Earthquake Rupture Zone Research Articles

Seismic evidence of bending and unbending of subducting oceanic crust and the presence of mantle megathrust in the 2004 Great Sumatra earthquake rupture zone

In subduction zones the plate interface (megathrust) is typically poorly imaged at depths>12km, however its precise geometry and nature as well as the positions of updip and downdip limits of the seismogenic zone are important elements to understand the generation of megathrust earthquakes. Using deep marine seismic reflection and refraction data, we observed discontinuous reflections off the top of the subducting oceanic crust down to 60km depth in the 2004 great Sumatra-Andaman earthquake rupture zone. We find that the top of the downgoing plate does not dip gently into the subduction zone but instead displays a staircase geometry with three successive, 5–15km vertical steps, spaced ~50km apart. Micro-earthquake data indicate that most of the seismicity lies below this interface, suggesting that the oceanic plate is deforming actively. Along part of the profile, we also image a second reflector located 8–10km below the top of the oceanic crust. The forward modelling of the gravity data along the profile supports the presence of a high-density material above this reflector. The presence of a staircase shape for the top of the oceanic crust, together with constraints from gravity data and earthquake data, require that the megathrust goes through this second reflector. This leads us to conclude that the megathrust is at least partly located in the oceanic mantle and that underplating of oceanic crust beneath the wedge and underplating of upper mantle beneath the forearc basin are taking place in this region.

Aftershock seismicity of the 27 February 2010 Mw 8.8 Maule earthquake rupture zone

On 27 February 2010 the Mw 8.8 Maule earthquake in Central Chile ruptured a seismic gap where significant strain had accumulated since 1835. Shortly after the mainshock a dense network of temporary seismic stations was installed along the whole rupture zone in order to capture the aftershock activity. Here, we present the aftershock distribution and first motion polarity focal mechanisms based on automatic detection algorithms and picking engines. By processing the seismic data between 15 March and 30 September 2010 from stations from IRIS, IPGP, GFZ and University of Liverpool we determined 20,205 hypocentres with magnitudes Mw between 1 and 5.5. Seismic activity occurs in six groups: 1.) Normal faulting outer rise events 2.) A shallow group of plate interface seismicity apparent at 25–35km depth and 50–120km distance to the trench with some variations between profiles. Along strike, the aftershocks occur largely within the zone of coseismic slip but extend ~50km further north, and with predominantly shallowly dipping thrust mechanisms. Along dip, the events are either within the zone of coseismic slip, or downdip from it, depending on the coseismic slip model used. 3.) A third band of seismicity is observed further downdip at 40–50km depth and further inland at 150–160km trench perpendicular distance, with mostly shallow dipping (~28°) thrust focal mechanisms indicating rupture of the plate interface significantly downdip of the coseismic rupture, and presumably above the intersection of the continental Moho with the plate interface. 4.) A deep group of intermediate depth events between 80 and 120km depth is present north of 36°S. Within the Maule segment, a large portion of events during the inter-seismic phase originated from this depth range. 5.) The magmatic arc exhibits a small amount of crustal seismicity but does not appear to show significantly enhanced activity after the Mw 8.8 Maule 2010 earthquake. 6.) Pronounced crustal aftershock activity with mainly normal faulting mechanisms is found in the region of Pichilemu (~34.5°S). These crustal events occur in a ~30km wide region with sharp inclined boundaries and oriented oblique to the trench. The best-located events describe a plane dipping to the southwest, consistent with one of the focal planes of the large normal-faulting aftershock (Mw=6.9) on 11 March 2010.

Spatial analysis of the frequency–magnitude distribution of aftershock activity of 2003 Bam earthquake: southeast Iran

The Bam earthquake (2003 December 26, MW = 6.6) was one of the largest earthquakes that occurred in southeast of Iran during last century. It took place along an N–S trending right-lateral strike-slip fault, almost near the southern end of Nyband–Gowk fault. In this study, we mapped the frequency–magnitude distribution of aftershock events spatially across the Bam aftershock zone. The b-value varies between 0.6 and 1.1 across the Bam rupture zone. The overall depth distribution of b-value in Bam aftershock zone reveals two distinct increases in b-value: (1) at depths of 8–10 km and (2) shallower than 4 km beneath the Bam city. There is no correlation between high b- value anomalies found in this study and the region of largest slip, whereas the spatial correlation between high b-value anomalies and the zone of low Vs and high σ (in earlier tomography study) is obvious. This correlation reveals that material properties and increasing heterogeneity are more important in controlling b-value distribution in Bam earthquake rupture zone. The high b-value anomaly near the surface of northern part of rupture zone may be related to unconsolidated and water-rich quaternary alluvial sediments and probable low-strength rocks beneath them. The high b-value anomaly at depth range 8–10 km can be correlated with fractured and fluid-filled mass, which may result from the movement of magma during Eocene volcanism in the Bam area. In this study, the induced changes in pore fluid pressure due to main shock are suggested as a mechanism for aftershock generation.

