https___theconversation.com_be-prepared-always-the-tsunami-message-from-new-zealands-latest-earthquake-68742.xml
https___www.apkhabar.com_2018_12_24_3350__1.xml
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The tsunami warnings from the latest New Zealand earthquake show how important it is for people to be prepared for such an event. The early messages from New Zealand’s Ministry of Civil Defence and Emergency Management (MCDEM) on its Twitter account @NZcivildefence said it was “assessing whether there is any tsunami threat”, but just 30 minutes later it changed this advice to say a tsunami was “possible”. So why was the message changed, and what can we learn from it? Tsunami risk New Zealand is in a very active earthquake zone and the risk of tsunami is high along almost every part of the coastline. Tsunamis are usually generated by the largest earthquakes, of about magnitude 7 or above. If an earthquake of this size is detected, emergency management services will send out an immediate warning to give people in risk areas the maximum amount of time to evacuate. But it can take several hours to work out the exact location and magnitude of an earthquake, so emergency services will continually update their message when new information becomes available. In the case of the recent earthquake and tsunami in New Zealand, the first reports suggested the earthquake had a magnitude of 6.5, below that expected to generate a tsunami. It also looked like the earthquake occurred onshore, which again lowered the risk of a tsunami. That’s probably why the MCDEM, which is responsible for alerting the New Zealand public of any tsunami risk, initially sent out a tweet saying it was only assessing any tsunami threat. This would have been based on the best available information at the time. Increased magnitude But in the minutes following the quake, the estimates of its magnitude were raised to 7.5, meaning that there was real risk of a tsunami. A tsunami wave was measured at Kaikoura, on the east coast of the South Island, only minutes after the earthquake. This would have prompted authorities to make a rapid change to the tsunami warning status. Messages were immediately sent out via Twitter and other means to warn of the tsunami risk. Tsunami waves of 2.5m were measured at Kaikoura, 1m at Christchurch, and 0.5m at Wellington. The tsunami wave reached the Chatham Islands, 800km east of New Zealand, in about 4 hours, and was measured at Easter Island after 12 hours. A warning was eventually issued for all of New Zealand’s coasts. This was then gradually downgraded over the next few hours as the tsunami threat passed. Although the threat from tsunami waves has passed, unpredictable currents can affect beaches for many days following a tsunami, so people are usually warned to stay out of the water. Land quake, sea tsunami So how was a tsunami generated by an earthquake with its epicentre onshore? Most tsunamis are generated when a fault ruptures on the sea floor, rapidly pushing up a wall of water which then moves outwards as a wave. When an earthquake occurs on land there is far less risk of this happening. But tsunamis can also be caused by landslides caused by nearby earthquakes. The east coast of New Zealand near Kaikoura is considered to have a high risk of submarine landslides. The Kaikoura Canyon just offshore from Kaikoura has very steep, unstable sides with a lot of loose sediment. During an earthquake this sediment can fall into the canyon, displacing water and generating a tsunami wave. We already know that large landslides did occur onshore to the north and south of Kaikoura. It is highly likely that landslides also occurred in the deep water of the Kaikoura Canyon, and that this is what caused the tsunami. The first lesson to learn from this is that tsunamis can often strike the coast with very little warning, and people in high-risk zones need to act quickly when an earthquake occurs and not wait for official warnings. The second lesson is that despite the incredible technology we now have to measure earthquakes and tsunamis, there are still limits on how quickly we can accurately measure and locate an earthquake and assess the likely tsunami risks. Events such as Monday’s, where the earthquake likely triggered a submarine landslide, make it even harder to quickly calculate the tsunami risk. Earthquakes and tsunamis are unpredictable although our understanding of the science is improving all the time. But it takes time to get accurate information to the public after an earthquake and tsunami, so those living closest to the hazard zones need to be aware of natural warning signs and not rely only on official warning messages.
