Magnitude estimation for early warning applications using the initial part of P waves: A case study on the 2008 Wenchuan sequence

A period parameter τ_c and an amplitude parameter Pd determined from the very beginning of P wave are important for earthquake early warning (EEW), yet their dependence on source mechanism, focal depth and epicentral distance has not been fully studied. After the devastating Mw7.9 Wenchuan earthquake, hundreds of M4-6 earthquakes occurred with diverse focal mechanisms and depth range of 2–20 km. We calculate τ_c and Pd of these aftershocks and examine their dependence on magnitude, τ_c, distance, and depth. We find that τ_c correlates well with magnitude, but joint regression including distance and depth does not significantly improve the correlation. The effect of focal mechanism on the τ_c-magnitude correlation is not obvious. When P wave is nodal, τ_c measurement becomes inaccurate. Also, τ_c is systematically greater for slow earthquakes, leading to a possible false alarm. Thus, more studies are required to discriminate slow earthquakes for robust early warning.