A P-wave is one of the two main types of elastic body waves, called seismic waves in seismology. P-waves travel faster than other seismic waves and hence are the first signal from an earthquake to arrive at any affected location or at a seismograph. P-waves may be transmitted through liquids, or solids; the name P-wave can stand for primary wave. Primary and secondary waves are body waves; the motion and behavior of both P-type and S-type in the Earth are monitored to probe the interior structure of the Earth. Discontinuities in velocity as a function of depth are indicative of changes in phase or composition. Differences in arrival times of waves originating in a seismic event like an earthquake as a result of waves taking different paths allow mapping of the Earth's inner structure. All the information available on the structure of the Earth's deep interior is derived from observations of the travel times, reflections and phase transitions of seismic body waves, or normal modes. P-waves travel through the fluid layers of the Earth's interior, yet they are refracted when they pass through the transition between the semisolid mantle and the liquid outer core.
As a result, there is a P-wave "shadow zone" between 103° and 142° from the earthquake's focus, where the initial P-waves are not registered on seismometers. In contrast, S-waves do not travel through liquids. Advance earthquake warning is possible by detecting the nondestructive primary waves that travel more through the Earth's crust than do the destructive secondary and Rayleigh waves; the amount of advance warning depends on the delay between the arrival of the P-wave and other destructive waves on the order of seconds up to about 60 to 90 seconds for deep, large quakes such as the 2011 Tohoku earthquake. The effectiveness of advance warning depends on accurate detection of the P-waves and rejection of ground vibrations caused by local activity. Earthquake early warning systems can be automated to allow for immediate safety actions, such as issuing alerts, stopping elevators at the nearest floors and switching off utilities. In isotropic and homogeneous solids, a P-wave travels in a straight line longitudinal.
The velocity of P-waves in such a medium is given by v p = K + 4 3 μ ρ = λ + 2 μ ρ where K is the bulk modulus, μ is the shear modulus, ρ is the density of the material through which the wave propagates, λ is the first Lamé parameter. In typical situations the interior of the Earth, the density ρ varies much less than K or μ, so the velocity is "controlled" by these two parameters; the elastic moduli P-wave modulus, M, is defined so that M = K + 4 μ / 3 and thereby v p = M / ρ Typical values for P-wave velocity in earthquakes are in the range 5 to 8 km/s. The precise speed varies according to the region of the Earth's interior, from less than 6 km/s in the Earth's crust to 13.5 km/s in the lower mantle, 11 km/s through the inner core. Geologist Francis Birch discovered a relationship between the velocity of P waves and the density of the material the waves are traveling in: V p = a + b ρ which became known as Birch's law. Earthquake early warning Lamb waves Love wave S-wave Surface wave "Photo Glossary of Earthquakes".
U. S. Geological Survey". Archived from the original on February 27, 2009. Retrieved March 8, 2009. Animation of a P-Wave P-Wave velocity calculator Purdue's catalog of animated illustrations of seismic waves Animations illustrating simple wave propagation concepts by Jeffrey S. Barker Detection of P-waves and Rejection of Environmental Noise for Accurate Earthquake Early Warning Bayesian Networks for Earthquake Magnitude Classification in a Early Warning System
Earthquake prediction is a branch of the science of seismology concerned with the specification of the time and magnitude of future earthquakes within stated limits, "the determination of parameters for the next strong earthquake to occur in a region. Earthquake prediction is sometimes distinguished from earthquake forecasting, which can be defined as the probabilistic assessment of general earthquake hazard, including the frequency and magnitude of damaging earthquakes in a given area over years or decades. Prediction can be further distinguished from earthquake warning systems, which upon detection of an earthquake, provide a real-time warning of seconds to neighboring regions that might be affected. In the 1970s, scientists were optimistic that a practical method for predicting earthquakes would soon be found, but by the 1990s continuing failure led many to question whether it was possible. Demonstrably successful predictions of large earthquakes have not occurred and the few claims of success are controversial.
For example, the most famous claim of a successful prediction is that alleged for the 1975 Haicheng earthquake. A study said that there was no valid short-term prediction. Extensive searches have reported many possible earthquake precursors, but, so far, such precursors have not been reliably identified across significant spatial and temporal scales. While part of the scientific community hold that, taking into account non-seismic precursors and given enough resources to study them extensively, prediction might be possible, most scientists are pessimistic and some maintain that earthquake prediction is inherently impossible. Predictions are deemed significant. Therefore, methods of statistical hypothesis testing are used to determine the probability that an earthquake such as is predicted would happen anyway; the predictions are evaluated by testing whether they correlate with actual earthquakes better than the null hypothesis. In many instances, the statistical nature of earthquake occurrence is not homogeneous.
