Theoretical arrival time: the moment of the earthquake + the distance between the station and the source/propagation velocity
Theoretical travel time: distance/propagation velocity between station and source
Basics of Seismology: https://wenku.baidu.com/view/1f24151b7375a417866f8f4c.html
Magnitude: local earthquake magnitude scale ML, surface wave magnitude scale Ms, body wave magnitude scale Mb.
ML=lg(A)+R(△)
A: The ground motion displacement (um) corresponding to the maximum amplitude of the seismic record should be calculated by taking the geometric mean value of the maximum amplitude of the two horizontal components. In practice, the arithmetic mean value of the maximum amplitude of the two horizontal components is often taken; R(△) gauge The function has a positive relationship with the epicentral distance △ and is related to the type of recorder.
The maximum amplitude is 23mm, and the maximum magnification of the instrument is 2000, converted into real ground displacement (11.5um)
Ms = lg(A/T)max+c1lg(△)+c2
A seismic displacement of the maximum surface wave amplitude (um, generally the maximum combined displacement of the two horizontal components of the Rayleigh wave), T period (s)
Mb = lg(A/T)max+Q(△,h)
The maximum body wave amplitude of A seismic record, the T period (s), and the Q(△,h) magnitude initiation function are functions of the epicentral distance △ and the depth h.
Moment magnitude Mw
Mw = 2/3lgM0 - 6.06 The unit of M0 is N*m
Due to the orientation of the seismic wave by the energy radiation pattern, the influence of the seismic wave propagation path, and the influence of the base effect of the recording station, the magnitude value of the same earthquake measured by different stations will be different. Accuracy 0.3
The time difference between the onset of main body wave phases is less than 3 minutes when the epicentral distance is D<10°; less than 16 minutes if D<60°; less than 30 minutes if D<100°; and less than 45 minutes if D<180°.
The coda duration mainly depends on the magnitude of the event, as it can be used to calculate the magnitude MD.
The signal duration, especially the time difference between the arrival time of the first body wave and the onset of the last identifiable body wave before the onset of surface waves, can be used to roughly estimate whether the event is a local, regional, or teleseismic event.
Picking of initial movement: signal-to-noise ratio, slope, amplitude
The first rapid epicenter and moment of the earthquake released by the data center is only a preliminary estimate, and usually only based on the first arrival. Their improvements, especially those related to focal depth, required picking up more reliable onset times and identifying subsequent arriving waves.
At long distances, surface waves are only visible on long-period and broadband seismograms, and the geometric spread is smaller than that of body waves propagating in three dimensions. In the recording of shallow seismic events, the amplitude of surface waves is better than that of body waves. However, as the focal depth increases, the surface wave amplitude decreases relative to the body wave amplitude, and the shorter the wavelength, the stronger the attenuation. While non-dispersive body waves form short-duration wave trains, surface-wave dispersion causes long-wave trains of slowly increasing and then decreasing amplitude, with longer-period waves arriving first.
If the epicentral distance is the same, the S-wave amplitude of natural earthquakes is about 5 times the P-wave amplitude
Shallow focus earthquake: depth: 0~70km, medium focus earthquake: depth 70~300km, deep focus earthquake 300~700km, and focal depth greater than 70km is often called deep focus earthquake.
To accurately determine the focal depth h, it is necessary to require the validity of the network (at least one station in the network is very close to the source, for example, the epicentral distance of the station D<h), or to be able to identify the depth phase.
Quickly broadcast the focal depth, assuming it is 0~33km, when there is no depth seismic phase travel time curve or travel time table, the travel time difference △t(pP-P) can be used to quickly and roughly estimate the focal depth according to the empirical relationship
hr[in km ]≈△t（pP-P）/2[in s]×7（for h<100km）or ×9(for h>300km)
natural earthquakes and nuclear explosions
The focal process of an underground nuclear explosion is much simpler than that of an earthquake shear rupture. Compared with an earthquake, the P-wave produced by a nuclear explosion has more high-frequency components and is more pulse-like and compressive in all directions. move. The amplitude of the S-wave and the long-period cottonseed meal produced by the underground nuclear explosion is small.
If the three-component broadband and/or long-period records are well calibrated, it is possible to locate sufficiently strong local events (Ml>3) and distant events (Mb>5). The mean square error of the epicenter located within the distance range of 20°~145° is less than 300km,
For teleseismic P waves, effective filters include Buster high-pass filter, whose inflection point frequency fc>0.5HZ; standard band-pass filter, center frequency f=1HZ; for local seismic sources, use high-pass filter, its inflection point frequency fc =1.0HZ.
