Left-lateral Slip Component Research Articles

Structural geometry and active faulting along the southern segment of the Longmenshan thrust belt: Insights into the seismotectonics of the 1327 Tianquan and 2013 Lushan earthquakes (eastern Tibetan Plateau, China)

The 2008 Mw 7.9 Wenchuan and 2013 Mw 6.7 Lushan earthquakes ruptured the central and southern segments of the Longmenshan thrust belt on the eastern Tibetan Plateau, respectively. The ∼M 7 Tianquan earthquake in 1327 is also thought to have occurred on the southern segment with effects spatially overlapping those of the Lushan earthquake, but the deformation style and seismogenic mechanism of the segment remain poorly understood. Satellite image interpretation, field investigations, and trenching reveal that the Shuangshi-Dachuan fault (SDF) on the southern segment displaces landforms, dips southeast, and is dominated by thrust motion with a left-lateral strike-slip component. The latest and penultimate events along the fault occurred in 1085–1457 CE and 1215 BCE-104 CE, respectively. Some ancient landslides exits along the fault, and one of them formed close to 703 ± 17 yr BP. The ages of the latest event and of the landslide agree well with the occurrence of the Tianquan earthquake, indicating that the earthquake was most likely generated by the SDF. Magnetotelluric imaging and the aftershock belt of the Lushan earthquake show that the Longmen Shan forms a wedge-shaped high-resistivity body protruding eastward. The SDF corresponds well to the east-dipping upper electrical boundary of the wedge-shaped body and is interpreted as a shallow backthrust; the seismogenic fault of the Lushan earthquake corresponds to the west-dipping lower boundary and is interpreted as a blind thrust fault. The left-lateral slip component of the SDF may be a response to the lateral extension of the weak lower crust.

Coseismic and early postseismic deformation of the 2020 Nima Mw 6.4 earthquake, central Tibet, from InSAR and GNSS observations

The 2020 Mw6.4 Nima earthquake is one of the largest normal-faulting earthquakes recently occurring north of the Banggong suture zone in remote central Tibet, where geologic investigation of active faults is extremely limited. We analyze jointly InSAR and GNSS observations over 9 months after the Nima earthquake and calculate the coseismic and postseismic displacement. The optimal coseismic slip model suggests this event is the result of moderate-angle down-dip slip on a complex reversed “S-shape” three-segmented structure at fault junctions of the West Yibu-Chaka fault, the Heishi fault, and an unmapped blind fault, with a small component of left-lateral slip. The superposition of seismic waves from faults with different strikes and dips accounted for a large non-double-couple component in the long-period point-source solutions. The geodetic moment released by the mainshock is 6.4 × 1018 N⋅m, equivalent to Mw 6.42. Coseismic rupture concentrated at a depth of 4–15 km, with a peak slip of 1.36 m at 8.5 km depth. The cumulative afterslip moment within 9 months after the mainshock is 1 × 1018 N m, about 15.6% of that released by the mainshock coseismic slip. The afterslips contributed largely to the release of additional strain energy. In addition, shallow creep on the northern part of the blind fault, and deep uplift on the east normal fault system are promoted by stress perturbations. A significant proportion of down-dip coseismic slip spreading to more than 20 km beneath the surface, and deep up-slip afterslip have implications for the rheology of down-dip extension of the dipping faults in northern Tibet. Two obvious stress loading zones of more than 1 bar highlight seismic hazards in the region, especially in the junction between normal faults and ends of the large-size sinistral Riganpei-Co and Jiangai-Zangbu faults. It is necessary to forecast accurately by longer-term afterslip observation over timescales of years for the faults. Compared with previous studies, our results suggest a more complex subsurface fault geometry linking the normal and strike-slip faults and dynamic stress adjustment in this poorly-known region of Tibet.

Two-Dimensional Boundary Element Method Application for Surface Deformation Modeling around Lembang and Cimandiri Fault, West Java

Lembang and Cimandiri fault are active faults in West Java that thread people near the faults with earthquake and surface deformation risk. To determine the deformation, GPS measurements around Lembang and Cimandiri fault was conducted then the data was processed to get the horizontal velocity at each GPS stations by Graduate Research of Earthquake and Active Tectonics (GREAT) Department of Geodesy and Geomatics Engineering Study Program, ITB. The purpose of this study is to model the displacement distribution as deformation parameter in the area along Lembang and Cimandiri fault using 2-dimensional boundary element method (BEM) using the horizontal velocity that has been corrected by the effect of Sunda plate horizontal movement as the input. The assumptions that used at the modeling stage are the deformation occurs in homogeneous and isotropic medium, and the stresses that acted on faults are in elastostatic condition. The results of modeling show that Lembang fault had left-lateral slip component and divided into two segments. A lineament oriented in southwest-northeast direction is observed near Tangkuban Perahu Mountain separating the eastern and the western segments of Lembang fault. The displacement pattern of Cimandiri fault shows that Cimandiri fault is divided into the eastern segment with right-lateral slip component and the western segment with left-lateral slip component separated by a northwest-southeast oriented lineament at the western part of Gede Pangrango Mountain. The displacement value between Lembang and Cimandiri fault is nearly zero indicating that Lembang and Cimandiri fault are not connected each other and this area is relatively safe for infrastructure development.

