Evidence for Strain Transfer
Mapped surface rupture sequences in each array in the PVTR are between 5 – 7 km. Displacement-length scaling relationships for strike slip and oblique systems suggest that total offsets of ~0.5 – 1 m generate surface ruptures that are 20 – 30 km long (Wells and Coppersmith, 1994; Wesnousky, 2008). This discrepancy in surface rupture length may be resolved by a substantial distance (~15 - 26 km) of unmapped faults along strike of these arrays. However, this is unlikely as these faults would bisect the Panamint Range and Panamint fault ~10 - 15 km to the north, and project into the Slate Range and the Searles Valley fault ~2 km to the south. While much of this distance is active playa that could rapidly obscure surface rupture trace after an event, currently there is no evidence of rupture in Holocene fans along the Panamint range front that shows that N-NE trending surface ruptures bisecting the NNW trending Panamint fault ~15 km to the north. The more probable explanation is that stress transfer is occurring between some combination of the PVTR, the Ash Hill, the Slate Range-Manly Pass fault, and the central-southern Panamint Valley fault, all with documented late Holocene paleoseismicity.
The Panamint fault, the Ash Hill fault, and the Slate Range-Manly Pass fault system all project into, or are parallel to, the faults in the transtensional relay. An overlap in earthquake ages and a similar kinematic style between the PVTR and the Panamint, Ash Hill or Slate Range faults may suggest a kinematic relationship in Panamint Valley that supports strain transfer between these fault systems. In the following sections, we present evidence correlating earthquake timing and kinematics in the transtensional relay to ruptures on either the Panamint or Ash Hill fault. As the Slate Range detachment and Manly Pass fault lack well constrained ages and kinematics of Holocene faulting (Numelin et al., 2007), this precludes any convincing correlation with the transtensional relay faults of this study.
The Panamint fault, the Ash Hill fault, and the Slate Range-Manly Pass fault system all project into, or are parallel to, the faults in the transtensional relay. An overlap in earthquake ages and a similar kinematic style between the PVTR and the Panamint, Ash Hill or Slate Range faults may suggest a kinematic relationship in Panamint Valley that supports strain transfer between these fault systems. In the following sections, we present evidence correlating earthquake timing and kinematics in the transtensional relay to ruptures on either the Panamint or Ash Hill fault. As the Slate Range detachment and Manly Pass fault lack well constrained ages and kinematics of Holocene faulting (Numelin et al., 2007), this precludes any convincing correlation with the transtensional relay faults of this study.
Comparison of Late Holocene Rupture TIming
The similarity in the timing of earthquakes between the PVTR, the Ash Hill and the Panamint faults supports a temporal correlation between earthquakes on each fault system. A strong temporal correlation of earthquakes could indicate paleoseismic evidence for strain transfer or loading of adjacent faults during a rupture event on either the Ash Hill or the Panamint fault. Below, we discuss the temporal range in which multi-fault rupture(s) may have occurred within Panamint Valley in the late Holocene.
The timing of Event 1 in the PVTR correlates with the timing of Event 1 on the Ash Hill fault, and the central and southern Panamint fault. Event 1 offsets alluvial surfaces in the PVTR that are correlated to offset alluvial surfaces along the Panamint and Ash Hill faults. Assuming these ruptures occurred in the same event or temporally similar events, the rupture(s) associated with Event 1 would have occurred ~500 – 600 years ago. The timing of Event 2 in the PVTR overlaps the timing of Event 2 on the Ash Hill, and the timing of Event 2 on the central and southern Panamint fault. If Event 2 ruptured all three fault systems in the same event or temporally similar events, ruptures associated with Event 2 can be constrained to ~1.8 – 2.2 ka. The timing of Event 3 in the PVTR is a very wide range between 1.6 – 3.6 ka. If we assume that Event 3 in the PVTR occurred prior to the deposition of Qf7 with a maximum age of ~ 2.6 ka, Event 3 likely occurred at the upper end of this age range. This age range of ~2.6 – 3.6 ka for Event 3 in the PVTR correlates well with dated earthquakes for Event 3 on the Ash Hill and southern Panamint fault. If these ruptures occurred in the same event or temporally similar events, the rupture(s) associated with Event 3 likely occurred around ~3.3 – 3.6 ka. The timing of Event 4 in the PVTR correlates with the minimum age bracket of Event 4 on the Panamint fault (Figure 22). While the timing of Event 3 on the Ash Hill overlaps with the timing of Event 4 in the PVTR and on the Panamint fault, the age range for Event 3 is large, and is inferred to be on the lower end of this age range (Regalla et al., 2022). As such, a fourth event is not recorded on the Ash Hill in the last ~4 ka.
