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Rethinking PSHA
Get PDFSince the early 1980s seismic hazard assessment in New Zealand has been based on Probabilistic
Seismic Hazard Analysis (PSHA). The most recent version of the New Zealand National Seismic Hazard
Model, a PSHA model, was published by Stirling et al, in 2012. This model follows standard PSHA
principals and combines a nation-wide model of active faults with a gridded point-source model based on
the earthquake catalogue since 1840. These models are coupled with the ground-motion prediction
equation of McVerry et al (2006). Additionally, we have developed a time-dependent clustering-based
PSHA model for the Canterbury region (Gerstenberger et al, 2014) in response to the Canterbury
earthquake sequence.
We are now in the process of revising that national model. In this process we are investigating
several of the fundamental assumptions in traditional PSHA and in how we modelled hazard in the past.
For this project, we have three main focuses: 1) how do we design an optimal combination of multiple
sources of information to produce the best forecast of earthquake rates in the next 50 years: can we
improve upon a simple hybrid of fault sources and background sources, and can we better handle the
uncertainties in the data and models (e.g., fault segmentation, frequency-magnitude distributions,
time-dependence & clustering, low strain-rate areas, and subduction zone modelling)? 2) developing
revised and new ground-motion predictions models including better capturing of epistemic uncertainty – a
key focus in this work is developing a new strong ground motion catalogue for model development; and 3)
how can we best quantify if changes we have made in our modelling are truly improvements? Throughout
this process we are working toward incorporating numerical modelling results from physics based
synthetic seismicity and ground-motion models
Rethinking PSHA
No full textSince the early 1980s seismic hazard assessment in New Zealand has been based on Probabilistic
Seismic Hazard Analysis (PSHA). The most recent version of the New Zealand National Seismic Hazard
Model, a PSHA model, was published by Stirling et al, in 2012. This model follows standard PSHA
principals and combines a nation-wide model of active faults with a gridded point-source model based on
the earthquake catalogue since 1840. These models are coupled with the ground-motion prediction
equation of McVerry et al (2006). Additionally, we have developed a time-dependent clustering-based
PSHA model for the Canterbury region (Gerstenberger et al, 2014) in response to the Canterbury
earthquake sequence.
We are now in the process of revising that national model. In this process we are investigating
several of the fundamental assumptions in traditional PSHA and in how we modelled hazard in the past.
For this project, we have three main focuses: 1) how do we design an optimal combination of multiple
sources of information to produce the best forecast of earthquake rates in the next 50 years: can we
improve upon a simple hybrid of fault sources and background sources, and can we better handle the
uncertainties in the data and models (e.g., fault segmentation, frequency-magnitude distributions,
time-dependence & clustering, low strain-rate areas, and subduction zone modelling)? 2) developing
revised and new ground-motion predictions models including better capturing of epistemic uncertainty – a
key focus in this work is developing a new strong ground motion catalogue for model development; and 3)
how can we best quantify if changes we have made in our modelling are truly improvements? Throughout
this process we are working toward incorporating numerical modelling results from physics based
synthetic seismicity and ground-motion models
The Apostolic Psha!
Get PDFMemory plays us scurvy tricks. I remember that Hilaire Belloc says somewhere that there are three things that a real man must be capable of saying. I remember also that the first one is: Credo in unum Deum, Patrem omnipotentem; and that the third one is: Psha
How different PSHA is different enough?
