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Geology topics

William H. K. Lee

Publications and source records attributed to William H. K. Lee.

10 recordsLinked to original sources
USGS2016

Rotational seismology

Rotational seismology is an emerging study of all aspects of rotational motions induced by earthquakes, explosions, and ambient vibrations. It is of interest to several disciplines, including seismology, earthquake engineering, geodesy, and earth-based detection of Einstein’s gravitation waves. Rotational effects of seismic waves, together with rotations caused by soil–structure interaction, have been observed for centuries (e.g., rotated chimneys, monuments, and tombstones). Figure 1a shows the rotated monument to George Inglis observed after the 1897 Great Shillong earthquake. This monument had the form of an obelisk rising over 19 metres high from a 4 metre base. During the earthquake, the top part broke off and the remnant of some 6 metres rotated about 15° relative to the base. The study of rotational seismology began only recently when sensitive rotational sensors became available due to advances in aeronautical and astronomical instrumentations.

Book chapter↗
USGS2016

Complexity Theory

A complex system consists of many interacting parts, generates new collective behavior through self organization, and adaptively evolves through time. Many theories have been developed to study complex systems, including chaos, fractals, cellular automata, self organization, stochastic processes, turbulence, and genetic algorithms.

Book chapter↗
USGS2011

Moment tensor inversions using strong motion waveforms of Taiwan TSMIP data, 1993–2009

Earthquake source parameters are important for earthquake studies and seismic hazard assessment. Moment tensors are among the most important earthquake source parameters, and are now routinely derived using modern broadband seismic networks around the world. Similar waveform inversion techniques can also apply to other available data, including strong-motion seismograms. Strong-motion waveforms are also broadband, and recorded in many regions since the 1980s. Thus, strong-motion data can be used to augment moment tensor catalogs with a much larger dataset than that available from the high-gain, broadband seismic networks. However, a systematic comparison between the moment tensors derived from strong motion waveforms and high-gain broadband waveforms has not been available. In this study, we inverted the source mechanisms of Taiwan earthquakes between 1993 and 2009 by using the regional moment tensor inversion method using digital data from several hundred stations in the Taiwan Strong Motion Instrumentation Program (TSMIP). By testing different velocity models and filter passbands, we were able to successfully derive moment tensor solutions for 107 earthquakes of M w >= 4.8. The solutions for large events agree well with other available moment tensor catalogs derived from local and global broadband networks. However, for M w = 5.0 or smaller events, we consistently over estimated the moment magnitudes by 0.5 to 1.0. We have tested accelerograms, and velocity waveforms integrated from accelerograms for the inversions, and found the results are similar. In addition, we used part of the catalogs to study important seismogenic structures in the area near Meishan Taiwan which was the site of a very damaging earthquake a century ago, and found that the structures were dominated by events with complex right-lateral strike-slip faulting during the recent decade. The procedures developed from this study may be applied to other strong-motion datasets to compliment or fill gaps in catalogs from regional broadband networks and teleseismic networks.

Tectonophysics↗
USGS2009

Review: Progress in rotational ground-motion observations from explosions and local earthquakes in Taiwan

Rotational motions generated by large earthquakes in the far field have been successfully measured, and observations agree well with the classical elasticity theory. However, recent rotational measurements in the near field of earthquakes in Japan and in Taiwan indicate that rotational ground motions are 10 to 100 times larger than expected from the classical elasticity theory. The near-field strong-motion records of the 1999 M w 7.6 Chi-Chi, Taiwan, earthquake suggest that the ground motions along the 100 km rupture are complex. Some rather arbitrary baseline corrections are necessary in order to obtain reasonable displacement values from double integration of the acceleration data. Because rotational motions can contaminate acceleration observations due to the induced perturbation of the Earth’s gravitational field, we started a modest program to observe rotational ground motions in Taiwan. Three papers have reported the rotational observations in Taiwan: (1) at the HGSD station ( Liu et al. , 2009 ), (2) at the N3 site from two TAiwan Integrated GEodynamics Research (TAIGER) explosions ( Lin et al. , 2009 ), and (3) at the Taiwan campus of the National Chung-Cheng University (NCCU) ( Wu et al. , 2009 ). In addition, Langston et al. (2009) reported the results of analyzing the TAIGER explosion data. As noted by several authors before, we found a linear relationship between peak rotational rate (PRR in mrad/sec) and peak ground acceleration (PGA in m/sec 2 ) from local earthquakes in Taiwan, PRR=0.002+1.301 PGA, with a correlation coefficient of 0.988.

Bulletin of the Seismological Society of America↗
USGS1985

Crustal structure of the southern Calaveras fault zone, central California, from seismic refraction investigations

A magnitude 5.7 earthquake on 6 August 1979, within the Calaveras fault zone, near Coyote Lake of west-central California, motivated a seismic-refraction investigation in this area. A northwest-southeast profile along the fault, as well as two fan profiles across the fault were recorded to examine the velocity structure of this region. The analysis of the data reveals a complicated upper crustal velocity structure with strong lateral variations in all directions. The near-surface layers consist of recent sediments with seismic P -wave velocities of 2.6 to 3.2 km/sec. These are underlain by rocks of the Great Valley Sequence which have an average velocity of 4.5 km/sec. The Great Valley Sequence is present along the whole profile; depths range from 4.3 to 4.8 km in the northwest near Anderson Lake and in the southeast of the profile line near Hollister. In the middle of the profile near Coyote Lake, however, this layer thins and we find a laterally limited higher velocity layer (5.1 km/sec) between depths of 2.8 to 4.8 km. The high-velocity zone, which coincides with a gravity high, also correlates spatially with the hypocentral area of the 6 August earthquake and its aftershocks and may therefore represent an asperity on the fault. Velocities within the fault zone were determined from the fan profiles. Near Anderson Lake, a pronounced delay of first arrivals on the fan records indicates a vertical 1- to 2-km-wide near-surface, low-velocity zone along the fault. Near Coyote Lake, the delays observed in the fan records correlate with two subsurface en-echelon fault planes which have been previously identified from lineations in the seismicity pattern. The structure of the lower crust is similar to the neighboring Diablo Range: a 8- to 9-km-thick upper crustal layer with a seismic velocity of 5.7 to 6.3 km/sec is underlain by a 3-km-thick layer with velocity 6.8 km/sec. In accordance with previous studies of the Diablo Range, there are indications of a pronounced lower crustal low-velocity zone between a depth of 17 and 23 km. The presence of this low-velocity zone suggests that higher velocity (igneous?) rocks of the middle crust have been thrust over sedimentary rocks.

California↗