Meso- and Microstructural Analysis of Coseismic Shear Zone of the 1999 MW 7.6 Chi-Chi Earthquake, Taiwan

The surface rupture of the 1999 M w 7.6 Chi-Chi (Taiwan) earthquake was controlled primarily by the pre-existing Chelungpu fault zone, which is distributed in a wide zone of up to 60 m, although the main coseismic slip is localized in a narrow shear zone of <0.3 cm. The Chelungpu fault zone is composed of cataclasite, fault breccia, and gouge zones that are well observed in both the fault outcrops and the drill cores taken throughout the fault zone. The foliations developed in the cataclasite and fault breccia zones are oriented parallel to that of the fault gouge zone where the main coseismic slip occurred. The structural analyses of the shear zone and fault rocks show that the Chelungpu fault zone has slipped as a thrust fault with a significant left-lateral slip component since its formation in the Pleistocene. This oblique thrust motion is caused by the southeast–northwest oblique convergence between the Philippine Sea plate and the Eurasian plate.

The development of the North Giudicarie fault zone (Insubric line, Northern Italy)

The kinematics of transpressive fault zones is generally characterized by a variable degree of strain partitioning. This process is enhanced by the presence of older faults, which partition the simple shear strain component more easily. The North Giudicarie line can be taken as a good example of strain partitioning because it is a left-lateral transpressive fault developed during the Oligocene–Miocene along an inherited normal fault system of late-Triassic to Cretaceous age. This paper analyzes two main points related to the kinematics of the North Giudicarie line: (1) the role of the inherited faults in the development of the North Giudicarie fault zone, and (2) the mechanisms of strain partitioning. These processes have been analyzed by structural analysis and detailed mapping of the North Giudicarie fault zone. The main tectonic contact of the North Giudicarie line is a northwest dipping reverse fault, which juxtaposes the Austroalpine basement rocks against the Southalpine cover. Oligocene foliated tonalites, together with basement- and limestone-derived mylonites, are found along the main fault plane. The brittle fault zone, up to 1 km thick, is mostly developed within the Southalpine cover rocks. This area is made up of several tectonic lenses, up to 1 km thick and several km long, bounded by anastomosing faults characterized by a normal offset. North-striking faults (e.g., the Trento-Cles line) branch from the North Giudicarie fault zone. Thrusting along the North Giudicarie line and left-lateral strike-slip along the anastomosing faults and the Trento-Cles line has been documented from shear-sense indicators observed in mylonites and small-scale fault surfaces. Stratigraphic data reveal that the Trento-Cles line reactivated pre-existing late Triassic or jounger normal faults. Therefore, it is suggested that the left-lateral slip component took place more easily along inherited faults. Deformation and metamorphic features of mylonites, and the estimated emplacement depth of the Oligocene foliated tonalites, can give a bulk estimate of the vertical displacement along the North Giudicarie line. The calculated thrust slip (∼21 km) is lower than the cumulative left-lateral slip along the whole Giudicarie fault system, as inferred from published regional data.

Source parameter determination of regional earthquakes in the Far East using moment tensor inversion of single-station data

The source mechanisms of the Kobe earthquake on 16 January 1995 and the Neftegorsk on 27 May 1995, including their strongest aftershocks as well as two recent South Korean earthquakes of Youngwol on 13 December 1996 and Kyongju on 25 June 1997 are calculated using moment tensor inversion technique (Green's functions are calibrated for path of source to receiver for these earthquakes). Results derived from data of only one or two stations for Kobe and Neftegorsk mainshocks show a good agreement with those of other authors. The inversion solutions for aftershocks of the Kobe and the Neftegorsk earthquakes turn out to be very similar to their mainshocks. The focal mechanism for the Yongwol earthquake on 13 December 1996 is found to be a right-lateral slip event with a NE strike, and the Kyongju earthquake on 25 June 1997 is found to be an oblique reverse fault with a slight component of left-lateral slip in the NE direction ( Kim and Kraeva, 1999).

Source parameter determination of local earthquakes in Korea using moment tensor inversion of single station data

AbstractThe purpose of this investigation is to determine source parameters such as focal mechanism, seismic moment, moment magnitude, and source depth from recent small earthquakes in the Korcan Peninsula using broadband records of three-component single station. It is very important and worthwhile to use a three-component single station in Korea because for most Korean earthquakes it is not possible to read enough first motions of P-wave arrivals because of the poor coverage of the seismic network and the small size (ML 5.0 or less) of the events. Furthermore the recent installation of the very broadband seismic stations in Korea and use of a 3D tomography technique can enhance moment tensor inversion to determine the source parameters of small earthquakes (ML 5.0 or less) that occur at near-regional distances (Δ ≤ 500 km).The focal solution for the Youngwol earthquake of 13 December 1996 is found to be a right-lateral strike slip event with a NE strike, and the Kyongju earthquake of 25 June 1997 is found to be an oblique reverse fault with a slight component of left-lateral slip in the SE direction.

