The 1992 Landers Earthquake Sequence: Seismological observations

Aftershocks from the 1990 Lake Tennyson earthquake (ML 5.8) recorded at nine temporary portable seismographs have been used to invert travel-time data simultaneously for both hypocentre and velocity parameters, resulting in a 1 -D velocity model and station terms for the Lake Tennyson region. The distribution of the best relocated aftershocks outlines a main fault lineation in a ENE direction, and several off-fault clusters. The main fault lineation is 8 km long, with a strike of about 60° and a dip that is nearly vertical. It is located between and subparallel to the Awatere and Fowler Faults, on a previously unknown fault. The mainshock has been relocated in the middle of this lineation zone, which suggests that the fault ruptured bilaterally. The distribution of aftershocks matches that expected from the Coulomb failure criterion, which identifies areas of increased and decreased stress levels due to the occurrence of the mainshock.Focal mechanisms for the mainshock and aftershocks that make up the main fault lineation are consistent with rightlateral strike-slip movement on this fault. Clusters that extend from each end of the main fault lineation predominantly do not have strike-slip focal mechanisms. Aftershocks located in the clusters to the north of the main fault lineation have various thrust mechanisms with no consistent orientation. Most focal mechanisms from this sequence had their P axes closely aligned with the regional axis of compression, and the main fault lineation is consistent with the relative plate motion direction in the Lake Tennyson region.

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“…The same effect has also been observed for the Lake Tennyson sequence (Fig. 6B) and the 1992 Joshua Tree earthquake (Hauksson et al 1993). …”

Section: Discussionsupporting

confidence: 79%

The 1990 Lake Tennyson earthquake sequence, Marlborough, New Zealand

McGinty

Robinson

Taber

et al. 1997

New Zealand Journal of Geology and Geophysics

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“…A deviation from homogeneity of dS = 11.27 is calculated for the north segment of the Landers (Mw = 7.3) aftershock sequence (cluster L6), significant at confidence levels in excess of 99.99%. Likewise, previous studies [Hauksson et al, 1993;Hauksson, 1994] In order to achieve accurate estimates of stress it is necessary to have well recorded events (each with, say, more than 20-30 first motions) with few polarity violations (say, ¾ < 0.05). For real data sets we find uncertainties in stress orientations typically to be 20-30 ø or more.…”

Section: Northern Landers Sequence: No Consistent Stressesmentioning

confidence: 99%

Direct inversion of earthquake first motions for both the stress tensor and focal mechanisms and application to southern California

Abers¹,

Gephart²

2001

J. Geophys. Res.

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Abstract. In inverting populations of focal mechanisms to estimate stress, misfits may arise owing to (1) errors in determining the fault plane solutions and (2) real variations in the state of stress. Among a number of techniques that invert for stress, most effectively regard only the latter and only indirectly describe the former through statistical means so make it difficult to differentiate real stress variations from noisy data. Ambiguity arises because realistic methods for determining focal mechanisms for small earthquakes depend on P wave first motions in a highly nonlinear manner, which is difficult to propagate into stress inversions. To address these issues, we present a new method for constraining stress tensors, in which firstmotion observations are directly inverted for stress without assuming that focal mechanisms are known. The technique produces estimates of four stress parameters, a suite of focal mechanisms consistent with both the stress tensor and the first motions, and estimates of the uncertainty in stress. It also provides a natural test of the stress homogeneity hypothesis and a means to identify those earthquakes whose first motions are not consistent with a homogeneous stress tensor. When tested against artificial data, the method correctly recovers input stresses and yields uncertainties that depend upon both data distribution and reliability of first motion picks, as expected. Utilizing first motions from the southern California catalog, the method is applied to several clusters and aftershock sets, from 1981 to 1992. Typical uncertainties in stress orientation exceed 20-30 ø at the 95% confidence level, resulting from-5% mispick rates typical of even the cleanest data. These uncertainties exceed some previous estimates for similar populations, presumably because they account for focal mechanism uncertainties, and perhaps indicating that some previously reported stress variations are not statistically significant. Most populations are found to be internally consistent with a homogeneous stress field, the most significant exceptions being aftershock sequences of some major earthquakes. In at least one case, the 1992 Landers sequence, aftershocks at the north end of the rupture zone appear to reflect exceedingly heterogeneous stresses. Overall, the method provides a useful and robust approach for evaluating stress constraints provided by P wave first motions.