Co-seismic deformation of 2011 Mw9. 0 Japan earthquake observed by InSAR technique

Abstract: Co-seismic line-of-sight displacements of the 2011 M w9. 0 Japan earthquake derived from InSAR data of Envisat ASAR, ALOS PALSAR and TerraSAR-X show a maximum value of about –245 cm to –221cm near the epicenter. This result is in good agreement with the result of GPS measurement. The observed displacement pattern suggests an earthquake-rupture zone over 500km long, with a ground-motion pattern in the vicinity of the northern segment more complex than that of the southern segment, possibly due to immediate aftershocks that occurred between satellite passes.

Seismites in the Kathmandu basin and seismic hazard in central Himalaya

Soft-sediment deformation structures have been analyzed at six sites of the Kathmandu valley. Microgranulometric study reveals that silty levels (60 to 80% silt) favor the development of soft-sediment deformation structures, while sandy levels (60 to 80% sand) are passively deformed. Nonetheless well sorted sand levels (more than 80% sand) generate over-fluid pressure during compaction if located beneath a silty cap, leading to fluidization and dike development. 3-D geometry of seismites indicates a very strong horizontal shearing during their development. Using a physical approach based on soil liquefaction during horizontal acceleration, we show that the fluidization zone progressively grows down-section during the shaking, but does not exactly begin at the surface. The comparison of bed-thickness and strength/depth evolution indicates three cases: i) no soft-sediment deformation occurs for thin (few centimeters) silty beds; ii) the thickness of soft-sediment deformation above sandy beds is controlled by the lithological contrast; iii) the thickness of soft-sediment deformation depends on the shaking intensity for very thick silty beds. These 3 cases are evidenced in the Kathmandu basin. We use the 30 cm-thick soft-sediment deformation level formed during the 1833 earthquake as a reference: the 1833 earthquake rupture zone extended very close to Kathmandu, inducing there MMI IX–X damages. A 90 cm-thick sediment deformation has therefore to be induced by an event greater than MMI X. From a compilation of paleo and historic seismology studies, it is found that the great (M ~ 8.1) historical earthquakes are not characteristic of the greatest earthquakes of Himalaya; hence earthquakes greater than M ~ 8.6 occurred. Kathmandu is located above one of the asperities that laterally limits the extent of mega-earthquake ruptures and two successive catastrophic events already affected Kathmandu, in 1255 located to the west of this asperity and in ~ 1100 to the east.

