Earthquake casualty estimation refers to the practice of estimating the number of fatalities and injuries caused by seismic events, often before comprehensive data from the field are available. Systems such as the United States Geological Survey's PAGER rapidly assess potential impacts by combining seismic data with population exposure and loss models.
Casualties are defined as fatalities and injuries. Recent advances are improving the speed and accuracy of loss estimates immediately after earthquakes. After major and great earthquakes, rescue agencies and civil defense managers need rapid estimates of the extent of the potential disaster, before information may be available. This process is more challenging in developing countries.
Purpose and use Earthquake casualty estimation is used by emergency management agencies, governments, and humanitarian organizations to obtain early assessments of potential human impacts following major seismic events. These estimates are typically produced before comprehensive field reports are available and are intended to support situational awareness, prioritization of response efforts, and allocation of resources. Rapid estimation models can provide actionable information before complete field assessments are available, although they are inherently uncertain.
Examples of the initial underestimation of the extent of earthquake disasters in developing as well as industrialized countries are shown in Figure 1. The responsible experts believed for 4 days that the death toll in the Wenchuan earthquake, Mw8 of May 12, 2008, was less than 10,000.
For this reason, the QLARM and the PAGER teams maintain around the clock capabilities to calculate earthquake damage and casualties within less than 1 hour of any earthquake worldwide. No other groups are capable of these detailed analyses.Wyss, M. (2004), Earthquake loss estimates in real-time begin to assist rescue teams, worldwide, EOS, 85(52), 567.Wyss, M. (2014), Ten years of real-time earthquake loss alerts, in Earthquake Hazard, Risk, and Disasters, edited by M. Wyss, pp. 143-165, Elsevier, Waltham, Massachusetts. This page can help medical and other responders understand how fast and how accurate loss estimates can be calculated after earthquakes and what should be added to be more helpful.
The estimates of fatalities distributed by email by the QLARM team of the International Centre for Earth Simulation Foundation (ICES) within 100 minutes of the Wenchuan earthquake was 55,000 ± 30,000, which includes the final toll of about 87,000.
For the 2009 L'Aquila earthquake, an M6.3 earthquake, QLARMs estimate of fatalities was 275 ±200, 22 minutes after the event.3. List of alerts can be found on www.wapmerr.org. The final death toll was 287. In both cases, the official fatality count was slow to reflect the true extent of the disasters. Thus, theoretical estimates of fatalities in real time can be useful for reacting with an appropriate disaster relief response, even though these estimates have large error margins. Current QLARM alerts can be found on the International Institute for Earth Simulation Foundation website, the alarms by the United States Geological Survey PAGER team are found on their website.
Pinpointing the hypocenter and magnitude The location of an earthquake (its epicenter and depth) needs to be known rapidly for estimating losses. It is calculated from the times at which the waves it generates arrive at seismographs surrounding the source. A computer moves the epicenter estimate close to those stations which record the waves first and far from stations that reported the waves later. This can be done within seconds to accuracies of 1 kilometer in regions where dense seismograph networks exist with inter-station distances of about 10 km.Allen, R. M., and H. Kanamori (2003), The potential for earthquake early warning in Southern California, Science, 300, 786-789 For most of the world, this luxury is not available and the worldwide seismograph network has to be used to estimate the location based on teleseismic data (recorded at distances of more than 1,000 km). This means that estimates of the location cannot be calculated before the waves have traveled hundreds and thousands of kilometers to stations that record them.
The race to know about a new earthquake The following agencies distribute estimates of latitude, longitude, depth, and magnitude of worldwide earthquakes rapidly and with high accuracy. The Geoforschungszentrum, Potsdam, Germany, delivers automatic solutions within 7 minutes (median) for all major earthquakes worldwide. The National Earthquake Information Center of the United States Geological Survey (USGS) delivers solutions that are reviewed by a seismologist within 18 minutes (median) for all major earthquakes worldwide. The European-Mediterranean Seismological Centre delivers reviewed parameters mostly in the European area within 21 minutes (median). The Pacific Tsunami Warning Center and the National Tsunami Warning Center of the National Oceanic and Atmospheric Administration (NOAA) delivers reviewed parameters for earthquakes in the wider Pacific area within 9 minutes (median). These are updated numbers, slightly shorter than discussed in detail earlier.
Epicenter If the epicenter is incorrect the loss estimate will be uncertain. Errors are introduced in the estimate of position mostly because of the heterogeneity of the Earth. Seismic waves travel with different speeds in different rocks. Uncertainties in real time epicenters estimated by teleseismic means are ±25 km (median).Wyss, M., Elashvili, M., Jorjiashvili, N. & Javakhishvili, Z. (2011). Uncertainties in teleseismic epicenter estimates: implications for real-time loss estimate, Bulletin of the Seismological Society of America, in press.
