Right-lateral Slip Component Research Articles

لرزه زمین ساخت پهنه های البرز خاوری و کپه داغ با استفاده از داده های شبکه لرزه نگاری محلی

The Kopeh Dagh and Binalud-Alla Dagh mountains are important structural elements located in the northeastern boundary of Arabia-Eurasia collision zone. Due to existence of large cities with a long history of civilization, there is a relatively rich body of data on historical seismicity in this area. Nonetheless, little adequate instrumental seismic data were available prior to this study. In this paper, we utilized the temporary China-Iran local seismic network data deployed in the area for 13 months. Based on this data, we determined 37 focal solutions through first motion polarity approach suggesting a combination of strike-slip, reverse and normal mechanisms. There is a significant concentration of epicenters and focal mechanisms around the eastern Alborz - Kopeh Dagh boundary along the Atrak River. The southern and western boundaries of the Binalud Mountains also show relatively high seismic activities. The Local magnitude (ML) of the events ranges from 3.5 to 4.9 with depths of up to 20 km, mostly concentrated at ~10 km. The principal stress axes and slip vectors obtained from the focal mechanisms agree well with kinematic state of main faults and tectonic regime of the area, confirming the seismogenic nature of these faults. In addition, they are consistent with right-lateral component of slip along thrust faults in eastern Kopeh Dagh and Binalud and both laft- and right-lateral motions in the central Kopeh Dagh.

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.

The August 1st, 2014 (M w 5.3) Moderate Earthquake: Evidence for an Active Thrust Fault in the Bay of Algiers (Algeria)

On August 1st, 2014, a moderate-sized earthquake struck the capital city of Algiers at 05:11:17.6 (GMT+1). The earthquake caused the death of six peoples and injured 420, mainly following a panic movement among the population. Following the main shock, we surveyed the aftershock activity using a portable seismological network (short period), installed from August 2nd, 2014 to August 21st, 2015. In this work, first, we determined the main shock epicenter using the accelerograms recorded by the Algerian accelerograph network (under the coordination of the National Center of Applied Research in Earthquake Engineering–CGS). We calculated the focal mechanism of the main shock, using the inversion of the accelerograph waveforms in displacement that provides a reverse fault with a slight right-lateral component of slip and a compression axis striking NNW–SSE. The obtained scalar seismic moment (Mo = 1.25 × 1017 Nm) corresponds to a moment magnitude of Mw = 5.3. Second, the analysis of the obtained aftershock swarm, of the survey, suggests an offshore ENE–WSW, trending and NNW dipping, causative active fault in the bay of Algiers, which may likely correspond to an offshore unknown segment of the Sahel active fault.

The role of basement-involved normal faults in the recent tectonics of western Taiwan

AbstractIn the foreland area of western Taiwan, some of the pre-orogenic basement-involved normal faults were reactivated during the subsequent compressional tectonics. The main purpose of this paper is to investigate the role played by the pre-existing normal faults in the recent tectonics of western Taiwan. In NW Taiwan, reactivated normal faults with a strike-slip component have developed by linkage of reactivated single pre-existing normal faults in the foreland basin and acted as transverse structures for low-angle thrusts in the outer fold-and-thrust belt. In the later stage of their development, the transverse structures were thrusted and appear underneath the low-angle thrusts or became tear faults in the inner fold-and-thrust belt. In SW Taiwan, where the foreland basin is lacking normal fault reactivation, the pre-existing normal faults passively acted as ramp for the low-angle thrusts in the inner fold-and-thrust belt. Some of the active faults in western Taiwan may also be related to reactivated normal faults with right-lateral slip component. Some main earthquake shocks related to either strike-slip or thrust fault plane solution occurred on reactivated normal faults, implying a relationship between the pre-existing normal fault and the triggering of the recent major earthquakes. Along-strike contrast in structural style of normal fault reactivation gives rise to different characteristics of the deformation front for different parts of the foreland area in western Taiwan. Variations in the degree of normal fault reactivation also provide some insights into the way the crust embedding the pre-existing normal faults deformed in response to orogenic contraction.

