2111 STRESS INVERSION USING DATA RECORDED BY A SMALL SEISMIC SYSTEM F. Calixto1, W. Suaña2 and A. Rigby1 1Institute of Mine Seismology 2Volcan Compañía Minera, Lima, Peru ABSTRACT Stress inversion from seismic data is a great complement to stress measurements performed by the mine. Seismically active mines, while sometimes facing the inherent hazards posed by episodes of dynamic rock mass failure, also have the opportunity to use recorded waveforms to perform advanced seismic analysis (e.g. calculation of source mechanisms and stress inversion). In this paper, we show an application of a recently developed method (Malovichko and Rigby, 2024) to data recorded by a small seismic system in South America. The method, called Stress Inversion from Mine Seismicity – SIMS, was successfully used to better understand the stress field. The agreement between the inferred principal stress orientations and stress measurements was good. In particular, it was found that the maximum principal stress near a seismically active decline is mainly controlled by the presence of a fault, causing its orientation to be close to vertical. In other areas, the maximum principal stress is mostly sub-horizontal and orthogonal to the orebody. The results improve confidence in the mine’s adopted stress state. KEYWORDS Stress inversion, seismicity, seismic monitoring 1. INTRODUCTION A mine in South America is monitoring seismicity using a small system that first started with six 4.5 Hz geophones and then was expanded to a total of ten. The waveforms used to calculate the source mechanisms were of high signal to noise ratio, which could be achieved thanks to good sensor installation procedures (e.g. installation in 10 m deep boreholes away from potential sources of mechanical and electrical noise; sensor placement providing good three-dimensional coverage and away from heterogeneities such as caves, stopes and their yielding zones; full grouting of the borehole with appropriate mix). In addition, the boreholes were surveyed and their orientations were confirmed electronically and with seismic data. Moreover, seismic velocity calibration was performed to ensure reliable hypocenter estimation. Figure 1 shows the seismicity recorded since 2020. Over 13,000 events were recorded with moment magnitudes in the range of mW -2.4 to mW 1.6. Source mechanisms were calculated for many events, and about 800 of these were selected for the SIMS analysis (Figures 2 and 3). The mechanisms observed are of both crush type and slip type. The crush-type events occur near the excavations and are interpreted to be sudden episodes of dynamic stress fracturing in the rock mass (Malovichko, 2020). The slip-type or doublecouple events can be interpreted as sudden slip along existing weaknesses or shear rupture of the intact rock mass (Malovichko and Rigby, 2024). The red lines in Figure 2 represent the P-axes (pressure axes) of the moment tensor solutions. Typically, P-axis orientations are assumed to be parallel to the orientation of the maximum principal stress, σ1, and T-axes parallel to the minimum principal stress, σ3. However, this is not necessarily accurate, as σ1 can be anywhere within the dilatational quadrant of the focal sphere (McKenzie, 1969). This means that the
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