بررسی پیش‌نشانگرهای لرزه‌ای زمینلرزه بـم با استفاده از داده‌های شبکه لرزه‌نگاری یزد

دسته لرزه زمین ساخت
گروه سازمان زمین شناسی و اکتشافات معدنی کشور
مکان برگزاری سومین گردهمائی علوم زمین
نویسنده نوید شاد منامن، عبدالرضا قدس و نوربخش میرزایی
تاريخ برگزاری ۲۶ بهمن ۱۳۸۳

Investigating precursory changes in seismicity before the Bam Earthquake using Yazd seismic network data


Abstract

Yazd telemetry seismic network of the Institute of Geophysics, University of Tehran is the closest local network to the Bam event of ۲۰۰۳/۱۲/۲۶ of ۶.۲ Mn. The network could detect larger number of earthquakes than other networks, thus allowing us to search for possible precursory changes of seismicity before the event. In this study we use the Yazd earthquake bulletin to detect possible seismicity activation before the events. Time evolution of integral measure of the intensity of earthquake flow, concentration of mainshocks, clustering of events and number of aftershocks are calculated for the Bam region. Our results show a significant increase of seismic activity about one year before the event happened. The results suggest that local network data might reduce the uncertainty in time of mid-term prediction algorithms to about one year.



Tectonics of Hormuz Island in the Persian Gulf


Pedram Aftabi

Abstract:

The high amount of potassium (۲۰%) content in the secondary salt come from the central dome show a volume of primary lenses as founded in Pohl.

The Hormuz Island seems to be mushroom diapir similar to Great Kavir but is not a simple mushroom. Dips of the layering in this typical dome was inward to upward and is different with a simple mushroom diapir.Althought in the mushroom diapirs in all levels the dips of layering might be down dip. If a salt layer intruded into another salt layer an overhang of salt formed into the youngest salt and intrusion of salt into competent layers formed a rim or collar around the dome. The change in the compression rate can active a dead hot salt and open a closed pull apart. The dead salt or droplet salt with make an internal mushroom. Interpretation of folds base of the field work and aerial photograph showed that the shape of the island is probably related to the internal structure of the island. The internal folds interoperations show possibility of the rotation in the island and complex shape of the mushroom called here Palm diapir.

Two type of structures or compositional layering seen in the field consist of upright to overturn down dip compositional layering in major to minor folds in the central part and southern new nanmkier and also up dip compositional layering in the old nanamkier.The new namkier showed up dip to down dip compositional layering and two direction flows. The island is consisting of three salts one in the middle is a concentric salt dome with major concentric folds and minor concentric sheath folds. Central part of the mountain is consisting of ۴ major concentric parts. These are ۴ mushrooms and may be to ۶ in some cases (major mushrooms).۴ to ۶ major mushrooms in concentric form are eroded by sea dissolution in the last period of time, when the pull apart was closed. It is believed that the pull apart at Hormuz is open now and the mountain rise because around the island sea water is very shallow. The holed salt in the triangle trap of closed stem curl after intrusion of the new salt and curls several times after intrusion of ۴ to ۶ major internal mushrooms in a big palm diapir.

چکیده

شبکه لرزه‌نگاری یزد وابسته به موسسه ژئوفیزیک دانشگاه تهران، نزدیک‌ترین پایگاه زلزله‌نگاری به رومرکز زمین‌لرزه بم (26/12/2003، 2/6= ) است. از اینرو، داده‌های این شبکه بدلیل ثبت زمینلرزه‌های کوچکتر، حاوی اطلاعات با ارزشی از روند وقوع زمینلرزه‌ها قبل از وقوع زلزله بم است. در این تحقیق، از داده‌های این شبکه برای بررسی پیش‌نشانگرهای لرزه‌ای استفاده شده است. پیش‌نشانگرهای تغییرات زمانی فراوانی زمینلرزه‌ها، انحراف فراوانی از روند بلندمدت، چگالی سطحی انرژی لرزه‌ای و تجمع یافتن زمینلرزه‌‌ها مورد مطالعه قرار گرفت. میانگین‌های زمانی پیش‌نشانگرهای مورد مطالعه یک رشد غیر عادی در مدت کمتر از یک سال قبل از وقوع زمینلرزه بم از خود نشان می‌دهند. از نتایج بدست آمده می‌توان امیدوار بود که مطالعه بیشتر داده های لرزه‌‌نگاری محلی ممکن است عدم قطعیت زمانی روش‌های پیش‌بینی میان مدت را تا یک سال کاهش دهد.


کلید واژه ها: یزد