Buck Ridge Research Articles

Spatial Variations of Upper Crustal Anisotropy Along the San Jacinto Fault Zone in Southern California: Constraints From Shear Wave Splitting Analysis

AbstractTo discern spatial and explore possible existence of temporal variations of upper crustal anisotropy in an ∼15 km section of the San Jacinto Fault Zone (SJFZ) that is composed of the Buck Ridge and Clark faults in southern California, we conduct a systematic shear wave splitting investigation using local S‐wave data recorded by three broadband seismic stations located near the surface expression of the SJFZ. An automatic data selection and splitting measurement procedure is first applied, and the resulting splitting measurements are then manually screened to ensure reliability of the results. Strong spatial variations in crustal anisotropy are revealed by 1,694 pairs of splitting parameters (fast polarization orientation and splitting delay time), as reflected by the dependence of the resulting splitting parameters on the location and geometry of the raypaths. For raypaths traveling through the fault zones, the fast orientations are dominantly WNW‐ESE which is parallel to the faults and may be attributed to fluid‐filled fractures in the fault zones. For non‐fault‐zone crossing raypaths, the fast orientations are dominantly N–S which are consistent with the orientation of the regional maximum compressive stress. A three‐dimensional model of upper crustal anisotropy is constructed based on the observations. An increase in the raypath length normalized splitting times is observed after the 03/11/2013 M4.7 earthquake, which is probably attributable to changes in the spatial distribution of earthquakes before and after the M4.7 earthquake rather than reflecting temporal changes of upper crustal anisotropy.

Diverse Volumetric Faulting Patterns in the San Jacinto Fault Zone

AbstractWe examine locations, magnitudes, and faulting types of post‐2000 earthquakes in the trifurcation area of San Jacinto fault zone to clarify basic aspects of failure processes in the area. Most M ≥ 3.5 events have strike‐slip mechanisms, occur within 1 km of the main faults (Clark, Buck Ridge, and Coyote Creek), and have hypocenter depths of 10–13 km. In contrast, many smaller events have normal source mechanisms and hypocenters in intrafault areas deeper than 13 km. Additional small events with hypocenter depth <13 km occur in off‐fault regions and have complex geometries including lineations normal to the main faults. Five moderate earthquakes with M 4.7–5.4 have high aftershock rates (~150 M ≥ 1.5 events within 1 day from the mainshock). To obtain more details on aftershock sequences of these earthquakes, we detect and locate additional events with the matched filter method. There are almost no aftershocks within 1 km from the mainshocks, consistent with large mainshock stress drops and low residual stress. The five aftershock sequences have almost no spatial overlap. While the mainshocks are on the main faults, most aftershocks are located in intrafault and off‐fault regions. Their locations and spatial distribution reflect the mainshock rupture directions, and many also follow structures normal to the main faults. The significant diversity of observed features highlights the essential volumetric character of failure patterns in the area. The increasing rate of moderate events, productive aftershock sequences, and large inferred stress drops may reflect processes near the end of a large earthquake cycle.

Short-term variations in slip rate and size of prehistoric earthquakes during the past 2000 years on the northern San Jacinto fault zone, a major plate-boundary structure in southern California

Most of the displacement across the North American−Pacific plate boundary in southern California is accommodated by the San Jacinto and the southern San Andreas fault zones. If and how the rate of displacement across these fault zones varies along strike and through time are still being resolved. Here, we present four calculations of late Holocene slip rate and average slip per event from the Claremont fault of the northern San Jacinto fault zone that show variations in strain distribution over the past 2000 yr and illustrate how plate-boundary displacement is distributed between the San Jacinto and southern San Andreas fault zones. We calculate a slip rate of 12.8–18.3 mm/yr and an average slip per event of 2.5 m from two measurements of streams offset by 9–11 earthquakes in the past 1500–2000 yr. Faster slip rates of 21–30 mm/yr and an average slip per event of 2.7–3 m were determined from measurements of a stream and a buried channel that were offset by three earthquakes in the past 400–500 yr. The 2000 yr slip rate is similar to the range in slip rates reported for the adjacent San Bernardino section of the San Andreas fault zone, suggesting that the northern San Jacinto accommodates a similar amount of displacement as the San Andreas fault zone at the same latitude. The rate is also slightly faster (by ∼2–3 mm/yr) than reported slip rates from the central San Jacinto fault zone to the southeast. A slip rate of 15 ± 2 mm/yr is within the range of uncertainty for almost all the geologic and geodetic data for the entire length of the San Jacinto fault zone and may be the best approximation for long-term average slip rate of the fault zone. Alternatively, 2–3 mm/yr of slip along the northern San Jacinto fault zone may be accommodated to the south along the lesser-studied Hot Springs, Thomas Mountain, Buck Ridge, and Santa Rosa faults, the lateral slip rates of which are not well known nor included in typical estimates of slip rate along the central San Jacinto fault zone. We infer that the faster slip rate over the past 500 yr is due to a cluster of earthquakes along the Claremont fault between A.D. 1400 and A.D. 1850 and larger-than-average surface displacement of 3 m or more during the third event back. The 3 m or more measurement of displacement in this event corresponds to rupture lengths that are slightly longer than the total length of the Claremont fault, and previously published paleoseismic data indicate that this event occurred coincident in time with an event on the adjacent Clark fault. We propose that this combination of slip per event data and paleoseismic data from adjacent fault strands is strong evidence for rupture through the releasing step over that separates these two segments of the San Jacinto fault zone.

