Seismicity Induced by the Cajati Open Pit Mine, SE Brazil: a case of stress concentration, weakness zone, and favorable geometry

doi:10.21203/rs.3.rs-4970087/v1

Fluid injection and reservoir-induced seismicity have been studied for decades. However, seismicity induced by crustal unloading in open-pit mines are seldomly reported. We describe a case associated with a large mine in SE Brazil exploiting carbonate rocks. The pit is 1.4 km long, 0.8 km wide, 300 m deep. 440 Mton of rock have been extracted. Eleven earthquakes (magnitudes 2.0 to 3.3 mb) have occurred since 2009. The largest event (3.2 Mw in 2015) caused expressive cracks in the mine benches. Relocated epicenters, using correlated P- and S-waves at regional stations, align in a single NNW-SSE oriented, 0.5 km long fault beneath the pit major axis. Focal mechanism shows reverse faulting, as expected for crustal unloading, with Coulomb stress increase of 2 MPa. In the coastal ranges of SE Brazil, low-velocities at lithospheric depths suggest stress concentration in the upper crust. The coast-parallel NE-SW P axis is consistent with stress rotation due to continent/ocean transition. Aeromagnetic data shows a NNW-SSE regional fault crossing the mine area. The Cajati mine is a classic case where several factors combine to induce seismicity: upper crustal high stresses, favorable orientation of a previous weak zone, and large Coulomb stress changes from unloading.

Anthropogenic seismicity is important for scientific studies (source physics, and fluid/fault interaction) as well as societal impact. Reported cases have been associated mainly to underground mining (38%), fluid injection at high pressures (26%), and water reservoir impoundment (24%), according to the surveys of Foulger et al.(2018) and Wilson et al. (2017). Fluid injection and underground mining can be .directly linked to the ensuing seismicity, almost as a controlled experiment. On the other hand, water reservoir and open pit mining ("quarrying") are often correlated with the seismicity by indirect arguments, such as changes in the historical seismicity, and spatial and temporal proximity (e.g., Davis & Frohlich, 1993), when attempting to distinguish them from natural causes.

For this reason, some cases of seismicity attributed to anthropogenic effects can be controversial. Direct links between the causes (reservoir filling, mining activity, for example) and the seismicity are not always clear. The physical explanation (pore-pressure diffusion, unloading, etc) is not always sufficient to discriminate between an anthropogenic or a natural cause, especially in seismically active areas. For example, the 2008 M ~ 8 Wenchuan earthquake has been associated to the Zipingpu reservoir, China, by several authors (e.g., Ge et al., 2009), but disputed by others (e.g., Gahalaut & Gahalaut, 2010). The 2019 4.9 Mw Le Teil earthquake in southern France has been claimed as induced by crustal unloading of a large nearby quarry (De Novelis et al., 2020; 2021), but disputed by Burnol et al. (2023) who proposed hydraulic pore-,pressure diffusion down to a shallow network of faults after a period of heavy rain.

Few cases of seismicity induced by open pit mining (often referred to as "quarrying") have been reported in the literature. The compilation of Klose (2013) and the HiQ ("Human induced earthQuake") database (Foulger et al., 2018; Wilson et al. 2017) report only four cases of quarrying induced seismicity, with maximum magnitude 5.2 Mw. Table 1 lists these four cases together with two additional ones. In Brazil, a few cases are known in the mining industry, but have not been reported yet in the literature. Here, we show a clear case of seismicity induced by quarrying in the Cajati mine, SE Brazil (maximum magnitude Mmax = 3.2 Mw). This should contribute to improve worldwide statistics and hopefully help improve estimates of seismic hazard in the development plans of large deep quarries.