Overall, the Ash Hill fault and the Panamint fault record two events since ~2.5 - 2.9 ka, and the transtensional relay records two events since ~2.4 ka. The PVTR and the Panamint fault record four earthquakes since ~4.2 ka, while only three earthquakes are bracketed on the Ash Hill since ~4.3 ka. The overlap in earthquake timing of the last three earthquakes on these three fault systems suggests that they may be rupturing in the same, or closely temporally related events, providing supporting evidence for strain transfer in the late Holocene (Figure 22). The temporal overlap of three late Holocene earthquakes on the PVTR, Ash Hill and Panamint Valley faults support the possibility of strain transfer in the late Holocene during the same event or temporally similar events. The lack of a recorded fourth event on the Ash Hill fault suggests that evidence for a fourth earthquake is poorly resolved on the Ash Hill or that a fourth event ruptured only the PVTR and the Panamint Valley fault.
The timing of Event 1 in the PVTR correlates with the timing of Event 1 on the Ash Hill fault, and the central and southern Panamint fault. Event 1 offsets alluvial surfaces in the PVTR that are correlated to offset alluvial surfaces along the Panamint and Ash Hill faults. Assuming these ruptures occurred in the same event or temporally similar events, the rupture(s) associated with Event 1 would have occurred ~500 – 600 years ago. The timing of Event 2 in the PVTR overlaps the timing of Event 2 on the Ash Hill, and the timing of Event 2 on the central and southern Panamint fault. If Event 2 ruptured all three fault systems in the same event or temporally similar events, ruptures associated with Event 2 can be constrained to ~1.8 – 2.2 ka. The timing of Event 3 in the PVTR is a very wide range between 1.6 – 3.6 ka. If we assume that Event 3 in the PVTR occurred prior to the deposition of Qf7 with a maximum age of ~ 2.6 ka, Event 3 likely occurred at the upper end of this age range. This age range of ~2.6 – 3.6 ka for Event 3 in the PVTR correlates well with dated earthquakes for Event 3 on the Ash Hill and southern Panamint fault. If these ruptures occurred in the same event or temporally similar events, the rupture(s) associated with Event 3 likely occurred around ~3.3 – 3.6 ka. The timing of Event 4 in the PVTR correlates with the minimum age bracket of Event 4 on the Panamint fault (Figure 22). While the timing of Event 3 on the Ash Hill overlaps with the timing of Event 4 in the PVTR and on the Panamint fault, the age range for Event 3 is large, and is inferred to be on the lower end of this age range (Regalla et al., 2022). As such, a fourth event is not recorded on the Ash Hill in the last ~4 ka.
Overall, the Ash Hill fault and the Panamint fault record two events since ~2.5 - 2.9 ka, and the transtensional relay records two events since ~2.4 ka. The PVTR and the Panamint fault record four earthquakes since ~4.2 ka, while only three earthquakes are bracketed on the Ash Hill since ~4.3 ka. The overlap in earthquake timing of the last three earthquakes on these three fault systems suggests that they may be rupturing in the same, or closely temporally related events, providing supporting evidence for strain transfer in the late Holocene (Figure 22). The temporal overlap of three late Holocene earthquakes on the PVTR, Ash Hill and Panamint Valley faults support the possibility of strain transfer in the late Holocene during the same event or temporally similar events. The lack of a recorded fourth event on the Ash Hill fault suggests that evidence for a fourth earthquake is poorly resolved on the Ash Hill or that a fourth event ruptured only the PVTR and the Panamint Valley fault.