Arc segmentation and seismicity in the Solomon Islands arc, SW Pacific

This paper evaluates neotectonic segmentation in the Solomon Islands forearc, and considers how it relates to regional tectonic evolution and the extent of ruptures of large megathrust earthquakes. We first consider regional geomorphology and Quaternary vertical displacements, especially uplifted coral reef terraces. Then we consider geographic seismicity patterns, aftershock areas and vertical displacements for large earthquakes, focal mechanisms, and along-arc variations in seismic moment release to evaluate the relationship between neotectonically defined segments and seismicity. Notably, one major limitation of using seismicity to evaluate arc segmentation is the matter of accurately defining earthquake rupture zones. For example, shoreline uplifts associated with the 1 April 2007 M w 8.1 Western Solomons earthquake indicate that the along-arc extent of rupture was about 50 km smaller than the aftershock area. Thus if we had relied on aftershocks alone to identify the 2007 rupture zone, as we do for most historical earthquakes, we would have missed the rupture's relationship to a major morphologic feature. In many cases, the imprecision of defining rupture zones without surface deformation data may be largely responsible for the poor mismatches to neotectonic boundaries. However, when a precise paleoseismic vertical deformation history is absent, aftershocks are often the best available tool for inferring rupture geometries. Altogether we identify 16 segments in the Solomon Islands. These comprise three major tectonic regimes or supersegments that correspond respectively to the forearc areas of Guadalcanal–Makira, the New Georgia island group, and Bougainville Islands. Subduction of the young and relatively shallow and buoyant Woodlark Basin and spreading system distinguishes the central New Georgia supersegment from the two neighboring supersegments. The physiographic expression of the San Cristobal trench is largely absent, but bathymetric mapping of the surface trace of the interplate thrust zone defines it adequately. The New Georgia supersegment has smaller arc segments, and more islands due to general late Quaternary forearc uplift very close to the trench where vertical displacement rates tend to be faster; prior to the 2007 earthquake it had much lower rates of seismic activity than the neighboring supersegments. Generally the mean along-arc lateral extent of Solomon arc segments is about 75 km, somewhat smaller than the segments reported in some other island arcs such as Japan (~ 100–260 km), but larger than those of the Tonga (30–80 km) and Central New Hebrides arcs (30–110 km). These differences may be real but it may occur simply because the coral-friendly tropical environment of the South Pacific arcs, numerous emerged forearc islands, and high seismicity rates provide an unusually favorable situation for observing variations in vertical tectonic activity and thus for identifying segment boundaries. Over the past century seismic slip in the Solomons, as indicated by seismic moment release, has corresponded to about half the plate convergence rate; however, there are notable variations along the arc. Even with the 2007 earthquake, the long-term moment release rate in the New Georgia supersegment is relatively low, and this may indicate that large earthquakes are imminent.

Enhanced reflectivity of backthrusts in the recent great Sumatran earthquake rupture zones

In the last five years, there have been three great megathrust earthquakes in the Sumatra subduction zone, and a part of the zone is still locked. We have carried out deep seismic reflection surveys along the SW Sumatran margin and have imaged backthrusts along four profiles down to 15–20 km depth. We find that the seismic images of the backthrusts in the 2004 and 2007 great earthquake ruptured zones are brighter than those in the regions where the subduction zone is still locked. We suggest that this enhanced reflectivity is due to the increase of fluid contents along the reactivated backthrusts during or soon after the great earthquakes. If this interpretation is valid, then the backthrusts observed in the forearc basins could be used to monitor fluid movement from mantle wedge and earthquake precursors before and after an imminent great earthquake in the Sumatra locked zone.

Laboratory-Based Maximum Slip Rates in Earthquake Rupture Zones and Radiated Energy

Laboratory stick-slip friction experiments indicate that peak slip rates increase with the stresses loading the fault to cause rupture. If this applies also to earthquake fault zones, then the analysis of rupture processes is simplified inasmuch as the slip rates depend only on the local yield stress and are independent of factors specific to a particular event, including the distribution of slip in space and time. We test this hypothesis by first using it to develop an expression for radiated energy that depends primarily on the seismic moment and the maximum slip rate. From laboratory results, the maximum slip rate for any crustal earthquake, as well as various stress parameters including the yield stress, can be determined based on its seismic moment and the maximum slip within its rupture zone. After finding that our new equation for radiated energy works well for laboratory stick-slip friction experiments, we used it to estimate radiated energies for five earthquakes with magnitudes near 2 that were induced in a deep gold mine, an M 2.1 repeating earthquake near the San Andreas Fault Observatory at Depth (SAFOD) site and seven major earthquakes in California and found good agreement with energies estimated independently from spectra of local and regional ground-motion data. Estimates of yield stress for the earthquakes in our study range from 12 MPa to 122 MPa with a median of 64 MPa. The lowest value was estimated for the 2004 M 6 Parkfield, California, earthquake whereas the nearby M 2.1 repeating earthquake, as recorded in the SAFOD pilot hole, showed a more typical yield stress of 64 MPa.

Strong aftershocks in the northern segment of the Wenchuan earthquake rupture zone and their seismotectonic implications

