hello great then welcome to the second week of our discussion our discussion will focus on the learning competency describe and relate the distribution of active volcanoes earthquake epicenter and major mounting belts of the blade tectonic theory now let's start with lesson one earthquakes triangulation method at the end of the lesson you should be able to one compute mathematically the distance of an epicenter and two locate the epicenter of an earthquake using the triangulation method earthquakes are vibrations caused by rock breaking under stress this vibration travels and reach the surface of the earth this movement is sudden and releases a lot of energy which is transmitted through rocks as seismic waves the point where an earthquake originate from underground is called focus the location directly above the surface of the earth is called the epicenter there are three types of seismic waves the p wave the s wave and the surface wave p-wave or primary waves are compression waves also known as push-pull weights they travel the fastest and the first wave to be detected in a seismic station s-wave or secondary waves or shear waves move to a side to side motion they are the second wave to be detected in a seismic station surface wave roll along the surface of the earth they are the most destructive waves as they move the ground up and down but lose their energy the fastest and cannot travel as far seismic waves are detected and recorded using seismometer to create seismogram seismologists can determine how far away the earthquakes strike by plotting the difference in arrival between p and s waves on the graph seismologists determine the earthquake's epicenter by drawing circles on the mouth showing the ridges from the distance of the earthquake of three or more seismic stations the point where these circles intersect determines the epicenter of the earthquake this method is known as the triangulation method now how do we determine the epicenter of an earthquake using triangulation method now for example we have a reading from a seismometer like this one the reading in a seismometer looks something like this we have the p waves the s waves and the surface waves the p waves this one is the first wave to arrive in a seismometer followed by the secondary waves or the s waves and the surface waves the report printed out by seismometer is called the seismogram now this diagram can tell us the distance of the epicenter from the seismic station now first thing that we need to look at is the first time the seismogram jumps and that is the arrival of the p wave followed by the second jump and that is the arrival of the s wave now we can determine the distance of the epicenter of the earthquake by simply calculating the time difference in the arrival of the s and b wave and we call that lag time now let's use the activity in your module as an example we have activity 1 lag time using the earthquake chart below compute for the lag time by subtracting s wave and p p wave so as you can see we have four records of earthquake in four seismic stations we have chicago kansas city santa barbara and seattle below are the time of arrival 0 15 30 45 60 75 92 150 now let's use chicago as an example so the first time the seismogram jumps is we have 18 seconds and that is the arrival of the p wave p wave is 18 seconds and then if you are going to look at the record we have minor quakes and the second jump is 153 so the arrival of the s wave is 153 seconds [Music] now to to compute for the log time what we need to do is to simply subtract p wave with s wave so if we have 153 seconds minus 18 seconds we have a lag time of 135 seconds so example chicago p wave 18 seconds s weigh 153 seconds lag time is 135 seconds now do the same for other three seismic stations next is determining the distance of the epicenter to the station now to compute let's use the formula d is equal to td over 8 seconds times 100 kilometer and let's use the lug time which is 135 seconds now how do we solve for the distance of the epicenter to compute follow the formula this one d is equal to let's use 135 seconds the lag time over eight seconds times 100 kilometer just multiply cancel the units we have d is equal to thirteen thousand five hundred kilometer divided by eight seconds and the distance is one thousand six hundred eighty seven point five kilometer now after determining the distance let's now do the triangulation method first we have the record of chicago we will be needing a compass and below the map we have a what's the scaling now now after getting the scale simply draw a circle with chicago as the center do this for the other seismic stations and the point where this three intersects that is the location of the epicenter of the earthquake [Music] now let's have another example you can find it in your module where is the epicenter we have three seismic stations one two and three the first seismic station records the distance of the earthquake 700 kilometers the second seismic station records a distance of three thousand and the third seismic station records 3500 the epicenter of the earthquake is located at the intersection of the three circles now that is how we determine the epicenter of an earthquake and that's the end of lesson one now let's start with lesson two distribution of earthquake in the world one of the evidences that the cross is divided into segments of landmasses is the distribution of earthquake in the world the yellow dots represents the earthquakes as you can see earthquakes are not randomly distributed in the world they tend to occur in the boundaries of plates however not all earthquakes occur at late boundaries there are interplayed earthquakes that occur because of the stress of rocks being released this is what caused the 1968 meccaring earthquake that happens in western australia that has a magnitude of 6.8 which caused major damage in structures and infrastructures scientists are researching on ways to predict the occurrence of earthquake but until now there are no devices that can measure when or where an earthquake may occur and that's the end of lesson two thank you for watching thank you [Music]