Aftershocks of the 1966 Parkfield-Cholame, California, earthquake: A detailed study
01 Aug 1970-Bulletin of the Seismological Society of America (GeoScienceWorld)-Vol. 60, Iss: 4, pp 1151-1197
TL;DR: The aftershocks were concentrated in patches on the slip surface, with large numbers of events in depth ranges of 2 to 4 km and 8 to 10 km and relatively few at depths of 5 to 7 km as discussed by the authors.
Abstract: Hypocenters and magnitudes of more than 600 aftershocks of the 1966 Parkfield-Cholame earthquake were determined from recordings of a dense network of portable seismograph stations operated in the epicentral region from 3 to 82 days after the main shock. Hypocenters were virtually confined to a nearly vertical zone extending downward from the zone of visible ground fracturing at the Earth9s surface to a depth of 12 to 14 km. Aftershocks were concentrated in patches on the slip surface, with large numbers of events in depth ranges of 2 to 4 km and 8 to 10 km and relatively few at depths of 5 to 7 km. First-motion patterns suggest that simple, nearly horizontal right-lateral strike-slip displacement was the source for an overwhelming majority of the aftershocks. The coefficient “ b ” in the frequency versus magnitude equation appears to be depth-dependent: it is about − 0.6 for events between 8- and 10-km depth but averages about −0.95 for events at other depths. Individual stations with consistently late P -wave arrivals were also found to record abnormally large amplitudes, with both anomalies increasing, apparently, with increasing thickness of sediments beneath the site.
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25 Jan 1991TL;DR: The connection between faults and the seismicity generated is governed by the rate and state dependent friction laws -producing distinctive seismic styles of faulting and a gamut of earthquake phenomena including aftershocks, afterslip, earthquake triggering, and slow slip events.
Abstract: This essential reference for graduate students and researchers provides a unified treatment of earthquakes and faulting as two aspects of brittle tectonics at different timescales. The intimate connection between the two is manifested in their scaling laws and populations, which evolve from fracture growth and interactions between fractures. The connection between faults and the seismicity generated is governed by the rate and state dependent friction laws - producing distinctive seismic styles of faulting and a gamut of earthquake phenomena including aftershocks, afterslip, earthquake triggering, and slow slip events. The third edition of this classic treatise presents a wealth of new topics and new observations. These include slow earthquake phenomena; friction of phyllosilicates, and at high sliding velocities; fault structures; relative roles of strong and seismogenic versus weak and creeping faults; dynamic triggering of earthquakes; oceanic earthquakes; megathrust earthquakes in subduction zones; deep earthquakes; and new observations of earthquake precursory phenomena.
3,802 citations
TL;DR: In this article, an empirical relation involving seismic moment M, energy E, magnitude M, and fault dimension L (or area S) is discussed on the basis of an extensive set of earthquake data (M_S ≧ 6) and simple crack and dynamic dislocation models.
Abstract: Empirical relations involving seismic moment M_o, magnitude M_S, energy E_S and fault dimension L (or area S) are discussed on the basis of an extensive set of earthquake data (M_S ≧ 6) and simple crack and dynamic dislocation models. The relation between log S and log M_o is remarkably linear (slope ∼ 2/3) indicating a constant stress drop Δσ; Δσ = 30, 100 and 60 bars are obtained for inter-plate, intra-plate and “average” earthquakes, respectively. Except for very large earthquakes, the relation M_S ∼ (2/3) log M_o ∼ 2 log L is established by the data. This is consistent with the dynamic dislocation model for point dislocation rise times and rupture times of most earthquakes. For very large earthquakes M_S ∼ (1/3) log M_o ∼ log L ∼ (1/3) log E_S. For very small earthquakes M_S ∼ log M_o ∼ 3 log L ∼ log E_S. Scaling rules are assumed and justified. This model predicts log E_S ∼ 1.5 M_S ∼ 3 log L which is consistent with the Gutenberg-Richter relation. Since the static energy is proportional to σL^3, where σ is the average stress, this relation suggests a constant apparent stress ησ where η is the efficiency. The earthquake data suggest ησ ~ 1/2 Δσ. These relations lead to log S ∼ M_S consistent with the empirical relation. This relation together with a simple geometrical argument explains the magnitude-frequency relation log N ∼ − M_S.