More than 28, 000 aftershocks have occurred since the 05/12/2008 Wenchuan earthquake, with dozens of them stronger than M 5. Since July, 2008, all the M > 5 earthquakes have occurred only in the northern segment of the rupture zone, suggesting obvious seismicity segmentation. We applied the double difference method to relocate all of the M > 3 aftershocks. After relocation, the aftershocks show a compact zone of seismicity, with a length of about 300 km and average width of 30 km, supporting that the hypothesis that the Beichuan-Yingxiu and Chaping-Linjiaan faults are the faults that ruptured in the earthquake. With the Cut and Paste (CAP) waveform inversion algorithm, we determined the source mechanism and focal depth of all the > M 5 aftershocks in the northern segments. The number of thrust events is close to the number of strike-slip events, but almost all of the events with thrust mechanism are distributed over the northern segment, while the aftershocks with strike-slip mechanism only occurred at the north-easternmost end, contrasting with field observations of a substantial strike-slip component of surface rupture over the northern segment. The events with strike-slip mechanism occurred at depths up to 18 km, consistent with the lack of surface rupture in the north-easternmost section. However, since early August, very shallow events (2 km) with thrust mechanism have occurred, probably releasing the strain energy of the unruptured fault in the north-easternmost section. It seems that the seismic hazard potential of the northern segment is still quite high, and more studies are needed to resolve some of the discrepancy suggested by aftershock patterns and other observations.

Low-Frequency 3D Wave Propagation Modeling of the 12 May 2008 Mw 7.9 Wenchuan Earthquake

The seismic potential of southern China is associated with the collision between the Indian and the Eurasian plates. This is manifested in the western Sichuan Plateau by several seismically active systems of faults, such as the Longmen Shan. The seismicity observed on the Longmen Shan fault includes recent events with magnitudes of up to 6.5, and the one of 12 May 2008 Mw 7.9 Wenchuan earth- quake. Herewith, as part of an ongoing research program, a recently optimized three- dimensional (3D) seismic wave propagation parallel finite-difference code was used to obtain low-frequency (≤ 0:3 Hz) 3D synthetic seismograms for the Wenchuan earthquake. The code was run on KanBalam (Universidad Nacional Autonoma de Mexico, Mexico) and HECToR (UK National Supercomputing Service) supercompu- ters. The modeling included the U.S. Geological Survey 40 × 315 km 2 kinematic description of the earthquake's rupture, embedded in a 2400 × 1600 × 300 km 3 phys- ical domain, spatially discretized at 1 km in the three directions and a temporal dis- cretization of 0.03 s. The compression and shear wave velocities and densities of the geologic structure used were obtained from recently published geophysical studies performed in the Sichuan region. The synthetic seismograms favorably compare with the observed ones for several station sites of the Seismological and Accelerographic Networks of China, such as MZQ, GYA, and TIY, located at about 90, 500, and 1200 km, respectively, from the epicenter of the Wenchuan event. Moreover, the com- parisons of synthetic displacements with differential radar interferometry (DinSAR) ground deformation imagery, as well as of maximum velocity synthetic patterns with Mercalli modified intensity isoseist of the 2008 Wenchuan earthquake, are acceptable. 3D visualizations of the propagation of the event were also obtained; they show the source rupture directivity effects of the Mw 7.9 Wenchuan event. Our results partially explain the extensive damage observed on the infrastructure and towns located in the neighborhood of the Wenchuan earthquake rupture zone.

Reply to comment by J. Déverchère et al. on “Zemmouri earthquake rupture zone (Mw 6.8, Algeria): Aftershocks sequence relocation and 3D velocity model”

Reply to comment by J. Déverchère et al. on “Zemmouri earthquake rupture zone (<i>M</i>_<i>w</i> 6.8, Algeria): Aftershocks sequence relocation and 3D velocity model”

Comment on “Zemmouri earthquake rupture zone (Mw 6.8, Algeria): Aftershocks sequence relocation and 3D velocity model” by A. Ayadi et al.

Although often difficult to characterize, the relationship between a seismic rupture, its aftershock sequence, and cumulative subsurface or surface faulting or folding is an important challenge to modern seismology and seismotectonics. Among other benefits, it helps document fault length, slip, and magnitude relationships, reconstruct the evolution of the rupture process through historical and prehistorical times and identify the complexity of the deformation in its path toward the surface. This approach is a prerequisite to any seismic hazard assessment but is particularly difficult for faults whose surface trace projects offshore. A specific effort to identify and quantify the source parameters of large earthquakes in coastal areas is therefore needed, not only in subduction zones but also in areas of slow rate and/or diffuse deformation.

