (ERH): 3.7 km; and the standard error of the focal 246 depth (ERZ): 2.0 Km. (Table 1). The local magnitude relation (Lay and Wallace, 1995) was 247 used in this study. 248 249 13 250 Num Yr mo day ho min sec Lat Long Depth (Km) Mag RMS ERH ERZ Gap Dist NL w0 16 5 11 22 35 18.8 20.78483 -103.43130 9.85 4.92 0.51 2.3 1.6 247 24.1 29 w1 16 5 11 22 45 8.65 20.74600 -103.46320 10.58 2.50 0.23 2.6 2.2 243 18.7 8 w2 16 5 11 22 47 25.6 20.75300 -103.46170 11.66 3.13 0.44 2.6 1.8 244 19.5 19 w3* 16 5 11 22 50 2.84 20.72133 -103.53020 7.55 2.70 0.61 5.7 6.9 238 14.0 10 w5* 16 5 11 23 15 53.3 20.70517 -103.52930 7.92 2.58 0.57 8.8 13.5 237 77.6 8 w6 16 5 11 23 22 43.5 20.75883 -103.54550 14.35 1.83 0.35 3.7 1.4 240 18.1 11 w7 16 5 11 23 45 9.48 20.67783 -103.46170 13.55 3.16 0.72 3.3 1.7 238 12.5 26 w8 16 5 11 23 56 23.8 20.78550 -103.45570 9.69 2.57 0.43 4.3 4.9 246 23.0 9 w10* 16 5 12 0 34 21.2 20.71017 -103.51470 18.47 2.27 0.67 5.7 2.0 238 13.1 11 w12 16 5 12 1 18 47.6 20.73250 -103.45330 12.64 3.03 0.55 2.5 1.6 243 17.9 25 w13 16 5 12 1 29 33.9 20.77550 -103.44130 5.00 2.95 0.49 2.3 3.5 246 22.6 21 w14 16 5 12 2 1 41.4 20.76267 -103.54750 16.62 2.53 0.23 3.4 1.1 320 18.6 8 w15 16 5 12 2 30 59.2 20.78017 -103.54380 17.01 2.43 0.19 3.7 1.3 321 20.5 7 w16* 16 5 12 3 21 36.2 20.64733 -103.54550 12.98 2.64 0.61 4.4 1.9 232 5.8 14 w17 16 5 12 4 28 29.7 20.73333 -103.46700 11.36 2.50 0.40 2.3 1.7 242 17.3 19 w19 16 5 12 4 49 49 20.78450 -103.44730 9.05 2.49 0.34 2.0 1.8 247 88.0 16 w20* 16 5 12 5 30 14.7 20.54383 -103.54550 11.78 2.17 0.50 4.6 4.3 139 5.7 19 w21* 16 5 12 5 32 37.3 20.66483 -103.54550 21.80 2.06 0.81 6.2 5.3 257 7.7 11 w24 16 5 12 8 29 25.3 20.76933 -103.44300 8.66 2.34 0.47 2.6 2.6 249 22.0 19 w26 16 5 12 10 50 25.7 20.81383 -103.41620 12.64 2.51 0.30 3.0 1.6 250 27.6 11 w28* 16 5 12 14 8 42.1 20.69383 -103.54550 14.75 2.11 0.67 5.4 2.9 235 10.9 9 w29* 16 5 12 15 26 27.9 20.59683 -103.57670 10.61 1.50 0.66 7.1 5.3 177 3.4 8 w31 16 5 12 18 22 8.53 20.74700 -103.46930 12.37 2.03 0.31 3.8 3.4 243 18.5 9 w32 16 5 12 18 24 7.21 20.80200 -103.54450 19.85 2.16 0.03 1.1 0.7 322 22.9 8 w33 16 5 13 15 17 9.59 20.45017 -103.54550 15.00 2.25 0.60 5.9 9.2 136 16.1 11 w35 16 5 13 19 7 32.3 20.72883 -103.45330 11.04 2.55 0.74 3.7 3.0 242 62.6 22 w36 16 5 13 10 37 59.1 20.77917 -103.45170 7.59 2.55 0.40 1.8 2.0 246 22.5 26 251 Table 1. Earthquakes relocated for the May 2016 seismic swarm. Mag: Local mag; RMS: Root mean 252 square error of time residuals in sec; ERH: Standard error of the epicenter in km; ERZ: Standard error 253 of the focal depth in km; Gap: Largest azimuthal separation in degrees between stations; Dist: 254 Epicentral distance in km to the nearest station; NL: number of phase readings. * Epicenters inside 255 La Primavera Caldera. 256 257 258 14 259 260 261 262 263 264 265 266 Figure 4. P-wave velocity model for Jalisco Block region (Núñez-Cornú et al., 2002). 267 268 The focal mechanism of the earthquakes was evaluated with the MEC93 code (Núñez-Cornú 269 and Sánchez-Mora, 1999) using the outputs obtained from the Hypo71. The program MEC93 270 is based on a probabilistic model proposed by Brillinger et al. (1980). This method for focal 271 mechanism estimation is also described by Udías et al. (1982), which uses a weighting 272 parameter p, and a relation between readings and theoretical amplitude values, as a measure 273 of the fit of a set of observations concerning the joint solution. A threshold value of p and a 274 minimum of readings are selected to form a group with a joint solution. Also, a pi value for 275 each i event is defined as a function of the minimum and the maximum number of readings 276 for all the events; this pi is the threshold value to accept the event into the group. 