Source Parameters of the 2003–2004 Bange Earthquake Sequence, Central Tibet, China, Estimated from InSAR Data

A sequence of Ms ≥ 5.0 earthquakes occurred in 2003 and 2004 in Bange County, Tibet, China, all with similar depths and focal mechanisms. However, the source parameters, kinematics and relationships between these earthquakes are poorly known because of their moderately-sized magnitude and the sparse distribution of seismic stations in the region. We utilize interferometric synthetic aperture radar (InSAR) data from the European Space Agency’s Envisat satellite to determine the location, fault geometry and slip distribution of three large events of the sequence that occurred on 7 July 2003 (Ms 6.0), 27 March 2004 (Ms 6.2), and 3 July 2004 (Ms 5.1). The modeling results indicate that the 7 July 2003 event was a normal-faulting event with a right-lateral slip component, the 27 March 2004 earthquake was associated with a normal fault striking northeast–southwest and dipping northwest with a moderately oblique right-lateral slip, and the 3 July 2004 event was caused by a normal fault. A calculation of the static stress changes on the fault planes demonstrates that the third earthquake may have been triggered by the previous ones.

Implications for the structure of the Hat Creek fault and transfer of right-lateral shear from the Walker Lane north of Lassen Peak, northern California, from gravity and magnetic data

Interpretation of magnetic and new gravity data provides constraints on the geometry of the Hat Creek fault, the amount of right-lateral offset in the area between Mount Shasta and Lassen Peak (northern California, USA), and confirmation of the influence of preexisting structure on Quaternary faulting. Neogene volcanic rocks coincide with short-wavelength magnetic anomalies of both normal and reversed polarity, whereas a markedly smoother magnetic field occurs over the Klamath Mountains and Paleogene cover there. Although the magnetic field over the Neogene volcanic rocks is complex, the Hat Creek fault, which is one of the most prominent normal faults in the region and forms the eastern margin of the Hat Creek Valley, is marked by the eastern edge of a north-trending magnetic and gravity high 20–30 km long. Modeling of these anomalies indicates that the fault is a steeply dipping (∼75°–85°) structure. The spatial relationship of the fault as modeled by the potential-field data, the youngest strand of the fault, and relocated seismicity suggest that deformation continues to step westward across the valley, consistent with a component of right-lateral slip in an extensional environment. Filtered aeromagnetic data highlight a concealed magnetic body of Mesozoic or older age north of Hat Creek Valley. The body’s northwest margin strikes northeast and is linear over a distance of ∼40 km. Within the resolution of the aeromagnetic data (1–2 km), we discern no right-lateral offset of this body. Furthermore, Quaternary faults change strike or appear to end, as if to avoid this concealed magnetic body and to pass along its southeast edge, suggesting that preexisting crustal structure influenced younger faulting, as previously proposed based on gravity data.

Co-seismic, geomorphic, and geologic fold growth associated with the 1978 Tabas-e-Golshan earthquake fault in eastern Iran

We describe the seismicity and late Cenozoic deformation associated with a blind thrust fault at Tabas-e-Golshan (hereafter referred to as Tabas), eastern Iran, which generated a devastating Mw 7.3 earthquake on the 16th September 1978. Measurements from a structural transect through the Sardar anticline segment indicate fault-propagation folding above a gently ~20° eastward-dipping blind thrust fault. The thrust flattens into a horizontal detachment at a depth of only ~2km. Tightening of the fold forelimb is accommodated by flexural slip along numerous bedding planes, with many of the slip surfaces showing fresh striations with a large component of right-lateral strike–slip. A steeply dipping fault zone showing almost pure strike–slip is also developed within the forelimb of the fold. Our field observations are consistent with the source parameters of the 1978 Tabas earthquake, and additional events in 1979 and 1980, which all involved slip on a shallowly-dipping thrust with a significant component of right-lateral slip. The surface of an alluvial fan, which is likely to have been abandoned at 8–10ka, has been folded as it crosses the Sardar anticline. The age constraints, combined with topographic profiles along the deformed fan surface and constraints on the dip of the fault at depth, provide an approximate rate of horizontal shortening of ~1.5mm/yr. Shortening at Tabas appears to result from transpressional bending at the north end of the Nayband strike–slip fault. A northward continuation of the Nayband Fault, which may be slipping at rates of >2mm/yr, is identified along the base of the Shotori Mountains ~10–20km east of the Tabas thrust. The range-front fault did not move in 1978 and constitutes an additional threat to local populations.

