تغییرات جانبی ضریب کیفیت امواج کدا قبل و بعد از زلزله بزرگ کوماموتو (Mw7.3) ژاپن

نوع مقاله : مقاله پژوهشی‌

نویسندگان

1 دانشجوی دکتری،گروه علوم زمین، دانشگاه علوم تحقیقات، دانشگاه آزاد اسلامی، تهران، ایران

2 استادیار، پژوهشگاه بین المللی ‌زلزله شناسی و مهندسی زلزله، تهران، ایران

3 استادیار، گروه علوم زمین، دانشگاه علوم تحقیقات، دانشگاه آزاد اسلامی، تهران، ایران

چکیده

کمیت­های سرعت انتشار و کاهندگی امواج لرزه­ای، از پارامترهای فیزیکی مهمی هستند که نحوه انتشار امواج زمین­لرزه را تحت‌تأثیر قرار می­دهند. بررسی دو پارامتر برای تعیین دقیق پارامترهای چشمه زمین­لرزه و نیز کاهش خطر زمین­لرزه ضروری هستند. در مطالعه حاضر، تغییرات جانبی زمانی کاهندگی امواج کدا قبل و بعد از زلزله کوماموتو با بزرگای 3/7 در منطقه کوماموتو ژاپن مورد بررسی قرار گرفته است. تخمین کاهندگی امواج کدا با استفاده از 474 زلزله محلی ثبت‌شده در 306 ایستگاه لرزه­نگاری با روش تک پراکنش به عقب صورت گرفت. فاصله رومرکزی داده­ها بین 1 تا 200 کیلومتر و بزرگای بین5/2 تا 5/5 است. سپس منطقه موردمطالعه به بلوک­هایی با ابعاد  درجه تقسیم‌بندی شد و ضریب کیفیت در فرکانس یک هرتز و در زمان گذشت 30 ثانیه برای کل منطقه محاسبه شد. کمترین مقدار ضریب کیفیت حاصل برابر با 52 و بیشترین مقدار برابر یا 200 است. نتایج نشان می‌دهد که منطقه مورد مطالعه از نظر تکونیکی به‌شدت فعال است و نرخ لرزه­خیزی بالایی دارد. تغییرات جانبی ضریب کیفیت نشان می­دهند که با دورشدن از کانون زلزله اصلی کوموماتو، ضریب کیفیت افزایش می­یابد. دلیل این‌گونه رفتار را می­توان در خردشدگی سنگ­ها به‌دلیل زلزله­های متعدد، اشباع‌شدگی رسوبات از سیالات و افزایش ناهمگنی در نزدیکی رومرکز زلزله دانست. بررسی مقادیر ضریب کیفیت قبل و بعد از زلزله اصلی، بیانگر کاهش این پارامتر بعد از وقوع زمین­لرزه است. بعد از زلزله اصلی، تعداد زلزله­های رخ داده (پس­لرزه­) در منطقه افزایش پیدا می­کند. افزایش زلزله در منطقه باعث افزایش شکستگی­ و خرد­‌‌شدگی سنگ­ها­ و همچنین اشباع‌‌شدگی رسوبات می­شود و در نتیجه کاهش ضریب کیفیت را به‌همراه دارد. وجود ناهمگنی­های زیاد موجود در منطقه به‌دلیل تعداد زیاد زلزله­ها و شکستگی فراوان بیانگر این موضوع است که قسمت اعظم این کاهندگی ناشی از پراکنش است.

کلیدواژه‌ها

موضوعات

عنوان مقاله [English]

Temporal variation of Q factor before and after Kumamoto earthquake

نویسندگان [English]

  • Maryam Saffari 1
  • Majid Mahood 2
  • Reza Heidari 3
1 Ph.D. Student, Department of Earth Sciences, Science and Research Branch, Islamic Azad University, Tehran, Iran
2 Assistant Professor, International Institude of Earthquake Engineering and Seismology(IIEES), Tehran, Iran
3 Assistant Professor, Department of Earth Sciences, Science and Research Branch, Islamic Azad University, Tehran, Iran
چکیده [English]

The Kumamoto earthquake with magnitude Mj=7.3 was one of the destructive earthquakes in Japan which had more than 273 dead and 2809 injured. Investigation of temporal variation of coda wave attenuation before and after the earthquake, and also evaluating the lateral variation give important information about the crust and upper mantel beneath the studied area. Likewise, Attenuation of the seismic waves along with the velocity of wave propagation are among the important physical parameters that affect the propagation of the seismic waves. Careful investigation of these two parameters is required to determine the exact parameters of the earthquake source and also reduce the risk of earthquakes in the region. In the present study, the lateral changes of coda wave attenuation with time have been investigated. Here we checked the coda wave attenuations before and after the Kumamoto earthquake with the magnitude of 7.3 in the Kumamoto region of Japan. The estimation of Coda wave attenuation was performed using 474 local earthquakes recorded in 306 seismic stations by single back scattering method. This approach assumes that source and station are located at one point. The data used for estimation of attenuation have the epicentral distance between 1 and 200 km and the magnitude between 2.5 and 5.5. In order to considering the lateral variation of coda wave attenuation, the study area was divided into blocks with 0.3×0.3 degree in both dimension and the quality factor was calculated at the frequency of 1 Hz and lapse time of 30 seconds for the entire area. The results show the low quality factor Q0 and high frequency parameter n. So, the study area is tectonically active and has a high seismicity rate. Lateral changes in the quality factor indicate that the quality factor increases as it moves away from the epicenter of the main Kumamoto earthquake. The reason for this behavior can be attributed to the crushing of rocks due to multiple earthquakes, saturation and increased heterogeneity near the earthquake epicenter. Lateral attenuation changes before and after the main earthquake explain the fact that the quality factor decrease after the earthquake. After the main earthquake, the number of aftershocks occurred is greater than the number of pre-earthquakes. Increased earthquake occurrence in the region increases the fracture, crushes and saturation and as a result, reduces the quality factor. Comparison of the obtained quality factor in the region with other active regions in the world indicates the fact that the studied area is highly tectonically active and has also a high seismicity.

کلیدواژه‌ها [English]

  • Kumamoto earthquake
  • attenuation
  • coda wave
  • lateral changes
  • time elapse

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مجله ژئوفیزیک ایران
دوره 16، شماره 2
شهریور 1401
صفحه 159-170

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