Seismic velocity structure in the Hot Springs and Trifurcation areas of the San Jacinto fault zone, California, from double-difference tomography

We present tomographic images of crustal velocity structures in the complex Hot Springs and Trifurcation areas of the San Jacinto Fault Zone (SJFZ) based on double-difference inversions of earthquake arrival times. We invert for V P , V S and hypocentre location within 50 × 50 × 20 km 3 volumes, using 266969 P and 148249 S arrival times. We obtain high-fidelity images of seismic velocities with resolution on the order of a few kilometres from 2 to 12 km depth and validate the results using checkerboard tests. Due to the relatively large proportion of S-wave arrival times, we also obtain stable maps of V P /V S ratios in both regions. The velocity of the Trifurcation Area as a whole is lower than adjacent unfaulted material. We interpret a 4-km-wide low velocity zone with high V P /V S ratio in the trifurcation itself as related to fault zone damage. We also observe clear velocity contrasts across the Buck Ridge, Clark and Coyote Creek segments of the SJFZ. The Anza segment of the SJFZ, to the NW of the trifurcation area, displays a strong (up to 27 per cent) contrast of V S from 2 to 9 km depth. In the Hot Springs area, a low velocity zone between the Claremont and Casa Loma Strands narrows with depth, with clear velocity contrasts observed across both segments. A roughly 10-km-wide zone of low velocity and low V P /V S ratio at the NW tip of the Hot Springs fault is indicative of either unconsolidated sediments associated with the San Jacinto basin, or fluid-filled cracks within a broad deformation zone. High V P /V S ratios along the Anza segment could indicate a preferred nucleation location for future large earthquakes, while the across-fault velocity contrast suggests a preferred northwest rupture propagation direction for such events.

Sedimentary geology of the Palaeoarchaean Buck Ridge (South Africa) and Kittys Gap (Western Australia) volcano-sedimentary complexes

The two Palaeoarchaean volcano-sedimentary complexes of the Buck Ridge (Barberton Greenstone Belt, South Africa) and Kittys Gap (Coppin Gap Greenstone Belt, East Pilbara, Australia) have a similar geological setting and age (∼3.45 Ga). The predominantly volcaniclastic sediments are concentrated at the top of these complexes, and experienced thorough, (very) early diagenetic silicification. In many places the silicification process has led to excellent preservation of the primary sedimentary structures. Elsewhere it has resulted in their obliteration or replacement by diagenetic structures. The Buck Ridge chert forms a regressive-transgressive succession, deposited around base level, with lacustrine and littoral marine facies. Deposition of the Kittys Gap Chert was also close to base level, almost exclusively subaqueous, with tidal influence and a regressive sequential trend. In both volcano-sedimentary complexes, these low-energy sediments are juxtaposed with high-energy breccia pods and layers, with often a high Fe-oxide content. The breccias are interpreted as being the result of explosive hydrothermal activity. Sedimentation was strongly controlled by normal faulting.

