Geology topics
Publications and source records attributed to W.H.K. Lee.
HYPO71, a computer program for determining hypocenter, magnitude, and first-motion pattern of local earthquakes, was first released in 1971. It is perhaps the first earthquake location program that achieved worldwide usage, as evidenced by the fact that about 1000 copies of the HYPO71 manual were requested and distributed (Lee, 1990). It is included here for historical documentation.
This chapter focuses on collective heritage from early seismologists. Scientists began systematic instrumental observation of earthquakes in the latter part of the 19 th century. Several authors describe the history of the development of an adequate instrumentation for seismology. In the 1880s, scientists in Italy, Japan, and Germany began to record more or less continuously the ground motion with their newly developed seismographs. Because of the limited ability to reproduce the original (analog) paper seismograms, seismologists had to describe their observations in words and numbers. The recording instruments were able to produce seismograms in which one could distinguish between the onsets of all three wave types (i.e., P, S, and surface waves), and a common vocabulary for the description of these records was developed. During the last half of the 19 th century, scientists in many countries began to systematically collect data of macro seismically observed earthquakes and the locations of these events, known only on the basis of such data. In some countries, scientists and/or their governments established special committees or commissions to do this work. The earthquake lists and bulletin data from the early seismic stations not only document the history of seismology but also have intrinsic scientific value. Even today, these old bulletins are needed for event relocation as our technique for earthquake location improves. They are also useful for magnitude analysis to establish a consistent magnitude scale.
Earthquake seismology became a quantitative scientific discipline after instruments were developed to record seismic waves in the late 19th century ( Dewey and Byerly, 1969 ; Chapter 1 by Agnew). Earthquake seismology is essentially based on field observations. The great progress made in the past several decades was primarily due to increasingly plentiful and high-quality data that are readily distributed. Our ability to collect, process, and analyze earthquake data has been accelerated by advances in electronics, communications, computers, and software (see Chapter 85 edited by Snoke and Garcia-Fernandez). Instrumental observation of earthquakes has been carried out for a little over 100 years by seismic stations and networks of various sizes, from local to global scales (see Chapter 87 edited by Lahr and van Eck). The observed data have been used, for example, (1) to compute the source parameters of earthquakes, (2) to determine the physical properties of the Earth's interior, (3) to test the theory of plate tectonics , (4) to map active faults, (5) to infer the nature of damaging ground shaking, and (6) to carry out seismic hazard analysis. Construction of a satisfactory theory of the earthquake process has not yet been achieved within the context of physical laws. Good progress, however, has been made in building a physical foundation of the earthquake source process, partly as a result of research directed toward earthquake prediction. This chapter is intended for a general audience. Technical details are not given, but relevant references and chapters in this Handbook are referred to. The first part of this chapter presents a brief overview of the observational aspects of earthquake seismology, concentrating on instrumental observations of seismic waves generated by earthquakes (i.e., seismic monitoring), and readers are referred to Chapter 49 by Musson and Cecic for noninstrumental observations. A few key developments and practices are summarized by taking a general view, since many national and regional developments have been chronicled in national and institutional reports (see Chapter 79 edited by Kisslinger). In the latter part of this chapter, the nature of seismic monitoring and some challenges in observational seismology are discussed from a personal perspective. Comments of a technical or philosophical nature are given in the Notes at the end of the chapter, and they are referenced by superscript numbers in the text.
The Chi-Chi earthquake occurred at 17:47 on 20 September 1999 and is the largest earthquake ( M W 7.6) to have occurred on land in Taiwan in the twentieth century. This earthquake caused considerable damage and was named the “921 Chi-Chi Great Earthquake” by the Taiwan government, as the local date was 21 September. Because an extensive strong-motion instrumentation program in Taiwan was completed by the Central Weather Bureau (CWB) in 1996, over 30,000 digital strong-motion records have been obtained from the Chi-Chi earthquake and thousands of its aftershocks. These records form the largest set of strong-motion data recorded from a major earthquake since strong-motion seismology studies began in the 1930s. This data set is important to seismology and earthquake engineering because it includes over 60 recording sites within 20 km of the fault ruptures, which provides a five-fold increase of such near-field records available for the entire world.
