3. Data Processing Procedures
3.4. Event Processing
Event processing is the procedure for measuring some basic parameters from the recorded seismic traces that describe the earthquake ground motion. These parameters are generally known as phase data and include:
( I ) the onset time and the direction of motion of the first P-arrival; (2) the onset time of later arrivals, such as the S-arrival (if possible); (3) the maximum trace amplitude and its associated period; and (4) the signal
3 A. Everit Processitig 61 duration of the earthquake. Precise measurement of the phase data is essential because they are used to compute the origin time, hypocenter, and magnitude of the earthquake, and to deduce its focal mechanism. In the following subsection, we give some general methods of measuring the phase data.
The goals of event processing are to measure the phase data as uni- formly and precisely as possible and to prepare the so-called phase list containing these data for each earthquake. In this section, we discuss processing the individual seismic traces, while in Section 3.5 we discuss processing the phase lists.
3.4.1. Visual Methods
The traditional way of measuring phase data is to read visually the phase time, amplitudes and periods, etc., from the seismograms using rulers or scales (see, e.g., Willmore, 1979). The phase data are usually recorded on a printed form to facilitate punching cards or otherwise enter- ing the data into a computer. The visual method has the advantage that it is straightforward. It is also a simple matter to read and merge data from a variety of visual recording media. For a small microearthquake network operating in an area of moderate seismicity, this may be the most cost- effective method to use. However, the visual method has the following disadvantages: ( 1) different analysts may read the seismograms differ- ently; (2) errors may be introduced in measuring, writing, or keypunching the data; and (3) it may be difficult to carry out such a tedious task over a long period of time. Also, visual recording media have a limited dynamic range and a limited recording speed, so that some of the phase data (such as first P-motion, S-arrival, and maximum trace amplitude) cannot be read precisely, if at all.
For a large microearthquake network where the data volume is large, the visual method of measuring phase data requires a considerable amount of checking to minimize errors in data handling. This includes systematic checking of measured phase data by another analyst before keypunching, and systematic verifying of the phase data after keypunching. Our experi- ence shows that analysts or keypunch operators occasionally enter the data in the wrong place or transpose digits in the data. For example, a P-arrival time of 13.55 sec may be written or keypunched as 31.55 sec.
3.4.2. Semiautomated Methods
Semiautomated methods usually are designed to eliminate the tasks of recording and keypunching data that are required in the visual methods.
62 3, Dritrr Processing Procedures
The analyst is still required to examine each seismic trace on the film or paper seismograms and to make decisions about what should be mea- sured. Instead of a ruler or scale, however, the analyst uses a digitizing cursor to indicate where each measurement should be made and a keyboard to enter additional information. In the simplest case, the output from the digitizing cursor and the keyboard is written automatically on punched cards by a standard keypunch machine. In the more elaborate case, a small computer is used to provide the analyst with an interactive mode of measuring phase data. We now give an example of each case.
A noninteractive semiautomatic system to measure phase data from the 16-mm microfilms recorded by the USGS Central California Microearth- quake Network has been in use since 1973. It has replaced the routine visual method of measuring phase data discussed in the previous subsec- tion. This system consists of an overhead projector, an electronic dig- itizer, a keyboard, and a standard keypunch machine. The electronic digitizer has a digitizing table about 90 by 130 cm in size and a digitizing cursor; its digitizing resolution is about 0.025 mm. About 60 sec of one 16-mm microfilm is projected onto the digitizing table from the overhead projector. The optical magnification is about 30x so that I sec in time is equivalent to 1.5 cm in length on the digitizing table. For each earthquake, the analyst uses the keyboard to enter the data and time of the event and the microfilm identification. The digitizing cursor is used to enter the coordinates of ( I ) time fiducials, (2) seismic trace levels, (3) P-phase on- sets, and (4) other measurements. The output from the keyboard and the digitizing cursor is punched on cards automatically as an unformatted string of alphanumeric characters.
Using the scan list, phase data for all earthquakes recorded on one roll of microfilm are processed sequentially. Because several rolls of micro- films per day are recorded by the USGS Central California Microearth- quake Network, this procedure is repeated for each roll of microfilm.
The resulting deck of punched cards is processed off-line by a computer.
The processing program translates the unformatted string of alphanumeric characters into the intended phase data, organizes the phase data by earthquakes, and outputs a phase list for earthquake location. Routine processing of phase data using this system is about two to three times faster than the visual method described in the previous subsection be- cause reading off a scale, writing down the data, and keypunching the data are eliminated.
A major drawback of this processing system is that it is a two-step procedure, and errors often are not detected until the punched cards are processed off-line. To remedy this difficulty, W. H. K. Lee and colleagues introduced a low-priced microcomputer to control the data processing of
3.4. Evetit Processing 63