2,648 citations
TL;DR: In this paper, the spectral decay parameter k shows little variation at a single station for multiple earthquakes at the same distances, but it increases gradually as the epicentral distance increases.
Abstract: At high frequencies f the spectrum of S-wave accelerations is characterized by a trend of exponential decay, e^(−πkf). In our study, the spectral decay parameter k shows little variation at a single station for multiple earthquakes at the same distances, but it increases gradually as the epicentral distance increases. For multiple recordings of the San Fernando earthquake, k increases slowly with distance, and k is systematically smaller for sites on rock than for sites on alluvium. Under the assumption that the Fourier spectrum of acceleration at the source is constant above the corner frequency (an ω^(−2) source model), the exponential decay is consistent with an attenuation model in which Q increases rapidly with depth in the shallow crustal layers.
1,110 citations
TL;DR: In this paper, the authors derived a two-dimensional solution for any number of nonintersecting cracks arbitrarily located in a homogeneous elastic material, including the elastic interaction between cracks.
Abstract: Fault traces consist of numerous discrete segments, commonly arranged as echelon arrays. In some cases, discontinuities influence the distribution of slip and seismicity along faults. To analyze fault segments, we derive a two-dimensional solution for any number of nonintersecting cracks arbitrarily located in a homogeneous elastic material. The solution includes the elastic interaction between cracks. Crack surfaces are assumed to stick or slip according to a linear friction law. For an array of echelon cracks the ratio of maximum slip to array length significantly underestimates the difference between the driving stress and frictional resistance. The ratio of maximum slip to crack length slightly overestimates this difference. Stress distributions near right- and left-stepping echelon discontinuities differ in two important ways. For right lateral shear and left-stepping cracks, normal tractions on the overlapped crack ends increase and inhibit frictional sliding, whereas for right-stepping cracks, normal tractions decrease and facilitate sliding. The mean compressive stress between right-stepping cracks also decreases and promotes the formation of secondary fractures, which tend to link the cracks and allow slip to be transferred through the discontinuity. For left-stepping cracks the mean stress increases; secondary fracturing is more restricted and tends not to link the cracks. Earthquake swarms and aftershocks cluster near right steps along right lateral faults. Our results suggest that left steps store elastic strain energy and may be sites of large earthquakes. Opposite behavior results if the sense of shear is left lateral.
974 citations
Abstract: The geometry of Turkish strike-slip faults is reviewed, showing that fault geometry plays an important role in controlling the location of large earthquake rupture segments along the fault zones. It is found that large earthquake ruptures generally do not propagate past individual stepovers that are wider than 5 km or bends that have angles greater than about 30 degrees. It is suggested that certain geometric patterns are responsible for strain accumulation along portions of the fault zone. It is shown that fault geometry plays a role in the characteristics of earthquake behavior and that aftershocks and swarm activity are often associated with releasing areas.
869 citations
References
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TL;DR: The magnitude of an earthquake was originally defined by the junior author of as discussed by the authors for shocks in southern California, as the logarithm of the maximum trace amplitude expressed in thousandths of a millimeter with which the standard short-period torsion seismometer (free period 0.8 sec., static magnification============2800, damping nearly critical) would register that earthquake at an detectable distance of 100 kilometers.
Abstract: The magnitude of an earthquake was originally defined by the junior author
(Richter, 1935), for shocks in southern California, as the logarithm of the
maximum trace amplitude expressed in thousandths of a millimeter with which
the standard short-period torsion seismometer (free period 0.8 sec., static magnification
2800, damping nearly critical) would register that earthquake at an
epicentral distance of 100 kilometers. Gutenberg and Richter (1936) extended
the scale to apply to earthquakes occurring elsewhere and recorded on other
types of instruments.
1,118 citations
TL;DR: In the course of historical or statistical study of earthquakes in any given region, it is frequently desirable to have a scale for rating these earthquakes in terms of their original energy, independently of the effects that may be produced at any particular point of observation as mentioned in this paper.
Abstract: In the course of historical or statistical study of earthquakes in any
given region it is frequently desirable to have a scale for rating these
shocks in terms of their original energy, independently of the effects
which may be produced at any particular point of observation. On the
suggestion of Mr. H. O. Wood, it is here proposed to refer to such a
scale as a "magnitude" scale. This terminology is offered in distinction
from the name "intensity" scale, now in general use for such scales as
the Rossi-Forel and Mercalli-Cancani scales, which refer primarily to the
local intensity of shock manifestation.
