Pinpointing the hypocenter and magnitude Earthquake casualty estimation

1 pinpointing hypocenter , magnitude

1.1 race know new earthquake
1.2 epicenter
1.3 depth
1.4 magnitude

pinpointing hypocenter , magnitude

the location of earthquake (its epicenter , depth) needs known rapidly estimating losses. calculated times @ waves generates arrive @ seismographs surrounding source. computer moves epicenter estimate close stations record waves first , far stations reported waves later. can done within seconds accuracies of 1 kilometer in regions dense seismograph networks exist inter-station distances of 10 km. of world, luxury not available , worldwide seismograph network has used estimate location based on teleseismic data (recorded @ distances of more 1,000 km). means estimates of location cannot calculated before waves have traveled hundreds , thousands of kilometers stations record them.

the race know new earthquake

the following agencies distribute estimates of latitude, longitude, depth, , magnitude of worldwide earthquakes rapidly , high accuracy. geoforschungszentrum, potsdam, germany, delivers automatic solutions within 7 minutes (median) major earthquakes worldwide. national earthquake information center of united states geological survey (usgs) delivers solutions reviewed seismologist within 18 minutes (median) major earthquakes worldwide. european-mediterranean seismological centre delivers reviewed parameters in european area within 21 minutes (median). pacific tsunami warning center , national tsunami warning center of national oceanic , atmospheric administration (noaa) delivers reviewed parameters earthquakes in wider pacific area within 9 minutes (median). these updated numbers, shorter discussed in detail earlier.

epicenter

if epicenter incorrect loss estimate uncertain. errors introduced in estimate of position because of heterogeneity of earth. seismic waves travel different speeds in different rocks. uncertainties in real time epicenters estimated teleseismic means ±25 km (median).

depth

the depth important, uncertain in top 50 km. depths of earthquakes range 0 700 km. generally, earthquakes in top 100 km close enough settlements cause casualties. decrease of wave amplitudes function of distance (figure 2) shows dangerous intensities, i≥vii, not exist beyond 30 50 km major earthquakes. thus, deep earthquakes not of interest alerts.

the depth of energy release can estimated accurately (to within 1 km) if seismograph station right above earthquake (or near it) records waves. not case , 1 has rely on teleseismic methods estimate depth.

the teleseismic method measure time delay wave reflected earth’s surface above earthquake arrives @ seismograph. surface of earth acts mirror. wave runs against cannot travel air, reflected down earth, traveling same seismograph recorded direct wave little bit earlier. time delay of reflected wave depends of course directly on distance has traveled: hypocenter surface , down depth of hypocenter.

this method works fine, if hypocentral depth z>50 km because, in case, direct , reflected phases (waves) separated on record. shallower depths, delay small 2 pulses on seismogram not readily recognizable separate pulses; takes filtering techniques separate , identify them.

it follows depth of shallow earthquakes, dangerous, must assumed 25 ±25 km, if there no other evidence available. uncertainty approximately same of epicenter. there exists possibility reduce error based on historic data, in cases. regions tectonic style , faults producing earthquakes known, 1 may chose depth assuming same in past earthquakes depth had been determined accurately.

magnitude

for earthquake magnitudes smaller m7.5, different agencies mentioned above issuing location estimates, distribute values of m within 0.2 units each other. these medium-sized earthquake, average of estimates reliable determination of earthquake size. however, great earthquakes approaching m8 , exceeding it, initial estimate of m small. because surface wave m, obtained, defined proportional 20 sec reighly surface wave, , wave has wavelength of 100 km. therefore short reliably measure m of earthquake rupture exceeding 100 km. in these cases, in depth analysis, takes time, needed arrive @ correct m.

as example, wenchuan earthquake of 12 may 2008 had been assigned m7.5 in real-time. later estimates m7.9 m8.0. based on first estimate, fatalities had been expected reach maximum of 4,000, based on second maximum had been calculated 100,000. observed number of fatalities in case 87,000, determined after months (see figure in introduction of page).