Shallow low-velocity zone of the San Jacinto fault from local earthquake waveform modelling

SUMMARY We developed a method to determine the depth extent of low-velocity zone (LVZ) associated with a fault zone (FZ) using S-wave precursors from local earthquakes. The precursors are diffracted S waves around the edges of LVZ and their relative amplitudes to the direct S waves are sensitive to the LVZ depth. We applied the method to data recorded by three temporary arrays across three branches of the San Jacinto FZ. The FZ dip was constrained by differential traveltimes of P waves between stations at two side of the FZ. Other FZ parameters (width and velocity contrast) were determined by modelling waveforms of direct and FZ-reflected P and S waves. We found that the LVZ of the Buck Ridge fault branch has a width of ∼150 m with a 30–40 per cent reduction in Vp and a 50–60 per cent reduction in Vs. The fault dips 70 ± 5 ◦ to southwest and its LVZ extends only to 2 ± 1 km in depth. The LVZ of the Clark Valley fault branch has a width of ∼200 m with 40 per cent reduction in Vp and 50 per cent reduction in Vs. The Coyote Creek branch is nearly vertical and has a LVZ of ∼150 m in width and of 25 per cent reduction in Vp and 50 per cent reduction in Vs. The LVZs of these three branches are not centred at the surface fault trace but are located to their northeast, indicating asymmetric damage during earthquakes.

Early Archaean hydrothermal fluids; a study of inclusions from the ∼ 3.4 Ga Buck Ridge Chert, Barberton Greenstone Belt, South Africa

Fluid inclusions have been studied from cavities and breccias in one of the oldest well-preserved, relatively low-grade metamorphosed sedimentary sequences on Earth. The fluid inclusion-bearing samples are from the uppermost part of the early Archaean (∼ 3.4 Ga) Buck Ridge volcano-sedimentary complex in the Barberton Greenstone Belt (South Africa), which was affected by hydrothermal alteration during and immediately after its deposition. In banded, silicified sandy sediments this alteration is characterised, among other features, by quartz-cemented hydrothermal breccias, layer-bound stockworks of cm-size quartz veins and quartz-filled cavities below stacked pebbles. Quartz grains in these hydrothermal deposits contain numerous fluid inclusions filled with either two-phase aqueous or mixed H 2O–CO 2 fluid. The aqueous inclusions are divided into three different groups, based on their microthermometric characteristics: type H A (relatively high salinity), type M A (intermediate salinity) and type L A (relatively low salinity). Optical and microthermometric characteristics of the fourth type of inclusions, the mixed H 2O–CO 2 fluid (type C) indicate that it was homogeneous at the time of trapping, with only some minor changes occurring after trapping. Type H A and type C fluids appear as immiscible fluids in their host crystal, most probably resulting from unmixing of the hydrothermal fluid. Phase relations in the CO 2–H 2O system show that the unmixing must have occurred at a relatively low pressure of ∼ 100 bars. This conclusion is in line with the geological context of the samples, which indicates a shallow environment of deposition for the sediments, (almost) contemporaneous hydrothermal activity, and a very low metamorphic grade. However, it is contrary to the first impression given by the homogeneous character of type C inclusions, which suggests much higher P– T conditions at the time of trapping. The aqueous component of type C fluids may have progressively mixed with high-salinity aqueous fluids (type H A), resulting in a dilution trend from H A to M A to L A fluids. The hydrothermal fluids from the Buck Ridge Chert in the Barberton Greenstone Belt are strikingly similar to those encountered in present-day systems. They show minor differences with older (ca. 3.8 Ga) fluids preserved in hydrothermally altered rocks from the Isua Greenstone Belt in Greenland.

Growth-fault structure and stratigraphic architecture of the Buck Ridge volcano-sedimentary complex, upper Hooggenoeg Formation, Barberton Greenstone Belt, South Africa

The ∼3.45–3.42Ga Buck Ridge volcano-sedimentary complex (BR-vsc) forms the uppermost part of the Hooggenoeg Formation, in the early Archaean Onverwacht Group of the Barberton Greenstone Belt, South Africa. The complex consists mainly of massive and pillowed basalts and felsic, quartz-plagioclase porphyritic rocks, which are capped by pervasively silicified sedimentary rocks. Deposition of the felsic extrusive and (volcani-)clastic sedimentary rocks occurred in shallow water to emersion, during an extensional tectonic regime. The extensional regime is expressed by a number of listric normal faults that transect the entire ∼2km thick complex. The syndepositional (i.e. growth fault) character of the faults is indicated by variations in thickness and facies of rock units across the faults, and by systematic rotation of the fault blocks. Broadly contemporaneous with the faulting, felsic rocks intruded the complex into the highest levels. The geometry of these intrusive bodies was controlled by the major growth faults. Extension also created space for the intrusion of approximately bedding-perpendicular felsic and mafic dykes, and black chert veins. The new field observations and U–Pb zircon SHRIMP data have pinned the timing of events during and after deposition of the BR-vsc. The growth-fault model accounts well for previously unexplained structural observations that include both extensional and compressional features. Recognition of an early extensional phase of deformation demonstrates further similarities between the tectonic histories of the Barberton Greenstone Belt and the greenstone belts of the east Pilbara in Australia.

