Landers Earthquake Research Articles

Probing the mechanical properties of seismically active crust with space geodesy: Study of the coseismic deformation due to the 1992 Mw7.3 Landers (southern California) earthquake

The coseismic deformation due to the 1992 Mw7.3 Landers earthquake, southern California, is investigated using synthetic aperture radar (SAR) and Global Positioning System (GPS) measurements. The ERS‐1 satellite data from the ascending and descending orbits are used to generate contiguous maps of three orthogonal components (east, north, up) of the coseismic surface displacement field. The coseismic displacement field exhibits symmetries with respect to the rupture plane that are suggestive of a linear relationship between stress and strain in the crust. Interferometric synthetic aperture radar (InSAR) data show small‐scale deformation on nearby faults of the Eastern California Shear Zone. Some of these faults (in particular, the Calico, Rodman, and Pinto Mountain faults) were also subsequently strained by the 1999 Mw7.1 Hector Mine earthquake. I test the hypothesis that the anomalous fault strain represents essentially an elastic response of kilometer‐scale compliant fault zones to stressing by nearby earthquakes [Fialko et al., 2002]. The coseismic stress perturbations due to the Landers earthquake are computed using a slip model derived from inversions of the InSAR and GPS data. Calculations are performed for both homogeneous and transversely isotropic half‐space models. The compliant zone model that best explains the deformation on the Calico and Pinto Mountain faults due to the Hector Mine earthquake successfully predicts the coseismic displacements on these faults induced by the Landers earthquake. Deformation on the Calico and Pinto Mountain faults implies about a factor of 2 reduction in the effective shear modulus within the ∼2 km wide fault zones. The depth extent of the low‐rigidity zones is poorly constrained but is likely in excess of a few kilometers. The same type of structure is able to explain high gradients in the radar line of sight displacements observed on other faults adjacent to the Landers rupture. In particular, the Lenwood fault north of the Soggy Lake has likely experienced a few centimeters of left‐lateral motion across <1‐km‐wide compliant fault zone having the rigidity reduction of more than a factor of 2. The inferred compliant fault zones are interpreted to be a result of extensive damage due to past earthquakes.

Triggered Deformation and Seismic Activity under Mammoth Mountain in Long Valley Caldera by the 3 November 2002 Mw 7.9 Denali Fault Earthquake

The 3 November 2002 Mw 7.9 Denali fault earthquake triggered defor- mational offsets and microseismicity under Mammoth Mountain (MM) on the rim of Long Valley caldera, California, some 3460 km from the earthquake. Such strain offsets and microseismicity were not recorded at other borehole strain sites along the San Andreas fault system in California. The Long Valley offsets were recorded on borehole strainmeters at three sites around the western part of the caldera that includes Mammoth Mountain—a young volcano on the southwestern rim of the caldera. The largest recorded strain offsets were 0.1 microstrain at PO on the west side of MM, 0.05 microstrain at MX to the southeast of MM, and 0.025 microstrain at BS to the northeast of MM with negative strain extensional. High sample rate strain data show initial triggering of the offsets began at 22:30 UTC during the arrival of the first Rayleigh waves from the Alaskan earthquake with peak-to-peak dynamic strain amplitudes of about 2 microstrain corresponding to a stress amplitude of about 0.06 MPa. The strain offsets grew to their final values in the next 10 min. The associated triggered seismicity occurred beneath the south flank of MM and also began at 22:30 UTC and died away over the next 15 min. This relatively weak seismicity burst included some 60 small events with magnitude all less than M 1. While poorly constrained, these strain observations are consistent with triggered slip and intrusive opening on a north-striking normal fault centered at a depth of 8 km with a moment of 10 16 N m, or the equivalent of a M 4.3 earthquake. The cumulative seismic moment for the associated seismicity burst was more than three orders of magnitude smaller. These observations and this model resemble those for the triggered deformation and slip that occurred beneath the north side of MM following the 16 October 1999 M 7.1 Hector Mine, California, earthquake. However, in this case, we see little post-event slip decay reflected in the strain data after the Rayleigh-wave arrivals from the Denali fault earthquake and onset of triggered seismicity did not lag the triggered defor- mation by 20 min. These observations are also distinctly different from the more widespread and energetic seismicity and deformation triggered by the 1992 M 7.3 Landers earthquake in the Long Valley caldera. Thus, each of the three instances of remotely triggered unrest in Long Valley caldera recorded to date differ. In each case, however, the deformation moment inferred from the strain meter data was more than an order of magnitude larger than the cumulative moment for the associated triggered seismicity.

