A seismological study of landquakes using a real-time broad-band seismic network

We detected 12 landquakes in Taiwan with collapse areas between 0.27 and 2.48 km² associatedwith rock collapse, rockslide and debris and/or rock avalanche during the passage of TyphoonMorakot in 2009. These events were recorded by seismic stations of the Broadband Arrayin Taiwan for Seismology. Their locations were determined by a cross-correlation techniquethat maximizes the coherency of horizontal envelope function among seismic stations with amean location error of 1.92 km. We applied time-frequency analysis to estimate the bandwidthof seismic energy generated by the landquakes. The predominant frequency ranges from 0.5to 5.0 Hz, with higher-frequency signals likely caused by block impact. We extracted signalduration (S_D), peak ground velocity, rise time (T_R), area of velocity envelope function (A_E)from the closest station and estimated the collapse area (A_C) and run-out distance (D_R) bymapping satellite images. Based on aforementioned seismic and geometrical parameters, wedefined the potential of initial impact (P_I), the frequency of rock impact signal (f_I) and themean quasi-front velocity (V_f) as indicators of landquake types, especially for events withdam formation (dam-formation-type events). We also derived an empirical linear relationshipbetween the envelope area (A_E) and collapse area (A_C) with a high correlation coefficient of0.83. Our automatic approach is very effective for rapid determination of landquake centroidlocation and collapse area, and for identifying dam-formation event using records from existingreal-time broad-band seismic networks, thus providing an important alternative for landquakehazard mitigation.