Shallow seismic trapping structure in the San Jacinto fault zone near Anza, California

SUMMARY We analyse fault zone trapped waves, generated by ∼500 small earthquakes, for high-resolution imaging of the subsurface structure of the Coyote Creek, Clark Valley and Buck Ridge branches of the San Jacinto fault zone near Anza, California. Based on a small number of selected trapped waves within this data set, a previous study concluded on the existence of a lowvelocity waveguide that is continuous to a depth of 15‐20 km. In contrast, our systematic analysis of the larger data set indicates a shallow trapping structure that extends only to a depth of 3‐5 km. This is based on the following lines of evidence. (1) Earthquakes clearly outside these fault branches generate fault zone trapped waves that are recorded by stations within the fault zones. (2) A traveltime analysis of the difference between the direct S arrivals and trapped wave groups shows no systematic increase (moveout) with increasing hypocentral distance or event depth. Estimates based on the observed average moveout values indicate that the propagation distances within the low-velocity fault zone layers are 3‐5 km. (3) Quantitative waveform inversions of trapped wave data indicate similar short propagation distances within the low-velocity fault zone layers. The results are compatible with recent inferences on shallow trapping structures along several other faults and rupture zones. The waveform inversions also indicate that the shallow trapping structures are offset to the northeast from the surface trace of each fault branch. This may result from a preferred propagation direction of large earthquake ruptures on the San Jacinto fault.

Characterization of the San Jacinto fault zone near Anza, California, by fault zone trapped waves

We installed three 350‐m‐long seismic arrays, each array consisting of 12 three‐component stations, across the Coyote Creek fault (CCF), Clark Valley fault (CVF), and Buck Ridge fault (BRF) of the San Jacinto fault zone (SJFZ) near Anza, California, to record fault zone trapped waves from microearthquakes. We observed trapped waves with relatively large amplitudes and long duration at stations close to the fault traces for earthquakes occurring within the fault zone. The coda‐normalized amplitude spectra of trapped waves showed peaks at 4–7 Hz, which decreased sharply with the distance from the fault trace. Observations and three‐dimensional finite difference simulations of trapped waves revealed low‐velocity and low‐Q waveguides on these active faults with the width of 75–100 m in which shear velocities are reduced by 25–30% from wall rock velocities and Q values are 40–90 at depths between the surface and 18 km. The locations of earthquakes for which we observed trapped waves delineate the most seismically active fault strands of the SJFZ in a region with complicated slip planes near Anza. The low‐velocity waveguides inferred from trapped waves extend 15 to 20 km in the length on these active faults and are segmented by the fault discontinuities. The waveguide on the BRF dips south westward to connect the waveguide on the CVF, which dips northeastward. This waveguide extends at the seismogenic depth through Anza slip gap to another low‐velocity waveguide on the Casa Loma fault (CLF), which has been delineated in our previous study of the SJFZ using trapped waves [Li et al., 1997]. The waveguide on the CCF in Coyote Mountain is nearly vertical and disconnected from the CLF at the south edge of Anza gap. We interpret the low‐velocity waveguides on these active strands to partly result from recent prehistoric significant earthquakes on them and evaluate the future earthquake in the Anza region

