Category Archives: earthquake size

Class Summary – 22 August – Analysis & Summary, Earthquake Experiences, +

During an extended seismicity briefing, I discussed the information available online following a large earthquake (yesterday’s M 7.3 earthquake north of Venezuela).

We briefly discussed the Analyze / Summarize reading and students described personal earthquake experiences. Many had experienced the 2001 Virginia Earthquake shaking, that rattled the eastern seaboard in August of that year. Here is the link to the USGS summary page for that earthquake:

https://earthquake.usgs.gov/earthquakes/eventpage/se609212#executive

I also briefly discussed textbooks and noted that you will need the required book for the class within a couple of weeks. So if you need to order it, do so soon. If you are having financial trouble with your textbooks (in general) see our syllabus for information on help with those costs.

I noted the PSU Involvement Fair is scheduled for the HUB Lawn Thursday 23 Aug 11AM – 4PM.  Click here for a list of clubs, organizations that are participating. Get involved in a club that interests you to provide an oft-needed break from study.

Next Class: Online EQ information and EQ Shaking

Class Summary – 23 Sep, 2015, Earthquake Size – Seismic Moment

Seismic moment is a quantity that combines the area of the rupture and the amount of fault offset with a measure of the strength of the rocks – the shear modulus.

Seismic Moment = (Shear Modulus) x (Rupture Area) x (Fault Offset)

Usually we measure the moment directly from seismograms, since the size of the very long-period waves generated by an earthquake is proportional to the seismic moment. The physical units of seismic moment are force x distance, such as newton-meter or dyne-centimeter. These are the same units as energy, but we use the explicit N-m or dyne-cm forms to distinguish the physical character of the quantity as a moment.

For scientific studies, the moment is the preferred measure we use to compare earthquake size since it has fewer limitations than the magnitudes, which often reach a maximum value (we call that magnitude saturation).

To compare seismic moment with magnitude we use a formula constructed by Hiroo Kanamori of the California Institute of Technology:

Mw = log10(Seismic Moment)/1.5 – 10.7

where the units of the moment are in dyne-cm. We call Mw the moment magnitude. Note that moment is often reported in N-m (newton-meters), to convert to dyne-cm, multiply the N-m value by 107.

Class Summary – 11 Sep, 2015 – Earthquake Rupture Sizes

We discussed earthquake ruptures and used data to explore the area of rupture for a large earthquake. I introduced the concept of earthquake magnitude as a logarithmic measure of the amplitude of earthquake-induced ground shaking. We will be using and discussing measures of earthquake size for several classes in the next week or so.

In-class activity: Students estimated the area of the 2015 Tibetan Earthquake Rupture using plots of the aftershocks.

Class Summary –10 Oct – Magnitude

I began a lecture on earthquake size; we will finish next class. Get the notes and read the appropriate sections in the class texts if you need more background. Some of the material online is very good on these subjects. Detail can be found in wikipedia, for example.

The earliest metrics of earthquake size were based on their impact upon societies. The suffering, social impact, and economic impacts are recorded historically for many earthquakes. Since the 1700’s we have tried to be more systematic about measuring earthquake size using the scale of damage to human-made structures. In the late 1800’s the development of the seismometer led to classic magnitude scales (developed in the 1930’s) and satellite geodesy has led to additional measures based on ground deformation.

Magnitude is based on observations of the size of the ground shaking produced by an earthquake. The simple idea is that larger earthquakes produce larger vibrations. To measure the ground vibrations precisely, we use seismometers. Seismometers are sensors used to measure ground motion; seismographs are instruments used to record the motions as a function of time; and seismograms are plots of the motion versus time.

Magnitude is usually computed using two observations from each seismogram – the largest amplitude and the period of the motion. We will talk about period and frequency later.  The magnitude of the earthquake can be measured using a number of different parts of a seismogram, and thus we have a number of different types of magnitudes.

Richter originally used the logarithmic (base 10) difference between the observed largest ground motion and a reference value $$M_L = log_{10} A – log_{10}~A_0~,$$where \(A\) now represents the amplitude of the ground motion. The subscript in \(M_L\) stands for “local”, and we call this measure, local magnitude because originally this measure was local to southern California, where Richter was working. The above formula works fine if the earthquakes are the same distance away. However,  we must also account the fact that vibrations observed farther from an earthquake are naturally smaller. Richter recognized that some type of distance-based correction would be necessary if seismometers at all distances were to produce consistent estimates for the same earthquake.

Having even a repeatable, easy-to-measure estimate of earthquake size is very valuable if you want to compare earthquakes. Richter recognized this quickly and in collaboration with a colleague at CalTech, Gutenberg, he developed other magnitude measures that were appropriate for waves recorded at greater distances from the earthquake.

In-class activity: Students started a calculation of the magnitude of the 2011 Virginia Earthquake.