The Rupture Process of the Manjil, Iran Earthquake of 20 June 1990 and Implications for Intraplate Strike-Slip Earthquakes (original) (raw)
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Journal of Earthquake Engineering, 2007
The Bojnurd region of NE Iran experienced a Mw 6.4 earthquake on February 4, 1997. By combining results from teleseismic body-waveform analysis, field observations of structural damage, coseismic deformation, geomorphology, and analysis of the resulting strong ground-motions, we build a coherent picture of the faulting associated with this earthquake. The earthquake resulted from almost pure right-lateral strike-slip motion (0.5–1.0 m), which ruptured a ∼15 km long section of fault, striking ∼340° at its northern end, which changes to ∼320° at its southern end. This fault can be seen clearly on the geological map and satellite imagery. The village of Sheikh lies ∼10–15 km SE of the fault rupture, yet was severely damaged during the earthquake. Analysis of strong-motion records, recorded by the Building and Hazard Research Center of Iran, particularly their significant duration, the polarization of the fault-normal component, and the velocity pulse, indicates a probable directivity effect, in which the rupture propagated from north to south, towards the village of Sheikh.
Journal of Geodynamics, 2012
From 2005 December 2 to 2006 February 26, a dense seismological network of 17 stations was installed in the epicentral region of the 2005 November 27 Qeshm sequence. The epicentral distribution of aftershocks, including an ENE-WSW trend of seismicity, is terminated on both sides by NW-SE alignments of events. The depth distribution of events (i.e., 8-20 km depth) is diffuse but in the eastern part of the aftershock zone reveals an alignment of seismicity dipping ∼40 • toward the NW. Focal mechanisms of the aftershocks are strike-slip, different from that of the mainshock, which had a reverse mechanism. The scenario of a mainshock at shallow depth which is followed by a separated, deeper aftershock sequence implies different mechanisms of deformation in the sedimentary layer and the basement at the western edge of the Hormuz Strait. The epicentral distribution of aftershocks and their focal mechanisms suggest that shortening in the basement, due to convergence between Arabia and Eurasia, is accommodated in this region mainly by strike-slip motions. (F. Yaminifard). the main-shock? What is the depth range of the aftershocks in this region of the Zagros? What is the reason for the existence of such different focal mechanisms for the mainshock and the largest aftershock? How is deformation distributed in the southeastern-most part of the Zagros? And, is high-resolution analysis using local data consistent with teleseismic and radar interferometry studies? 0264-3707/$ -see front matter
Physics of the Earth and Planetary Interiors, 2018
Seismotectonic features of the Ahar-Varzaghan complex fault system are reviewed by the mainshocks/aftershocks study of the 2012 Ahar-Varzaghan earthquake doublet (Mw=6.5 & Mw=6.3). The early aftershocks were accurately monitored by a dense local seismological network. The mainshocks hypocenters were relocated using a new velocity model for the area. The regional displacement waveforms of the mainshocks and large aftershocks were jointly inverted for their moment tensors and centroids. Precisely located aftershocks are mostly distributed in three distinct clusters. These E-W trending clusters are situated to the north of the surface rupture over a ~30 km long and 10 km wide zone, extending from the ground surface down to the depth of ~15 km. Based on our results, the first mainshock (M1) with Mw=6.5 nucleated 3 km east of the surface rupture at a depth of 10 km. The rupture evolved toward the west and shallower depths on an almost E-W striking rightlateral strike-slip vertical fault plane and produced ~12 km surface rupture. Its main energy was released at about 5 km west of the hypocenter at an average depth of 5 km. About 11 minutes later, the second mainshock (M2) with Mw=6.3 nucleated 5 km northwest of the M1 hypocenter and at a depth of 15 km. It occurred on an ENE-WSW striking, north-dipping (65° to 70°) fault plane with a dominant reverse mechanism and strike-slip component. The M2 related rupture 2 also expanded to the west and to shallower depths and released most of its energy ~5 km west of the respective hypocenter at an average depth of 11 km. The eastern aftershocks mainly show the right-lateral strike-slip mechanism on almost E-W trending fault, close to the location of the first main centroid (M1). The northern aftershocks mostly include reverse mechanisms with strikeslip component near the M2 centroid. A big gap of aftershock activity is observed close to the M1 centroid location, most likely associated with the area of maximum slip. We do not observe any N-S lineament of aftershocks neither near the M2 hypocenter and nor its centroid, thus making the suggestion of some previous investigators that M2 ruptured an N-S trending fault unlikely.