277 278 15 Results and Discussions 279 The analysis of the first 115 earthquakes identified on helicorder records allowed us to obtain 280 54 events that could be read for enough stations; the remainder were too small and/or had 281 station high signal-noise levels. Of the earthquakes processed, 39 achieved the standards 282 mentioned before, but only 27 were in the area of interest (Table 1). All earthquakes relocated 283 for the study period are shown in Fig. 2 and in Fig. 5a with error bars, while the depth profile 284 projected along the EW line is shown in Fig. 5b. Figures 5a and 5b include the activity 285 reported by Marín-Mesa et al. (2019). Our results show that there are two sub-parallel 286 alignments with an approximate N-S direction at the western edge of the urban area (Fig. 2) 287 lying in part of the GMZ and La Primavera Caldera, and from figure 5b it is possible to 288 estimate a fault width (in-depth) of 10 -12 km in both. The western alignment crosses La 289 Primavera Caldera, and the eastern one marks the "boundary" of the urban area and the 290 contact between Atemajac and Tesistan volcanic structures (Fig. 2). The separation distance 291 between the alignments is above 10 km (Fig. 5a). 292 The magnitude of the largest earthquake (ML= 4.9) suggests a rupture area of 6 km2 (4.0 x 293 1.5 km) with an approximate slip of 10 cm (Wyss, 1979; Hanks and Kanamori, 1979). 294 Furthermore, we analyzed the distribution of the epicentral location error bars plotted in 295 figure 5, where we can distinguish two groups. The first one is characterized by having errors 296 in the epicentral location less than 5 km and corresponds to those epicenters outside La 297 Primavera Caldera. The second group of epicenters is located within La Primavera Caldera 298 with epicentral errors between 5 and 8 km and RMS values between 0.50 and 0.81. The 299 difference in the residuals is due to the volcanic complex and heterogenic structure of La 300 Primavera Caldera. 301 16 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 Figure 5. (A) Reported earthquakes (Triangles) for the period December 5 20 Marín-Mesa et al. 317 (2019). Relocated earthquakes (circles) for period May 11-13, 2016 (Table 1) Error bars in km; 318 Caldera La Primavera marked with dashed magenta line. Inverted triangle: Seismic station CPRJ. (B) 319 Cross section along Profile P1 showing the hypocenters projected in a line E W. Error bars in km. 320 17 321 To compare the differences in the source, we analyzed the waveforms recorded by MZCJ 322 station for the largest earthquakes of the seismic swarm, which are shown in figure 6. This 323 station is located near the city of Mazamitla (Fig. 3) on the east side of the Jalisco Block. The 324 pathway to MZCJ station is very similar for all earthquakes, which are located at a distance 325 between 95 and 112 km and with a 13° directional gap. It is possible to observe differences 326 in the P and S waveforms, indicating different types of source processes and radiation 327 patterns. At least five groups with waveform similarities can be found (Fig. 6). Earthquakes 328 w20, w16, and w14 are located in the western alignment, the rest in the eastern one. In the 329 other groups, the epicenters are at a distance less than 5 km between them. 330 The focal mechanisms at the epicentral zone were evaluated from the take-off and azimuth 331 angles generated by Hypo-71. Ten solutions, four individual mechanisms, and six composite 332 mechanisms, or groups, (including 14 earthquakes) were obtained for a total of 18 333 earthquakes (Table 2), all with a P-polarity score (or success rate) of 80% or higher, and focal 334 planes well-controlled (Fig. 7). From the total of the obtained mechanisms, 12 earthquakes 335 show a normal fault mechanism, and 6 of them show an inverse fault mechanism. In both 336 cases, the directions of the fault planes are congruent with the directions of the two seismic 337 alignments observed from the epicentral locations that can also be observed at the figures 8a 338 and 8b, where the data reported by Marín-Mesa et al. (2019) are included. 339 340 341 18 342 343 344 345 346 347 348 349 350 351 352 353 354 355 Figure 6. Waveforms (vertical component, not filtered) from the largest earthquakes of the May 11, 356 2016 earthquake swarm recorded at MCZJ Station (Fig. 3). For each trace, left side: above the trace 357 T0 (origin time) of the earthquake, below the trace, the initial second of the trace. Above the trace: 358 name of the earthquake, local magnitude, distance and azimuth from the epicenter. Right side: 359 maximum and minimum amplitude in microvolts. 360 19 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 Figure 7. Focal Mechanism Solutions: four individual earthquakes (G07, G11, G12, G14) and five 376 composite focal mechanisms (G01, G02, G03, G04, G06, G13) for 14 earthquakes. Solid black dots 377 compressions, solid green dots dilatations (Table 2). 