Secondary earthquake hazards



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SECONDARY EARTHQUAKE HAZARDS

S. M. Govorushko1



ABSTRACT
Very often earthquakes are the cause of secondary hazards. The factors of the earthquakes’ impact on other natural processes are the following: 1) vertical movement of crustal blocks; 2) shaking; 3) ground properties alteration; 4) longitudinal and rotational waves. The first factor is necessary for emergence of tsunamis, seiches, and internal waves. We know that far not each submarine earthquake generates tsunami. For that, vertical movement of sea bed and certain depth of the earthquake center are necessary. Submarine earthquakes are the most frequent cause of tsunamis, and especially strong ones. The role of submarine earthquakes in creation of internal waves and seiches is of minor importance. The shakes are important factor of awakening of slope processes. This factor is important for emergence of rock and ice falls, landslides, submarine landslides, taluses, rock streams, avalanches. Quite often slope processes caused by shaking are the most dimensioned. Ground properties alteration impacts on other natural processes through ground liquefaction. Vibration transforms ground from solid to viscous semi-fluid condition which resembles quick sand. Above all, this factor is important for mudflows; it is also of some importance for landslides. The impact on volcanic eruptions lies in exposure of one and closing of other cracks in rocks, proportionally to the distribution of impulses of extension-compression through those cracks.

Introduction

It is known that there is a correlation between earthquakes and other natural processes. Some natural processes, namely volcanic eruptions, karst collapses, rock falls, are capable of triggering earthquakes. In one’s turn, earthquakes trigger such dangerous processes as tsunamis, rock and ice falls, landslides and submarine landslides, mudflows, avalanches, taluses, rock streams, volcanic eruptions, internal waves, and seiches. The article investigates intensity and frequency of above-mentioned correlation, considers mechanism of natural processes’ interaction, and describes factual cases of secondary earthquake hazards.



Natural processes causing earthquakes
Earthquakes’ creation is conditioned not only on tectonic processes but on surface shaking that can be of internal or external origin. Internal shaking is explosion of phreatic gases during volcanic eruptions; external one is roof caving and great rock falls. However, occasions mentioned above are rarely observed, and the power of such earthquakes is not high. Thus all strong earthquakes and 99% small ones are caused by tectonic processes (Shcetnikov, 1981). However, volcanic, pit collapse, and rock falls earthquakes exist, too.

Earthquakes triggered by volcanic eruptions occur only if magmatic gases explode, sometimes due to heating of subterranean water by magma centre heat. Such volcanic eruptions are also named phreatic eruptions. The earthquake of November 29, 1975 in the southern coast of Hawaii Island, and the earthquake of May 18, 1980 preceding the Saint-Helens Volcano eruption in Washington St., USA, are typical examples of volcanic eruptions (Gere, Shah, 1988).

Pit collapse earthquakes are caused by roof caving (photo 1) and abandoned excavations caving (photo 2). First reason accounts for the earthquake of 1802 caused by karst collapse nearby Yablonovets Village upon Matyra River in the west of today’s Tambov Region (Russia). Roof caving triggered local magnitude 5-6 earthquake (Kurbatova et al., 1997). Strong earthquakes caused by excavations caving are rare. Not more than 10 of them were noted worldwide, 4 of them witnessed at potash mines in Germany (July 8, 1958; June 23, 1975; March 13, 1989; September 11, 1996), 1 at dolomite mine in Austria (May 2, 1993), 1 at kyanite mine in Ukraine (June 26, 1986), and 1 at mine in the USA (February 3, 1995).


1 Professor, Main Research Officer, Pacific Geographical Institute, 7 Radio St., Vladivostok, Russia, 690041


e-mail: sgovor@tig.dvo.ru

Picture 1. Collapse sinkhole in Winter Park, Florida.

Photo credit: USGS, 1981.

Picture 2. Caving of pit’ roof in Kansas, USA.

Photo credit: G. Ohlmacher, June 2002.

Rockfall-triggered earthquakes do not occur frequently either. A bright illustration of such is the rockfall of July 11, 1996 in Yosemite National Park (California, USA). The rock, 38,000 m3 in volume, separated and disintegrated when it landed, creating an air blast that was so powerful that it flattened as many as 2,000 trees in the area (photo 3). The dust kicked up from the pulverized granite blocked out sunlight and coated tents and recreational vehicles, similar to ashfall from volcano. Seismographs caught the seismic wave as far as 200 km away from the rockfall (Wieczorek et al, 2000).