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“…The Mw 6.1 Joshua Tree earthquake preceded the Landers earthquake by 2 months and is interpreted to be a preshock of the larger sequence [Hauksson et al, 1993] striking nodal planes or sinistral strike-slip faulting on ENE striking nodal planes. No surface rupture was observed following the earthquake [Hauksson et al, 1993].…”

Section: Joshua Tree Earthquake Sequencementioning

confidence: 99%

“…No surface rupture was observed following the earthquake [Hauksson et al, 1993]. Based on a visual inspection of the aftershock distribution, we divided the events into three groups ( Figure 5): (1) the southern part of the northerly alignment, which trends slightly west of north; (2) the northern part of the northerly alignment, which trends slightly east of north; and (3) the east-northeast alignment.…”

Section: Joshua Tree Earthquake Sequencementioning

confidence: 99%

Seismogenic deformation field in the Mojave block and implications for tectonics of the eastern California shear zone

Unruh¹,

Twiss²,

Hauksson³

1996

J. Geophys. Res.

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Abstract. From the aftershocks of the 1992 Landers earthquake, we infer the orientation of the principal strain rate axes (dt>t:h>ll]; dt lengthening), their relative magnitude, and the relative spin of fault blocks by using a micropolar continuum model to invert the seismic P and Taxes. The seismogenic deformation is consistent with the geodetic measurements of the coseismic displacement and with the secular deformation of the central Mojave block. Regionally, the aftershock data define two major domains within the central Mojave block: (1) the western Mojave block, including the San Bernardino Mountains and the epicentral area of the Big Bear earthquake, which is characterized by E-W d1 Oengthening) and N-S d3 (shortening); and (2) the central Mojave block, including the Landers surface rupture zone, which is characterized by NW-SE d1 and NE-SW d3. Inversion for the principal strain axes of geodetically measured coseismic displacements across the Big Bear and Landers seismogenic zones gives results similar to the aftershock inversions for those areas, indicating that the aftershocks accommodate a deformation similar to the main rupture and do not reflect elastic rebound or residual stresses. The background seismicity for 1981 to 1991 shows the same characteristic d1 and d3 orientations for the two domains, indicating that the secular seismogenic strain has the same regional geometry as the 1992 coseismic deformation. The micropolar inversion also provides values of the relative vorticity parameter W, which reflects a difference between the vorticity of a shearing continuum and the vorticity of fault-bounded blocks rotating within tabular seismogenic shear zones. The observed fault geometry along the K.ickapoo fault suggests a pinned-block model for the local block rotation that is consistent with the values of W obtained from our inversions. We interpret the regional NW-SE orientation of d1 in the central Mojave block to be characteristic of the dextral eastern California shcm zone, which transfers approximately 22% of the Pacific-North American plate motion from the San Andreas system to the Walker Lane Belt in eastern California. Our results and geodetic determinations of the secular shear strain in the central Mojave block indicate that the locus of NW dextral shear generally lies between the San Bernardino Mountains and the Pisgah fault.

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The 1992 Landers Earthquake Sequence: Seismological observations

Cited by 234 publications

References 28 publications

The 1990 Lake Tennyson earthquake sequence, Marlborough, New Zealand

The 1990 Lake Tennyson earthquake sequence, Marlborough, New Zealand

Direct inversion of earthquake first motions for both the stress tensor and focal mechanisms and application to southern California

Seismogenic deformation field in the Mojave block and implications for tectonics of the eastern California shear zone

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