تعیین ساختار سه بعدی پوسته در منطقۀ بم به روش توموگرافی زمین لرزه های محلی

نوع مقاله: مقاله پژوهشی

نویسندگان

1 کارشناسی ارشد ژئوفیزیک، پژوهشگاه بین المللی زلزله شناسی و مهندسی زلزله، تهران، ایران

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

3 استادیار، پژوهشگاه استاندارد، اداره کل استاندارد و تحقیقات صنعتی خراسان رضوی، مشهد، ایران

چکیده

در تاریخ 5 دی ماه 1382 (26دسامبر 2003) زمین لرزه ای با بزرگی 6/6 MW =  بخش گسترده‌ای از جنوب استان کرمان را به لرزه در آورد. با توجه به تلفات زیاد این رویداد، توجه زیادی روی این زمین‌لرزه معطوف شد. به‌رغم مطالعات گسترده، ابهامات زیادی در مورد هندسه و محل گسل مسبب زمین لرزۀ بم باقی مانده است. در تحقیق حاضر، برای به نقشه در آوردن تصویر سه بعدی ساختار سرعتی منطقه،  تعیین هندسه گسل بم در ژرفا،  و در صورت امکان نقش آن در ایجاد زمین لرزۀ دی ماه 1382، داده‌های زمان سیر پس‌لرزه‌های زمین‌لرزۀ بم که توسط یک شبکه لرزه‌نگاری محلی و متراکم ثبت شده‌اند، به‌طور همزمان برای تعیین ساختار سرعتی و کانون زمین‌لرزه‌ها وارون‌سازی شدند. در این مطالعه 6201 زمان رسید موج P حاصل از 544 پس‌لرزه ثبت شده در 23 ایستگاه موقت نصب شده پس از زمین‌لرزۀ بم، مورد استفاده قرار گرفتند. این داده‌ها توسط برنامه  SIMULPS14  مورد برگردان سه‌بعدی قرار گرفتند. تغییرات سه‌بعدی سرعت موج P به‌دست آمده برای ژرفای کمتر از 20 کیلومتر در مطالعه حاضر نشان می‌دهد که در زیر ژرفای10 کیلومتر سرعت کمی در منطقه حاکم است و در ژرفاهای بالای 14 کیلومتر،  یک بی‌هنجاری با سرعت بالا در قسمت باختری منطقه دیده می‌شود. تفاوت در سرعت مشاهده‌ای که در دو طرف گسل بم قرار می‌گیرند، می‌تواند به عملکرد گسل وارون بم- بروات نسبت داده  شود. به نظر می‌رسد که گسل بم- بروات ساختار فعال اصلی منطقه، در طی زمین لرزۀ مخرب  5 دی ماه 1382 بوده باشد. 

کلیدواژه‌ها


عنوان مقاله [English]

Three Dimensional Structure of the BAM Region from Local Earthquake Tomography

نویسندگان [English]

  • Z. Jeddi 1
  • Mohammad Tatar 2
  • B. Saeedi Razavi 3
1 M.Sc. Student of Geophysics, International Institute of Earthquake Engineering and Seismology, Tehran, Iran
2 Associate Professor, International Institute of Earthquake Engineering and Seismology, Tehran, Iran
3 Assistant Professor, Institute of standard and Industrial research - Khorasan-e Razavi, Mashhad, Iran
چکیده [English]

The catastrophic December 26, 2003 Mw 6.6 Bam earthquake is one of the most disastrous earthquakes in Iran. This earthquake attracted much attention, and has been far more studied that which would be expected from a moderate magnitude earthquake. Nevertheless, there are doubtful results related to geometry and location of the causative fault that produced the Bam earthquake. As very little is known about the 3-D structure of the region and in order to define the 3-D velocity structure and the geometry of the active fault in the Bam area travel time data from the aftershock series of the Bam earthquake  are  inverted simultaneously for both hypocenter locations and 3-D Vp structure. The data used for this purpose are 6201 P-wave arrival times from 544 selected local earthquakes recorded by temporary 23 short-period seismic stations. 3-D P-wave velocity variations down to 20 km depth were obtained. The acquired tomographic images show that the 3-D velocity structure beneath the region is heterogeneous in that low velocity appears throughout the region down to ~10 km depth, and high velocities occur in western part from ~14 km depth. Velocity structure of seismogenic region is well resolved to a depth of 20 km. Difference in observed velocities in two sides of  the Bam fault in depth of 10-20 km is clear in tomographic images and on cross sections. We relate this difference in velocity to the effect of the reverse Bam-Baravat fault which seems influenced the deeper layer down to 20 km depth. This implies that the pre-existing reverse Bam-Baravat fault is a major active structure in the region that could be caused the December 2003 Bam earthquake.

کلیدواژه‌ها [English]

  • Bam
  • Local Tomography
  • 3-D Structure
  • Aftershocks
  • Reverse faulting
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