Picture 3. Rockfall, Yosemite National Park, California.

Photo credit: E. Harp, July 12, 1996.

El Nino phenomenon was also proven to correlate with earthquakes. Basing upon investigations of seismic activity in the south-western part of the Pacific Ocean in 1975-1994, it was found out that seismic activity increased in the moments of El Nino action. In particular, January 1976, December 1982, and October 1986 brought multiple shallow-focus earthquakes in Tonga and Kermadec trenches. Those earthquakes could be perceived around 100 km from the epicentre and occurred within 5-10 days, similar mechanisms involved in all cases. There is a supposition that dynamical force to lithosphere induced by El Nino provoked the earthquake (Khomutov et al., 1997).

Factors of the earthquakes impact on other natural processes

Factors of the earthquake impact on other natural processes are: 1) vertical movement of crustal blocks; 2) shaking; 3) ground properties alteration; 4) longitudinal and rotational waves. The first factor is important for creation of tsunami, seiches and internal waves. It is known that far not each submarine earthquake begets tsunami. To trigger it, vertical movement of crustal blocks and sufficient depth of earthquake center are necessary, since tsunami usually do not occur when the depth of earthquake center is less than 50 km. Submarine earthquakes is the most frequent cause of tsunami, being responsible for all the strong ones. Earthquakes do not play a considerable part in creation of internal waves and seiches.

Shaking is the important factor of slope processes intensification. Since stability of the slope is determined by the balance of forces of resistance to displacement and shifting forces, shaking not infrequently upsets this balance. Shaking plays a large part in creation of rock and ice falls, landslides and submarine landslides, taluses, rock streams, and avalanches. Slope processes induced by shaking are often the most powerful ones.

Ground properties alteration impacts on other natural processes through ground liquefaction. Such liquefaction occurs under following conditions: 1) at least 10 second duration of the earthquake; 2) certain frequency of tremors; 3) certain composition of ground; 4) water saturation. Vibration transforms ground from solid condition to semi-fluid one which resembles quick sand. This factor is preeminently important for mudflows creation and is of some importance for landslides, too.

The impact on volcanic eruptions is arguably explained by exposure of some cavities and closing of others proportionally to the distribution of impulses of extension/compression through those cavities

Examples of secondary earthquakes hazards

Tsunamis are the obvious illustration of secondary earthquake hazards (photo 4). Submarine earthquakes are the most frequent cause of its emergence. According to one data, they account for 90% of all tsunamis (Seismic hazards, 2000); other sources argue for 99% (Belozersky et al., 1994). Tsunamis catalogue issued by National Geophysical Data Center, USA, defined 108 of 2400 tsunamis as volcanogenic (http://www.ngdc.noaa.gov/seg/hazard/tsu_db.shtml). Hence, other tsunami-triggering factors, namely landslides and submarine landslides, rockfalls, and mudflows, account for very insignificant number of cases and do not need special attention.

Picture 4. Tsunami near Hilo, Hawaii generated earthquake of April, 1 1946, Aleutian Islands, Alaska.

Photo credit: University of California, at Berkeley

Far not each submarine earthquake begets tsunami. Certain depth of the earthquake center (not deeper than 50 km) and certain mechanism are necessary. Vertical movement of sea bed crustal blocks is considered to be obligatory factor (Gere, Shah, 1988). Earthquakes are also considered to be one of the causes of internal waves which are oscillating movements emerging between water layers with different density, and seiches which are standing vibrations emerging in closed or partially closed water bodies, namely lakes, gulfs, bays, and harbors. For instance, mass seiches emergence induced the Cape-Yakataga earthquake of September 3, 1899 in Alaska (Aprodov, 2000). In comparison with tsunamis, earthquakes do not contribute considerably to creation of internal waves and seiches (Gevorkyan, 2000; Korgen, 1995; de Jong et al., 2003).