Only those data that can enhance the amplitude of short-period signals are suitable for studying the fine structure of the earth's interior and accurately reading out the first arrival time and amplitude of short-period P waves. In contrast, broadband seismograms proportional to displacement and long-period filtered seismograms suppress the high-frequency components of the signal. Such data are more suitable for conventional surface wave magnitude estimation and identification of most subsequent seismic phases.
Beamforming: Beamforming can improve the signal-to-noise ratio (SNR) of seismic signals by stacking coherent signals from the array. The signal at each station is time-shifted relative to the delay of the reference station. The delay depends on the slowness and azimuth, and the slowness and azimuth can be obtained by fk analysis. The delayed signals are added seismically to create the beam. SNR will increase root N
Polarization analysis: The task of polarization analysis is to convert a trisectoral seismogram into a coordinate system of ray directions. To identify wave polarization and study shear wave splitting, the traditional triads N, E, Z are rotated to the coordinate system of the ray direction, or R (radial, towards the epicenter), T (perpendicular to the epicenter direction) Coordinate System. This is especially suitable for identification of subsequent seismic phases.
Conventional Earthquake Analysis Software
1 SHM can handle local area and teleseismic events, and is used for data analysis of Grafenberg array and German regional array.
2 SEISAN regional or global seismic phase picking, spectral analysis, azimuth determination, mapping and some other applications.
3 PITSA digital signal processing and routine analysis
4 GIANT is a software system for heterogeneity analysis of a large number of non-uniform seismic data .
5 The combination of PCEQ and positioning software HYPO71 is widely used in local event processing
Seismogram Analysis
The main characteristics of seismograms of local earthquakes are short recording duration (usually from a few seconds to one minute), rich high-frequency components, and obvious envelope characteristics of the waveform.
Teleseismic (distance > 13°) recordings recorded low-frequency waves (because high-frequency energy is attenuated by elastic attenuation and scattered waves) with durations ranging from 15 minutes to several hours.
Phase: The wave groups arriving at the station along different paths are called phases.
For near-seismic events, there are multiple localization methods (ORFEUS software). Seismic phase picking, plane wave method and fk method can be used for teleseismic . Using the data of the array and network, and according to the travel time difference of the seismic phase, the epicentral distance can be estimated quite reliably . Combining the slowness values with the azimuth estimates derived from the trigonometric records, hypocenters can be localized using only single-station records . Depth phases can be used to determine focal depth . The amplitude and period of the different seismic phases are used to calculate the magnitude . Both body waves and surface waves can be used to calculate magnitude.
Near earthquake (0°<D<13°)
The P-wave and S-wave seismic phases that travel along different paths through the crust are determined, and these seismic phases are called "crustal phases". The typical speeds of Pg and Sg are 5.5~6.5km/s and 3.2~3.7km/s respectively
Seismic Phases Observed Near Earthquakes
Pg, Sg is the direct wave transmitted to the station by the source located in the upper or middle part of the crust ; the first arriving P wave and S wave within the range of 120km
Both Pmp and Sms are reflected waves of the Moho interface;
Pn and Sn are the critical refracted waves along the Moho interface or closely along the flag, respectively. Pn arrives before Pg when the distance exceeds 200km, and Pn and Sn become the main seismic phase after the epicentral distance exceeds 600~800km;
The critical refraction of Pb and Sb at the interface in the middle of the crust or the seismic phase at the double turning point in the lower crust
Sg and Sms are the strongest body waves in the local seismic record
Pg and Pmp have maximum amplitudes at the beginning of the record at least up to a distance of 200-400km;
1 The lateral variation of the crustal velocity structure makes the actual arrival time of some stations consistent with the theoretical arrival time, but not of other stations.
2 The amplitude ratio of Pg and Sg varies greatly with azimuth. This is due to the different patterns of P-wave and S-wave radiation. This fact can be used for the fault-plane solution of short-legged earthquakes.
Usually, for near-surface earthquakes and the distance is less than 400KM, the amplitude of Pn is much smaller than that of Pg. However, for longer distances, the relative amplitudes of Pn and Sn increase and become the main seismic phase.
Teleseismic (13°<D<180°)