
,A Figure 1 . (A) More than 100,000 fatalities from earthquakes have occurred on the Indian Plate in the past two centuries. Prior to 1800 the historical record is incomplete, although we know of no event in the past millennium that has claimed as many lives as the Republic Day event (Bapat et aL 1983; lyengar and Sharma, 1998) . Government estimates following the Kangra 1905 earthquake indicate 19,500 fatalities (Ambraseys and Bilham, 2000) . The 1737 Calcutta cyclone is mistakenly included in many lists of Indian earthquakes (Bilham, 1994) . (B) The rate of occurrence of fatal earthquakes in the past two decades is more than double its mean value in the past two centuries.

remains as a monsoon-filled depression to this day. A lake also formed north of the Bund in 1819, which drained in 1826 when a torrent broke through several artificial dams on the Puran and cut a gorge through the Bund, flooding regions downstream. The geometry of this gorge was investigated briefly by Sir Alexander Burnes, whose two accounts (1835) differ in numerical detail. Captain William Baker, a canal engineer of considerable integrity (Phillimore, 1968) , undertook a precise spirit leveling line across the Bund in 1844, but his important cross-section was mislaid in Bombay in 1846 and not discovered for more than a half-century (Baker, 1846; Oldham, 1898) . From the details of Baker's leveling line and the assumption that the measured topography is entirely due to the 1819 earthquake, it is possible to infer 11.5+1 m of reverse slip on a north-northeast-dipping fault that terminated 300-600 m below the surface (Bilham, 1998) . Damage to Bhuj and Anjar during the 1819 earthquake was substantial (MacMurdo, 1823 (MacMurdo, , 1839 . Much Iike those from the 2001 event, damage reports from 1819 describe widespread collapse of structures on the Kachchh mainland, accompanied by substantial liquefaction features in the Rann of Kachchh. Damaging earthquakes also occurred in 1845, 1856, 1857, 1864, 1903, 1927, 1940, 1956, and 1970 in the Kachchh region but with less severity (5 < M < 6) (Oldham, 1883; Bapat et al., 1983) . The most recent of these are well located (Tandon, 1956; Chung and Gao, 1995) ; the others are known only by accounts of local damage. The events of ] 845 (mistakenly attributed to 1844; see Bilham, I998) were felt in Karachi (Phillimore, 1968) and are reputed to have generated a tsunami and to have caused changes in channel depth near and north of Lakput on the western edge of the Kachchh Peninsula (LeGrand Jacob, 1860) . Accounts describe numerous aftershocks, so these events may have exceeded M7. Events in 1856 and 1857 are known from reports in accounts of the Great Trigonometrical Survey of India (Phillimore, 1968; Biiham and Gaur, 2000) . The 1956 Anjar earthquake killed 152 people.

The mechanism of the 2001 Republic Day earthquake is currently unresolved, although, as in the 1819 and 1897 earthquakes, the event apparently occurred on a steeply dipping thrust fault that did not break the surface. An unusual feature of the event is that aftershocks have occurred at considerable depth (20-30 km), suggesting rupture through much of the lithosphere. This appears to have similarities to the great 1897 Assam earthquake, which occurred on a deep, steeply south-dipping reverse fault beneath the northern edge of the Shillong Plateau (Bilham and England, 2001) . Aftershocks appear to favor a south-dipping rupture for the nodal plane of the 2001 main shock (Johnston, personal communication, 2001 ).

Slip appears to have been less in 2001 than in 1819, although it is possible that a careful examination of the recent earthquake will reveal new insights into the mechanism and magnitude of the 1819 event. Despite the lack of a surface scarp, there are numerous surface cracks and localized compression and tension features near the epicenter. Geodetic measurements and epicentral aftershock studies currently underway, and anticipated InSAR imagery, will ultimately resolve which of the nodal planes was active during the earthquake (Figure 2A ). From the estimated seismic moment of the earthquake (Harvard/USGS/NEIC) we envisage a downdip rupture width of 15-30 km and an along-strike length of 50-100 km east-west with 1-4 m of slip. The surface manifestation of this rupture is likely to be a broad zone of distributed uplift with a corresponding region of subsidence south of the surface projection of the subsurface fault.
