Subvertical Fault Planes Research Articles

Tectonic setting of the northwestern andes Constrained by a high-resolution earthquake catalog: Block Kinematics

A high-precision earthquake catalog was generated using source-specific station terms and waveform cross-correlation techniques. This detailed catalog serves to interpret the crustal structure and deformation related to the tectonic setting in Northwestern South America. The Panamá-Chocó Block (PCB) is in contact with the Northwestern Andes causing crustal deformation and hence, fault interaction in the brittle regime leading to a high earthquake production concentrated in the Murindó Seismic Cluster (MSC) which is located in the northeastern corner of the PCB. Slip on the Uramita Fault Zone (UFZ) is a response to the accommodation of the PCB against the North Andean block (NAB), splitting the fault into two segments, marked by an abrupt change in its strike. The north segment of the UFZ, a dextral transform fault (strike = S52°E and near vertical plane), which produced the Mutatá earthquake (2016-09-14, Mw = 6.2). The central segment of the UFZ strikes N12°E and exhibits mainly reverse slip with a subvertical fault plane which produced the 1987-03-19 Mw = 5.4 and 1987-11-11 Mw = 5.3 events. The Murindó sinistral strike-slip fault (strike = N9°W), which produced the great Murindó earthquake (1992-10-18, Mw = 7.1, epicentral intensity XI) is characterized by its wide zone of brittle deformation as a response to internal deformation of the PCB under a compressional NW-SE stress. Intermediate depth earthquakes are widely observed in the area, where we find evidence of the Caribbean slab subducting southwards beneath Panamá. The subduction of the Malpelo microplate to the south, shows high productivity within the Cauca Cluster and the presence of at least three finger-shaped mantle zones of seismicity.

Active Faults Revealed and New Constraints on Their Seismogenic Depth from a High-Resolution Regional Focal Mechanism Catalog in Myanmar (2016–2021)

ABSTRACTWe derive a new earthquake focal mechanism catalog for 86 Mw>4.0 earthquakes that occurred in the Myanmar region from 2016 to 2021. We apply the generalized Cut-and-Paste inversion method to a new set of regional broadband waveform data to obtain the earthquake focal mechanism and centroid depth with uncertainties estimated in a bootstrapping manner. Compared with global earthquake catalogs, our results are better aligned with mapped, active faults and reveal seismic activity along unmapped, blind faults. Our new catalog shows that the Sagaing Fault is more active in its northern segment with deeper seismogenic zone (∼27 km) compared to its southern segment that has a shallower seismogenic zone (∼10 km), sandwiching a seismic gap in its central segment. Earthquakes that occurred on the unmapped, blind faults beneath the Central Myanmar Basin at shallow depths (3–12 km) suggest a dominating northeast–southwest compressional stress field. Shallow earthquakes beneath the Indo-Myanmar Range (IMR) are rare, instead, north–south-oriented strike-slip faults are active within the deep accretionary wedge or lower crust of the Myanmar plate between depths of 20 and 40 km. At the eastern edge of the IMR, earthquakes with high-angle thrust mechanisms occurred between depths of 30 and 48 km, likely along steep faults separating the accretionary wedge from the Myanmar forearc crust. High-resolution intraslab focal mechanisms show that to the north of 22° N, slab deformation is dominated by strike-slip earthquakes with subvertical fault planes down to a depth of ∼25 km beneath the slab, suggesting lateral shear within the slab due to the northward motion of the Indian plate. To the south, more normal-faulting earthquakes suggest a stronger role of plate-bending processes in the slab deformation.

Variable vertical movements and their deformation behaviors at convergent plate suture: 14-year-long (2004–2018) repeated measurements of precise leveling around middle Longitudinal Valley in eastern Taiwan

