We identify two interrelated but independent species of microcracks with differentorigins and different distributions. One species is the classic high-stress microcracksidentified in laboratory stress-cells associated with acoustic emissions as microcracks openwith increasing stress. The other species is the low-stress distributions of closely-spacedstress-aligned fluid-saturated microcracks that observations of shear-wave splitting （SWS）demonstrate pervade almost all in situ rocks in the upper crust, the lower crust, and theuppermost 400 km of the mantle. On some occasions these two sets of microcracks may beinterrelated and similar （hence ＇species＇） but they typically have fundamentally-differentproperties, different distributions, and different implications. The importance for hydrocarbonexploration and recovery is that SWS in hydrocarbon reservoirs monitors crack alignmentsand preferred directions of fluid-flow. The importance for earthquake seismology is that SWSabove small earthquakes monitors the effects of increasing stress on the pervasive low-stressmicrocrack distributions so that stress-accumulation before, possibly distant, earthquakes canbe recognised and impendin~ earthquakes stress-forecast.
The linear Gutenberg-Richter relationship is well-established. In any region of the Earth,the logarithm of the number of earthquakes,greater than any magnitude,is proportional to magnitude. This means that the underlying physics is non-linear and not purely elastic. This nonlinear physics has not been resolved. Here we suggest that a new understanding of fluid-rock deformation provides the physics underlying Gutenberg-Richter: where the fluid-saturated microcracks in almost all in situ rocks are so closely-spaced that they verge on failure and fracture,and hence are critical-systems which impose fundamentally-new properties on conventional sub-critical geophysics. The observation of linear Gutenberg-Richter relationship in moonquakes suggests that residual fluids exist at depth in the Moon.
When propagating through anisotropic rocks in the crust,shear-waves split into faster and slower components with almost orthogonal polarizations.For nearly vertical propagation the polarization of fast shear-wave(PFS)is parallel to both the strike of the cracks and the direction of maximum horizontal stress,therefore it is possible to use PFS to study stress in the crust.This study discusses several examples in which PFS is applied to deduce the compressive stress in North China,Longmenshan fault zone of east edge of Tibetan plateau and Yunnan zone of southeast edge of Tibetan plateau,also discusses temporal variations of PFS orientations of 1999 Xiuyan earthquake sequences of northeastern China.The results are consistent to those of other independent traditional stress measurements.There is a bridge between crustal PFS and the crustal principal compressive stress although there are many unclear disturbance sources.This study suggests the PFS results could be used to deduce regional and in situ principal compressive stress in the crust only if there are enough seismic stations and enough data.At least,PFS is a useful choice in the zone where there are a large number of dense seismic stations.
Systematic analyses of seismic data recorded by the Yunnan regional seismograph network reveal significant crustal and upper mantle anisotropy.Splitting of the S phase of local earthquakes and teleseismic SKS,PKS,and SKKS phases indicates time-delays from 1.60 ms/km to 2.30 ms/km in the crust,and from 0.55 s to 1.65 s in the upper mantle which corresponds to an anisotropic layer with a thickness about between 55-165 km.The polarization orientations of fast shear waves in the crust are complicated with a predominantly north-south direction,and the mantle anisotropy has a nearly west-east direction.Our results show different deformation styles and mechanisms exist between the crust and upper mantle.
To investigate the relationship between velocity structure and earthquake activity on the southeastern front of the Tibetan Plateau, we make use of continuous observations of seismic ambient noise data obtained at 55 broadband stations from the regional Yunnan Seismic Network. These data are used to compute Rayleigh wave Green's Functions by cross-correlating between two stations, extracting phase velocity dispersion curves, and finally inverting to image Rayleigh wave phase velocity with periods between 5 and 34 s by ambient noise tomography. The results show significant lateral variations in crustal and uppermost mantle structures in the studied region. Phase velocity anomalies at short periods(5–12 s) are closely related to regional tectonic features such as sediment thickness and the depth of the crystalline basement. The Sichuan-Yunnan rhombic block, enclosed by the Honghe, Xiaojiang and Jianchuan faults, emerges as a large range of low-velocity anomalies at periods of 16–26 s, that inverts to high-velocity anomalies at periods of 30–34 s. The phase velocity variation in the vicinity of the Sichuan-Yunnan rhombic block suggests that the low-velocity anomaly area in the middle-lower crust may correspond to lower crustal channelized flow of the Tibetan Plateau. The spatial distribution of strong earthquakes since 1970 reveals that the Yunnan region is inhomogeneous and shows prominent characteristics of block motion. However, earthquakes mostly occur in the upper crust, with the exception of the middle-Yunnan block where earthquakes occur at the interface zone between high and low velocity as well as in the low-velocity zones, with magnitudes being generally less than 7. There are few earthquakes of magnitude 5 at the depths of 15–30 km, where gather earthquakes of magnitude 7 or higher ones which mainly occur in the interface zone between high and low velocities with others extending to the high-velocity abnormal zone.
