Living on an Active Earth Perspectives on Earthquake Science (2003) / Chapter Skim
Currently Skimming:
4. Observing the Active Earth: Current Technologies and the Role of the Disciplines
4. Observing the Active Earth: Current Technologies and the Role of the Disciplines
Pages 176-255
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 176... ...
The Global Seismic Network (GSN) , initiated with the founding of the National Science Foundation (NSF)
|
From page 177... ...
was constructed in 1992. Paleoseismologists produced a preliminary 1000-year history of major ruptures on the San Andreas fault in 1995 and discovered a prehistoric moment magnitude (M)
|
From page 178... ...
Global Seismic Networks State-of-the-art seismic stations for global seismic networks comprise three-component sensors with high dynamic TABLE 4.1 Scales of Seismic Monitoring Typical Typical Detection Type Network Size Station Spacing Thresholda Global Global 1000 km 4.5 Regional 500 km 25-50 km 2.0 Local 10 km <1000 m -1.0 aMagnitude of smallest event with a high probability of detection; see examples in Figure 4.2.
|
From page 179... ...
digital seismometers, as well as for the older short-period and long-period analog seismometers of the World Wide Standardized Seismographic Network. Broadband instruments are capable of faithfully recording ground motions ranging from ambient noise at quiet sites (line labeled low Earth noise)
|
From page 180... ...
Dotted line is for events in southern California during the same interval recorded by the Southern California Seismic Network. Dashed line is for mining-induced seismicity recorded during 1997-1999 by a local network in the Elandrands gold mine, South Africa.
|
From page 182... ...
The GSN data acquired over the last 15 years have facilitated many advances in the study of global Earth structure and earthquake sources. Seismic tomography has provided dramatic images of subducting slabs, plume-like upwellings, and other features of the mantle convective flow responsible for plate-tectonic motions (Figure 4.4~.
|
From page 183... ...
van Heijst, and l. Woodhouse, Complex shear wave velocity structure imaged beneath Africa and Iceland, Science, 286, 1925-1928, 1999.
|
From page 184... ...
Regional Seismic Networks Owing to their sparse station coverage, global networks do a poor job of detecting and locating events with magnitudes less than about 4.5 (Figure 4.2) , and their sampling is too crude for investigating how waves are produced by fault ruptures, especially the near-fault radiation that generates the complex patterns of strong ground motions observed in large earthquakes.
|
From page 185... ...
, but the dynamic range of these instruments was too low to furnish useful recordings of large regional events. In the last decade, deployments of broadband, high-dynamic-range seismometers have begun to transform the regional networks into much more powerful tools for investigating the basic physics of the earthquake source, the detailed structure of the Earth's crust and deep interior, and the patterns of potentially destructive ground motions.
|
From page 186... ...
FIGURE 4.6 Vertical along-strike cross sections of earthquakes located along a 30-kilometer section of the San Andreas fault. Circles are an estimate of event size.
|
From page 189... ...
They have also yielded a surprising and strikingly detailed picture of the strength, strength distribution, and evolution of the deep San Andreas fault; the scaling of the earthquake source (18~; and the strain accumulation on the Parkfield locked zone at depth. The discovery of numerous characteristically repeating microearthquake sequences at Parkfield has contributed significantly to the development of earthquake recurrence models currently being used to estimate earthquake hazard in California (19~.
|
From page 190... ...
Strong-motion databases are maintained by both the USGS and the CGS, as well as by the Southern California Earthquake Center (SCEC) and the Pacific Earthquake Engineering Research (PEER)
|
From page 191... ...
They are also used to image the architecture of fault systems and other aspects of crustal structure, such as sedimentary basins, that affect the amplitude and duration of strong motions. Until recently, this mode of operation was limited to short-period seismometers with low dynamic range, but large pools of broadband instruments are now efficiently organized within the IRIS Program of Array Seismic Studies of the Continental Lithosphere (PASSCAL)
|
From page 192... ...