No full textProbabilistic seismic hazard analysis (PSHA) is widely employed worldwide as the rational way to
quantify the uncertainty associated to earthquake occurrence and effects. National-scale PSHA
has its results typically expressed in the form of maps of ground motion measures intensities
that all have the same exceedance return period. Classical PSHA relies on data that continuously
increase due to instrumental seismic monitoring, and on models that continuously evolve with
the knowledge on each of its many aspects. Therefore, it can happen that different, equally
legitimate, hazard maps for the same region can show apparently irreconcilable differences,
sparking public debate. This situation is currently ongoing in Italy, where the process of
governmental enforcement of a new hazard map is delayed. The discussion is complicated by the
fact that the events of interest to hazard assessment are intentionally rare at any of the sites the
maps refer to, thus impeding empirical validation at any specific site. The presentation will show
the result of two recent studies, which pursue a regional approach, regarding three different
authoritative PSHA studies for Italy. The first one entailed formal tests on the output of PSHA
against the observed ground shaking exceedance frequencies, obtained from about fifty years of
continuous monitoring of seismic activities across the country (Iervolino et al., 2023a). The
second compares the areas in which exceedance of PSHA-postulated ground motion intensity
threshold is estimated according to ShakeMap for twelve years of instrumental earthquakes,
with what expected from the considered PSHA models (Iervolino et al., 2023b). The bulk of
analyses reveals that, apparently alternative hazard maps are, in fact, hardly distinguishable in
the light of observations and ShakeMap estimations. This perspective, which may be relevant for
the current debate, may be strengthened by the fact that recent studies (Baltzopoulos et al.,
2023) also show that structural design, for example for reinforced concrete moment-resisting
frames, is strictly dominated by seismic actions only in a fraction of the country, owing to the
effect of building-code-prescribed minima and design for gravity loads
Simulation-Based PSHA for the Canterbury Region(cybershake v17.8)
Get PDFThis poster presents the computational components of physics-based broadband ground motion simulations to perform probabilistic seismic hazard analysis (PSHA) for Canterbury, New Zealand. Ground motions are simulated using the hybrid broadband simulation approach of Graves and Pitarka (2010, 2015). The workflow for the nation-wide simulation-based PSHA (i.e., NZ Cybershake) comprises three components
Near-Field PSHA with Directivity – Dorud
No full textThis study investigates the probabilistic seismic hazard in Dorud city, located near the active Dorud fault, with a specific focus on incorporating near-field and rupture directivity effects into the hazard modeling framework. Near-field ground motions – particularly those influenced by rupture directivity – can generate long-period velocity pulses, posing serious risks to long-period structures such as bridges and tall buildings near faults. To realistically capture these effects, this study integrates empirical directivity models (Somerville et al., 1997; Abrahamson, 2000) into the probabilistic seismic hazard assessment (PSHA). The seismicity parameters were derived using the Kijko (2004) method based on a carefully declustered earthquake catalog; the suitability of this catalog for PSHA was statistically confirmed through a Kolmogorov‑Smirnov (K‑S) test, validating the Poissonian nature of inter-event times.Seismic hazard calculations were performed for vibration periods of 0.75, 1, 2, 3, and 4 seconds and return periods of 50, 475, and 2475 years. The study further includes deaggregation analysis to examine how near-field and directivity effects influence magnitude and distance contributions to hazard. The results show that the influence of directivity increases with both return period and vibration period. The most significant amplification – a 17.16% increase in acceleration – occurs when directivity is included for a 2475-year return period at a 4-second vibration period. A regional comparison of seismicity parameters with previous PSHA studies supports the robustness of the selected input values. This study demonstrates the importance of explicitly incorporating directivity in PSHA for fault-adjacent urban areas, especially for engineering design of critical long-period structures
PSHA validated by quasi observational means
Get PDFIt might seem odd to be writing about confirmation of the validity of probabilistic seismic hazard assessment (PSHA) in 2011, given that the method has been successfully applied in countless studies worldwide over the last 40 years. However, the fact that papers still occasionally find their way into print attacking the method as mathematically invalid seems to indicate that there is still some requirement, if small, to demonstrate the soundness of the method. A number of mathematical arguments have been advanced over the last few years purporting to show mathematical or logical flaws in the standard PSHA methodology that invalidate the results. A comprehensive summary of these objections can be found in Klügel (2008)
Why is Probabilistic Seismic Hazard Analysis (PSHA) still used?
