ALBERT

Description: We detected 12 landquakes in Taiwan with collapse areas between 0.27 and 2.48 km 2 associated with rock collapse, rockslide and debris and/or rock avalanche during the passage of Typhoon Morakot in 2009. These events were recorded by seismic stations of the Broadband Array in Taiwan for Seismology. Their locations were determined by a cross-correlation technique that maximizes the coherency of horizontal envelope function among seismic stations with a mean location error of 1.92 km. We applied time-frequency analysis to estimate the bandwidth of seismic energy generated by the landquakes. The predominant frequency ranges from 0.5 to 5.0 Hz, with higher-frequency signals likely caused by block impact. We extracted signal duration ( 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 ) by mapping satellite images. Based on aforementioned seismic and geometrical parameters, we defined the potential of initial impact ( P I ), the frequency of rock impact signal ( f I ) and the mean quasi-front velocity ( V f ) as indicators of landquake types, especially for events with dam formation (dam-formation-type events). We also derived an empirical linear relationship between the envelope area ( A E ) and collapse area ( A C ) with a high correlation coefficient of 0.83. Our automatic approach is very effective for rapid determination of landquake centroid location and collapse area, and for identifying dam-formation event using records from existing real-time broad-band seismic networks, thus providing an important alternative for landquake hazard mitigation.