SEISMIC PRODUCTIVITY IN KACHCHH
The Kachchh region is more than 200 km from the closest plate boundary, hence the Republic Day earthquake occurred within the Indian Plate, albeit near a transform boundary. Geodetic measurements of plate deformation between southernmost India and the northern edge of the Gangetic Plain indicate a contraction rate of the Indian plate of 3+2 mm/yr (Paul et al., 2001) Figure 2D indicates postseismic changes predicted to occur in the year following the postseismic survey. The model consists of a layered viscous rheology shown in the inset with a fault terminating 2.5 km below the surface, using the focal mechanism solution in 2A (Pollitz, personal communication, 2001 ).
SEISMIC PRODUCTIVITY IN KACHCHH
causative faults). Historic data record at least one severe earthquake in the region west of Kachchh in about A.D. 1000 and perhaps another to the northeast of the Allah Bund in 1250-1300 (Iyengar and Sharma, 1998) . Paleoseismic investigations of the 1819 Allah Bund revealed several prehistoric earthquakes in the past three millennia consistent with this view Rajendran, 1998, 2001 ). However, since contractile strain almost certainly occurs elsewhere in India (Subrahmanya, 1996; Bendick and Bilham, 1999) , it is possible that the deformation rate in western India is higher than in eastern and central India. Survey-mode GPS measurements that might confirm or refute this possibility were initiated in this region only in the past few years.
SEISMIC PRODUCTIVITY IN KACHCHH
The 188-year interval between the 1819 and 2001 events cannot be explained purely by a simple failure-strain replenishment processes. In Figures 2A and 2B we estimate approximately the dilatational strain fields that may have accompanied these events, calculated using boundary element methods (Toda et al., 1998) . We approximate the Allah Bund rupture as a 90-kin-long, 20-km-wide planar dislocation with an average slip of 6 m with N 18~ strike. The geometry of the Allah Bund event is much simplified in this model since its relict surface expression is quite sinuous and possibly segmented (Oldham, 1898 (Oldham, , 1926 . We apply a regional compressive strain to the Gujarat region to emulate the regional compressive stress azimuth characterized by recent seismicity and measured principal stress directions. The result of this model suggests that the earthquakes of 1845-1865 were partly in response to the westerly-directed lobe of compression from 1819, although this is clearly speculative given our poor understanding of the epicentral regions of these events. The Anjar 1956 and Republic Day 2001 earthquakes occurred in the easterly-directed lobe of increased compressive stress from the 1819 event.
SEISMIC PRODUCTIVITY IN KACHCHH
The region now inferred to be subject to enhanced strain contraction (compressive stress) has extended eastward, although its detailed geometry depends critically on the availability of the coseismic deformation field, which is currently ambiguous. We know of no specific earthquake in the Ahmedabad region in the historical record; however, a large tsunami is reported to have occurred in 1524 that caused considerable alarm to the Portuguese fleet assembled offshore at 17~ ' (Bendick and Bitham, 1999) . A violent earthquake occurred in southern Gujarat in 1705, and minor shaking is reported at Surat in 1663 (iyengar and Sharma, 1998) and 1684 (Oldham, 1883; Bapat et al., 1983) .
GEODESY
The 2001 Republic Day earthquake occurred near the northern edge of a dense network of primary and secondary triangulation points of two first-order triangulation networks (Figure 2 )~the Cutch Coast Series (Strahan, 1893) and the Kathiawar MeridionaI Series (Thullier, 1894) . These triangulation networks form part of the Great Trigonometrical Survey of India (GTS) that was completed toward the end of the 19th century (Phillimore, 1968) . The principal triangulation of the Kachchh region was completed in May 1858 with a mean angle measurement accuracy of 7 pradians, with the exception of a small number of angles in western Kachchh that included lines of sight that grazed hillsides (• Iarad). Anticipated coseismic epicentral angular changes are expected to exceed 100 larad. The original survey monuments consist of subsurface marks on bedrock, or on a buried stone tablet, surmounted by masonry columns with cemented upper marks. Each mark consists of a 1-cmdiameter chiseled depression surrounded by a 15-cm chiseled circle. Our field teams discovered some of the points intact, but in many cases new points were established consisting of pins cemented into exposed rock outcrops (Figure 2) .
GEODESY
Two moderate earthquakes occurred during the initial surveys. According to survey officer D. J. Nasmyth, the November 1856 earthquake distorted angles by up to 24 prad in a triangle of the Kathiawar series approximately 25 km east of Bhachau (Strahan, 1893) . Two months later (in January 1857) an earthquake was sufficiently severe to damage the 4-m-high column at Khari Rohar 15 km east-southeast of Anjar (2305 ' 70~ threatening to overturn the 18-inch theodolite upon it (Strahan, 1893, p. viii) . A leveling network from Ahmedabad through Bhachau to Anjar included some of these triangulation points in 1874-1875 and in 1889-1890. When these heights were remeasured after the 1956 Anjar earthquake, it was discovered that elevation changes of up to a meter had occurred (Nagar and Singh, 1992) .