Clustering occurs in both time. In southern California about 6% of M≥3.0 earthquakes are "followed by an earthquake of larger magnitude within 5 days and 10 km." In central Italy 9.5 % of M ≥ 3.0 earthquakes are followed by a larger event within 30 km. While such statistics are not satisfactory for purposes of prediction they will skew the results of any analysis that assumes that earthquakes occur randomly in time, for example, as realized from a Poisson process, it has been shown that a "naive" method based on clustering can predict about 5% of earthquakes. As the purpose of short-term prediction is to enable emergency measures to reduce death and destruction, failure to give warning of a major earthquake, that does occur, or at least an adequate evaluation of the hazard, can result in legal liability, or political purging. For example, it has been reported that members of the Chinese Academy of Sciences were purged for "having ignored scientific predictions of the disastrous Tangshan earthquake of summer 1976."
Wade 1977. Following the L'Aquila earthquake of 2009, seven scientists and technicians in Italy were convicted of manslaughter, but not so much for failing to predict the 2009 L'Aquila Earthquake as for giving undue assurance to the populace – one victim called it "anaesthetizing" – that there would not be a serious earthquake, therefore no need to take precautions, but warning of an earthquake that does not occur incurs a cost: not only the cost of the emergency measures themselves, but of civil and economic disruption. False alarms, including alarms that are canceled undermine the credibility, thereby the effectiveness, of future warnings. In 1999 it was reported that China was introducing "tough regulations intended to stamp out ‘false’ earthquake warnings, in order to prevent panic and mass evacuation of cities triggered by forecasts of major tremors." This was prompted by "more than 30 unofficial earthquake warnings... in the past three years, none of, accurate." The acceptable trade-off between missed quakes and false alarms depends on the societal valuation of these outcomes.
The rate of occurrence of both must be considered. In a 1997 study of the cost-benefit ratio of earthquake prediction research in Greece, Stathis Stiros suggested that a excellent prediction method would be of questionable social utility, because "organized evacuation of urban centers is unlikely to be accomplished", while "panic and other undesirable side-effects can be anticipated." He found that earthquakes kill less than ten people per year in Greece, that most of those fatalities occurred in large buildings with identifiable structural issues. Therefore, Stiros stated that it would be much more cost-effective to focus efforts on identifying and upgrading unsafe buildings. Since the death toll on Greek highways is more than 2300 per year on average, he argued that more lives would be saved if Greece's entire budget for earthquake prediction had been used for street and highway safety instead. Earthquake prediction is an immature science—it has not yet led to a successful prediction of an earthquake from first physical principles.
Research into methods of prediction therefore focus on empirical analysis, with two general approaches: either identifying distinctive precursors to earthquakes, or identifying some kind of geophysical trend or pattern in seismicity that might precede a large earthquake. Precursor methods are pursu
Shear wave splitting
Shear wave splitting called seismic birefringence, is the phenomenon that occurs when a polarized shear wave enters an anisotropic medium. The incident shear wave splits into two polarized shear waves. Shear wave splitting is used as a tool for testing the anisotropy of an area of interest; these measurements reflect the degree of anisotropy and lead to a better understanding of the area’s crack density and orientation or crystal alignment. We can think of the anisotropy of a particular area as a black box and the shear wave splitting measurements as a way of looking at what is in the box. An incident shear wave may enter an anisotropic medium from an isotropic media by encountering a change in the preferred orientation or character of the medium; when a polarized shear wave enters a new, anisotropic medium, it splits into two shear waves. One of these shear waves will be faster than the other and oriented parallel to the cracks or crystals in the medium; the second wave will be slower than the first and sometimes orthogonal to both the first shear wave and the cracks or crystals in the media.
The time delays observed between the slow and fast shear waves give information about the density of cracks in the medium. The orientation of the fast shear wave records the direction of the cracks in the medium; when plotted using polarization diagrams, the arrival of split shear waves can be identified by the abrupt changes in direction of the particle motion. In a homogeneous material, weakly anisotropic, the incident shear wave will split into two quasi-shear waves with orthogonal polarizations that reach the receiver at the same time. In the deeper crust and upper mantle, the high frequency shear waves split into two separate shear waves with different polarizations and a time delay between them that may be up to a few seconds. Hess made the first measurements of P-wave azimuthal velocity variations in oceanic basins; this area was chosen for this study because oceanic basins are made of large uniform homogeneous rocks. Hess observed, from previous seismic velocity experiments with olivine crystals, that if the crystals had a slight statistical orientation this would be evident in the seismic velocities recorded using seismic refraction.
