In laboratory tests, in rock, elastic wave conversion occurs when shear cracks initiate. In the experiments, the samples are continuously probed with seismic waves during loading that caused shear crack initiation and growth. The initiation of shear cracks is not detected with either transmitted P- or S-waves across the crack. However, shear (S) waves, propagated at normal or oblique incidence on an emerging shear crack, are partially partitioned into compressional (P) waves. The emergence of S–P converted modes is theoretically predicted to arise from wave interaction with an array of similarly oriented microcracks. Wave conversions occur before the initiation of shear cracks can be observed on the rock surface using direct imaging measurements. As shear cracks propagate, until crack coalescence, the converted waves amplitude increases. The trend of changes in amplitude of the S–P waves with respect to load for both normally and obliquely incident shear waves on a shear crack is consistent with the extended displacement discontinuity theory. The consistent observation in all the tests of emergent S–P waves during crack initiation and the detection of peak amplitude prior to crack coalescence provides a basis for the development of seismic monitoring techniques to characterize shear crack initiation, crack growth, and crack coalescence.

中文翻译：

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剪切裂纹萌生和扩展引起的地震波转换

在实验室测试中，在岩石中，当剪切裂缝开始时会发生弹性波转换。在实验中，在加载过程中用地震波连续探测样品，导致剪切裂纹萌生和扩展。剪切裂纹的产生不能通过穿过裂纹的透射 P 波或 S 波检测到。然而，在新出现的剪切裂纹上以垂直或斜入射传播的剪切 (S) 波被部分划分为压缩 (P) 波。理论上预测 S-P 转换模式的出现是由波与一系列类似取向的微裂纹的相互作用引起的。波转换发生在剪切裂缝开始之前，可以使用直接成像测量在岩石表面观察到。随着剪切裂纹的传播，直到裂纹合并，转换波的振幅增加。对于剪切裂纹上的垂直和斜入射剪切波，S-P 波振幅随载荷变化的趋势与扩展位移不连续理论一致。在裂纹萌生期间出现的 S-P 波的所有测试中的一致观察和裂纹合并前峰值振幅的检测为开发地震监测技术以表征剪切裂纹萌生、裂纹扩展和裂纹合并提供了基础。