U.S. DEPARTMENT OF THE INTERIOR U. S. GEOLOGICAL SURVEY
THE SCOTTS MILLS, OREGON, EARTHQUAKE OF MARCH 25,1993: INTENSITIES, STRONG-MOTION DATA, AND TELESEISMIC DATA
by
James W. Dewey *, B. Glen Reagor*, Dennis Johnson ^, George L. Choy *, and Frank Baldwin*
Open-File Report 94-163
National Earthquake Information Center U.S. Geological Survey, MS 967 Box 25046, Denver Federal Center Denver, CO 80225
2 National Strong-Motion Program U.S. Geological Survey 19604 Pacific Highway So. Seattle, WA 98188
This report has not been reviewed for conformity with U.S. Geological Survey editorial standards and stratigraphic nomenclature.
1994 CONTENTS
Introduction...... 1 Seismotectonk Framework...... ^ Modified Mercalli Intensity Scale of 1931...... 2 Description of Earthquake Effects ...... 3 Strong Motion Records...... ^ Focal Mechanism And Depth Determined from Digital Teleseismic Data...... 12 References Cited...... ^
FIGURES
1. Seismicity of western Oregon and southwestern Washington, from 1872 through the occurrence of the 1993 Scotts Mills earthquake ...... ^ 2. Isoseismal map for the Scotts Mills, Oregon, earthquake of March 25,1993 ...... 4 3. Near-source intensities and near-source sites at which U.S. Geological Survey Cooperative Strong-Motion Network accelerographs were triggered in the Scotts Mills earthquake ...... 5 4. Accelerograms from theUSGS Cooperative Strong-Motion Network...... l5 5. Strong-motion record obtained from the Oregon earthquake of November 6,1962...... 24 6. P-wave displacements recorded at stations of the Federation of Digital Seismograph Networks...... 25 7. Observed P-wave displacements together with synthetic displacements...... 26 8. Focal mechanism of the Scotts Mills earthquake, showing the distribution of data used to construct the focal mechanism...... 26
TABLES
1. Earthquakes that produced intensities of VII and earthquakes that triggered strong-motion accelerographs within 100 km of the 1993 Scotts Mills earthquake...... 2 2. USGS strong-motion data for the Scotts Mills, Oregon earthquake of March 25,1993,13:34:35.4 UTC...... 13
This is the first of a series of reports on significant U.S. earthquakes to be produced jointly by the National Earthquake Information Center and the National Strong-Motion Program. We present basic observational data on the Scotts Mills, Oregon, earthquake of March 25,1993: source parameters, prior seismicity from the epicentral region, damage reports, isoseismal maps, strong-motion data, and digital teleseismic data. 1
INTRODUCTION their interpretation are presented in the section entitled "FOCAL MECHANISM AND DEPTH DETERMINED FROM DIGITAL TELESEISMIC DATA." On March 25, 1993, 5:35 A.M. Pacific Standard Time, western Oregon and Washington were shaken by a moderate earthquake centered near the community of Scotts Mills, Oregon. Property damage was estimated to SEISMOTECTONIC FRAMEWORK be about $28.4 million (Madin and others, 1993). Most economic loss resulted from damage to old unreinforced masonry structures. A reinforced-concrete and steel The hypocenter and magnitudes of the Scotts Mills highway bridge also suffered damage and was closed for earthquake are as follows (U.S. Geological Survey, several days. There was economic loss due to fall of 1993; depth as determined in present study). items from shelves and to closure of businesses in condemned buildings. Origin time: 13:34:35.4 UTC As part of a routine intensity-monitoring procedure, (05:34:35.4 PST) the Branch of Earthquake and Geomagnetic Information, Epicenter 45.035°N., 122.607° W. U.S. Geological Survey, sent out 460 mail questionnaires Depth: 16 km to post-offices, police stations, and fire stations at towns Magnitude: 5.5 mb (GS), 5.4 MS(GS), in and near the region within which the Scotts Mills 5.5 ML(BRK), 5.7 ML (SEA), 5.6 Mw (HRV) earthquake was felt. Two of us, B. G. Reagor and F. W. Baldwin, conducted a reconnaissance field trip in the region of strongest shaking. Damage in the region of The shock occurred in the Cascadia subduction maximum shaking corresponds to a Modified Mercalli zone, where the North American tectonic plate is being Intensity of VII. The earthquake was felt over an area of underthrust by the east-dipping Juan de Fuca tectonic approximately 97,000 square kilometers (37,500 square plate. The hypocenter lies within the North American miles). Intensity observations and their interpretation are plate; the Juan de Fuca plate is several tens of kilometers discussed in the section entitled "DESCRIPTION OF deeper than the focus of the Scotts Mills earthquake EARTHQUAKE EFFECTS." (Michaelson and Weaver, 1989). Shocks lying within the North American plate, similarly situated well above the The National Strong-Motion Program of the U. S. Juan de Fuca plate, have been observed elsewhere in Geological Survey collected records from five strong- northwest Oregon and western Washington (fig. 1, motion accelerographs that were triggered by the Ludwin and others, 1991; Yelin and Patton, 1991). earthquake and that were situated at distances of 43 to These shocks are probably related to each other in a 106 km from the earthquake's epicenter. The broad sense, perhaps corresponding to a region of high accelerograph in the upper gallery of the Detroit Dam crustal strain resulting from the subduction of the Juan (epicentral distance of 43 km) recorded a peak horizontal de Fuca plate beneath the North American plate. In acceleration of .18g. The accelerograph immediately detail, the earthquakes within the North American plate downstream of Detroit Dam recorded a peak horizontal occur on many different geologic faults. One dense acceleration of .06g. The strong-motion data are cluster of epicenters in figure 1 corresponds to shocks presented in the section entitled "STRONG-MOTION that accompanied eruptions of Mount St. Helens, but RECORDS". earthquakes at most other inland locations of the Pacific Northwest are not directly related to volcanic activity. Digital seismographs of institutions in the There is no evidence that the Scotts Mills earthquake Federation of Digital Seismograph Networks obtained was related to volcanic processes. on-scale recordings of the Scotts Mills earthquake at epicentral distances ranging from 74 km to over 10,000 km. Data from these stations imply that the earthquake Several previous earthquakes that have occurred nucleated at a depth of approximately 16 km and since the mid-nineteenth century produced intensity VII involved a combination of strike-slip and reverse effects within 100 km of the epicenter of the Scotts Mills faulting. The focal mechanism and focal depth implied earthquake (table 1). Intensities higher than VII have not by the digital data are consistent with the earthquake been documented within 100 km of the Scotts Mills occurring on the ML Angel fault, as had been suggested earthquake during the period since the mid-nineteenth previously by other investigators. The digital data and century for which information is available. SCOTTS MILLS, OREGON, EARTHQUAKE OF MARCH 25,1993
Table 1. Earthquakes that produced intensities of VII and earthquakes that triggered strong-motion accelerographs within 100 km of the 1993 Scotts Mills earthquake.