Late Holocene Kinematics and Slip Rates for For Faults IN panamint Valley
The Panamint Valley transtensional relay (PVTR) is a right-lateral, oblique-normal fault system. Late Holocene ruptures in the PVTR have kinematics varying from pure strike slip to strike-slip, dip-slip ratios of 5:1 (up to ~1.0 m of lateral slip and ~.20 m of dip-slip recorded in two separate events). Measured total offsets of up to ~.5 - 1.0 m of slip in the PVTR support total surface ruptures between ~20 - 33 km in length and produce earthquakes of Mw ≈ 6.7 - 6.9.Additionally, the PVTR has a slip rate of 0.6 - 0.9 mm/yr for the late Holocene (< 5 ka).
The Ash Hill fault is a right-lateral, oblique-normal fault system, with a strike slip to dip slip ratio between 5:1 and 8:1 (Regalla et al., 2022). On the Ash Hill fault, three late Holocene earthquakes have been constrained by the age of the deposit they offset and the cumulative |
offset on older deposits, each with a lateral and vertical offset of ~1.0 m and ~0.20 m, respectively (Regalla et al., 2022). The average fault orientation for the Ash Hill is 240° 70°W, with a slip vector of ~335°, 15°. The kinematics of the Ash Hill fault support a total strike length of ~40 km, correlative to Mw 6.9 – 7.0 earthquakes (Wells and Coppersmith, 1994; Wesnousky, 2008; Regalla et al., 2022). The late Holocene slip rate for the Ash Hill fault is ~0.7-1.4 mm/yr (Regalla et al., 2022)
The Panamint fault is a right-lateral, oblique-normal fault system, with kinematic variations ranging from pure normal slip in the north to dominantly strike-slip in the south (Figure 2) (McAuliffe et al., 2013; Sethanant, 2019). Southeast of the PVTR, at Goler Canyon, the Panamint fault has an average strike of 160°, with a slip vector of ~2.5 m towards 334° (Figure 2) (Sethanant, 2019). Here, the strike slip to dip slip ratio is 4:1, equating to about ~2.5 m lateral offset and ~.6 m of dip slip in a single event (Sethanant, 2019). North of Manly Peak Canyon (Figure 2), changes in the slip vector and fault orientations result in a change from predominately dextral slip along the southern Panamint fault to predominately normal slip along the northern Panamint fault. At Manly Peak Canyon, the slip vector is 2.2 m toward 310° on faults with an average strike of 330°, equating to a strike slip to dip slip ratio of ~1:1 and about ~1.7 m of vertical, and ~1.7 m of horizontal slip in a single event (Figure 2) (Sethanant, 2019). West of the PVTR, the Panamint fault shallows to a dip of ~15 – 30° approximately north of Big Horn Canyon (Numelin, 2005; Walker et al., 2005; Hoffman, 2009; Sethanant, 2019). Northwest of the PVTR, at Happy Canyon and Pleasant Canyon, the Panamint fault strikes 215° and has a slip vector of 310° (Figure 2) (Sethanant, 2019). This fault and slip vector orientation results in nearly pure dip-slip on this section of the Panamint fault, with 2.5 – 4.2 m of vertical offset measured for a single event and no horizontal offsets measured. Measured single-event offsets support a minimum of 45 – 50 km of surface rupture, equating to a Mw6.9 – 7.2 event (Sethanant, 2019). Additionally, considering the paleoseismic evidence that Event 1 ruptured from Hall Canyon to the Playa Verde paleoseismic trench, this rupture produced a Mw 6.9 – 7.2 earthquake (McAuliffe et al., 2013; Sethanant, 2019). The late Holocene slip rate for the Panamint Valley fault is 2 – 5 mm/yr in the last 5 ka (Sethanant, 2019).