Table 1

Main published cases of seismicity induced by open-pit ("*quarry*"). Fault type: "R"= reverse (horizontal compression); "depth" and "length x width" refer to the open pit. ΔM = quantity of excavated rock (in 10⁹kg). Magnitudes: b = mb, L = ML, w = Mw (preferably from ISC).
Largest Earthquake				Name	Mine and Open-Pit Data				Refs.
Year	Mag	depth (km)	type	Locality	begin	depth (m)	length x width (km)	ΔM (Mton)
1974	3.3 b	0.8	R	Wappinger Falls, USA	1952	50	2.1 x 0.8	70(*)	1, HiQ
1981	2.5 L	0.	R	Lompoc, CA, USA	1940	50–100	~ 1 x 0.5	25(+)	2
1994	4.3 w	1.5	R	Cacoosing, PA, USA	1992	50	0.75 x 0.6	11	3, HiQ
2013	5.2 w	4	R	Bachat, Kuzbass, Russia	1948	320	10 x 2.2	4.5(+)	4, HiQ
2015	3.1 L	0.5	R	Plainfield, CT, USA	1970?	~ 20	0.6 x 0.4	11	5
2015	3.2 w	~ 0.5	R	Cajati, SE Brazil	1946	300	1.4 x 0.8	440	this paper
2019	4.9 w	1	R	Le Teil, France	1833	60–80	1.5 x 1.5	100	6, 7, HiQ
(*) Klose (2013) mentions 25 "Mton", which is probably volume; Pomeroy et al.(1976) gives unloading of 70 Mton.
(+) = order of magnitude estimate based on mine area, maximum depth, and typical density.
Refs: 1) Pomeroy et al., 1976; 2) Yerkes et al., 1983; 3) Seeber et al.(1998); 4) Emanov et al.(2014; 2021), ISC;
5) Kondas, 2020; 6) De Novelis et al. (2020, 2021); 7) Ritz et al.(2020); HiQ) Foulger et al.(2018).

	Table 2. Cajati events recorded at RSBR in SE Brazil. Coordinates are absolute location given by the regional network. Event 2 (2015 mainshock) was used as reference in the relative location. "Int" is intensity in Cajati town, 3 to 5 km from the mine. Events with bold numbers were used for the P- and S-wave correlations and relative location
No	Date	Origin	latitude (°)	longitude (°)	mag (mR)	Int (MM)
1	2009-06-08	17:16:04	-24.74	-48.12	2.9	-
2	2015-10-23	06:53:11	-24.73	-48.12	3.3	IV
3	2016-05-20	23:27:22	-24.75	-48.11	2.8	III
4	2016-11-18	06:34:21	-24.78	-48.13	2.4	-
5	2018-05-25	21:16:33	-24.77	-48.05	2.8	IV
6	2018-06-05	20:22:47	-24.82	-48.05	2.3	II?
7	2019-01-31	08:22:35	-24.77	-48.12	2.7	IV
8	2020-04-16	16:59:50	-24.72	-48.12	2.3	III
9	2020-07-23	14:23:12	-24.68	-48.12	2.0	II
10	2022-07-15	21:12:26	-24.66	-48.17	2.3	-
11	2023-09-06	23:02:12	-24.70	-48.12	2.3	II

The Cajati Mine, officially called "Morro da Mina" (Mine Hill), is located in the coastal ranges of SE Brazil (Fig. 1) and explores carbonate rocks of the mafic/ultramafic Jacupiranga Complex, a magmatic intrusion composed of ultramafic, mafic, alkaline rocks and carbonatites (Huang et al., 1994; Rugenski, 2006; Marangoni & Mantovani, 2013). With a Lower Mesozoic age of ~ 130 Ma (Amaral, 1978), it was part of a widespread magmatic event with several other intrusions in SE Brazil that accompanied the initial stages of the South Atlantic rifting. The complex has an oval shape, NNW-SSE oriented, with a gravity anomaly of 70 mGal above the regional values. At the surface, rock densities vary from 2.8 g/cm3 (carbonatites) to 3.5 g/cm3 (jacupiranguites and pyroxinites). Gravity modeling indicates a deep body (down to ~ 16 km) with an average density of 3.3 g/cm3 above a 2.7 g/cm3 of the host rocks (Rugenski, 2006).

Operations started in 1946 by mining surface layers of residual ore, going to deeper layers in the 1960's (Brasil Mineral, 1989). The present (2024) pit border is 1.4 km long in the NNW and 0.8 km across. The present deepest point in the pit has an altitude of -200 m (with respect to mean sea level), which gives a maximum depth of 270 m with respect to the present border around + 60 to + 80 m altitude. In the 1980's, the mine area still included a small hill (about 79 m above the border (IBGE, 1987)), which was completely excavated down to 180m below m.s.l., as of 2024. So, the maximum excavated depth ranges between 270 m to 330m, depending on which criteria is used.

