Strain Partitioning
Given that displacement-length scaling relationships support that the PVTR cannot rupture alone without coseismic rupture on a larger fault system, and the PVTR is kinematically more similar to the Ash Hill fault, I propose that the PVTR must be rupturing in the same event as the Ash Hill fault. Additionally, an overlap in timing of late Holocene earthquakes between the PVTR, Ash Hill and Panamint faults indicates that strain transfer may be occurring between the Ash Hill and Panamint faults.
One piece of evidence for strain transfer between the Panamint fault and the Ash Hill fault is the change in kinematics of the Panamint fault between the central and southern segments. Cumulative offset and paleoseismic trenching shows that the most recent event, Event 1, on the Panamint fault ruptured from Happy Canyon to the Playa Verde trench (McAuliffe et al., 2013; Sethanant, 2019). However, the kinematics of the Panamint fault change from primarily normal to primarily dextral along the length of the fault (Figure 2). South of the PVTR, at Goler Canyon, the main slip vector on the Panamint fault is transtensional, with a 4:1 strike slip to vertical slip ratio (Figure 2). At Manly Peak Canyon (north of Goler Canyon), where the strike slip to dip slip ratio is 1:1, ~1.7 m of right lateral and ~1.7 m of vertical offset is recorded for a single event (Figure 2). Northwest of the PVTR, at Happy Canyon, the main slip vector resolves on the central Panamint fault as pure normal slip, with pure dip slip measured for Event 1 at this location (Figure 2) (Sethanant, 2019). As a result, at Happy Canyon the total slip for a single event is ~2.5 – 4.2 m of vertical offset, while ~15 km to the south the total slip for a single event is resolved into ~1.7 m of right lateral and ~1.7 m of vertical offset. If the paleoseismic records are correct and the central and southern sections of the Panamint fault have ruptured in the same events, only the westerly (normal) component of the regional transtensional (NW-SE) slip vector is being accommodated by the low angle normal fault north of the transition between southern and central Panamint fault kinematics. Therefore, it is reasonable to assume that the more northerly component of the slip vector is being accommodated elsewhere in Panamint Valley.
Conceptually, according to Mohr-Coulomb failure criteria, it is not kinematically favorable for lateral motion to occur on a low angle normal fault, and normal slip is also unlikely. Firstly, the low angle Panamint fault must be exceptionally weak to accommodate observed dip-slip (Numelin, 2005). Secondly, if the low angle Panamint fault cannot accommodate lateral motion, the lateral component resolved on Panamint Valley faults by the regional slip vector is more likely accommodated elsewhere. Given the change in kinematics between the central and southern Panamint fault, the kinematic similarity between the Ash Hill and the PVTR, and the similarity in orientation between the PVTR and central Panamint fault, I suggest that regional slip is being partitioned between the Ash Hill-PVTR fault systems and the Panamint fault. The partitioning of slip into thrust and strike-slip components on different fault systems commonly occurs at mature, oblique-convergent subduction zones (Northern Andes, Schütt and Whipp, 2020; Southern Andes, Rosenau et al., 2006; Southwest Japan, Gutscher, 2001; Northern Sumatra, McCaffrey et al., 2000). While less attention has been given to strain partitioning in transtensional zones, modern and geologic examples of local (North Aegean Trough; McNiel et al., 2004) and regional (central Walker Lane; Surpless, 2008) strain partitioning in the brittle regime and the ductile regime (Mesozoic Atacama fault system, Chile; Cembrano et al., 2005) may be analog to the partitioning of strain in Panamint Valley during the late Holocene.
In the scenario of strain partitioning in Panamint Valley, the normal and lateral components resolved on faults by the regional slip vector are being accommodated by the Panamint fault and the Ash Hill-PVTR faults, respectively. In this case, the PVTR, through strain localization onto a pre-existing weakness in the lower crust, is acting as a relay to transfer strain between the PVTR and the Ash Hill fault. This idea is supported by comparable magnitudes of ~ 1m of dextral slip on the Ash Hill fault and ~ 1.7 m of dextral slip on the Manly Peak section of the Panamint fault and the Ash Hill fault (Figure 2). While the exact mechanism of this strain transfer cannot be resolved by this study, there are a few mechanisms that could support strain transfer through the PVTR.
