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In order to image the 3-D velocity structure of its shallow crust in Kunming region, China, finite-difference seismic tomography is used to invert the seismic data selected carefully from six-shot data. The result lays a foundation for the discussion of the relationship between the obtained velocity structure and the hidden faults, and for the illumination of the depth extents of main active faults surrounding Kunming city. Puduhe-Xishan fault lies on the western margin of the Kunming basin and is just situated on the west edge of the low velocity anomaly zone found at all depth levels. This indicates that this fault is a borderline fault of the Kunming basin. It can be concluded that the fault dips eastwards with a steep angle and its depth extent is large. Puji-Hanjiacun fault and Heilongtan-Guandu fault play a role in controlling the low velocity anomaly zone in middle basin. The depth extents of the two faults are comparatively small, without traversing the interface of basin floor.

The method of radiation energy (E R) of the earthquake wave measured by the peak velocity (r·v) of the ground motion is applied to a detailed study on the temporal and spatial distribution of the seismic apparent stresses (σ a) for the moderate and small earthquakes and two aftershock sequences in Yunnan area. The results show that there exists an obvious non-homogeneity for the seismic apparent stresses in the spatial distribution. The concentrated regions of the high apparent stresses are related to the active places of the moderate and small earthquakes. Before the Dayao M=6.2 earthquake, there was a period in which the apparent stresses were high and the value was 5 times of the average value, 0.25 MPa. The relatively high values of apparent stresses distribute around the epicentral area of the major shock and nearby. It indicates that the variation characteristics of the apparent stresses can be taken as a new kind of criterion for the earthquake-risk forecast. Usually the ratio of the apparent stresses of the aftershock sequence σ aA to the ones σ aM of main shock is less than 1.0.

On the basis of about 300 earthquake wave forms observed in the Shidian MS=5.9 sequences on April 12, 2001 recorded in Kunming Digital Seismic Network, the spectra of shear wave have been used to estimate the focal parameters of these earthquake sequences. The results show that within the magnitude range of 1.5–5.3, the seismic moments are 1010–1016 N·m, the corner frequencies are 0.2–0.8 Hz, radii of the focal rupture are 200–2 500 m and the stress drops are 0.1×105–20×105Pa. Through the statistical analyses of variation of corner frequency fc and stress drop Δσ with time, it is discovered that the average corner frequency of the foreshock sequences is obviously lower than that of the aftershock sequences. Contrarily, the average stress drops Δσ of the foreshock sequences are clearly higher than that of the aftershocks. It is considered that these variation characteristics of average corner frequency and stress drops before and after the main shock have index significance to the precursory information before a strong earthquake. The higher stress drops for the foreshock sequences show that the higher shear stresses have been stored in the area of main shock. After the main shock, most of the stresses have been released, so the aftershock sequences show a rupture process of lower stresses.

The medium-short term forecast for a certain kinds of main earthquake events might be possible with the time-to-failure method presented by Varnes (1989), Bufe and Varnes (1993), which is to simulate an accelerative releasing model of precursory earthquake energy. By fitting the observed data with the theoretical formula, a medium-short term forecast technique for the main shock events could be established, by which the location, time and magnitude of the main shock could be determined. The data used in the paper are obtained from the earthquake catalogue recorded by Yunnan Regional Seismological Network with a time coverage of 1965-2002. The statistical analyses for the past 37 years show that the data ofM≥2.5 earthquakes were fairly complete. In the present paper, 30 main shocks occurred in Yunnan region were simulated. For 25 of them, the forecasting time and magnitude from the simulation of precursory sequence are very close to the actual values with the precision of about ±0.57 (magnitude unit). Suppose that the last event of the precursory sequence is known, then the time error for the forecasting main shock is about ±0.64 year. For the other 5 main shocks, the simulation cannot be made due to the insufficient precursory events for the full determination of energy accelerating curve or disturbance to the energy-release curve. The results in the paper indicate that there is no obviously linear relation in the optimal searching radius for the main shock and the precursory events because Yunnan is an active region with damage earthquakes and moderate and small earthquakes. However, there is a strong correlation between the main shock moment and the coefficientk/m. The “optimal fitting range“ for the forecasting time and magnitude can be further reduced using the relation between the main shock moment lgM 0 and the coefficientlgk/m and the value range of the restricting indexm, by which the forecast precision of the simulated main shock can be improved. The time-to-failure method is used to fit 30 main shocks in the paper and more than 80% of them have acquired better results, indicating that the method is prospective for its ability to forecast the known main shock sequence. Therefore, the prospect is cheerful to make medium-short term forecast for the forthcoming main shocks by the precursory events.

