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# Earthquake Design Magnitude2

## Earthquake Design Magnitude

(OP)
What determines the magnitude of earthquake is used when coming up with earthquake loads? Obviously this varies with region, but is there a table that the design magnitude is pulled from or is there some chart that connects spectral acceleration to magnitudes? I'm trying to connect the values given on the USGS reports with something more widely understood such as "Magnitude 7" or something similar (not that the general public understands that much better, it just helps give some sort of frame of reference).

### RE: Earthquake Design Magnitude

The earth is a molten sort of spherical (oblate sphereoid) object with a surfacial crust that varies in thickness to approx 15 miles thick. This crust is 'broken into little areas' (some several thousand square miles) that are slowly moving; these are called tectonic plates. Movement is small. Depending on the area, maybe a few parts of an inch per year. The plates are either moving under the adjacent plate or over it usually. When these plates are restricted from moving, they build up energy and eventually the forces are great enough to allow movement. When this happens, the energy is released in the form of an earthquake. The greater the restriction, the more energy is stored and the greater amount of energy is released. This is a measure of the intensity of an earthquake. Earthquakes usually occur on the edge of one of these plate boundaries and can be on the surface or anywhere on the depth of the plate. Others can add to this info, but, it's a simplified description of the 'making' of an earthquake (seismic event).

Dik

### RE: Earthquake Design Magnitude

There's a variety of equations relating Richter Magnitude to Peak Ground Acceleration. Blume 1965 gives one: http://www.iitk.ac.in/nicee/wcee/article/vol3_IV-53.pdf

Here's a table of PGA and magnitude for California earthquakes:

The relationship between PGA (what the ground experiences) and Spectral Acceleration (what a building would experience) is established empirically (I believe ATC 3-06 does this?), but I think Sa tends to be around 1.5-2.5 PGA.

Brian C Potter, PE
Simple Supports - Back at it again with the engineering blog.

### RE: Earthquake Design Magnitude

There really isn't a concrete correlation between the magnitude of a Earthquake [M] and PGA or duration. The 1989 Loma Prieta earthquake (7.1 on the Richter scale) registered a PGA of 0.65g while the (1994) Northridge earthquake (6.7 on the Richter scale) registered a PGA of 1.8g. Obviously site conditions play a role in this from place to place.

The accelerations used are based on observation, probabilities of occurring, etc.

As BP noted, there are a few empirical relations (that can satisfy a question from a client looking for a correlation).....but it's not 100% accurate.

### RE: Earthquake Design Magnitude

then after an earthquake how does one determine the seismic loads a particular bldg was subjected to??

### RE: Earthquake Design Magnitude

You can use the so called 'ground motion prediction equations', which link spectral values with parameters such as magnitude, distance to epicenter or fault, soil type, etc.

As far as I know they are a statistical device with little mechanistic reasoning.

I wouldn't trust them for design (or anything) since they are typically expressed in log scale (where 1 is near the 10) and show high variability. We all know it's easy to lie with statistics.

Boore and Atkinson (2007) GMPE enjoy some popularity I believe.

### RE: Earthquake Design Magnitude

SAIL3, where I live most new buildings require seismic monitoring equipment to be installed.

### RE: Earthquake Design Magnitude

I've heared a few times questions like what magnitude of earthquake can this building or bridge resist? Because usually the strength of earthquake is measured by magnitude at least that's how they report it in the news.

What is the best answer to this question? We design based on asce/ibc code but neither of the code discuss the magnitude of earthquake.

### RE: Earthquake Design Magnitude

I think that the magnitude of the earthquake is related to the released energy and its relation with design accelerations is not common.

If you want to explain the significance of the earthquakes based on the acceleration values given on the different codes (i.e. IBC, ASCE 7-10), you could indicate that these values are for the "maximum considered earthquake" (MCE) for a specific area, which is an earthquake that is expected to occur once in approximately 2,500 years. I think that people will understand this as the "big one" regardless its "magnitude".

However, the actual design earthquake is a different history because it applies a factor (2/3) to the MCE which makes the accelerations to be equivalent to accelerations for a return period of 475 years (for some parts for the US).

I also agree with WARose: the registered PGA depends on site soil conditions. Also, the table from briancpotter is interesting. It correlates well with the assumed PGA for liquefaction analyses in Japan (0.35g) which is based on the 1995 Great Hanshin EQ (6.9 Mw).

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