Locating earthquakes

How do we know where the earthquakes happened? This map shows 445 earthquakes occurring in California within a period of one week. How do we know where the earthquakes happened? The students can brainstorm ideas – how do we know where earthquakes occurred? People feel earthquakes, seismometers record ground motion, sometimes damage occurs. What information is used to determine where an earthquake happened? Figure source: Screen-capture from http://earthquake.usgs.gov

Create a map of the stations. Earthquake Location Step 1: Find seismograms from a single earthquake that have been recorded by at least 3 stations. Step 2: Create a map of the stations. Map of stations A, B, and C = 1 cm on this page (if printed)

Step 3: For each seismogram at each station, determine what time the P wave appears on the seismogram. Also determine the time for the S wave.

Step 3: For each seismogram at each station, determine what time the P wave appears on the seismogram. Also determine the time for the S wave. P S P S Furthest seismometer from earthquake P-wave time S-wave time A 0.1 0.88 B 1.35 C 0.66 P S Closest seismometer to earthquake

Step 4: The difference in timing of the P-wave and S-wave on the seismogram tells us how far away the earthquake is from the station. Fill in the table provided to determine how far away each earthquake occurred, and then convert these distances (in km) to distances in cm on the map. Station P-wave time S-wave time P minus S times Convert to distance Distance (in km) Scaled distance (in cm) A 0.1 0.88 0.78 x 8 = 6.24 3.12 cm B 1.35 1.25 10.00 5.00 cm C 0.66 0.56 4.48 2.24 cm

Step 5: Draw a circle around each station with a radius matching the distance in the final column. Station A: 3.12 cm Station B: 5.00 cm Station C: 2.24 cm The 3 circles should come close to meeting up if the earthquake has been properly located. Mark the intersection of the 3 earthquakes with a star. Map of stations A, B, and C

See worksheet for activity; break here.

How deep do earthquakes occur?

Earthquakes occur in the lithosphere Students should already be familiar with diagrams of Earth’s internal structure. Where do they think earthquakes occur within Earth? Lithosphere = crust and upper mantle Along transform and divergent plate boundaries, the deepest earthquakes are generally around 25km deep. At subduction zones, earthquakes generally occur down to depths of ~70km. Occasionally earthquakes occur at very deep depths, down to ~700km! Seismologists think these are due to changes in the mineral structure of certain minerals in the mantle. Figure source: http://pubs.usgs.gov/gip/dynamic/graphics/FigS1-1.gif

Earthquakes occur on faults beneath the surface of Earth Figure source: http://www.uwsp.edu/geo/faculty/ritter/images/lithosphere/tectonics/normal_fault_labelled_diagram.jpg

What do the terms ‘focus’ and ‘epicenter’ mean? The focus (also called the hypocenter) of an earthquake is where the earthquake occurred beneath Earth’s surface. The epicenter is the location as projected onto Earth’s surface, which is necessary for plotting the earthquake on a map! If the fault is not vertical (like in the picture shown here), then earthquakes occurring deeper on the fault will have epicenters that do not line up with the fault line on the surface. Figure source: http://earthguide.ucsd.edu/mystery_detectives/teach/triangulation/images/epicenter_focus.gif

Focus: the location within Earth along a fault at which the first motion of an earthquake occurs

Epicenter: the point on Earth’s surface directly above an earthquake’s focus

We plot epicenters when we look at maps of earthquakes. How many stations do you need to figure out the depth at which the earthquake occurred? 4 stations are necessary to determine earthquake depth! Figure source: Screen-capture from http://earthquake.usgs.gov

This is called a seismic gap. Sometimes there are spots that we expect to have earthquakes but that don’t seem to have very many. This map shows over 20 years of earthquakes on the San Jacinto Fault. Orange dots: small earthquakes Red stars: larger earthquakes What’s happening here? This is called a seismic gap. Figure from the ANZA Seismic Network (UCSD)

Seismic gap: an area along a fault where relatively few earthquakes have occurred recently but where strong earthquakes are known to have occurred in the past