1,017 citations
TL;DR: In this paper, a method was devised to extract useful information about the earthquake source from the coda of local small earthquakes based on the assumption that the power spectrum of coda waves of a local earthquake is only a function of time measured from the earthquake origin time and independent of distance and details of wave path to the station.
Abstract: A method was devised to extract useful information about the earthquake source from the coda of local small earthquakes. The method is based on the assumption that the power spectrum of coda waves of a local earthquake is only a function of time measured from the earthquake origin time and independent of distance and details of wave path to the station. Evidence supporting this assumption is presented, using the data on aftershocks of the Parkfield earthquakes of June 28, 1966. A simple statistical model of the wave medium that accounts for the observations on the coda is proposed. By applying the method to many Parkfield aftershocks, the relation between the seismic moment M0 and local magnitude ML is determined as log M0 (dyne cm) = 15.8 + 1.5ML. The size of a microearthquake with magnitude zero is estimated as 10×10 meters.
910 citations
TL;DR: In this paper, the authors studied the source mechanism of earthquakes in the California-Nevada region using surface wave analyses, surface displacement observations in the source region, magnitude determinations, and accurate epicenter locations.
Abstract: The source mechanism of earthquakes in the California-Nevada region was studied using surface wave analyses, surface displacement observations in the source region, magnitude determinations, and accurate epicenter locations. Fourier analyses of surface waves from thirteen earthquakes in the Parkfield region have yielded the following relationship between seismic moment, M0 and Richter magnitude, ML: log M0 = 1.4 ML + 17.0, where 3 < ML < 6. The following relation between the surface wave envelope parameter AR and seismic moment was obtained: log M0 = log AR300 + 20.1. This relation was used to estimate the seismic moment of 259 additional earthquakes in the western United States. The combined data yield the following relationship between moment and local magnitude: log M0 = 1.7 ML + 15.1, where 3 < ML < 6. These data together with the Gutenberg-Richter energy-magnitude formula suggest that the average stress multiplied by the seismic efficiency is about 7 bars for small earthquakes at Parkfield and in the Imperial Valley, about 30 bars for small earthquakes near Wheeler Ridge on the White Wolf fault, and over 100 bars for small earthquakes in the Arizona-Nevada and Laguna Salada (Baja California) regions. Field observations of displacement associated with eight Parkfield shocks, along with estimates of fault area, indicate that fault dimensions similar to the values found earlier for the Imperial earthquake are the rule rather than the exception for small earthquakes along the San Andreas fault. Stress drops appear to be about 10% of the average stress multiplied by the seismic efficiency. The revised curve for the moment versus magnitude further emphasizes that small earthquakes are not important in strain release and indicate that the zone of shear may be about 6 km in vertical extent for the Imperial Valley and even less for oceanic transform faults.
498 citations
TL;DR: The Parkfield earthquake of June 27, 1966 and its aftershock and creep sequences are examined as a detailed example of fault slippage that includes both types, seismic and aseismic as discussed by the authors.
Abstract: Field and experimental evidence are combined to deduce the mechanism of slip on shallow continental transcurrent faults, such as the San Andreas in California. Several lines of evidence portray the central section of the San Andreas fault as a very smooth and flat surface, with a very low frictional strength in comparison to the breaking strength of intact rock. The Parkfield earthquake of June 27, 1966, and its aftershock and creep sequences are examined as a detailed example of fault slippage that includes both types, seismic and aseismic. It is shown from considerable number of field data that during the main shock a region from about 4 to 10 km in depth slipped approximately 30 cm. In response to this slippage, creep and aftershocks were generated. The creep and aftershocks are not directly interrelated, but they are microscopically identical processes of time-dependent brittle friction occurring in parallel in different regions. The creep occurred by time-dependent stable frictional sliding in the 4-km-thick surface layer; the aftershocks, by time-dependent stick-slip at the ends of the initial slipped zone. This model is in good agreement with laboratory results which show that slippage should occur by stable (aseismic) friction in the upper 4 km, by stick-slip accompanied by earthquakes from about 4 to 12 km, and by stable sliding or plastic friction below 12 km on the fault. One feature not observed in the laboratory is the episodic nature of creep. These episodes can be predicted with an accuracy of about 1 week.
175 citations