Allah Bund Research Articles

Unveiling the Mechanisms of the 1819 M 7.7 Kachchh Earthquake, India: Integrating Physics‐Based Simulation and Strong Ground Motion Estimates

AbstractThis study provided a comprehensive understanding of the source process of the 1819 M 7.7 Kachchh Indian earthquake using physics‐based dynamic rupture modeling and strong ground motion simulations. We successfully simulated the spontaneous dynamic rupture along a curved non‐planar fault using the 3‐D curved‐grid finite‐difference method (CGFDM). The estimated earthquake magnitude is around 7.6, consistent with previous estimations. Our simulations accurately replicated macroscopic rupture patterns and surface deformation, showing agreement with observed data along the Allah Bund fault (ABF) with a maximum displacement ∼5.5 m at the Earth's surface. The maximum modeled coseismic slip on the fault was approximately 7.5 m. Notably, the ABF exhibited characteristics of a weak barrier (leaky barrier) at the bending part, allowing the rupture to propagate further. Despite limitations in surface deformation calculations, the modeled values aligned with the trend of surface fault slip, with a slight deviation in the epicenter toward the east compared to earlier studies. We observed a homogeneous principal stress oriented N25°E, consistent with the present day Indian plate motion. The estimated horizontal peak ground velocities (PGVh) and the maximum value of Intensity X+ aligns well with observations. Furthermore, conducting thorough case studies on significant earthquakes and potential seismic scenarios in stable continental regions is crucial. Such studies play a vital role in validating and improving dynamic rupture models. When combined with statistical methods, this research holds great promise for advancing seismic hazard assessments, earthquake engineering, and strategies for disaster management.

Holocene surface-rupturing paleo-earthquakes along the Kachchh Mainland Fault: shaping the seismic landscape of Kachchh, Western India

This study explores the seismotectonics of Kachchh in western India, a region with a low-to-moderate strain rate and a history of significant earthquakes, notably the 1819, Mw 7.8 Allah Bund, and the 2001, Mw 7.6 Bhuj. Despite its substantial seismic risk, comprehensive studies on Kachchh’s seismogenic sources are scarce. This is attributed to the concealed nature of active structures, hindering definitive age constraints in paleoseismological research. Our research comprises a detailed paleoseismic analysis of the north-verging, reverse Jhura Fault underlying the Jhura anticline, a segment of the Kachchh Mainland Fault. This fault segment shows evidence of surface-rupturing earthquakes in the area south of the Great Rann of Kachchh. The investigation reveals three paleoseismic events: Event I before 9.72 ka B.P., Event II between 8.63–8.20 ka B.P., and Event III between 6.20–6.09 ka B.P. The elapsed time since the last event on this fault is > 8000 years, suggesting that the area is exposed to a significant earthquake hazard. This highlights the need for more precise characterization of individual seismogenic sources for future earthquake preparedness.

200 years’ of Commemoration of Allah Bund Earthquake of Kachchh

200 years’ of Commemoration of Allah Bund Earthquake of Kachchh

Active tectonics of eastern segment of the South Wagad Fault Zone, Kachchh, Western India

The present study describes the active tectonic features of the eastern segment of the South Wagad Fault Zone (SWFZ) of the Kachchh Rift Basin (KRB). The SWFZ defines the southern margin of the Wagad uplift with a series of small domes and anticlines forming a low hill range. Geomorphologically, the SWFZ is expressed by the presence of the South Wagad Hill range to the north and the south sloping Samkhiyali and Lakadia plains to the south. The easternmost part of the SWFZ consists of domes and anticlines having a rugged topography, which comprises of Mesozoic to Neogene rocks and is practically unexplored as far as their active tectonic characterizations are concerned. The geomorphic expression of the SWFZ in the eastern segment consists of the Gagodar, Kanmer, Guan and Mardak domes and anticlines. They are exposed as a series of linear mounts projected amidst the Little Rann of Kachchh and surrounded by the alluvial sediments of easterly flowing rivers as well as the Rann sediments. The active nature of the eastern SWFZ is portrayed by warping, flexuring, alluvial fans, tilted Quaternary deposits, scarps, gullied surfaces, slickensides and knickpoints. Presence of wave cut cliff and notches at Mardak Bet suggests that the eastern segment of SWFZ is marginally uplifted during Middle to Late Holocene period. We provided a detailed morphotectonic map of the eastern segment of SWFZ, which can be used for future infrastructural development, town planning and seismic hazard assessment in the region. Further, the study suggests that the seismically silent eastern segment of the active SWFZ is capable of generating a large magnitude earthquake similar to the 2001 Bhuj earthquake or the 1819 Allah Bund earthquake in the future, owing to long-term stress accumulation.