The Central Weather Bureau (CWB) of Taiwan completed a deployment of 1200 modern digital strong-motion instruments in 1996 at free-field sites and in buildings and bridges. Consequently, a very extensive set of strong-motion records were obtained for the M W = 7.6 Chi-Chi earthquake in 1999, including over 60 near-field records within 20 km of the fault ruptures. For documentation purposes, we included all relevant data files on the attached CD-ROM from our article, “CWB Free-Field Strong-Motion Data from the 21 September Chi-Chi, Taiwan, Earthquake” in this issue. We presented the data in four different ways to make the data more user friendly: (1) the original recorded data by accelerograph type; (2) the processed data in ASCII text format; (3) the processed data in SUDS format; and (4) the processed data in SAC format. (1)-(3) are exactly the same as in Lee et al. ( 2001 ), except the files are zipped (or compressed). (4) is provided for users using the SAC software on a Unix computer, and it also contains waveform plots of the data in PDF files for ease of viewing. The attached CD contains the full report of Lee et al. ( 2001 ) in 25 PDF files for viewing or printing. This full report described the data set and the data processing performed in details. It also contains plots of all processed data with an index map showing the fault, mainshock location, and the station location. We also provided a readme.txt on the CDROM to augment this Short Note with further explanations. We classify the recorded data files into four quality groups. Since the recorded strong-motion data are based on a triggering algorithm, a main concern is the amount of pre-event data and whether or not the record is long enough to cover the entire duration of the ground shaking at that station. In addition, we are concerned whether or not the record has defects (e.g., spikes, or a component was not recorded), and whether or not the record has absolute timing. In general, QA-class records are the best and can be used for any studies. QB-class records are the next best, except most of them do not have absolute timing. QC-class records cover the principal strong motions but may not have adequate pre-event data and/or post strong-motion data. QD-class records have some defects; they are included here for completeness and should not be used for most studies. Since most A800 accelerographs have a colocated A900 or A900A accelerograph, the user should use the A900 or A900A data files whenever possible. There are a readme file and six subdirectories containing the data files described in our article on the attached CDROM, under the directory of \LeeWHK: (1) readme.txt: an ASCII text file of this Short Note and additional explanations. (2) DOCfiles (subdirectory): documentary files in PDF format for viewing, and some files in ASCII text format for use in data processing. User should view or print the Reportxx.pdf files which describe the Chi-Chi strong-motion data set in details (xx is a number from 01 through 25). (3) Software (subdirectory): containing computer programs from the accelerograph manufacturers for viewing and converting the original recorded data; they are grouped by manufacturer. (4) ORGfiles (subdirectory): the original recorded data files grouped by accelerograph types; these files are for archival purposes only. (5) SUDfiles (subdirectory): the processed strong-motion data files in PC-SUDS format, and are grouped by quality class. (6) ASCfiles (subdirectory): the processed strong-motion data files are in ASCII text format and are grouped by quality class. (7) SACfiles (subdirectory): the processed strong-motion data files are in SAC format and are grouped by quality class. Within each quality class, the data are given by station. Each station has two files—xxx.pdf for viewing the three-component strong-motion data, and xxx_SAC.zip, a WINZIP file containing three SAC data files corresponding to the three components of the recorded acceleration. We use “xxx” here to denote the station name.
No abstract available.