This concept was tested using seismic refraction profiles from the Mendocino Fracture Zone. Hess found that the slow shear waves propagated perpendicular to the plane of slip and the higher velocity component was parallel to it, he inferred that the structure of oceanic basins could be recorded and understood better if these techniques were used. Ando focused on identifying shear-wave anisotropy in the upper mantle; this study focused on shear wave splitting recorded near the Chubu Volcanic Area in Japan. Using newly implemented telemetric seismographic stations, they were able to record both P-wave and S-wave arrivals from earthquakes up to 260 km beneath the volcanic area; the depths of these earthquakes make this area ideal for studying the structure of the upper mantle. They noted the arrivals of two distinct shear waves with different polarizations 0.7 seconds apart. It was concluded that the splitting was not caused by the earthquake source but by the travel path of the waves on the way to the seismometers.
Data from other nearby stations were used to constrain the source of the seismic anisotropy. He found the anisotropy to be consistent with the area directly below the volcanic area and was hypothesized to occur due to oriented crystals in a deep rooted magma chamber. If the magma chamber contained elliptical inclusions oriented N-S the maximum velocity direction would be N-S, accounting for the presence of seismic birefringence. Crampin proposed the theory of earthquake prediction using shear wave splitting measurements; this theory is based on the fact that microcracks between the grains or crystals in rocks will open wider than normal at high stress levels. After the stress subsides, the microcracks will return to their original positions; this phenomenon of cracks opening and closing in response to changing stress conditions is called dilatancy. Because shear wave splitting signatures are dependent on both the orientation of the microcracks and the abundance of cracks, the signature will change over time to reflect the stress changes in the area.
Once the signatures for an area are recognized, they may be applied to predict nearby earthquakes with the same signatures. Crampin first acknowledged the phenomenon of azimuthally-aligned shear wave splitting in the crust, he reviewed the current theory, updated equations to better understand shear-wave splitting, presented a few new concepts. Crampin established. If a corresponding solution for an isotropic case can be formulated the anisotropic case can be arrived at with more calculations; the correct identification of body and surface wave polarizations is the key to determining the degree of anisotropy. The modeling of many two-phase materials can be simplified by the use of anisotropic elastic-constants; these constants can be found by looking at recorded data. This has been observed in several areas worldwide; the difference in the travel velocities of the two shear waves can be explained by comparing their polarizations with the dominant direction of anisotropy in the area. The interactions between the tiny particles that make up solids and liquids can be used as an analogue for the way a wave travels through a medium.
Solids have tightly bound particles that transmit energy quickly and efficiently. In a li
A seismometer is an instrument that responds to ground motions, such as caused by earthquakes, volcanic eruptions, explosions. Seismometers are combined with a timing device and a recording device to form a seismograph; the output of such a device — recorded on paper or film, now recorded and processed digitally — is a seismogram. Such data is used to locate and characterize earthquakes, to study the earth's internal structure. A simple seismometer, sensitive to up-down motions of the Earth, is like a weight hanging from a spring, both suspended from a frame that moves along with any motion detected; the relative motion between the weight and the frame provides a measurement of the vertical ground motion. A rotating drum is attached to the frame and a pen is attached to the weight, thus recording any ground motion in a seismogram. Any movement of the ground moves the frame; the mass tends not to move because of its inertia, by measuring the movement between the frame and the mass, the motion of the ground can be determined.
Early seismometers used optical levers or mechanical linkages to amplify the small motions involved, recording on soot-covered paper or photographic paper. Modern instruments use electronics. In some systems, the mass is held nearly motionless relative to the frame by an electronic negative feedback loop; the motion of the mass relative to the frame is measured, the feedback loop applies a magnetic or electrostatic force to keep the mass nearly motionless. The voltage needed to produce this force is the output of the seismometer, recorded digitally. In other systems the weight is allowed to move, its motion produces an electrical charge in a coil attached to the mass which voltage moves through the magnetic field of a magnet attached to the frame; this design is used in a geophone, used in exploration for oil and gas. Seismic observatories have instruments measuring three axes: north-south, east-west, vertical. If only one axis is measured, it is the vertical because it is less noisy and gives better records of some seismic waves.
