Tomales Bay to Bolinas BayBy G. K. Gilbert. The following data upon intensity were gathered, with slight exceptions, between April 26 and May 12, 1906. In their arrangement the order followed is: (1) The line of the fault from south to north; (2) the towns of the Rift belt; (3) the peninsula west of the Rift; (4) routes of travel east of the Rift; and (5) distribution. Along the Fault. — Mrs. Steele's farm buildings, near the head of Tomales Lagoon, stood in a very narrow fault-sag which was traversed by the fault-trace. At this point the trace consists of a group of cracks 10 to 20 feet broad. The barn, resting partly on the ground traversed by these cracks, was demolished so that, as I saw it, it lay in ruins. The house, standing only a few feet to the east of the fault, was thrown from its underpinning and a wing was partly separated from it. ― 192 ―
The buildings of E. R. Strain, 1.5 miles north of Bolinas Lagoon, stand about 20 rods east of the fault-trace, the house being on a hill and the other buildings on sloping alluvium at its base. The house did not leave its brick foundation, but the foundation was cracked. Chimneys were thrown down. The other buildings were thrown from their underpinning, moving eastward. Milking was in progress in the barnyard. Some cows were thrown down, and Mr. Strain himself was thrown to the ground, but rose and grasped a tree, of which he retained hold with much difficulty. Daniel Bondietti lives 3.5 miles north from the head of the lagoon, and his buildings are about 20 rods east of the main crack. His house was shifted 3 feet toward the fault and his barn moved in the same direction. Men engaged in milking were thrown in a direction away from the fault — that is, to the northeast — and cows were also thrown in this direction. At a barnyard near Bondietti's, and east of the fault, a milker was thrown to the west — that is, toward the fault. At Beisler's ranch, a short distance north of Bondietti's, the fault-trace is in two parts, of which the western or main part passes under the barn, and the eastern between the house and the barn. Mr. Beisler was milking a cow at a point within 6 feet of the west branch, and on the southwest side. He was thrown to the southwest, arose, and started to go in the opposite direction, when he saw the crack in the ground; he then turned and was again thrown, but with difficulty reached a fence 10 or 15 feet away before the end of the shock. His house and buildings were strained, but they did not collapse, and their shifting was slight. The greatest shifting was of the main part of his barn, which stood between the branches of the fault and moved about 2 feet to the northwest. A water-tank near the fault was shifted slightly but did not overturn. At both the Bondietti and Beisler ranches the surface of the ground has considerable slope and it is probable that bedrock is not far below the surface. The buildings of the Dickson ranch, 2.5 miles south of Olema, are about 0.25 mile east of the fault-trace, standing on a hillside presumably on firm ground. They nearly all slid southwest — that is, downhill and toward the fault. The barn, an old building, collapsed. At the Bloom place, a mile south of Olema, the buildings stand 30 or 40 rods east of the fault, and are on firm ground. The injury to buildings was here comparatively small. A water-pipe by which water was brought from a point on the opposite side of the fault was broken in many places, being at some points pulled apart and at others telescoped. At one place it buckled so as to project several feet above the ground. After being repaired, the pipe was found to be shorter than before, the difference being estimated at about 5 feet. I did not examine the course of the pipe, but from its general direction I infer that it crost the fault obliquely from south to north, and that the shortening was the direct result of the horizontal throw of the fault. Mr. Payne J. Shafter's place is near the village of Olema. The fault-trace is close to the house and other buildings. These stand on a bed of alluvium which is probably supported by bedrock at a short distance below the surface. In the barnyard men were milking, and were thrown violently to the ground, along with the cattle. The buildings were much damaged. During the earthquake a cow fell into the fault-crack and the earth closed in on her, so that only the tail remained visible. At the time of my visit the tail had disappeared, being eaten by dogs, but there was abundant testimony to substantiate the statement. As the fault-trace in that neighborhood showed no cracks large enough to receive a cow, it would appear that during the production of the fault there was a temporary parting of the walls. Mr. Skinner's ranch is 0.5 mile west of Olema and on the line of the fault. The trace passes within about 10 feet of the house and within 2 or 3 feet of the dairy, and runs under a portion of a large cow-barn. The house stands southwest of the fault-line, and is on the block which moved northwest. The house itself was shifted northwest with reference ― 193 ―
to the ground. (See fig. 22.) A granary standing 100 feet farther west than the house was shifted southward about 3 feet.
