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مقدِّر التسارع الأرضي الأقصى

Estimate Peak Ground Acceleration at your location for a given earthquake scenario.

Calculation

ما هو ذروة التسارع الأرضي (PGA)؟

تسارع الأرض الأقصى (PGA) هو أقصى تسارع يُعاش على سطح الأرض خلال زلزال، يُقاس ككسر من تسارع الجاذبية (g = 9.81 م/ث²). هو أحد أهم المعلمات في الهندسة الزلزالية لأنه يرتبط مباشرة بالقوى التي يجب أن تتحملها المباني والبنية التحتية. PGA بقيمة 0.1g يعني أن الأرض تسارعت بنسبة 10% من الجاذبية — كافٍ ليُشعر به بقوة ويسبب أضراراً طفيفة. عند 0.5g، التسارع نصف الجاذبية، وتصبح الأضرار الإنشائية للمباني جيدة التصميم محتملة. أعلى PGA مسجل كان 2.7g خلال زلزال توهوكو 2011 في اليابان.

معادلات التنبؤ بحركة الأرض (GMPEs)، وتُسمى أيضاً علاقات التخميد، هي نماذج تجريبية تُقدّر PGA بناءً على قوة الزلزال والمسافة من الصدع والعمق وظروف الموقع. تستخدم هذه الأداة GMPE مبسطة لتوفير تقديرات تعليمية. يستخدم التحليل المهني للخطر الزلزالي GMPEs متعددة ويراعي عدم اليقين من خلال أشجار المنطق. ظروف الموقع مهمة جداً — يمكن للتربة الرخوة تضخيم PGA بـ 2-3 أضعاف مقارنة بالصخر، وهي ظاهرة تُلتقط بعوامل تضخيم الموقع المحددة في قوانين البناء مثل ASCE 7 وEurocode 8.

PGA والتصميم الإنشائي

  • تحدد قوانين البناء قيم PGA الأساسية للتصميم: يجب أن تقاوم المنشآت مستوى معيناً من تسارع الأرض بدون انهيار. في المناطق ذات النشاط الزلزالي العالي، هذا عادة 0.3-0.4g.
  • PGA وحده لا يصف بالكامل شدة حركة الأرض — المدة ومحتوى التردد مهمان أيضاً. قد يكون نبض قصير من PGA العالي أقل ضرراً من اهتزاز معتدل مطول.
  • تصنيف الموقع (A إلى F في ASCE 7) يؤثر على طيف التصميم: مواقع التربة الرخوة (فئة D/E) تتطلب قوى تصميم أعلى بسبب تأثيرات التضخيم.
  • خرائط الخطر الزلزالي، مثل تلك التي ينتجها USGS وGSHAP، تُظهر قيم PGA باحتمال 10% للتجاوز في 50 عاماً — المرجع المعياري لقوانين البناء.

الاستخدامات الشائعة

  • تقدير مستوى الاهتزاز الأرضي في موقع محدد لسيناريو زلزال افتراضي.
  • فهم كيف تؤثر المسافة وظروف التربة على تسارع الأرض.
  • مقارنة PGA المقدّر بمتطلبات قوانين البناء لتقييم الأضرار المحتملة.
  • عروض تعليمية لتخميد الموجات الزلزالية وتأثيرات تضخيم الموقع.

How to Use

  1. 1
    Define the Earthquake Scenario

    Enter the earthquake magnitude (Mw), epicenter coordinates, and focal depth. For site-specific design purposes, you may also use a return period (e.g., 475-year) to retrieve the probabilistic PGA from the USGS seismic hazard map.

  2. 2
    Specify Your Site Location and Soil Class

    Enter your coordinates and select your site soil class (A through F per ASCE 7 / Eurocode 8). Site class D (stiff soil, Vs30 = 180–360 m/s) is the reference class; softer soils amplify PGA, harder rock reduces it.

  3. 3
    Review the PGA Estimate and Design Implications

    Read your estimated PGA in g-units and its corresponding approximate MMI intensity. The tool notes the relevant building code seismic design category (SDC) for US locations and the equivalent Eurocode 8 PGA design value.

About

Peak Ground Acceleration emerged as the primary seismic engineering parameter in the 1950s–60s when strong-motion accelerographs first became widely deployed following the 1940 El Centro earthquake (which produced the first complete accelerogram used in engineering, with PGA = 0.33g). PGA's appeal lies in its direct measurement from instruments, its physical intuition (how hard the ground shakes), and its correlation with observations from historical earthquakes. The USGS National Seismic Hazard Maps, first produced in the 1970s and updated through PSHA methodology, express seismic hazard primarily as PGA at specific probability levels.