Origin Date Time Epicenter Depth Magnitude Source (UTC) (°N) (°W) (km) mb MS ML
1877 Oct. 12 2153..... 45.750 122.500 ...... 1 Intensity VII at Cascades and Portland, Oregon. 1949 Apr. 13 195542.0 47.167122.617 70 ...... 6.9 1 Intensity VII atHillsboro, North Portland, Oregon City, Oswego, Portland, Pratum, Sandy, and Shedd, Oregon. Intensity VII at Camasa, Orchards, Stevenson, and Vancouver, Washington. 1961 Nov. 7 012908.4 45.700 122.400 33 ...... 4.5 1 Maximum intensity VI. Triggered strong-motion accelerograph. 1962 Nov. 6 033643.6 45.642 122.588 16 ...... 5.2 1 Intensity VII at Portland, Oregon. Triggered strong-motion accelero graph. 1965 Apr. 29 152843.7 47.400 122.300 59 6.5 6.5 6.6 1 Maximum intensity VI at sites within 100 km of Scotts Mills, Oregon. Triggered strong-motion accelerograph. 1993 Mar. 25 133435.4 45.035 122.607 16 5.5 5.4 5.7 2 Scotts Mills earthquake
[Sources (hypocenter and magnitude): 1, Stover and Coffman, 1993; 2, U.S. Geological Survey, 1993, and this study]
MODIFIED MERCALLI INTENSITY SCALE OF 1931 (Abridged; Wood and Neumann, 1931)
I. Not felt except by a very few under especially favorable circumstances. n. Felt only by a few persons at rest, especially on upper floors of buildings. Delicately suspended objects may swing, m. Felt quite noticeably indoors, especially on upper floors of buildings, but many people do not recognize it as an earthquake. Standing motor cars may rock slightly. Vibration like passing of truck. Duration estimated. IV. During the day felt indoors by many, outdoors by few. At night, some awakened. Dishes, windows, doors disturbed; walls made cracking sound. Sensation like heavy truck striking building. Standing motor cars rocked noticeably. V. Felt by nearly everyone; many awakened. Some dishes, windows, etc., broken; a few instances of cracked plaster; unstable objects overturned. Disturbance of trees, poles and other tall objects sometimes noticed. Pendulum clocks may stop. VI. Felt by all; many frightened and run outdoors. Some heavy furniture moved; a few instances of fallen plaster or damaged chimneys. Damage slight VII. Everybody runs outdoors. Damage negligible in buildings of good design and construction; slight to moderate in well-built ordinary structures; considerable in poorly built or badly designed structures; some chimneys broken. Noticed by persons driving motor cars. Vm. Damage slight in specially designed structures; considerable in ordinary substantial buildings with partial collapse; great in poorly built structures. Panel walls thrown out of frame structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture overturned. Sand and mud ejected in small amounts. Changes in well water. Disturbed persons driving motor cars. DC. Damage considerable in specially designed structures; well designed frame structures thrown out of plumb; great in substantial buildings, with partial collapse. Buildings shifted off foundations. Ground cracked conspicuously. Underground pipes broken. X. Some well-built wooden structures destroyed; most masonry and frame structures destroyed with foundations; ground badly cracked. Rails bent. Landslides considerable from river banks and steep slopes. Shifted sand and mud. Water splashed (slopped) over banks. XI. Few, if any (masonry), structures remain standing. Bridges destroyed. Broad fissures in ground. Underground pipe lines completely out of service. Earth slumps and land slips in soft ground. Rails bent greatly. XII. Damage total. Waves seen on ground surfaces. Lines of sight and level distorted. Objects thrown upward into the air. DESCRIPTION OF EARTHQUAKE EFFECTS
126 124 120 Figure 1. Seismicity of western Oregon and southwestern Washington, from 1872 through the occurrence of the 1993 Scotts Mills earthquake. Earthquakes plotted are those with magnitudes 4.5 and larger and earthquakes for which no magnitudes were computed but for which intensities of VI or greater were observed. Almost all shocks lying inland of the coastline and having depths less than 40 km occurred in the North American (NOAM) plate. Shocks with depths exceeding 40 km lay within the Juan de Fuca (JDF) plate, as did most shallow offshore earthquakes. The mapped plate boundary shows
EXPLANATION where the JDF plate underthrusts DEPTH MAGNITUDE/MTENSITY the NOAM plate near the surface; c40km >40km from this surficial boundary the subducted JDF plate dips to the east beneath the NOAM plate. Seismicity data are from Stover and Coffman (1993) and the Preliminary Determination of Epicenters of the National Earthquake Information Center. 42 122 120