Geophysical Journal International, 2006
The catastrophic 2003 M w 6.6 Bam earthquake in southern Iran attracted much attention, and has been studied with an abundance of observations from synthetic aperture radar, teleseismic seismology, aftershock studies, strong ground motion, geomorphology, remote sensing and surface field work. Many reports have focused on the details of one or other data type, producing interpretations that either conflict with other data or leave questions unanswered. This paper is an attempt to look at all the available data types together, to produce a coherent picture of the coseismic faulting in 2003 and to examine its consequences for active tectonics and continuing seismic hazard in the region. We conclude that more than 80 per cent of the moment release in the main shock occurred on a near-vertical right-lateral strike-slip fault extending from the city of Bam southwards for about 15 km, with slip of up to 2 m but mostly restricted to the depth range 2-7 km. Analysis of the strong ground motion record at Bam is consistent with this view, and indicates that the extreme damage in the city can be attributed, at least in part, to the enhancement of ground motion in Bam because of its position at the end of the northward-propagating rupture. Little of the slip in the main shock reached the Earth's surface and, more importantly, aftershocks reveal that ∼12 km vertical extent of a deeper part of the fault system remained unruptured beneath the coseismic rupture plane, at depths of 8-20 km. This may represent a substantial remaining seismic hazard to the reconstructed city of Bam. We believe that some oblique-reverse slip (up to 2 m, and less than 20 per cent of the released seismic moment) occurred at a restricted depth of 5-7 km on a blind west-dipping fault that projects to the surface at the Bam-Baravat escarpment, an asymmetric anticline ridge that is the most prominent geomorphological feature in the area. This fault did not rupture significantly at shallow levels in 2003, and it may also represent a continuing seismic hazard. Widespread distributed surface ruptures north of the city are apparently unrelated to substantial slip at depth, and may be the result of enhanced ground motion related to northward propagation of the rupture. The faulting at Bam may be in the early stages of a spatial separation ('partitioning') between the reverse and strike-slip components of an oblique convergence across the zone. Such a separation is common on the continents, though in this case the slip vectors between the two faults differ only by ∼20 • as a substantial strike-slip component remains on the obliquereverse fault. The Bam earthquake is one in a series of large earthquakes involving faulting along the western edge of the Lut desert. In addition to the unruptured parts of the faults near Bam itself, continuing and substantial hazard is represented by unruptured neighbouring faults, particularly blind thrusts along the Jebel Barez mountains to the south and strike-slip faulting at Sarvestan to the west.
The rupture process of the Armenian earthquake from broad-band teleseismic body wave records
Geophysical Journal International, 1992
The destructive earthquake of December 1988 in Armenia (M, = 6.9) was recorded on broad-band and very long-period channels at teleseismic distances by Geoscope and GDSN networks. These records are well distributed in azimuth, and allow a detailed study of the rupture process of this earthquake. The average focal mechanism obtained by Pand SH-wave modelling ( C#I = 300" f lo", 6 = 63" f 5", A = 100" f 20") is compatible with the mechanism obtained from very long-period surface waves and intermediate-period single-station determinations, as well as field observations. The mean depth of the rupture is also fixed by waveform modelling between 5 and 7 km which means that the rupture surface extends from the surface to a depth between 10 and 14 km, in agreement with aftershock depth distribution. The rupture is found to be complex, composed of a weak beginning or a small foreshock and two pulses well separated in time. The time delay between the two events is estimated for each station by waveform modelling and by spectral analysis. The azimuthal variation of this time delay is interpreted in terms of direction and velocity of rupture on the fault plane. A more detailed analysis of the source implies the use of additional information coming from aftershock studies and tectonics. We use forward modelling to investigate several rupture mechanisms. A three-source model gives an acceptable fit to the observed records but the western mechanism is at odds with observed tectonics and, furthermore, rupture propagation is not well simulated. A five-segment model of the source obtained from field seismotectonic data gives a better waveform fit, a time sequence of individual breaks that simulates a rupture propagating away from the hypocentre, and subsource mechanisms that are compatible with surface tectonics.
