Joshua Tree Research Articles

Gas exchange and chlorophyll fluorescence responses of three south‐western Yucca species to elevated CO2 and high temperature

The ability of seedlings to tolerate temperature extremes is important in determining the distribution of perennial plants in the arid south‐western USA, and the manner in which elevated CO2 impacts the ability of plants to tolerate high temperatures is relatively unknown. Whereas the effects of chronic high temperature (30–38°C) and elevated CO2 are comparatively well understood, little research has assessed plant performance in elevated CO2 during extreme (> 45 °C) temperature events. We exposed three species of Yucca to 360 and 700 μmol CO2 mol–1 for 8 months, then 9 d of high temperature (up to 53 °C) to evaluate the impacts of elevated CO2 on the potential for photosynthetic function during external high temperature. Seedlings of a coastal C3 species (Yucca whipplei), a desert C3 species (Yucca brevifolia), and a desert CAM species (Yucca schidigera), were used to test for differences among functional groups. In general, Yuccas exposed to elevated CO2 showed decreases in carboxylation efficiency as compared with plants grown at ambient before the initiation of high temperature. The coastal species (Y. whipplei) showed significant reductions (33%) in CO2 saturated maximum assimilation rate (Amax), but the desert species (Y. brevifolia and Y. schidigera) showed no such reductions in Amax. Stomatal conductance was lower in elevated CO2 as compared with ambient throughout the temperature event; however, there were species‐specific differences over time. Elevated CO2 enhanced photosynthesis in Y. whipplei at high temperatures for a period of 4 d, but not for Y. brevifolia or Y. schidigera. Elevated CO2 offset photoinhibition (measured as Fv/Fm) in Y. whipplei as compared with ambient CO2, depending on exposure time to high temperature. Stable Fv/Fm in Y. whipplei occurred in parallel with increases in the quantum yield of photosystem II (ΦPSII) at high temperatures in elevated CO2. The value of ΦPSII remained constant or decreased with increasing temperature in all other treatment and species combinations. This suggests that the reductions in Fv/Fm resulted from thermal energy dissipation in the pigment bed for Y. brevifolia and Y. schidigera. The greater efficiency of photosystem II in Y. whipplei helped to maintain photosynthetic function at high temperatures in elevated CO2. These patterns are in contrast to the hypothesis that high temperatures in elevated CO2 would increase the potential for photoinhibition. Our results suggest that elevated CO2 may offset high‐temperature stress in coastal Yucca, but not in those species native to drier systems. Therefore, in the case of Y. whipplei, elevated CO2 may allow plants to survive extreme temperature events, potentially relaxing the effects of high temperature on the establishment in novel habitats.

Earthquake source parameters and fault kinematics in the eastern California shear zone

AbstractBased on waveform data from a profile of aftershocks following the north-south trace of the 28 June 1992 Landers rupture across the Mojave desert, we construct a new velocity model for the Mojave region that features a thin, slow crust. Using this model, we obtain source parameters, including depth and duration, for each of the aftershocks in the profile and, in addition, any significant (M > 3.7) Joshua Tree-Landers aftershock between April 1992 and October 1994 for which coherent TERRAscope data were available. In all, we determine source parameters and stress drops for 45 significant (Mw > 4) earthquakes associated with the Joshua Tree and Landers sequences, using a waveform grid-search algorithm. Stress drops for these earthquakes appear to vary systematically with location, with respect to previous seismic activity, proximity to previous rupture (i.e., with respect to the Landers rupture), and with tectonic province. In general, for areas north of the Pinto Mountain fault, stress drops of aftershocks located off the faults involved with the Landers rupture are higher than those located on the fault, with the exception of aftershocks on the newly recognized Kickapoo (Landers) fault. Stress drops are moderate south of the Pinto Mountain fault, where there is a history of seismic swarms but no single throughgoing fault. In contrast to aftershocks in the eastern Transverse ranges, and related to the 1992 Big Bear, California, sequence, Landers events show no clear relationship between stress drop and depth. Instead, higher stress-drop aftershocks appear to correlate with activity on nascent faults or those that experienced relatively small slip during mainshock rupture.