• We provide 14-year-long leveling results across plate suture of Philippine Sea and Eurasia. • Along the LVF, shallow locked in north, and rapid creeping without locking in south. • The leveling data imply blind slip on the deeper part of the Central Range fault. • Active creeping occurs on a nearly vertical contact along the Chimei fault. To better characterize the vertical movements and the deformation behaviors across the plate suture of an arc-continent collision, we conducted annual repeated measurements on two precise leveling routes in a length of 34 and 37 km, respectively, across the middle part of the Longitudinal Valley in eastern Taiwan in 2004–2018. The 14-year-long results showed that the Longitudinal Valley fault (LVF) dominates the surface deformation: a) the middle LVF (Juisui fault) exhibited partially locked in the upper few kilometers, with a cumulative uplift rate of 9–10 mm/yr in a distance of 4 km; b) the southern LVF (Chihshang fault) showed a creeping behavior with a vertical rate of 24–27 mm/yr. In addition, we are able to characterize other features, including 1) tilting upward to the west in the eastern Central Range, suggesting activity on the west-dipping Central Range fault; 2) the hanging wall of the LVF showed tilting downward behavior to the east; 3) the Chimei fault, a suspected active fault, revealed active slip on the sub-vertical fault plane, that caused a vertical rate of 8–9 mm/yr. Putting the results under global ITRF system, the whole Juisui route was moving downward, supporting the notion that NNW subduction of the Philippine Sea plate starts around the latitude of the middle of the Longitudinal Valley. Finally, the co-seismic vertical deformation of the 2013 M L 6.4 Juisui earthquake was characterized by tilting upward to the west, consistent with stick–slip on the deeper part of a west-dipping interface of the forearc basement.

Source characteristics of the March 16, 2014 Mw 6.7 earthquake and its implications for the Mw 8.2 Pisagua mainshock

The March 16, 2014 Mw 6.7 earthquake played an important role in the complex seismic sequence leading to the nucleation of the April 1st, 2014 Mw 8.2 Pisagua earthquake in northern Chile. The Mw 6.7, an upper plate reverse faulting event with nodal planes highly rotated counterclockwise with respect to the strike of the megathrust at its location, is analyzed. For each nodal plane of the regional W-phase centroid moment tensor solution, the spatiotemporal slip distribution is inverted using near-field records. The optimal space and time smoothing constraints are determined objectively based on the Akaike's Bayesian Information Criterion. We analyze seismicity on the region affected by the Mw 6.7 and Mw 8.2 Pisagua mainshock, with accurate hypocenter locations obtained using a 3D heterogeneous medium. We also performed regional moment tensor inversion of events (M≥4.0) occurred before the Mw 8.2. Our results suggest the sub-vertical fault plane dipping southward (dip∼70°) as being the causative fault of the March 16 Mw 6.7 earthquake. The rupture propagated to the northwest, lasted 15s, and yielded a total seismic moment of 1.36×1019Nm (Mw 6.7). Estimated slip is characterized by total rupture length of ∼22km, and suggest shallow slip distributed between ∼ 3km, down to 17km depth. A comparison between coastal tide gauge records and forward tsunami modeling show similarity of the time series, thus supporting the plausibility of our estimated slip models. Our interpretation suggests that the rupture of the Mw 6.7 intraplate could be explained by stress accommodation between two segments along-dip with different mechanical properties at the interplate boundary, and inside the upper plate.

Evidence for Rupture Through a Double Benioff Zone During the 2017 Mw 8.2 Chiapas, Mexico Earthquake

AbstractWe combine multi‐array relative back‐projection of high‐frequency P waves and finite‐fault modeling of low‐frequency P waves from the 8 September 2017 Mw 8.2 Chiapas, Mexico earthquake to image unilateral rupture on a southeast‐northwest striking subvertical fault plane over depths of 10–70 km. Improved multi‐array relative back‐projection estimates of rupture speed and fault geometry provide key refinements to the slip model from finite‐fault modeling. Compilation of prior seismicity and aftershocks reveals the earthquake initiated in the lower plane of a double Benioff zone (DBZ) and ruptured with large slip across the entire DBZ. The previously aseismic interior does not appear to be due to lack of stress as both planes show downdip extension stresses. Instead, tomographic imaging of DBZs find high Poisson's ratio with fast velocities in the DBZ interior, so we propose that highly hydrated harzburgite generates velocity‐strengthening behavior that inhibits earthquake nucleation, yet it can slip seismically during dynamic weakening from a large throughgoing rupture.

