India subducts beneath Burma microplate at about 50 mm/yr Earthquakes occur at plate interface along the Sumatra arc (Sunda trench) These are spectacular & destructive results of many years of accumulated motion
STAGE 1: INTERSEISMIC: India subducts beneath Burma microplate at about 50 mm/yr Fault interface is locked STAGE 2: EARTHQUAKE (COSEISMIC): Fault interface slips, overriding plate rebounds, releasing accumulated motion HOW OFTEN? Fault slipped ~ 10 m = 10000 mm 10000 mm / 50 mm/yr = 200 yr Longer if some slip is aseismic Faults aren’t exactly periodic for reasons we don’t understand.
Examples of seismograms, showing the ground motions recorded at different locations around the world as a function of time. Note how long in duration these P waves are.
MODELING THESE SEISMOGRAMS shows how slip varied along the fault plane. Most slip along an area ~400 km long Maximum slip ~ 20 m
TWO VIEWS OF THE PART OF THE SUMATRA SUBDUCTION ZONE THAT SLIPPED Seismogram analysis shows most slip in southern 400 km Aftershocks show slip extended almost 1200 km
For comparison, a magnitude 5 earthquake would rupture a patch roughly the size of New York City's Central Park.
EARTHQUAKE MAGNITUDE 9 Energy released ≈ 20 × 10 17 Joules, or 475,000 kilotons (475 megatons) of TNT, or the equivalent of 23,000 Hiroshima bombs.
LocationDate Magnitude 1.Chile1960 05 22 9.5Chile 2.Prince William Sound, Alaska1964 03 28 9.2Prince William Sound, Alaska 3.Andreanof Islands, Alaska1957 03 09 9.1Andreanof Islands, Alaska 4/5.Kamchatka1952 11 04 9.0Kamchatka 4/5.West Coast of Northern Sumatra2004 12 26 9.0West Coast of Northern Sumatra 6.Off the Coast of Ecuador1906 01 31 8.8Off the Coast of Ecuador 7.Rat Islands, Alaska1965 02 04 8.75Rat Islands, Alaska 8.Assam – Tibet1950 08 15 8.6Assam – Tibet 9/10.Kamchatka1923 02 03 8.5 9/10.Banda Sea, Indonesia1938 02 01 8.5Banda Sea, Indonesia Top Ten List of Earthquakes in the Past 100 Years.
This one earthquake comprises about ¼ of the total energy released by earthquakes in the past 30 years.
Atmospheric Angular Momentum (AAM, in red) and VLBI length of day (LOD, in blue) measurements are strongly correlated over periods of one week to several years (storms, atmospheric currents, El Nino, etc.). Ocean tides and currents modulate the Earth's orientation on periods of a day and shorter. The model (red) follows the VLBI measurements (blue) closely. Predicted change in length of day due to the Sumatra earthquake: -2.676 microseconds (not verifiable?) Changes in Length of Day (due to mass redistribution)
The change in polar motion excitation from the earthquake is predicted to be very small, about 0.82 milliarcseconds (also not observable?.
TSUNAMI - water wave generated by earthquake NY Times
Four Stages of a Tsunami: Initiation Wave Split Amplification Runup
IN DEEP OCEAN tsunami has long wavelength, travels fast, small amplitude - doesn’t affect ships AS IT APPROACHES SHORE, it slows. Since energy is conserved, amplitude builds up - very damaging
TSUNAMI SPEED IN DEEP WATER of depth d c = (gd) 1/2 g = 9.8 m/s 2 d = 4000 m c = 200 m/s = 720 km/hr = 450 m/hr Tsunami generated along fault, where sea floor displaced, and spreads outward Reached Sri Lanka in 2 hrs, India in 2-3
Tsunami waves bend around “obstacles” like islands and peninsulas in a process called DIFFRACTION.
SLOW FAST Tsunami waves also bend in a process called REFRACTION (according to “Snell’s Law”) as water depth & thus speed changes.
TRACE RAY PATHS USING SNELL’S LAW RAYS BEND AS WATER DEPTH CHANGES FIND WHEN WAVES ARRIVE AT DIFFERENT PLACES DENSITY OF RAYS SHOWS FOCUSING & DEFOCUSING 1 hour
1. 1797: A magnitude 8.4 earthquake near the central part of the western Sumatra generated a tsunami that flooded Padang. More than 300 fatalities. 2. 1833: A magnitude 8.7 earthquake near the south coast of the western Sumatra triggered a huge tsunami that flooded the southern part of western Sumatra. Numerous victims. 3. 1843: A tsunami that came from the southeast and flooded the coast of the Nias Island. Many fatalities. 4. 1861: A magnitude 8.5 earthquake affected all the western coast of Sumatra. Several thousand fatalities. 5. 1881: A magnitude 7.9 earthquake in the Andaman Island region generated a 1 m high tsunami on India’s eastern coast. 6. 1883: Krakatau explosion. 36,000 fatalities, primarily on the islands of Java and Sumatra. 7. 1941: A magnitude ~7.7 Adaman Islands earthquake. Anecdotal accounts exist of a tsunami, however, no official records exist. Previous Tsunamis in the Sumatra Region
Because seismic waves travel much faster (km/s) than tsunamis, rapid analysis of seismograms can identify earthquakes likely to cause major tsunamis and predict when waves will arrive. TSUNAMI WARNING SYSTEM Deep ocean buoys can measure wave heights, verify tsunami and reduce false alarms.
The 1960 Chilean tsunami radiated outward from a subduction zone along the coast of Chile. Its waves reached Hawaii in 15 hours and Japan in 22 hours.
Seen safely from high ground, a wave of the 1960 Chilean tsunami pours into Onagawa, Japan.
Aftermath of the 1960 Chilean tsunami in Hilo, Hawaii, where the tsunami caused 61 deaths.
The 1960 Chile earthquake produced a series of tsunami waves that crossed the Pacific Ocean. This record shows measurements of water levels beneath the Wailuku River Bridge made by seismologist Jerry Eaton and his companions during the first few hours of the tsunami in Hilo, Hawaii.
Waterfront area in Crescent City, California, flooded by the 1960 Chilean tsunami. Here, the tsunami caused more than $30,000 in damage. However, damage was much more severe 600 miles to the south in the Los Angeles area, where the tsunami killed one person and caused from $500,000 to $1,000,000 in damage, including the sinking of 30 boats.
“CIVILIZATION EXISTS BY GEOLOGICAL CONSENT” The same geologic processes that make our planet habitable also make it dangerous