Effects of Rock Climbing on Cliff Plant Communities at Joshua Tree National Park, California

We compared the plant communities on cliffs used for rock climbing with cliffs not used for climbing. Eighteen cliffs in Joshua Tree National Park, California (six each with no climbing, moderate climbing, and intensive climbing) were sampled for plant diversity and community structure. Plants were sampled on cliff faces and at the base of cliffs. The dominant taxa were Quercus cornelius‐mulleri, Ericameria cuneata var. cuneata, and Bromus madritensis ssp. rubens. Plant species richness on cliff faces and at the base of cliffs was greater for cliffs with no evidence of climbing and lowest for cliffs with intensive use. Numbers of individual plants decreased with increased climbing use. Trees, shrubs, forbs, and cacti had greater relative abundances on cliffs without climbing than on cliffs with climbing. Plant cover on cliff faces decreased with increased climbing use. Adaptive management will be necessary to minimize the deleterious effects of rock climbing on plant communities associated with cliffs.Efectos de Alpinismo sobre las Comunidades de Plantas en Acantilados en el Parque Nacional Joshua Tree, CaliforniaComparamos las comunidades de plantas en acantilados usados para alpinismo contra acantilados no usados para esta actividad. Dieciocho acantilados del Parque Nacional Joshua Tree, California (seis con alpinismo, seis con alpinismo moderado y seis sin alpinismo) fueron muestreados para determinar la diversidad de plantas y la estructura de la comunidad. Las plantas fueron muestreadas en las caras y bases de los acantilados. Los taxones dominantes fueron Quercus cornelius‐mulleri, Ericameria cuneata var. cuneata y Bromus madritensis ssp. rubens. La riqueza de especies en las caras y bases de los acantilados fue mayor en aquellos sin evidencia de alpinismo y menor para aquellos acantilados con uso intensivo. Los números individuales de plantas disminuyeron con un incremento en el uso para alpinismo. Árboles, arbustos, pastos y cactus tuvieron una abundancia relativa mayor en acantilados sin alpinismo que en aquellos con alpinismo. La cobertura de las plantas en las caras de los acantilados disminuyó con un incremento en el alpinismo. Será necesario emplear un manejo adaptable para minimizar los efectos dañinos del alpinismo en roca sobre comunidades de plantas asociadas con acantilados.

Effects of Rock Climbing on Cliff Plant Communities at Joshua Tree National Park, California

Effects of Rock Climbing on Cliff Plant Communities at Joshua Tree National Park, California

Spatial patterns of mortality in a Colorado desert plant community

Spatial pattern and patterns of mortality were analyzed from a five year re-census of over 7000 mapped perennial shrubs within a one hectare study plot in Joshua Tree National Park. A five year interval revealed evidence for negative intraspecific interactions which was not evident after a one year evaluation of the same plants within the study area. Juvenile mortality was a positive function of conspecific density for Ambrosia dumosa, Eriogonum fasciculatum and Sphaeralcea ambigua. In addition, juvenile E. fasciculatum that died occurred closer to conspecific adults than would be expected by chance. The two censuses are similar in that uniform patterns of distribution among conspecifics are rare, particularly for non-succulent species.

A statistical technique for validating velocity models

Abstract This study investigates the use of a station influence statistic to identify velocity model shortcomings in the earthquake hypocenter location problem. Two groups of microearthquake events are examined. The first is a group of 81 events from the Mount St. Helens region that occurred between November 1987 and September 1991; the second, 110 well-located events from the 1992 Joshua Tree after-shock sequence. We describe a method for validating a postulated earth model. Let λ denote the hypocenter estimates that Geiger's method obtains. Systematically remove each station observation from the location problem, and recompute the location estimate. Call this estimate λ(i) when the ith station is removed. For a single event, define a station's influence (SI) as a weighted difference between λ and λ(i). Distributional summaries of SI statistics across events are used to identify model shortcomings: Given a specified velocity model, SI distributions that are not homogeneous across stations provide evidence of model inadequacies and/or failures in the weighting scheme. We show that velocity model shortcomings detected using SI statistics for the Mount St. Helens sequence under a one-dimensional model appear to correlate with known physical anomalies; while SI distributions evaluated under a three-dimensional model are more homogeneous and reflect a modest improvement over the one-dimensional model. SI distributions provide evidence of model failure for the Joshua Tree sequence under a one-dimensional model, but no evidence of failure under a three-dimensional model. Finally, the weighting scheme's validity is verified for the Joshua Tree sequence under the three-dimensional model.