The movements of the house and granary were thus in nearly opposite directions. The dairy remained on its foundations. The
barn was not shifted on the earth block supporting its greater part, but was dragged along over the other block. Movables
in the buildings were thrown about with violence; dishes, etc., were broken; but no buildings were destroyed and all were
afterward repaired and used. A circular water-tank standing on a trestle about 12 feet high, approximately 100 feet northeast
of the fault, was uninjured, and seemed to be absolutely undisturbed. In the barn-yard, which was traversed by the fault,
cows were assembled and several men were engaged in milking. Cows and men were all thrown to the ground, the direction of
their fall being northeastward and away from the fault. This direction was also downhill.
The road from the Skinner place to Olema crosses a small creek, and near the bridge is a deep pool. Water from this pool was thrown out to the southwest, being carried across the road a total distance of 3 or 4 rods. Bolinas. — At the south end of the peninsula is a sloping plain carved by the sea when the land stood lower than it now does. Its general form and relations are shown by the contours of the map, fig. 10. This plain originally extended at least as far as the shore of Bolinas Lagoon, but east of Paradise Valley it has been modified by changes associated with the Rift. The line of Paradise Valley, when extended southeastward parallel to the fault-trace, marks approximately the limit of the Rift in that direction, and all the land between it and the fault-trace is broken into blocks which have been diversely faulted and tilted. As some of these blocks retain the smooth upper surface which they received as parts of the plain of marine denudation, their present attitudes serve to express the nature of the dislocations. Two small blocks facing the southern part of Bolinas Lagoon retain approximately their original height, but are tilted at different angles toward the northeast. A third block, too narrow to be caught by the map contours, has dropt 50 feet lower and is tilted at a still higher angle toward the northeast. A fourth and much larger block, itself involving minor dislocations, slopes southward from a point opposite the head of Paradise Valley to the delta of Pine Gulch Creek. The upper part of the village of Bolinas lies in a curving fault-sag among these dislocated blocks, and another portion stands on the delta of Pine Gulch Creek. In the fault-sag, where the ground was much cracked, nearly all the houses were either shifted on their foundations or else thrown from their foundations. There was great destruction of furniture and other breakable articles. In some cases people were thrown from their beds, but none were seriously injured. Three buildings which had stood on stilts along the shore of the lagoon were tipt toward it so that their lower edges came within reach of the tide. Several buildings were so badly injured that they were afterward torn down by their owners instead of being repaired. Just outside the fault-sag, and only a few rods distant, a group of houses stand on higher ground, and these were comparatively uninjured. They were not moved on their foundations, and in one instance the chimneys were not thrown down. In the northern part of the town, standing on the delta of Pine Gulch Creek, about half the buildings were thrown from their foundations, and here also the destruction was greater on low flat land than on higher ground. Olema. — The village of Olema is about 0.5 mile east of the fault-trace and at the edge of the Rift belt, the greater part being included within the Rift. The residence of Mr. Peace, standing on alluvium, was shifted south about 2 feet, falling from its supports. It was very badly wracked, and was eventually torn down. A neighboring piece of alluvial land bordering Olema Creek sank about 2 feet. The hotel owned by Mr. Nelson, standing on higher ground, was somewhat wracked but was not shifted. A house next door moved 2.5 feet to the northeast. A house opposite moved 2 feet to the northwest. Another house opposite fell from its supports, moving southwest. A neighboring stable ― 194 ―
was wracked so as to lean to the southwest. A church moved 3 feet to the southwest, that direction being downhill. Probably
half the houses in the town were not shifted from their foundations. Of two bridges over Olema Creek, one was shaken to pieces.
A lady in the hotel was thrown from her bed by the shock.
