STUDIES MADE BETWEEN ARAUCO AND VALDIVIA WITH RESPECT TO THE EARTHQUAKES OF 21 AND 22 MAY 1960

ABSTRACT

The earthquake of 21 May was felt as a brusque horizontal movement; the large shock of 22 May was felt as a suave movement of large amplitude similar to a ship at sea. Lebu was extensively damaged. The shoreline at Lebu was elevated 1.5 meters, rendering docks and waterside facilities useless; a large expanse of new beach was exposed. Rio Lebu is no longer navigable because of the uplift. Details of tsunami are reported for several towns on the Arauco Peninsula and south to Tolten.

INTRODUCTION

In connection with the earthquakes that occurred on 21 and 22 May 1960, the author, in company with Roberto Morales and Arturo Valenzuela, was sent to make observations of the effects of the earthquakes in the coastal region between Arauco and Valdivia. Observations were also made of the magnitude of the maremoto at different points on the coast. The commission, directed by the author, left Santiago on the 25th of May, stopped overnight in the locality of Los Angeles (110 km east of Lebu) and went to Lebu, passing by Canete (figure 1). Leaving Lebu, 3 days were spent traveling to Arauco and Lota, briefly visiting Curanilahue and Pilpilco. Later, the commission continued south, to Pto. Saavedra, Tolten (figure 2) and Valdivia.
All persons interrogated agreed that the earthquake of 21 May had brusque, ample movements of a horizontal variety, and the earthquake of 22 May was of suave, large movements such as those of a ship on the high seas, and was of a long duration. All of the earthquakes thereafter had more or less brusque, horizontal movements. By personal observations of aftershocks during our stay in Lebu, the preferential direction of the initial movements of the P waves was N10degreeW.
Little destruction was observed in Collipulli, 90 km east of Canete. In the Fundo el Salto, located 16 kilometers to the southeast of Collipulli, the movement of 21 May had a preferential direction of N 15ーW, as indicated by the displacement of furniture in the house of the patron, the fall of the chimney and of the north wall of the house. The owner of this fundo described the seism of 21 May as undulatory, with large horizontal movements. He reported seeing waves in the surface of the "trumao" (a local name for the reddish clay-like soil produced by the weathering of the underlying rock, characteristic of the south of Chile).

LEBU AREA

The commission arrived at Lebu on the 27th of May and stayed in the house of the administrator of the Victoria Coal Co. of Lebu. This house did not suffer much damage. It was constructed over concrete foundations and located on a slightly inclined terrain of sedimentary rocks of Tertiary age. The walls were panels of a network of laths covered with clay and papered. From some of the walls, the mud and paper fell; and on others, one could only see cracks in the paper, which crossed the horizontal at 45 degrees. The fall of tiles and chimneys was principally toward the east, and the second story of the house had moved 20 to 30 centimeters (with respect to the first floor) in a northerly direction (figure 3.)
In Lebu, there was considerable destruction (figure 4-7), and only the new buildings did not incur damage; for example, the new school.
The most important effect of the earthquakes in the Peninsula of Arauco was the elevation of the continent at Lebu of 1ス meters. This elevation was measured on a post that serves to indicate the tides; the post had been established for many years in firm rock of Tertiary age. The uplift manifested itself in the following manner: (1) The level of the present high tide is now located at the level of the former low tide, the difference between the levels being about 1ス meters, and (2) the sea has retired from the beach, leaving uncovered a new beach of an average width of 50 meters all along the coast of the bay of Lebu (figure 8-10). In Arauco, one could not measure this uplift exactly, but on the coast of Lota a continental uplift of 40 to 50 centimeters was noted. To the south of Lebu, some 25 kilometers from Morguilla, it was noticed that the level of the sea was 1.5 to 1.8 meters lower than before the earthquakes. This calculation was based on a measurement, by Brunton compass, between the level of the present high tide and that of the high tide before the earthquakes. The beach is 20 to 30 meters wider than before.
On the beach, polygonal cracks were observed in the sand, especially in places where trees grew. These cracks were aligned in a general direction N30degreeE and N50degreeW. In a poorly cemented conglomerate, probably of upper Tertiary age, with round clasts of some 2 centimeters in diameter, numerous landslides with a few cracks N20degreeE to N30degreeE were observed. Some 3 to 5 kilometers to the interior from this part of the coast, in the Fundo Quinahue, there were cracks in the "trumao" of several hills; they were found on the western side of the summits and were in line in the north-south direction. They had directions that varied between N10degreeE and N10degreeW, with a variable extension between 50 and 200 meters, widths 1/2 of to 30 centimeters, irregularities in their surface of 1 to 15 centimeters, and depths of perhaps 50 centimeters.
On the northern edge of the beach of Lebu there were large landslides along the cliffs in rocks of Tertiary age. The effects of the tidal waves on these also may be observed. One is able to see that the water reached a height, of 5 or 6 meters above the normal level.
The Lebu River was visibly affected by the uplift of the continent: it is shallower than before, and boats are no longer able to navigate the river, a thing that they did before without difficulty. The north half of the bridge (direction N30degreeE) over the the Lebu River was displaced 10 centimeters with respect to the other half, measured at the central junction of the bridge. (The sense of displacement was not recorded.). In the mouth of the Lebu River, there were cracks in the recent fill with the directions N65degreeW and N45degreeW.Some of these cracks were about 1 meter in depth, but their width was not more than 5 centimeters.