Detailed observations of California foreshock sequences: Implications for the earthquake initiation process

We find that foreshocks provide clear evidence for an extended nucleation process before some earthquakes. In this study, we examine in detail the evolution of six California foreshock sequences, the 1986 Mount Lewis (ML = 5.5), the 1986 Chalfant (ML = 6.4), the 1986 Stone Canyon (ML = 4.7), the 1990 Upland (ML = 5.2), the 1992 Joshua Tree (MW = 6.1), and the 1992 Landers (MW = 7.3) sequence. Typically, uncertainties in hypocentral parameters are too large to establish the geometry of foreshock sequences and hence to understand their evolution. However, the similarity of location and focal mechanisms for the events in these sequences leads to similar foreshock waveforms that we cross correlate to obtain extremely accurate relative locations. We use these results to identify small‐scale fault zone structures that could influence nucleation and to determine the stress evolution leading up to the mainshock. In general, these foreshock sequences are not compatible with a cascading failure nucleation model in which the foreshocks all occur on a single fault plane and trigger the mainshock by static stress transfer. Instead, the foreshocks seem to concentrate near structural discontinuities in the fault and may themselves be a product of an aseismic nucleation process. Fault zone heterogeneity may also be important in controlling the number of foreshocks, i.e., the stronger the heterogeneity, the greater the number of foreshocks. The size of the nucleation region, as measured by the extent of the foreshock sequence, appears to scale with mainshock moment in the same manner as determined independently by measurements of the seismic nucleation phase. We also find evidence for slip localization as predicted by some models of earthquake nucleation.

A scan‐angle correction for thermal infrared multispectral data using side lapping images

Thermal infrared multispectral scanner (TIMS) images, acquired with side lapping flight lines, provide dual angle observations of the same area on the ground and can thus be used to estimate variations in the atmospheric transmission with scan angle. The method was tested using TIMS aircraft data for six flight lines with about 30% sidelap for an area within Joshua Tree National Park, California. Generally the results correspond to predictions for the transmission scan‐angle coefficient based on a standard atmospheric model although some differences were observed at the longer wavelength channels. A change was detected for the last pair of lines that may indicate either spatial or temporal atmospheric variation. The results demonstrate that the method provides information for correcting regional survey data (requiring multiple adjacent flight lines) that can be important in detecting subtle changes in lithology.

Observational constraints on earthquake source scaling: understanding the limits in resolution

I examine the resolution of the type of stress drop estimates that have been used to place observational constraints on the scaling of earthquake source processes. I first show that apparent stress and Brune stress drop are equivalent to within a constant given any source spectral decay between ω 1.5 and ω 3 (i.e., any plausible value) and so consistent scaling is expected for the two estimates. I then discuss the resolution and scaling of Brune stress drop estimates, in the context of empirical Green's function results from recent earthquake sequences, including the 1992 Joshua Tree, California, mainshock and its aftershocks. I show that no definitive scaling of stress drop with moment is revealed over the moment range 10 19–10 25; within this sequence, however, there is a tendency for moderate-sized ( M 4–5) events to be characterized by high stress drops. However, well-resolved results for recent M > 6 events are inconsistent with any extrapolated stress increase with moment for the aftershocks. Focusing on corner frequency estimates for smaller ( M < 3.5) events, I show that resolution is extremely limited even after empirical Green's function deconvolutions. A fundamental limitation to resolution is the paucity of good signal-to-noise at frequencies above 60 Hz, a limitation that will affect nearly all surficial recordings of ground motion in California and many other regions. Thus, while the best available observational results support a constant stress drop for moderate- to large-sized events, very little robust observational evidence exists to constrain the quantities that bear most critically on our understanding of source processes: stress drop values and stress drop scaling for small events.

Rupture directivity and slip distribution of theM4.3 foreshock to the 1992 Joshua Tree earthquake, Southern California

AbstractDetails of the M 4.3 foreshock to the Joshua Tree earthquake were studied using P waves recorded on the Southern California Seismic Network and the Anza network. Deconvolution, using an M 2.4 event as an empirical Green's function, corrected for complicated path and site effects in the seismograms and produced simple far-field displacement pulses that were inverted for a slip distribution. Both possible fault planes, north-south and east-west, for the focal mechanism were tested by a least-squares inversion procedure with a range of rupture velocities. The results showed that the foreshock ruptured the north-south plane, similar to the mainshock. The foreshock initiated a few hundred meters south of the mainshock and ruptured to the north, toward the mainshock hypocenter. The mainshock (M 6.1) initiated near the northern edge of the foreshock rupture 2 hr later. The foreshock had a high stress drop (320 to 800 bars) and broke a small portion of the fault adjacent to the mainshock but was not able to immediately initiate the mainshock rupture.