The second factor of the earthquake impact is important for intensification of gravity slope processes. The typical case of the earthquake impact is rockfall generation (photo 5). Remembering earthquakes entailing mass rockfalls one should name the Sarez earthquake of February 18, 1911 in Central Pamir, the earthquake of September 25, 1988 in Northern Pamir, the Bihar-Nepal earthquake of January 15, 1934 in India, the Central Peruan earthquake of May 24, 1940, the Alaska earthquake of March 28, 1964, and many others. The rockfall of February 1911 in Pamir, 2.2 billion km3 in volume, deserves special mention since it buried Usoy Kishlak, modified Murgab River and generated Sarez Lake 80 km long and 500 meters deep (Basics …, 1994).

Picture 5. Rocfall in Lefkada Island, Greece triggered earthquake.

Photo credit: Geobrugg Protection Systems, 2004

Earthquakes that caused huge landslides were the Great Assam earthquake of August 15, 1950, the earthquake of June 16, 1929 in Australia, the Tokachi-oki earthquake of May 16, 1968 in Japan, etc. For instance, the Las Colinas landslide (photo 6) was one of thousands landslides triggered by the 13th El Salvador earthquake (MW 7.6). The landslide was highly destructive. It led to the death of 585 people when it swept into a residential area of Santa Tecla, a suburb of San Salvador. Another well-known example of such kind is slipping down of nearly 1 km3 of land in the northern Peloponnesus coast in the winter of 373-372 B.C. This landslide destroyed ancient Greek city of Gelios. In 1920, approx. 100,000 were killed by landslides generated by the earthquake in Gansue Province, China (Landslides …, 1984).

Picture 6. Las Colinas landslide triggered earthquake in Salvador.

Photo credit: USGS, January, 2001.

Submarine landslides are another typical result of earthquakes. One cannot omit mentioning the South-Calabrian earthquake of March 11, 1979 which generated submarine landslides in the Ionian Sea, and the Alaskan earthquake of March 28, 1964 entailing similar consequences (Aprodov, 2000). In the similar way, for instance, port structures of Valdez (Alaska, USA) were destroyed. The town is located on the brim of delta composed of water-saturated sand-and-gravel sediments. The earthquake of March 28, 1964 provoked a landslide moving along the coastline. The territory 1120 meters long and 183 meters wide slipped into sea, volume of the landslide to be 75 million m3. As a result, a considerable part of the coast together with piers and other port infrastructure plunged in water. The similar submarine landslide occurred in 1975 nearby Vancouver (British Columbia, Canada). Since the territory was not populated, it did not cause serious property damage, yet modified the coastline and generated tsunami (Schuster, Highland, 2001). To make another example, one should remember the submarine landslide generated by the Grand Banks earthquake of November 18, 1929 in the vicinity of Newfoundland, North Atlantic. This landslide not only killed 27 (Nisbet, Piper, 1998), but induced a turbidity current which firstly moved down continental slope through several channels but later on merged into one sizeable stream (Kennet, 1987). This huge turbidity current torn asunder seven underwater cables one by one (Vinogradov, 1980).

In regard to ice falls, the most well-known ones were generated by the earthquake of September 10, 1899 in Yakutat Bay, Alaska, and the Huaskaran earthquake of May 31, 1970 in Peru (Gere, Shah, 1988). So, Huaskaran calamity caused falling down of approx. 5 million m3 of ice and snow that rushed from the slopes of a mountain 6655 meters high (photo 7). This mass hit the glacier and chopped off a considerable part of it. Rushing downwards, the flow augmented in mass and velocity, mounting to 50 million m3 and 320 km/hour respectively. Yungay and Ranrahirka towns were annihilated, more than 15,000 killed (Kotlyakov, 1994).

Picture 7. Glacier fall due to Huascaran earthquake in Peru.

Photo credit: NGDC, 1970.

Earthquake-triggered ground liquefaction contributes to mudflows. The earthquake of March 6, 1987 in Ecuador brightly illustrates that. Multiple earthquake-triggered mudflows destroyed 40 km of the country’s central oil pipeline, damaged communications and roads, ruined several communities with a total population of approx. 1,000 (Aprodov, 2000). To continue the list, the Khait earthquake of July 10, 1949 in Tajikistan triggered a series of mud and debris flows that buried 33 villages (Landslides …, 1996).