The kinematics of the PVTR, Ash Hill and Panamint Valley faults show that each zone is capable of producing surface rupture between 20 – 50 km, and hosting earthquakes up to Mw 6.9 – 7.2. However, the PVTR is kinematically more similar to the Ash Hill fault compared to the Panamint fault. Strike-slip to dip-slip ratios of 5:1 in the PVTR, and between 5:1 and 8:1 on the Ash Hill, are comparable. Additionally, Event 3 and Event 4 in the PVTR produced the same lateral and vertical slip (1.0 m and 0.2 m, respectively) that the Ash Hill fault produced in all three late Holocene events. However, the PVTR deviates from this magnitude of slip in Event 1 and 2, which produced lateral offsets of ~0.6 m and ~0.8 m, respectively, and negligible dip slip. If these two fault systems are rupturing in tandem, this discrepancy in total offset magnitude may be a result of dissipating slip at the southern tip of the Ash Hill fault. If the PVTR is acting as a fault tip for the Ash Hill in Events 1 and 2, smaller offsets in the PVTR for Events 1 and 2 may indicate that rupture of Ash Hill ended at the PVTR near the southern extent of ruptures mapped for Events 1 and 2 (Figure 20). If the PVTR is acting as a fault tip for the Ash Hill, this would be consistent with smaller observed offsets, and a smaller length of mapped ruptures in the PVTR for Event 1 and 2 compared to Event 3 and 4 (Figure 20). A further line of evidence for coseismic rupture of the PVTR and Ash Hill faults is the similarity in the late Holocene slip rate of 0.6 - 0.9 mm/yr for the PVTR and ~0.7 - 1.4 mm/yr for the Ash Hill. Furthermore, the slip rates of the PVTR and Ash Hill faults do not overlap with the late Holocene slip rates of the Slate Range detachment (~0.17 - 0.29 mm/yr) or the Panamint detachment (2 – 5 mm/yr). These rates show that in the Panamint Valley, only these two fault zones are slipping at approximately the same rates. Small discrepancies between the PVTR and Ash Hill slip rates may be a function of a smaller magnitude of slip in the PVTR during Events 1 and 2.
The trend of the PVTR faults ranges between N-NNE, which is similar to the NNE-NE trend of the Panamint Valley detachment (north of Middle Park Canyon; Walker et al., 2005; Sethanant, 2019) and the approximately N-S orientation of the Searles Valley fault to the south (Figure 2; Numelin et al., 2007). However, the late Holocene kinematics of the PVTR are not similar to late Holocene kinematics of the Panamint fault. The Panamint fault hosts a larger percentage of normal motion ranging from 4:1 right lateral to vertical offset to pure dip slip, compared to right lateral to vertical offset ratios between 8:1 and 5:1 for the Ash Hill fault and the PVTR. Additionally, the Panamint fault has greater total offset of 2.5 – 4.2 m per event, and has faster slip rates of 2 – 5 mm/yr, while the Ash Hill fault and PVTR have a maximum total offset of ~1.00 m per event and slip rates between 0.6 – 1.4 mm/yr. Yet, if the PVTR ruptures in the same earthquake as the Ash Hill fault, this NNE-SSW orientation of faulting is inconsistent with typical fault-tip horsetail structures on a fault with an average strike of ~ 160° (Figure 2; Sylvester, 1988). This orientation of PVTR faulting suggests that while the PVTR may be rupturing in the same events as the Ash Hill fault, it is likely taking advantage of a different orientation of preexisting weakness at depth. Therefore, I propose that there is a preexisting weakness trending N-NNE below the surface trace of the PVTR faults, possibly associated with the plane of the low angle Panamint fault, allowing for rupture associated with the Ash Hill ruptures to initiate at this orientation in the PVTR. If we assume that the Panamint Valley detachment and the Searles Valley detachment were once continuous, at depth there may be a remnant preexisting weakness related to the relic detachment where strain is localizing at depth.
Our offset data provides constraints on a dextral, oblique-normal slip rate for the Panamint Valley transtensional relay in the late Holocene. Assuming that the transtensional relay has experienced ~3.4 m of cumulative right lateral slip from four events since the deposition of Qf6b (~3.85 – 5.89 ka), this suggests a late Holocene slip rate of 0.6 – 0.9 mm/yr. This slip rate is similar to the late Holocene total slip of ~0.7 – 1.4 mm/yr for the Ash Hill fault (Regalla et al., 2022). The Slate Range fault records a similar Quaternary slip rate of 0.5 – 1.6 mm/yr (Smith et al., 1968; Wesnousky, 1986), but a much smaller late Holocene total slip rate of ~0.17 – 0.29 mm/yr (Numelin et al., 2007). The Panamint fault has the largest late Holocene total slip rate of Panamint Valley at 2 – 5 mm/yr in the last 5 ka (Sethanant, 2019), which is ~1.5 – 7 times faster than the slip rate of the Ash Hill fault and ~2 – 8 times faster than the slip rate of the PVTR faults.