Small‐scale stress heterogeneity in the Anza seismic gap, southern California

Focal mechanism inversions reveal significant lateral variations in stress orientations along the Anza segment of the San Jacinto fault zone. The most notable stress anomaly is within the 20‐km aseismic (seismic gap) portion of the fault zone, where , the maximum compressive stress, is nearly horizontal and is oriented at 74°±13° relative to the fault strike. This contrasts with orientations ranging from 62°±11° to 49°±7° along the more seismically active portions of the fault zone immediately to the northwest and southeast of the seismic gap. Regional stress results, found by inverting all focal mechanisms simultaneously, indicate that is horizontal and trends north‐south, while is horizontal and trends east‐west. Approximately, 15 km west of the seismic gap, in the off‐fault Cahuilla swarm area, and solutions are rotated clockwise by about 25° relative to the regional model. Roughly, 10 km southeast of the seismic gap near the Buck Ridge fault, and are rotated counterclockwise by about 10° relative to the regional solution. Northwest of the seismic gap along the fault zone, plunges about 30° from the horizontal, correlating with a local increase in reverse faulting between the Hot Springs and San Jacinto faults. Southeast of the seismic gap, plunges about 45° from the horizontal, correlating with a local increase in normal faulting in the trifurcation region of the Buck Ridge, Clark, and Coyote Creek faults. We propose a simple mechanical model in which a block rotation superimposed on the dominant right‐lateral strikeslip motion of the fault zone satisfies the first‐order observations of stress orientation, faulting, and horizontal surface strain. Under this model the Anza seismic gap is the region of zero convergence between the northeast and southwest sides of the fault, and the fault zone strength within the seismic gap is either comparable to or exceeds the fault zone strength adjacent to the gap.

Fault plane determinations for three small earthquakes along the San Jacinto Fault, California: Search for cross faults

Using rupture directivity and aftershock locations, three earthquakes (ML 4.2 – 4.3) were studied to determine which of the two nodal planes of the focal mechanism was the rupture plane. All three events occurred near the San Jacinto fault zone during December 1989 and February 1990 and had a large component of strike‐slip motion with nodal planes oriented in the northeast and northwest directions. Smaller earthquakes were used as empirical Green functions to deconvolve the path and site complications from the short‐period P waveforms. The resultant far‐field source time functions were inverted for slip on a finite fault and tested to see which nodal plane provided a better fit to the data. The results of the waveform inversion showed that the earthquake near the Anza Gap (December 2, 1989) and the earthquake near San Bernardino (December 28, 1989) ruptured northwest trending planes. The earthquake near Buck Ridge (February 18, 1990) appeared to rupture a northeast trending plane, but the waveform results for this event were not considered as reliable. Available aftershock data for the three events were also examined using master event relocations. The aftershock sequence following the Buck Ridge event showed a clear lineation trending toward the northeast. The results of this study indicate that the Anza and San Bernardino earthquakes ruptured areas on or parallel to the San Jacinto fault, while the Buck Ridge event ruptured a northeast trending cross fault. In addition to determining the fault plane, the waveform analysis resolved details of the slip distribution for these relatively small earthquakes.

Source characteristics of earthquakes along the southern San Jacinto and Imperial fault zones (1937 to 1954)

Abstract Body waveform inversion techniques are used to study the source parameters of four earthquakes occurring between 1937 and 1954 along the southern San Jacinto and Imperial faults (1937 Buck Ridge, 1940 Imperial Valley, 1942 Borrego Mountain, and 1954 Salada Wash events). All earthquakes had simple rupture histories with the exception of the 1940 Imperial Valley main shock, which consisted of at least four subevents whose relative locations indicate unilateral rupture toward the southeast. Earthquakes in regions of high heat flow (>80 mW/m2) had focal depths near the base of the seismogenic zone (8 to 10 km). The 1937 Buck Ridge earthquake, located in a region of lower heat flow, however, appears to have occurred at a shallow (3 ± 2 km) depth. The location, mechanism, and aftershock distribution for the 1942 Borrego Mountain earthquake suggest it could have occurred along the Split Mountain fault, a recently identified northeast-trending cross fault located between the Elsinore and Coyote Creek faults or along an unnamed fault that parallels the trend of the Coyote Creek fault. Moment and rupture length estimates obtained from this study agree well with estimates obtained in previous studies that used different data sets.