One piece of evidence for strain transfer between the Panamint fault and the Ash Hill fault is the change in kinematics of the Panamint fault between the central and southern segments. Cumulative offset and paleoseismic trenching shows that the most recent event, Event 1, on the Panamint fault ruptured from Happy Canyon to the Playa Verde trench (McAuliffe et al., 2013; Sethanant, 2019). However, the kinematics of the Panamint fault change from primarily normal to primarily dextral along the length of the fault (Figure 2). South of the PVTR, at Goler Canyon, the main slip vector on the Panamint fault is transtensional, with a 4:1 strike slip to vertical slip ratio (Figure 2). At Manly Peak Canyon (north of Goler Canyon), where the strike slip to dip slip ratio is 1:1, ~1.7 m of right lateral and ~1.7 m of vertical offset is recorded for a single event (Figure 2). Northwest of the PVTR, at Happy Canyon, the main slip vector resolves on the central Panamint fault as pure normal slip, with pure dip slip measured for Event 1 at this location (Figure 2) (Sethanant, 2019). As a result, at Happy Canyon the total slip for a single event is ~2.5 – 4.2 m of vertical offset, while ~15 km to the south the total slip for a single event is resolved into ~1.7 m of right lateral and ~1.7 m of vertical offset. If the paleoseismic records are correct and the central and southern sections of the Panamint fault have ruptured in the same events, only the westerly (normal) component of the regional transtensional (NW-SE) slip vector is being accommodated by the low angle normal fault north of the transition between southern and central Panamint fault kinematics. Therefore, it is reasonable to assume that the more northerly component of the slip vector is being accommodated elsewhere in Panamint Valley.
Conceptually, according to Mohr-Coulomb failure criteria, it is not kinematically favorable for lateral motion to occur on a low angle normal fault, and normal slip is also unlikely. Firstly, the low angle Panamint fault must be exceptionally weak to accommodate observed dip-slip (Numelin, 2005). Secondly, if the low angle Panamint fault cannot accommodate lateral motion, the lateral component resolved on Panamint Valley faults by the regional slip vector is more likely accommodated elsewhere. Given the change in kinematics between the central and southern Panamint fault, the kinematic similarity between the Ash Hill and the PVTR, and the similarity in orientation between the PVTR and central Panamint fault, I suggest that regional slip is being partitioned between the Ash Hill-PVTR fault systems and the Panamint fault. The partitioning of slip into thrust and strike-slip components on different fault systems commonly occurs at mature, oblique-convergent subduction zones (Northern Andes, Schütt and Whipp, 2020; Southern Andes, Rosenau et al., 2006; Southwest Japan, Gutscher, 2001; Northern Sumatra, McCaffrey et al., 2000). While less attention has been given to strain partitioning in transtensional zones, modern and geologic examples of local (North Aegean Trough; McNiel et al., 2004) and regional (central Walker Lane; Surpless, 2008) strain partitioning in the brittle regime and the ductile regime (Mesozoic Atacama fault system, Chile; Cembrano et al., 2005) may be analog to the partitioning of strain in Panamint Valley during the late Holocene.
In the scenario of strain partitioning in Panamint Valley, the normal and lateral components resolved on faults by the regional slip vector are being accommodated by the Panamint fault and the Ash Hill-PVTR faults, respectively. In this case, the PVTR, through strain localization onto a pre-existing weakness in the lower crust, is acting as a relay to transfer strain between the PVTR and the Ash Hill fault. This idea is supported by comparable magnitudes of ~ 1m of dextral slip on the Ash Hill fault and ~ 1.7 m of dextral slip on the Manly Peak section of the Panamint fault and the Ash Hill fault (Figure 2). While the exact mechanism of this strain transfer cannot be resolved by this study, there are a few mechanisms that could support strain transfer through the PVTR.