In summary, the similarities between the PVTR and the Ash Hill faults with respect to late Holocene earthquake timing, number of events since ~ 3.6 ka, fault kinematics, and late Holocene slip rates suggest that the transtensional relay is rupturing in the same earthquake as the Ash Hill fault. However, a fourth event is not constrained on the Ash Hill in the last ~4 – 5 ka. We favor an interpretation that a rupture either localized on a nearby, parallel fault strand, or that offsets associated with this fourth event are poorly preserved. A similarity in fault orientation in the PVTR compared to the low angle Panamint fault provides evidence that the PVTR may be localizing on a preexisting weakness related to the remnant low angle fault at depth.
The Panamint fault is a right-lateral, oblique-normal fault system, with kinematic variations ranging from pure normal slip in the north to dominantly strike-slip in the south (Figure 2) (McAuliffe et al., 2013; Sethanant, 2019). Southeast of the PVTR, at Goler Canyon, the Panamint fault has an average strike of 160°, with a slip vector of ~2.5 m towards 334° (Figure 2) (Sethanant, 2019). Here, the strike slip to dip slip ratio is 4:1, equating to about ~2.5 m lateral offset and ~.6 m of dip slip in a single event (Sethanant, 2019). North of Manly Peak Canyon (Figure 2), changes in the slip vector and fault orientations result in a change from predominately dextral slip along the southern Panamint fault to predominately normal slip along the northern Panamint fault. At Manly Peak Canyon, the slip vector is 2.2 m toward 310° on faults with an average strike of 330°, equating to a strike slip to dip slip ratio of ~1:1 and about ~1.7 m of vertical, and ~1.7 m of horizontal slip in a single event (Figure 2) (Sethanant, 2019). West of the PVTR, the Panamint fault shallows to a dip of ~15 – 30° approximately north of Big Horn Canyon (Numelin, 2005; Walker et al., 2005; Hoffman, 2009; Sethanant, 2019). Northwest of the PVTR, at Happy Canyon and Pleasant Canyon, the Panamint fault strikes 215° and has a slip vector of 310° (Figure 2) (Sethanant, 2019). This fault and slip vector orientation results in nearly pure dip-slip on this section of the Panamint fault, with 2.5 – 4.2 m of vertical offset measured for a single event and no horizontal offsets measured. Measured single-event offsets support a minimum of 45 – 50 km of surface rupture, equating to a Mw6.9 – 7.2 event (Sethanant, 2019). Additionally, considering the paleoseismic evidence that Event 1 ruptured from Hall Canyon to the Playa Verde paleoseismic trench, this rupture produced a Mw 6.9 – 7.2 earthquake (McAuliffe et al., 2013; Sethanant, 2019). The late Holocene slip rate for the Panamint Valley fault is 2 – 5 mm/yr in the last 5 ka (Sethanant, 2019).
The kinematics of the PVTR, Ash Hill and Panamint Valley faults show that each zone is capable of producing surface rupture between 20 – 50 km, and hosting earthquakes up to Mw 6.9 – 7.2. However, the PVTR is kinematically more similar to the Ash Hill fault compared to the Panamint fault. Strike-slip to dip-slip ratios of 5:1 in the PVTR, and between 5:1 and 8:1 on the Ash Hill, are comparable. Additionally, Event 3 and Event 4 in the PVTR produced the same lateral and vertical slip (1.0 m and 0.2 m, respectively) that the Ash Hill fault produced in all three late Holocene events. However, the PVTR deviates from this magnitude of slip in Event 1 and 2, which produced lateral offsets of ~0.6 m and ~0.8 m, respectively, and negligible dip slip. If these two fault systems are rupturing in tandem, this discrepancy in total offset magnitude may be a result of dissipating slip at the southern tip of the Ash Hill fault. If the PVTR is acting as a fault tip for the Ash Hill in Events 1 and 2, smaller offsets in the PVTR for Events 1 and 2 may indicate that rupture of Ash Hill ended at the PVTR near the southern extent of ruptures mapped for Events 1 and 2 (Figure 20). If the PVTR is acting as a fault tip for the Ash Hill, this would be consistent with smaller observed offsets, and a smaller length of mapped ruptures in the PVTR for Event 1 and 2 compared to Event 3 and 4 (Figure 20). A further line of evidence for coseismic rupture of the PVTR and Ash Hill faults is the similarity in the late Holocene slip rate of 0.6 - 0.9 mm/yr for the PVTR and ~0.7 - 1.4 mm/yr for the Ash Hill. Furthermore, the slip rates of the PVTR and Ash Hill faults do not overlap with the late Holocene slip rates of the Slate Range detachment (~0.17 - 0.29 mm/yr) or the Panamint detachment (2 – 5 mm/yr). These rates show that in the Panamint Valley, only these two fault zones are slipping at approximately the same rates. Small discrepancies between the PVTR and Ash Hill slip rates may be a function of a smaller magnitude of slip in the PVTR during Events 1 and 2.
