Earthquake Dynamics

When we initially feel the movement of an earthquake, we know nothing for certain concerning the actual event that has taken place or what is taking place. We cannot be certain if we are experiencing a foreshock, mainshock or the beginning of the main event. We have no knowledge as to the earthquake's depth, focus (where rupture has occurred), epicenter (directly above focus), amount of energy released, seismic waves generated, or velocity (speed in relation to time) of seismic waves radiating toward our location. Our best chance for surviving a destructive earthquake then, is found in early detection and taking immediate and appropriate action, based on our knowledge of what may and can occur.

Early Detection

Often times precious seconds are lost when a person thinks but is not sure if earthquake activity has taken place. Survivor Alert, in addition to alerting those who are asleep, is a confirmation to those awake - that indeed seismic activity has ocurred and immediate action should be taken. An understanding of the character of seismic waves reveal that warning times are often possible.

Energy Released

Seismic waves are caused by the sudden breaking and slippage of rock within the earth's crust and upper mantle. The amount of energy released is dependent upon a number of factors, such as the area of the fault surface, amount of slippage, focal depth and shear modulus of elasticity (the ability of rock under pressure to bend before breaking). Magnitude is proportional to the amount of energy released and as data is received and computed at seismograph stations, a logarithmic (Richter scale) value is assigned. A difference of just one number on the Richter scale equates to about 32 times more energy released. For instance, a magnitude 6.5 earthquake releases energy that would be equivalent in energy to about 31,550 tons of T.N.T., whereas a magnitude 7.5 earthquake would release energy in the equivalent of 1,000,000 tons of T.N.T.

Types of Seismic Waves

A portion of the energy released radiates outward in the form of seismic waves - waves of energy that often brings devastating results to both life and property. Within the scope of our concern, there are two types of waves: body waves and surface waves.

The first of the two body waves that will arrive is the P-wave or Primary wave. It is the fastest, typically moving a 5 miles per second through the upper mantle and 3.5 miles per second through the continental crust. This compressional wave is detectable as it pushes and pulls rock in the direction of travel. However, we normally only feel the bump and rattle of these waves as they pass.

The second body wave to arrive is the S-Wave or Shear wave, traveling about 1.75 slower than the P-wave. It is with this differential in arrival times that may be our best key to survival in an earthquake. In general, the Shear waves are larger and do much of the damage, especially close to the rupturing fault, which results from strong shaking. These waves travel in a shear motion, vibrating rocks transversely (side-to-side) in the wave's direction of travel.

The second type are surface waves. As the name implies, they travel along the surface of the earth. Since the waves travel half in rock and half in the low density of air, they are usually slower and often larger in aplitude (especially from shallow depth events). They have a tendency to physically move the surface around more violently than body waves.

Love waves are the fastest of the two surface saves, have a horizontal motion that is shear or transverse to the direction of travel and can be expected to arrive after the S-wave.

Raleigh waves are the slowest but often the largest and most destructive. when of sufficient amplitude, these complex waves physically move the ground up and down and from side-to-side in the direction of travel.

Warning Times

Although P and S wave speeds vary by a factor of ten or so, as they travel through the earth, the ratio between the two waves is quite constant. The time interval between these waves increase with distance. This time difference allows an estimate to be made in a distance range of 31-313 miles. The established rule-of-thumb for this range is 1 second of P and S wave separation for each 5 miles traveled. As an example, a shallow earthquake, 100 miles away would have a wave separation of about 20 seconds - 20 seconds of warning time. If 150 miles away, about 30 seconds. If the focus is deeper in the earth or otherwise further in distance, precious seconds of warning will be gained.

Foreshock

A foreshock is a small tremor that comonly precedes a larger earthquake. It originates in or near the rupture zone. About a fourth of the time, it can be expected within one hour prior to the main shock. The historic San Francisco earthquake experienced a foreshock that was felt throughout the San Francisco Bay area about 20-25 seconds before the major earthquake struck with violent shocks and strong shaking that lasted 45-60 seconds. The mainshock could be felt in southern Oregon, to south of Los Angeles and as far inland as central Nevada. Foreshocks do not always preceded a large earthquake but when they do and are detected, they can be considered another key to survival - especially for those in close proximity to the rupture zone where the smaller tremor (foreshock) is more easily detected.

Prediction

Very few advancements have been made in the realm of earthquake predictability. Only after an earthquake strikes in a given area over a period of time are seismologists able to study the earthquake's reoccurrence intervals and make a rough estimate as to when another will strike in the same area.

Late Night - Early Morning Earthquakes

Earthquakes occur about half the time between the hous of 8 p.m. and 8 a.m. this period of time not only covers a period of darkness but is a time when most people are fast asleep- a time in our home when we are most vulnerable. A foreshock or the beginning of a main shock may only rock us into a deeper sleep. If awakened, we may feel disoriented or wonder why we were awakened. Conversely, with Survivor Alert, the alarm will immediately let us know that the detector has sensed movement of the building structure and the lantern instantaneously brings visual orientation as well as a lighted path for escape or to seek cover. The historical earthquake of San Francisco, mentioned earlier, took place seconds after the 5:12 a.m. foreshock, when most people were fast asleep. Times have not changed. A 7.6 magnitude earthquake that struck Taiwan took place at 1:47 a.m., resulting in over 2,000 deaths. With Survivor Alert, some lives would have been spared by early detection and by individuals taking cover. Since electrical power was immediately severed, those affected, that had survived the mainshock had to make their way in darkness, through damaged buildings and debris to exit buildings - Survivor Alert would have permitted a quicker and more secure exit. The same might be said of several of the aftershocks that followed - over one thousand, 22 being in the magnitude of 5.0 - 6.8.

Optimum Intensity

Survivor Alert has been shop tested over a wide range of sensitivity with excellent results. However, since reproduction of the prototype will involve a different manufacturing process necessary to reduce cost and increase speed for mass production, further testing by TimeLine is not feasible and will not be forthcoming. Sensitivity is accomplished through the medium of an electromechanical system that either increases or decreases sensitivity by rotation of the sensitivity control dial. It is recommended that the "set mark" beside the sensitivity control, reflect an optimum setting that corresponds to an Intensity IV earthquake.