Waveform Characteristics of Earthquakes Induced by Hydraulic Fracturing and Mining Activities: Comparison with Those of Natural Earthquakes

Some industrial activities, such as underground mining, hydraulic fracturing (HF), can cause microearthquakes and even damaging earthquakes. In recent years, with the rapid development of mining and HF, many induced damaging earthquakes have been reported and studied. The seismic waves contain rich information of the seismic sources and wave propagation path, which are the valuable data for studying the natural and induced earthquakes. Therefore, the non-linear and time-frequency characteristics of different seismic waveforms are studied by using multifractal theory and the Hilbert-Huang transform method. In general, for both the natural and induced earthquakes, the larger the magnitude is, the longer the duration of the waveform is, and the lower the frequency is. In addition, the amplitude and frequency of natural and induced seismic wave show non-linear decreasing trend with the increase in propagation distance. Both the natural and induced earthquakes' waveforms show clear multifractal features. The multifractal parameters Delta alpha and Delta f(alpha) can be used as indicators to reflect the non-linear characteristics of the seismic waveforms. The seismic waveforms' multifractal and time-frequency characteristics recorded by the specific sensors are affected by not only the magnitudes, types and rupture mechanism of seismic sources, but also the distances from sensors to seismic sources, wave propagation path and the sensors' properties. Because the coal is soft and unconsolidated rock with developed joints and fractures, the seismic waveforms induced by mining are more complex and variable than the ones with the same magnitude induced by HF of shale and hot dry rock.