Volcano Watch — Tsunami, landslide cause most quake damage

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A great earthquake, with a magnitude of 7.8, struck northern Japan during the night on Monday, July 12. Most of the deaths were attributed to a tsunami that was generated by the earthquake, but other people were killed in a landslide and by falling buildings.

Tsunami, landslide cause most quake damage...

Tsunami, landslide cause most quake damage

(Public domain.)

A great earthquake, with a magnitude of 7.8, struck northern Japan during the night on Monday, July 12. Most of the deaths were attributed to a tsunami that was generated by the earthquake, but other people were killed in a landslide and by falling buildings. In any large earthquake, falling buildings and other structures cause death and destruction. However, tsunami and landslides, which commonly accompany large earthquakes, cause more widespread damage.

Tsunami are generated by large earthquakes that occur beneath the sea, or by large land masses which slide into the sea. These huge ocean waves travel at speeds of about 435 miles per hour and cause destruction when they enter shallow water and break. The tsunami generated by the Japanese earthquake this past week crested at about 16 feet and caused extensive damage along the shore of the Sea of Japan to the west of Hokkaido Island and as far away as South Korea.

Initially, the Pacific Tsunami Warning Center issued a tsunami watch for the entire Pacific Basin, but that watch was cancelled a short time later. The tsunami watch was called because the location of the earthquake was initially placed on the east (Pacific Ocean) side of Hokkaido rather than on the west (Sea of Japan) side of the island, where the earthquake was redetermined to be beneath the Sea of Japan. Had the earthquake been located beneath the sea on the Pacific side of Hokkaido, any tsunami generated would have taken about 7 to 8 hours to reach Hawai`i (see figure). 

During large earthquakes, power and communications are almost inevitably disrupted near the epicenter. The loss of electrical power and communications makes it difficult to obtain the data from nearby seismometers, which are those that best pinpoint the location of the earthquake. The initial location of large earthquakes is almost always determined from the time of arrival of seismic waves at distant stations because of these difficulties. Because these stations are located far from the earthquake, the waves take some minutes to reach them, and the location of the earthquake cannot be determined accurately because the waves travel at speeds that vary due to variations in the geology. Only after arrival times are obtained for a number of stations, including some relatively close to the epicenter, can the location and depth be accurately determined.

The disruption of fundamental services, including electrical power, telephones, water, natural gas lines, and even transportation along highways, also causes enormous problems in responding to an earthquake once it occurs. When the October 1989 Loma Prieta earthquake occurred in the San Francisco Bay area, many of these services were disrupted over large areas and for long periods of time. Natural gas lines broke in the Marina district of San Francisco, which led to fires that could not be controlled because the water lines were disrupted. Similar fires destroyed much of Aonae Village on Okushiri Island in Japan this week.

In San Francisco, many bridges and freeway overpasses sustained damage, which disrupted movement of emergency vehicles in the area immediately following the earthquake. Many of these bridges and overpasses continued to disrupt traffic for many months afterwards. Many areas were without electrical power for days following the earthquake, and the regional telephone system was overwhelmed and essentially out of service for nearly a day following the earthquake.

For the people living in the San Francisco area, there was little news to be had because of the difficulties in moving around the area (traffic lights, as well as some bridges and freeway overpasses, were out). It was many hours before the extent of the damage was known and broadcast to the public. Several of the local television and most of the radio stations were off the air due to damage at their broadcast facilities and the lack of electrical power. Keep in mind that the Loma Prieta earthquake was not a great earthquake like the one that occurred this week in Japan. The Hokkaido earthquake, with a magnitude of 7.8, was roughly 5 times larger than the Loma Prieta earthquake, with a magnitude of 7.1.

Similar difficulties in communications apparently followed the Hokkaido earthquake, as can be seen in the time it has taken to determine the number of people killed and missing. The estimates and reports have varied widely since the earthquake occurred.

Hawai`i, as we have reported numerous times, is an extremely active earthquake region. The largest earthquake to have occurred here in historic times was even larger than the Hokkaido earthquake, having an estimated magnitude of 8.0. This great earthquake occurred in 1868 and was located between Pahala and Volcano. Because the south flank of Hawai`i moved toward the sea, a tsunami was generated that accounted for most of the people killed. Similarly, in 1975, another large (magnitude 7.2) earthquake beneath Kīlauea Volcano generated a local tsunami that accounted for the only deaths attribued to the earthquake.

Tsunami generated by nearby earthquakes allow almost no time to evacuate. In Japan, particularly for people living on Okushiri Island located nearly at the epicenter, many of the people killed simply did not have [time to get to high ground. Here in Hawai`i, if you are located close to sea level and feel a strong earthquake (one where the shaking continues for more than a minute or so), do not wait to hear the tsunami sirens, but instead head for higher ground immediately, as there may be only a few minutes between the earthquake and the ensuing tsunami.]*

*bracketed section not published in Hawaii Tribune-Herald