Rupture process of the Miyagi-Oki, Japan, earthquake of June 12, 1978
Physics of the Earth and Planetary Interiors, 1980
The faulting mechanism and multiple rupture process of theM = 7.4 Miyagi-Oki earthquake are studied using surface and body wave data from local and worldwide stations. The main results are as follows. (1) P-wave first motion data and radiation patterns of long-period surface waves indicate a predominantly thrust mechanism with strike Nb 0 E, dip 20°W,and slip angle 76°.The seismic moment is 3.1 X 1027 dyne-cm. (2) Farfield SH waveforms and local seismograms suggest that the rupture occurred in two stages, being concordant, with the two, zones of afteishock activity revealed by the microearthquake network of Tohoku University. The upper and lower zones, located along the westward-dipping plate interface, are separated by a gap at a depth of 35 km and have dimensions of 37 X 34 and 24 X 34 km2, respectively. Rupture initiated at the southern end of the upper aftershock zone and propagated at N20°Wsubparallel to the trench axis. About 11 s later, the second shock, which was located 30 km landward (westward) of the first, initiated at the upper corner of the lower aftershock zone and propagated down-dip N80°W.Using Haskell modelling for this rupture process, synthetic seismograms were computed for teleseismic SH waves and nearfield body waves. Other parameters determined are: seismic moment M 0 = 1.7 Xl 027 dyne-cm, slip dislocation iT = 1.9 m, i~o= 95 bar, rupture velocity v = 3.2 km~rise time r = 2 s, for the first event; M0= 1.4 x 1027 dyne-cm, iT= 2.4 m,~a = 145 bar, for the second event; and time separation between the two shocks~T = 11 s. The above two-segment model does not explain well the sharp onsets of the body waves at near-source stations. An initial break of a small subsegment on the upper zone, which propagated down-dip, was hypothesized to explain the observed near-source seismograms. (3) The multiple rupture of the event and the absence of aftershocks between the two fault zones suggests that the frictional and/or sliding characterisitics along the plate interface are not uniform. The rupture of the first event was arrested, presumably by a region of high fracture strength between the two zones. The fracture energy of the barrier was estimated to be 1010 erg cm 2. (4) The possible occurrence of a large earthquake has been noted for the region adjacent to and seaward of the area that ruptured during the 1978 event. The 1978 event does not appear to reduce the likelihood of occurrence of this expected earthquake.
Control of rupture by fault geometry during the 1980 El Asnam (Algeria) earthquake
Geophysical Journal International, 1985
= 7.3) occurred on a segmented reverse fault in the Atlas Mountains of Algeria. This report reexamines the teleseismic data for the main shock and major aftershocks in the light of detailed studies of the surface breaks, geodetic changes and aftershock distribution. The observed thrust fault is split into southern, central and northern segments by distinct offsets and changes in trend. The southern and central segments are each about 12 kin long, but the northern segment showed only 3-4 km of surface thrust breaks. However, widespread tensional faulting on ridges in the northeast area, together with focal mechanisms of locally-recorded aftershocks, indicates that a series of imbricate listric thrusts exists to the north of the northern fault segment. Using ISC arrival-time lists, a relative relocation scheme is applied to the main shock and major aftershocks to improve their locations with respect to the mapped faults. The main shock epicentre was at the southwest end of the fault system, in a zone where the thrust has a zigzag surface trace. By contrast most of the major aftershocks were located further northeast , and were associated with the northern thrust segment and the imbricate thrusts to the north. The largest aftershock (mb = 6.1), which occurred about 3 hr after the main shock, was located to the east of the main thrust fault, beneath the Chelif alluvial basin. First-motion fault plane solutions are presented for the main shock and largest aftershock. The main shock solution indicates a fault plane dipping 54" NW, with strike 040". The aftershock solution, however, has a more E-W trend (080"), in agreement with the trends of the basin edges near its location. The rupture history of the main shock is investigated by forward modelling of the long-period P-waves. Four subevents are clearly distinguishable. The first three subevents occurred in rapid succession and contributed to the first cycle of the P-wave radiation. These three subevents represent the successive rupturing of the southern, central and northern fault segments. In order to match the observed waveforms, these three ruptures must have different