DESCRIPTION OF EARTHQUAKE that, in addition to being constructed of unreinforced EFFECTS masonry, most damaged buildings had other characteristics that made them vulnerable to earthquake Intensities assigned by the U. S. Geological Survey shaking: "unanchored gable end walls, tall story heights, are based on the Modified Mercalli Intensity Scale of interaction with adjacent buildings of different height, 1931 (Wood and Neumann, 1931). The abridged version and high parapets all were factors" (Bonneville and of the scale is given on the adjacent page; the scale is others, 1993, p. 5). Buildings constructed in recent reproduced in its entirety in the annual U.S. Earthquakes decades, including those of unreinforced masonry, publication of the U.S. Geological Survey and in Stover sustained only minor damage directly from the and Coffman (1993). We follow Stover and Coffman earthquake, although some were damaged by falling (1993) and U.S. Geological Survey practice of the last bricks from adjacent older buildings. In the residential decade and assign intensities of V and above principally parts of communities, the most characteristic type of on the basis of effects on human structures and contents damage was chimney damage. of structures. Area newspapers noted four cases of people needing The distribution of intensities associated with the medical attention for minor injuries suffered in the Scotts Mills earthquake is shown in figures 2 and 3. earthquake and three cases of people needing attention for inhalation of noxious fumes (see entry below under The most severe damage from the Scotts Mills Woodburn). People have been killed in other U.S. earthquake occurred to old (pre-1950), unreinforced earthquakes from falls of masonry similar to those that masonry buildings. Bonneville and others (1993) state occurred during the Scotts Mills earthquake; the absence SCOTTS MILLS, OREGON, EARTHQUAKE OF MARCH 25,1993
124° 122° 120° 118° Figure 2. Isoseismal map for the Scotts Mills, Oregon, earthquake of March 25, 1993. Roman numerals represent average Modified Mercalli intensities for the regions between isoseismals; Arabic numerals represent intensities in individual towns or zip-code regions. Squares represent towns identified on the figure. Shown also are three sites at which the U.S. Geological Survey Cooperative Strong-Motion Network accelerographs were triggered in the Scotts Mills earthquake. Peak accelerations in units of g (1g local acceleration of gravity, ~ 980 cm/sec2) are shown in parentheses next to the accelerograph sites. Multiple values are those recorded by different accelerographs triggered at that site (table 2). Near-source intensities and accelerograph sites are plotted in figure 3, which is an enlarged 6 FMtatlnlenftty6 view of the area within the box. F FWt O Not FWt Accetorograph (.07) acntonrtton of serious injuries or fatalities in the case of the Scotts Intensity VII Mills earthquake may be attributable in pan to few Oregon people being in or near damaged buildings when the Dayton (SH 18 bridge over Yamhill River)- One shock occurred end of a span dropped, leaving a 6 to 8 inch (15 to 20 cm) vertical offset between that span and the The U.S.G.S. Earthquake Report questionnaire asks adjacent span. The drop resulted from the toppling respondents about the presence and strength of earth of a pedestal on which the span rested (Bonneville noise that may have accompanied the earthquake. More and others, 1993). A car encountering the offset than half of the questionnaires returned from within the from the downdropped side had blowouts in all intensity VI and VII isoseismals reported strong or tires, and the driver nearly lost control of the moderate earth noise associated with the earthquake. For vehicle (press report). In addition to the damage intensity V and all lower intensities, more than half the due to the overturned pedestal, the asphalt respondents either explicitly reported not hearing noise approach to the bridge settled several inches due or did not answer the question about earth noise. to soil compaction. The bridge was given a temporary repair after having been out of service Descriptions of damage attributed to the press were for three days. taken from the Canby Herald, the Molalla Pioneer, the Newberg Graphic, The Oregonian, The Outlook Molalla- The Molalla Union High School South (Gresham), the Sandy Post, the Statesman Journal Campus building, constructed in 1925, suffered (Salem), the Wilsonville Spokesman, and the Woodburn structural damage to its brick and mortar walls. Independent. Bricks from the south and west gables were DESCRIPTION OF EARTHQUAKE EFFECTS
123.3 121.9 45.8 A EXPLANATION K Epicenter Figure 3. Near-source intensities and 7 Felt at Intensity 7 F Felt sites at which U.S. Geological o Not felt Survey Cooperative Strong-Motion (.06) Accelerograph ^^^acceleration) Network accelerographs were 4 triggered in the Scotts Mills earthquake. Roman numerals represent average Modified Mercalli intensities for the regions between isoseismals; Arabic numerals represent intensities in individual towns or zip-code regions. Squares represent towns identified on the figure. Peak accelerations in units of g (1g » local acceleration of gravity, -980 cm/sec2) are shown in parentheses next to the accelerograph sites. Multiple values shown for Detroit Dam are those recorded by three different accelerographs triggered at that site (table 2). MAP - Mount Angel fault.