Earthquakes oftentimes intensify volcanic activity. Thus, in two days after the Chilean earthquake of May 21, 1960, the two weeks long Pueue Volcano eruption started. The Ecuador earthquake of March 6, 1987 awoke Reventador Volcano; the South-Kamchatka earthquake of November 4, 1952 stimulated Klutchevskiy, Karymskiy, Avachinskiy and other volcanoes. All above-mentioned holds true for mud volcanism as well. For instance, some of the earthquakes nearby Shemakha in Azerbaijan, such as of June 11, 1859, January 28, 1872, and others, generated mud eruptions (Aprodov, 2000).
Examples of many-step impacts
Earthquakes frequently impact on other natural processes in several steps creating a chain reaction. One can emphasize the following most obvious chains: 1) earthquake – landslide – flood; 2) earthquake – rockfall – tsunami; 3) earthquake – landslide – mudflow; 4) earthquake – tsunami – seiches; 5) earthquake – submarine landslide – internal waves; 6) earthquakes – rockfalls – seiches; 7) earthquakes – icefall – tsunami, etc. For instance, the earthquake of July 9, 1958 in Alaska made approx. 300 million km3 of ice and rock fall down from Lithuaya glacier, 900 meters high, to the namesake bay. It brought about the wave 530 meters high (Natural and anthropogenic processes …, 2004). This earthquake also generated a series of submarine landslides that produced 30 meter tsunamis in fiords (Myagkov, 1995). The Khait magnitude 7.5 earthquake of 1949 is also a very pertinent example since it generated a big landslide which later on transformed into a mudflow which buried part of town Khait (photo 8).

Picture 8. Consequences of Khait earthquake, 1949. Landslide (top right part of photo) transformed into mudflow that buried part of Khait town.

Photo credit: L. Desinov, 1986.

Impact of earthquakes on natural components

Besides impact on human activity, earthquakes influence several natural constituents which are: 1) surface water; 2) ground water; 3) geomorphologic environment; 4) geological environment; 5) wild life.

River stemming induced by earthquake-triggered landslides, mudflows, and etc. is a typical example of the impact on surface water. River stemming is very widespread. Sometimes earthquakes alter inland freshwater system. Thus several rivers vanished and reappeared in new sites due to the Buyin-Zara earthquake of September 1, 1962 in North-West Iran. Lakes also disappear, as the case of the earthquake of July 9, 1905 in North-West Mongolia proves. On the other hand, new lakes, and quite large ones, can be formed when sunken sites are inundated. So, the New-Madrid earthquake of December 16, 1812 in the USA inundated approx. 500 km3. Reelfoot Lake which is the largest of the new-formed water reservoirs had 50 km3 in square (Aprodov, 2000). Cases of earthquake-triggered waterfall generation are also known (photo 9).

Picture 9. Vertical offset along the fault during Taiwan earthquake has formed new waterfall 8 meters high on Tachia River.

Photo credit: NOAA

The impact on ground water lies in exposure of some cavities and closing of others proportionally to the distribution of impulses of extension/compression through those cavities. Spasmodic changes in water level in wells and boreholes often occur before an earthquake. Such phenomena were witnessed during the Haicheng earthquake of February 4, 1975 in China and the Khankai earthquake of August 15, 1967 in Primorsky Region. Not infrequently old water sources vanish, and new one emerge. That was a distinctive feature of the earthquake of November 4, 1946 in Turkmenistan and the Argun earthquake of March 2, 1966 in the North Caucasus. Thermal springs in areas with geothermal activity often vanish, too. That was proven by the Iturup earthquake of November 6, 1958 in Kuril Islands, the Gediz earthquake of March 28, 1970 in Turkey, etc. (Aprodov, 2000). Sometimes new geysers emerge, as it happened on June 30, 1975 in Yellowstone National Park in the USA (Gere, Shah, 1988). Very frequently earthquakes modify chemical composition of ground water adding helium and radon into it. Such phenomena were noted before the Tashkent earthquake of April 25, 1966 and the Sarykamysh earthquake of June 5, 1970 in Kirgyzstan.