Currently (2024), the size of the open pit (1.4 km long, 0.8 km wide, depth down to about 300 m). has an approximate volume of 1.5 10⁸ m³. The excavated carbonatites and ultramafic rocks have an average density of 2910 kg/m³, which would correspond to a total excavated rock mass of 440 Mton (Table 1; written communication from Mosaic Fertilizantes S.A.)

The largest historical event in the southernmost part of the state of São Paulo is a 4.6 mb in 1946 (Sisbra 2024), about 60 km SE of the Cajati mine (Fig. 1). Regional earthquakes have been recorded by the Brazilian Seismographic Network (RSBR; Bianchi et al., 2018) since 2009 (Fig. 1, Table 2). The largest event associated with the mine occurred in 2015 with magnitude 3.3 mb and 3.2 Mw (Table 2): it was strongly felt in the Cajati Mine, in the Cajati town (3 to 5 km distance), and up to about 10 km away. This event caused cracks in one of the quarry benches, with up to 10 cm displacement, as reported by Coppedê (2018).

The uncertainties of the absolute locations of the regional Brazilian network, around 10 km when only few stations between 100 and 300 km distance are used, prevented the recognition that the other events had occurred at the same epicenter of the 2015 mainshock. Absolute differences between the regional epicenters (Table 2) and the mine pit ranged from 2 to 12 km.

2.1 Relative epicenter location

All events detected by the Brazilian seismic network (Table 2) had very similar P- and S-wave signal at all stations, as seen in the examples of Fig. 2. This allowed a relative location by picking P and S arrivals for all events using cross-correlation with the mainshock of 2015 (Nogueira, 2023). The resulting relative epicenters of the six best recorded events align in the NNW-SSE direction (Fig. 3b) which is the same direction of the open pit (Fig. 3a). This relative location used stations in the 100–300 km range. Although epicentral uncertainties were small enough to show the NNW-SSE alignment, the uncertainties in the hypocentral depths did not allow a well constrained dip of the fault plane.

Since 2018, the Cajati mine operates a local seismic network to monitor the microseismicity. Most of the microseismicity is observed right beneath the open pit, as seen in Fig. 3c. In addition, three of the events used in the relative location (Table 2) were also located by the Cajati local net with epicenters right beneath the open pit and aligned in the same NNW-SSE direction (Dias, 2022).

2.2 Focal mechanism

The focal mechanism of the 2015 mainshock was determined using the three closest stations at distances between 100 and 160 km. Two different methods were used: a) inversion of waveform envelope and b) full waveform inversion.

a) The envelope inversion method of Carvalho et al. (2018), based on the ISOLA code, was used with additional constraint from three clear dilatational polarities. The polarities are needed to resolve the ambiguity of the slip direction when only the envelope is used. On the other hand, the envelope method is less dependent on the accuracy of the velocity model. The inversion was carried out in the frequency band 0.05–0.3 Hz. The best-fitting solution was obtained for a source depth of 0.3 km, in excellent agreement with the depth ranges of the hypocentral locations by the local mine network (Dias, 2022). Figure 4 shows the resulting reverse faulting mechanism with the nodal planes oriented in the NW-SE and NNW-SSE directions, consistent with the alignment of the epicenters.

b) The FMNEAR (Delouis, 2014) code was also used with the same three stations (Fig. 5). A frequency band of 0.1 to 0.2 Hz was used attempting to fit P and S waves as well as the surface waves. Again, a reverse mechanism was obtained with NW-SE and NNW-SSE nodal planes. Different depths were tested, and the best hypocenter was found at around 1 km. The reverse faulting mechanism with NE-SW oriented P axis was a stable solution.
In addition to the two waveform modeling inversions above, a composite solution using all P-wave polarities from all events of the series was tried. Figure 6a shows that only three clear dilatational polarities were observed (all other P waves had weak Pn arrivals at more than 200 km distance) and a focal mechanism was not possible. However, the P-wave polarities are consistent with a reverse faulting mechanism with NW-SE striking nodal planes. An inversion using SH polarities and SH/P amplitude ratios was also tried using the FOCMEC code (Snoke at al., 2003). The solution (Fig. 6b) could not be constrained but a family of reverse faulting solutions with NW-SE nodal planes is also indicated.