The Zandjan fault system: Morphological and tectonic evidences of a new active fault network in the NW of Iran

The Zandjan province is situated in the NW of Iran, at the junction between the southern termination of the Tabriz right-lateral strike-slip fault and the western end of the Alborz active mountain range. Despite this key location, in terms of active tectonics, there is no destructive earthquake reported in available historical seismicity catalogues. Even if a few low magnitude instrumental earthquakes have been recorded indicating that this area is actively deforming, the Zandjan region can be considered as a seismic gap. Based on detailed field investigations, digital elevation models (DEMs), satellite image and aerial photo interpretations, we have identified a major active fault network affecting this regions. It is mainly constituted by several west and est verging thrust faults, trending NW–SE to N–S, that clearly cut Late Quaternary to Holocene deposits. Evidences in the morphology indicate also that they accommodate some right-lateral slip component. This fault network can be divided into four major fault zones affecting Zandjan city (present day population over 500,000) and part of the Zandjan–Mianeh basin more to the west. Based on fault geometrical characteristics (fault length, segmentation…) and kinematics, the Zandjan fault system is certainly capable to produce moderate to large earthquakes (Mw = 6.5–7.0) and serious damages in the Zandjan city and surrounding areas. The study of these faults is, then, of major interest for seismic hazard assessment in Zandjan province where total population exceed 1.5 million but also to better understand present day geodynamic of NW Iran.

Refining locations of the 2005–2006 recurrent earthquakes in Mukacheve, West Ukraine, and implications for their source mechanism and the local tectonics

Three groups of recurrent events have been identified within the 2005–2006 Mukacheve series of about forty M SH 0.3–1.8 near-surface earthquakes. After a cross-correlation analysis of their waveforms and subsequent relocation with application of the adjusted arrivals and station terms it turned out that they most likely belong to at least two distinct crustal layers separated by an interface at about 4 km; the seismic activation started deeper and progressed upwards. As concerns polarities of first arrivals, the same source mechanism was inferred within individual groups, indicating a normal faulting along the Az ≈ 120° with a slight component of right lateral slip; this finding is consistent with data available on some other local sources, as well as with an ambient field of extensional stresses in the local crust and a significant horizontal gradient of vertical movements across the epicenter area of the Mukacheve series.

Surface Fault Slip Associated with the 2004 Parkfield, California, Earthquake

Surface fracturing occurred along the San Andreas fault, the subparallel Southwest Fracture Zone, and six secondary faults in association with the 28 September 2004 ( M 6.0) Parkfield earthquake. Fractures formed discontinuous breaks along a 32-km-long stretch of the San Andreas fault. Sense of slip was right lateral; only locally was there a minor (1–11 mm) vertical component of slip. Right-lateral slip in the first few weeks after the event, early in its afterslip period, ranged from 1 to 44 mm. Our observations in the weeks following the earthquake indicated that the highest slip values are in the Middle Mountain area, northwest of the mainshock epicenter (creepmeter measurements indicate a similar distribution of slip). Surface slip along the San Andreas fault developed soon after the mainshock; field checks in the area near Parkfield and about 5 km to the southeast indicated that surface slip developed more than 1 hr but generally less than 1 day after the event. Slip along the Southwest Fracture Zone developed coseismically and extended about 8 km. Sense of slip was right lateral; locally there was a minor to moderate (1–29 mm) vertical component of slip. Right-lateral slip ranged from 1 to 41 mm. Surface slip along secondary faults was right lateral; the right-lateral component of slip ranged from 3 to 5 mm. Surface slip in the 1966 and 2004 events occurred along both the San Andreas fault and the Southwest Fracture Zone. In 1966 the length of ground breakage along the San Andreas fault extended 5 km longer than that mapped in 2004. In contrast, the length of ground breakage along the Southwest Fracture Zone was the same in both events, yet the surface fractures were more continuous in 2004. Surface slip on secondary faults in 2004 indicated previously unmapped structural connections between the San Andreas fault and the Southwest Fracture Zone, further revealing aspects of the structural setting and fault interactions in the Parkfield area.