The impact of earthquakes on relief lies in intensification of relief-forming processes, elevation of one sites and lowering of others, etc. Such relief-building modifications can be very sizeable. For instance, the Gobi-Altay earthquake of December 4, 1957 in Mongolia elevated Gurban-Bogdo ridge 275 km long and 30 km wide, and the shift was 10 meters up and 9 meters east (Aprodov, 2000).

Earthquakes affect fauna increasing animal mortality. The same effect do seaquakes have. Submarine tremors create powerful acoustic waves that kill fish and sea animals (Seismic hazards, 2000).


Evaluation of correlation between earthquakes and other natural processes
The matrix mentioned below shows the possible correlation between natural processes (table 1). Matrix lines contain earthquake-triggering processes. Matrix columns contain earthquake-triggered processes. Squares located in the intersection of columns and lines contain the parameters of correlation, or interaction. Interaction is explained through numerator and denominator of a fraction. Numerator indicates the frequency of correlation while denominator characterizes the intensity of such correlation.

Under frequency of correlation we imply the ratio of cases caused by the particular process to the total number of such cases; we use three-point scale for measuring that frequency. 3 points mean that the process plays the prevailing part in generating another process; 1 point means that the processes correlate rarely and faintly.

From this table it becomes obvious that earthquakes account for absolute majority of tsunamis. Hence, we recognize frequency of correlation for 3 points. At the same time, earthquakes do not contribute much to generating volcanic eruptions, therefore we estimate this correlation at 1 point.

The denominator indicates the intensity, or, to be more exact, the ratio of the intensity of triggered process to the intensity of natural process. Here we also apply three-point scale. We give three points to the process in case the intensity of triggered process is equal or close to the intensity of the natural process. For instance, the intensity of dangerous earthquake-triggered geomorphologic processes, namely landslides, rockfalls, mudflows, avalanches, taluses, is usually equal or close to the intensity of all geomorphologic processes regardless of their origin. In other words, the most powerful and large-scaled, say, landslides are usually earthquake-triggered.



Thus the correlation of processes can be all-embracing if both frequency and intensity are equal to 3 points. The interaction of earthquakes and tsunamis illustrates that. Correspondingly, the correlation can be weak in case both frequency and intensity are estimated at 1 point. The examples of that is the impact of earthquakes on volcanic eruptions (and vice versa) and the impact of karst collapses on earthquakes, etc.
TABLE 1. The matrix of correlation between earthquakes and other natural processes



Triggering processes

Triggered processes



Earthquakes

Volcanic eruptions

Tsunamis

Rockfalls

Taluses

Avalanches

Landslides

Submarine landslides

Mudflows

Rock streams

Internal waves

Seiches

Glaciers

Earthquakes




1/1

3/3

2/3

1/3

1/3

2/3

2/3

2/3

1/3

1/3

1/3

1/3

Volcanic eruptions

1/1





































Karst

1/1





































Rockfalls

1/1





































El Nino phenomenon

1/1







































Conclusion

Thus earthquakes are interlinked with many other widespread natural processes. In a sense, earthquakes can be considered as the ‘leading’ process. Under the word ‘leading’ we imply that earthquakes are the most ‘independent’ process since other natural processes contribute very little to earthquakes, both from the point of frequency of their emergence and the strength of their passing. At the same time, earthquake-triggered processes gain maximum possible strength. So, the intensity of dangerous earthquake-triggered geomorphologic processes, namely landslides, rockfalls, mudflows, avalanches, taluses, is usually equal or close to the intensity of all geomorphologic processes regardless of their origin. Undoubtedly, the issue of the correlation between earthquakes and other natural processes cries for further thorough research.



References

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Turner A.K., Schuster R.L. 1996. Landslides: investigation and mitigation. National Academy Press, Washington, D.C., USA.
Natural-antropogenic processes and ecological risk, 2004. Gorodets, Moscow, Russia.
Nisbet E.G., Piper D.J.W., 1998. Giant submarine landslides, Nature 392, 329-330.
Schuster R.L., Highland L.M., 2001. Socioeconomic and environmental impacts of landslides in the Western Hemisphere. Proceedings of 3 rd Panamerican Symposium on Landslides. 1. Cartagena, Columbia, 1-50.
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Vinogradov Yu. B., 1980. Sketches about mudflows, Gidrometeoizdat, Leningrad, Russia.
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