Detroit Dam (.03, .06, .18) 5 5
KILOMETERS 44.6
shaken down; the gable walls were not attached to There was chimney damage in residential the wood frame of the gable roofs (Bonneville and districts, as well as minor cracking of interior others, 1993). The west gable bricks fell onto an walls and items shaken off shelves or walls. Most entrance porch, in an area where students usually instances of chimney damage were associated congregate before and after school. School with older, two-story, wood-frame houses. hallways were littered with bits of acoustical tile Damage included chimneys being separated from and plaster, and there were cracks in the stairwells houses, chimneys sustaining cracks or having (press report). Cement-block maintenance bricks fall from them, and chimneys shaken down buildings behind the school building were also to roof line. Some sidewalks and roads sustained cracked (press report). large cracks or were buckled; some masonry fences or retaining walls were cracked or destroyed; some windows were broken out; heavy Several unreinforced masonry commercial furniture was displaced; light furniture or small buildings sustained structural damage. Exterior appliances overturned; many glassware items or walls of the 44-year-old Odd Fellows Hall dishes broke; many small objects overturned and became detached from the roof and tilted outward fell; residents experienced difficulty walking or over adjacent buildings; authorities ordered the standing. hall and two adjacent buildings closed. Bricks and mortar fell from the walls of a three-bay carwash. Mount Angel- Saint Mary's Catholic church Eight businesses located in or adjacent to sustained damages estimated at $4 to $6 million damaged buildings were ordered temporarily (press report). Exterior walls were cracked, as closed (press report). were buttresses; the 200-foot high bell tower SCOTTS MILLS, OREGON, EARTHQUAKE OF MARCH 25,1993
listed about 5 degrees from vertical (press report); Silverton- Several chimneys were cracked, lost a stained-glass window cracked The church was bricks, or were broken at the roof line; masonry built in 1912 and had been declared a national exterior walls cracked or bulged outward; many historic landmark. items were thrown from store shelves; many glassware items or dishes were broken; many Elsewhere in town, many chimneys were small objects overturned and fell; windows were cracked or had bricks fallen from them; many broken out; a few people ran out of buildings. items were shaken off store shelves; many glassware items or dishes broke; some windows Woodburn- Three people received medical attention were broken out; many small objects overturned as a consequence of exposure to noxious fumes at and fell. the Woodburn Wal-Mart; the fumes arose from spilling and mixing of garden chemicals, paint, Newberg- Loss from earthquake damage was and battery acid (press report). The tops of preliminarily estimated to be about $700,000. unreinforced brick exterior walls fell. Brick third- Damage was heaviest in the old part of town story walls of the Salud Medical Center collapsed along First Street; at least 14 buildings were and fell through the second story ceiling, forcing appreciably damaged, among them the Francis temporary closure of the center. Structural Theatre building, the Bank of America building, damage was heaviest along Front Street, where the Newberg Fitness Center, and the Portland authorities closed seven buildings because of General Electric building. At George Fox College, quake damage (press report). Wood-Mar Hall had fallen plaster; three sets of library shelving collapsed. Residents of a four- Many chimneys were cracked, lost bricks, or unit apartment building were evacuated, (press fell; interior walls sustained large cracks; reports) underground pipes cracked; masonry fences or retaining walls were cracked; some windows were Interior walls sustained large cracks; broken out; many items were shaken off store sidewalks and roads sustained large cracks; shelves; many glassware items or dishes broke; underground pipes were broken; masonry fences many people ran out of buildings. or retaining walls were cracked; some windows were broken out; many items were shaken off store shelves; furniture was displaced; many small Intensity VI: objects overturned and fell. The following effects were commonly noted on Salem (State Capitol building)- There was returned questionnaires, and we do not list them for each significant damage to the old section of the capital location: trees and bushes shook moderately; standing building, which was built in 1935. The 106 foot vehicles rocked moderately; moving vehicles rocked high rotunda cracked and bulged out by three feet; moderately; vibration was described as moderate or the dome over the rotunda was extensively strong; a few people ran out of buildings; felt by and cracked and fallen plaster littered the Rotunda awakened many or all. floor; the ten-ton "Oregon Pioneer" statue on a pedestal atop the dome was slightly rotated, so Oregon that mortar between the pedestal and the dome Aumsville- A few chimneys were cracked; brick and proper was cracked (press reports). concrete-block exterior walls sustained hairline cracks; plaster and drywall sustained a few large Scotts Mills- Several chimneys were cracked, lost cracks; a few windows cracked; a few items were bricks, or were broken at the roof line; masonry shaken off store shelves; a few glassware items or exterior walls sustained large cracks; interior dishes broke; a few small objects overturned and walls sustained large cracks; masonry fences and fell. retaining walls were cracked or partially fallen; many items were shaken off store shelves; Aurora- Interior walls split at seams; brick exterior furniture was displaced; many small objects walls sustained hairline cracks; chimneys were overturned and fell; some windows were broken broken at the roof line; bricks that had fallen from out. a chimney damaged a car parked below (press DESCRIPTION OF EARTHQUAKE EFFECTS
report); many items were shaken off store shelves foundations; a few items were shaken off store (press report); several glassware items or dishes shelves; a few glassware items or dishes broke; broke; water splashed onto sides of swimming many small objects overturned and fell; hanging pools; a few tombstones were displaced. pictures fell.
Beaver Creek- Drywall sustained large cracks; Donald- A few chimneys were cracked or had bricks masonry exterior walls sustained hairline cracks; a fallen from them; exterior walls sustained hairline few small objects overturned and fell. cracks; a few large cracks occurred in drywall; masonry fences or retaining walls were cracked; Boring- A few chimneys were cracked or had bricks several items were shaken off store shelves; a few fallen from them; plaster and drywall sustained glassware items or dishes broke; many small hairline cracks; exterior brick walls sustained objects overturned and fell; hanging pictures fell; hairline cracks; masonry fences and retaining water splashed onto sides of swimming pools. walls were cracked; a few windows cracked; a few items were shaken off store shelves; a few Dundee- A few chimneys were cracked or had small objects overturned and fell; hanging bricks fallen from them; the top ten feet of a pictures swung out of place; water splashed onto chimney at Dundee Elementary school was sides of swimming pools. rotated (press report); ceiling tiles fell in the Dundee Elementary gymnasium (press report); Canby- Some chimneys were cracked or had bricks plaster and drywall sustained hairline cracks and a fallen from them; plaster and drywall sustained few large cracks; masonry fences or retaining hairline cracks; ceiling tiles fell in the city hall; walls were cracked; a few windows cracked; some windows were broken out; many items were many items were shaken off store shelves; shaken off store shelves (press report); many furniture was displaced; many small objects glassware items or dishes broke; furniture was overturned and fell; hanging pictures fell. displaced; many small objects overturned and fell; hanging pictures swung out of place or fell; a Estacada- A few chimneys were cracked; brick clock fell to the floor (press report); trees and exterior walls sustained hairline cracks; plaster bushes shook strongly; moving vehicles rocked and drywall sustained hairline cracks and a few moderately; water splashed onto sides of large cracks; masonry fences or retaining walls swimming pools; masonry fences or retaining were cracked; a few windows cracked; a few walls were cracked; people had difficulty walking items were shaken off store shelves; a few or standing. glassware items or dishes broke; a few small objects overturned and fell. Cascadia- Interior walls sustained large cracks; exterior walls sustained hairline cracks; a few Forest Grove- The Forest Grove Fire Hall, in poor windows cracked; a few glassware items or dishes condition before the earthquake, sustained heavy broke; several small objects overturned and fell; damage and will have to be replaced (press hanging pictures swung out of place. report). Elsewhere exterior and interior walls sustained hairline cracks; several items were Colton- At one residence a wall was detached from shaken off store shelves; several small objects the ceiling; interior walls sustained cracks; the overturned and fell; hanging pictures swung out chimney pulled away from a house; masonry of place; windows, doors, and dishes rattled fences or retaining walls were cracked; many loudly. items were shaken off store shelves; small appliances were displaced; several dishes and Gervais- A few chimneys fell; interior walls knickknacks were broken; several small objects sustained hairline cracks; a few windows cracked overturned and fell; hanging pictures fell; water or were broken out; several items were shaken off splashed onto banks of ponds; small rock falls store shelves; small appliances were displaced; a occurred; wells were muddied. few small objects overturned and fell.