Palaeoseismic investigations along the Kachchh Mainland Fault: A comprehensive review and new insights of the past earthquakes in the Kachchh basin, western India

The intraplate Kachchh Rift basin has been hit by several devastating earthquakes in the historic past including the 1819 Allah Bund Earthquake and the 2001 Bhuj earthquake. The source region of these earthquakes, within the basin have been studied by several workers in the last two decades to understand their potential for earthquake recurrence. However, very little information is available on the palaeoseismic and geomorphic characterization of Kachchh Mainland Fault (KMF). Therefore, in the present study, six trenches were excavated across the KMF, between Lakhpat and Nirona to understand ground deformation pattern and timing of the historic earthquakes. Based on geomorphic and palaeoseismic investigations, five palaeoearthquakes of Early to Late Holocene have been identified between 10,000–890 yrs. Apart from the Holocene, an earthquake of Late Quaternary period was also identified, which possibly occurred around 19,800 yrs BP. Fault related parameters were analyzed to understand the geometry of the active fault scarp along KMF. The results of the analyzed fault geometric parameters show that the vertical displacement along the KMF is higher than the horizontal displacement. The slip rate of the KMF from the western portion to the middle portion decreases from 0.08 to 0.04 mm/yr, and increases towards east from 0.22 mm/yr to 0.36 mm/yr. As the Kachchh district of the Gujarat state is rapidly developing in terms of infrastructural development, the outcome of this research might provide significant inputs for micro zonation studies and also in the evaluation of the seismic hazard in the Kachchh region.

Intensity estimation for the 2001 Bhuj earthquake, India on ESI-07 scale and comparison with historical 16th June 1819 Allah Bund earthquake: A test of ESI-07 application for intraplate earthquakes

On 26th January 2001, an earthquake of magnitude Mw 7.7 occurred near Bhuj, in northwestern India, resulting in severe environmental effects. No unequivocal primary surface rupture was observed for the earthquake, but it caused widespread liquefaction and lateral spreading in the Rann of Kachchh and Little Rann. After the earthquake, several researchers collected field evidence of secondary surface rupture, rockfall, dry craters, and surface manifestations of liquefaction, including the formation of mud volcanoes and lateral spreads, in the meizoseismal area. Analysis of pre- and post-earthquake satellite images suggests that several “dry” streams in the Rann of Kachchh began to flow due to extensive liquefaction induced by the earthquake. In this present study, the macroseismic intensity of the Bhuj earthquake is evaluated by considering these environmental effects and applying the ESI-07 intensity scale to the affected area. As an outcome, the epicentral intensity of the 2001 Bhuj earthquake was determined to be XI. According to historical records and seismic catalogs, 16th June 1819 Allah Bund earthquake caused prominent surface rupture which was not so clear in the case of 2001 Bhuj earthquake, but the secondary effects were similar for both earthquakes. Considering the environmental effects caused by the 1819 Allah Bund earthquake, an intensity of XI was estimated for the epicentral area. For both earthquakes, the ESI scale yields a significant difference of one to two degrees with the traditional intensity scales. The 2001 Bhuj earthquake and 1819 Allah Bund earthquake shows similar ESI-07 intensity of XI despite of different epicentral locations. This implies the reliability of ESI-07 scale application for different earthquakes of similar dimensions in the same geological setting. This study contributes to the application of ESI-07 scale for Indian earthquakes, especially reverse faulting events, and to the future improvement of the ESI scale with emphasis on its applicability to historical earthquakes on the Indian subcontinent. Also, this study may help in future land use planning in the meizoseismal area of 1919 Allah Bund and 2001 Bhuj earthquakes.