Quantitative analysis of seismic fault zone waves in the rupture zone of the 1992 Landers, California, earthquake: evidence for a shallow trapping structure

SUMMARY We analyse quantitatively a waveform data set of 238 earthquakes recorded by a dense seismic array across and along the rupture zone of the 1992 Landers earthquake. A grid-search method with station delay corrections is used to locate events that do not have catalogue locations. The quality of fault zone trapped waves generated by each event is determined from the ratios of seismic energy in time windows corresponding to trapped waves and direct S waves at stations close to and off the fault zone. Approximately 70 per cent of the events with S‐P times of less than 2 s, including many clearly off the fault, produce considerable trapped wave energy. This distribution is in marked contrast with previous claims that trapped waves are generated only by sources close to or inside the Landers rupture zone. The time difference between the S arrival and trapped waves group does not grow systematically with increasing hypocentral distance and depth. The dispersion measured from the trapped waves is weak. These results imply that the seismic trapping structure at the Landers rupture zone is shallow and does not extend continuously along-strike by more than a few kilometres. Synthetic waveform modelling indicates that the fault zone waveguide has depth of approximately 2‐4 km, a width of approximately 200 m, an S-wave velocity reduction relative to the host rock of approximately 30‐40 per cent and an S-wave attenuation coefficient of approximately 20‐30. The fault zone waveguide north of the array appears to be shallower and weaker than that south of the array. The waveform modelling also indicates that the seismic trapping structure below the array is centred approximately 100 m east of the surface break.

Near‐field postseismic deformation associated with the 1992 Landers and 1999 Hector Mine, California, earthquakes

After the Landers earthquake (Mw = 7.3, 1992.489) a linear array of 10 monuments extending about 30 km N50°E on either side of the earthquake rupture plus a nearby off‐trend reference monument were surveyed frequently by GPS until 2003.2. The array also spans the rupture of the subsequent Hector Mine earthquake (Mw = 7.1, 1999.792). The pre‐Landers velocities of monuments in the array relative to interior North America were estimated from earlier trilateration and very long baseline interferometry measurements. Except at the reference monument, the post‐Landers velocities of the individual monuments in the array relaxed to their preseismic values within 4 years. Following the Hector Mine earthquake the velocities of the monuments relaxed to steady rates within 1 year. Those steady rates for the east components are about equal to the pre‐Landers rates as is the steady rate for the north component of the one monument east of the Hector Mine rupture. However, the steady rates for the north components of the 10 monuments west of the rupture are systematically ∼10 mm yr−1 larger than the pre‐Landers rates. The relaxation to a steady rate is approximately exponential with decay times of 0.50 ± 0.10 year following the Landers earthquake and 0.32 ± 0.18 year following the Hector Mine earthquake. The postearthquake motions of the Landers array following the Landers earthquake are not well approximated by the viscoelastic‐coupling model of Pollitz et al. [2000]. A similar viscoelastic‐coupling model [Pollitz et al., 2001] is more successful in representing the deformation after the Hector Mine earthquake.