An earthquake withM=6.5 happened on January 15, 2000 in Yao’an of Yunnan Province. After the earthquake, a temporary digital network with 6 detectors around the epicenter area was set up. 402 aftershocks were located more precisely. According to coda short recording observed, the coda averaging quality factor has been acquired via Sato’s single scattering model analyses,Q c(f)=49f 0.95,f=1.5∼20.0 Hz, which has the attenuation characteristics of high structural active region.

Based on data collected by deep seismic sounding carried out in 1999, a three-dimensional P wave velocity structure is determined with tomographic inversion. The tomographic result shows that there is a P wave low velocity zone (LVZ) in the upper crust beneath the Tengchong volcanic area. The LVZ is in the depth of 7–8 km and may be a small magma chamber or a partial melting body. The result also shows that the LVZ is in the northeastern side of the Rehai hydrothermal field, which is located in another LVZ near the surface. The shallow LVZ may represent a well-developed fracture zone. The strong hydrothermal activity in Rehai area can attribute to the existence of fractures between two LVZs. These fractures are the channels for going upwards of the deep hot fluid.

Based upon the deep seismic sounding profile conducted in the Tengchong volcanic-geothermal area, a two-dimensional crustal P velocity structure is obtained by use of the finite-difference inversion and the forward travel-time fitting method. The crustal model shows that there is a low velocity zone in upper crust in the Tengchong area, which may be related to the volcanic-geothermal activities, and two intracrustal faults (the Longling-Ruili fault and Tenchong fault) exist on the profile, where the Tengchong fault may extend to the Moho discontinuity. Meanwhile, based on teleseismic data recorded by a temporary seismic network, we obtained the S-wave velocity structures beneath the Rehai-Retian region in the Tengchong area, which show the low S-wave velocity anomaly in upper crust. The authors discuss the causes of Tengchong volcanic eruption based on the deep crustal structure. The crustal structure in the Tengchong volcanic-geothermal area is characterized by low P-wave and S-wave velocity, low resistivity, high heat-flow value and low Q value. The P-wave velocity in the upper mantle is also low. For this information, it can be induced that the magma in the crust is derived from the upper mantle, and the low velocity anomaly in upper crust in the Tengchong area may be related to the differentiation of magma. The Tengchong volcanoes are close to an active plate boundary and belong to ‘plate boundary’ volcanoes.

According to dispersion theory of coda of local events, using near-field data of aftershocks recorded by two mobile digital stations at the Tianxin and Yunlong after the Wuding, Yunnan M S=6.5 main-shock and sampling at different central frequencies (1.5–20 Hz), spatial distribution features of coda attenuation ratio β(f) are studied. The results show that there exists obvious non-uniformity of coda attenuation ratio β(f) given by the two stations in small region. When f