The digital array at Anza, California: Processing and initial interpretation of source parameters

Seismicity along the San Jacinto fault suggests that a 40‐km‐long section near the town of Anza may constitute a gap in the occurrence of ML = 6–7 earthquakes. The potential for a gap‐filling shock and the high rate of seismicity at the southern end of the gap (five events of 4.0 ≤ ML ≤ 5.5 since 1970) provided the impetus for deploying a digital seismic array to collect high‐quality ground motion recordings of all events 2 ≤ ML ≤ 4.5 (on‐scale recording for shocks with magnitudes above ML = 4 can be obtained from an existing array of eight strong motion accelerographs). The Anza site also had the advantage of being in the southern California batholith, which appears to be relatively homogeneous compared to the Franciscan/Gablian contrast of the central San Andreas; we expected that the granitic rocks of the batholith would yield relatively accurate earthquake locations and efficiently propagate high frequencies. The field instrumentation is specifically designed for broadband recording (up to 70 Hz) and high dynamic range (96 dB in the on‐site digitizer alone), since both are necessary for determining the rupture history of earthquakes. Both local VHF and microwave digital telemetry transmit the data from the Anza region to San Diego for computer data logging. In the first 30 months of the array's operation, approximately 292 events have been recorded, located, and processed for source parameters. Most events occur in one of five clusters or in a diffuse zone near the Buck Ridge fault. Two of these clusters are located at right‐stepping en echelon offsets (Coyote Creek‐San Jacinto and San Jacinto‐Hot Springs); two others are directly below and about 8 km west of Anza, respectively. The fifth cluster is just to the northeast of the Hot Springs fault. Although event depths are generally between 11 and 14 km, at the southern end of the Hot Springs fault, depths extend to 18 km; these are some of the deepest strike‐slip earthquakes on the San Andreas system. We calculate source parameters such as the scalar moment and stress drop for the analysis of high‐frequency waves, scaling relations, and earthquake interaction. The largest event recorded thus far had a moment of 4.4 × 1021 dyn cm (M = 3.8) and a stress drop of 55 bars. Both arms and Brune stress drops increase with moment; source radius increases only slowly with moment. The maximum values of both the Brune and arms stress drops increase with depth down to 10 km, remain approximately constant to 14 km, and may decrease below 14 km. The data suggest that stress drops of a group of earthquakes can be related to the strength of the upper crust calculated from frictional and quasi‐plastic flow laws, although individual events may not be related. Stress drops on the San Jacinto fault are high compared to those on the central creeping section of the San Andreas, where stress drops are about 10 bars or less. This observation is consistent with the relative rupture area of events of equal ML (5.5) and may be related to the lithologie differences between the two fault segments and the amount of normal stress compared to shear stress.

Rupture patterns and preshocks of large earthquakes in the southern San Jacinto fault zone

AbstractWe relocated the large 1937, 1942, and 1954 earthquakes in the San Jacinto fault zone. The epicenters of the main shocks, aftershocks, and some preshocks were determined using empirical station corrections from recent small events in the study areas. The 1937 (ML 5.9) earthquake has an epicenter between the surface traces of the San Jacinto and Buck Ridge faults, and aftershocks suggest about 7 km of rupture predominantly to the northwest. A significant increase in small earthquake activity occurred about 112 yr before this event. The 1954 (ML 6.2) earthquake is located at the southeast end of the mapped trace of the San Jacinto fault, and aftershocks suggest about 15 km of rupture further southeast into an area of folded young sediments with no surface fault trace. This event was preceded by a cluster of small earthquakes which occurred within an 8-hr period 10 weeks before the main event and in the eventual rupture zone. The 1942 (ML 6.3) earthquake is located southwest of the southeast end of the Coyote Creek fault. Large aftershocks of this event are spread over a 15 by 18 km area southwest of the Coyote Creek fault and are not associated with any one fault. The relation of the 1942 event to the San Jacinto fault zone is not simple.

Terpenoid and morphological variability of Pinus quadrifolia and the natural hybridization with Pinus monophylla in the San Jacinto Mountains in California

Monoterpenoids from wood of Pinus quadrifolia and Pinus monophylla from south of the San Jacinto Mountains in southern California were analysed by gas-liquid chromatography (GLC). The number of needles per fascicle, the number of resin canals, and the number of abaxial and adaxial stomatal rows in the needles were determined. Percentages of mycrene, α-pinene, and to a minor extent of camphene, β-pinene, limonene, and β-phellandrene, the above mentioned morphological characteristics and previously obtained data were all used for identification and characterisation of species intermediacy in the San Jacinto area. It was concluded that many P. quadrifolia trees in the mixed Buck Ridge stand and some trees in two pure stands of the same area were hybrids and that sympatric and to a lesser extent allopatric introgression of P. monophylla into P. quadrifolia takes place in the San Jacinto region. Introgression of P. quadrifolia into P. monophylla could not be demonstrated.