Geodetic Evidence of High Compression Across Seismically Active Kachchh Paleorift, India

AbstractIn the past 200 years, the Kachchh paleorift in western India, a plate interior region, has witnessed unusually large number of strong earthquakes, namely, the 2001 Bhuj (Mw 7.6), 1956 Anjar (Mw 6.0), 1845 Lakhpat (Mw ~6), and 1819 Allah Bund (Mw ~7.7) earthquakes. We report continuous GPS measurements from the Kachchh and adjoining region. We find that the ongoing deformation during 2009–2016 is inconsistent with the models of postseismic deformation due to the 2001 Bhuj earthquake. The current widespread deformation pattern implies that the paleorift, bounded by the Kachchh Mainland fault and several other faults along its southern flank and the Allah Bund fault and Nagar Parker fault along its northern flank, is under long‐term compression from both sides at a rate of ~4–5 mm/year. We show that most of this compression is released seismically, making this one of the most seismically active paleorifts in the world.

Present-day crustal deformation and geodetic strain in the vicinity of Dholavira - Harappan civilization site, Kachchh, western part of the Indian plate

To understand the geodynamics in the vicinity of Dholavira, a Harappan Civilization Site (HCS), in Kachchh - western India, GPS data from 2009 to 2015 were processed and analyzed. Tectonically, this part of northern Kachchh is in the influence of Island Belt Fault (IBF) and Allah Bund Fault (ABF) and is seismically one of the most active intra-plate regions of India. The motion with respect to Indian plate is used for the estimation of deformation and strain rate while fault associated slip rate has been estimated by inverse modeling of observed site motions. The analysis reveals that the ABF deforms with an average rate of 1.8 mm/yr. However, the segmented IBF has a maximum deformation rate of 2.9 mm/yr towards the Pachham Island and minimum towards the Khadir Island. The computation discloses a maximum seismic moment (M0) of 2.1 × 1024 dyne-cm in this part which corresponds to an earthquake of ≈Mw 6.0. The calculated maximum strain of 0.04 micro-strain/year is low but significant in the intra-plate region. The postseismic deformation, after the Mw 7.7, 2001 Bhuj earthquake is estimated to be low in this part. The reverse along with strike-slip motion of faults builds up stress in the area and accumulating more strain.

Moment Tensor Solutions of some Selected Local Events: Implications towards the Present-day Tectonics of the Kachchh Rift zone

ABSTRACT The Kachchh rift zone of the northwestern India lies near to the India–Arabia and the India–Eurasia plate boundaries, which has experienced many devastating earthquakes in the past, namely the 1819 Allah Bund earthquake, the 1956 Anjar earthquake and the 2001 Bhuj earthquake. These earthquakes claimed the lives of about 17,000 people. To understand the current seismo-tectonic scenario, moment tensor inversion on the broadband data of fourteen Kachchh events of Mw 3.5-4.6 (during 2009-2015) from 5-12 three-component seismograph stations of the National Geophysical Research Institute (NGRI), Hyderabad, India was applied. Here deviatoric moment tensor inversion of multiple point sources (10–20s) for regional (or local) earthquakes, developed by Zahradnik et al. (2005) was used. The study reveals that modeled focal mechanisms range between reverse and normal oblique strike-slip while no pure normal dip-slip mechanism is found. However, only four out of fourteen events show oblique normal faulting with a minor strike-slip component. Thus, the modeling proposed in this study suggests that the oblique-reverse strike-slip, reverse and strike-slip type focal mechanisms are found to be dominant in the Kachchh rift zone. This observation indicates that the region is presently under compression.

Paleo-earthquake signatures from the South Wagad Fault (SWF), Wagad Island, Kachchh, Gujarat, western India: A potential seismic hazard

