Question

Distinguish between focus and epicentre.




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Answer 1

Answer:

epicentre- the point on the earth's surface located directly above the focus of the earthquake

focus-the location where the earthquake begins.The ground reptures at this spot,then seismic waves radiate outside in all directions

Answer 2

EPICENTER

The epicenter, epicentre /ˈɛpɪsɛntər/ or epicentrum in seismology is the point on the Earth's surface directly above a hypocenter or focus, the point where an earthquake or an underground explosion originates.

In most earthquakes, the epicenter is the point where the greatest damage takes place, but the length of the subsurface fault rupture may indeed be a long one, and damage can be spread on the surface across the entire rupture zone. As an example, in the magnitude 7.9 2002 Denali earthquake in Alaska, the epicenter was at the western end of the rupture, but the greatest damage was about 330 km (210 mi) away at the eastern end. Focal depths of earthquakes occurring in continental crust mostly range from two to twenty kilometres (1.2 to 12.4 mi). Continental earthquakes below 20 km (12 mi) are rare whereas in subduction zones earthquakes can originate at depths deeper than 600 km (370 mi).

Epicentral distance

During an earthquake, seismic waves propagate in all directions from the hypocenter. Seismic shadowing occurs on the opposite side of the Earth from the earthquake epicenter because the planet's liquid outer core refracts the longitudinal or compressional (P-waves) while it absorbs the transverse or shear waves (S-waves). Outside the seismic shadow zone, both types of wave can be detected but, because of their different velocities and paths through the Earth, they arrive at different times. By measuring the time difference on any seismograph and the distance on a travel-time graph on which the P-wave and S-wave have the same separation, geologists can calculate the distance to the quake's epicenter. This distance is called the epicentral distance, commonly measured in ° (degrees) and denoted as Δ (delta) in seismology.

Once distances from the epicenter have been calculated from at least three seismographic measuring stations, the point can be located, using trilateration.

Epicentral distance is also used in calculating seismic magnitudes as developed by Richter .

FOCUS

Focus of an earthquake

The focus is also called the hypocenter of an earthquake. The vibrating waves travel away from the focus of the earthquake in all directions. The waves can be so powerful they will reach all parts of the Earth and cause it to vibrate like a turning fork.