عنوان مقاله [English]
Rapid estimation of the epicentral distance and magnitude is of fundamental importance for real time earthquake detection and earthquake early warning systems (EEWS). Earthquake magnitude and P-wave amplitude are important parameters for EEWS, yet their dependence on source mechanism, focal depth and epicentral distance (Δ) has not been fully studied. We examined a method to estimate an earthquake’s magnitude and epicentral distance using the initial part of P-wave data (within 3 s) for application in EEWS. The B-Δ method is used to estimate the epicentral distance from a single station data in a short time. In order to quantitatively evaluate the difference in observed seismic waveforms, we used a simple function with the form of y(t) = B.t.exp(-At) and determined A and B in terms of the least-squares method by fitting this function to the initial part of the waveform envelope. logB is inversely proportional to logΔ, where Δ is the epicentral distance. This relation holds true regardless of earthquake magnitude. By using this relation, we can roughly estimate the epicentral distance nearly immediately after the P-wave arrival. Then, we can readily estimate the magnitude from the maximum amplitude observed within a given short time interval after the P-wave arrival by using an empirical magnitude–amplitude relation that includes the epicentral distance as a parameter. B values are calculated on the basis of 76 vertical-component accelerograms of the Alborz region in a magnitude range Mw 4.5-6.2 and epicentral distances less than 100 km. By using this method, we could estimate the epicentral distance and earthquake magnitude by specific relations for this region. We showed the amplitude of the large earthquake increases gradually with time, whereas that of the small earthquake decreases soon after P-wave arrival, which is consistent with the observation by other researchers.
This method, as a whole, works well for estimating an earthquake magnitude from a B value and the maximum amplitude observed within a quite short time (e.g., 3 sec) from the P-wave arrival. However, some improvements may be required for near earthquakes and for ill-natured earthquakes for which the fault rupture process is rather complicated, such as the Mosha fault with different segments. One measure that we can take to cope with this difficulty is to estimate the magnitude repeatedly with time as the amplitude increases. The term logB may be replaced with other functions such as log(log B). In order to determine the best functional formula for this term, we need further investigations with more earthquake data covering a larger range of magnitudes, depths, and distances. The other parameter A, may be useful for distinguishing shallow and deep earthquakes and large and small earthquakes. This can be an option for future studies.
This method can apply as a new stand-alone seismographic system that detects an earthquake and issues a warning immediately after the arrival of P-wave. The greatest advantage of this method is its accuracy and rapidness.