Epicentral distance
On the contrary, with the increase of epicentral distance, the damage caused by the earthquake is gradually reduced.
[2] Due to the limitation of seismometers designed in the early years, some seismic magnitude scales began to show errors when the epicentral distance exceeded a certain range from the observation points.
[4] If the source is very far away, that is, when the epicenter distance is greater than 105 °,[Notes 4] the epicenter distance cannot be determined according to the S-P move out method so it must be determined by P, PKP, PP, SKS, PS, and other waves.
[4] In 1935, in the absence of a mature seismic magnitude scales, two seismologists from the California Institute of Technology, Charles Francis Richter and Bino Gutenberg, designed the Richter magnitude scale to study the earthquakes that occurred in California, USA.
[Notes 5] In order to keep the result from being negative, Richter defined an earthquake with a maximum horizontal displacement of 1 μ m (which is also the highest accuracy and precision of the Wood Anderson torsion seismometer) recorded by the seismometer at the observation point at the epicentral distance of 100 km as a magnitude 0 earthquake.
According to this definition, if the amplitude of the seismic wave measured by the Wood Anderson torsion seismometer at the epicentral distance of 100 km is 1 mm, then the magnitude is 3.
[Notes 6] Although Richter et al. attempted to make the results non-negative, modern precision seismographs often record earthquakes with negative scales due to the lack of clear upper or lower limits on the magnitude of nearby earthquakes.
[5] Moreover, due to the limitation of the Wood Anderson torsion seismometer used in the original design of the Richter scale, if the local earthquake scale ML is greater than 6.8 or the epicentral distance exceeds about 600 km the observation point, it is not applicable.
represents the maximum particle displacement in the surface wave (sum of two horizontal Euclidean vectors), in micrometers; T represents the corresponding period, in seconds; Δ Is the epicentral distance, in degrees; and
This is also a meaningful quantitative work for carrying out research on the measurement of Body wave magnitude MB recorded by short period instrument DD-1 and VGK.
Due to the inability to determine the precise range of the polar region, errors were often caused.
When an earthquake occurs in a certain place, the analyst can measure the time difference of various waves of the earthquake from the seismogram and calculate the epicentral distance by comparing it with the prepared travel timetable or applying the formula.
After the azimuth and epicentral distance are calculated, the epicenter position can be easily found.
[Notes 10] When the epicentral distance is calculated by at least three seismic stations, the location of the epicenter can be determined by trilateral measurement.
Given the epicentral distance from the observation points, it is easier to distinguish complex and different seismic phases, which are generally judged according to the overall situation of seismic records on the record map.