Follow Chen and Duda’s model of spectral fall-off of ω3, the dependence of peak parameters of ground motion, peak displacement dm, peak velocity vm and peak acceleration am, upon the environment stress τ0-values are studied using near source seismic digital recordings for the sequence of the Wuding, Yunnan, M=6.5 earthquake, in which, as a new thought, the peak parameters are assumed to be related to the medium Q-value. Three formulae for estimating the environment stress τ0-values by the peak parameters of three types of ground motions are derived. Using these formulae, the environment stress τ0-values are calculated for the sequence of the Wuding earthquake. The result show that τ0-values calculated by the three formulae are constant largely, the averages of τ0 are in the range of 5.0–35 MPa for most earthquakes. It belongs to the high-stress earthquakes sequence: the high-stress values are restricted to the relatively small area closely near to the epicenter of the main shock. The fine distribution structure for the contours of the environment stress τ0-values is related closely to the strong aftershocks. The analysis of spatial and temporal feature of τ0-values suggests that the earthquakes sequence in a rupture process generated at the specific intersection zone of seismo-tectonics under high-stress background.

According to the source dislocation model suggested by Brune(1970), the authors have calculated the displacement spectra of S-wave and source parameters of the Heqing Ms 5.3 earthquake sequence, using the digital data of this sequence obtained in the Western Yunnan Earthquake Prediction Experimental Field (WYEPEF). Based on this calculation we have studied the dependence of the peak velocity (rv) of ground motion on the seismic stress drop Δσ. From the seismic scaling law we obtained (rv) ∞Δσ2/3, thus the three formulae of calculating seismic stress drop Δσ using the peak velocity parameters can be derived: lg(rv)=d1+1/3lgM0+2/lgΔσ lg(rv)=d2+1/3ML+2/3lgΔσ=-1.0+1.5lg(rv). Assuming that the average stress drop Δσ=3.0×106 Pa for great and small earthquakes, then the constants d1= −3.88 and d2=−0.38 are determined by the observational data of the Heqing Ms5.3 sequence. Results of the source parameters for this sequence show that the seismic moment M0 is between 1011 N · m and 1015 N · m, the rupture radius a of the source is between 200 m and 600 m, the stress drop Δσ is between 0.1 MPa and 10 MPa and the average stress drop Δσ=3.7 MPa calculated from the peak velocity parameter of the ground motion. Δσ values measured from these scaling relations are basically in agreement with the results given by Brune’s method (1970). Results of this study show that the dependence of the ground motion peak velocity parameter (rv) on the stress drop Δσ is even stronger than that on the seismic moment M0.

According to the source velocity spectrum model suggested by Chen and Duda (1993), the method of directly measuring the maximum value max of the velocity spectrum from the seismic momentM 0 has been studied. The authors have measured the seismic momentM 0 and source mechanical parameters of three microearthquake sequences, the Er’ yuan, Maidi and Songgui sequences, by use of the digital data of these sequences obtained by near-field observation in the Western Yunnan Earthquake Prediction Experimental Field (WYEPEF). The magnitudesM L of the three micro-earthquake sequences are mostly between 1.0 and 2.5, the measured seismic momentM 0 is between 1010N · m and 1013N · m, and the rupture radius a of the source between 90 m and 270 m. The study of the ambient stress parametersτ 0 and Δ σ shows that the ground motion peak velocityr ·v is a function of the ambient stress parametersτ 0 and Δ σ , while the ambient stresses are related to the seismotectonics and the media states in turn. The values of ambient stress parametersτ 0 and Δ σ for the three earthquake sequences are between 0.5–5.0 MPa for the high stress earthquake swarm (Maidi sequence), between 0.3–1.5 MPa for the moderate stress swarm (Songgui sequence), and between 0.01–0.5 MPa for the low stress swarm (Er’ yuan sequence). We have further discussed the dependence of the high, moderate and low stress states on the tectonics and on the difference in media state. This study can help us to understand deeply the relation that the features of ambient stress states bear with the seismogenic process and the potential earthquake risk zone. Besides, the authors have also found that there is an obvious difference between the moment scaling laws for large and small earthquakes. The scaling law between the peak velocityr ·v and the seismic momentM 0 isr ·v∞M 00.6 whenM 0>1014 (N · m), but isr ·v∞M 0 1/2 whenM 0