Figure 6 compares all focal mechanism solutions above. All attempts show a nodal plane consistent with the NNW-SSE orientation of the fault as determined by the relative locations (Fig. 3b). Despite the small number of stations and few impulsive polarities, it is clear that the Cajati earthquake series occurs on a reverse fault oriented in the same NNW-SSE direction of the open pit (Fig. 3a). Interestingly, the worldwide compilation of open-pit mine-induced earthquakes (Table 1) shows that the largest event (Bachatsky mine in Russia, magnitude 5.2 Mw) was also a reverse faulting event with the fault plane aligned with the 10km long open pit.

Open-pit mining seismicity is usually attributed to stress perturbations related to vertical unloading of the crust due to the removed rock mass (e.g., Foulger et al., 2018). All reported cases show reverse faulting (e.g., Klose, 2013; Table 1), consistent with high horizontal stresses being the major principal stress (S1), and the vertical overburden being the least compressive stress (S3). In this scenario, the removal of the rock mass reduces the vertical overburden (decreasing S3), which increases shear stresses in pre-existing faults.

Despite being under reported in the literature, earthquakes induced by quarrying only occur in exceptional cases. The vast majority of large quarries do not produce any noticeable seismicity (M > 3) monitored by regional networks. This means that special conditions must exist to favor the occurrence of induced earthquakes. We now examine which special conditions could be listed to explain the Cajati case.

3.1 Stress concentration due to lithospheric thinning

The causes of intraplate seismicity are varied and still in debate. In general, most models used to explain seismicity within stable continental regions rely on mechanisms of stress concentration or weak zones, or a combination of both effects (e.g., Mazzoti et al., 2007). In Brazil, lithospheric thinning has been proposed as a contributing factor to increase stresses in the upper crust: a shallow and weak asthenosphere cannot support the regional plate-wide compressive stresses, which tend to concentrate in the upper brittle crust (e.g., Assumpção et al., 2004; Rocha et al. 2016). A similar model has been proposed for the British Isles by Lebedev (2023).

Seismic velocity anomalies from regional tomography are often used as proxies for variations of lithospheric thickness. Figure 7 shows the seismicity in the continental part of SE Brazil together with the P-wave anomalies at 165 km depth from the teleseismic tomography of Rocha et al. (2019). There is a trend of higher seismicity to occur in areas with lower velocities, presumably where the lithosphere is thinner. The Cajati area is a relatively active zone in SE Brazil and also an area with low P-wave velocities. In addition, it can be seen that almost all Mesozoic magmatic intrusions (green triangles in Fig. 7) occur in areas with low velocity anomalies, as previously noted by Assumpção et al. (2004) and Rocha et al. (2011). This observation is consistent with lithospheric thin spots being more favorable to originate magmatic intrusions.

3.2 Coast parallel compression

Intraplate stresses in South America have been mapped by Assumpção et al. (2016) by compiling results mainly from focal mechanism studies. Figure 8 shows the available stress data from focal mechanisms (colored solid bars), in-situ measurements (blue open bars) and a few breakout data (black bars). In the northern part of the area of Fig. 8 (San Francisco craton and NE part of the Paraná basin) stress data indicates a general E-W compressional stress regime (shown by the large black arrows). An average E-W oriented compression is observed in most of the mid-plate region in South America with a trend towards SE-NW rotation in northern Brazil (Assumpção et al., 2016). E-W compression in mid-plate South America is usually explained as due to the Mid-Atlantic ridge-push force combined with convergence motion between the Nazca and South American plates. However, local effects due to the continent/ocean structural transition can rotate the maximum horizontal stress (SHmax) making it parallel to the coast (e.g., Tingay et al, 2006; Heidbach et al., 2018). In Brazil, such rotation, making SHmax parallel to the coast, has been observed in the coast of Maranhão, northern equatorial margin (Dias et al., 2018) and Bahia northeastern margin (Assumpção et al., 2024).

Figure 8 shows that the P axis of the Cajati focal mechanism is also parallel to the coast, roughly consistent with other observations along the coast of São Paulo state.

