Systematic examination of ‘precursor waves’ of strong earthquakes by using the wavelet method

Based on the borehole strainmeter recordings immediately before the strong earthquakes （Ms〉7.5） during the period from 2001 to 2005 observed at the Taian observation station, Shandong Province, the authors make a systematic and objective examination of the precursor waves of the quakes. The effects of Earth tides with periods larger than 128 min are eliminated through high-pass filtering; and atmospheric-pressure inferences are removed by linear regression. The 2-128 min signals are then separated into six frequency bands by employing the wavelet method. Results indicate that the wavelet method is capable of picking out information of weak variations in the signals. According to the characteristics of the ＇precursor waves＇ obtained from wavelet transformation, the method of overrun ratio analysis is put forward for examination. All the detailed components of the wavelets have been analyzed. For the time series of the overrun rate in all these components, statisitical calculation has been made for the slopes of fit curves, and mean values and standard deviations were obtained and positive-negative slope ratios were analyzed. The three statistical data show that ＇precursor waves＇ are not widely recorded by borehole strainmeter within 15 days before remote strong earthquakes.

Discussion on the Reproducibility of Precursor Waves in the Water Level Records of Jingle Well

Choosing the Jingle well in Shanxi as the research object, this paper discusses the reproducibility problem of well water level precursor waves. The results show that the start time and fluctuation cycle before similar earthquakes are highly reproducible,the ratio of which is up to 63. 1%. The fluctuation form and amplitude before some similar earthquakes are of certain similarity,but earthquakes with all precursor wave features being similar were not found in the research scope. The reason may be related to the origin and transmission route of the precursor wave as well as the properties of the seismic source area of similar earthquakes.

A Discussion on the Generality of Precursor Waves on Well Water Level Records

Taking five underground water level observation wells with high earthquake response capability and located in Shanxi,Hebei and Inner Mongolia as study objects,this paper studies and discusses the generality of precursor waves in well water levels before strong earthquakes. It is indicated that the precursor waves often happen before strong earthquakes,with the ratio of occurrence up to 65% ( 41 /63 ) ; the precursor waves have poor generality in different observation wells,which may be related to the different well-aquifer system of different observation wells and the different source parameters of earthquakes. Compared to the seismic water level fluctuations in a single well,the number of precursor wave times recorded in well water levels before strong earthquakes is slightly less than that of seismic water level fluctuations,with a ratio between 39. 7% and 50. 7%, respectively. The number of observation wells with precursor wave recordings before earthquakes accounts for up to 66. 7% of the total number of observation wells. The precursor wave in well water level records has a certain generality in terms of the number of times recorded and the spatial distribution,except for its manifestation characteristics.

The electromagnetic wave experiment for CSES mission： Search coil magnetometer

The seismic activities on the Earth can produce a disturbance of the electromagnetic field and particles in the ionosphere. The search coil magnetometer(SCM) mounted on China Seismo-Electromagnetic satellite(CSES) is designed to measure the magnetic field fluctuation of low frequency electromagnetic waves in the frequency range of 10 Hz–20 k Hz. The SCM comprises a three-axis search coil sensor mounted on a 4.5 m boom and an electronic box inside satellite module. The sampling rate of the SCM is 51.2 k Hz and the time resolution of the power spectrum density(PSD) is 2 s. The frequency resolution is 12.5 Hz.There are three operation modes: survey, detailed survey and calibration. In the survey mode, the SCM can provide a PSD in the whole frequency range of 10 Hz–20 k Hz and wave forms in the low frequency range below 2 k Hz while in the detailed survey mode the SCM can provide both PSD and wave forms in the whole frequency range of 10 Hz–20 k Hz. The sensitivity of the SCM instrument is 5.0×10^(-4) n T Hz^(-1/2) at 10 Hz, 5.0×10^(–5) n T Hz^(-1/2) at 200 Hz, 3.4×10^(-5) n T Hz^(-1/2) at 2 k Hz and 1.1×10^(-4) n T Hz^(-1/2) at 20 k Hz. The telemetry rate is ~0.85 Mbps in the survey mode and ~3.0 Mbps in the detailed survey mode. The phase difference between three axes can be made generally with a precision of less than 1.0°. The dynamic range of the SCM instrument is over 100 d B. The orthogonality of three mechanical axes of search coil senor is better than 0.13°. The performance of SCM can satisfy the requirement of scientific objectives of CSES mission.

Large-scale experimental investigation of the effects of gas explosions in underdrains

This study involved the construction and explosion of a large-scale(80-meter-long)underdrain and detailed investigations of the damaging impacts of a gas explosion to provide an experimental foundation for similarity modeling and infrastructural designs.The experiment vividly recreated the scene and explosion damage of the"11.22″explosion accident in Qingdao,China,thus allowing for evaluations of the movements and destruction of the cover plates.The damage mechanism was determined by analyzing the overpressure curves inside and outside the underground canal.It was determined that the cover plates were first lifted by the precursor wave,which induced a maximum overpressure of 0.06 MPa and resulted in explosion venting.The pressure entered the deflagration stage at the end of the explosion.The combustion wave overpressure reached 3.115 MPa close to the initiation point,and had a significant influence on the projectile energy of the cover plates there.Overall,64%of the cover plates were only affected by the precursor wave,while 36%of the cover plates were subjected to both the precursor wave and the combustion wave;these cover plates were severely damaged.The results of this study provide fundamental insights relevant to the prevention and control of underdrain gas explosions.