Seismicity and stress‐drop in the Eastern Transverse Ranges, southern California

Stress‐drops for small to moderately sized earthquakes in Southern California are found to vary systematically with source‐depth and location (tectonic environment). We determine high‐quality fault‐plane solutions, plus depth and source duration, for 17 significant (M &gt;3.9) aftershocks associated with the June 28, 1992 Big Bear sequence, including the more recent April 4, 1994 19:04 GMT Mw 4.6 Lake Arrowhead aftershock, and a Mw 4.2 Banning Pass event which occurred on May 31, 1993 at 08:55 GMT. Given source durations and moments obtained from long‐period source estimations, and assuming a circular fault model, we estimate stress‐drop for each event. Big Bear aftershocks are moderate to high (&gt;100 bars) stress‐drop. Events deeper than 12 km are generally high stress‐drop (&gt;100 bars), while shallower events exhibit moderate to high stress‐drops. These results are compared with a similar analysis of Landers aftershocks in the Mojave block. For the Big Bear region, stress drops appear to correlate with depth, with the deepest events yielding the highest stress‐drops. In general, events in this region yield higher stress‐drops than events occuring in the Mojave block and those associated with the Landers and Joshua Tree sequences. Comparisons of ML to MO are consistent with the stress‐drop results: deep, high stress‐drop events show elevated ML to MO ratios.

Multiple precursors to the Landers earthquake

AbstractWe present evidence for several temporally related premonitory movements preceding the Landers earthquake of 28 June 1992. Premonitory movements were measured at alignment arrays spanning the northeast, Painted Canyon, and San Andreas faults in Mecca Hills, California. A transitory fault normal extension of 30 ± 3 mm to eight targets was measured twice, on 8 and 9 March 1992 and 7 and 8 June 1992, across the northeast and Painted Canyon faults at the Hidden Spring Wash site (HSWS). Four kilometers northwest of the HSWS simultaneous fault normal extensions of 24, 21, and 6 ± 6 mm were measured across a previously locked 12-km segment of the San Andreas fault on 6 June 1992, 22 days prior to the Landers earthquake. A linear regression fit of horizontal displacement at HSWS showed a distinct increase in horizontal angles for those targets spanning active faults on the same dates as the fault normal extension. For the first time, other than a few small events on the North Shore creepmeter, dextral slip was measured on a nontranspressive, parallel slip segment of the San Andreas. On 6 June 1992, we measured offsets of 14, 8, and 3 arc sec ±3 sec at the Indian Stone Ring site (ISRS) on the San Andreas fault, which translates into movement of 20, 18, and 9 mm. Triggered slip was not observed on the main trace of the San Andreas fault after the Joshua Tree earthquake, but 17 mm of dextral slip, possibly coseismic with the Landers earthquake, was measured along cracks on 5 July 1992. This amount of slip supports the magnitude of premonitory movement. Taken together, substantial simultaneous fault normal extension and lateral slip at three alignment arrays across three faults followed by a large triggered slip event provide strong evidence for the occurrence of precursors to the Landers earthquake.

Stress/strain changes and triggered seismicity following the Mw7.3 Landers, California earthquake

Calculations of dynamic stresses and strains, constrained by broadband seismograms, are used to investigate their role in generating the remotely triggered seismicity that followed the June 28, 1992 Mw7.3 Landers, California earthquake. I compare straingrams and dynamic Coulomb failure functions calculated for the Landers earthquake at sites that did experience triggered seismicity with those at sites that did not. Bounds on triggering thresholds are obtained from analysis of dynamic strain spectra calculated for the Landers and Mw6.1 Joshua Tree, California earthquakes at various sites, combined with results of static strain investigations by others. I interpret three principal results of this study with those of a companion study by Gomberg and Davis [this issue]. First, the dynamic elastic stress changes themselves cannot explain the spatial distribution of triggered seismicity, particularly the lack of triggered activity along the San Andreas fault system. In addition to the requirement to exceed a Coulomb failure stress level, this result implies the need to invoke and satisfy the requirements of appropriate slip instability theory. Second, results of this study are consistent with the existence of frequency‐ or rate‐dependent stress/strain triggering thresholds, inferred from the companion study and interpreted in terms of earthquake initiation involving a competition of processes, one promoting failure and the other inhibiting it. Such competition is also part of relevant instability theories. Third, the triggering threshold must vary from site to site, suggesting that the potential for triggering strongly depends on site characteristics and response. The lack of triggering along the San Andreas fault system may be correlated with the advanced maturity of its fault gouge zone; the strains from the Landers earthquake were either insufficient to exceed its larger critical slip distance or some other critical failure parameter; or the faults failed stably as aseismic creep events. Variations in the triggering threshold at sites of triggered seismicity may be attributed to variations in gouge zone development and properties. Finally, these interpretations provide ready explanations for the time delays between the Landers earthquake and the triggered events.

