(a) An example image of a seismogram from Harvard‐Adam Dziewoński Observatory. The recording starts at 13:56 (UTC) on Sunday, 13 November 1938, and ends at 13:55 (UTC) on Tuesday, 15 November 1938. Pseudocolorization of the seismogram with light shades exists, especially near the edges of the seismograms. The black stripe at the bottom of the image is introduced during the scanning procedure. (b) The region enclosed by rectangle 1 in (a) where stains are visible. (c) The region enclosed within rectangle 2 in (a), showing handwritten notes about the timing of the traces. Short segments that are offset from the main traces are time marks where single segments are minute marks and the occurrence of two segments indicates the hour.The color version of this figure is available only in the electronic edition.
Comparison of the image in red, green, and blue channels and the hue, saturation, and value channels. (a) A portion of the left edge of a seismogram and (b) a zoom in on one trace. Red, green, and blue channels all contain trace information and other features such as stains or chromatic aberration. In contrast, when the image is decomposed into hue, saturation, and value attributes, the trace is separated from noise and is stored in the Value channel.The color version of this figure is available only in the electronic edition.
Histogram correction to reduce noise such as stains, smudges, and fingerprints from the image. (a) The initial gray‐scale image (negative of the original) contains stains of medium‐intensity values. (b) Histogram corresponding to the image. The vertical lines show the intensity range that has been chosen by a user for remapping. (c) The image after intensity remapping. (d) The histogram after remapping for which all the pixels with intensity below the lower bound shown in (b) are given the value of 0 (hence the spike at 0) and those above the upper bound are set to 255.The color version of this figure is available only in the electronic edition.
Example of seismogram distortion. (a) The solid line in the middle of the image represents the second‐degree polynomial that fits the observed distortion. (b) The two insets show the larger view of the two vertical strips containing time marks, and the dashed lines are identical in both windows, drawn to follow the position of the time marks in the left strip. The misalignment of the dashed line with the time marks on the right illustrates the arbitrary distortion that must be corrected before horizontal position can be converted to time.The color version of this figure is available only in the electronic edition.
Examples from (a) the beginning and (b) the end of the traces shown in Figure 1. The curves above the images show the sum of the intensity values in the vertical direction.The color version of this figure is available only in the electronic edition.
A section of the image from Figure 1 illustrating the timing difficulties. Dotted and solid vertical lines correspond to the estimated and actual locations of ending pixel of the time marks, respectively. The rectangles are the bounding boxes of time marks. The thin solid curves show the digitization result superimposed on the image.The color version of this figure is available only in the electronic edition.
(a) A gray‐scale 8‐bit image containing the main trace and two time marks. (b) Binary mask associated with time marks. (c) Mask for the main trace. (d) Fused image using (b) and (c).The color version of this figure is available only in the electronic edition.
(a) Illustration of the digitization approach. The background is a gray‐scale image. The grid shows pixels around the horizontal position of interest. The curve is the variation in intensity (increasing values to the right) for the column at the center of the grid. The final point (circle) is computed as the weighted average of the individual intensities of the specific column (equation 3). (b) Example from the digitization result. The digitized seismogram is shown on top of the gray‐scale image for comparison. The offset of the time marks is corrected.The color version of this figure is available only in the electronic edition.
(a) The digitized seismogram from the image that is shown in Figure 1 (left) and the corresponding spectrogram (right). Horizontal lines mark the origin time (solid lines) and the associated surface‐waves arrival times (dashed lines) for five earthquakes that are identified in the International Seismological Center catalog (Table 1). The numbers beside the horizontal lines are the indices of the earthquakes from Table 1. (b) Digital broadband north–south seismogram recorded at the HRV station with similar length and dates of the year as in (a) except that it is from year 2014 (left) and its spectrogram (right). The earthquake signal appearing around 2:00 on 15 November corresponds to an Mw 7.1 earthquake in Indonesia. The noise peaks at approximately 0.14, and 0.25 Hz are present in both spectrograms.The color version of this figure is available only in the electronic edition.