Elevation Changes Preceding the San Fernando Earthquake of February 9, 1971

Castle, Robert O.; Alt, John N.; Savage, J. C.; Balazs, Emery I.

doi:10.1130/0091-7613(1974)2<61:ecptsf>2.0.co;2

Cited by 60 publications

(13 citation statements)

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“…Strain and rotation fields at the free surface are the integrated response of all fault slip and should show a fairly sudden transition to higher rates near the epicenter prior to an earthquake. Evidence of such anomalous deformation is the epicentral uplift prior to the 1971 ML = 6.4 San Fernando, California, earthquake which had a left lateral thrust mechanism [Castle et al, 1974]. The anomalous uplift rate commenced about 3 years before the earthquake, but data are insufficient to determine the suddenness of onset A simple and efficient algorithm which exploits the intuitive notion that fault displacements should be monotonic in time is used to satisfy the nonlinear friction law at the fault.…”

Section: Model Solutionsmentioning

confidence: 99%

“…Reid said nothing about precursory fault slip, having no reason to, yet some evidence exists for such motion prior to stick slip events in the laboratory [Scholz et al, 1972;Byedee and Summers, 1975] and in earthquakes, as inferred from study of geodetic data [Castle et al, 1974;Thatcher, 1976] and ground breakage [Allen and Smith, 1966]. Little is known about the relation between precursory fault slip and fault stress, however.…”

Section: Introductionmentioning

confidence: 99%

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Strain softening prior to two‐dimensional strike slip earthquakes

Stuart¹

1979

J. Geophys. Res.

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A model for two‐dimensional strike slip faulting in which the fault friction is displacement softening is analyzed to estimate preearthquake crustal deformation and conditions for an earthquake instability. The friction law is initially displacement hardening but becomes softening after the peak stress is surpassed. The peak stress increases with depth to a maximum before decreasing again. The material surrounding the fault is represented by elastic plates. Several deformation histories are calculated by solving numerically the quasi‐static, nonlinear problem subject to displacement boundary conditions simulating relative plate motion. When the crust is compliant or the fault is rapidly softening, an inertia‐limited instability results. Prior to the instability the point of maximum fault slip rate moves upward toward the greatest peak stress position. The slip causes a rapid increase in shear strain rate at the free surface near the fault trace. Although fault slip is monotonic, the average fault stress reaches a maximum and then decreases before the instability. When instability is not possible, a similar but smoother deformation episode results. Qualitative extrapolation of the computed results suggests that crustal earthquakes may be preceded by accelerating fault slip near the focus, particularly below, and that the enhanced slip rate may cause recognizable strain and tilt anomalies at the free surface.

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Section: Model Solutionsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Strain softening prior to two‐dimensional strike slip earthquakes

Stuart¹

1979

J. Geophys. Res.

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“…Interpretation of vertical geodetic measurements has re-ceived considerable attention in the literature in relation to tectonic changes that occur in the crust both in the zone of earthquake preparation and in the zone of the earthquake and associated aftershocks [e.g., Tsubokawa et al, 1964;Whitcomb et al, 1973;Scholz et al, 1973;Castle et al, 1974;Nur, 1974;Kisslinger, 1975;Stuart and Johnston, 1975;Oliveret al, 1975]. Most of these studies have not estimated the model dependence of their interpretations of leveling, tilt, and gravity changes.…”

Section: So Far Two Classes Of Measurements Have Been Discussedmentioning

confidence: 99%

“…the approximate horizontal dimension of the anomalous zone and M is the earthquake magnitude. For an M = 6.4 earthquake like the 1971 San Fernando, California, event, L is 133 km, giving an a of 66 km Castle et al [1974]. have interpreted leveling data obtained prior to the San Fernando earthquake and have found a change in d of at least l0 cm near the epicentral area in the 10 years prior to the earthquake.…”

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confidence: 95%

New vertical geodesy

Whitcomb¹

1976

J. Geophys. Res.

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Vertical geodesy is undergoing a revolution because of two factors. First, new precise three-dimensional position measurement techniques used over very long distances and based on extraterrestrial reference systems provide a new class and precision of geometric data previously unavailable for geophysical investigations. Second, physical models in tectonic theory for large earthquakes predict crustal distortions that violate the conventional assumptions used to interpret gravity and leveling data. Leveling and geometric elevation measurements are not directly comparable because the interpretation of leveling data is density-model dependent. Estimates of pre-1971 San Fernando earthquake elevation changes based on leveling of about l0 cm may be as much as 3 cm, or 40%, too large. Pre-1964 Niigata earthquake leveling surveys, previously used as confirmation of the dilatancy model, do not require dilatancy as an explanation and easily allow an alternative model with a subsurface density increase. Gravity is also not a dependable estimator of elevation change. But a combination of gravity with either leveling, if the dimensions of the distorted body are known or small, or geometric elevation measurements is essential for the determination of crustal density and strain changes. The 1965-1967 Matsushiro earthquake swarm leveling and gravity data show a significant dilatant strain of 0.6-1.8 x l0 -4 if the proper model dimensions are used. This dilatant strain would be adequate to cause the observed drop in V,o/Vs, even if the crust were initially saturated prior to distortion. The combination of gravity, leveling, and the new geometric elevation measurements provides a useful parameter, gravitational potential, for the inversion of subsurface density distributions. Use of this parameter, defined as the free-air elevation anomaly, is illustrated for a nearly compensated mountain root structure and shows that this technique holds significant promise for the study of large, deep structures in the crust and upper mantle.

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“…The vast store of leveling and triangulation data gathered over many decades by public agencies and bureaus is proving to be a virtual gold mine of useful information (Castle et al 1974). An example is the dramatic discovery of southern California's "Palmdale Bulge" (Castle et al 1976), perhaps too hastily christened, as additional investigations (Bennett 1977, Castle et a11977) show it to be an elongated ridge extending from Point Arguello on the coast at least 580 km eastward to the Arizona border with the point of greatest uplift, 45 cm, between TwentyNine Palms and Amboy, a considerable distance east of Palmdale.…”

Section: Geodesy and Recent Deformationmentioning

confidence: 99%

Geomorphological Processes on Terrestrial Planetary Surfaces

Sharp¹

1980

Annu. Rev. Earth Planet. Sci.

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Elevation Changes Preceding the San Fernando Earthquake of February 9, 1971

Cited by 60 publications

References 0 publications

Strain softening prior to two‐dimensional strike slip earthquakes

Strain softening prior to two‐dimensional strike slip earthquakes

New vertical geodesy

Geomorphological Processes on Terrestrial Planetary Surfaces

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