The station coverage of the Central Weather Bureau Seismic Network is still poor in relatively inaccessible central mountainous areas. Bottom: Depth cross section showing the slip distribution derived from this data set using a finite-fault inversion methodology.
|
From page 193... ...
Various forms of telemetry are making it possible to monitor state of health and to retrieve ground-motion data in near real time, allowing portable arrays to be integrated with permanent seismic monitoring systems for a wide range of seismic applications. Imaging the Earth Investigations of Earth structure have always figured prominently in the study of earthquakes because they frame the interpretation of seismograms in terms of source processes.
|
From page 194... ...
194 o ~~ o ·bC ~ ~' au au ~ ~ ~ ~3 .
|
From page 195... ...
SOURCE: U.S. Geological Survey, Fact Sheet 110-99, |
From page 196... ...
, based at Thatcham in Berkshire, United Kingdom, is a nongovernmental organization charged with producing a standard global catalog (29~; it provides the most comprehensive compilations of short-period arrival times and amplitudes from the largest, most globally distributed set of seismic stations (approximately 3000) , including earthquake reports from a number of regional seismic monitoring agencies.
|
From page 198... ...
A few regional monitoring systems routinely catalog all seismicity above M 2. Broadband regional networks routinely produce moment tensor solutions for regional earthquakes greater than M 4 (e.g., Figure 4.11~.
|
From page 199... ...
These solutions are obtained automatically and in quasi-real time using broadband data for a subset of events of M 4 and larger. Robustness of the solution is assessed by comparing the results of two independent inversionsone in the time domain, the other in the frequency domain.
|
From page 201... ...
4.2 TECTONIC GEODESY The elastic strain energy unleashed in earthquakes accumulates in the Earth's crust through the imperceptibly slow motions of plate tectonics. The strain rates in tectonically active areas such as the western United States are only few parts in 10 million per year (35~.
|
From page 202... ...
to monitor the strain accumulation between large earthquakes (36~. It allowed the USGS to confirm that slip rates observed over decades across major faults in California are quite similar to geological estimates, which are averaged over thousands to millions of years.
|
From page 203... ...
The campaign mode involves temporary occupation of geodetic benchmarks in much the same way as for earlier triangulation and trilateration surveys. A series of issues the decreasing costs of receivers, the high labor costs of campaign measurements, the loss of precision caused by antenna setup has motivated the installation of permanent GPS stations in configurations similar to seismic monitoring networks.
|
From page 204... ...
SOURCE: Southern California Earthquake Center and U.S. Geological Survey.
|
From page 205... ...
The most recent innovation in tectonic geodesy is InSAR, which has imaged earthquake deformations at a level of detail unanticipated only 10 years ago (45~. InSAR measures deformation by comparing reflected radar waves recorded on successive passes of a satellite from nearly identical positions.
|
From page 206... ...
Amelung, Fault slip distribution of the Mw 7.1 Hector Mine, California, earthquake estimated from satellite radar and GPS measurements, Bull.
|
From page 207... ...
InSAR also reveals unexpected transient strain accumulation along the Blackwater-Little Lake fault system within the eastern California shear zone in a 120-kilometer-long, 20-kilometer-wide zone of concentrated shear between the southern end of the 1872 Owens Valley earthquake surface break and the northern end of the 1992 Landers earthquake surface break. The shear zone is continuous through the Garlock fault, which does not show any evidence of left-lateral slip during the same period.
|
From page 208... ...
The discovery in 1960 of aseismic slip or "creep" on a segment of the San Andreas fault in central California (48) led to methods for extremely localized measurements of fault displacement using invar tapes, wire creep
|
From page 209... ...
The proposed Plate Boundary Observatory would deploy several hundred borehole and perhaps several long-baseline strainmeters at strategically chosen sites along the San Andreas fault system, as well as at several volcanic systems (53~. Geodetic Observations of Earthquake Processes The increasing precision and density of geodetic measurements are furnishing new constraints on how complex fault systems are loaded, how earthquakes interact, the nature of aseismic deformation transients, and how the theological structure of the Earth's crust controls the earthquake process.
|
From page 210... ...