GEODESY
Despite these indications of significant and repeated deformation and despite important recent progress in implementing GPS geodesy in India (Kulkarni, 1998; Bilham and Gaur, 2000) , no GPS control specifically for earthquakerelated studies had been established in the Kachchh region prior to the earthquake. To our knowledge no published record exists of remeasurement of the Kachchh angles or positions, although unpublished remeasurements for mapping and control purposes are available in government archives that may now be available (Gulatee, 1950) . Subsequent to the earthquake groups of geodesists from the Indian Institute of Geomagnetism (IIG); the Indian Institute of Technology Bombay (IITB), the Centre for Mathematical Modelling and Computer Simulation (CMMACS), Bangalore; and the University of Colorado at Boulder undertook a series of measurements in the Kachchh region. A subset of measurement points measured by these several groups is shown in Figure 2 . The measured points extend approximately 100 km east, west, and south of the epicenter.
GEODESY
The GPS data are currently being processed to determine coseismic changes during the earthquake. However, the number of GTS control points currently recovered is insufficient to constrain more than simple subsurface dislocation geometries (Figure 2A ). We also recognize that the several previous earthquakes since 1860 in the region will have contributed to the cumulative deformation field. We anticipate that planned InSAR imagery using recent images may distinguish between the coseismic strain change from this earthquake and deformation from previous earthquakes and interseismic movements.
GEODESY
In addition to the recovery of coseismic strain changes, new GPS points have been distributed far from the epicenter in order to capture future postseismic deformation resulting from viscous flow in the lower crust ( Figure 2D ). Preliminary aftershock depths and inferred rupture dimensions suggest that the Republic Day earthquake will have significantly perturbed the lower crust. Whereas the viscosity of the lower crust has been investigated near plate boundaries, it has not been possible to do this in a midcontinent setting due to the absence of well recorded events in these areas. The surface response to anticipated viscous flow will lead to a broadening of the coseismic deformation field that will be detectable over periods of decades. Preliminary estimates of viscoelastic vertical and horizontal deformation rates are projected to exceed 1 cm/yr ( Figure 2D ) for a simple starting model of an elastic crust overlying viscous lower crust and mantle (Pollitz, personal communication, 2001) . A network of new geodetic points has been selected for their long-term stability to record these changes over the next century, to distances of more than 100 km from the epicenter. The surface strain rates in the next several years will give an estimate of the viscosity of the weak lower crust in a continental setting.
ISOSEISMAL INTENSITIES
A full compilation of shaking effects for the 2001 earthquake will not be available for some time (e.g., Narula and Chaubey, 2001 ), but we have compiled news accounts from the immediate aftermath of the earthquake and interpreted them to obtain Modified Mercalli intensities (MMI) at over 180 locations throughout India and Pakistan. Maps of the resulting intensity distribution are shown in generate these maps we use a simple mathematical approach to interpolate between locations with known intensity values. Away from the constrained data points, the maps therefore do not reflect geologic site conditions, which influence shaking levels substantially. Previously the largest recorded intraplate earthquake in India occurred in 1897, and its magnitude (M = 8.1 • 0.1) was determined only recently from the sparse global seismic network of the time (Ambraseys and Bilham, 2000) and from geodetic estimates of seismic moment (Bilham and England, 2001 ). The 1967 M _> 6.5 Koyna intraplate earthquake was not felt sufficiently far from the epicenter to calibrate the far-field perceived intensity scale. The magnitudes of numerous other Indian earthquakes are known only from their intensity distributions, and an accurate attenuation versus distance relationship for Modified Mercalli Intensities on the Indian craton is currently unavailable.
ISOSEISMAL INTENSITIES
For the first time, the seismological community will be able to estimate the attenuation of acceleration across the indian craton using intensity observations from an intraplate M_> 7.6 earthquake with a well resolved instrumental magnitude. Although strong-motion accelerograph recordings of the Republic Day earthquake are sparse, we anticipate that the distribution of strong-motion instruments in India will be adequate to calibrate Modified Mercalli Intensity (MMI) values relative to acceleration and frequency data. Isoseismal intensities obtained in the last few weeks already provide an important data set with which to initiate this study. Armed with a newly calibrated attenuation curve we anticipate that we will be able re-evaluate intensity data from historic Indian earthquakes in order to refine their estimated magnitudes (e.g., Johnston, 1996a, b; Ambraseys and Bilham, 2001) . Results from the Republic Day earthquake may also provide a critical calibration for events that occurred in similar tectonic settings worldwide, including the 1811-1812 New Madrid earthquake (see below).