Redefining Medlicott–Wadia's main boundary fault from Jhelum to Yamuna: An active fault strand of the main boundary thrust in northwest Himalaya

The MBT demarcates a tectonic boundary between the Tertiary Sub Himalaya and the pre-Tertiary Lesser Himalaya. South of the MBT, another tectonically important fault extends from Muzaffarabad and Riasi in Jammu–Kashmir to Bilaspur and Nahan in Himachal. Medlicott and Wadia had designated this fault the Main Boundary Fault (MBF) in Simla Hills and Jammu region respectively. In between these two areas, later workers gave local-area names to the MBF as the Riasi Thrust in Jammu, Palampur Thrust in Kangra, Bilaspur Thrust in Simla Hills and Nahan Thrust in Sirmur. We have reviewed and established the tectonostratigraphic framework and physical continuity of the lower Tertiary belt and the MBF. The lower Tertiary belt, lying south of the MBT, has characteristic tectonostratigraphic setting with discontinuous bodies of stromatolite-bearing Proterozoic limestone overlain with depositional contact by the Paleocene-lower part Middle Eocene marine Subathu/Patala formation which in turn overlain by the Upper Oligocene–Lower Miocene non-marine Dharamsala/Murree Formation. To avoid confusion with the MBT, we designate collectively the MBF and related faults as the Medlicott–Wadia Thrust (MWT). The MWT extends east of Hazara–Kashmir syntaxis to river Yamuna, covering a distance of ∼ 700 km. Further east of Yamuna, the lower Tertiary belt pinches out and the MWT merges with the sensuo-stricto MBT. The Proterozoic limestone represents the basement over which the lower Tertiary sediments were deposited. The limestone basement with its cover was detached by the MWT, exhuming to the surface and thrusting over largely the Siwalik group. The reactivated Balakot–Bagh Fault, causative fault for the 2005 Kashmir earthquake, extends southeast with right-step to the Riasi Thrust. The Riasi Thrust shows evidence of reactivation and active tectonic activity in Jammu region. It extends further east to the Palampur Thrust in Kangra reentrant, which lies within the 1905 Kangra earthquake rupture zone. The Bilaspur Thrust, continuation of the Palampur Thrust, shows active faulting south of Simla hills between Sataun and Yamuna River. These observations indicate that the MWT represents a southern strand of the sensuo-stricto MBT and shows active faulting in some segments.

The 2006–2007 Kuril Islands great earthquake sequence

The southwestern half of a ∼500 km long seismic gap in the central Kuril Island arc subduction zone experienced two great earthquakes with extensive preshock and aftershock sequences in late 2006 to early 2007. The nature of seismic coupling in the gap had been uncertain due to the limited historical record of prior large events and the presence of distinctive upper plate, trench and outer rise structures relative to adjacent regions along the arc that have experienced repeated great interplate earthquakes in the last few centuries. The intraplate region seaward of the seismic gap had several shallow compressional events during the preceding decades (notably an MS 7.2 event on 16 March 1963), leading to speculation that the interplate fault was seismically coupled. This issue was partly resolved by failure of the shallow portion of the interplate megathrust in an MW = 8.3 thrust event on 15 November 2006. This event ruptured ∼250 km along the seismic gap, just northeast of the great 1963 Kuril Island (Mw = 8.5) earthquake rupture zone. Within minutes of the thrust event, intense earthquake activity commenced beneath the outer wall of the trench seaward of the interplate rupture, with the larger events having normal‐faulting mechanisms. An unusual double band of interplate and intraplate aftershocks developed. On 13 January 2007, an MW = 8.1 extensional earthquake ruptured within the Pacific plate beneath the seaward edge of the Kuril trench. This event is the third largest normal‐faulting earthquake seaward of a subduction zone on record, and its rupture zone extended to at least 33 km depth and paralleled most of the length of the 2006 rupture. The 13 January 2007 event produced stronger shaking in Japan than the larger thrust event, as a consequence of higher short‐period energy radiation from the source. The great event aftershock sequences were dominated by the expected faulting geometries; thrust faulting for the 2006 rupture zone, and normal faulting for the 2007 rupture zone. A large intraplate compressional event occurred on 15 January 2009 (Mw = 7.4) near 45 km depth, below the rupture zone of the 2007 event and in the vicinity of the 16 March 1963 compressional event. The fault geometry, rupture process and slip distributions of the two great events are estimated using very broadband teleseismic body and surface wave observations. The occurrence of the thrust event in the shallowest portion of the interplate fault in a region with a paucity of large thrust events at greater depths suggests that the event removed most of the slip deficit on this portion of the interplate fault. This great earthquake doublet demonstrates the heightened seismic hazard posed by induced intraplate faulting following large interplate thrust events. Future seismic failure of the remainder of the seismic gap appears viable, with the northeastern region that has also experienced compressional activity seaward of the megathrust warranting particular attention.