A major earthquake occurred near the town of Chi-Chi in Nantou County, Taiwan, at 1:47 am (local time), 21 September 1999, about 150 km south of Taipei. This is the largest earthquake to have occurred on land in Taiwan during the 20th century. Although Taiwan has an earthquake building code, thousands of buildings collapsed due to the earthquake, leaving more than 100,000 people homeless. The death toll exceeded 2,300 with more than 10,000 injured. Within 102 seconds after the earthquake's origin time, a good estimate of the hypocenter (23.87° N, 120.75° E, Depth = 10 km) and magnitude ( M L = 7.3), and a shaking map were determined automatically by the RTD system. The result was immediately disseminated to governmental emergency response agencies electronically in four ways, by e-mail, World Wide Web, fax, and pager. This rapid information system has been successfully operating in Taiwan for more than four years. During the Chi-Chi earthquake, the rapid availability of earthquake information facilitated the emergency response. The RTD system worked very well throughout the whole aftershock sequence (several aftershocks had local magnitude of 6 or larger). Again this timely information was useful to the emergency response teams. In this paper, we briefly describe the RTD system and summarize its performance during the Chi-Chi earthquake.
In order to explore the feasibility of an earthquake early warning system for Taipei, two prototype seismic early warning systems have been implemented in Hualien, about 120 km away. The Taiwan Rapid Earthquake Information Release (TREIRS) can routinely determine earthquake location and magnitude in about one minute after the earthquake occurred. However, this reporting time is too long for earthquake early warning purposes. A dense, real-time monitoring system composed of 8 strong-motion stations was installed in Hualien area for testing earthquake early warning capability. For the 27 earthquakes occurred during August 1998 to April 1999, this system has successfully reported earthquake information in about 18 sec after the origin time. Therefore, it provided about 15 sec of early warning time before shear waves arrival in the Taipei urban area. Based on our experience of these two systems, we are encouraged to proceed forward in the development of an earthquake early warning system.
Apparent southward-dipping, reverse-fault zones are imaged to depths of about 1.5 km beneath Potrero Canyon, Los Angeles County, California. Based on their orientation and projection to the surface, we suggest that the imaged fault zones are extensions of the Oak Ridge fault. Geologic mapping by others and correlations with seismicity studies suggest that the Oak Ridge fault is the causative fault of the 17 January 1994 Northridge earthquake (Northridge fault). Our seismically imaged faults may be among several faults that collectively comprise the Northridge thrust fault system. Unusually strong shaking in Potrero Canyon during the Northridge earthquake may have resulted from focusing of seismic energy or co-seismic movement along existing, related shallow-depth faults. The strong shaking produced ground-surface cracks and sand blows distributed along the length of the canyon. Seismic reflection and refraction images show that shallow-depth faults may underlie some of the observed surface cracks. The relationship between observed surface cracks and imaged faults indicates that some of the surface cracks may have developed from nontectonic alluvial movement, but others may be fault related. Immediately beneath the surface cracks, P-wave velocities are unusually low (<400 m/sec), and there are velocity anomalies consistent with a seismic reflection image of shallow faulting to depths of at least 100 m. On the basis of velocity data, we suggest that unconsolidated soils (<800 m/sec) extend to depths of about 15 to 20 m beneath our datum (<25 m below ground surface). The underlying rocks range in velocity from about 1000 to 5000 m/sec in the upper 100 m. This study illustrates the utility of high-resolution seismic imaging in assessing local and regional seismic hazards.
