Essay Questions Key

1. When was the first earthquake detection instrument invented?
 
 The earliest known earthquake detection instrument was invented in 132 A.D. by Zhang Heng, a Chinese philosopher. The instrument was a large (2 meters or 6.5 feet in diameter) bronze jar, with a central pendulum inside. Decorating the jar on the outside were a series of dragon heads connected to a pendulum, each with a ball in a hinged mouth. Directly beneath each dragon head, on the surface of the stand, was a bronze toad, head up, mouth open to receive a ball from the dragon's mouth.
 
 
2. If a seismograph station has 3 pendulums, at what orientation are they placed for maximum sensitivity?  What four measurements can scientists estimate from this setup?
 
 One seismograph station, having three different pendulums sensitive to the north-south, east-west, and vertical motions of the ground, will record seismograms that allow scientists to estimate the distance, direction, Richter Magnitude, and type of faulting of the earthquake.  Seismologists use networks of seismograph stations to determine the location of an earthquake, and better estimate its other parameters.
 
 
3. What are the differences and similarities between tidal waves and tsunamis?
 
 In the past, tsunamis were sometimes referred to as "tidal waves" by the general public, and as "seismic sea waves" by the scientific community.  The term "tidal wave" is a misnomer; although a tsunami's impact upon a coastline is dependent upon the tidal level at the time a tsunami strikes, tsunamis are unrelated to the tides. Tides result from the unbalanced, extraterrestrial, gravitational influences of the moon, sun, and planets. The term "seismic sea wave" is also misleading. "Seismic" implies an earthquake-related generation mechanism, but a tsunami can also be caused by a non-seismic event, such as a landslide or meteorite impact.
 
 
4. Explain what happens when a tsunami approaches then just reaches the coast.

As a tsunami leaves the deep water of the open ocean and travels into the shallower water near the coast, it transforms. If you read the "How do tsunamis differ from other water waves?" section, you discovered that a tsunami travels at a speed that is related to the water depth - hence,  as the water depth decreases, the tsunami slows. The tsunami's energy flux, which is dependent on both its wave speed and wave height,
remains nearly constant. Consequently, as the tsunami's speed diminishes as it travels into shallower water, its height grows. Because of this shoaling effect, a tsunami, imperceptible at sea, may grow to be several meters or more in height near the coast. When it finally reaches the coast, a tsunami may appear as a rapidly rising or falling tide, a series of breaking waves, or even a bore.
 

5.    What are the similarities and differences between wind generated and tsunami waves?

Tsunamis are unlike wind-generated waves, which many of us may have observed on a local lake or at a coastal beach, in that they are characterized as shallow-water waves, with long periods and wave lengths. The wind-generated swell one sees at a California beach, for example, spawned by a storm out in the Pacific and rhythmically rolling in, one wave after another, might have a period of about 10 seconds and a wave length of 150 m. A tsunami, on the other hand, can have a wavelength in excess of 100 km and period on the order of one hour.