Possible (Preliminary) Models for Strain Transfer
Understanding how strain is accommodated and transferred between adjacent faults is important to understanding the local and regional stress field and how earthquakes in the ECSZ can be spatio-temporally clustered. Temporal variations in the stress field, affected by post-seismic processes, may change how faults are loaded through time, and thus the recurrence intervals of large earthquakes (Kenner and Simons, 2004). In the following sections, we propose a few different scenarios by which the PVTR may act as a zone for strain transfer between the Ash Hill and Panamint Valley fault, leading to spatio-temporal clustering of earthquakes on these two fault systems.
Firstly, strain accommodation in Panamint Valley may be facilitated by co-seismic, dynamic, or static stress transfer if the Ash Hill, and kinematically-related PVTR faults sole into the low angle Panamint Valley fault at depth (Supplemental Figure 3a-d). This is possible given the high-angle (~70 – 90°) westward dipping geometry of the Ash Hill fault, the high-angle (~70 – 90°) westward and eastward dipping PVTR surface ruptures, and the westward, gently to moderately dipping (~15 – 35°) Panamint Fault (Burchfiel et al., 1987; Hoffman, 2009; Sethanant, 2019). These fault geometries would suggest that the Ash Hill fault and the PVTR ruptures are physically connected to the Panamint fault at depth, providing a direct pathway for rupture to propagate along, or trigger, connecting faults from ~3 – 12 km depth to the surface (Supplemental Figure 3a-d) (Regalla et al., 2022). One possible scenario is that rupture initiates on a weak Panamint Valley detachment that (primarily) accommodates normal motion (Supplemental Figure 3a). As rupture propagates up dip, the high angle Ash Hill and PVTR faults are triggered to rupture in the same earthquake and (primarily) accommodate the dextral strike-slip component as strain is partitioned between multiple fault networks. Additionally, the Panamint fault, the Ash Hill fault and the PVTR faults do not need to rupture in the same earthquake if they are geometrically linked at depth, as failure of the low angle detachment could load strain and facilitate failure on the Ash Hill and PVTR faults in another earthquake that occurs shortly after the initial Panamint fault rupture.
Firstly, strain accommodation in Panamint Valley may be facilitated by co-seismic, dynamic, or static stress transfer if the Ash Hill, and kinematically-related PVTR faults sole into the low angle Panamint Valley fault at depth (Supplemental Figure 3a-d). This is possible given the high-angle (~70 – 90°) westward dipping geometry of the Ash Hill fault, the high-angle (~70 – 90°) westward and eastward dipping PVTR surface ruptures, and the westward, gently to moderately dipping (~15 – 35°) Panamint Fault (Burchfiel et al., 1987; Hoffman, 2009; Sethanant, 2019). These fault geometries would suggest that the Ash Hill fault and the PVTR ruptures are physically connected to the Panamint fault at depth, providing a direct pathway for rupture to propagate along, or trigger, connecting faults from ~3 – 12 km depth to the surface (Supplemental Figure 3a-d) (Regalla et al., 2022). One possible scenario is that rupture initiates on a weak Panamint Valley detachment that (primarily) accommodates normal motion (Supplemental Figure 3a). As rupture propagates up dip, the high angle Ash Hill and PVTR faults are triggered to rupture in the same earthquake and (primarily) accommodate the dextral strike-slip component as strain is partitioned between multiple fault networks. Additionally, the Panamint fault, the Ash Hill fault and the PVTR faults do not need to rupture in the same earthquake if they are geometrically linked at depth, as failure of the low angle detachment could load strain and facilitate failure on the Ash Hill and PVTR faults in another earthquake that occurs shortly after the initial Panamint fault rupture.
