Kashmir Seismic Gap
Seismologists, Earthquake Geologist and Engineers must work hard to reduce the risk to life and property from seismic shaking
SAFETY BY DR. BASHIR AHMAD BILAL
Himalayan mountain belt is divided in to three main segments based on the earthquake recurrence in the last 100 years or so. These segments have the potential to generate large earthquakes. Such segments are referred to as seismic gaps. Three main seismic gaps have been identified in the Himalaya: the gap between the 1950 Assam and 1934 Bihar-Nepal earthquake known as the Assam Gap: the gap between the 1905 Kangra and 1934 Bihar-Nepal earthquakes known as Central Gap, and the Kashmir Gap which lies west of the 1905 Kangra earthquake rupture. Here we focus on Kashmir seismic gap: Historical evidence suggested that the earthquake of 1885 is the last great earthquake which struck the Kashmir seismic Gap; instead of 1555, rather there is no robust information about deformation type including surface rupture and the damage trajectory from the 1555 event. I strongly rebut the recent claims published in various news papers which suggested that the largest historical event in the northwestern Himalaya occurred in 1555. One often hears statements in the popular media like, “the Big One is overdue” and “the longer it waits, the bigger it will be.” Surprisingly, data to critically test the variability in recurrence intervals rupture displacements, and relationships between the two are almost nonexistent. Instead historical sources suggest that The most recent large (M >7) earthquake known in and around the 2005 epicentral area is the 1885 Baramulla earthquake of M 6.7, but the damage from the earthquake is reported to have been concentrated around Baramulla to the southeast Indo Kohistan Seismic Zone IKSZ suggesting that the 1885 and 2005 events were perhaps on the same causative faults. The 2005 meizoseismal area is also located just southeast of a northwest-trending belt of high microseismicity, called the Indus–Kohistan seismic zone (IKSZ) (Fig) The 1974 M 6.2 Pattan earthquake occurred within the IKSZ claiming 5300 lives despite its moderate size. The 2005 aftershocks were also concentrated in this zone, so it appears that the large part of the strain energy has already been released and there is seldom any chance for a big earthquake in Kashmir Valley in the near future.
Generally, short intervals tend to coincide with large displacements and long intervals with small displacements the more strain released in an earthquake, the longer it will take to build up the required strain for the next one. The lack of displacement data precludes direct comparison of recurrence interval and slip for individual earthquake cycles. The Kashmir Valley has been an active region and until now has not had long records of displacements and enough sites with long records to seriously consider the extent of paleoearthquakes which inhibits us to predict an earthquake and its damage scenario. The first and foremost requirement for the better seismic hazard assessment of the Valley is to prepare a fault map based on modern techniques of trenching and OSL dating of the events, rather than issuing the press releases where by creating public panic. Scientists should come up with research findings instead publishing mere speculations.
SEISMIC HAZARD ASSESSMENT
One of the main problems in our case is the lack of fault map (structural map) of Kashmir which can provide us an insight about the ground shaking close to the causative fault of an earthquake. This raises the possibility that large numbers of buildings and other structures are not sufficiently resistant for the intense levels of shaking that can occur close to the fault. Damage patterns in recent moderate-sized earthquakes occurring in or adjacent to urbanized areas, however, indicate that many structures, including some modern ones designed to meet earthquake code requirements, cannot withstand the severe shaking that can occur close to a fault. New structures situated close to active faults should be designed on the basis of ground motion estimates greater than those used in the past. Scientists and engineers must inform the public about earthquake shaking and its effect on structures. The exposure to damage from seismic shaking is steadily increasing because of continuing urbanization and the increasing complexity of lifeline systems, such as power, water, transportation, and communication systems. Over a longer time span, however, we can significantly reduce the risk to life and property from seismic shaking through better land utilization, improved building codes and construction practices, and at least the gradual replacement of poor buildings by more resistant buildings. For example, lacking observational data, seismologists must rely on simplified theoretical and numerical models of the earthquake process to estimate near-fault ground motion, especially for earthquakes as large as magnitude 7 and 8. Further developments in computer technology and in computer modeling techniques will permit more realistic simulations of the seismic response of soils and structures that take into account their inelastic behavior and their strain-dependent properties. Earthquake design codes will continually be revised to better utilize existing knowledge concerning the nature of strong ground motion and the dynamic behavior of buildings during earthquakes and to incorporate new knowledge. We believe that application of new knowledge; improvements in earthquake-resistant design and construction, and remedial strengthening or replacement of weak existing structures can significantly reduce our current level of exposure to earthquake hazards in Kashmir.
Lastupdate on : Fri, 23 Sep 2011 21:30:00 Makkah time
Lastupdate on : Fri, 23 Sep 2011 18:30:00 GMT
Lastupdate on : Sat, 24 Sep 2011 00:00:00 IST
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