A Structural Interpretation of the Aftershock "Cloud" of the 1992 Mw 7.3 Landers Earthquake

A Structural Interpretation of the Aftershock "Cloud" of the 1992 Mw 7.3 Landers Earthquake

OpenSHA: A Developing Community-modeling Environment for Seismic Hazard Analysis

Probabilistic seismic hazard analysis (PSHA) provides the conceptual framework for estimating the likelihood that something of concern related to earthquake shaking will occur over a specified time period. Based on more than thirty years of research and development ( e.g. , Cornell, 1968; Algermissen et al. , 1982; SSHAC, 1997), PSHA has become a standard tool for combining information on earthquake occurrence, seismic radiation, and shaking response to produce hazard estimates, including the U.S. Geological Survey's national seismic hazard maps (Frankel et al. , 1996, 1997). PSHA methods, while now mature, continue to evolve as scientists improve the characterization of earthquake hazards and engineers develop new measures of seismic shaking for performance-based design. The Southern California Earthquake Center (SCEC), in collaboration with USGS, the California Geological Survey (CGS), and other partners, has undertaken a series of studies aimed at improving the regional application of PSHA methods. Phase I examined the implications of the 1992 Landers earthquake sequence for regional seismic hazards (WGCEP, 1992), Phase II developed a probabilistic earthquake forecast model (WGCEP, 1995), and Phase III assessed the wave-propagation and site effects that give rise to local variations in seismic shaking (see Field et al. [2000] for an overview). The Phase III study found that accounting for some site attributes ( i.e. , the 30-meter shear-wave velocity and basin depth) can lead to significant improvements in PSHA. It was also found that making such corrections does not significantly reduce the prediction uncertainty associated with empirical ground-motion relations. In fact, 3D waveform modeling presented in the same report (Olsen, 2000) implied that this residual uncertainty represents an intrinsic variability caused by complex propagation effects that are unique to each earthquake-rupture/site combination. The Phase III study therefore concluded that significant improvements in PSHA will require replacing the standard empirical-regression …

A Structural Interpretation of the Aftershock "Cloud" of the 1992 Mw 7.3 Landers Earthquake

We analyze the spatial relationship of relocated aftershocks to the principal rupture planes of the M w 7.3 1992 Landers mainshock from a structural point of view. We find that the aftershocks constitute primarily a several-kilometer-wide damage zone centered on the mainshock rupture planes. The intensity of damage decreases away from the principal faults. Less than half of the aftershocks occurred within 1 km of the mainshock planes, and perhaps only 5% of the aftershocks are candidates for rerupture of the mainshock faults. Moreover, it seems that aftershocks along the Landers rupture have b -values that correlate well with the complexity of the mainshock rupture. Low b -values occur along segments that are simple, whereas higher b -values correlate with sections that are more complex. Thus, structural complexity appears to correlate with a greater relative abundance of small earthquakes. These observations imply that aftershock populations reflect fault populations in the medium surrounding the principal faults rather than the behavior of the mainshock planes themselves. Online material : Arcview information about the surface rupture of the Landers mainshock.

When and where the aftershock activity was depressed: Contrasting decay patterns of the proximate large earthquakes in southern California

Seismic quiescence has attracted attention as a possible precursor to a large earthquake. However, sensitive detection of quiescence requires accurate modeling of normal aftershock activity. We apply the epidemic‐type aftershock sequence (ETAS) model that is a natural extension of the modified Omori formula for aftershock decay, allowing further clusters (secondary aftershocks) within an aftershock sequence. The Hector Mine aftershock activity has been normal, relative to the decay predicted by the ETAS model during the 14 months of available data. In contrast, although the aftershock sequence of the 1992 Landers earthquake (M = 7.3), including the 1992 Big Bear earthquake (M = 6.4) and its aftershocks, fits very well to the ETAS up until about 6 months after the main shock, the activity showed clear lowering relative to the modeled rate (relative quiescence) and lasted nearly 7 years, leading up to the Hector Mine earthquake (M = 7.1) in 1999. Specifically, the relative quiescence occurred only in the shallow aftershock activity, down to depths of 5–6 km. The sequence of deeper events showed clear, normal aftershock activity well fitted to the ETAS throughout the whole period. We argue several physical explanations for these results. Among them, we strongly suspect aseismic slips within the Hector Mine rupture source that could inhibit the crustal relaxation process within “shadow zones” of the Coulomb's failure stress change. Furthermore, the aftershock activity of the 1992 Joshua Tree earthquake (M = 6.1) sharply lowered in the same day of the main shock, which can be explained by a similar scenario.