Revised hypocentre and fault rupture geometry for the 1980 November 23 Campania-Basilicata earthquake in southern Italy

SUMMARY The fault rupture nucleation point of the 1980 November 23 Campania-Basilicata earthquake is relocated, following the recent identification of ∼20 km of surface faulting, the Carpineta and Picentini fault scarps, in addition to the ∼15 km previously documented on the Marzano and San Gregorio faults, all of which have ∼NW strike and ∼60° NE dip. This relocation indicates a range of revised origin times and nucleation point positions at 10-12 km depth that are ∼5–9 km SE of previously documented coordinates, between 18:34:52.0 ± 0.3 s, with latitude 40.724°± 1.4 km and longitude 15.414°± 1.4 km (preferred), and 18:34:52.5 ± 0.3 s, with latitude 40.742°± 1.4 km and longitude 15.373°± 1.4 km. My preferred nucleation point coincides with a downdip projection of the SE end of the Carpineta fault and indicates that this fault ruptured first, rather than the Marzano fault as was previously thought. With the fault rupture nucleation point adjusted to this new preferred position, field and seismological estimates of seismic moment match well, both overall and for individual fault scarps, and suggest the following sequence of fault rupture subevents. The initial fault rupture nucleated at or near the SE end of the Carpineta fault and propagated NW, releasing ∼2.5 × 1018 N m seismic moment. Rupture continued apparently without interruption onto the adjoining Marzano fault farther NW, where ∼6.5 × 1018N m of seismic moment was released. Rupture then paused for ∼0.5s, before continuing NW along the Picentini fault, where ∼4.5 × 1018N m more seismic moment was released. Probably early in this sequence of NW-propagating ruptures, a SE-propagating rupture released ∼2 × 1018 N m seismic moment on the San Gregorio fault. Each of these fault scarps and the corresponding rupture subevent was associated with intense aftershock activity. The existence of another aftershock cluster NW of the Picentini scarp suggests a fifth mainshock rupture subevent, at Castelfranci, which released up to ∼2 × 1018 N m more seismic moment. Rupture at this locality appears necessary to explain the form of ground acceleration recorded nearby, also. Following these ruptures on steep NE-dipping normal faults, two additional ruptures occurred on faults with different orientations, ∼20 s and ∼40 s after the initial mainshock rupture. The 40 s subevent involved release of ∼3 × 1018 N m of seismic moment on a steep normal fault that dips SW at ∼70° and projects to the earth's surface ∼11 km NE of the Marzano fault. The 20 s subevent apparently involved release of ∼4 × 1018 N m of seismic moment on a surface dipping NE at ∼20°, at the base of the brittle upper crust beneath this steep antithetic fault. Points where normal fault ruptures nucleated on the steep NE-dipping normal faults coincide with en echelon steps of < ∼1 km and abrupt ∼15° changes in strike. The Marzano and Carpineta faults, which have strike ∼315°, took up a small component of left-lateral slip, as is revealed by the first-motion focal mechanism, teleseismic waveform modelling, striations measured in-the field, and consistent rightward stepping: their slip vector azimuth is ∼N37°E. Assuming the same slip vector azimuth, a component of right-lateral slip is expected on the San Gregorio and Picentini faults that have strike ∼300°.

Geological constraints on the kinematics and fault-plane solution of the quiches fault zone reactivated during the 10 November 1946 Ancash earthquake, northern Peru

The Ancash earthquake of 10 November 1946 ( Mw = 6.8) produced surface faulting along the Quiches normal-slip fault. Field studies reported discontinuous NW-SE to NNW-SSE-striking seismic scarps that displace Quaternary glacial moraines and outwash, as well as Mesozoic sedimentary rocks. Structural analyses show that the 1946 normal displacement was accompanied by a small left-lateral slip component. Morphological analysis of the fault scarp and observations in a trench excavated across the scarp show evidence of a single normal-fault displacement before the 1946 seismic event that offset late Pleistocene glacial moraines. This indicates an approximate recurrence interval of more than 13,000 ± 1,000 years for the major events, suggesting a rate of net vertical displacement on the fault of less than 0.25 mm/yr. Kinematic analysis along the Quiches fault, in comparison with other regional studies, suggests that the state of stress in the High Andes is regionally and locally homogeneous with only minor changes in the stress directions adjacent to major structures such as the Maranon Thrust and Fold Belt. The minimum horizontal stress (σ3 axis) is oriented N10-25°E, the intermediate horizontal stress (σ2 axis) is oriented N100-115°E, and the maximum stress (σ1 axis) is vertical. We suggest that the 60°SW-dipping Quiches surface fault plane is connected at depth to a low-angle-dipping fault plane. This seismic fault is a reactivated thrust fault of the Tertiary Maranon Thrust and Fold Belt.