Supplemental Figure 3: Three possible scenarios for how strain transfer may occur in Panamint Valley during the Holocene. If fault geometries allow for a physical link at depth, the Ash Hill and PVTR faults may sole into the Panamint Valley fault at depth (a-d), suggesting that all three fault zones may be capable of rupturing in the same earthquake. However, if the Panamint Valley detachment is no longer active (relict), then the PVTR still provides a zone for strain to transfer across <5 km distances a. Rupture initiates downdip on the Panamint Valley detachment. b. Rupture propagates up dip, triggering rupture on the high angle Ash Hill fault resulting in strain partitioning of dextral slip on the Ash Hill fault and normal slip on the Panamint Valley detachment. c. Rupture continues to propagate up dip, triggering the PVTR faults, continuing to partition strain on multiple fault systems. d. Rupture reaches the high angle faults imaged along the uppermost crust along the base of the Panamint range. e. If the Panamint fault is no longer active and is cut by high angle faults (Gold et al., 2020), rupture may initiate on these high angle faults, accommodating both normal and dextral motion. f. Seismic waves from the Panamint fault earthquake reach the PVTR, potentially transferring some strain onto these faults. g. Pushed to failure by added strain from the Panamint fault earthquake, the PVTR ruptures sometime later. h. The PVTR and Ash Hill rupture together in a single event, as the rupture propagates north. i. If rupture instead initiates on the Ash Hill fault, the rupture can propagate south towards the PVTR. j. The PVTR ruptures in the same event as the Ash Hill fault, creating seismic waves from the rupture front that can transfer strain and load the nearby Panamint Valley fault. k. Sometime later, the additional strain resolved on the Panamint fault caused by the Ash Hill-PVTR rupture critically stresses the Panamint fault, leading to failure and earthquake initiation.
If instead the Panamint Valley detachment is no longer active, and both normal and dextral slip is distributed on the eastern side of Panamint Valley along high-angle faults at the surface (Gold et al., 2020), strain may be transferred from the Panamint fault to the Ash Hill fault across the en échelon stepping PVTR fault network (Supplemental Figure 3e-k). I propose a model that depicts the PVTR acting as a strain transfer zone, decreasing the distance between adjacent fault segments in the stepover between the Ash Hill fault and the Panamint fault. In this model, the PVTR localizes rupture on pre-existing weaknesses at depth, possibly associated with the relict low angle detachment, capable of initiating earthquakes and decreasing the distance between the southern tip of the Ash Hill fault and westernmost trace of the central Panamint Valley fault. A smaller distance in stepovers between adjacent fault segments supports rupture transfer and earthquake “jumping”, assisting in rupture nucleation on the adjacent fault segments in either the same event or closely temporally spaced events. This model is supported by the orientation of the PVTR faults, such that they parallel the surface trace of the central Panamint fault and display inconsistent geometries with respect to Ash Hill-related fault tip horsetail splays. Comparable surface orientations of the low angle Panamint fault and the low angle Slate Range fault (Figure 2) suggest that these faults were once a continuous low angle fault plane that has been subsequently torn apart by Pliocene-Quaternary transtension. A similar orientation of the PVTR faults to this proposed relict Panamint - Slate Range fault zone suggests that the PVTR faults could be reoccupying a section of the relict low-angle fault at depth. While the PVTR ruptures may initiate on this relict fault plane, new, high-angle fault pathways form across the basement-sedimentary basin strength contrast that correspond to more kinematically favorable, dominantly dextral-oblique slip on high-angle, faults at the surface (Figure 23e-k). In my proposed scenarios, rupture may initiate on either the Panamint fault (Supplemental Figure 3e-h) or the Ash Hill fault (Supplemental Figure 3i-k), propagating along or triggering rupture on PVTR faults, allowing for fault interaction between adjacent fault segments over distances <5 km. By decreasing the distance between the Ash Hill and Panamint faults, the PVTR may assist in coseismic loading and strain partitioning in Panamint Valley.