Source parameters of the 23 April 1992 M 6.1 Joshua Tree, California, earthquake and its aftershocks: Empirical Green's function analysis of GEOS and TERRAscope data

Source parameters of the M 6.1 23 April 1992 Joshua Tree mainshock and 86 M 1.8 to 4.9 aftershocks are determined using an empirical Green's function methodology. For the aftershocks, deconvolved P - and S -wave spectra are calculated for 126 pairs of closely spaced events recorded on portable GEOS stations; S -wave spectra from the two horizontal components are averaged. The deconvolved spectra are fit by a ratio of omega-square source models, yielding an optimal (least-squares) corner frequency for both the large and the small event in each pair. We find no resolved difference between the inferred P - and S -wave corner frequencies. Using the standard Brune model for stress drop, we also find no resolved nonconstant scaling of stress drop with moment, although we also conclude that detailed scaling systematics would be difficult to resolve. In particular, a weak increase of stress drop with moment over a limited moment/magnitude cannot be ruled out. For magnitudes smaller than M 3 to 3.5, the inferred stress-drop values will be limited by the maximum observable corner frequency value of 60 Hz. For the mainshock, source-time functions are obtained from mainshock recordings at three TERRAscope stations (PFO, PAS, and GSC) using an M 4.3 foreshock as an empirical Green's function. The results indicate a fairly simple, single-pulse source-time function, with clear south-to-north directivity and an inferred rupture radius of 5 to 6 km. The deconvolved source-time functions are inverted to obtain a finite-rupture model that gives a robust estimate of rupture dimension. Early aftershocks are found to lie along the perimeters of regions with high mainshock slip. The inferred mainshock stress-drop value, 56 bars, is within the range determined for the aftershocks. Our derived mainshock source spectra do not show resolvable deviation from the omega-square model.

Relation of the 1992 Landers, California, Earthquake Sequence to Seismic Scattering

Measurements of crustal scattering for the area surrounding the 1992 Landers earthquake sequence obtained from regional array recordings of teleseismic events for the 10-year period before the sequence showed that the slip distribution on faults could be deducible from the preshock elastic structure. Scattering intensity correlated strongly with the distribution of aftershocks and slip of the moment magnitude ( M w ) 7.3 Landers main shock, M w 6.1 Joshua Tree, and M w 6.2 Big Bear events, which implies that aftershocks and slip are structurally controlled and broadly predictable. Scattering within the fault zones was directional and consistent with variable along-strike alignment of stress-induced cracks.

Elemental composition of globoids in the perisperm tissue of various seeds

The mature seeds of some angiosperms contain perisperm tissue derived from remains of the nucellus tissue of the ovule. In our transmission electron microscopy studies, the perisperm tissues of the seeds of Yucca brevifolia, Coffea arabica, Beta vulgaris, Piper nigrum, and Zostera capricorni contained naturally electron-dense globoids. Energy dispersive X-ray analysis of globoids from the perisperm tissues of these seeds revealed varying levels of phosphorus, potassium, magnesium, and calcium, results that are consistent with the presence of the mineral nutrient store called phytate. Phosphorus, potassium, magnesium, calcium, and other mineral nutrients, likely stored as phytate, have routinely been located in globoids of endosperm, female gametophyte, and embryo tissues of seeds. Key words: perisperm, globoids, mineral nutrient reserves, phosphorus, seeds, energy dispersive X-ray analysis.