Precise relative earthquake depth determination using array processing techniques

AbstractPrecise determination of hypocentral depth remains one of the most relevant problems in earthquake seismology. It is well known that using depth phases allows for significant improvement in event depth determination; however, routinely and systematically picking such phases, for teleseismic or regional arrivals, is problematic due to poor signal‐to‐noise ratios around the pP and sP phases. To overcome this limitation, we have taken advantage of the additional information carried by seismic arrays. We use velocity spectral analysis to precisely measure pP‐P times. The individual estimates obtained at different subarrays, for all pairs of earthquakes, are combined using a double‐difference algorithm, in order to precisely map seismicity in regions where it is tightly clustered. We illustrate this method by relocating intermediate‐depth earthquakes in the Nazca subducting plate, beneath northern Chile, where we confirm the existence of a narrowly spaced double seismic zone, previously imaged using a local dedicated deployment. As a second example we relocate the aftershock sequence of the 2014 Mw 7.9 intermediate depth, Rat Islands earthquake, and provide evidence of a subvertical fault plane for the main shock. Finally, we show that the resulting relative depth errors are typically smaller than 2 km.

The Red Rock Fault zone (northeast New South Wales): kinematics, timing of deformation and relationships to the New England oroclines

The formation of the northern part of the New England oroclines has previously been linked to dextral strike-slip faulting, but hitherto no concrete evidence has been shown to support this suggestion. We studied an exposure of a fault zone in the Red Rock headland, northeastern New South Wales, and we present here structural observations from the fault zone complemented by geochronological constraints on the timing of faulting and geochemical data that inform us on the nature of the co-seismic fluids. Our observations show evidence for dextral strike-slip faulting, with a reverse kinematic component, along a subvertical fault plane oriented NNE–SSW. Rb–Sr and 40Ar/39Ar ages from fault gouge samples indicate that brittle faulting occurred at 286.5 ± 1.5 Ma with possible reactivations at 267.0 ± 9.6 Ma and 264 ± 11 Ma. Oxygen and hydrogen stable isotope geochemistry indicates that the fluids that circulated in the fault zone were associated with a deep crustal source. Based on these results, we conclude that the Red Rock Fault zone is likely an exposed segment of a larger fault system, that was active at 288–285 Ma (Early Permian). The timing of faulting was contemporaneous with the development of the New England oroclines, raising the possibility that oroclinal bending was accompanied by strike-slip faulting. Whether faulting was associated with local deformation at the limb of the Coffs Harbour Orocline, or with a larger-scale wrench tectonic zone remains unresolved.

The Skovorodino, 2011, earthquake

On October 14, 2011, an earthquake with the magnitude Mw = 6.0 occurred 15 km northwest of the town of Skovorodino in East Siberia. This earthquake aroused interest among geophysicists. In this paper, we present some results on the prior seismicity in the region, which are based on the macroseismic and instrumental data. The assessment of the seismic regime immediately before the main shock and the analysis of the aftershock sequence rely on the data provided by the field network of the Schmidt Institute of Physics of the Earth of the Russian Academy of Sciences. The network was in operation from the end of July 2011 to January 2012. The analysis of the entire dataset suggests that the Skovorodino earthquake was outstanding in a vast region: only seismic events with low magnitudes (at most 4) occurred in the epicentral zone of this earthquake and its nearest neighborhood. The positions of the source of the main shock and the hypocenters of the after-shocks (more than 1300 events) are determined. The aftershock sequence is anomalous—it lacks sufficiently strong seismic events (M ≥ 3.5). The spatiotemporal distribution of the aftershocks shows two clusters of the epicenters to the west and east of the epicentral zone. These clusters drastically differ in their parameters, indicating that the source has a complex structure. Generally, the source can be understood as a shear displacement on the subvertical fault plane trending approximately along the latitude. The size of the source is 17 × 17 km; the average slip is 63 cm, which is at least 30% larger than the characteristic values for the earthquakes with M = 6.0.