Get PDFEven though it has never been validated by objective testing, Probabilistic Seismic Hazard Analysis (PSHA) has been widely used for almost 50 years by governments and industry in applications with lives and property hanging in the balance, such as deciding safety criteria for nuclear power plants, making official national hazard maps, developing building code requirements, and determining earthquake insurance rates. PSHA rests on assumptions now known to conflict with earthquake physics; many damaging earthquakes, including the 1988 Spitak, Armenia, event and the 2011 Tohoku, Japan, event, have occurred in regions relatively rated low-risk by PSHA hazard maps. No extant method, including PSHA, produces reliable estimates of seismic hazard. Earthquake hazard mitigation should be recognized to be inherently political, involving a tradeoff between uncertain costs and uncertain risks. Earthquake scientists, engineers, and risk managers can make important contributions to the hard problem of allocating limited resources wisely, but government officials and stakeholders must take responsibility for the risks of accidents due to natural events that exceed the adopted safety criteria
Perhitungan Desain Struktur Gedung Perkantoran 12 Lantai di Kota Surabaya dengan Beban Gempa PSHA Berdasarkan Data USGS
Get PDFGempa merupakan suatu tantangan tersendiri oleh para perencana gedung dalam mendesain sebuah bangunan terutama bangunan gedung tinggi di Indonesia. Sebab, Indonesia merupakan negara yang berada dalam kawasan Ring of Fire. Seringkali bangunan gedung masih kuat dalam menerima beban gravitasi namun masih jarang sekali gedung yang mampu bertahan terdapat beban gempa saat terjadinya bencana alam tersebut, sehingga beban gempa pada desain bangunan tidak boleh diabaikan begitu saja terlebih lagi pada kota-kota besar yang memiliki gedung-gedung tinggi. Saat ini telah adanya peta gempa Indonesia terbaru yakni peta gempa Indonesia 2017. Namun, belum adanya peraturan SNI yang dimutahirkan, hal ini yang melatarbelakangi penulis untuk menggunakan beban gempa rencana yang dihitung menggunakan Metode PSHA (Probabilistic Seismic Hazard Analysis) pada Kota Surabaya dengan data pengambilan dari laman internasional yankni USGS. Data yang akan diambil yakni riwayat terjadinya gempa pada radius pengaruh 500 km dari Kota Surabaya yang memilik magnitude lebih dari 5 skala richter. Perhitungan untuk mendapatkan nilai PGA akan didapatkan dengan cara menentukan faktor Atenuasi dan amplikasi. Dalam kasus ini penulis akan menggunaka fungsi atenuasi Matuscha 1980 dan dapat terbentuk sebuah respons spektra untuk Kota Surabaya dengan metode PSHA. Sehingga, penulis akan membandingkan desain gedung yang menggunakan SNI 1726-2012 dengan desain gedung menggunakan beban gempa metode PSHA. Berdasarkan hasil analisa perhitungan, beban gempa desain dengan metode PSHA memiliki nilai Sa sebesar 0,689 g dan dibandingakan dengan nilai Sa pada PUSKIM 2012 didapatkan nilai sebesar 0,607 g. Sehingga pada analisis ini didapatkan bahwa menghitung beban gempa desain dengan metode PSHA lebih besar 4,9% dari beban gempa PUSKIM. Setelah dihitung didapatkan bahwa volume dan berat kedua beban gempa tersebut memiliki selisih 1,29%, sehingga dengan beban gempa desain PSHA akan lebih membutuhkan secara keungan namun akan lebih aman untuk menjadi bangunan lebih tahan gempa.
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The earthquake was a challenge by building planners in designing the main buildings in Indonesia. Cause, Indonesia is a country that is in the Ring of Fire region. The building series is still strong in accepting the burden, there is still one building that is able to withstand the earthquake, so the earthquake load on the design of the building should not be ignored in large cities that have tall buildings.
Lately there has been the latest Indonesian earthquake map on the Indonesian earthquake map 2017. However, because there are no SNI regulations decided because of the earthquake map, this is the background for the author to use earthquakes made using the PSHA Method (Probabilistic Seismic Hazard Analysis) in the city of Surabaya by retrieving data from the international page that is USGS. The data taken was taken from the example taken by the earthquake at a radius of influence of 500 km from the city of Surabaya which has a greater than 5 Richter scale. The value for obtaining a PGA value will be obtained by determining the attenuation and amplification factors. In this case the writer will use the Matuscha 1980 attenuation function and can form a spectra response for the City of Surabaya using the PSHA method. Comparing, the writer will compare the building design using SNI 1726-2012 with the building design using the PSHA earthquake method.
Based on the results of the assessment, the earthquake load design with the PSHA method has a Sa value of 0.689 g and compared with the Sa value in PUSKIM 2012 obtained a value of 0.607 g. So in this analysis it is necessary to calculate the earthquake with the PSHA method greater than 4.9% of the PUSKIM earthquake load. After being calculated, the volume and weight of the two earthquakes had a difference of 1.29%, so with the earthquake load, the design of the PSHA would require more cash, but it would be safer to be an earthquake resistant building
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