Historic Surface Faulting and Paleoseismicity in the Area of the 1954 Rainbow Mountain-Stillwater Earthquake Sequence, Central Nevada

The Rainbow Mountain area was the site of three surface-rupturing earthquakes on 6 July and 23 August 1954. More than 50 field measurements of surface offsets constrain the distribution of slip along the discontinuous and distrib- uted rupture zone that formed during the earthquake sequence. Vertical offsets reach a maximum of 0.8 m with the average vertical offset being 0.2 m. In contrast to original reports, we see evidence for a right-lateral component of slip along portions of the rupture zone, including offset stream channels (0.5-1.0 m), left-stepping en echelon scarps, and a well-preserved, 100-m-long mole track. The right-slip com- ponent is consistent with focal plane solutions for the events and recent geodetic results. Previously unmapped surface ruptures now extend the known rupture length of the sequence by 25 km to a total of 70 km. Surface ruptures along the previously unmapped Fourmile Flat fault are subparallel to and form a 10-km left step to the southeast of the Rainbow Mountain fault. Event locations and anecdotal information indicate that the Fourmile Flat ruptures represent minor, primary surface rupture associated with the large 6 July aftershock, triggered 11 hr after the initial 6 July Rainbow Mountain event. The paleoseismic histories of the Rainbow Mountain and Fourmile Flat faults, as recorded in natural and trench exposures, are different although both faults experi- enced three post 15-ka surface rupturing events, including 1954. Bracketing ages for triultimate events on both faults do not overlap. However, constraints on the penultimate event for the Rainbow Mountain and triultimate event for the Fourmile Flat fault do overlap slightly, allowing the possibility that they may have ruptured close in time as in 1954. The Holocene slip rate for the Fourmile Flat fault (0.40 mm/yr) is similar to the post-latest Pleistocene rate for the Rainbow Mountain fault (0.20-0.46 mm/yr) even though the total length of the Fourmile flat (10 km) is much shorter than the overall length of the Rainbow Mountain rupture zone (60 km), indicating that even minor faults can be important for assessing regional strain rates and patterns.

First results of the CRLN seismic network in the western Corinth Rift: evidence for old-fault reactivation

Abstract The 12 stations Corinth Rift Laboratory Seismological Network (CRLNET) aims at monitoring the seismicity (Ml>1) in the CRL area and at constraining the geometry of active structures at depth. Two years of microseismicity (2000–2001) recorded by the CRLNET in the Aigion area shows: (1) background seismicity inside the Corinth rift at depth of 4.5–11 km, deepening towards the north and no activity in the upper 4 km of the crust – this seismicity is not clearly related to major faults observed at the surface –; (2) a swarm, 6 km south of the city of Aigion, associated with the Mw=4.2, 8 April 2001 earthquake. This earthquake occurred at 6 km depth, on a SW–NE oriented fault dipping 40° to the northwest and corresponds to normal faulting with a right lateral component of slip. It likely occurred on an old structure reactivated in the present stress field. To cite this article: H. Lyon-Caen et al., C. R. Geoscience 336 (2004).

