Kate Hutton - extended dialogue page

> >Receipt of data .. Every bit of the data the comes from the remote stations via telemetry is saved for seven days. Immediately after it arrives, a computer program called a P-Picker looks through it for sudden increases in signal strength that might indicate the P-wave, or beginning of an earthquake. Another computer program, the Associator tries to fit all the P-Picker's results together and associate them into earthquakes. A third program, the Locator, figures out the best time, latitude, longitude, and depth to explain the data.

> >Location ... Locating an earthquake depends on timing. Think of a rock dropping into a pond, a disturbance analogous to an earthquake. Ripples spread outward from the epicenter, causing each lily pad to bob up and down at a particular time, depending on how far it is from the impact. Each frog is a P-Picker who notes the arrival time at his lily pad. Knowing the arrival time for each pad, and the speed of the wave (which depends only on the pond, not on the rock, we can figure out where the impact occurred.

With earthquakes, the problem is even easier, since each quake generates two types of sound waves: a P wave or Primary wave and an S or secondary wave. The S wave is always slower than the P wave, and gets farther and farther behind as they both travel outward. If we note the time lag between the P and the S waves, we can figure out just from that how far we are from the quake, much like a weather observer counting the seconds between the lightning and the thunder.

> >Magnitude ... You will almost never hear a seismologist talk about "the Richter scale". Seismologists, including the late Charles Richter and his collaborator Beno Gutenburg, talk about earthquake magnitude. Earthquake magnitude is an arbitrary number intended to describe the inherent size of an earthquake. It is computed from the amplitude, or peak deflection of the pen, on the seismogram (which is, of course, the recording made by the seismograph!), correcting for the distance between the quake and the seismographic station.Think about this: a 40-watt light bulb is still a 40-watt light bulb, no matter how far away you are and how dim it appears.

> >Saving data ...Once an earthquake's location and magnitude have been determined, yet another computer program copies the raw data for the appropriate time period from the seven-day data pool into a permanent archive. Several different web sites are automatically updated, e-mails and pages sent, and so forth. Usually, a human analyst checks the computer's work sometime during the following business day, just to make sure everything is right.

> >Causative fault ... Once we know the location of the earthquake, it is easy to figure out how far away it is from the nearest mapped fault. Surprisingly, though, the nearest fault that we know about is not always the one the caused the earthquake! Small quakes, like magnitude one's, two's, three's, or even four's, can happen on faults that are too small to even extend to the surface of the Earth, much less be mapped by geologists. For this reason, we usually require more information before we blame a particular fault. This other information could include observed ground rupture, or so-called focal mechanism data, which consists of the orientation (on the map) of the causative fault and the direction that the movement occurred. If this information matches a known fault, famous or not, then we may make an accusation. Otherwise, we usually do not.

> >Distinquishing blasts ... In addition to the 25 or 30 local southern California earthquakes that we record every day, we also pick up several large blasts at the local mines and quarries, plus regional earthquakes and teleseisms, or distant earthquakes. We also sometimes record prominent sonic booms, for example, if the Space Shuttle lands at Edwards Air Force Base.

> >Intensity ... Unlike magnitude, earthquake intensity measures the perceived shaking of an earthquake at a particular location. An earthquake ideally has only one magnitude (like the wattage of that light bulb), but it has a whole map full of intensities, indicating where the shaking was the strongest. For intensity, we use the Modified Mercalli scale, which is based on descriptive reports and questionnaires from the public and on damage surveys. An analogous instrumental intensity can be computed from the recordings of so-called strong-motion accelerographs, which are seismographs specifically designed to record felt and damaging earthquakes.