Coseismic fault slip associated with the 1992 Mw 6.1 Joshua Tree, California, earthquake: Implications for the Joshua Tree‐Landers earthquake sequence

Coseismic surface deformation associated with the Mw 6.1, April 23, 1992, Joshua Tree earthquake is well represented by estimates of geodetic monument displacements at 20 locations independently derived from Global Positioning System and trilateration measurements. The rms signal to noise ratio for these inferred displacements is 1.8 with near‐fault displacement estimates exceeding 40 mm. In order to determine the long‐wavelength distribution of slip over the plane of rupture, a Tikhonov regularization operator is applied to these estimates which minimizes stress variability subject to purely right‐lateral slip and zero surface slip constraints. The resulting slip distribution yields a geodetic moment estimate of 1.7 × 1018 N m with corresponding maximum slip around 0.8 m and compares well with independent and complementary information including seismic moment and source time function estimates and main shock and aftershock locations. From empirical Green's function analyses, a rupture duration of 5 s is obtained which implies a rupture radius of 6–8 km. Most of the inferred slip lies to the north of the hypocenter, consistent with northward rupture propagation. Stress drop estimates are in the range of 2–4 MPa. In addition, predicted Coulomb stress increases correlate remarkably well with the distribution of aftershock hypocenters; most of the aftershocks occur in areas for which the mainshock rupture produced stress increases larger than about 0.1 MPa. In contrast, predicted stress changes are near zero at the hypocenter of the Mw 7.3, June 28, 1992, Landers earthquake which nucleated about 20 km beyond the northernmost edge of the Joshua Tree rupture. Based on aftershock migrations and the predicted static stress field, we speculate that redistribution of Joshua Tree‐induced stress perturbations played a role in the spatio‐temporal development of the earthquake sequence culminating in the Landers event.

Thirty Thousand Years of Vegetation Changes in the Alabama Hills, Owens Valley, California

Twenty packrat (Neotoma) middens recovered from three sites (1265-1535 m) in the Alabama Hills, Inyo County, California, provide a ca. 31,450-yr record of vegetation change. Located ca. 7 km east of the Sierra Nevada, the middens document that Utah juniper (Juniperus osteosperma), Joshua tree (Yucca brevifolia), and bitterbush (Purshia tridentata) occupied the site between 31,450 and 19,070 yr B.P. Joshua tree and bitterbush departed by ca. 17,760 yr B.P., with cliffrose (Purshia mexicana) and joint-fir (Ephedra viridis) appearing. By 13,350 yr B.P., blackbush (Coleogyne ramosissima) and cholla (Opuntia echinocarpa) entered the record. Between 9540 and 7990 yr B.P., Utah juniper and other species now extralocal to the sites departed and modern components such as wolfberry (Lycium andersonii) and rubber rabbitbrush (Chrysothamnus teretifolius) appeared. The middle Holocene records little variation in plant macrofossil composition; however, pollen analysis reflects an increase in aquatic pollen types which might suggest more-open conditions. The transition to the modern vegetation associations at the sites occurred after ca. 2800 yr B.P. The record from the Alabama Hills correlates well with that of other regional vegetation data but documents conditions of increasing aridity earlier than many other packrat midden sites. A shift in understory vegetation between 19,070 and 17,760 yr B.P. may reflect a transition from glacial maximum to post-maximum conditions in the eastern Sierra Nevada.

Global positioning system resurvey of Southern California Seismic Network stations

Abstract Systematic errors in travel-time data from local earthquakes can sometimes be traced to inaccuracies in the published seismic station coordinates. This prompted a resurvey of the stations of the Caltech/USGS Southern California Seismic Network (SCSN) using the Global Positioning System (GPS). We surveyed 241 stations of the SCSN using Trimble and Ashtech dual-frequency GPS receivers and calculated positions accurate to 3 m using differential positioning from carrier phase measurements. Twelve percent of the stations that were surveyed were found to be mislocated by more than 500 m. Stations of the TERRAscope and USC networks were also surveyed, as well as a network of portable seismic stations deployed shortly after the 1992 Joshua Tree and Landers earthquakes. The new coordinates and the offsets from the old coordinates are given below. The new coordinates are being used in SCSN locations as of 1 January 1994.

Slip triggered on southern California faults by the 1992 Joshua Tree, Landers, and Big Bear earthquakes : P. Bodin, R. Bilham, J. Behr, J. Gomberg & K. W. Hudnut, Bulletin - Seismological Society of America, 84(3), 1994, pp 806–816

Slip triggered on southern California faults by the 1992 Joshua Tree, Landers, and Big Bear earthquakes : P. Bodin, R. Bilham, J. Behr, J. Gomberg & K. W. Hudnut, Bulletin - Seismological Society of America, 84(3), 1994, pp 806–816