Dayton- Bricks fell from the tops of a few Gladstone- A few chimneys were cracked; exterior chimneys; a fireplace crumbled in one residence; walls sustained hairline cracks; interior walls hairline cracks occurred in exterior walls and in sustained hairline cracks and a few large cracks; SCOTTS MILLS, OREGON, EARTHQUAKE OF MARCH 25,1993
masonry fences or retaining walls were cracked; a few windows cracked; streets and highways were few windows cracked; many items were shaken cracked; many items were shaken off store off store shelves; many glassware items or dishes shelves; small appliances were displaced; a few broke; many small objects overturned and fell; glassware items or dishes broke; many small hanging pictures fell. objects overturned and fell; hanging pictures fell.
Grand Ronde- Bricks fell from a few chimneys; Mulino- Exterior and interior walls sustained masonry exterior walls sustained large cracks; hairline cracks; several items were shaken off interior walls sustained hairline cracks and some store shelves; furniture was displaced; several separated from ceilings or floors; a few items glassware items or dishes were broken; many were shaken off store shelves; a few glassware small objects overturned and fell. items or dishes broke; a few small objects overturned and fell; water splashed onto sides of Oregon City- Several chimneys were cracked; swimming pools. exterior walls sustained hairline cracks; interior walls sustained hairline cracks and a few large Gresham- A clay-brick wall of the Flav-R-Pac cracks; masonry fences or retaining walls were building, a warehouse built in 1920 and vacant at cracked; sidewalks and streets sustained a few the time of the earthquake, was cracked and cracks; a few windows cracked; a few items were pulled away from the perpendicular wall at a shaken off store shelves; furniture was displaced; corner (press report). Elsewhere, exterior and a few glassware items or dishes broke; a few interior walls sustained hairline cracks; a few small objects overturned and fell; many small windows cracked; a few items were shaken off objects overturned and fell. store shelves; a few glassware items or dishes broke; a few small objects overturned and fell; Portland (Rose City Park)- Bricks fell from several hanging pictures swung out of place. chimneys; a few interior and exterior walls sustained large cracks; a few windows cracked; a Hubbard- A few chimneys were cracked; brick few items were shaken off store shelves; light exterior walls sustained hairline cracks; plaster furniture was displaced; several small objects walls sustained hairline cracks and a few large overturned and fell; small landslides and rockfalls cracks; a few windows cracked; many items were occurred. shaken off store shelves; small appliances were displaced; a few glassware items or dishes broke; Saint Paul- Interior walls of the 147-year-old Saint many small objects overturned and fell; hanging Paul Catholic church sustained cracks, and a few pictures fell. items fell off walls. Elsewhere there were isolated instances of the top bricks being knocked off Lafayette- Bricks were shaken from the tops of chimneys. chimneys. Salem- With the exception of the intensity VII Lake Oswego- Several chimneys were cracked; damage at the state capitol building, listed exterior and interior walls sustained hairline separately above, effects in Salem are those cracks; masonry fences or retaining walls were associated with intensities of VI or lower. Several cracked; a few windows cracked; a few items acoustical ceiling tiles fell with fluorescent light were shaken off store shelves; a few small objects fixtures in a public library and in city hall. A few overturned and fell. chimneys were cracked; exterior and interior walls sustained hairline cracks; a few windows Mehama- Underground pipes cracked; some cracked; several items were shaken off store windows were broken out; a few items were shelves; several small objects overturned and fell; shaken off store shelves; a few glassware items or hanging pictures fell. dishes broke; several small objects overturned and fell. Sandy- A few chimneys were cracked; exterior walls sustained hairline cracks; interior walls Mill City- A few chimneys were cracked; exterior sustained hairline cracks and a few large cracks; a and interior walls sustained hairline cracks; few small objects overturned and fell; hanging masonry fences or retaining walls were cracked; a pictures swung out of place. DESCRIPTION OF EARTHQUAKE EFFECTS
Stayton- A few chimneys were cracked or had Blodgett- A few small objects overturned and fell. bricks fallen from them; exterior and interior walls sustained hairline cracks; several items Brightwood- A few items were shaken off store were thrown from store shelves; a few windows shelves; water splashed onto sides of swimming cracked; several small objects overturned and pools; a few people ran out of buildings. fell; hanging pictures fell. Carlton- A few items fell from walls; some pictures West Linn- A few chimneys were cracked or fallen; were tilted. brick exterior walls sustained hairline cracks; masonry fences or retaining walls were cracked; Cascade Locks- A few items were shaken off store a few windows cracked; a few items were shaken shelves; a few small objects overturned and fell; off store shelves; a few glassware items or dishes hanging pictures swung out of place. broke; a few small objects overturned and fell; hanging pictures swung out of place. Cornelius- Light furniture or small appliances were displaced; a few glassware items or dishes broke; Wilsonville- An exterior wall of the Wilsonville a few small objects overturned and fell; hanging Bowling Lanes sustained a large crack; pictures swung out of place; windows, doors, and acoustical ceiling tile fell in the bowling alley; dishes rattled loudly; standing vehicles rocked pins were down in every lane (press report). In moderately. the Mentor Graphics Building, a security guard was trapped for an hour inside a damaged Corvallis- A few windows cracked; a few elevator (press report). Exterior brick walls glassware items or dishes broke; several small sustained hairline cracks; drywall sustained objects overturned and fell; hanging pictures fell; hairline cracks; a few small objects overturned buildings shook strongly; walls creaked loudly; and fell; hanging pictures swung out of place; windows, doors, and dishes rattled loudly. moving vehicles rocked slightly. Crabtree- A few windows cracked; hanging objects Yamhill- A few chimneys were cracked, twisted, or or doors swung violently; small appliances were had bricks fallen from them; a few bricks were displaced; a few small objects overturned and loosened in the wall of the Yamhill Video store fell; hanging pictures swung out of place; but did not fall; several items were thrown from buildings shook strongly; walls creaked loudly; store shelves; several knickknacks were broken; windows, doors, and dishes rattled loudly; water several items overturned and fell. splashed onto sides of swimming pools.