Earth and Planetary …, 2010
We investigate the depth and geometry of faulting within a cluster of buried, reverse faulting earthquakes that struck Qeshm island, in the Zagros fold-and-thrust belt, over a four year period between November 2005 and July 2009. Of particular interest is our observation that there was a vertical separation between the largest two earthquakes (Mw 5.8 and 5.9), which ruptured the lower parts of a ∼ 10-km thick sedimentary cover, and microseismicity recorded by a local network after the first, Mw 5.8 event, which was concentrated within the underlying basement at depths of 10–20 km. Although measured in different ways — the largest three earthquakes using radar interferometry, moderate-sized events with teleseismically-recorded, long-period waveforms, and the microseismicity using data from a local seismic network — we used consistent velocity and elastic parameters in all our modelling, and the observed vertical separation is robust and resolvable. We suggest that it reflects the influence of the Proterozoic Hormuz salt, a weak layer at the base of the sedimentary cover across which rupture failed to propagate. Because the full thickness of the seismogenic layer failed to rupture during the largest earthquakes in the sequence, the lower, unruptured part may constitute a continued seismic hazard to the region. Considering the rarity of earthquakes larger than Mw 6.2 in the Zagros Simply Folded Belt, we suggest that the Hormuz salt forms an important, regional barrier to rupture, not just a local one. Finally, we note that buried faulting involved in the largest earthquakes is almost perpendicular to the trend of an anticline exposed at the surface immediately above them. This suggests that locally, faulting and folding are decoupled, probably along a weak layer of marls or evaporites in the middle part of the sedimentary cover.
Tectonophysics, 2004
Large earthquakes in strike-slip regimes commonly rupture fault segments that are oblique to each other in both strike and dip. This was the case during the 1999 Izmit earthquake, which mainly ruptured E-W-striking right-lateral faults but also ruptured the N608E-striking Karadere fault at the eastern end of the main rupture. It will also likely be so for any future large fault rupture in the adjacent Sea of Marmara. Our aim here is to characterize the effects of regional stress direction, stress triggering due to rupture, and mechanical slip interaction on the composite rupture process. We examine the failure tendency and slip mechanism on secondary faults that are oblique in strike and dip to a vertical strike-slip fault or bmasterQ fault. For a regional stress field well-oriented for slip on a vertical right-lateral strike-slip fault, we determine that oblique normal faulting is most favored on dipping faults with two different strikes, both of which are oriented clockwise from the strike-slip fault. The orientation closer in strike to the master fault is predicted to slip with right-lateral oblique normal slip, the other one with left-lateral oblique normal slip. The most favored secondary fault orientations depend on the effective coefficient of friction on the faults and the ratio of the vertical stress to the maximum horizontal stress. If the regional stress instead causes left-lateral slip on the vertical master fault, the most favored secondary faults would be oriented counterclockwise from the master fault. For secondary faults striking F308 oblique to the master fault, right-lateral slip on the master fault brings both these secondary fault orientations closer to the Coulomb condition for shear failure with oblique right-lateral slip. For a secondary fault striking 308 counterclockwise, the predicted stress change and the component of reverse slip both increase for shallower-angle dips of the secondary fault. For a secondary fault striking 308 clockwise, the predicted stress change decreases but the predicted component of normal slip increases for shallower-angle dips of the secondary fault. When both the vertical master fault and the dipping secondary fault are allowed to slip, mechanical interaction produces sharp gradients or discontinuities in slip across their intersection lines. This can effectively constrain rupture to limited portions of larger faults, depending on the locations of fault intersections. Across the fault intersection line, predicted rakes can vary by N408 and the sense of lateral slip can reverse. Application of these results provides a potential explanation for why only a limited portion of the Karadere fault ruptured during the Izmit (J.R. Muller).