378 20 Table 2. Parameters of the focal mechanism showed in Figure 8. NPol: Number of polarities readings. 379 %: score (or success rate) for P polarities. 380 381 The May 2016 seismic sequence began within the eastern alignment with the largest 382 earthquake, south of the swarm reported by Marín-Mesa et al. (2019), and the fourth locatable 383 Group Earthq. Mag Npol % Planes Strike (°) Dip (°) Slip (°) Axis Plunge (°) Azim (°) 01 w00 4.92 31 83 Plane A 140 47 -88 P: 2 90 w21 2.06 Plane B 317 43 -88 T: 88 229 w32 2.16 02 w01 2.5 11 100 Plane A 155 34 -65 P: 19 311 w16 2.64 Plane B 5 60 -74 T: 77 83 03 w06 1.38 27 85 Plane A 157 38 -53 P: 25 335 w08 2.57 Plane B 20 60 -65 T: 78 92 w19 04 w13 2.95 18 83 Plane A 175 33 -42 P: 32 353 w14 2.53 Plane B 48 69 -64 T: 70 119 06 w31 2.03 13 100 Plane A 129 41 -77 P: 11 344 w26 2.51 Plane B 329 50 -75 T: 85 284 07 w35 2.55 12 92 Plane A 207 56 24 P: 80 158 Plane B 103 71 37 T: 51 60 11 w24 2.34 11 82 Plane A 157 35 42 P: 72 101 Plane B 30 68 63 T: 32 339 12 w07 3.16 18 83 Plane A 161 33 51 P: 73 98 Plane B 25 65 68 T: 27 331 13 w15 2.43 9 100 Plane A 158 38 80 P: 82 61 w29 1.5 Plane B 326 53 82 T: 10 202 14 w10 2.27 10 80 Plane A 162 35 78 P: 84 63 Plane B 327 56 81 T: 6 201 21 earthquake occurred 15 min later within the western alignment (Figs. 8a and 8b). After that, 384 both features were simultaneously active, but no hypocentral migration pattern was observed 385 (Fig. 8b); the eastern alignment agrees with the alignment reported by Marín-Mesa et al. 386 (2019). The main event is included in composite focal mechanism Group 01, showing a 387 normal fault focal mechanism and suggesting an NNW SSE fault with a P (090, 02) and T 388 (229, 88) axis. The western alignment can be observed crossing La Primavera Caldera with 389 NS direction (Fig. 8). Of the waveforms shown in figure 6, the earthquakes w0, w13, w16, 390 w14, and w19 have a normal fault mechanism; meanwhile, the earthquakes w35 and w24 391 have an inverse fault mechanism, with apparent differences in the waveforms. 392 The focal mechanisms within and adjacent to the study area have been obtained by recent 393 seismicity and by ancient faults. Singh et al. (2017), using data from SSN, which had its 394 closest station beyond 100 km from the epicenter, studied this earthquake and suggested that 395 most of the events during the entire sequence were tightly clustered, probably within a 396 volume of 1 to 2 km radius; this does not agree with our results. They also obtained normal 397 fault mechanisms oriented NNE-SSW (azimuth of 021°) with tension axis at 110°. 398 Yamamoto et al. (2018) also studied this earthquake using data from the SSN and other 399 station; they suggested a strike-slip fault mechanism solution oriented SSE - NNW (azimuth 400 of 158º) with a P (021°, 29°) and T (289°, 04°) axis. However, we obtain a normal fault 401 mechanism with P (090°, 02°) and T (229°, 88°). Differences in these results may be due to 402 the different models of velocities and datasets (azimuth coverage, distance to the epicenter, 403 and the number of stations) were used in each case. 404 405 22 406 407 408 409 410 411 412 413 414 415 416 417 418 419 Figure 8. Time sequence: (A) Map of the epicenters with focal mechanism. (B) Cross section along 420 Profile P1. 421 422 23 423 Most of the RESAJ stations are less than 200 km from the epicenter; meanwhile, most of the 424 SSN stations have epicentral distances farther than 200 km. Our focal mechanism solution 425 and the obtained by Singh et al. (2017) are a bit similar, and the polarity data of their focal 426 mechanism are congruent with our results. The 3 orientations from ESE-WNW to ENE-427 WSW were also obtained from the preferred orientation the geological structures measured 428 near the study area (Ferrari and Rosas-Elguera, 1999; Duque-Trujillo et al., 2014). The N-S 429 trend of the proposed Zapopan Graben follows the same trend as the Basin and Range 430 structures defined in central Mexico (Nieto-Samaniego et al., 1999; Aranda-Gómez et al., 431 2000). So, this graben is apparently formed on reactivated structures (Moore et al., 1994; 432 Frey et al., 2007). On the other hand, the reverse focal mechanisms could be related to 433 wrenching faults due to the right lateral movement component along the faults that define the 434 Tepic-Zacoalco rift (Nieto et al., 1985; Rodríguez-Castañeda and Rodríguez-Torres, 1992). 