Intensity V Dallas- A brick exterior wall sustained large cracks; moving vehicles rocked slightly. The following effects were commonly noted on returned questionnaires, but we do not list them for each Detroit- A few items were shaken off store shelves; location: trees and bushes shook slightly; vibration was a few glassware items or dishes broke; several described as moderate or strong; felt by and awakened small objects overturned and fell, hanging many or all. pictures swung out of place; windows, doors, and dishes rattled loudly; a few people ran out of Oregon buildings; trees and bushes shook moderately. Amity- Intensity assigned from field evidence. Mail questionnaire reported that a few chimneys Drain- A few small objects overturned and fell. were cracked; exterior brick walls sustained hairline cracks; plaster walls sustained large Eagle Creek- A few windows cracked; a few items cracks; highways sustained large cracks; were shaken off store shelves; a few glassware underground pipes were broken; a few windows items or dishes broke; a few small objects cracked; a few items were shaken off store overturned and fell; hanging pictures fell; shelves; several small objects overturned and moving vehicles rocked slightly. fell; hanging pictures fell; water splashed onto sides of swimming pools and ponds. Eddyville- A few small objects overturned and fell. 10 SCOTTS MILLS, OREGON, EARTHQUAKE OF MARCH 25,1993
Elmira- A few items were shaken off store shelves; Marion- A few windows cracked; several a few glassware items or dishes broke; a few knickknacks were broken; several small objects small objects overturned and fell. overturned and fell; hanging pictures swung out of place or fell; windows, doors, and dishes rattled Falls City- Exterior and interior walls sustained loudly. hairline cracks; a few windows cracked; several small objects overturned and a few fell; hanging Marylhurst- A few glassware items or dishes broke; pictures swung out of place; a few people ran out a few small objects overturned and fell; hanging of buildings. pictures swung out of place; windows, doors, and dishes rattled loudly. Gaston- A few items fell from walls. McMinnville- A few small objects overturned; items Gates- Several items were shaken off store shelves; were thrown from store shelves; a few people ran several small objects overturned and fell; hanging out of buildings. pictures swung out of place; a few people ran out of buildings. Milwaukie- A few items were shaken off store shelves; a few small objects overturned and fell; Government Camp- A few small objects overturned standing vehicles rocked moderately. and fell; windows, doors, and dishes rattled loudly. Monroe- Bricks fell from a few chimneys; drywall sustained hairline cracks; a few small objects Hillsboro- A few items were shaken off store overturned and fell. shelves; a few glassware items or dishes broke; several small objects overturned and fell; hanging Nehalem- A few items were shaken off store pictures fell; moving vehicles rocked slightly; a shelves; a few glassware items or dishes broke; a few people ran out of buildings. few small objects overturned and fell.
Idanha- A few items were shaken off store shelves; North Plains- Drywall sustained hairline cracks; a several small objects overturned and a few fell; few windows cracked; a few small objects hanging pictures swung out of place or fell; a few overturned and fell. people ran out of buildings. Oakridge- Interior walls sustained a few large Independence- A few glassware items or dishes cracks. broke; a few small objects overturned and fell; hanging pictures swung out of place or fell; Otis- A few small objects overturned and fell. windows, doors, and dishes rattled loudly. Pacific City- A few items were shaken off store Jefferson- A few small objects overturned and fell; shelves; a few small objects overturned and fell. hanging pictures swung out of place. Portland (Brooklyn)- Drywall sustained hairline Liberal (5 km [3 miles] north of Molalla)- Items fell cracks; a few windows were cracked; several from store shelves and a few knick-knacks items were shaken off store shelves; a few overturned and fell. glassware items or dishes broke; a few small objects fell. Lyons- A few windows cracked; a few items were shaken off store shelves; light furniture and small Portland (Multnomah)- Drywall split at seams; a appliances were displaced; several small objects few items were shaken off store shelves; a few overturned and fell; hanging pictures swung out glassware items or dishes broke; a few small of place; windows, doors, and dishes rattled objects overturned and fell; hanging pictures were loudly. swung out of place.