This article reports the recent progress on real-time seismic monitoring in Taiwan, particularly the real-time strong-motion monitoring by the Taiwan Central Weather Bureau's telemetered seismic network ( CWBSN ), which is presently aiming at rapid reporting immediately after a large earthquake occurrence. If rapid reporting can be achieved before the arrival of the strong shaking, earthquake early warning will become possible. CWBSN has achieved the generation of the intensity map, epicenter, and magnitude within 1 min of the occurrence of a large earthquake. Both rapid reporting and early warning are principally applied to large ( M ≫ 5) events; the requirement of on-scale waveform recording prompted CWBSN in 1995 to integrate strong-motion sensors (e.g., force-balance accelerometers) into its telemetered seismic monitoring system. Time-domain recursive processing is applied to the multi-channel incoming seismic signals by a group of networked personal computers to generate the intensity map. From the isoseismal contours, an effective epicenter is immediately identified that resides in the middle of the largest (usually the 100-gal) contour curve of the intensity map. An effective magnitude is also defined that can be derived immediately from the surface area covered by the largest (usually the 100-gal) contour curve. For a large event with a finite rupture surface, the epicenter and magnitude so derived are more adequate estimates of the source location and of the strength of destruction. The effective epicenter gives the center of the damage area; it stands in contrast with the conventional epicenter location, which only gives the initial point of rupture nucleation. The effective magnitude reflects more closely the earthquake damage potential, instead of the classical magnitude definition that emphasizes the total energy release. The CWBSN has achieved in obtaining the above crucial source information well within 1 min. This time can further be reduced to better than 30 sec, as illustrated by the example in this article, showing that earthquake early warning is indeed an achievable goal. The rapid reporting and early warning information is electronically transmitted to users to allow rapid response actions, with or without further human intervention.
The 17 January 1994, Northridge, California, earthquake produced strong ground shaking at the Cedar Hills Nursery (referred to here as the Tarzana site) within the city of Tarzana, California, approximately 6 km from the epicenter of the mainshock. Although the Tarzana site is on a hill and is a rock site, accelerations of approximately 1.78 g horizontally and 1.2 g vertically at the Tarzana site are among the highest ever instrumentally recorded for an earthquake. To investigate possible site effects at the Tarzana site, we used explosive-source seismic refraction data to determine the shallow (<70 m) P -and S -wave velocity structure. Our seismic velocity models for the Tarzana site indicate that the local velocity structure may have contributed significantly to the observed shaking. P -wave velocities range from 0.9 to 1.65 km/sec, and S -wave velocities range from 0.20 and 0.6 km/sec for the upper 70 m. We also found evidence for a local S -wave low-velocity zone ( LVZ ) beneath the top of the hill. The LVZ underlies a CDMG strong-motion recording site at depths between 25 and 60 m below ground surface ( BGS ). Our velocity model is consistent with the near-surface (<30 m) P - and S -wave velocities and Poisson's ratios measured in a nearby (<30 m) borehole. High Poisson's ratios (0.477 to 0.494) and S -wave attenuation within the LVZ suggest that the LVZ may be composed of highly saturated shales of the Modelo Formation. Because the lateral dimensions of the LVZ approximately correspond to the areas of strongest shaking, we suggest that the highly saturated zone may have contributed to localized strong shaking. Rock sites are generally considered to be ideal locations for site response in urban areas; however, localized, highly saturated rock sites may be a hazard in urban areas that requires further investigation.
The Northridge earthquake caused 1.78 g acceleration in the east-west direction at a site in Tarzana, California, located about 6 km south of the mainshock epicenter. The accelerograph was located atop a hill about 15-m high, 500-m long, and 130-m wide, striking about N78°E. During the aftershock sequence, a temporary array of 21 three-component geophones was deployed in six radial lines centered on the accelerograph, with an average sensor spacing of 35 m. Station C00 was located about 2 m from the accelerograph. We inverted aftershock spectra to obtain average relative site response at each station as a function of direction of ground motion. We identified a 3.2-Hz resonance that is a transverse oscillation of the hill (a directional topographic effect). The top/base amplification ratio at 3.2 Hz is about 4.5 for horizontal ground motions oriented approximately perpendicular to the long axis of the hill and about 2 for motions parallel to the hill. This resonance is seen most strongly within 50 m of C00. Other resonant frequencies were also observed. A strong lateral variation in attenuation, probably associated with a fault, caused substantially lower motion at frequencies above 6 Hz at the east end of the hill. There may be some additional scattered waves associated with the fault zone and seen at both the base and top of the hill, causing particle motions (not spectral ratios) at the top of the hill to be rotated about 20° away from the direction transverse to the hill. The resonant frequency, but not the amplitude, of our observed topographic resonance agrees well with theory, even for such a low hill. Comparisons of our observations with theoretical results indicate that the 3D shape of the hill and its internal structure are important factors affecting its response. The strong transverse resonance of the hill does not account for the large east-west mainshock motions. Assuming linear soil response, mainshock east-west motions at the Tarzana accelerograph were amplified by a factor of about 2 or less compared with sites at the base of the hill. Probable variations in surficial shear-wave velocity do not account for the observed differences among mainshock acceleration observed at Tarzana and at two different sites within 2 km of Tarzana.