In last 500 years, Kachchh experienced several large magnitude earthquakes (6.0 ≥ M ≤ 7.8), however, not all accompanied surface rupture. The 1819 Allah Bund earthquake (Mw7.8) accompanied surface rupture, whereas, the 2001 Bhuj event (Mw7.6) occurred at a depth of 23 km on E-W striking south dipping thrust fault remained blind. Discontinuities between the denser-brittle basement (?) and overlying ductile-softer Mesozoic-Tertiary-Quaternary succession resulted in a different geometry of faulting. Normal faults associated with rift were reactivated as reverse faults during inversion tectonics, propagated in sedimentary succession and arrested. Thrust-ramps developed along the discontinuities accompanied surface ruptures. Folded structures along the South Wagad Fault (SWF) – an active thrust, exhibits lateral-propagation of fold segments and linkage, suggestive of fault-related-fold growth. Paleoseismic investigations revealed evidence of at least three paleo-earthquakes. Event I occurred before BCE 5080; Event II between BCE 4820 and 2320, and was probably responsible for a massive damage at Dholavira – Harappan site. Event III was between BCE 1230 and 04, most likely caused severe damage to Dholavira. Archaeo-seismological Quality Factor (AQF) of 0.5 suggests that the Dholavira is vulnerable to earthquakes from nearby active faults. With 1500–2000 yr of recurrence interval, occurrence of a large magnitude earthquake on SWF cannot be ruled out.

Seismic-Hazard Assessment in the Kachchh Region of Gujarat (India) through Deterministic Modeling Using a Semi-Empirical Approach

Online Material: Peak ground accelerations for scenario earthquakes. The Kachchh and adjoining region in western India have been affected in the past by large damaging earthquakes. In 1668, an M 7 earthquake razed Samaji, a town 200 km west of Kachchh on the Indus delta in Pakistan (Chandra, 1977). In 1819, an M w 7.6 earthquake 100 km northwest of Bhuj formed a 90 km long and 6 m high scarp along the Allah Bund fault (Bilham and Gaur, 2000). In 1845, an earthquake of modified Mercalli intensity VIII hit Lakhpat in the western part of Kutch (Rajendran and Rajendran, 2001) and an M w 6 intensity VIII earthquake struck Anjar in 1956, causing 115 deaths (Tandon, 1959; Chung and Gao, 1995). Nine damaging earthquakes of M w 5–6 have occurred during the past 155 years, on an average of every 17 years. Some of these were in the years 1864, 1903, 1940, and 1950. The last very high devastating earthquake of M w 7.6 occurred in 2001, causing 14,000 deaths and an economic loss of $10 billion U.S. (Rastogi, 2004). The major international trade traffic in oil/and gas (energy sector), bulk chemicals, and construction materials is through Gujarat coast because of its superior infrastructure in involving harbors, ports, jetties, and oil and gas pipelines. There are 42 ports, numerous jetties, several refineries, and a large number of chemical industries. Gujarat coast also lies in the most severe seismic zone (zone V) of India. In case of a major seismic event, large‐scale damage to these facilities will severely disrupt the trade traffic and adversely affect the national economy. Apart from the coastal development, Gujarat state is expanding its industrial development in other parts of Kachchh. Therefore seismic‐hazard assessment of the region is important and essentially required. The earthquake scenarios describe the effect of …

Evidence for right-lateral strike-slip environment in the Kutch basin of northwestern India from moment tensor inversion studies

Abstract The Kutch region located in northwestern part of India is an ancient rift basin that was active until Cretaceous period. The region falls close to the India–Arabia and the India–Eurasia plate boundaries and has experienced devastating earthquakes in the past, namely the 1819 Allah Bund earthquake, the 1956 Anjar earthquake and the 2001 Bhuj earthquake. To understand the tectonics of this region with respect to the adjacent plate boundaries, we invert seismic waveform data of 11 earthquakes in this region recorded by a network of the Institute of Seismological Research (ISR) during 2007–2009. The study yields focal mechanism solutions of reverse fault and strike-slip type mechanism. The inferred fault planes correlate well with the local trends of the known tectonic faults while the principal stress directions derived from stress inversion based on a linearized least squares approach, trend agreeably with the ambient stress field directions. A consistently right-lateral sense of shear is found on all the local faults as derived from each of the matching planes of the focal mechanism solutions computed in the present study. It is inferred that in the Kutch region a right-lateral strike-slip environment prevails along predominantly EW to NW-SE oriented deep-seated pre-existing faults in an otherwise compressive stress regime. This, in conjunction with the left-lateral movements along the Girnar mountain in southern Saurashtra, inferred from previous studies, indicates a westward escape of the Kutch–Saurashtra block as a consequence of the northward collision of the Indian plate with respect to the Eurasian landmass.