Fault contact of the Pieniny Klippen Belt with the Central Carpathian Paleogene Basin (Western Carpathians): new data from a unique temporary exposure in L̓utina village (Eastern Slovakia)

Abstract An exceptional, briefly exposed section across the boundary between the Pieniny Klippen Belt (PKB) and the Central Carpathian Paleogene Basin (CCPB) in Eastern Slovakia showed that the contact is formed by a single, subvertical fault plane. Whereas the Oligocene CCPB deposits exhibit only weak deformation with S-verging fold-and-thrust structures, the Cretaceous PKB sediments are internally strongly sheared with scaly fabric and numerous calcite veins. It is inferred that the PKB suffered strong thrusting- and wrenching-related deformation in pre-Oligocene times, while the Early Miocene oblique backthrusting resulted in exhumation of the PKB and formation of its steep fault boundaries

Fault plane orientations of intermediate‐depth earthquakes in the Middle America Trench

Intermediate‐depth earthquakes are often attributed to dehydration embrittlement reactivating preexisting weak zones. The orientation of presubduction faults is particularly well known offshore of Middle America, where seismic reflection profiles show outer rise faults dipping toward the trench and extending >20 km into the lithosphere. If water is transported along these faults and incorporated into hydrous minerals, the faults may be reactivated later when the minerals dehydrate. In this case, the fault plane orientations should be the same in the outer rise and at depth, after accounting for the angle of subduction. To test this hypothesis, we analyze the directivity of 54 large (MW ≥ 5.7) earthquakes between 35 and 220 km depth in the Middle America Trench. For 12 of these earthquakes, the directivity vector allows us to identify the fault plane of the focal mechanism. Between 35 and 85 km depth, we observe both subhorizontal and subvertical fault planes. The subvertical fault planes are consistent with the reactivation of outer rise faults, whereas the subhorizontal fault planes suggest the formation of new faults. Deeper than 85 km, we only observe subhorizontal faults, indicating that the outer rise faults are no longer being reactivated. The similarity with previous results from the colder Tonga‐Kermadec subduction zone suggests that the mechanism generating these earthquakes, and controlling fault plane orientations, depends on pressure rather than temperature or other tectonic parameters and that the observed rupture characteristics constitute a basic feature of intermediate‐depth seismicity. Exclusively subhorizontal faults may result from isobaric rupture propagation or the hindrance of seismic slip on preexisting weak subvertical planes.

Earthquake mechanics and deformation in the Tonga‐Kermadec subduction zone from fault plane orientations of intermediate‐ and deep‐focus earthquakes

We make use of rupture directivity to analyze 82 deep earthquakes (≥100 km depth) in the Tonga‐Kermadec subduction zone. Identifying the fault planes for 25 of them, we are able to place new constraints on both the physical mechanism of intermediate‐ and deep‐focus earthquakes and deformation within the subducting slab. We find that half of deep earthquakes with MW ≥ 6 have detectable directivity. We compare the obtained fault orientations with those expected for the reactivation of outer‐rise normal faults and with those expected for the creation of new faults in response to the ambient stress field. Earthquakes >300 km depth match the patterns expected for the creation of a new system of faults: we observe both subhorizontal and subvertical fault planes consistent with a downdip‐compressional stress field. Slip along these faults causes the slab to thicken. Rupture propagation shows no systematic directional pattern. In contrast, at intermediate depths (100–300 km), all ruptures propagate subhorizontally and all identified fault planes, whether in the upper or lower region of the double seismic zone, are subhorizontal. Rupture propagation tends to be directed away from the top surface of the slab. After accounting for the angle of subduction, the subhorizontal fault plane orientation is inconsistent with the orientation of outer‐rise normal faults, allowing us to rule out mechanisms that require the reactivation of these large surface faults. Subhorizontal faults are consistent with only one of the two failure planes expected from the slab stress field, suggesting that isobaric rupture processes or preexisting slab structures may also influence the fault plane orientation. If all deformation takes place on these subhorizontal faults, it would cause the slab to thin. Assuming the slab is incompressible, this implies that the slab is also lengthening and suggests that slab pull rather than unbending is the primary force controlling slab seismicity at intermediate depths.