The foundation of a seismic station is critical. A professional station is sometimes mounted on bedrock; the best mountings may be in deep boreholes, which avoid thermal effects, ground noise and tilting from weather and tides. Other instruments are mounted in insulated enclosures on small buried piers of unreinforced concrete. Reinforcing rods and aggregates would distort the pier as the temperature changes. A site is always surveyed for ground noise with a temporary installation before pouring the pier and laying conduit. European seismographs were placed in a particular area after a destructive earthquake. Today, they are concentrated in high-risk regions; the word derives from the Greek σεισμός, seismós, a shaking or quake, from the verb σείω, seíō, to shake. Seismograph is another Greek term from γράφω, gráphō, to draw, it is used to mean seismometer, though it is more applicable to the older instruments in which the measuring and recording of ground motion were combined, than to modern systems, in which these functions are separated.
Both types provide a continuous record of ground motion. The technical discipline concerning such devices is called seismometry, a branch of seismology; the concept of measuring the "shaking" of something means that the word "seismograph" might be used in a more general sense. For example, a monitoring station that tracks changes in electromagnetic noise affecting amateur radio waves presents an rf seismograph, and Helioseismology studies the "quakes" on the Sun. The first seismometer was made in China during the 2nd Century; the first Western description of the device comes from the French physicist and priest Jean de Hautefeuille in 1703. The modern seismometer was developed in the 19th century. In December 2018, a seismometer was deployed on the planet Mars by the InSight lander, the first time a seismometer was placed onto the surface of another planet. In AD 132, Zhang Heng of China's Han dynasty invented the first seismoscope, called Houfeng Didong Yi; the description we have, from the History of the Later Han Dynasty, says that it was a large bronze vessel, about 2 meters in diameter.
When there was an earthquake, one of the dragons' mouths would open and drop its ball into a bronze toad at the base, making a sound and showing the direction of the earthquake. On at least one occasion at the time of a large earthquake in Gansu in AD 143, the seismoscope indicated an earthquake though one was not felt; the available text says that inside the vessel was a central column that could move along eight tracks. The first earthquake recorded by this seismoscope was "somewhere in the east". Days a rider from the east reported this earthquake. By the 13th century, seismographic devices existed in the Maragheh observatory in Persia. French physicist and priest Jean de Hautefeuille built one in 1703. After 1880, most seismometers were descend
August 2016 Central Italy earthquake
An earthquake, measuring 6.2 ± 0.016 on the moment magnitude scale, hit Central Italy on 24 August 2016 at 03:36:32 CEST. Its epicentre was close to Accumoli, with its hypocentre at a depth of 4 ± 1 km 75 km southeast of Perugia and 45 km north of L'Aquila, in an area near the borders of the Umbria, Lazio and Marche regions; as of 15 November 2016, 299 people had been killed. The central Apennines is one of the most seismically active areas in Italy; the Apennines mountain belt were formed in the Miocene to Pliocene as a result of the ongoing subduction of the Adriatic Plate beneath the Eurasian Plate, forming a fold and thrust belt. During the Quaternary, thrust tectonics gave way to extensional tectonics, with the development of a zone of normal faulting running along the crest of the mountain range; the extension is the opening of the Tyrrhenian Sea. In the Central Apennines the zone of extension is about 30 km wide matching the zone of observed extensional strain as shown by GPS measurements.
Recent large earthquakes in this area have been caused by movement on SW-dipping normal faults. This was the largest tremor since 2009, when an earthquake near L'Aquila in the Abruzzo region killed over 300 people and displaced about 65,000; the earthquake was reported by INGV to have occurred at a depth of 5 km, with a magnitude of 6.0 Mw and epicentre in the comune of Accumoli. The USGS first reported an earthquake at a depth of 10.0 km with a magnitude of 6.4 Mw and epicentre southeast of Norcia, but subsequently revised the magnitude to 6.2 Mw. The European-Mediterranean Seismological Centre put the magnitude at 6.1. The discrepancies between the different estimates of the magnitude led INGV to explain in a blog post that they use a crustal velocity model calibrated for Italy and give more weight to the seismometric stations situated close to the epicentre. Using global models, INGV further stated that it can reproduce the values reported by foreign agencies; as of 30 August 2016, the initial earthquake was followed by at least 2,500 aftershocks.
The tremor and a number of aftershocks were felt across the whole of central Italy, including Rome and Bologna. As of 26 August 2016, the official figures of the Protezione Civile report that the earthquake caused the death of 297 people: 234 in Amatrice, 11 in Accumoli and 49 in Arquata del Tronto. At least 365 injured had to be treated in hospitals in Rieti and Ascoli Piceno, while people with less serious injuries were treated on the spot. In addition to those rescued with the help of other inhabitants or escaped by themselves, 238 people were pulled alive from the rubble by the timely intervention of the authorities, 215 by the Vigili del Fuoco and 23 by the Soccorso Alpino. 2,100 people found shelter in the emergency camps. 4,400 people were involved in the search and rescue operations, including 70 teams with rescue dogs. Logistics made use of 12 helicopters, with 9 more in stand-by; the earthquake killed 276 Italians, 11 Romanians, several others. The complete list is seen to the right.