The trend of the PVTR faults ranges between N-NNE, which is similar to the NNE-NE trend of the Panamint Valley detachment (north of Middle Park Canyon; Walker et al., 2005; Sethanant, 2019) and the approximately N-S orientation of the Searles Valley fault to the south (Figure 2; Numelin et al., 2007). However, the late Holocene kinematics of the PVTR are not similar to late Holocene kinematics of the Panamint fault. The Panamint fault hosts a larger percentage of normal motion ranging from 4:1 right lateral to vertical offset to pure dip slip, compared to right lateral to vertical offset ratios between 8:1 and 5:1 for the Ash Hill fault and the PVTR. Additionally, the Panamint fault has greater total offset of 2.5 – 4.2 m per event, and has faster slip rates of 2 – 5 mm/yr, while the Ash Hill fault and PVTR have a maximum total offset of ~1.00 m per event and slip rates between 0.6 – 1.4 mm/yr. Yet, if the PVTR ruptures in the same earthquake as the Ash Hill fault, this NNE-SSW orientation of faulting is inconsistent with typical fault-tip horsetail structures on a fault with an average strike of ~ 160° (Figure 2; Sylvester, 1988). This orientation of PVTR faulting suggests that while the PVTR may be rupturing in the same events as the Ash Hill fault, it is likely taking advantage of a different orientation of preexisting weakness at depth. Therefore, I propose that there is a preexisting weakness trending N-NNE below the surface trace of the PVTR faults, possibly associated with the plane of the low angle Panamint fault, allowing for rupture associated with the Ash Hill ruptures to initiate at this orientation in the PVTR. If we assume that the Panamint Valley detachment and the Searles Valley detachment were once continuous, at depth there may be a remnant preexisting weakness related to the relic detachment where strain is localizing at depth.
Our offset data provides constraints on a dextral, oblique-normal slip rate for the Panamint Valley transtensional relay in the late Holocene. Assuming that the transtensional relay has experienced ~3.4 m of cumulative right lateral slip from four events since the deposition of Qf6b (~3.85 – 5.89 ka), this suggests a late Holocene slip rate of 0.6 – 0.9 mm/yr. This slip rate is similar to the late Holocene total slip of ~0.7 – 1.4 mm/yr for the Ash Hill fault (Regalla et al., 2022). The Slate Range fault records a similar Quaternary slip rate of 0.5 – 1.6 mm/yr (Smith et al., 1968; Wesnousky, 1986), but a much smaller late Holocene total slip rate of ~0.17 – 0.29 mm/yr (Numelin et al., 2007). The Panamint fault has the largest late Holocene total slip rate of Panamint Valley at 2 – 5 mm/yr in the last 5 ka (Sethanant, 2019), which is ~1.5 – 7 times faster than the slip rate of the Ash Hill fault and ~2 – 8 times faster than the slip rate of the PVTR faults.
In summary, the similarities between the PVTR and the Ash Hill faults with respect to late Holocene earthquake timing, number of events since ~ 3.6 ka, fault kinematics, and late Holocene slip rates suggest that the transtensional relay is rupturing in the same earthquake as the Ash Hill fault. However, a fourth event is not constrained on the Ash Hill in the last ~4 – 5 ka. We favor an interpretation that a rupture either localized on a nearby, parallel fault strand, or that offsets associated with this fourth event are poorly preserved. A similarity in fault orientation in the PVTR compared to the low angle Panamint fault provides evidence that the PVTR may be localizing on a preexisting weakness related to the remnant low angle fault at depth.