Analysis of Izmit aftershocks 25 days before the November 12th 1999 Düzce earthquake, Turkey

We investigate spatial clustering of 2414 aftershocks along the Izmit M w = 7.4 August 17, 1999 earthquake rupture zone. 25 days prior to the Düzce earthquake M w = 7.2 (November 12, 1999), we analyze two spatial clusters, namely Sakarya (SC) and Karadere–Düzce (KDC). We determine the earthquake frequency–magnitude distribution ( b-value) for both clusters. We find two high b-value zones in SC and one high b-value zone in KDC which are in agreement with large coseismic surface displacements along the Izmit rupture. The b-values are significantly lower at the eastern end of the Izmit rupture where the Düzce mainshock occurred. These low b-values at depth are correlated with low postseismic slip rate and positive Coloumb stress change along KDC. Since low b-values are hypothesized with high stress levels, we propose that at the depth of the Düzce hypocenter (12.5 km), earthquakes are triggered at higher stresses compared to shallower crustal earthquake. The decrease in b-value from the Karadere segment towards the Düzce Basin supports this low b-value high stress hypothesis at the eastern end of the Izmit rupture. Consequently, we detect three asperity regions which are correlated with high b-value zones along the Izmit rupture. According to aftershock distribution the half of the Düzce fault segment was active before the 12 November 1999 Düzce mainshock. This part is correlated with low b-values which mean high stress concentration in the Düzce Basin. This high density aftershock activity presumably helped to trigger the Düzce event ( M w = 7.2) after the Izmit M w 7.4 mainshock.

Space-time variations of the shear wave attenuation field in the upper mantle of seismic and low seismicity areas

This paper deals with characteristics of the short period S -wave attenuation field in the rupture zones of 37 large and great earthquakes with M s = 7.0-8.6, as well as in low seismicity areas. We estimate the effective quality factor from Sn and Lg coda envelopes in two time intervals ( Q 1 and Q 2 ). The quantity Q 1 is a measure of shear wave attenuation in the uppermost mantle, at depths of down to approximately 200-250 km, while Q 2 is relevant to deeper horizons of the upper mantle. We studied variations in the attenuation field in the rupture zone of the 1950 Assam earthquake. We examined the parameters Q 1 , Q 2 , and Q 1 / Q 2 as functions of the time ∆ T elapsed after a large earthquake. It is shown that the parameter Q 2 in rupture zones is practically inde- pendent of ∆ T , while the quantities Q 1 and Q 1 / Q 2 increase until ∆ T ~ 20-25 years, especially rapidly for normal, normal-oblique, and strike-slip earthquake mechanisms. This analysis provides evidence that, as ∆ T increases, so does the quality factor in the upper mantle for shear waves. It is supposed that this is related to the rise of mantle fluids to the crust. Geodynamic mechanisms are discussed that can support a comparatively rapid "dry- ing" of the upper mantle beneath earthquake rupture zones.

The influence of great earthquakes on volcanic eruption rate along the Chilean subduction zone

Seismic activity has been postulated as a trigger of volcanic eruption on a range of timescales, but demonstrating the occurrence of triggered eruptions on timescales beyond a few days has proven difficult using global datasets. Here, we use the historic earthquake and eruption records of Chile and the Andean southern volcanic zone to investigate eruption rates following large earthquakes. We show a significant increase in eruption rate following earthquakes of M W > 8, notably in 1906 and 1960, with similar occurrences further back in the record. Eruption rates are enhanced above background levels for ~ 12 months following the 1906 and 1960 earthquakes, with the onset of 3–4 eruptions estimated to have been seismically influenced in each instance. Eruption locations suggest that these effects occur from the near-field to distances of ~ 500 km or more beyond the limits of the earthquake rupture zone. This suggests that both dynamic and static stresses associated with large earthquakes are important in eruption-triggering processes and have the potential to initiate volcanic eruption in arc settings over timescales of several months.