Point Reyes Station. — The village at the railroad station of Point Reyes is about 0.5 mile northeast of the fault-trace, and stands on a low bench of apparently firm ground. It is probably just outside the Rift belt. The schoolhouse, a 2-story building standing on a brick foundation wall, was shifted 2.5 feet to the south. A stone building used as a store was thrown down, the walls falling toward the southeast. The hotel barn was shifted 20 inches toward the south and a few other buildings were shifted, the distances, so far as observed, being less. Brick chimneys were generally thrown down. A large shed was wrecked. In all buildings furniture was shifted, objects on shelves were thrown down, dishes were broken, etc. An engine and three cars standing on the track were overturned toward the southwest. A long wood-pile was thrown down toward the southwest. Inverness. — Inverness is a village of summer residences on and near the southwest shore of Tomales Bay. The upland of the peninsula there closely approaches the bay. The village occupies two narrow valleys normal to the shore, and a mesa between them. Its site is within the Rift, and both valleys and mesa were traversed by many cracks, of which some had the character of branch faults. All the houses were of wood. About half of them were shifted on their foundations. To a certain extent the direction of shifting was determined by the slopes of the ground, the houses moving downhill; but where that factor did not control, the movement was toward the west or southwest. In one instance I noted a southwestward movement of several feet uphill. A few houses in the southern or "first valley" near the beach were demolished, or so badly injured as to be torn down. Several houses on the mesa were so badly injured as to require practical reconstruction. As the most serious injury was to houses thrown from their foundations, it is probable that the jar of falling was an important factor. It is related that a number of persons were thrown violently from their beds, but there were no serious personal injuries. Of a series of bath-houses standing on the beach, some remained unmoved; others were tilted because of the yielding of their slender supports; and one was turned over on its side without the breaking of the pins on which it stood. It fell to the northwest. A water-pipe following an east-west (or northeast-southwest) road on the mesa, and buried about 1 foot, was buckled at two points so as to be lifted above the ground. I saw no earth-cracks near these points. (See plate 71A.) The phenomena connected with five water-tanks seem worthy of special mention, because the simplicity and symmetry of the structures were such that the directions of displacement must represent closely directions of earth movement. A large tank containing water for the village supply stood on the mesa about 0.5 mile from the shore of the bay, its foundation rising a little above the ground. It was thrown in a direction almost due west and completely demolished, the planks and staves constituting its sides and bottom being strewn over a space of 50 feet. (Plate 72A.) The other four tanks were situated along the base of the hill between Inverness and the head of the bay, and held water for sprinkling the road. Each one stood on a square pedestal of braced timbers about 10 feet high. The tank nearest Inverness fell to the west, its pedestal yielding and being crusht. (Plate 72B.) The next fell to the southwest, and tank and pedestal were both crusht. The third was shifted 4.5 feet westward on its pedestal, both tank and pedestal remaining uninjured. The pedestal of the fourth stood unchanged, and the tank was thrown from it toward the west-northwest, being overturned as it fell. (Plate 71B.) Inverness to Point Reyes Light-house. — For the first 2 miles of travel, covering a right-line distance of about 1.5 miles, road-cracks were numerous and often large. There were also numerous small falls of earth from the road cliffs. Beyond that point there was a rapid falling off of such evidence, and tho road-cracks were frequently seen they were all ― 195 ―
small. In the neighborhood of Limantour Bay (indicated on some maps as Drake's Estero) there are a number of ranches. Most
of these showed broken chimneys; but at a ranch west of the head of the bay 2 brick chimneys stood uninjured. At Point Reyes
Post-office, the main residence building was thrown from its foundation of props and shifted 2 feet westward, being badly
wrecked. Other buildings of the same group were not shifted, and 2 water-tanks on high frames seemed to be uninjured. At Mr.
Claussen's ranch, south of the Post-office, 2 buildings were shifted a few inches to the south, that direction not being determined
by their structure but being diagonal to their sides. The chimneys were thrown down, plastering cracked, furniture shifted,
and many dishes broken. A picture was reversed so as to hang face to the wall. Mr. Claussen, being out-of-doors at the time,
was thrown down. Some cows were also thrown down.