However, PGA has known limitations as a sole damage predictor. It reflects high-frequency energy that governs rigid structural response but may not capture the damage potential for taller, more flexible structures sensitive to long-period energy. The 1985 Mexico City earthquake illustrated this dramatically: soft sediment resonance amplified long-period (2 second) waves while PGA on those same sediments was not extreme, yet mid-rise (8–15 story) buildings resonating at the site's natural period collapsed while shorter and taller buildings survived. This observation drove the adoption of design response spectra and, more recently, spectral acceleration at specific periods (Sa(1.0s), Sa(0.2s)) as primary design parameters in modern codes.

The emergence of broadband seismic networks and dense strong-motion arrays has dramatically expanded the observational database underpinning PGA prediction models. The Next Generation Attenuation (NGA) project, coordinated by PEER (Pacific Earthquake Engineering Research Center), compiled an international database of over 21,000 ground motion records from 600+ earthquakes to develop the NGA-West2 model suite—five independent GMPEs now used as the foundation of USGS hazard maps and incorporated into building codes in the US and internationally. These models explicitly account for magnitude, distance, depth, style of faulting, hanging wall effects, basin depth, and Vs30, providing PGA predictions with quantified uncertainty at any site worldwide.

FAQ

ما هو ذروة التسارع الأرضي (PGA)؟
Peak Ground Acceleration (PGA) is the maximum acceleration experienced by the ground surface during an earthquake, measured in units of gravitational acceleration (g, approximately 9.81 m/s²) or as a percentage of g (%g). PGA is the most widely used parameter for characterizing seismic hazard in building codes because it correlates reasonably well with damage to short-period structures such as low-rise buildings. A PGA of 0.05g is typically the threshold for human perception; 0.1g can cause non-structural damage; 0.3g represents severe shaking that damages poorly designed structures; and PGA values exceeding 1.0g have been recorded near fault ruptures (1.8g was recorded in the 1994 Northridge earthquake). PGA is measured by accelerographs (strong motion seismometers) rather than standard seismographs.
How does PGA relate to structural damage?
PGA is most relevant for predicting damage to stiff, low-period structures. For flexible structures (tall buildings, bridges) with natural periods greater than 0.5–1.0 seconds, spectral acceleration at the structure's period is a more accurate damage predictor than PGA. This is why modern building codes use design response spectra rather than PGA alone: the spectrum describes the maximum acceleration experienced by oscillators of different natural periods during the earthquake, capturing the full range of structural response. PGA corresponds approximately to the spectral acceleration at zero period (a completely rigid structure). The correlation between PGA and Modified Mercalli Intensity is approximate: MMI VI ≈ 0.06–0.10g PGA; MMI VII ≈ 0.10–0.18g; MMI VIII ≈ 0.18–0.34g.
What is Vs30 and why does it matter for PGA?
Vs30 is the time-averaged shear-wave velocity of the top 30 meters of soil, calculated as 30 meters divided by the summed travel time of shear waves through each layer. It is the internationally standardized proxy for site characterization in seismic hazard analysis and building codes (ASCE 7, Eurocode 8, Japanese seismic code). Higher Vs30 indicates stiffer, harder ground that amplifies shaking less. ASCE 7 site classes range from A (hard rock, Vs30 > 1,500 m/s) through F (potentially liquefiable soils). A change from rock (Site Class B, Vs30 ~760 m/s) to soft soil (Site Class E, Vs30 < 180 m/s) can amplify PGA by a factor of 3–5 at low spectral periods, with even larger amplification at mid-periods (0.1–0.5 s) relevant to 2–5 story buildings.
What is the difference between PGA, PGV, and PGD?
PGA (Peak Ground Acceleration), PGV (Peak Ground Velocity), and PGD (Peak Ground Displacement) are the three fundamental ground motion intensity measures derived from accelerograph recordings. PGA is the maximum of the acceleration time history and governs response of stiff, low-period structures. PGV—obtained by integrating the acceleration record—correlates best with damage to medium-period structures (0.5–2.0 s) such as mid-rise buildings and bridges, and is considered by many researchers to be the best single predictor of overall structural damage. PGD—the double integral of acceleration—governs response of long-period structures, buried pipelines, and dams, where absolute displacement matters. Modern ground motion attenuation models (Next Generation Attenuation, NGA-West2) simultaneously predict PGA, PGV, and spectral accelerations.
How do engineers use PGA in building design?
Engineers use probabilistic seismic hazard analysis (PSHA) to determine design PGA values at specified return periods. The ASCE 7-22 standard uses Risk-Targeted Maximum Considered Earthquake (MCER) maps, which represent ground motions with a 1% probability of collapse in 50 years for a code-conforming building (approximately a 5,000-year return period). The design-level ground motion is two-thirds of MCER. Site-specific hazard analysis, required for critical or large structures, integrates contributions from all potential earthquake sources (faults and distributed seismicity) to produce a hazard curve—the annual probability of exceeding different PGA levels. The design spectrum is then constructed from uniform hazard spectra, and dynamic analysis of the structural model under ground motions scaled to the design spectrum is used to verify code compliance.