San Fernando Earthquake series, 1971: Focal mechanisms and tectonics

Whitcomb, James H.; Allen, Christian B; Garmany, Jan; Hileman, James A.

doi:10.1029/rg011i003p00693

Cited by 132 publications

(79 citation statements)

References 7 publications

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“…deformation is accommodated by west or north-northwest striking thrust faults dipping both south and north. The absence of a large component of oblique faulting in the mainshock or significant secondary strike-slip faulting in the aftershocks as was reported for the 1971 San Fernando [Whitcomb et al, 1973] and 1987 Whittier Narrows [Hauksson and Jones, 1989] earthquakes, is consistent with slip partitioning, or decoupling of strike-slip and thrust faulting. The slip partitioning model postulates that future moderate-sized or large earthquakes may be caused by rightlateral faulting along northwest striking faults or left-lateral faulting along northeast striking faults.…”

Section: Seismic Hazardsupporting

confidence: 51%

“…This is in contrast to the large aftershocks of the 1971 Mw 6.7 San Fernando earthquake that had a variety of mechanisms at the edge of the mainshock rupture as well as a trend of left-lateral strike-slip deformation extending to the southsouthwest toward the Chatsworth fault [Whitcomb et al, 1973]. The 1971 San Fernando and the 1994 Northridge earthquakes ruptured partially abutting fault surfaces on opposite sides of a ridge [Mori et al, 1995] …”

Section: Relation To the 1971 San Fernando Earthquakementioning

confidence: 80%

“…Using additional arrival times from strong motion instruments, Hadley and Kanamori [1977] obtained an improved hypocentral location of 34°25.45'N and l18°22.63'W at a depth of 11.5 km. The San Fernando aftershocks were distributed around the mainshock fault plane, which had a strike ofN67°W, dip of 5r, and rake of 72° based on the first-motion focal mechanism [Whitcomb et al, 1973]. In addition, Whitcomb et aL [1973] used aftershock hypocenters and focal mechanisms to identify a westside-down step in the mainshock rupture plane along a northeast trending tear (the Chatsworth trend).…”

Section: Relation To the 1971 San Fernando Earthquakementioning

confidence: 99%

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The 1994 Northridge earthquake sequence in California: Seismological and tectonic aspects

Hauksson¹,

Jones²,

Hutton³

1995

J. Geophys. Res.

187

111

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Abstract. The Mw 6.7 Northridge earthquake occurred on January 17, 1994, beneath the San Fernando Valley. Two seismicity clusters, located 25 km to the south and 35 km to the north-northwest, preceded the mainshock by 7 days and 16 hours, respectively. The mainshock hypocenter was relatively deep, at 19 km. depth in the lower crust. It had a thrust faulting focal mechanism with a rake of 100° on a fa.ult plane dipping 35° to the south-southwest and striking N75°W. Because the mainshock did not rupture the surface, its association with surficial geological features remains difficult to resolve. Nonetheless, its occurrence reemphasized the seismic hazard of concealed faults associated with the contractional deformation of the Transverse Ranges. The Northridge earthquake is part of the temporal increase in earthquake activity in the Los Angeles area since 1970. The mainshock was followed by an energetic aftersh<>ek sequence. Eight aftershocks of M 2: 5.0 and 48 aftershocks of 4 ~ M < 5 occurred between January 17 and September 30, 1994. The aftershocks extend over most of the western San Fernando Valley and Santa Susana Mountains. They form a diffuse spatial distribution around the mainshock rupture plane, illunrinating a previously unmapped thrust ramp, extending from 7-10 km. depth into the lower crust to a depth of 23 k:m. No flattening of the aftershock distribution is observed near its bottom. At shallow depths, above 7-10 km, the thrust ramp is topped by a dense distnbution of aftershock hypocenters bounded by some of the surlicial faults. The dip of the ramp increases from east to west. The west side of the aftershock zon.e is characterized by a dense, steeply dipping, and north-northeast striking planar cluster of aftershocks that exhibited mostly thrust faulting. These events coincided with the Gillibrand Ca.nyon lateral ramp. Along the east side of the aftershock zone the aftershocks also exhibited primarily thrust faulting focal mechanisms. The focal mechanisms of the aftershocks were dominated by thrust faulting in the large aftershocks, with some strike-slip and normal faulting in the smaller aftershocks. The 1971 San Fernando and the 1994 Northridge earthquakes ruptured partially abutting fault surfaces on opposite sides of a ridge. Both earthquakes accommodated north-south contractional deformation of the Transverse Ranges. The two earthquakes differ primarily in the dip direction of the faults and the depth of faulting. The 1971 northnortheast trend of left-lateral faulting (Chatsworth trend) was not activated in 1994.Introduction .