Postseismic Fault Healing on the Rupture Zone of the 1999 M 7.1 Hector Mine, California, Earthquake

We probed the rupture zone of the October 1999 M 7.1 Hector Mine earthquake using repeated near-surface explosions in October 2000 and November 2001. Three dense linear seismic arrays were deployed across the north and south Lavic Lake faults (LLFs) that broke to the surface in the mainshock and across the Bullion fault (BF) that experienced minor slip in that event. Two explosions each year were detonated in the rupture zone, one on the middle and one on the south LLF. We found that P and S velocities of fault-zone rocks increased by 0.7%-1.4% and 0.5%-1.0% between 2000 and 2001, respectively. In contrast, the velocities for P and S waves in surrounding rocks increased much less. This trend indicates that the Hector Mine rupture zone has been healing by strengthening after the main- shock, most likely due to the closure of cracks that opened during the 1999 earth- quake. The observed fault-zone strength recovery is consistent with an apparent crack density decrease of 1.5% within the rupture zone. The ratio of travel-time decrease for P to S waves was 0.72, consistent with partially fluid-filled cracks near the fault zone. This restrengthening is similar to that observed after the 1992 M 7.4 Landers earthquake, which occurred 25 km to the west (Li and Vidale, 2001). We also find that the velocity increase with time varies from one fault segment to another at the Hector Mine rupture zone. We see greater changes on the LLFs than on the BF, and the greatest change is on the middle LLF at shallow depth. We tentatively conclude that greater damage was inflicted, and thus greater healing is observed, in regions with larger slip in the mainshock.

Damage to the shallow Landers fault from the nearby Hector Mine earthquake.

Crustal faults have long been identified as sites where localized sliding motion occurs during earthquakes, which allows for the relative motion between adjacent crustal blocks. Although there is a growing awareness that we must understand the evolution of fault systems on many timescales to relate present-day crustal stresses and fault motions to geological structures formed in the past, fault-zone damage and healing have been documented quantitatively in only a few cases. We have been monitoring the healing of damage on the shallow Johnson Valley fault after its rupture in the 1992 magnitude-7.3 Landers earthquake, and here we report that this healing was interrupted in 1999 by the magnitude-7.1 Hector Mine earthquake rupture, which occurred 20-30 km away. The Hector Mine earthquake both strongly shook and permanently strained the Johnson Valley fault, adding damage discernible as a temporary reversal of the healing process. The fault has since resumed the trend of strength recovery that it showed after the Landers earthquake. These observations lead us to speculate that fault damage caused by strong seismic waves may help to explain earthquake clustering and seismicity triggering by shaking, and may be involved in friction reduction during faulting.

A strong correlation between induced peak dynamic Coulomb stress change from the 1992 M7.3 Landers, California, earthquake and the hypocenter of the 1999 M7.1 Hector Mine, California, earthquake

The 1992 M7.3 Landers earthquake may have played a role in triggering the 1999 M7.1 Hector Mine earthquake as suggested by their close spatial (∼20 km) proximity. Current investigations of triggering by static stress changes produce differing conclusions when small variations in parameter values are employed. Here I test the hypothesis that large‐amplitude dynamic stress changes, induced by the Landers rupture, acted to promote the Hector Mine earthquake. I use a flat layer reflectivity method to model the Landers earthquake displacement seismograms. By requiring agreement between the model seismograms and data, I can constrain the Landers main shock parameters and velocity model. A similar reflectivity method is used to compute the evolution of stress changes. I find a strong positive correlation between the Hector Mine hypocenter and regions of large (>4 MPa) dynamic Coulomb stress changes (peak Δσf(t)) induced by the Landers main shock. A positive correlation is also found with large dynamic normal and shear stress changes. Uncertainties in peak Δσf(t) (1.3 MPa) are only 28% of the median value (4.6 MPa) determined from an extensive set (160) of model parameters. Therefore the correlation with dynamic stresses is robust to a range of Hector Mine main shock parameters, as well as to variations in the friction and Skempton's coefficients used in the calculations. These results imply dynamic stress changes may be an important part of earthquake trigging, such that large‐amplitude stress changes alter the properties of an existing fault in a way that promotes fault failure.