CURANILAHUE

In Curanilahue, there was a much higher percentage of destroyed buildings. All the ancient buildings suffered considerable damage, but new constructions were not affected by the earthquakes. The bridge at the entrance south of Curanilahue was cracked, and its base of concrete has fallen towards the east.

PILPILCO

Pilpilco did not incur much damage, and the mine there is functioning almost normally. There were landslides in the hills but without any serious consequences, and the activity of the region was normal.

ARAUCO AREA

About 8O% of the construction in Arauco incurred some damage (figure 11). Nevertheless, and in spite of the strong seismic movements of the earthquake, statues and posts in the city remained standing. On 21 May the movement was of preferential direction N60degreeE, and also perpendicular to that direction. The inhabitants of Arauco indicated that there was a brusque vertical movement. The destruction was caused by the horizontal movements, which threw down walls, roofs, and furniture. In the government house a heavy strong box fell from its pedestal.
In Arauco, the sea wave occurred on Sunday, 22 May, at about 1830 hours. The sea advanced slowly for a distance of 700 to 1,000 meters up the extensive beach without any destruction of houses, which were a considerable distance from the beach. The water only flooded the floors of the houses, without carrying them away. In this locality, a previous retreat of the sea was not observed as in the rest of the localities visited. Nevertheless in Llico, all the buildings were destroyed; and in Laraquete, there was about 70% destruction according to information obtained from the Carabineros and from the governor.

PUERTO SAAVEDRA AREA

The commission went to Puerto Saavedra on 30 May. This village was totally washed away by the waters of the sea wave of 22-May, but with little loss of life or injury because of the slow onset of the first wave, permitting the populace to seek higher ground. There were three advances of the sea, with the third carrying away the houses and animals. The sea flowed approximately 5 kilometers inland, carrying several houses for 3 or 4 kilometers and leaving them in the middle of the fields. The water reached a level of approximately 2(1/2) meters above its natural level (figures 12-14). The off-shore bar, or spit, that deviates the flow of the Rio Imperial from the original east-west course to one north-south was broken by the tidal wave in the northern part where it was joined to the continent, leaving the southern part transformed into an island. (Translator's note: For a more complete description, see report by Wright and Mella.)

TOLTEN AND QUEULE AREAS

In Tolten, where the water ran about 3 meters above the normal level of the river, there had occurred seven waves of the water. The first rising was at about 1545 hours, Sunday. The second, at 1645 hours, was the biggest, and it flowed inland approximately 4 kilometers from the coast to the locality of Callulfi. This produced inundations in the Fundo los Boldos (figure 1.5), which ruined the road that leads to Queule (figure 16, a coastal pueblo that was completely carried away by the water. In the Fundo los Boldos, water remained, on 31 May, at a depth of about 1/2 meter.
In the alluvial terrain that surrounds Tolten, a few cracks, some 15 centimeters wide, were produced in the preferential direction N25degreeE, with some minor cracks perpendicular to them (figure 17). The swinging bridge between Queule and Tolten, across the Rio Tolten, failed because of the displacement of its foundations produced by the movement of the soils affected by the earthquake. The reinforced concrete pillars of a firm mix were bent about 30 centimeters from the base. Fragments spalled off, leaving the twisted reinforcing irons visible (figure 18). The commission could not go to Queule because of the disappearance of the road below the waters, and because launches were not available for crossing at this time.

VALDIVIA AREA

On Tuesday, 1 June, the commission arrived in Valdivia. In this city, the destruction was produced principally along the edges of the rivers Valdivia and Calle-Calle. There were numerous landslides in the direction of the river from the banks (figures 19-22), and many houses were put out of plumb by compaction of the sediments over which the city was constructed. The terrain in the center of the city sunk l(1/2) to 2 meters. The bridges that cross the rivers Valdivia and Calle-Calle were destroyed at their extremities, because the concrete of which they were constructed failed; in part because of tension produced by the seismic movements, and in part because of the sliding of the margins of the rivers, so that the pillars were tilted, creating a horizontal component of tension that destroyed the concrete. The pillars in the center of the bridge Pedro de Valdivia were not damaged, nor were those of the bridge Calle-Calle (figure 23).
The great drinking-water tank, which is about 100 meters from the regional hospital, and which fortunately was empty during the earthquake, showed shear cracks in the structure of its pedestals. These shear cracks crossed each other at approximately 30-degree angles. At a level about 1/5 the height of the tank, there were many horizontal cracks.
In Valdivia, the level of the river increased about 1(1/2) to 2 meters, probably caused by the sinking of the continent. The cracks that were seen in the lands near the river were caused by the sliding of water impregnated sediments by the impulses of the earthquakes.

CONCLUSIONS

The most outstanding effects of the earthquakes were in the coastal region between Arauco and Valdivia. The great coastal block moved, as on a hinge, with an uplift on the Peninsula of Arauco of about 1.3 to 1.5 meters and, to the south, a sinkage of about 1.5 to 1.8 meters on an east-west axis, located between Lebu and Pto. Saavedra.
Superficial evidence of active faults was not observed.
The destruction of buildings increased from north to south, betwen Chillan (175 km northeast of Lebu) and Valdivia. The destruction was greater in buildings recently constructed over unconsolidated sediments, artificial fills, and inclined lands such as on the banks of the river and on the coast. The buildings on firm rock and flat land did not suffer greatly.
On the Mercalli scale, the intensity of the greatest earthquake of the series was approximately IX in Valdivia, VIII in Lebu, VIII in Curanilahue, and VII in Arauco. In the localities washed away by the waters, it was impossible to calculate the intensities. Manuscript received on February 6, 1963.