Magnitude 2.1 Earthquake

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Earthquakemagnitude, energy release, and shaking intensity are all related measurements of an earthquake that are often confused with one another. Their dependencies and relationships can be complicated, and even one of these concepts alone can be confusing.

The time, location, and magnitude of an earthquake can be determined from the data recorded by seismometer. Seismometers record the vibrations from earthquakes that travel through the Earth. Each seismometer records the shaking of the ground directly beneath it. Sensitive instruments, which greatly magnify these ground motions, can detect strong earthquakes from sources anywhere in the world. Modern systems precisely amplify and record ground motion (typically at periods of between 0.1 and 100 seconds) as a function of time.

Magnitude is the size of the earthquake. An earthquake has a single magnitude. The shaking that it causes has many values that vary from place to place based on distance, type of surface material, and other factors. See the Intensity section below for more details on shaking intensity measurements.

Magnitude is expressed in whole numbers and decimal fractions. For example, a magnitude 5.3 is a moderate earthquake, and a 6.3 is a strong earthquake. Because of the logarithmic basis of the scale, each whole number increase in magnitude represents a tenfold increase in measured amplitude as measured on a seismogram.

When initially developed, all magnitude scales based on measurements of the recorded waveform amplitudes were thought to be equivalent. But for very large earthquakes, some magnitudes overestimate true earthquake size, and some underestimate the size. Thus, we now use measurements that describe the physical effects of an earthquake rather than measurements based only on the amplitude of a waveform recording. More on that later.

The Richter Scale (ML) is what most people have heard about, but in practice it is not commonly used anymore, except for small earthquakes recorded locally, for which ML and short-period surface wave magnitude (Mblg) are the only magnitudes that can be measured. For all other earthquakes, the moment magnitude (Mw) scale is a more accurate measure of the earthquake size.

Although similar seismographs had existed since the 1890's, it was only in 1935 that Charles F. Richter, a seismologist at the California Institute of Technology, introduced the concept of earthquake magnitude. His original definition held only for California earthquakes occurring within 600 km of a particular type of seismograph (the Woods-Anderson torsion instrument). His basic idea was quite simple: by knowing the distance from a seismograph to an earthquake and observing the maximum signal amplitude recorded on the seismograph, an empirical quantitative ranking of the earthquake's inherent size or strength could be made. Most California earthquakes occur within the top 16 km of the crust; to a first approximation, corrections for variations in earthquake focal depth were, therefore, unnecessary.

The Richter magnitude of an earthquake is determined from the logarithm of the amplitude of waves recorded by seismographs. Adjustments are included for the variation in the distance between the various seismographs and the epicenter of the earthquakes.

Moment Magnitude (MW) is based on physical properties of the earthquake derived from an analysis of all the waveforms recorded from the shaking. First the seismic moment is computed, and then it is converted to a magnitude designed to be roughly equal to the Richter Scale in the magnitude range where they overlap.

where rigidity is the strength of the rock along the fault, area is the area of the fault that slipped, and slip is the distance the fault moved. Thus, stronger rock material, or a larger area, or more movement in an earthquake will all contribute to produce a larger magnitude.

Another way to measure the size of an earthquake is to compute how much energy it released. The amount of energy radiated by an earthquake is a measure of the potential for damage to man-made structures. An earthquake releases energy at many frequencies, and in order to compute an accurate value, you have to include all frequencies of shaking for the entire event.

The energy can be converted into yet another magnitude type called the Energy Magnitude (Me). However, since the Energy Magnitude and Moment Magnitude measure two different properties of the earthquake, their values are not the same.

Whereas the magnitude of an earthquake is one value that describes the size, there are many intensity values for each earthquake that are distributed across the geographic area around the earthquake epicenter. The intensity is the measure of shaking at each location, and this varies from place to place, depending mostly on the distance from the fault rupture area. However, there are many more aspects of the earthquake and the ground it shakes that affect the intensity at each location, such as what direction the earthquake ruptured, and what type of surface geology is directly beneath you. Intensities are expressed in Roman numerals, for example, VI, X, etc.

Traditionally the intensity is a subjective measure derived from human observations and reports of felt shaking and damage. The data used to be gathered from postal questionnaires, but with the advent of the internet, it's now collected using a web-based form. However, instrumental data at each station location can be used to calculate an estimated intensity.

The reason is shown by the two cartoon cross-sections below. There was more shaking in the Northridge earthquake because the earthquake occurred closer to the surface (3-11 miles), as opposed to the Nisqually earthquake's deeper hypocenter (30-36 miles).

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A1: The reverberations from the earthquakes were felt in Iraq, Israel, Lebanon, and Jordan, though initial estimates suggest the greatest devastation occurred in southeastern Turkey and northern Syria. Home to over 2 million people, Gaziantep is the sixth largest city in Turkey. Within 24 hours, an estimated 5,600 buildings were destroyed in Turkey and over 5,000 people had died in Turkey and Syria, though these figures are likely to grow in the coming days.

A2: The biggest concerns are and will continue to be the loss of life and providing humanitarian relief to survivors. But the already high death toll could continue to grow for at least four reasons.

First, since the initial earthquake struck when many people were still at home, most were likely to have been in the thousands of buildings that were destroyed. Search and rescue operations will continue for the next few days, after which efforts will shift to recovery and ultimately to rebuilding damaged and destroyed infrastructure. The physical and psychological human impact will be far greater and longer lasting. In the coming days and weeks and months, international donors and NGOs will need to draw on lessons from other rapid onset disasters (e.g., tsunami and hurricane relief) which share similar destructive qualities. These lessons include the critical need to coordinate assistance, to build local resilience, and to draw from and strengthen local response structures.

Third, the strength of the earthquakes resulted in entire neighborhoods being reduced to rubble. In addition to the loss of life this has caused, the magnitude of the destruction means that all relief efforts will be challenging thanks to blocked roads, damaged bridges, communications and power outages, food and water shortages, and other critical disruptions. Many of the local and international NGOs that would typically respond to such a disaster have (or have in the recent past had) a presence in Gaziantep supporting Syrian refugees; this could help them understand the terrain and the language, but it also means that many of their own operations could have been destroyed or damaged.

Critical Questions is produced by the Center for Strategic and International Studies (CSIS), a private, tax-exempt institution focusing on international public policy issues. Its research is nonpartisan and nonproprietary. CSIS does not take specific policy positions. Accordingly, all views, positions, and conclusions expressed in this publication should be understood to be solely those of the author(s).

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A World Vision cash transfer program made it possible for Juan Baptista to build an interim house and shop in Jaramij after the 2016 Ecuador earthquake. (2016 World Vision/photo by Sofia Ziessler-Korppi)

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