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काठमाडौं , पुष ९ : प्राकृतिक विपत्ति बाजा बजाएर आउने भए सायद क्षणभरमै सयौँ जीवन माटोमा मिल्ने थिएन । सजगता अपनाउँदा अपनाउँदै पनि विपतले निम्त्याउने आपत अकल्पनीय भइदिन्छ । शनिबार राति इन्डोनेसियामा आएको सुनामीले २ सय २२ जनाको मृत्यु भइसकेको छ । सुनामीमा ८ सय ४३ घाइते भएका छन् । ज्वालामुखीका कारण आएको सुनामीले इन्डोनेसियाको जावा र सुमात्रा गाउँ बस्ती नै लथालिङ्ग भएको छ । बिना कुनैसंकेत शनिबार मध्यराति आएको सुनामीका कारण मृत्यु हुनेको संख्या बढेको हो । अनक कराकातु आइल्याण्डमा भएको ज्वालामुखी विस्फोटका कारण समुन्द्री सतहमा गएको पहिरोले सुनामी निम्त्याएको विज्ञहरुले बताएका छन् । भूगर्भसम्बन्धी अध्ययनकर्ताका अनुसार ८ हजार वर्षअघि सिसिलीमा विनासकारी सुनामी गएको थियो । ज्वालामुखीपछि विस्फोटले निम्त्याएको हिमपहिरो समुन्द्रमा खस्दा गएको सुनामी १० तलाको भवनभन्दा पनि अग्लो देखिएको थियो । कहाँ कहाँ गएको थियो ? बिनाशकारी सुनामी – २६ डिसेम्वर सन २००४ मा भारतको समुन्द्री सतहमा आएको भूइँचालोका कारण श्रीलंका, भारत, इन्डोनेसिया र थाइल्याण्डलगायतका ११ देशमा गएको सुनामीले २ लाख ३० हजारको ज्यान गएको थियो । यो सम्भवतः सुनामीको इतिहासमै सबैभन्दा ठूलो मानवीय क्षति हो । – ११ मार्च सन २०११ मा जापानमा गएको सुनामीमा १६ हजारको ज्यान गएको थियो भने २ सय ३५ बिलियन डलर बराबरको क्षति भएको थियो । समुन्द्री सतहमा ९ रेक्टर स्केलको भूइँचालोले निम्त्याएको सुनामीले १८ हजारको ज्यान गएको थियो भने ४ लाख ५२ हजार घरबारविहीन भएका थिए । – १ नोभेम्बर सन १७५५ मा लिस्वन, पोर्चुगलमा गएको सुनामीमा ६० हजारले ज्यान गुमाएका थिए । सुनामीले पोर्चुगल, स्पेन र मोरक्कोमा मानवीय क्षति पुर्याएको थियो । ८ दशमलव ५ रेक्टर स्केलको भूइँचालोपछि आएको सुनामीको लहर ३० मिटरभन्दा बढी उचाइमा फैलिएको अनुमान छ । – २७ अगस्ट सन १८८३ क्राकताउ काल्डेरामा बिस्फोट भएको ज्वालामुखीका कारण क्राकताउ, इन्डोनेसियामा सुनामी आएको थियो । यो सुनामीमा ४० हजारले ज्यान गुमाएका थिए । – २० सेप्टेम्वर १४९८ मा इन्सुनाडा, जापानमा आएको सुनामीमा ३१ हजारको ज्यान गएको थियो । समुन्द्री सतहमा आएको ८ दशमलव ३ रेक्टर स्केलको भूइँचालोले निम्त्याएको सुनामीले ठूलो जनधन क्षति भएको थियो । – २८ अक्टोबर १७०७ मा नान्काइदो, जापानमा सुनामी गएको थियो । ८ दशमलव ४ रिक्टरस्केलको भूकम्पले निम्त्याएको सुनामीमा ३० हजारको ज्यान गएको थियो भने झण्डै ३० हजार भवन क्षतिग्रस्त भएका थिए । – १५ जुन १८९६ मा सान्क्रिकु जापानमा सुनामी गएको थियो । ७ दशमलव ६ रिक्टरस्केलको भूइँचालोले निम्त्याएको सुनामीले ११ हजार भवन क्षतिग्रस्त भएका थिए भने २२ हजारको ज्यान गएको थियो । – १३ अगस्ट १८६८ मा उत्तरी चिलीमा गएको सुनामीमा २५ हजारको ज्यान गएको थियो, तीनसय मिलियन डलर बराबर क्षति भएको थियो । – २४ अप्रिल १७७१ मा जापानको रिउकु आइस्ल्याण्डमा गएको सुनामीमा १२ हजारको ज्यान गएको थियो भने ३ हजार १ सय ३७ भवनहरु क्षतिग्रस्त भएका थिए । – १८ जनवरी १५८७ मा जापानको इस्वेमा गएको सुनामीमा ८ हजारको ज्यान गएको थियो । ८ दशमलव २ रिक्टरस्केलको भूइँचालोले निम्त्याएको सुनामीले ठूलो जनधन क्षति भएको थियो । के हो सुनामी ? सुनामी जापानी शब्द हो जसको अर्थ बन्दरगाह नजिकको लहर हो । सुनामी एक किसिमको अनियन्त्रित पानीको लहर हो । जुन समुन्द्री सतहमा जाने भूकम्प र ज्वालामुखी विस्फोटका कारण आउने गर्दछ । सामान्यतया सुनामीको गति प्रतिघण्टा चारसय किलोमिटर र उचाई १० देखि १७ मिटरसम्म हुन्छ । जनता समाचार