Fault creep along the southern San Andreas from interferometric synthetic aperture radar, permanent scatterers, and stacking

Interferometric synthetic aperture radar (InSAR) provides a practical means of mapping creep along major strike‐slip faults. The small amplitude of the creep signal (<10 mm/yr), combined with its short wavelength, makes it difficult to extract from long time span interferograms, especially in agricultural or heavily vegetated areas. We utilize two approaches to extract the fault creep signal from 37 ERS SAR images along the southern San Andreas Fault. First, amplitude stacking is utilized to identify permanent scatterers, which are then used to weight the interferogram prior to spatial filtering. This weighting improves correlation and also provides a mask for poorly correlated areas. Second, the unwrapped phase is stacked to reduce tropospheric and other short‐wavelength noise. This combined processing enables us to recover the near‐field (∼200 m) slip signal across the fault due to shallow creep. Displacement maps from 60 interferograms reveal a diffuse secular strain buildup, punctuated by localized interseismic creep of 4–6 mm/yr line of sight (LOS, 12–18 mm/yr horizontal). With the exception of Durmid Hill, this entire segment of the southern San Andreas experienced right‐lateral triggered slip of up to 10 cm during the 3.5‐year period spanning the 1992 Landers earthquake. The deformation change following the 1999 Hector Mine earthquake was much smaller (<1 cm) and broader than for the Landers event. Profiles across the fault during the interseismic phase show peak‐to‐trough amplitude ranging from 15 to 25 mm/yr (horizontal component) and the minimum misfit models show a range of creeping/locking depth values that fit the data.

Aftershocks and pore fluid diffusion following the 1992 Landers earthquake

We model the evolution of regional stress following the 1992 Landers earthquake in order to test the importance of pore fluid flow in producing aftershocks. Rising fluid pressure due to pore fluid flow and the resulting Coulomb stress changes were found to be strongly correlated with the time and location of aftershock events. Regional aftershock frequencies computed by integrating pore pressure decay rates also agreed quite well with aftershock data. Calculations show that regions of rising postseismic poroelastic Coulomb stress overlap considerably with regions of positive coseismic Coulomb stress. Thus pore fluid flow, which affects pore pressure within faults and causes regional poroelastic stress evolution following earthquakes, gradually evolves the initial coseismic stress changes. Together these changes provide a reasonable physical mechanism for aftershock triggering which agrees with data for the 1992 Landers earthquake.

Triggering of the 1999 MW 7.1 Hector Mine earthquake by aftershocks of the 1992 MW 7.3 Landers earthquake

There is strong observational evidence that the 1999 MW 7.1 Hector Mine earthquake in the Mojave Desert, California, was triggered by the nearby 1992 MW 7.3 Landers earthquake. Many authors have proposed that the Landers earthquake directly stressed the Hector Mine fault. Our model of the Landers aftershock sequence, however, suggests there is an 85% chance that the Hector Mine hypocenter was actually triggered by a chain of smaller earthquakes that was initiated by the Landers main shock. We perform our model simulations using the Monte Carlo method based on the Gutenberg‐Richter relationship, Omori's law, Båth's law, and assumptions that all earthquakes, including aftershocks, are capable of producing aftershocks and that aftershocks produce their own aftershocks at the same rate that other earthquakes do. In general, our simulations show that if it has been more than several days since an M ≥ 7 main shock, most new aftershocks will be the result of secondary triggering. These secondary aftershocks are not physically constrained to occur where the original main shock increased stress. This may explain the significant fraction of aftershocks that have been found to occur in main shock stress shadows in static Coulomb stress triggering studies.