At the U. S. Life Saving Station, on the coast 3 or 4 miles from the light-house, brick chimneys were broken but not thrown down, furniture was moved, dishes were broken, and the filled ground about the house settled several inches. A mast standing in the sand was said to have been heaved up several feet, but its position had been restored before my visit. My informant said that he was standing when the shock came, and sat down to avoid falling. At Point Reyes Light-house the heavy mechanism controlling the light was shifted several inches on its base. A lens "jumped" from its ways. It was so held in place by dowel pins that its movement required a lift of about 2 inches. The only injury to buildings was from the cracking of chimneys. Wooden tanks with water were not shifted. One of the light-house keepers stated that after the shock he lookt from the window of his room, which commanded a portion of the sea near the beach, and saw the water "boiling," but there was no change of the nature of a wave. Sunshine Ranch and Vicinity. — I drove to the summit of the ridge southwest of the head of Tomales Bay, finding abundant and strong road-cracks all the way to the crest, which is about 1.5 miles from the fault-trace. There were also a number of landslides in this region, and a considerable number of trees were broken or uprooted. There were few houses. The only ranch visited, known as the Sunshine Ranch, and occupied by Mr. Silver, suffered as severely as the houses of Inverness and Bolinas. The house moved southwest 3 feet and was badly wracked. The dairy was thrown from its foundation and wrecked beyond repair. The barn, a large building, fell northward downhill and collapsed. Bear Valley. — I drove from Skinner's ranch southwestward thru a pass in the upland, covering two-thirds of the distance to the coast, and reaching a point about 3.5 miles in a direct line from the fault. The most striking evidence of violence was shown by the trees. A few were thrown down, including oaks and spruces; branches were broken from others and some spruces had lost their tops. Most of these phenomena were seen within 0.5 mile of the fault. In the same region are a few summer cottages, which sustained little injury, only the fall of chimneys being noted. The club-house of the Country Club, situated about 1 mile from the fault, lost chimneys but was not shifted. One of its barns was wrecked, falling downhill in a southerly direction. In this region I saw only a few cracks other than road-cracks, and the road-cracks were unimportant. Seven Lakes. — Crossing the main divide of the peninsula near the head of Pine Gulch Creek, I followed a road to the vicinity of the coast, a district known as Seven Lakes. As the trip was made 5 months after the earthquake, the evidence from road-cracks had disappeared. There were a few landslides, and a number of cracks already mentioned (page 75) testified to movements of large blocks of ground; but I think these were due to a peculiarly sensitive condition of the country rather than to the violence of the shock. At 2 ranch-houses not far from the ocean, chimneys were broken but buildings were not shifted. A few dishes were thrown down, but otherwise there was no injury to movables or houses. ― 196 ―
West of Bolinas. — Driving 2 miles west of Bolinas, and looking at buildings from the road, I saw very little evidence of injury. At a distance of about 0.5 mile from the fault a chimney was broken at the roof, but not lower down. North of Point Reyes Station. — I drove a few miles north and east from the station, over a high terrace separating the upland from the bay at the east. The injury to buildings was found to be much less there than at the station, and not all chimneys were thrown down. A large barn was seen to lean as tho some of its props had given out; two water-tanks were wrecked. A few cracks were seen in the ground, but they were much smaller and less numerous than at a similar distance on the opposite side of the fault. Sausalito to Point Reyes Station. — Observation was made only from the car-window. The towns from Sausalito to Fairfax showed no damage more serious than the loss of a portion of the chimneys. The same remark applies to buildings seen along Papermill Creek as far as Garcia. Beyond Garcia the creek has several reaches of alluvial bottom, and some of these were so badly shaken that the railway embankments and trestles had to be repaired. Railway traffic to Point Reyes was interrupted for about 10 days. Ross to Bolinas. — This road was driven over 8 days after the earthquake. In the village of Ross houses were not shifted. The principal injury is to brick chimneys, of which probably more than one-half fell. A group of stone buildings on a hill lost heavy stone chimneys, and there was injury to a tower. Some stone fences on alluvial ground were in part thrown down. These fences were of undrest stone, loosely piled. In San Anselmo most of the brick chimneys were broken, but other injuries in that town and in Fairfax appear to have been slight. Along the road from Fairfax to Bolinas Ridge, the only evidence of the earthquake consisted of small road-cracks, with occasional stones fallen from the road-cuts. These evidences of moderate disturbance continued down the western slope of Bolinas Ridge to the edge of Bolinas Lagoon. A house standing in the middle of the valley, probably 0.25 mile from the main fault, showed from a distance evidence of considerable disturbance. Its chimneys were broken, the house itself had probably been shifted on its foundations, and one of the outhouses was out of plumb, apparently having slidden downhill toward the northward. The house was not visited, but was merely seen from the road. The general fact brought out in this drive was that the region about Ross and Fairfax experienced a shock comparable with that at Berkeley, and there was no evidence of high intensity until the fault-trace was closely approached. Landslides were not seen east of the lagoon, and the road-cracks east of the lagoon were not important. Mill Valley to Bolinas. — At Mill Valley the visible injury was chiefly to chimneys. Extended enquiries were not made; but no reports were heard of destruction to furniture. The houses