Improvements in seismic event locations in a deep western U.S. coal mine using tomographic velocity models and an evolutionary search algorithm

Methods of improving seismic event locations were investigated as part of a research study aimed at reducing ground control safety hazards. Seismic event waveforms collected with a 23-station three-dimensional sensor array during longwall coal mining provide the data set used in the analyses. A spatially variable seismic velocity model is constructed using seismic event sources in a passive tomographic method. The resulting three-dimensional velocity model is used to relocate seismic event positions. An evolutionary optimization algorithm is implemented and used in both the velocity model development and in seeking improved event location solutions. Results obtained using the different velocity models are compared. The combination of the tomographic velocity model development and evolutionary search algorithm provides improvement to the event locations.

Analysis on the master event method and precise location of 1997 Jiashi strong earthquake swarm in western China

Master event location method was described in detail in this paper. Some problems in the application of master event method have been analyzed and some improvements of the method have been made. As compared the location results of MS>=3.0 earthquakes of Jiashi swarm obtained by using this method with that by the traditional absolute method, the result obtained by using the master event method shows more reasonable and more consistent with that from the focal mechanism solutions. After relocation, we can see, the epicenters of M>=5.0 earthquakes show an echelon-type alignment along NNW-SSE direction, and all earthquakes concentrate nearly in a volume region about 30 km (N-S) × 15 km (E-W) × 15 km (U-D). Earthquake focal depths are mainly in the range of 15-28 km.

Fast Marching Method for Microseismic Source Location in Cavern-Containing Rockmass: Performance Analysis and Engineering Application

Microseismic(MS)event locations are vital aspect of MS monitoring technology used to delineate the damage zone inside the surrounding rock mass.However,complex geological conditions can impose significantly adverse effects on the final location results.To achieve a high-accuracy location in a complex cavern-containing structure,this study develops an MS location method using the fast marching method(FMM)with a second-order difference approach(FMM2).Based on the established velocity model with three-dimensional(3D)discrete grids,the realization of the MS location can be achieved by searching the minimum residual between the theoretical and actual first arrival times.Moreover,based on the calculation results of FMM2,the propagation paths from the MS sources to MS sensors can be obtained using the linear interpolation approach and the Runge–Kutta method.These methods were validated through a series of numerical experiments.In addition,our proposed method was applied to locate the recorded blasting and MS events that occurred during the excavation period of the underground caverns at the Houziyan hydropower station.The location results of the blasting activities show that our method can effectively reduce the location error compared with the results based on the uniform velocity model.Furthermore,the obtained MS location was verified through the occurrence of shotcrete fractures and spalling,and the monitoring results of the in-situ multipoint extensometer.Our proposed method can offer a more accurate rock fracture location and facilitate the delineation of damage zones inside the surrounding rock mass.

Fracture evolution around pre-existing cylindrical cavities in brittle rocks under uniaxial compression

The development of fracture around pre-existing cylindrical cavities in brittle rocks was examined using physical models and acoustic emission technique. The experimental results indicate that when granite blocks containing one pre-existing cylindrical cavity are loaded in uniaxial compression condition, the profiles of cracks around the cavity can be characterized by tensile cracking （splitting parallel to the axial compression direction） at the roof-floor, compressive crack at two side walls, and remote or secondary cracks at the perimeter of the cavity. Moreover, fracture around cavity is size-dependent. In granite blocks containing pre-existing half-length cylindrical cavities, compressive stress concentration is found to initiate at the two sidewalls and induce shear crack propagation and coalescence. In granite blocks containing multiple parallel cylindrical cavities, the adjacent cylindrical cavities can influence each other and the eventual failure mode is determined by the interaction of tensile, compressive and shear stresses. Experimental results show that both tensile and compressive stresses play an important role in fracture evolution process around cavities in brittle rocks.

The Characteristic Analysis and Seismic Triggering Study of the M6.2 and M6.1 Dayao Earthquake Sequences in 2003

The high-resolution hypocenter locations of the mainshocks on July 21 （M6.2） and October 16, 2003 （M6.1） and their aftershock sequences are determined in Dayao, Yunnan by using a double-difference earthquake location algorithm. The results show that the epicenters of the two mainshocks are very close to each other and the distribution of the aftershock sequence appears to be very linear. The distribution of the earthquake sequence is very consistent with the focal mechanism, and both mainshocks are of nearly vertical right-lateral fault. Unlike most other double earthquakes in the Yunmm area, the aftershock distribution of the M6.2 and M6.1 Dayao earthquakes does not appear to be a conjugated distribution but to be in a line, and there are some stacks in the two earthquake sequences. It can be inferred that they are all controlled by the same fault. The distribution of aftershocks is asymmetrical with respect to the mainshock location and appears to be unilateral. The aftershocks of the M6.2 mainshock centralize in the northwest of M6.2 earthquake and the aftershocks of the M6.1 earthquake are in the southeast of the mainshock, moreover, the M6.1 earthquake appears to be another rupture on the southeastern extensiou of the same fault as the M6.2 earthquake. The results of Coulomb failure static stress changes △σf show that the earthquake on July 21 （M6.2） apparently triggered the earthquake on October 16 （M6.1）, the two mainshocks have stress triggering to their off-fault aftershocks to different extents, and the M6.5 earthquake that occurred in Yao＇an in 2000 also triggered the occurrence of the two Dayao earthquakes.

Relocating ruptures of two M6 earthquakes in Zhongba, South-central Tibet in 2004 and 2005 with seismological and geodetic methods

To better understand repeatability of strong earthquakes in previously ruptured zones during one seismogenic period, we studied the rupture zones of the doublet of M6 earthquakes in Zhongba region of southcentral Tibet, China, in 11 July 2004 and 7 April 2005, respectively. We focused on the overlapping degree of two strong quakes’ aftershock areas one week after the mainshocks by using the SQH station in China Seismic Network and a 68-stations temporary broadband seismic array, a part of the international HI-CLIMB project. About 115 local earthquakes were recorded in one week after the mainquakes, and we located these earthquakes by master event relative location （MERL） method. We also used this method to relocate 31 other M3.7 ＋ earthquakes from 1 July 2004 to 1 July 2005. Meanwhile, we studied two mainshocks’ coseismic ruptures with satellite interferometric synthetic aperture radar （InSAR）. Our results show that the ruptured zones of the two earthquakes do not overlapp substantially, either from early aftershock data or from InSAR inversions.