Early reports indicated severe damage in the town of Amatrice, near the epicentre, in Accumoli and Pescara del Tronto. Sergio Pirozzi, the mayor of Amatrice, stated that "Amatrice is not here anymore, half of the town is destroyed." Photos of the destruction depicted a massive pile of rubble in the town's centre with only a few structures still standing on the outskirts. It cost an estimated economic loss between $1 billion to $11 billion. In addition to the loss of human life, widespread destruction of cultural heritage is reported. In Amatrice, the facade and rose window of the Church of Sant'Agostino were destroyed, the museum dedicated to the painter Nicola Filotesio and companion of Raphael, collapsed; the earthquake created cracks in the Baths of Caracalla in Rome. The earthquake was so broad that authorities made structural tests on the Colosseum as well, not damaged; the Basilica of Saint Francis of Assisi – a UNESCO World Heritage site with frescoes by Giotto and Cimabue that were destroyed by an earthquake in 1997 – was declared safe after an extensive survey by the head restorer.
3D computer models were used to help damage assessment of the Basilica of Saint Francis of Assisi and the Church of Sant'Agostino. The data for building the models was collected by robots deployed by the European project TRADR. Two ground robots and one drone were used inside the San Francesco Basilica, one drone was used inside the Sant'Agostino church, two drones were used on the outside of both churches. After the earthquake in Central Italy, the court of Rieti discovered that not all the buildings of those cities were constructed or renovated under the antiseismic law of 1974 in which it explained all the construction techniques of an earthquake resistant building. In fact, a family was killed that night by the rubble of a church, not renovated under that law; the Romolo Capranica elementary school in Amatrice collapsed if in 2013 the town spent 160,000 euros in a seismic retrofit operation that improved the building's seismic resistance, but wasn't enough to comply with 2012 earthquake standards in Italy.
The investigation is ongoing to discover the causes that allowed buildings to become reduced to rubble instead of sustaining damage attributed to buildings following anti-seismic regulations Amatrice. French satirical magazine Charlie Hebdo published a cartoon depicting Italian earthquake victims as pasta dishes, causing "shock and outrage." In response to the reaction of Italians unleashed on social networks, the cartoonist Coco pointed out wit
East Pacific Rise
The East Pacific Rise is a mid-oceanic ridge, a divergent tectonic plate boundary located along the floor of the Pacific Ocean. It separates the Pacific Plate to the west from the North American Plate, the Rivera Plate, the Cocos Plate, the Nazca Plate, the Antarctic Plate, it runs south from the Gulf of California in the Salton Sea basin in southern California to a point near 55° S, 130° W where it joins the Pacific-Antarctic Ridge trending west-southwest towards Antarctica near New Zealand. Much of the rise lies about 3200 km off the South American coast and rises about 1,800–2,700 m above the surrounding seafloor; the oceanic crust is moving away from the East Pacific Rise to either side. Near Easter Island the rate is over 15 cm per year, the fastest in the world. However, on the northern end, it is much slower at only 6 cm per year. On the eastern side of the rise the eastward moving Cocos and Nazca plates meet the westward moving South American Plate and the North American Plate and are being subducted under them.
The belt of volcanoes along the Andes and the arc of volcanoes through Central America and Mexico are the direct results of this collision. Due east of the Baja California Peninsula, the Rise is sometimes referred to as the Gulf of California Rift Zone. In this area, newly formed oceanic crust is intermingled with rifted continental crust originating from the North American Plate. Near Easter Island, the East Pacific Rise meets the Chile Rise at the Easter Island and Juan Fernandez microplates, trending off to the east where it subducts under the South American Plate at the Peru–Chile Trench along the coast of southern Chile; the southern extension of the East Pacific Rise merges with the Southeast Indian Ridge at the Macquarie Triple Junction south of New Zealand. Parts of the East Pacific Rise have oblique spreading, seafloor spreading, not orthogonal to the nearest ridge segment. Along the East Pacific Rise the hydrothermal vents called black smokers were first discovered and have been extensively studied.
These vents are forming volcanogenic massive sulfide ore deposits on the ocean floor. Many strange deep-water creatures have been found here; the southern stretch of the East Pacific Rise is one of the fastest-spreading sections of the Earth's mid-ocean ridge system. Overlapping Spreading Centers East Pacific Rise 2004 – Scripps Institution of Oceanography Columbia University Researchers Find Key to the Formation of New Seafloor Spreading Centers – Columbia University