Seismic Waves Earthquake Earthquake: Release of the built up stress in the once the is passed Release of this energy is called Most earthquakes occur along a Fault : crack in the Earth s crust where can occur. Energy is released at a in the form of seismic waves elastic fault. limit elastic fault lithosphere rebound movement Earthquake Seismic waves focus originate at the of the earthquake. Epicenter : the location on Earth s surface directly focus above the. all Seismic Waves Move in directions from the. Carry the energy of an earthquake away from the focus, interior focus surface through the Earth s, and across its. Seismic Waves Body Waves strongest Felt the at the epicenter. Two categories of seismic waves: Body waves 1. Surface waves 2. Travel through the ground in directions. Primary Waves (P Waves) First waves to arrive compression all Produce waves Travel from one end to the other and back in a straight line 1
expanding P Waves contracting Series of & waves Push Pull Also known as waves solids liquids gases Travel through,, and Secondary Body Waves Waves (S Waves) Second waves to arrive Produce waves up shear down Move and solids Travel through only Body Waves Surface Waves Primary Waves Fast, Arrive 1 st Push - Pull Wave (Compression) Solids, Liquids & Gases Secondary Waves Slow, Arrive 2 nd Shear Wave Solids ONLY Travel along only Slower surface than body waves earthquake (focus) While P and S waves are generated by the, P-Waves surface waves are made by. Surface Waves Surface Waves Produce the ground movements. Two wave actions at the same time: Rayleigh most severe roll waves: causes ground to like ocean waves Rayleigh Love waves: shake ground from side to side Love 2
Earthquake Strength ground Seismograph: Records movements seismic waves caused by. Earthquake Strength Richter Scale: Measure of earthquake (released energy from earthquake) Logarithm of the maximum wave amplitude upper No limit decimal Reported using numbers MAGNITUDE 2.0 3.0 : Felt by people (minor) 6.0 : Strong 4.0 : Light 7.0 : Major (18/yr) 5.0 : Moderate (800/yr) 8.0 9.0: Great 10.0+ : Epic (Never) Earthquake Strength Moment Magnitude Scale: version of the Richter scale. In addition to the amplitude of the seismic waves, it also considers: surface 1. Amount of along the fault rocks 2. Strength of that were broken decimal Modern movement (Also uses numbers) Earthquake Strength Mercalli Scale: Measure of Not a precise measurement using instruments, but based on Scale of 1 to 12 using numerals: I human XII to INTENSITY observations roman Earthquake Strength Seismograph records the arrival of P and S waves on a 24 hour clock using Greenwich Mean Time (GMT) The prime meridian runs through Greenwich, England Greenwich, England is 5 hours ahead of New York State. May also be called Universal Coordinated Time (UTC) 3
Seismograph records the arrival of P and S waves on a 24 hour clock using Greenwich Mean Time (GMT) The prime meridian runs through Greenwich, England Seismograph records the arrival of P and S waves on a 24 hour clock using Greenwich Mean Time (GMT) Times reported as hh:mm:ss (uses a 'military time' format) Greenwich, England is 5 hours ahead of New York State. May also be called Universal Coordinated Time (UTC) Ex: 09:14:32 14:52:17 9:14:32 in the morning 2:52:17 in the afternoon New York is GMT -5 (UTC -5): If it is 4:00:00 pm in New York, what is GMT? 4:00:00 pm + 5 hours = 9:00:00 pm If it is 4:00:00 pm in New York, what is the time in Chicago (central time zone)? 3:00:00 pm or, 21:00:00 GMT Dec 10 12:29 PM Dec 10 12:29 PM Chicago is GMT -6 (UTC -6): If it is 3:00:00 pm in Chicago, what is GMT? If a seismograph location records the arrival of a P-wave at 10:21:30 am (hours : minutes : seconds)...and the earthquake occurred 8,000 km away, what time did the earthquake originate? 3:00:00 pm + 6 hours = 9:00:00 pm or, 21:00:00 GMT Dec 10 12:29 PM Jan 5 2:26 PM 4
Dec 14 7:25 AM Dec 14 7:25 AM Must determine the difference between the P and S wave arrival times, then use ESRT (Page 11) to determine the distance. Determine the Difference to get the Distance between P and S-Wave arrival times to the Epicenter Dec 14 7:25 AM In class example: Say what? P (arrival) = GMT S (arrival) = GMT 5
Step 1: Determine the Difference between the P and S-Wave arrival times. Step 1: Determine the Difference between the P and S-Wave arrival times. Subract the P-Wave arrival (the lower number) from the S-Wave arrival 00: Subract the P-Wave arrival (the lower number) from the S-Wave arrival 00: 02: Step 1: Determine the Difference between the P and S-Wave arrival times. Subract the P-Wave arrival (the lower number) from the S-Wave arrival 00:02: -11 The negative value means we need to borrow 60 seconds (one minute) from the minutes. Subract the P-Wave arrival (the lower number) from the S-Wave arrival 00: 02: -11 Reduce the minutes by 1 minute and then subtract the negative value from 60 seconds: 00: 01: (60-11) Subract the P-Wave arrival (the lower number) from the S-Wave arrival 00: 02: -11 Reduce the minutes by 1 minute and then subtract the negative value from 60 seconds: 00: (60-11) 01: = 00:01:49 Use some paper to supplement your notes: Find the difference between the following P and S-wave arrival times: 1. P arrival : 07:30:24 GMT S arrival : 07:35:42 GMT 2. P A : 19:24:46 UTC S A : 19:27:02 UTC Dec 16 7:46 AM 6
Dec 16 7:54 AM Dec 16 7:54 AM Dec 16 7:54 AM Dec 16 7:55 AM Dec 16 7:55 AM Dec 16 7:55 AM 7
Dec 16 7:55 AM Dec 16 7:55 AM one minute 8
one minute; 20 seconds one minute; 40 seconds one minute; 49 seconds one minute; 49 seconds 'tic' mark Step 2: Determine the Distance to the Epicenter. Use the ESRT (page 11) and the "Slip and Slide" method Use the ESRT (page 11) and the "Slip and Slide" method: Slide the scrap paper with the 'tic' mark on it up and between the "P" and "S" curves (keeping it verticle) until the bottom of the paper is touching the "P" curve at the same time the "S" curve is touching the 'tic' mark. 9
'tic' mark Slip and Slide to get the distance The greater the difference between the P and S-Wave arrival, the greater the distance to the Epicenter Epicenter Distance (x10 3 km) What is the time difference between these arrival times? What is the distance to the epicenter? Dec 10 10:35 AM Dec 10 10:35 AM Let's Practice... Jan 5 8:51 AM 10