These models portraying that the Ash Hill, PVTR and Panamint faults rupture in the same or closely temporally spaced events are supported by the change in Panamint Valley kinematics from north to south. If the lateral motion identified along the southern Panamint fault is not accommodated along the northern and central Panamint fault segments, this strain may be loaded onto other faults in the upper crust with more preferred geometries to accommodate dextral slip. Strain partitioning on this scale is kinematically favorable, as reactivation of basement and upper crustal weaknesses in favorable orientations minimizes the amount of work done by faults in unfavorable orientations (such as the low angle Panamint fault) in the current transtensional stress regime (Jones and Tanner, 1995). Historical earthquakes in the ECSZ (1992 Mw 7.3 Landers, 1999 Mw 7.2 Hector Mine, 2010 Mw 7.2 El Mayor-Cucapah) displayed fault “jumping” characteristics across kilometer-scale separations between fault tips, previously assumed to be barriers to rupture. Pre-existing weaknesses in the upper crust, such as those that may initiate rupture in the PVTR, may act to transfer strain, assisting in multi-fault rupture and linkage of separate fault zones across zones previously assumed to act as rupture barriers. Therefore, areas where complex and diffuse faulting occurs between larger fault strands must be identified to locate where multi-fault rupture may occur. The Uniform California Earthquake Rupture Forecast (UCERF3) model currently only considers the hazard of multi-fault ruptures for fault separations < 5 km (Madden et al., 2013). While the surface trace of the Panamint and the Ash Hill fault are roughly ~ 10 km apart, the PVTR cuts the distance between adjacent fault segments in half to ~4.6 - 5.8 km between the termination of the Ash Hill and the northern extent of PVTR ruptures, and ~ 4 km between the PVTR and the trace of the Panamint fault. The PVTR thus provides a structural link between adjacent fault segments that conform to the current UCERF3 model for multi-fault rupture and earthquake “jumping”. Therefore, zones such as the PVTR are crucial to locate in zones of large fault separations to characterize seismic hazard in areas previously assumed to act as rupture barriers.
These models portraying that the Ash Hill, PVTR and Panamint faults rupture in the same or closely temporally spaced events are supported by the change in Panamint Valley kinematics from north to south. If the lateral motion identified along the southern Panamint fault is not accommodated along the northern and central Panamint fault segments, this strain may be loaded onto other faults in the upper crust with more preferred geometries to accommodate dextral slip. Strain partitioning on this scale is kinematically favorable, as reactivation of basement and upper crustal weaknesses in favorable orientations minimizes the amount of work done by faults in unfavorable orientations (such as the low angle Panamint fault) in the current transtensional stress regime (Jones and Tanner, 1995). Historical earthquakes in the ECSZ (1992 Mw 7.3 Landers, 1999 Mw 7.2 Hector Mine, 2010 Mw 7.2 El Mayor-Cucapah) displayed fault “jumping” characteristics across kilometer-scale separations between fault tips, previously assumed to be barriers to rupture. Pre-existing weaknesses in the upper crust, such as those that may initiate rupture in the PVTR, may act to transfer strain, assisting in multi-fault rupture and linkage of separate fault zones across zones previously assumed to act as rupture barriers. Therefore, areas where complex and diffuse faulting occurs between larger fault strands must be identified to locate where multi-fault rupture may occur. The Uniform California Earthquake Rupture Forecast (UCERF3) model currently only considers the hazard of multi-fault ruptures for fault separations < 5 km (Madden et al., 2013). While the surface trace of the Panamint and the Ash Hill fault are roughly ~ 10 km apart, the PVTR cuts the distance between adjacent fault segments in half to ~4.6 - 5.8 km between the termination of the Ash Hill and the northern extent of PVTR ruptures, and ~ 4 km between the PVTR and the trace of the Panamint fault. The PVTR thus provides a structural link between adjacent fault segments that conform to the current UCERF3 model for multi-fault rupture and earthquake “jumping”. Therefore, zones such as the PVTR are crucial to locate in zones of large fault separations to characterize seismic hazard in areas previously assumed to act as rupture barriers.