Structural Configuration of the Otates Fault (Southern Basin and Range Province) and Its Rupture in the 3 May 1887 MW 7.5 Sonora, Mexico, Earthquake

Abstract During the great 3 May 1887 Sonoran earthquake (surface-rupture end-to-end length: 101.8 km; M W 7.5±0.3), an array of three north–south-striking Basin and Range Province faults (from north to south: Pitaycachi, Teras, and Otates) slipped along the western margin of the Sierra Madre Occidental Plateau of northern Mexico. This detailed field survey of the previously undocumented 1887 earthquake rupture zone along the Otates fault includes mapping the rupture scarp and measurements of surface deformation at 25 sites. The surface rupture coincides with the mapped trace of the Otates fault, dips 68° W, and has an end-point-to-end-point length of 18.2 km. The lateral rupture terminations are controlled by cross faults. Striation measurements and the style of rupture indicate extensional dip slip without significant lateral displacement. The average shortening ( P ) and extension ( T ) kinematic axes inferred from the striations are similar to published composite focal mechanisms of the microseismicity recorded in the epicentral region of the 1887 earthquake. Based on surface deformation, maximum slip along the Otates fault in the 1887 earthquake was 270 cm, and the average slip was 189 cm. A quadratic best fit to the along-rupture surface offsets indicates an asymmetric distribution with the maximum offset near the northern end of the rupture segment, which can be explained by the mechanical interaction of the Teras and Otates surface-rupture segments through their stress fields. The average recurrence interval of 1887-sized earthquakes on the Otates fault is 30–42 kyr based on the long-term geologic dip-slip rate of the fault (0.06 mm/yr) and its amount of slip in the 1887 earthquake.

Rupture behavior and deformation localization of the Kunlunshan earthquake (M w 7.8) and their tectonic implications

Earthquake surface rupture is the result of transformation from crustal elastic strain accumulation to permanent tectonic deformation. The surface rupture zone produced by the 2001 Kunlunshan earthquake (Mw7.8) on the Kusaihu segment of the Kunlun fault extends over 426 km. It consists of three relatively independent surface rupture sections: the western strike-slip section, the middle transtensional section and the eastern strike-slip section. Hence this implies that the Kunlunshan earthquake is composed of three earthquake rupturing events, i.e. the Mw=6.8, Mw=6.2 and Mw⩽=7.8 events, respectively. The Mw=7.8 earthquake, along the eastern section, is the main shock of the Kunlunshan earthquake, further decomposed into four rupturing subevents. Field measurements indicate that the width of a single surface break on different sections ranges from several meters to 15 m, with a maximum value of less than 30 m. The width of the surface rupture zone that consists of en echelon breaks depends on its geometric structures, especially the stepover width of the secondary surface rupture zones in en echelon, displaying a basic feature of deformation localization. Consistency between the Quaternary geologic slip rate, the GPS-monitored strain rate and the localization of the surface ruptures of the 2001 Kunlunshan earthquake may indicate that the tectonic deformation between the Bayan Har block and Qilian-Qaidam block in the northern Tibetan Plateau is characterized by strike-slip faulting along the limited width of the Kunlun fault, while the blocks themselves on both sides of the Kunlun fault are characterized by block motion. The localization of earthquake surface rupture zone is of great significance to determine the width of the fault-surface-rupture hazard zone, along which direct destruction will be caused by co-seismic surface rupturing along a strike-slip fault, that should be considered before the major engineering project, residental buildings and life line construction.

An Algorithm of Fault Parameter Determination Using Distribution of Small Earthquakes and Parameters of Regional Stress Field and Its Application to Tangshan Earthquake Sequence

AbstractIt is known that clustered small earthquakes often occur in the fault plane vicinities of large earthquakes. Based on the simulated annealing and Gauss‐Newtonian nonlinear inversion algorithms, we develop a robust algorithm to estimate fault plane parameters of strong earthquakes such as strike, dip, and location using data of densely populated small events. Rake angle of the fault plane is further inferred from regional tectonic stress parameters. This method can also be employed for determination of fault plane parameters of an active fault using data of densely populated small earthquakes.We apply the algorithm to investigate the fault planes of the 1976 Tangshan, Luanxian, and Ninghe earthquakes and the Lulong fault, using precisely relocated earthquakes which occurred near the earthquake rupture zone between 1 April 2002 and 31 May 2006, and obtain the parameters of strike, dip, location, and rake angle of these fault planes. Our estimated fault planes are close to the results of previous studies, attesting the validity of our method. In addition, we discover an aligned seismic belt east of the Luanxian seismic zone trending NE‐SW, which can be ascertained as a fracture zone. The faulting parameters of the fracture zone such as the strike, dip and rake are determined accurately. It is still an open question whether the Luanxian and Ninghe faults have ruptured during the Tangshan earthquake sequence, answers to the question await for further multi‐disciplinary studies and bear great significance for the understanding of dynamic processes of the Tangshan earthquake sequence.