Seismogenesis of the uninterrupted occurrence of the aftershock activity in the 2001 Bhuj earthquake zone, Gujarat, India, during 2001–2010

Seismogenesis of aftershocks occurring in the Kachchh seismic zone for more than last 10 years is investigated through modeling of fractal dimensions, b-value, seismic velocities, stress inversion, and Coulomb failure stresses, using aftershock data of the 2001 Bhuj earthquake. Three-dimensional mapping of b-values, fractal dimensions, and seismic velocities clearly delineate an area of high b-, D-, and Vp/Vs ratio values at 15–35 km depth below the main rupture zone (MRZ) of the 2001 mainshock, which is attributed to higher material heterogeneities in the vicinity of the MRZ or deep fluid enrichment due to the release of aqueous fluid/volatile CO2 from the eclogitisation of the olivine-rich lower crustal rocks. We notice that several aftershocks are occurred near the contacts between high (mafic brittle rocks) and low velocity regions while many of the aftershocks including the 2001 Bhuj mainshock are occurred in the zones of low velocity (low dVp, low dVs and large Vp/Vs) in the 15–35 km depth range, which are inferred to be the fractured rock matrixes filled with aqueous fluid or volatiles containing CO2. Further support for this model comes from the presence of hydrous eclogitic layer at sub-lithospheric depths (34–42 km). The depth-wise stress inversions using the P- and T-axes data of the focal mechanisms reveal an increase in heterogeneity (i.e., misfit) with an almost N–S σ1 orientation up to 30 km depth. Then, the misfit decreases to a minimum value in the 30–40 km depth range, where a 60o rotation in the σ1 orientation is also noticed that can be explained in terms of the fluid enrichment in that particular layer. The modeling of Coulomb failure stress changes (ΔCFS) considering three tectonic faults [i.e., NWF, GF, and Allah bund fault (ABF)] and the slip distribution of the 2001 mainshock on NWF could successfully explain the occurrences of moderate size events (during 2006–2008) in terms of increase in positive ΔCFS on GF and ABF. In a nutshell, we propose that the fluid-filled mafic intrusives are acting as stress accentuators below the Kachchh seismic zone, which generate crustal earthquakes while the uninterrupted occurrence of aftershocks is triggered by stress transfer and aqueous fluid or volatile CO2 flow mechanisms. Further, our results on the 3-D crustal seismic velocity structure, focal mechanisms, and b-value mapping will form key inputs for understanding wave propagation and earthquake hazard-related risk associated with the Kachchh basin.

Mid-Holocene sedimentation and landscape evolution in the western Great Rann of Kachchh, India

This study is an attempt to reconstruct the history of sedimentation and landform evolution in the western Great Rann of Kachchh. Field stratigraphy, sedimentology, geochemistry, optical and radiocarbon dating suggest dominance of tidal flat sedimentation with subordinate contribution coming from the southern-draining streams during 5.5 to 2ka. Variability in the geochemical proxies are interpreted as a surrogate for tidal energy (viz. Zr and Cr) which, along with the major elemental ratios (TiO2/Al2O3 and K2O/Al2O3), indicate enhanced tidal energy condition during 5.5 to 5ka and around 3ka. A close correspondence of the major and trace elements with those of the Indus River sediment and the dominance of illite suggest the Indus River was a major contributor for western Rann sedimentation during the mid-Holocene. These sediments were routed through the Kori Creek during marginally high sea level until around 2.2ka and the present landscape was largely sculptured after the 1819 Allah Bund earthquake.

Intensity, Magnitude, Location, and Attenuation in India for Felt Earthquakes since 1762