Marcola- A few items were shaken off store shelves; Portland (Parkrose)- A few glassware items or a few small objects overturned and fell; hanging dishes broke; a few small objects overturned and pictures swung out of place. fell; hanging pictures swung out of place. DESCRIPTION OF EARTHQUAKE EFFECTS 11
Portland (Southgate)- A few small objects few small objects overturned and fell. overturned and fell. Tualatin- Drywall sustained hairline cracks; a few Rainier- A few small objects overturned and fell. small objects overturned and fell; hanging pictures swung out of place; windows, doors, and Rockaway- Interior walls sustained hairline cracks; dishes rattled loudly. a few items were shaken off store shelves; a few glassware items or dishes broke; a few small Walterville- A few chimneys were cracked; drywall objects overturned and fell; hanging pictures fell; sustained hairline cracks; masonry fences or moving vehicles rocked slightly; a few people ran retaining walls were cracked; a few items were out of buildings. shaken off store shelves; a few glassware items or dishes broke; a few small objects overturned and Saint Paul- Plaster walls sustained a few large fell; trees and bushes shook moderately; water cracks; a few items were shaken off store shelves; splashed onto sides of swimming pools. a few glassware items or dishes broke; a few small objects overturned and fell; hanging Welches- A few glassware items or dishes broke; pictures fell; moving vehicles rocked slightly. several small objects overturned and fell; hanging pictures swung out of place. Scio- A few chimneys were cracked; interior walls sustained hairline cracks or separated from ceiling Westfir- Ceiling tile loosened and sagged; small or floors; a few tombstones were cracked; a few appliances were displaced; a few small objects glassware items or dishes broke; several small overturned and fell; hanging pictures swung out objects overturned and a few fell; hanging of place; windows, doors, and dishes rattled pictures swung out of place; windows, doors, and loudly. dishes rattled loudly; a few people ran out of buildings; trees and bushes shook moderately; Willamina- A few windows cracked; a few items moving vehicles rocked moderately. were shaken off store shelves; a few small objects overturned and fell; hanging pictures swung out Shaw (3 km [2 miles] north of Aumsville)- Light of place. damage to chimneys; possible thin cracks in foundations. Washington Amboy- Underground pipes were broken at one Siletz- Exterior and interior walls sustained hairline location; a few items were shaken off store cracks; a few items were shaken off store shelves; shelves; hanging pictures swung out of place. a few glassware items or dishes broke; a few small objects overturned and fell; hanging Camas- A few items were shaken off store shelves; pictures swung out of place; windows, doors, and a few small objects fell; hanging pictures swung dishes rattled loudly. out of place. Sisters- A few windows cracked; several objects Cathlamet- A few items were shaken off store overturned. shelves; a few small objects overturned and fell.
Sublimity- Masonry fences or retaining walls were Kalama- A few small objects overturned and fell; cracked; a few windows cracked; a few items hanging pictures swung out of place; windows, were shaken off store shelves; a few glassware doors, and dishes rattled loudly. items or dishes broke; a few small objects overturned and fell; hanging pictures swung out Longview- Many items were shaken off store of place; windows, doors, and dishes rattled shelves; hanging pictures fell; standing vehicles loudly; moving vehicles rocked moderately. rocked moderately; moving vehicles rocked moderately. Terrebonne- A few items were shaken off store shelves; a few glassware items or dishes broke; a Underwood- A few windows cracked; a few small few small objects fell. objects overturned and fell.
Tidewater- A few items were shaken off store Intensity IV shelves; a few glassware items or dishes broke; a Oregon Albany, Alsea, Arch Cape, Banks, 12 SCOTTS MILLS, OREGON, EARTHQUAKE OF MARCH 25,1993
Beaverton, Bend, Blue River, Brownsville, recorded a peak acceleration of .18 g. At other Corps of Cannon Beach, Clatskanie, Columbia City, Engineers facilities, Green Peter, Blue River, and Cottage Grove, Creswell, Dexter, Eugene, Cougar Dams, only 4 of the existing 14 accelerographs Fairview, Fall Creek, Florence, Foster, Hood were triggered by the earthquake. Ground accelerations River, Junction City, Keizer, Lebanon, Lincoln were low in the Portland-Vancouver area, reaching .02 g City, Manzanita, Monmouth, Mount Hood, (table 2), barely sufficient to trigger the instrumentation. Oakridge, Odell, Philomath, Redmond, Rhododendron, Rickreall, Scholls, Seaside, Previous strong-motion records were obtained in Shedd, Sheridan, Springfield, Sweet Home, Oregon at the Portland State Office building for Tangent, The Dalles, Thurston, Tillamook, earthquakes in 1961, 1962, and 1965 (table 1). Among Tolovana Park, Turner, Vernonia, Vida, these shocks, only the 1962 event produced a significant Waldport, Warm Springs, Warrenton. record, with a peak horizontal acceleration of .10g (fig. 5). The epicenter of the 1962 earthquake (Yelin and Washington Battle Ground, Bingen, Carson, Patton, 1991) was located at a distance of approximately Centralia, Cosmopolis, Glenwood, Husum, Kelso, 15 km from the accelerograph. Lyle, Randle, Ridgefield, Stevenson, Toledo, Toutle, Washougal, White Salmon, Winlock, FOCAL MECHANISM AND DEPTH Wishram, Woodland. DETERMINED FROM DIGITAL DATA