Using simulated coda waves, the resolution of the single-scattering model to extract coda Q ( Q c ) and its power law frequency dependence was tested. The back-scattering model of AKI and CHOUET (1975) and the single isotropic-scattering model of SATO (1977) were examined. The results indicate that: (1) The input Q c models are reasonably well approximated by the two methods; (2) almost equal Q c values are recovered when the techniques sample the same coda windows; (3) low Q c models are well estimated in the frequency domain from the early and late part of the coda; and (4) models with high Q c values are more accurately extracted from late code measurements.
Coda wave data from California microearthquakes were studied in order to delineate regional fluctuations of apparent crustal attenuation in the band 1.5 to 24 Hz. Apparent attenuation was estimated using a single back scattering model of coda waves. The coda wave data were restricted to ???30 s following the origin time; this insures that crustal effects dominate the results as the backscattered shear waves thought to form the coda would not have had time to penetrate much deeper. Results indicate a strong variation in apparent crustal attenuation at high frequencies between the Franciscan and Salinian regions of central California and the Long Valley area of the Sierra Nevada. Although the coda Q measurements coincide at 1.5 Hz (Qc=100), at 24 Hz there is a factor of four difference between the measurements made in Franciscan (Qc=525) and Long Valley (Qc=2100) with the Salinian midway between (Qc=900). These are extremely large variations compared to measures of seismic velocities of comparable resolution, demonstrating the exceptional sensitivity of the high frequency coda Q measurement to regional geology. In addition, the frequency trend of the results is opposite to that seen in a compilation of coda Q measurements made worldwide by other authors which tend to converge at high and diverge at low frequencies, however, the worldwide results generally were obtained without limiting the coda lengths and probably reflect upper mantle rather than crustal properties. Our results match those expected due to scattering in random media represented by Von Karman autocorrelation functions of orders 1/2 to 1/3. The Von Karman medium of order 1/3 corresponding to the Franciscan coda Q measurement contains greater amounts of high wavenumber fluctuations. This indicates relatively large medium fluctuations with wavelengths on the order of 100 m in the highly deformed crust associated with the Franciscan, however, the influence of scattering on the coda Q measurement is currently a matter of controversy.
Since the 1950s, the Academia Sinica in Peking, People's Republic of China, has carried out extensive research on the Chinese earthquake history. With a historical record dating back some 3000 years, a wealth of information on Chinese earthquakes exists. Despite this monumental undertaking by the Academia Sinica, much work is still necessary to correct the existing earthquake data for historical changes in population, customs, modes of communication, and dynasties. In this paper we report on the status of our investigation of Chinese earthquake history and present some preliminary results. By applying point process analysis of earthquakes in 'Central China', we found suggestions of (1) lower earthquake activity at intervals of about 175 years and 375 years, and (2) higher earthquake activity at an interval of about 300 years.
A microearthquake network is a group of highly sensitive seismographic stations designed primarily to record local earthquakes of magnitudes less than 3. Depending on the application, a microearthquake network will consist of several stations or as many as a few hundred . They are usually classified as either permanent or temporary. In a permanent network, the seismic signal from each is telemetered to a central recording site to cut down on the operating costs and to allow more efficient and up-to-date processing of the data. However, telemetering can restrict the location sites because of the line-of-site requirement for radio transmission or the need for telephone lines. Temporary networks are designed to be extremely portable and completely self-contained so that they can be very quickly deployed. They are most valuable for recording aftershocks of a major earthquake or for studies in remote areas.
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