A comprehensive, consistently interpreted new catalog of felt intensities for India (Martin and Szeliga, 2010, this issue) includes intensities for 570 earth- quakes; instrumental magnitudes and locations are available for 100 of these events. We use the intensity values for 29 of the instrumentally recorded events to develop new intensity versus attenuation relations for the Indian subcontinent and the Himalayan region. We then use these relations to determine the locations and mag- nitudes of 234 historical events, using the method of Bakun and Wentworth (1997). For the remaining 336 events, intensity distributions are too sparse to determine mag- nitude or location. We evaluate magnitude and location accuracy of newly located events by comparing the instrumental- with the intensity-derived location for 29 cali- bration events, for which more than 15 intensity observations are available. With few exceptions, most intensity-derived locations lie within a fault length of the instrumen- tally determined location. For events in which the azimuthal distribution of intensities is limited, we conclude that the formal error bounds from the regression of Bakun and Wentworth (1997) do not reflect the true uncertainties. We also find that the regression underestimates the uncertainties of the location and magnitude of the 1819 Allah Bund earthquake, for which a location has been inferred from mapped surface deformation. Comparing our inferred attenuation relations to those developed for other regions, we find that attenuation for Himalayan events is comparable to intensity attenuation in California (Bakun and Wentworth, 1997), while intensity attenuation for cratonic events is higher than intensity attenuation reported for central/eastern North America (Bakun et al., 2003). Further, we present evidence that intensities of intraplate earth- quakes have a nonlinear dependence on magnitude such that attenuation relations based largely on small-to-moderate earthquakes may significantly overestimate the magnitudes of historical earthquakes.

Estimation of static stress changes after the 2001 Bhuj earthquake: Implications towards the northward spatial migration of the seismic activity in Kachchh, Gujarat

Spatial-temporal patterns of aftershocks of the 2001 Mw7.7 Bhuj earthquake during 2001-2008 reveal a northward spatial migration of seismic activity in the Kachchh seismic zone, which could be related with the loading stresses caused by the continued occurrences of aftershocks on the north Wagad fault (NWF), the causative fault of the 2001-mainshock. Aiming at explaining the observed northward migration of activity, we modelled the Coulomb failure stress change (DCFS) produced by the 2001-mainshock, the 2006 Mw5.6 Gedi fault (GF) and the 2007 Mw4.5 Allah bund fault (ABF) events on optimally oriented plane. A strong correlation between occurrences of earthquakes and regions of increased DCFS is obtained on the associated three faults i.e. NWF, ABF and GF. Predicted DCFS on the GF increased by 0.9 MPa at 3 km depth, where the 7 th March 2006 Mw5.6 event occurred, whereas predicted DCFS on the ABF increased by 0.07 MPa at 30 km depth, where the 15 th December 2007 Mw4.5 event occurred. Focal mechanism solutions of three events on the ABF have been estimated using the iterative inversion of broadband data from 5-10 stations, which are also constrained by the first P-motion data from 8-12 stations. These focal mechanism solutions for the ABF events reveal a dominant reverse movement with a strike-slip component along a preferred northwest or northeast dipping plane (∼50-70°). Focal mechanisms of the events on all the three fault zones reveal an N-S oriented P- axis or maximum principal stress in the region, which agrees with the prevailing N-S compression over the Indian plate. It is apparent that the northward migration of the static stress changes from the NWF, resulting from the occurrence 2001 Bhuj mainshock, might have caused the occurrence of the events on the GF and ABF during 2006-08.

Active Low-Angle Reverse Fault and Wide Quaternary Deformation Identified in Jhura Trench across the Kachchh Mainland Fault, Kachchh, Gujarat, India

The Kachchh region has suffered from at least four damaging moderate to large earthquakes since the 17th century. However, none of these earthquakes except the 1819 Allah Bund earthquake accompanied surface rupture. Even the recent 2001 Bhuj earthquake with Mw7.6 occurred on a blind fault. Several faults in the Kachchh viz. the Island Belt Fault, the Kachchh Mainland Fault (KMF), and the Katrol Hill Fault were suggested to be active during Late Quaternary time by previous studies. But there is no such supportive evidence available in the historical documents; also none of recent studies except ours (Morino et al., 2007, Malik et al., 2008, and Morino et al., 2008) reported ground truth that these fault are active. We in our earlier paper reported faulting in Late Pleistocene to Holocene age sediment near the Lodai Village along the KMF. To confirm further active faulting along the KMF, paleoseismic investigation near Jhura Village about 30 km west of Lodai revealed an active fault displacing overbank deposits of Kaila River. Two fault strands F1 and F2 were identified in the trench. The northern F1 shows a low-angle reverse fault with inclination of 15° towards the south. At least two faulting events were inferred on the basis of upward fault termination with clear angular unconformity. The net-slip during a single faulting event considering deformation on the hanging wall of F1 fault is over 5 m.