Determinações da deformação finita em rochas metassedimentares da faixa ribeira na região de Iporanga e Apiaí, SP

Quantitative finite strain estimates were performed in metasedimentary units belonging to the Lajeado and Ribeira subgroups of the Acungui Supergroup in the region of the Upper and Medium Ribeira Valley, Sao Paulo State. The main purposes were to determine the original thickness of the formations, the amount of tectonic shortening of the area and map the principal strain axes and types of local strain ellipsoids. Fourteen complete determinations were obtained including the orientations of the X , Y , Z principal strain axes as also the respective stretch ratios. The main method used was that of Fry by defining the strain ellipses on three concurrent rock sections for each sample; one of the samples was also submitted to the R f / f , preferential orientation and harmonic mean determinations. The grain distributions in coarse clastics and siltstones as also those of rutile crystals were analyzed. A graphic method based on the determination of circular sections was applied for the deduction of the strain ellipsoid. The majority of the samples showed low X / Z strain ratios (between 2 and 1.4), two values were high (greater than 5) and two showed medium values (between 2 and 5). The majority of the determined ellipsoids resembles those related to planar strain with k=1, three were strongly prolate with k>>1 and one was strongly oblate (k<<1). The results confirmed the idea that the rocks of the Lajeado Subgroup are little deformed with exception of areas next to large shear zones, in contrast with those of the Ribeira Subgroup with higher strain ratios. Almost the totality of the Z axes is coherent with a NE oriented plane of maximum flattening dipping to NW at high angles, parallel to the mean directions of the schistosity and slaty cleavage of these rocks. The X axes show a general NE-SW sub horizontal trend underlining the importance of the regional transcurrent tectonics, while three samples next to the Figueira and Agudos Grandes Lineaments showed strong plunges to NW suggesting inverse or oblique slip. The kinematic analysis of the geometry of the Ribeira Lineament shear zone indicated an amount of ductile right-lateral component of slip of about 18 km. The total inferred slip using regional correlations could be in the order of 50 km.

Triggered Surface Slips in the Salton Trough Associated with the 1999 Hector Mine, California, Earthquake

Surface fracturing occurred along the southern San Andreas, Superstition Hills, and Imperial faults in association with the 16 October 1999 (Mw 7.1) Hector Mine earthquake, making this at least the eighth time in the past 31 years that a regional earthquake has triggered slip along faults in the Salton Trough. Fractures associated with the event formed discontinuous breaks over a 39-km-long stretch of the San Andreas fault, from the Mecca Hills southeastward to Salt Creek and Durmid Hill, a distance from the epicenter of 107 to 139 km. Sense of slip was right lateral; only locally was there a minor (∼1 mm) vertical component of slip. Dextral slip ranged from 1 to 13 mm. Maximum slip values in 1999 and earlier triggered slips are most common in the central Mecca Hills. Field evidence indicates a transient opening as the Hector Mine seismic waves passed the southern San Andreas fault. Comparison of nearby strong-motion records indicates several periods of relative opening with passage of the Hector Mine seismic wave—a similar process may have contributed to the field evidence of a transient opening. Slip on the Superstition Hills fault extended at least 9 km, at a distance from the Hector Mine epicenter of about 188 to 196 km. This length of slip is a minimum value, because we saw fresh surface breakage extending farther northwest than our measurement sites. Sense of slip was right lateral; locally there was a minor (∼1 mm) vertical component of slip. Dextral slip ranged from 1 to 18 mm, with the largest amounts found distributed (or skewed) away from the Hector Mine earthquake source. Slip triggered on the Superstition Hills fault commonly is skewed away from the earthquake source, most notably in 1968, 1979, and 1999. Surface slip on the Imperial fault and within the Imperial Valley extended about 22 km, representing a distance from the Hector Mine epicenter of about 204 to 226 km. Sense of slip dominantly was right lateral; the right-lateral component of slip ranged from 1 to 19 mm. Locally there was a minor (∼1–2 mm) vertical component of slip; larger proportions of vertical slip (up to 10 mm) occurred in Mesquite basin, where scarps indicate long-term oblique-slip motion for this part of the Imperial fault. Slip triggered on the Imperial fault appears randomly distributed relative to location along the fault and source direction. Multiple surface slips, both primary and triggered slip, indicate that slip repeatedly is small at locations of structural complexity.