Mechanical coupling between the Landers and Hector Mine (California) fault systems

A 2D finite element approach is used to model the mechanical coupling between faults and the interaction between large earthquakes. The regional tectonic stress, the fault geometry and the Coulomb friction law control the resulting slip patterns. To test the potential of this approach we investigate the Landers and Hector Mine earthquakes which occurred in southern California. The resulting coseismic displacement satisfactorily fits the available GPS data. We show that the stress changes induced by the 1992 MW = 7.3 Landers earthquake modulate the applied regional tectonic stress reducing the slip on the faults of the subsequent 1999 MW = 7.1 Hector Mine rupture. We emphasize that fault interaction due to stress transfer can alter the slip distribution of impending earthquakes.

Interactions between the Landers and Hector Mine, California, Earthquakes from Space Geodesy, Boundary Element Modeling, and Time-Dependent Friction

The M w 7.1 Hector Mine earthquake happened 7 years after the 1992 M w 7.3 Landers earthquake, within a system of right-lateral strike slip faults comprising the Eastern California Shear Zone. Because estimates of recurrence intervals of M 7 earthquakes on these faults range anywhere from 1500 to 50,000 years, the close temporal spacing of these two earthquakes suggests that there is some interaction between the two. Current models of simple Coulomb static stress interactions between the earthquake ruptures do not predict an obvious cause-and-effect relationship. The interaction between the computed normal and right-lateral shear stress reductions induced by the Landers earthquake at the Hector Mine hypocenter is unclear. We use a combination of space geodesy, boundary element modeling, and computer modeling of time-dependent fault friction to investigate the interaction between the two earthquakes. We compare stress changes on the Hector Mine rupture plane induced by the Landers earthquake with the detailed slip distribution inferred using a combination of GPS and InSAR data. The slip distributions of both earthquakes are also used to infer the magnitude of the shear stress drop on each earthquake rupture and the orientation of the remote background stress consistent with these two recent events. For each earthquake the azimuth of the remote maximum compressive stress was approximately 17° ± 6°, and the magnitude of the shear stress drop was 8 ± 1 MPa on the Landers rupture and 10 ± 2 MPa on the Hector Mine rupture. Using a simple spring-and-slider model as a proxy for the rate-and-state frictional response of the Hector Mine faults to Landers earthquake-imposed stress steps, we find that, while a decrease in normal stress at the Hector Mine hypocenter would, by itself, have caused the Hector Mine earthquake to nucleate immediately, the simultaneous decrease in shear stress may have caused a delay in the peak shear loading. Consideration of an acceptable range of rate-and-state friction parameters, secular strain rates, and crustal stiffnesses leads us to conclude that this delay could have been 0 to 40 years.

TriNet Strong-Motion Data from the M 7.1 Hector Mine, California, Earthquake of 16 October 1999

The M_w 7.1 Hector Mine earthquake of October 16, 1999 was recorded by more than 300 stations of TriNet, which is administered cooperatively by the California Division of Mines and Geology's California Strong Motion Instrumentation Program (CDMG/CSMIP), California Institute of Technology, and the U.S. Geological Survey (USGS). The earthquake occurred in a remote part of the Mojave Desert, approximately 190 km northeast of downtown Los Angeles, and there were no strong-motion stations close to the surface rupture. The nearest station, Hector, is about 27 km north of the epicenter; it recorded a peak horizontal ground acceleration of 0.33g. The two next closest stations, Amboy and Joshua Tree, are to the east and south, both at epicentral distances of about 50 km; each recorded peak ground accelerations of about 0.2g. The new digital instruments installed for the TriNet project recorded a large set of reliable data at epicentral distances up to 275 km. These data can significantly improve empirical peak ground motion attenuation relationships, which are usually developed for distances only up to 100 km (Boore et al., 1993, 1997) because adequate data have not been available at greater distances. Hector Mine peak ground motions demonstrate reasonable agreement with empirical attenuation relationships for acceleration. In contrast, higher than expected ground velocities and displacements were recorded at epicentral distances of about 150 to 220 km, especially in the Los Angeles sedimentary basin, where anomalously high-amplitude displacements with periods of 6 to 7 sec were recorded in Los Angeles, Long Beach, and other areas. These long-period surface- or basin-generated waves can have significant effects on large structures. The M_w 7.3 Landers earthquake of 1992 similarly produced strong, long-period waves in the basin. The peak ground motions produced by the Landers earthquake were on average 1.6 times higher than for the Hector Mine earthquake in the Los Angeles area. Ground-motion data recorded by digital instruments were uniformly processed in the frequency band 0.067 to 46 Hz (0.022–15 sec). The processed data set includes records from 213 ground-response stations. In an effort to make strong-motion data available quickly to the engineering and scientific communities, important records from this event were made available by file transfer protocol (ftp) beginning the day of the earthquake.