3.3 Favourable stress conditions

The occurrence of mine induced seismicity in Cajati can be explained by the occurrence of several favourable factors:

a) the area is probably subject to high crustal stresses concentrated in the upper crust due to lithospheric thinning, b) the orientation of the maximum compressional stress (S1) is probably coast parallel, which makes it favorably oriented in relation to a NNW-SSE trending fault.
c) removal of a large quantity of rock (estimated at the order of 440 Mton) causes stress changes from unloading of the overburden pressure. The reverse faulting mechanism is consistent with stress changes from vertical unloading in a region subject to horizontal compression. Coulomb stress changes in a 45^o dipping NNW-SSE oriented fault are estimated as around 2 MPa, as shown in Fig. 9. This is larger than the average stress-drop of shallow Brazilian earthquakes (Ciardelli & Assumpção, 2019).

d) in addition to the favorable stress conditions, the Cajati mine seems to be crossed by a major fault as indicated by aeromagnetic anomalies in Fig. 10. In this part of the Ponta Grossa Arch, the aeromagnetic map shows a swarm of NW-SE oriented dikes related to an extensional phase during the South Atlantic opening. The odd NNW-SSE dike suggests that a local deep fault may have facilitated the ascending of magma in a different direction. In fact, gravity modeling by Rugenski (2006) shows maximum densities distributed along a NNW-SSE direction. This deep fault crossing the area of the Cajati mine may be a weak zone, which would be a third favorable factor for inducing seismicity.

Induced seismicity in the Cajati mine has been observed since at least 2009. Most of the larger events occur in the same NNW-SSE oriented fault system parallel to the pit orientation. This was observed both from the records of regional stations as well as from the local mine network. Several factors contribute positively to facilitate the occurrence of induced seismicity: high stresses in the upper crust, favorable orientation of a pre-exiting large fault with respect to the stress field, and a significant stress perturbation due to extraction of a large quantity of rock mass. The Cajati earthquake series is an excellent example of all factors that contribute to increase the chances of induced seismicity in an open pit mine.

Author contributions

Marcelo Assumpção: conceptualization, analysis of seismic data, focal mechanism studies,

writing, editing, figure preparation.

J.A. Nogueira: data processing and analysis, relative epicenter location.

L.S. Dias: data processing of local data, editing.

J.M. Carvalho: data processing for focal mechanism, editing

L.A. Schirbel: data processing, focal mechanism studies, Coulomb stress calculation, editing.

M.B. Bianchi: data curation, discussions, editing

Data availability statement (mandatory)

All data used for the relative location of epicenters and focal mechanisms are from the Brazilian Seismographic Network (RSBR), which is open. Data is available at https://www.sismo.iag.usp.br/rq/

Additional Information (including a Competing Interests Statement)

We have no competing or financial interests with the subject of this paper. We have not used or analyzed any data from the mine local network, but mentioned some of their results which are already open. The mine company is aware of our results in this paper and plans for future cooperation have started. Interpretation of all results in this paper were made independently of the Mine company.

Acknowledgments

J.A. Nogueira thanks CAPES (Brazilian Ministry of Education) for an M.Sc. scholarship, L.S. Dias thanks Mosaic Fertilizantes S.A. for permission to use data from the local seismic network of the Cajati mine. We thank Bertrand Delouis for the FMNEAR code, and Elías Heimisson for the code to calculate Coulomb stresses. Mosaic Fertilizantes S.A. provided information on the size of the open pit. Plots and figures prepared with GMT software (Wessel & Smith, 1998). Seismic data retrieved from the open Brazilian Seismograhic Network at https://www.sismo.iag.usp.br/rq/

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No competing interests reported.

Seismicity Induced by the Cajati Open Pit Mine, SE Brazil: a case of stress concentration, weakness zone, and favorable geometry

Status:

Version 1

Abstract

Figures

1. Introduction

2. Seismicity

2.1 Relative epicenter location

2.2 Focal mechanism

3. Discussion

3.1 Stress concentration due to lithospheric thinning

3.2 Coast parallel compression

3.3 Favourable stress conditions

4. Conclusions

Declarations

References

Additional Declarations

Status:

Version 1