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Section: Seismic Hazardsupporting

confidence: 51%

Section: Relation To the 1971 San Fernando Earthquakementioning

confidence: 80%

Section: Relation To the 1971 San Fernando Earthquakementioning

confidence: 99%

See 1 more Smart Citation

The 1994 Northridge earthquake sequence in California: Seismological and tectonic aspects

Hauksson¹,

Jones²,

Hutton³

1995

J. Geophys. Res.

187

111

View full text Add to dashboard Cite

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“…Jackson and Fitch [1978] mention this effect at Gediz, Turkey and it has also been observed by local networks [e.g. Whitcomb et al, 1973]. The smaller aftershocks may represent internal deformation in blocks whose overall motion is controlled by larger faults.…”

Section: Discussion Of the Mainshockmentioning

confidence: 98%

Characteristics of foreshocks and short-term deformation in the source area of major earthquakes

Molnár¹

1981

Open-File Report

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We have examined the locations and radiation patterns of the foreshocks to the 4 February 1978 Haicheng earthquake. Using four stations, the foreshocks were located relative to a master event. They occurred very close together, no more than 6 kilometers apart. Nevertheless, there appear to have been too clusters of foreshock activity. The majority of events seem to have occurred in a cluster to the east of the master event along a NNE-SSW trend. Moreover, all eight foreshocks that we could locate and with a magnitude greater than 3.0 occurred in this group. The're also "appears to be a second cluster of foresfiocks located to the northwest of the first. Thus it seems possible that the majority of foreshocks did not occur on the rupture plane of the mainshock, which trends WNW, but on another plane nearly perpendicualr to the mainshock. plane. This inference is supported by differences among the radiation patterns of the foreshocks.We also examined the radiation patterns of the foreshocks, primarily through the amplitude ratios of P and S waves (Ap/As) recorded at each station. While the ratios were relatively constant at the more distant (A=100-200 kilometers) stations, two very different waveforms were recorded throughout the sequence at Yingkou (A=20 kilometers). Calculations of the amplitude ratios expected from the (not very different) fault plane solutions of the mainshock and some of the largest foreshocks showed that two different waveforms should be recorded at Yingkou. A definite correlation could not be made between the two mechanisms and the different location clusters. 1) IntroductionThe successful prediction of the 4 February, 1975 Haicheng earthquake (M=7.3) was due, in a large part, to the occurrence of a large f oreshock sequence of more than 500 events in the four days preceding the mainshock (Wu, 1976). Such a large sequence is a rare event -indeed, the Haicheng series is the only large foreshock sequence to have been well recorded -and we cannot hope that many other mainshocks will have such sequences. Many large earthquakes are preceded by a few foreshocks (Jones and Molnar, 1979) but so far we have been able to recognize them only in hindsigl}£. To increase the number of earthquakes that can be successfully predicted, we need to learn how to differentiate foreshocks from "normal" earthquake swarms before the mainshock occurs and when the shear number of events is not an overpowering consideration as it was at Haicheng. But precisely because ofits great length, the Haicheng sequence provides a unique opportunity to study this problem. The large number of events allows the characteristics of "this foreshock sequence to be seen more clearly. These characteristics could then be compared with those of earthquake swarms to the benefit of all people engaged in earthquake prediction research. Thus, the first part of the Chinese-American exchange in earth sciences has been a study of the Haicheng foreshock sequence resulting in this report.There are two purposes of this research. There is,...

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“…Within the western Transverse Ranges province, these secondary cross faults displace the major east-west trending faults and folds (Yeats, 1983;Gurrola and Kamerling, 1996) and appear to act primarily as tear-faults or lateral ramps, usually with left-lateral displacement, The importance of these faults is that they may act to segment the major east-trending faults and thus limit the size of expected fault rupture and earthquake magnitudes. For example, the predominantly reverse 1970 M6.7 San Fernando (Whitcomb et al, 1973), 1978 M5.9 Santa Barbara (Corbett, and Johnson, 1982), 1987 M6.0 Whittier Narrows (Hauksson and Jones, 1989) and 1994 M6.7 Northridge earthquakes (Seeber and Armbruster, 1995) all appear to be bounded, at least in part, by well defined aftershock zones with strike-slip focal mechanisms. Segmentation is especially important for estimating the hazard from local faults because it not only limits the characteristic size of earthquake a fault may produce, but it may also limit the distance a rupture may propagate towards a particular area of interest.…”

Section: Cross Faults Tear Faults and Fault Segmentationmentioning

confidence: 99%

Initial source and site characterization studies for the U.C. Santa Barbara campus

Archuleta¹,

Nicholson²,

Steidl³

et al. 1997

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San Fernando Earthquake series, 1971: Focal mechanisms and tectonics

Cited by 132 publications

References 7 publications

The 1994 Northridge earthquake sequence in California: Seismological and tectonic aspects

The 1994 Northridge earthquake sequence in California: Seismological and tectonic aspects

Characteristics of foreshocks and short-term deformation in the source area of major earthquakes

Initial source and site characterization studies for the U.C. Santa Barbara campus

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