By integrating the slip rate over the areas of faults, one can estimate the rate at which seismic moment is accumulating. If it can be assumed 34° 305 34° Do 50 mm ~ 1 0 km i, ~ I, ~ ~ ~ ~ I, OIL jPN A. a,,,, ,, ARK a-.
|
From page 211... ...
For example, spatial variations of strain rate along the San Andreas fault near Parkfield, California, show details of the slip rate on the fault plane. The San Andreas is primarily locked to a depth of about 15 kilometers to the southeast of that location and is creeping to the northwest.
|
From page 212... ...
, and 1989 Loma Prieta (70~. The 1992 Landers earthquake, where the cumulative postseismic deformation may have equaled 10 to 20 percent of the M 7.3 mainshock, was the first to be observed by a full suite of modern geodetic methods: GPS (71)
|
From page 213... ...
Gwyther, and R.G. Bilham, A slow earthquake sequence on the San Andreas fault, Nature, 383, 65-68, 1996.
|
From page 214... ...
Although the latter has not yet been fully tested owing to measurement limitations and the lack of the longterm observations, Reid's notion of an earthquake "cycle" appears to be at odds with the observed complexity of the earthquake process and the inherent irregularity of stick-slip behavior. Fault-friction models derived from laboratory data predict that observable aseismic slip in the nucleation zone of an earthquake might precede the seismic phase of fault rupture on time scales of minutes to days.
|
From page 215... ...
. Triangles show postseismic deformation from the Landers earthquake observed by strainmeters, GPS, and InSAR.
|
From page 216... ...
Such events have been observed within several plate boundary fault zones (see above) , but so far no convincing relationship to seismic fault slip has been demonstrated.
|
From page 217... ...
LIDAR systems can be mounted on light aircraft equipped with inertial and GPS guidance systems to obtain vertical resolution at the decimeter level (79~. An example of data from the 1999 Hector Mine earthquake is shown in Figure 4.23.
|
From page 219... ...
219 E ~ o' ~ ~ ¢ E ~ ~ E E ° ~ ~ ° X ° ., ~ ~ ~ ~ ~ ~ be, au _,` - ` ° ~ ~ ~ ~ o o ~ ~ o ~ o ~ ~ ~ ° -I a,, ~ ~ =~ ~ .Q ~ ~ o ~ o ~ ° ~ o au baas ~ .> .° ~ ,~~ E-1 CL~ ¢ Ed ~ ~ ~ ~ ~ % = o ~ ~ ~ ~ o au be, ~ ~ 0 c E En ~ ~ to ~ o -~ ¢ o ~ ° o ~ ~ C ~ ~ ~ ° ~ ~ ~ ~ '= got ~ C.~ ° ~ ~ ~ -I ~ ~ ~ ~ .= ~ V
|
From page 220... ...
220 au ~ 0 := au ~ ~ of ¢ > e ~~ ° beo 0 =' O .Q .= ~ i= .m 0 ~ ~ ~ au ~ ~ ~ 0 ~ e at, Liz e ·5- ~ X C., 0 V)
|
From page 221... ...
OBSERVING THE ACTIVE EARTH 221 FIGURE 4.22 Shaded-relief map made from a 40-meter digital elevation model of Taiwan, used in mapping active faults and folds. Geomorphic expression of the Chelungpu fault (red line)
|
From page 222... ...
has pinned down the dates and locations of fault ruptures in Alaska and along the San Andreas fault, and has been used to determine the dates of subduction-related submergence of coastal Washington and massive seismically induced landslides in urban Seattle (82~. Other dating methods used in earthquake geology include tephrochronology, thermoluminescence,
|
From page 223... ...
OBSERVING THE ACTIVE EARTH 223 FIGURE 4.24 Three-dimensional rendering of the bathymetry offshore southern California, illustrating the quality of data now achievable from swath-mapping by surface ships. The view is toward the northwest.
|
From page 224... ...