Quantifying the Media Bias in Intensity Surveys: Lessons from the 2001 Bhuj, India, Earthquake

Many seismologists have looked at the 26 January 2001 Bhuj earthquake as a key modern calibration event that could be used to improve estimates of magnitudes of large historic mainshocks in stable continental regions. Since no instrumental data are available for important historic events such as the 1819 Allah Bund, India, and the 1811–1812 New Madrid, central U.S. mainshocks, calibration hinges on comparisons of the macroseismic effects of these earthquakes with those of comparable modern earthquakes for which a reliable, instrumentally determined moment magnitude is available. However, although such a comparison is conceptually straightforward, in practice it is complicated by potentially significant inconsistencies in methods used to quantify macroseismic effects in different regions and/ or times. For the Bhuj earthquake, extensive intensity data sets have been compiled and published from both media accounts and detailed direct surveys. Comparing the two provides a quantification of the previously suspected media bias, whereby earthquake effects can be exaggerated in media accounts. This bias is a strong function of intensity level, with substantial bias at the highest shaking levels and significantly less bias at low intensities. Because only sparse documentary data are in general available for older historic earthquakes, the results of this study suggest that their inferred intensity distributions might be similarly biased. We further use the survey- based intensity values to develop a new relationship between intensities and ground motions.

Fractal Dimensions of Blocks Using a Box-counting Technique for the 2001 Bhuj Earthquake, Gujarat, India

Several destructive earthquakes have occurred in the Kachchh region of Gujarat during the past two centuries, among them Allah Bund earthquake (M7.8) in 1819, Anjar earthquake (M6) in 1956 and the recent Bhuj earthquake (M7.6) in 2001. The Anjar earthquake was on KMF (Kachchh Mainland Fault) and the recent Bhuj events were caused by a hidden fault north of KMF. The present study discusses the fractal analysis of tectonics governing seismic activity in the region. The region has been divided into five blocks and the fractal dimension of each block has been calculated using the box-counting technique. The results show significantly low value of fractal dimension of the Kachchh rift block consisting of the KMF compared to the other surrounding blocks, which also contain faults and rifts of higher fractal dimension. This indicates that the cause of earthquakes in this block may be asperities and barriers. However, the predominance of aftershocks over foreshocks signifies that barriers may be the main cause. The other results, such as the lower value of dimension of fault clustering show that the Kachchh rift block has faults which are distributed in a clustered manner. In this context, the seismicity of this block seems to be high.

Historical Constraints on Previous Seismic Activity and Morphologic Changes near the Source Zone of the 1819 Rann of Kachchh Earthquake: Further Light on the Penultimate Event

The Kachchh (also spelled Kutch and Cutch) province in the northwestern part of India continues to be remarkably active seismically, the 2001 Bhuj ( Mw 7.7) earthquake being the most recent example of the region's seismogenic potential (Figure 1). Among past events, the 1819 earthquake ( M > 7.5) is well known and is quoted in the early geologic literature for the dramatic coseismic topographic changes associated with it, including the formation of the elevated tract of land called the Allah Bund (Lyell, 1857). Occurrence of two large earthquakes during a short span of 180 years gives an exceptional status to this seismic zone, considered to be part of the stable peninsular shield of India (Johnston, 1989). In addition to these, several moderate events are reported to have occurred in this region. These earthquakes are suggestive of the existence of multiple seismogenic faults in this region, but very little is known about their past histories, adding more uncertainty to the regional hazard assessment. Our search in the epicentral area of the 1819 earthquake led to the identification of another event between 800 and 1,000 years B.P. (Rajendran and Rajendran, 2001a). Based on the relative size and frequency of older sandblows, we interpreted that the earlier earthquake may have also originated from the same source. However, we recognize that isolating earthquake sources using paleoliquefaction features is not an easy task, particularly where large areas of high liquefaction susceptibility are affected by each event. The problem is more complex in regions where multiple sources exist. For example, until the occurrence of the 2001 earthquake, the Allah Bund region was the only known source of large earthquakes in the Kachchh region (Figure 2). Rajendran et al. , (2001) reported large sandblow craters formed by the Bhuj earthquake close to the 1819 source. If distant …