435 436 Conclusions 437 The hypocenters of the December 2015 and May 2016 seismic sequences show two apparent 438 NS alignments west of the GMZ, and these structures have the same direction of the CR. The 439 epicenters suggest the existence of two active quasi-parallel structures or faults separated 440 about 10 km. The fault strikes obtained from the focal mechanisms coincide with the 441 direction of the alignments. Using the waveforms recorded at MZCJ station, it is also possible 442 to conclude that different types of sources took place during the seismic sequence. 443 24 Furthermore, the solutions of most of these mechanisms correspond to a normal fault, 444 suggesting the existence of a graben, which we propose as Zapopan Graben. The western 445 fault crosses La Primavera Caldera. In the central part of the eastern fault, the structural 446 features are not visible on the ground owing to the urban development. The proposed length 447 of the eastern fault is approximately 21 km (including the December 2015 segment), and the 448 western fault is 28 km. From the hypocentral depths of the seismic sequence, it is possible to 449 assume a fault width of approximately 10 km, so the maximum earthquakes that could be 450 generated by these faults would be approximate of magnitudes 6.3 and 6.5, respectively. 451 Moreover, these faults are shallow so that these earthquakes could cause significant damage 452 as the 1847b Ocotlán earthquake (Suter, 2018) and the historical damage reported. More 453 geophysical and geological studies are needed to confirm the existence of these tectonic 454 structures. However, the macroseismic reports from these seismic sequences are not 455 comparable to the historical reports about seismic sequence previously mentioned; only the 456 mainshocks were felt by most of population, and not relevant damages were reported. Recent 457 microseismicity studies at La Primavera Caldera (Quinteros-Cartaya et al., 2020) reports 458 microearthquake activity (ML <3.0) inside the proposed Zapopan Graben. This proposed 459 graben could be the source of the historical seismic sequences that occurred at GMZ. 460 The GMZ is the second-largest urban area of Mexico, and unlike Mexico City, where there 461 are historical reports of local seismicity up to at least magnitude 6.0 representing a significant 462 seismic hazard that must be studied and evaluated in great detail. Particularly noteworthy are 463 these studies for the development of the Cerritos Colorados Geothermal Field in La 464 Primavera Caldera. As a result of this study, the research group CA-UDG-276 (SisVOc) is 465 25 carrying out a joint project with the Zapopan Civil Defense Authorities to install a seismic 466 network in the municipality of Zapopan. 467 468 DATA AND RESOURCES 469 All seismic data collected by RESAJ are in a database at CA-UDG-276 SisVOc Research 470 Group. The data may be available for use in collaborative research projects between SisVOc 471 and other interested institutions by specific agreements. For information, contact 472 pacornu77@gmail.com. 473 474 Funding 475 This Research is funded by Projects: Centro Mexicano de Innovación en Energía 476 Geotérmica (CeMIE-Geo). P24. Proyecto Secretaría de Energía - Consejo Nacional de 477 Ciencia y Tecnología (SENER-CONACyT) 201301-207032; Consejo Nacional de Ciencia y 478 Tecnología Fondos Mixtos Jalisco (CONACyT-FOMIXJal) 2008-96567 (2009); 479 CONACyT FOMIXJal 2008-96539 (2009); CONACyT-FOMIXJal 2010-149245 (2010) 480 and Universidad de Guadalajara internal Projects; Walter M. Rengifo was funded by a Master 481 scholarship from CONACyT, CVU 596343, Reg. 572768. 482 Acknowledgments 483 The authors are grateful to anonymous reviewers for a thorough review and constructive 484 suggestions that improve the quality of this article, also would like to thank Quiriat J. 485 Gutiérrez-Peña for his assistance during the data selection and processing stages of this work. 486 487 References 488 Allan, J.F. (1986). 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