Neotectonics Although plate tectonics furnishes the first-order framework for understanding global seismicity, most tectonically active plate boundaries exhibit significant second-order complexities that are responsible for a large percentage of the destructive earthquakes of the twentieth century (see Chapter 2~. Placing the resulting diversity of fault structures in a consistent kinematical framework is the program of neotectonics.
|
From page 225... ...
At the San Andreas fault, where a 4000-year-old channel and a 14,000-year-old alluvial fan are offset right-laterally, the derived slip rate is about 33 millimeters per year (Figure 4.25~. Here, offset during the great earthquake of 1857 was about 9 meters.
|
From page 226... ...
yield a Holocene slip rate for the San Andreas fault of 34 + 3 millimeters per year.
|
From page 227... ...
disarticulation at a fault rupture, including fault scarps, wedges of debris at their base, fissures, and disarticulated strata of various origins; 2. changes in sea level or disruption of fluvial gradients, including ponded sediment and deformed fluvial and marine terraces; and 3.
|
From page 228... ...
Fault Rupture Many important active faults ruptured most recently in prehistoric or pre-instrumental time. In the case of southern California's great 1857 earthquake, the approximate length of the rupture could be deduced from written accounts of shaking, but the actual slip as a function of distance along the San Andreas fault was determined only by measurement of offset landforms more than a century after the event (97~; these measurements implied a moment magnitude of about 7.9.
|
From page 229... ...
Deformed lake beds of the Dead Sea not only provide a record of strong shaking, but also show that ruptures cluster into 10,000-year sequences separated by similarly lengthy periods of quiescence (110~. 4.4 FAULT AND ROCK MECHANICS In the context of earthquake science, the study of fault and rock mechanics aims to describe the macroscopic phenomena of fault slip and rock deformation in terms of the microscopic transport processes that operate on crystalline and atomic scales.
|
From page 230... ...
laboratory research to characterize the properties of rocks and faults under the pressure, temperature, chemical, and strain-rate conditions that operate during the earthquake cycle such observations are basic ingredients for the investigation of earthquake processes and the formulation of mechanistic and phenomenological models of rock friction; and 2. field research to elucidate the structure and processes of real fault zones, accounting for differences in rock types and tectonic regimesthese observations provide information on the tectonic stresses that drive lithospheric deformation and on the scaling of laboratory-based models to the parameter range of tectonic earthquakes.
|
From page 231... ...
The size of the nucleation zone generating the precursory sliding is proportional to the critical slip distance Dc, so that the small laboratory values of DC—less than a few hundred microns for gouge thicknesses up to 3 millimeters (117) imply that the nucleation process will typically involves slip on a fault patch with a radius less than a few tens of meters, equivalent to only a magnitude-zero earthquake.
|
From page 233... ...
Combining the laboratory data on brittle, semibrittle, and ductile behavior of rocks with temperature profiles, compositional models, and scaling laws provides estimates of the theological structure of the lithosphere that can be tested with field observations of deformation (121~.
|
From page 234... ...
Kilgore, Direct observation of frictional contacts: New insights for state-dependent properties, Pure Appl. Geophys., 143, 283-302, 1994.
|
From page 235... ...
Mora, and D Place, Simulation of the influence of rate- and state-dependent friction on the macroscopic behavior of complex fault zones with the lattice solid model, Pure Appl.
|
From page 236... ...
236 no ~ o t1J J // I_ :~ 1 ~ A_ ~— L' ~ ~ ~ _ 0 0—~ _ ~ ~ m ~C;< n: ~ ~ ~5_ ' Lo m ~ 49 a:— = £ V ~ —LU Cal ~ 0 :D J ~ 11,} AL 1 ~ ~ ~ I,JJ '_ .
|
From page 237... ...
The notable exceptions are shallow-dipping extensional detachment faults and major transform faults like the San Andreas, which are anomalously weak and may require some mechanism to maintain fluid overpressures. Fault-Zone Petrology Field-scale observations provide important constraints on the genesis of fine-grained rocks in fault zones (129~.
|
From page 238... ...
