تعیین سازوکار کانونی زمین‌لرزه‌ها با بزرگی 5≤ Mn ≤ 2 در منطقه البرز با استفاده از قطبش اولین رسید موج P و نسبت دامنه موج S به دامنه موج P

نوع مقاله : مقاله پژوهشی‌

نویسندگان

1 گروه فیزیک زمین، موسسه‌ ژئوفیزیک، دانشگاه تهران

2 گروه فیزیک زمین، موسسه ژئوفیزیک دانشگاه تهران

چکیده

با اندازه‌گیری سازوکار کانونی زمین‌لرزه‌ها می‌توان جهت انتشار گسیختگی، ساختار گسل و میدان تنش منطقه را تعیین کرد. در دسترس بودن حجم وسیعی از زمین‌لرزه‌های کوچک، استفاده از آنها را برای این منظور حائز اهمیت کرده است. هدف این مطالعه، تعیین سازوکار کانونی 4204 زمین‌لرزه با بزرگای 5≤ Mn ≤  2 ثبت شده در چهل‌و‌یک ایستگاه از ایستگاه‌های شبکه‌های لرزه‌نگاری مرکز لرزه‌نگاری کشوری در منطقه البرز با استفاده از قطبش اولین رسید موج P و نسبت دامنه موج S به دامنه موج P است. این زمین‌لرزه­ها از سال 1997 تا ماه مه سال 2015 رخ داده‌اند. در این تحقیق از شکل‌موج‌های گیرنده‌های سه مؤلفه‌ای استفاده شده است. شکل‌موج‌های سرعت با انتگرال‌گیری، به شکل‌موج‌های جابه‌جایی تبدیل شدند. به‌دلیل ضعیف بودن کیفیت شکل‌موج‌های زمین‌لرزه‌های کوچک، کاتالوگی از سازوکار کانونی 277 زمین‌لرزه با بزرگای Mn بیشتر از 5/3، بر‌اساس زوایای امتداد، شیب و لغزش فراهم شد. به‌منظور بررسی عملکرد این روش در تعیین سازوکار گسلش در منطقه موردنظر، دوازده زمین‌لرزه مصنوعی نیز تولید شد که نتایج بررسی آنها ارائه خواهد شد.
گسلش غالب در نواحی شمالی منطقه موردمطالعه، معکوس و در نواحی جنوبی امتداد‌لغز است. در امتداد باختری گسل طالقان، اکثر سازوکارها امتداد‌لغز چپگرد تعیین شده‌اند. راستای گسل ایندس، شمال‌باختری‌- جنوب‌خاوری است و در امتداد جنوب‌خاوری آن با توجه به سازوکارهای رویداد 18/6/2007 و تعدادی از پس‌لرزه‌های آن، گسلش منطقه معکوس است. نتایج این مطالعه با بیشتر نتایج مطالعات قبلی در این منطقه هماهنگی دارد.

کلیدواژه‌ها

موضوعات

عنوان مقاله [English]

Determining focal mechanism of earthquakes with magnitude 2≤ Mn≤5 in Alborz region by using the polarity of P wave first motion and S/P amplitude ratios

نویسندگان [English]

  • Fatemeh Hajimirza alian 1
  • Mohammad Reza Hatami 2
  • Vahid Maleki 1
1 Earth Physics Department Institute of Geophysics, University of Tehran
2 Department of Seismology, Institute of Geophysics, University of Tehran
چکیده [English]

Calculating the focal mechanism of earthquakes, helps us to investigate the direction of rupture propagation, the style of faulting, stress field and the seismicity potential of an area. Focal mechanisms classically are determined by using the polarity of P- wave first motion. This method depends on the number of P wave polarities per event and the data set dispersion distribution Significant improvement of long period instruments caused advantages in using waveform inversion method for determining focal mechanism of earthquakes. In this method outspread outlier data can be used.
Extensive availability of small earthquakes makes them useful in determining focal mechanisms. The current study accomplished to determine focal mechanisms of 4204 events with magnitude 2≤ Mn ≤5. The earthquakes were recorded in 41 stations of Iranian Seismological Center (IRSC) in the Alborz region. The focal mechanisms were prepared by using the polarity of P wave first motion and S/P amplitude ratios.
In this study, waveforms of three-component seismograms were filtered with bandpass filter in a 0/1–10 Hz frequency range. Seismograms with velocity components were integrated to the corresponding displacement components. P-wave first motion polarities are revised by reviewing the waveforms carefully. For calculating the S/P ratios, P wave, S wave and background noise amplitudes were measured by selecting 2 second time windows from waveforms. Maximum amount of S/P ratio is near the nodal planes and the minimum amount is far from the nodal planes. Using P wave polarities and ratios, the final focal mechanisms were obtained. With the progressive improvement of IRSC networks since 2012, more P wave first motion polarities can be recorded and the number of the calculated S/P ratios for each event has increased.
In this study, data set of 1997 to May 2015 was investigated. Because of weak qualities of small earthquakes waveforms, small events were omitted and a focal mechanism catalogue containd strike, dip and rake angels for earthquakes with Mn greater than 3.5 was produced. The method also was tested with 12 synthetic earthquakes in the Alborz rigion. Most of the focal mechanisms at north of the region have thrust-faulting components and at south they are strike-slip. Along the left side of the Talaghan fault, most of the focal mechanisms are left-lateral strike-slip which shows the left-lateral motion along the Mosha fault. The Indes fault has north west-south east strike. Along the south east side of this fault, the focal mechanisms of 18/6/2007 event and its aftershocks show the inverse faulting. The results of this study are compatible with most of the previous studies in this region.

کلیدواژه‌ها [English]

  • focal mechanism
  • S/P amplitude ratio
  • Alborz rigion

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مجله ژئوفیزیک ایران
دوره 12، شماره 3
آذر 1397
صفحه 122-144

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