were not shifted. The buildings at West Point, on the Tamalpais Railway, did not suffer; and I was told that there was no injury from the earthquake at the hotel on the summit of the mountain. From crags on the south slope of Mount Tamalpais, stones were detached and rolled down the slope. The same thing occurred near Willow Camp. From West Point to Willow Camp there are no buildings, road-cracks were small, and no landslides were seen. A few stones fell to the road from the side of the roadcut. A ranch 0.5 mile east of Willow Camp showed no injury to buildings. At Willow Camp all brick chimneys fell, several houses moved a few inches toward the southeast, and dishes were thrown from shelves. A tall house 0.5 mile to the northeast was apparently not disturbed, and retained its brick chimney. Farther up the shore of the lagoon, and nearly opposite Dipsea, some farm buildings seemed to have been so disturbed as to be thrown out of plumb. They were not visited. At Dipsea 2 summer cottages were moved a few inches to the southwest, or were wracked in that direction. The hotel was swayed in the same direction, but the building withstood the shock. The barn, a rather large building, was thrown from its underpinning, falling toward the lagoon. ― 197 ―
Distribution. — The variation of intensity with the character of the geologic formation is evident at various localities, but most conspicuously at Bolinas, where the destruction on alluvium at the bottom of the little valley was very much greater than on the hills immediately adjacent. Nevertheless, the data are not sufficiently full for a satisfactory discussion of this phase of the distribution of intensity, and I have therefore tried to make allowance for differences of formation, and in that way obtain a general conception of the distribution of intensity with reference to the fault and the Rift. The intensity was greatest on the line of the fault, but did not diminish rapidly toward the east and west within the Rift belt. In a general way the intensity was greater in the Rift belt than on either side. On the east it fell off rapidly — almost suddenly — at the limit of the Rift. On the west it fell off gradually, being nearly as high at a distance of 0.5 mile or 0.75 mile from the rift as at the edge of the Rift. In a general way the intensity west of the Rift was greater than at the east. My conception of the distribution on a line normal to the Rift is exprest by the following curve (fig. 50), but this should not be subjected to measurement, as its elements are not definitely quantitative. It is a generalization from data that are heterogeneous and by no means complete. In a general way the distribution of high intensity follows the distribution of bedrock cracks. Inverness, where the injury to structures on firm ground reached a maximum, is traversed by important bedrock cracks, some of which are to be accounted as branches of the main fault. The high ridge west of the main valley, over which the intensity was nearly as great as along the Rift, was also characterized by many important bedrock cracks, and by a general derangement of the underground circulation of water. The district east of the Rift, where the intensity rapidly diminished, was practically exempt from bedrock cracks, and its underground circulation was not disturbed. Notes by other observers (R. S. Holway). — A bridge about 0.75 mile southeast of Point Reyes (toward San Francisco) went completely down, causing several days' delay to trains. The track had had several horizontal bends of a few inches. The "fills" across the arms of Tomales Bay generally sank from 2 to 8 feet. The 1,000-yard fill about 2 miles north of Point Reyes Station sank from 6 to 8 feet; as did the next fill, which is some 500 feet long. In one or two instances the pile-supported bridge in the middle of the fill remained at grade. Just above Hamlet a trestle-work which had been filled in settled, leaving the trestle-work some 2 feet above. The bottom of the bay in these arms is usually sand. At Hamlet quite an extensive landslide has started in the hillside above the track. The railroad cut is in old rock, and the arch of the head of the slide is some 70 feet above the track. The country wagon road has been carried away by the slide for possibly 100 yards. Miss Margaret Keating, a teacher at Marshall's, just at the close of the earthquake saw two waves coming from the opposite side across the Bay; that is, the length of the wave was parallel to the main Rift. The waves were from 6 to 8 feet high. The waves came nearly to the top of the trestle, and also up to certain willows which she indicated, both points roughly indicating a wave of the height she mentioned. At Marshall's a hotel and a stable built on the west side of the track and on underpinning, resting in the tidal flat, went easily and gently into the bay. The occupants of the hotel did not realize that the hotel had fallen, but at first throught the water had risen. At the post-office store goods were thrown from the west wall, but scarcely at all from the east. ― 198 ―
George H. Covert, of Cypress Grove, about 0.5 mile north of Marshall's states that on the morning of April 18 he saw a wave 8 to 10 feet high, and white-capped, come broad-side on to the east side of the bay immediately after the shock. That is, the wave-crest was parallel to the axis of the bay. The ground has a gentle slope here, and the wave did no harm. Mr. Covert gave a clear, intelligent account, and fully corroborated the testimony of the teacher at Marshall's. The island in the bay nearly opposite Hamlet was visited, but no sign of the fault was found. Tomales Point, west of the bay, was crost at the "Gum Trees." Small landslides were found on the bay shore on the ocean side of the point at various places. On the peninsula no cracks were found. At one place on the ocean shore a projecting granitic, rocky spur was much crusht and ground in the narrow neck connecting it with the mainland. The spur is about 30 feet high and 50 feet long. Prof. E. Knowlton gave an account of the damage caused by the earthquake at Bolinas and vicinity in the public press, extracts from which are here quoted:
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