These large frictional stresses are at odds with the surface heat-flow measurements along the San Andreas fault, which show no significant anomaly due to strain heating (see Section 2.5~.
|
From page 239... ...
Methods for Estimating Stress Stress orientations can be mapped using a number of stress-field indicators that sample the stress regime of the upper crust. The geological indicators include fault slip data (143)
|
From page 240... ...
For example, it is difficult to assess the importance of fluids in earthquake generation and rupture based solely on studies of exhumed fault zones, because the complex history of uplift and denudation severely alters, or even destroys, the evidence on deformation mechanisms, fault-zone mineralogy, and fluid compositions during the actual faulting. Drilling holes to relatively shallow seismogenic depths (less than 5 kilometers)
|
From page 241... ...
Am., 72, 2349-2367, 1982~. Initially, the digitizing electronics were limited to 16 bits, which required electronic gain-ranging to record high-amplitude ground motions, but 24-bit digitizers became available by 1985, providing the full 140 dB of dynamic range.
|
From page 242... ...
That report emphatically recommended that "the federal government should establish a more rational, coordinated, and stable means of support for the seismic networks of the United States." Several important steps have been taken toward improving and coordinating seismic monitoring efforts at the regional level. Two coordinating bodies have been formed: the Council of the National Seismic System, comprising primarily the operators of conventional weak-motion monitoring networks ( |
From page 243... ...
29. The ISC was formed in Edinburgh in 1964 to continue the work of the British Geological Survey in producing the International Seismological Summary (described in Section 2.3~.
|
From page 244... ...
In California, 100-kilometer baselines that cross the San Andreas fault are observed to change at rates of 20-35 millimeters per year, corresponding to strain rates of 2.0-3.5 x 10-7 per year.
|
From page 245... ...
The International GPS Services for Geodynamics, the Universities NAVSTAR Consortium, and the Southern California Earthquake Center, among many organizations, have made great progress in publishing GPS data freely over the Internet, contributing greatly to scientific progress in tectonic geodesy.
|
From page 246... ...
Res., 87, 6977-6982, 1982; R.O. Burford, The response of creeping parts of the San Andreas fault to earthquakes on nearby faults; Two examples, Pure Appl.
|
From page 247... ...
Harris and P Segall, Detection of a locked zone at depth in the Parkfield, California segment of the San Andreas fault, J
|
From page 248... ...
They also inferred slip at depth below the Banning segment of the San Andreas fault, which could have serious consequences for possible earthquakes on the San Andreas. Savage and Svarc inferred that postseismic slip was concentrated on the northern half of the mainshock rupture zone and that postseismic deformation is still ongoing Q.C.
|
From page 249... ...
and R.E. Wallace, Evaluation of effects of trees on past movements on the San Andreas fault, northern California, Geol.
|
From page 250... ...
Brillinger, A more precise chronology of earthquakes produced by the San Andreas fault in southern California, J Geophys.
|
From page 251... ...
Brillinger, A more precise chronology of earthquakes produced by the San Andreas fault in southern California, J Geophys.
|
From page 252... ...
Res., 81, 3579-3587, 1976) , the critical slip distance for fault zones of finite width W may obey Dc = ~cw, where ~c is a "critical strain." Experiments by C
|
From page 253... ...
See R.H. Sibson, Earthquakes and rock deformation in crustal fault zones, Ann.
|
From page 254... ...
The horizontal component of normal-fault slip is taken as the direction of least principal stress, while the horizontal component of reverse-fault slip determines the direction of greatest principal stress. As in the case of seismologically derived focal mechanisms, these geological interpretations can be biased if the faulting occurs on misoriented planes of weakness; however, the bias associated with normal and reverse faults tends to be small, because strength anisotropy tends to be in the m1-~3 plane and not in planes containing the m2 axis.
|
From page 255... ...
Forsyth, An improved method for determining the regional stress tensor using earthquake focal mechanism data: Application to the San Fernando earthquake sequence, J Geophys.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.