Applcation Of Wave Velocity Anomalies In The Prediction Of Earthquakes

The previous research about seismic velocity anomalies were often focused on the time course of seismic wave velocity ratio, and the selection of local regions such as the epicenter and the vicinity as the study area. For more comprehensive investigation of the relation between the foreshock’s wave velocity or wave velocity ratio and the main shock, this paper takes the recent Wenchuan earthquake and Lushan earthquake as an example, which occurred in the Longmen belt. The single station method and multi-station methods are used to study the anomalies of the generalized apparent velocity of p wave and s wave before the earthquake, from the viewpoints of the time course and the extended spatial evolution image. By doing this, we hope to strength the understanding of the features of the abnormal spatial and temporal changes of pre-seismic wave velocity or wave velocity ratio. This is important significant to the prediction of the earthquake based on the seismic velocity or velocity ratio.In order to obtain the abnormal change characteristics of the generalized apparent velocity and seismic wave velocity ratio in spatial evolution image, namely the time and spatial distribution image, the spatial scan method is employed, which utilizes a1.5°×1.5°rectangular window to scan in0.2°sliding step sequentially. In addition, in order to ensure the data accuracy, this paper employs the near earthquake travel-time curve and Wadati curve to select data. Because some discrete points could be the outcome of the seismic velocity anomalies, it is not permitted to diverge in excess of4seconds from travel-time curve.This paper analyzes the influence of hypocenter depth on generalized apparent velocity and observation error on seismic velocity ratio, reduces the impact of the two factors on generalized apparent velocity and seismic velocity ratio by increasing the epicentral distance, and compares with the results at the distance25～125km. The results show that the abnormal changes of seismic wave velocity and seismic wave velocity ratio are more obvious at the distance80-200km.Using the above methods, this paper observes clearly the anomalies of the generalized apparent velocity of P wave and S wave, seismic wave velocity ratio before the two strong earthquakes. In the time course, both the generalized apparent velocity of P wave and S wave have obvious decline, and when the earthquake is stronger in magnitude, the amplitude of abnormal decrease is greater and the duration is longer as well. All the seismic wave velocity ratios show obvious abrupt change. Before the Wenchuan earthquake, the generalized apparent velocity of P wave dropped from the early5.62km/s at the beginning of the year2004to5.55km/s by0.07km/s. At the same time, the generalized apparent velocity of s wave dropped from the early3.50km/s to3.40km/s by0.10km/s。Then, the pair of the apparent velocities fluctuated around the low value after the drops until the end of the year2007. Before Lushan earthquake, the generalized apparent velocity of p wave dropped from early5.66km/s at the beginning of the year2012to5.64km/s by0.02km/s。At the same time, the generalized apparent velocity of s wave dropped from the early 3.39km/s to3.36km/s by0.03km/s. Then, the pair of the apparent velocities dropped slowly until the earthquake happened. In spatial evolution, the two earthquakes occurred in the area where the apparent velocities had obviously declined, and the two earthquakes were located at the junction of high wave velocity ratio zone and low wave velocity ratio. In the Wenchuan earthquake area, the generalized apparent velocity was obviously high before2004, and turned to low value at the end of the year2007. The Lushan earthquake occurred in the low generalized apparent velocity zone generated after the Wenchuan earthquake, where the generalized apparent velocity was further reduced before the Lushan earthquake.Based on the analysis above, this paper concludes that the anomalies of the generalized apparent velocity in time course fit with the interpretation of the expansion-inlet mode. That is, when the rocks are under stress action, it will produce micro crack expansion, leading the wave velocity to decrease significantly. On the other side, this model demonstrates that the logarithm of abnormal duration has approximately linear relationship with the magnitude. And this can explain why the durations of anomaly before the two earthquakes are different. But further research is needed as there is no model to explain the anomalies in spatial evolution of the generalized apparent velocity and the wave velocity ratio and the anomalies of wave velocity ratio in time course.Finally, we focus on the near earthquakes that happened in North China from2002to2013, and research the changes of P-and S-wave apparent velocities as well as the velocity ratios using the multi-station and spatial scanning methods. Results show that the P-and S-wave apparent velocities in the Shanxi fault zone and nearby show obvious signs of rises and are different from the patterns prior to the Wenchuan and Lushan earthquakes. After falling abruptly in early2011, the apparent velocities in Jiangsu have been keeping rising fluctuantly, bearing resemblance to the situation before the Wenchuan earthquake. Special attention should be paid to other seismic precursors of that region. We should also follow precursors in Beijing, Langfang and Baoding of Hebei province, where the apparent velocities do go down.