The Hector Mine, California, Earthquake of 16 October 1999: Introduction to the Special Issue

The Hector Mine, California, earthquake (M_w 7.1) struck the Mojave Desert at 09:46 UTC, 16 October 1999. The earthquake occurred approximately 55 km northwest of the town of Twentynine Palms, California, and about 200 km east-northeast of Los Angeles (Fig. 1). The shock was widely felt throughout southern California, southern Nevada, western Arizona, and northernmost Baja California, Mexico. The Hector Mine earthquake, like the M_w 7.3 Landers earthquake seven years earlier, was associated with fault rupture in the eastern California shear zone (ECSZ) (Fig. 1), which is an approximately 80-km-wide zone of deformation that accommodates about 24% of the relative Pacific–North American plate motion (Sauber et al., 1986, 1994; Dokka and Travis, 1990; Savage et al., 1990, 2001; Gan et al., 2000; Miller et al., 2001). A block diagram highlighting some of the basic aspects of the Hector Mine earthquake is presented in Figure 2. A preliminary summary of the Hector Mine earthquake, its effects, and the response of the geoscience community is presented by Scientists from the U.S. Geological Survey; Southern California Earthquake Center, and California Division of Mines and Geology (USGS, SCEC, and CDMG, 2000).

The Coseismic Displacement Fields for the 1992 Landers and 1999 Hector Mine Earthquakes in California, from Regional GPS Observations

The fortuitous occurrences of the 1992 Landers and the 1999 Hector Mine earthquakes within a regional GPS network established during 1991 in the Mojave Desert allows assessment of earthquake-related deformation for both events. Annual and earthquake-response campaign-mode GPS observations record active deformation in the Mojave Desert over the past decade. We present new displacement fields for both the Landers earthquake and the Hector Mine earthquake, using corrections for well-established 1993 to 1999 velocities to provide robust estimates of coseismic displacement. Each displacement field is compared to a forward elastic dislocation model for the earthquake that integrates geologically and seismologically determined offsets along the Landers rupture and is then used to geodetically constrain an inversion for elastic dislocation. The geodetic data sets provide better resolution of moment magnitude for a given fault plane, and systematically underestimate seismologically determined M w by 0.06 to 0.07. In addition, during the 1992 to 1999 time interval, the post-Landers earthquake velocities at stations within 40 km its surface rupture provide direct observations that support kinematic loading of the Hector Mine rupture plane. This observation substantiates predictions from recent viscoelastic models.

Lack of Precursory Slip to the 1999 Hector Mine, California, Earthquake as Constrained by InSAR

We looked for evidence of interseismic strain occurring between the 1992 Landers earthquake and the 1999 Hector Mine earthquake near the Lavic Lake and Bullion faults by using interferometric synthetic aperture radar (InSAR). Interferograms covering the Hector Mine epicentral region were studied for possible slip along the Bullion and Lavic Lake faults by both visual inspection and a matched filter technique intended to emphasize slip located at the nucleation point. Some indications of possible deformation associated with the 5 July 1992 M L 5.4 Pisgah event was observed, but high decorrelation prevented a conclusive determination. We have seen no evidence for precursory slip in the epicentral region up to 30 days before the Hector Mine event. We estimated that the slip equivalent to a M w 4.5 event would have been observable in the months before the Hector Mine event, and this places an upper bound on the long-term precursory slip, had it occurred. We have noted that InSAR is well suited for detecting precursory slip in general due to the high spatial resolution and the lack of ground instrumentation required but that the detection level depends on the depth and orientation of the slip. [ Back ]