Neotectonics of the Thakkhola graben and implications for recent activity on the South Tibetan fault system in the central Nepal Himalaya

The Thakkhola graben is one of many north-trending rifts that define the Neogene structural pattern of the southern Tibetan Plateau. Lying at the southern margin of the plateau and extending to the crest of the Himalaya, the graben provides an opportunity to evaluate the kinematic relationships between east-west extensional strain in southern Tibet and north-south extensional strain in the Himalaya. Neotectonic and structural mapping of the Dangardzong fault along the western margin of the graben reveals a southward-decreasing component of normal slip coupled with a southward-increasing component of right-lateral slip that affects Pleistocene basin-fill sediments. We present 14C ages for river terraces in the Thakkhola graben that provide a ca. 17.2 ka minimum age on the latest stage of Dangardzong fault movement. Near the southern termination of the graben, the Dangardzong fault apparently offsets the Annapurna detachment, an early (Miocene) strand of the east-striking South Tibetan fault system. However, the Dangardzong fault itself terminates against a young (i.e., younger than ca. 17.2 ka) strand of the South Tibetan fault system, the Dhumpu detachment. Structural relationships among the Dangardzong, Annapurna, and Dhumpu faults suggest that the Dangardzong structure is a tear fault in the South Tibetan allochthon that accommodates differential amounts and rates of displacement along the South Tibetan fault system. Thus, although the South Tibetan fault system first developed as part of the structural architecture of the Himalaya in Miocene time, at least some strands have been active as recently as the Pleistocene. In a regional context, the South Tibetan fault system serves to accommodate the strain gradient between extension in Tibet and shortening in the Himalaya.

Active faulting at Delphi, Greece: Seismotectonic remarks and a hypothesis for the geologic environment of a myth

Research Article| July 01, 2000 Active faulting at Delphi, Greece: Seismotectonic remarks and a hypothesis for the geologic environment of a myth Luigi Piccardi Luigi Piccardi 1Consiglio Nazionale delle Ricerche, Centro di Studio di Geologia dell'Appennino e delle Catene Perimediterranee, Firenze, Italy Search for other works by this author on: GSW Google Scholar Geology (2000) 28 (7): 651–654. https://doi.org/10.1130/0091-7613(2000)28<651:AFADGS>2.0.CO;2 Article history received: 23 Nov 1999 rev-recd: 30 Mar 2000 accepted: 20 Apr 2000 first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Luigi Piccardi; Active faulting at Delphi, Greece: Seismotectonic remarks and a hypothesis for the geologic environment of a myth. Geology 2000;; 28 (7): 651–654. doi: https://doi.org/10.1130/0091-7613(2000)28<651:AFADGS>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Historical data are fundamental to the understanding of the seismic history of an area. At the same time, knowledge of the active tectonic processes allows us to understand how earthquakes have been perceived by past cultures. Delphi is one of the principal archaeological sites of Greece, the main oracle of Apollo. It was by far the most venerated oracle of the Greek ancient world. According to tradition, the mantic proprieties of the oracle were obtained from an open chasm in the earth. Delphi is directly above one of the main antithetic active faults of the Gulf of Corinth Rift, which bounds Mount Parnassus to the south. The geometry of the fault and slip-parallel lineations on the main fault plane indicate normal movement, with minor right-lateral slip component. Combining tectonic data, archaeological evidence, historical sources, and a reexamination of myths, it appears that the Helice earthquake of 373 B.C. ruptured not only the master fault of the Gulf of Corinth Rift at Helice, but also the antithetic fault at Delphi, similarly to the Corinth earthquake of 1981. Moreover, the presence of an active fault directly below the temples of the oldest sanctuary suggests that the mythological oracular chasm might well have been an ancient tectonic surface rupture. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

The MW=8.1 Antofagasta (North Chile) Earthquake of July 30, 1995: First results from teleseismic and geodetic data

