پاسخ لرزه ای رسوبات آبرفتی ناشی از مؤلفه قائم زلزله های حوزه ی نزدیک گسل

نوع مقاله : Articles

نویسندگان

دانشکده مهندسی عمران، دانشگاه تهران

چکیده

یکی از راهکارهای متداول برای تعیین خصوصیات مؤلفه قائم زلزله طرح به­صورت ویژه ساختگاه استفاده از روابط تجربی نسبت طیفی شتاب قائم به افقی (V/H) است. با این ‌حال اثرات ساختگاهی تاکنون به­صورت دقیق در این روابط وارد نشده است. در این مقاله اثرات محلی ساختگاه بر روی خصوصیات مؤلفه قائم شتاب زلزله‌های میدان نزدیک بررسی شده است.  به این منظور ساختگاه­های مختلف با ویژگی­های دینامیکی متفاوت به­جای دسته‌بندی‌های کلی روابط تجربی در نظر گرفته شده و تحت تحریک مجموعه‌ای از زلزله‌های میدان نزدیک به ‌صورت دو بعدی تحلیل دینامیکی شده‌اند. نتایج به‌دست‌آمده نشان می‌دهد که نسبت طیفی شتاب قائم به افقی (V/H) از عواملی نظیر پریود اساسی ارتعاشی ساختگاه در حالت انتشار موج فشاری، نسبت پواسون خاک (υ) و همچنین مشخصات تحریک ورودی تأثیرپذیر است. مقایسه‌ی نسبت­های V/H به­دست­آمده از تحلیل‌های عددی با روابط تجربی موجود نشان می‌دهد که همخوانی نتایج در حالت اشباع بودن خاک نسبت به حالت خشک بیشتر است و با افزایش نسبت پواسون از میزان تأثیر نوع خاک بر نسبت‌های طیفی شتاب V/H کاسته می‌شود. در نهایت پیشنهاد می‌شود از معکوس امپدانس لرزه‌ای یک‌چهارم طول ‌موج بر مبنای سرعت موج فشاری، در روابط کاهندگی تجربی V/H برای لحاظ نمودن اثرات ساختگاهی استفاده شود.

کلیدواژه‌ها


عنوان مقاله [English]

Seismic Response of Alluvial Deposits due to Vertical Component of Near-Fault Earthquakes

نویسندگان [English]

  • Ali Kavand
  • S. Ahmad Sarkeshikzadeh Motlagh
  • Abbas Ghalandarzadeh
School of Civil Engineering, College of Engineering, University of Tehran, Iran
چکیده [English]

The characteristics of vertical component of earthquakes are essentially different from those of their horizontal components in terms of amplitude and frequency content. This difference is mainly due to the fact that the vertical component is rather influenced by P-waves while the horizontal component is associated with S-waves. It is widely accepted that the energy of the vertical component of earthquakes is usually concentrated over a narrower range of high frequencies compared to that of the horizontal component. This concentration is destructive to engineering structures having vertical fundamental frequency within the high frequency range. During past earthquakes, the collapse of several buildings and bridges has been found to be directly caused by vertical excitations. Previous researches have illustrated that vertical ground motions are mainly influenced by earthquake magnitude, source-to-site distance as well as local site conditions such as geometry and material properties of subsurface soil layers.
Current paper aims to investigate the effects of various local site conditions on vertical component of near-fault earthquakes. To this end, a series of seismic site response analyses based on time-domain equivalent-linear approach were carried out using Quake-W to determine the ground response under simultaneous horizontal and vertical excitations. The site condition was categorized into different soil profiles comprising of different soil types with different Vs30 values rather than the generic site classifications used in seismic design codes. The input seismic motions used in the analyses comprised of 23 acceleration time histories from near-fault earthquakes recorded worldwide in tectonically active regions within distances less than 10 Km from the active faults (RJB ≤ 10 km) and with magnitudes ranging from M = 5.6 to 7.6.
The results of numerical analyses were finally presented in terms of 5% damped V/H spectral ratio at the ground surface. The results were also compared with recent empirical V/H attenuation equations developed by other researchers. It was found that the obtained V/H spectral ratios show better agreement with empirical equations in case of higher-magnitude earthquakes at stiff soils than lower-magnitude ones at soft soils. In all of these cases, Poisson ratio (υ) of the soil that controls the ratio of S-waves to P-waves velocities was considered equal to 0.35 as a general value. It should be added that although υ is influenced by parameters like soil type, confining pressure and void ratio, it critically depends on the degree of saturation of the ground as well as the drainage conditions during loading. The saturation of a soil can result in rapid increase in υ, which in turn increases the P-wave velocity. To better understand this issue, some models were analyzed with a constant Vs30 value of 230 m/s but with different υ values or alternatively different P-wave velocity profiles. It was observed that as υ increased, the agreement between V/H spectral ratio predicted by numerical analysis and those obtained based on empirical equations enhanced. It can be concluded that the degree of saturation of the ground can profoundly affect V/H spectral ratio. This issue is not reflected in current V/H empirical relationships.
In conclusion, the results of current study demonstrate that V/H spectral ratio depend on parameters such as the natural period of the ground for P-wave, Poisson ratio of the soil and the frequency content of the input shaking. Moreover, it was revealed that the inverse of quarter wavelength impedance contract of P-waves shows a good correlation with V/H spectral ratios and can be used for the implementation of site effects into V/H empirical equations.

کلیدواژه‌ها [English]

  • Near-fault earthquakes
  • Vertical component
  • Site Effects
  • V/H Acceleration Spectral Ratio
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