ISOSEISMAL INTENSITIES
Although preliminary, the intensity maps already show several interesting features. The event was felt only lightly at the higher-elevation cities on Deccan lavas throughout central and southern India. Away from the Kachchh region, intensities were dearly amplified significantly in areas that are along rivers, within deltas, or on coastal alluvium. One example is the Narmada River Valley in the province of Madhya Pradesh, where MMI values up to VI were reached at distances of over 600 km. Significant site effects were also observed within Mumbai (Bombay). Most of the city experienced shaking at the MMI V level, but intensities of VI-VII were reached at areas built on landfill in southern and central Mumbai as well as along Bombay Harbour.
ISOSEISMAL INTENSITIES
Variations in the distribution of felt intensity are also apparent in the Kachchh region. The most heavily damaged villages are concentrated toward the western edge of the inferred fault, implying substantial western directivity from the epicenter. Significant sediment-induced amplification is also suggested at a number of locations around the Gulf of Kachchh, including Kandla and many of the villages on mudflats. Liquefaction features produced in the Rann of Kachchh induded sand volcanoes and fissures that expelled enormous volumes of sediment, infilling broad river channels and spreading outward as much as 3-5 kin.
IMPLICATIONS FOR FUTURE HAZARD: INDIA AND BEYOND
The 2001 Republic Day earthquake has important implications for earthquake hazard, not only in India, but also in other parts of the world where the source zones and/or the wave travel paths are similar.
IMPLICATIONS FOR FUTURE HAZARD: INDIA AND BEYOND
The occurrence of the Republic Day earthquake less than 200 years after the severe 1819 event provides further evidence that large intraplate earthquakes can occur in clusters in regions of the crust where the strain rate is relatively low. The event highlights the potential hazard faced by areas that lie outside more rapidly deforming plate boundary regions.
IMPLICATIONS FOR FUTURE HAZARD: INDIA AND BEYOND
The parallels between Kachchh earthquakes and the 1811-1812 New Madrid earthquakes are remarkable. All of these earthquakes occurred within failed rift systems. The largest New Madrid earthquake may also have been similar in size to the Republic Day event. A recent reinterpretation of the 7 February 1812 New Madrid event obtained an M~ 7.5 (Hough et al., 2000) ; previous studies had inferred M w values as high as 8.0 (Johnston, 1996a, b) . Finally, both the Republic Day and the largest New Madrid events occurred on thrust faults favorably oriented for slip in the current stress regime that failed to produce either extensive or pronounced surface ruptures.
IMPLICATIONS FOR FUTURE HAZARD: INDIA AND BEYOND
The distribution of shaking intensity in the Republic Day and New Madrid earthquakes was remarkably similar (Figure 4 ). Both events were felt at coastal regions as far as 2,000 km from the epicenter, both caused light damage at sediment sites as far as 600 km away, and both generated substantial liquefaction over an extremely broad region. A thorough analysis of shaking effects from the Republic Day earthquake may help us better constrain the magnitude of the New Madrid earthquakes.
IMPLICATIONS FOR FUTURE HAZARD: INDIA AND BEYOND
MacMurdo in his many accounts of the 1819 earthquake drew attention to the relative immunity of structures on bedrock compared to those on sediments. A similar observation has been made in the 2001 Republic Day earthquake. it is clear that had this been included as a post-1819 earthquake reconstruction guideline, damage in many areas in 2001 would have been reduced; however, much of the severe damage in villages was the result of construction methods based on unreinforced, undressed masonry. Since 1819 the population has increased by a factor of ten in Gujarat, and seismic-resistant building codes have been implemented. Yet in 1819 2,000 people were killed, and in 2001 the death toll was close to 20,000. It is difficult to escape the grim observation that the same fraction of the Kachchh population lost their lives in the two earthquakes. Yet the long-term legacy of the Republic Day earthquake need not be only one of tragedy if the seismological and earthquake engineering commu- nities can exploit the available data to the fullest extent possible and make use of the lessons learned. The Republic Day earthquake provides an important opportunity to better understand the hazard posed by earthquakes that occur in or affect intracratonic regions. E:I

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