A strong (Mw = 8.1) subduction earthquake occurred on July 30, 1995 in Antofagasta (northern Chile). This is one of the largest events during this century in the region. It ruptured the southernmost portion of a seismic gap between 18°S and 25°S. In 1992 we had used GPS to survey a network with about 50 benchmarks covering a region nearly 500 km long (N‐S) and 200 km wide (E‐W). Part of these marks were re‐surveyed with GPS after the 1995 earthquake. Comparison with 1992 positions indicate relative horizontal displacement towards the trench reaching 0.7 m. The inland subsided several decimeters. The Mejillones Peninsula was uplifted by more than 15 cm. Teleseismic body‐wave modelling of VBB records gives a subduction focal mechanism and source time function with three distinct episodes of moment release and southward directivity. Modelling the displacement field using a dislocation with uniform slip in elastic half‐space suggests a rupture zone with 19°–24° eastward dip extending to a depth no greater than 50 km with N‐S length of 180 km and an average slip of about 5 m. The component of right‐lateral slip inferred both from the teleseismic and geodetic data does not require slip partitioning at the plate boundary. That the well‐constrained northern end of the 1995 rupture zone lies under the southern part of the Mejillones Peninsula increases the probability for a next rupture in the gap north of it.

Tectonic implications of Paleomagnetic data from the Northland ophiolite, New Zealand

Paleomagnetic sampling of 12 sites from two separate massifs of the Northland ophiolite, New Zealand, yields consistent results. Two characteristic NRM directions are recognised: one of normal polarity associated with weakly magnetised rocks (particularly intrusive dolerites), and another of reversed polarity associated with strongly magnetised rocks (particularly basaltic pillow lavas). Thermal and AF demagnetisation shows that the normal directions are due to a secondary viscous component which reflects the present-day field direction, whilst the reversed directions are stable (typically to 500°C) reflecting either the primary TRM or a hard secondary component. The declination and inclination of this stable magnetisation differ substantially from that of the present-day field implying significant tectonic movement or deformation of the ophiolite since formation. Anomalous declinations and inclinations are interpreted here in terms of rotations about vertical axes and, tentatively, latitudinal translation since no widespread significant tilting has been recognised within the massifs. Declinations are consistent with clockwise rotations of 50° and 120° for the Cape Reinga and Mangakahia massifs, respectively, probably associated with obduction at a convergent margin with a right-lateral slip component, situated to the northeast of Northland. The anomalously low inclination vectors (30–40°) are consistent with an origin at lower than present latitudes which, if true, implies long-lived N-S translation of oceanic crust in the southwest Pacific region between Cretaceous and Oligocene times.

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°.

Near-fault measurement of postseismic slip associated with the 1989 Loma Prieta, California, Earthquake

Five small-aperture (0.5 to 7.7 m) quadrilaterals were installed across the San Andreas fault and newly formed extensional cracks shortly after the October 17, 1989 Loma Prieta M7.1 earthquake. Two quadrilaterals were installed across the San Andreas fault near the southeast and northwest ends of the 1989 rupture, and three were installed across dominantly extensional cracks in the Summit Road area near the main-shock epicenter and off the San Andreas fault. Measurements of line-length changes from as early as 4 d up to 184 d after the earthquake in the nail quadrilaterals indicate a small amount of right-lateral postseismic slip on the San Andreas fault. The site near the southeast end of the 1989 afterstock zone on the San Andreas fault showed about 5±2 mm of postseismic right-lateral component of slip in addition to 23 mm of right-lateral coseismic movement. The site near the northwest end of aftershocks likewise showed about 5±2 mm of postseismic slip, but after only 5 mm of coseismic slip. Measurements at sites across extensional cracks indicate possible minor left-lateral postseismic slips, and possible extensional/contractional motion. No measurements were made of possible vertical movements. Sites on extensional cracks showed similarly small amounts of possible postscismic lateral slip, from ∼2 to 10 mm, even though coseismic lateral slips were much greater (up to 410 mm) than on the surface trace of the San Andreas fault. The small slip values, in spite of uncertainties, clearly show that the lack of coseismic surface slip associated with the earthquake was not followed by large postseismic slip.