Faulted hybrid joints: an example from the Campo de Dalias (Betic Cordilleras, Spain)

The development of hybrid and faulted joints has not been studied in detail in natural outcrops. This field study in the Campo de Dalias (Betic Cordilleras) establishes the distinctive features of these structures as compared with those predicted by theoretical studies. Hybrid joints appear as two sets of vertical joints forming variable angles, but generally about 25°, with opening directions orthogonal to joint planes. Their development requires low differential stresses with a tensile minimum stress. The main criteria that indicate the existence of faulted joints are: (1) the presence of subvertical fault planes with oblique slip forming a complex pattern of dextral and sinistral faults developed under a single stress regime; (2) faults with similar geometries as joints in the same outcrop, showing an inconsistent cross-cutting relationship; and (3) the development of small basins with variable polarity of asymmetric wedge filling. A permutation in stress orientation since the Tortonian in the Campo de Dalias was responsible for the development of a faulted hybrid joint system, with a constant ENE–WSW extension trend and a switch of σ 1 between NNW–SSE and vertical.

The 1998 Bovec‐Krn Mountain (Slovenia) Earthquake Sequence

We study the 1998 Bovec‐Krn mountain (Slovenia) earthquake sequence by combining hypocenters relocation, strong motion inversion, digital elevation modelling and field geology. The main shock (Ms=5.7), a 12 km right lateral strike‐slip event on the Dinaric fault system, occurred on a sub‐vertical fault plane. The rupture, confined between 3 and 9 km depth, with no evidence of surface faulting, propagated bilaterally within two structural barriers. The northwestern barrier is at the junction between Dinaric and Alpine structures where there is a sharp change in the geometry of faulting. The southeastern barrier is within the Dinaric system and its surface expression corresponds to the Tolminka‐spring perched basin, a 1 km restraining step‐over. At this site, the Bovec‐Krn earthquake‐fault overlaps with a 30 km strike‐slip fault segment that is free of aftershocks and could be undergoing an increase of stress. This fault system represents the northern branch of the Idrija right‐lateral fault.

Three‐dimensional velocity structure, seismicity, and fault structure in the Parkfield Region, central California

This study examines the three‐dimensional velocity structure in a 60‐ by 80‐km region containing the Parkfield segment of the San Andreas fault. We use local earthquake and shot P arrival times in an iterative simultaneous inversion for velocity and hypocentral parameters. Using the three‐dimensional model, we relocated 5251 events that occurred from 1969 to 1991, as well as the 1966 aftershocks, and computed 664 fault plane solutions. The San Andreas fault (SAF), characterized by a sharp across‐fault velocity gradient, is the primary feature in the velocity solution. There is a 5–20% lateral change in velocity over a 4‐km width, the contrast being sharper where there is better resolution. The model also shows significant variations in the velocity and in the complexity of the velocity patterns along the SAF. The largest across fault velocity difference is below Middle Mountain, where a large volume of low‐velocity material impinges on the SAF from the northeast. This material is inferred to be overpressured and may be key to understanding the unusual behavior in the Parkfield preparation zone. A 20‐km‐long high‐velocity slice is imaged northeast of the SAF near Gold Hill. Its along‐fault length corresponds to the length of the maximum slip in 1966. The relocated seismicity shows that the San Andreas fault is a planar vertical fault zone at seismogenic depths. Ninety percent of the fault plane solutions that are on, or near, the SAF were right‐lateral strike‐slip on subvertical fault planes that parallel the SAF. Thus the surface fault complexities do not appear to extend to depth and therefore do not explain the rupture character at Parkfield. At Parkfield, variations in material properties play a key role in fault segmentation and deformation style. Our observations suggest that there may be a general relation between increasing velocity and increasing ability of the rocks to store strain energy and release it as brittle failure.

Aftershock focal mechanisms of the Spitak earthquake

The focal mechanisms of foreshock, main shock, and 190 aftershocks of the Spitak earthquake, which occurred on December 7, 1988, were obtained using the signs of the first motion of body waves. The focal mechanisms of the aftershocks with magnitude M > 4 are completely representative ones and about 70% of the focal mechanisms of the aftershocks with M > 3.3 have been determined. The change in types of focal mechanisms of the aftershocks was observed from the second day of the main shock onwards. The prevailing types of motion on the first day (December, 7, 1988) were normal and strike-slip motions. The majority of the aftershocks beginning December, 8th were characterized by reversed and strike-slip motions. Vertical motions along a subvertical fault plane were observed during the aftershocks. Some characteristic types of focal mechanisms in different parts of the aftershock zone have been selected. Peculiarities of the aftershock distribution and types of focal mechanisms for each part of the zone are discussed. The result of this investigation may create a basis for a model of the aftershock processes.