최대지반가속도 추정기

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

Calculation

최대지반가속도(PGA)란?

최대지반가속도(PGA)는 지진 발생 시 지구 표면에서 경험되는 최대 가속도로, 중력 가속도(g = 9.81 m/s²)의 분율로 측정됩니다. 이는 건물과 인프라가 견뎌야 하는 힘과 직접 관련되므로 지진 공학에서 가장 중요한 매개변수 중 하나입니다. PGA 0.1g는 지반이 중력의 10%로 가속되었음을 의미하며, 강하게 느껴지고 경미한 피해를 유발할 수 있습니다. 0.5g에서는 가속도가 중력의 절반이며, 잘 설계된 건물에도 구조적 손상이 발생할 수 있습니다. 기록된 최고 PGA는 2011년 일본 도호쿠 지진에서의 2.7g입니다.

지반운동예측방정식(GMPE)은 감쇠 관계라고도 하며, 지진 규모, 단층으로부터의 거리, 깊이, 부지 조건을 기반으로 PGA를 추정하는 경험적 모델입니다. 이 도구는 교육적 추정치를 제공하기 위해 단순화된 GMPE를 사용합니다. 전문적인 지진 위험 분석에서는 여러 GMPE를 사용하고 논리 트리를 통해 불확실성을 고려합니다. 부지 조건은 매우 중요합니다. 연약한 토양은 기반암 대비 PGA를 2~3배 증폭시킬 수 있으며, 이는 ASCE 7과 유로코드 8과 같은 건축 기준에서 정의된 부지 증폭 계수로 포착됩니다.

PGA와 구조 설계

건축 기준은 설계 기준 PGA 값을 명시합니다: 구조물은 붕괴 없이 일정 수준의 지반 가속도를 견뎌야 합니다. 고지진 지역에서 이는 통상 0.3~0.4g입니다.
PGA만으로는 지반 운동의 심각성을 완전히 설명할 수 없습니다. 지속 시간과 주파수 내용도 중요합니다. 높은 PGA의 짧은 펄스는 장기간의 중간 정도 진동보다 피해가 적을 수 있습니다.
부지 분류(ASCE 7의 A~F등급)는 설계 스펙트럼에 영향을 미칩니다: 연약 토양 부지(D/E등급)는 증폭 효과로 인해 더 높은 설계 하중이 필요합니다.
USGS와 GSHAP에서 생산하는 것과 같은 지진 위험 지도는 50년 동안 10%의 초과 확률을 가진 PGA 값을 보여주며, 이는 건축 기준의 표준 참조입니다.

일반적인 용도

가상 지진 시나리오에 대해 특정 위치의 지반 진동 수준을 추정합니다.
거리와 토양 조건이 지반 가속도에 미치는 영향 이해.
잠재적 피해를 평가하기 위해 추정 PGA를 건축 기준 요구 사항과 비교.
지진 감쇠 및 부지 증폭 효과의 교육적 시연.

How to Use

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
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
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

최대지반가속도(PGA)란 무엇인가요?

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.