Depth The depth is important, but uncertain in the top 50 km. The depths of earthquakes range from 0 to about 700 km. Generally, only the earthquakes in the top 100 km are close enough to settlements to cause casualties. The decrease of the wave amplitudes as a function of distance (Figure 2) shows that dangerous intensities, I≥VII, do not exist beyond 30 to 50 km for major earthquakes. Thus, deep earthquakes are usually not of interest for alerts.
The depth of the energy release can be estimated accurately (to within 1 km) if a seismograph station right above the earthquake (or near it) records the waves. This is usually not the case and one has to rely on teleseismic methods to estimate the depth.
The teleseismic method is to measure the time delay with which the wave reflected from the Earth's surface above the earthquake arrives at a seismograph. The surface of the Earth acts like a mirror. A wave that runs up against it cannot travel into the air, so it is reflected back down into the Earth, traveling to the same seismograph that recorded the direct wave a little bit earlier. The time delay of the reflected wave depends of course directly on the extra distance it has traveled: from the hypocenter up to the surface and back down to the depth of the hypocenter.
This method works fine, if the hypocentral depth Z>50 km because, in that case, the direct and reflected phases (waves) are clearly separated on the record. For shallower depths, the delay is so small that the two pulses on the seismogram are not readily recognizable as separate pulses; it takes filtering techniques to separate and identify them.
It follows that the depth of the shallow earthquakes, those most dangerous, must be assumed to be 25 ±25 km, if there is no other evidence available. This uncertainty is approximately the same as that of the epicenter. There exists a possibility to reduce this error based on historic data, in some cases. For regions where the tectonic style and the faults producing the earthquakes are well known, one may choose a depth assuming it is the same as in past earthquakes for which the depth had been determined accurately.
Magnitude
For earthquake with magnitudes smaller than M7.5, the different agencies mentioned above as issuing location estimates, usually distribute values of M within 0.2 units from each other. For these medium-sized earthquake, the average of the estimates is a reliable determination of the earthquake size. However, for great earthquakes approaching M8 and exceeding it, the initial estimate of M is often significantly too small. This is so because the surface wave M, which is quickly obtained, is defined as proportional to the 20 sec Reighly surface wave, and this wave has a wavelength of about 100 km. It is therefore too short to reliably measure the M of an earthquake rupture exceeding 100 km. In these cases, an in depth analysis, which takes time, is needed to arrive at the correct M.
As an example, the Wenchuan earthquake of 12 May 2008 had originally been assigned M7.5 in real-time. Later estimates were M7.9 to M8.0. Based on the first estimate, fatalities had been expected to reach a maximum of 4,000, based on the second the maximum had been calculated as 100,000. The observed number of fatalities in this case was 87,000, determined after months (see Figure in the introduction of this page).
Estimates of shaking
The standard teleseismic measure of the 'size' of an earthquake is the surface-wave magnitude, , which has to be derived by definition from the surface waves with 20 second period. A more reliable and more modern scale is that of the moment magnitude, .
Variations of the amplitudes recorded at different seismograph stations are due to many reasons, but the mean magnitude derived from reports by many stations that have recorded the earthquake should be fairly stable. Nevertheless, the agencies which report source parameters (GFZ, NEIC, TWC. EMSC) differ in their magnitude estimates by 0.2 units, on average. This value is taken as the uncertainty of the magnitude estimate in real time.
There exists a special problem for great earthquakes; those with M>8. The waves with 20 seconds period, which define , have wavelengths of only about 100 km. This means, they are too short a yardstick to measure the size of ruptures that significantly exceed 100 km in length. For this reason was introduced, being based on wavelengths of about 1000 km. Unfortunately, these long wavelengths do not become available as fast as shorter ones, resulting in initial underestimates of the magnitude of great earthquakes. As an example, for the Tohoku, M9 earthquake of 11 March 2011, the initial estimates were: GFZ M8.5, NEIC M7.9, TWC M7.9, and EMSC M8.0.
Intensity
Strong ground motions damage buildings, sometimes bringing about collapse. Shaking of the ground decreases with distance from the release of energy, the hypocenter, or, more accurately expressed, from the entire area of rupture. To calculate the intensity of shaking at a given settlement, the computer looks up the attenuation (decrease in amplitude) for seismic waves that travel the distance to the settlement in question. Such calculations are similar to those made to assess the seismic hazard, part of the field of engineering seismology.
Errors are again introduced through the heterogeneity of the Earth. The loss of energy along the wave path is not exactly the same in all parts of the world.Seismic Waves and Earth's Interior, An Introduction to Earthquakes, Saint Louis University.Project Activity and Findings , Pacific Earthquake Engineering Research Center (PEER), in partnership with the U.S. Geological Survey and the Southern California Earthquake Center. Examples are shown in Figure 2. For poorly studied regions in developing countries, the uncertainty of the estimated intensities can be substantial, as shown by the different curves, because attenuation is poorly known.