Intensity III Figures 6 and 7 show broadband P-wave Oregon Astoria, Coos Bay, Culver, Dufur, displacements recorded at stations of the Federation of Gardiner, Greenleaf, Heppner, Lowell, Mapleton, Digital Seismograph Networks; the plotted seismograms Maupin, North Bend, Prineville, Roseburg, are from stations with the best signal-to-noise ratio. Scappoose, Sutherlin, Tygh Valley. The shape of the P waveform is strongly influenced Washington Carrolls, Chehalis, Dallesport, Forks, by the focal depth of the earthquake, by the orientation Lacey, Long Beach, Raymond, Seahurst, South of the causative fault that produced the earthquake, and Bend, Vancouver, Vashon. by the slip-direction that the crustal block on one side of Intensity II the fault has moved with respect to the crustal block on the other side of the fault during the earthquake. Thus Oregon Glendale, Harrisburg. one may determine the depth, orientation, and slip direction of the causative fault by analysis of the Washington Marysville, Tacoma. waveform. Felt Washington Graham. We have performed a preliminary analysis of the waveforms and obtained a depth of 16 km. The data imply two P-wave nodal-planes, one of which is the fault STRONG MOTION RECORDS plane and one of which is the conjugate plate. Plane 1 has strike 316° (N44°W); dip 63° (NE); slip 152° (right- Strong-motion accelerographs operated by the lateral oblique-reverse slip); plane 2 has strike National Strong-Motion Program (NSMP) were 60°(N60°E); dip 65°(SE); slip 30° (left-lateral oblique- triggered at the following stations (table 2; fig. 4): reverse slip). The focal mechanism implies that the Detroit Dam (3 records) shock was caused by compressional tectonic stress Portland State University ((1 record) directed approximately north-south. Figure 8 shows how Green Peter Dam (2 records) the data used in the analysis are distributed on the lower- Vancouver VA Hospital (1 record) hemisphere of the focal sphere. Dashed lines Blue River Dam (1 record) superimposed on the seismograms of figure 7 are Cougar Dam (1 record) synthetics, showing how well the inferred focal depth and source orientation account for the shape of those Maximum recorded accelerations occurred at seismograms. Detroit Dam. An accelerograph located immediately downstream of the dam recorded a peak acceleration of The depth and focal-mechanism orientation inferred .06 g; an accelerograph in the upper gallery of the dam for the Scotts Mills earthquake are consistent with a STRONG MOTION RECORDS 13
Table 2. USGS strong-motion data for the Scotts Mills, Oregon, earthquake of March 25, 1993, 13:34:35.4 UTC
EPICENTRAL STATION ACCELERATION DISTANCE * LOCATION COMPONENT MAXIMUM (km) (OWNER) DIRECTION (g)
43 Detroit Dam, Oregon 198° 0.06 Downstream Up 0.05 (ACOE) 108° 0.06
43 Detroit Dam, Oregon 198° 0.15 Gallery, Gallery Level #7 Up 0.07 (ACOE) 108° 0.18
43 Detroit Dam, Oregon 198° 0.03 Gallery, Gallery Level #1 Up 0.02 (ACOE) 108° 0.02
53 Portland State Univ., Oregon 291° 0.02 Cramer Hall, Basement Up 0.01 (USGS) 201° 0.02
64 Green Peter Dam, Oregon 042° 0.01 Lower Gallery Up 0.01 (ACOE) 312° 0.02
64 Green Peter Dam, Oregon 042° undamped Upper Gallery Up 0.03 (ACOE) 312° 0.07
65 Vancouver, Washington 018° 0.02 VA Hostpital Up 0.02 (VA) 288° 0.02
100 Blue River Dam, Oregon 329° 0.03 Toe Up 0.02 (ACOE) 239° 0.03
106 Cougar Dam, Oregon 076° 0.03 Center Crest Up 0.02 (ACOE) 346° 0.02
* Epicentral distances in this table and in figure 4 are measured with respect to a preliminary epicenter at 45.06°N., 122.52°W. Distances with respect to the current preferred epicenter differ by up to 2 km. ACOE=Army Corps of Engineers, VA=Veterans Administration 14 SCOTTS MILLS, OREGON, EARTHQUAKE OF MARCH 25,1993 suggestion that the shock involved slip at depth on the Madin, I.P., Priest, G.R., Mabey, M.A., Malone, S., northwest-trending Mt. Angel fault (Madin and others, Yelin, T.S., and Meier, D., 1993, March 23, 1993, 1993). The ML Angel fault had been previously mapped Scotts Mills earthquake western Oregon's wake- as a right-lateral oblique-reverse fault and identified as up call: Oregon Geology, v. 55, no. 3, p. 51-57. possibly being seismogenic (Werner and others, 1992). The seismologically determined fault orientation and sense of slip are consistent with the geologically Michaelson, C.A., and Weaver, C.S., 1989, Upper determined orientation and sense of slip. A plane having mantle structure from teleseismic P wave arrivals in the orientation of our northwest-striking nodal plate Washington and northern Oregon: JGR, v. 91, p. (plane 1) and passing through the hypocenter of the 2077-2094. earthquake would intersect the ground surface within several kilometers of the surface trace of the geologically Richter, C. F., 1958, Elementary seismology, W. H. mapped Mt. Angel fault. Even with the focal Freeman and Company, San Francisco and London, mechanism, however, presently available evidence 768 p. linking the Mt. Angel fault with the Scotts Mills earthquake is not as strong as evidence linking specific Stover, C.W., and Coffman, J.L., 1993, Seismicity of the faults with earthquakes in some other parts of the United States, 1568-1989 (Revised): U. S. western U.S. Rather than occurring on the Mt. Angel Geological Survey Professional Paper 1527,418 p. fault, the Scotts Mills earthquake may have occurred on a mid-crustal fault that does not extend as a continuous U.S. Geological Survey, 1993, Preliminary dislocation up into the shallow crust (Madin and others, Determination of Epicenters, Monthly Listing, 1993). March 1993,36 p.
REFERENCES CITED Werner, K., Ndbelek, J., Yeats, R., and Malone, S., 1992, The Mount Angel fault: implications of seismic- reflection data and the Woodburn, Oregon, Bonneville, D., Del Pino, A., and Kelly, D., 1993, The earthquake sequence of August 1990: Oregon March 25,1993, Scotts Mills earthquake - effects on Geology, v. 54, no. 5, p. 112-117. structures: EERI Special Earthquake Report May 1993,p.5-8. Wood, H.O., and Neumann, Frank, 1931, Modified Mercalli Intensity Scale of 1931: Seismological Ludwin, R.S., Weaver, C.S., and Crosson, R.S., 1991, Society of America Bulletin, v.21, no.4, p. 277-283. Seismicity of Washington and Oregon, in Slemmons, D.B., Engdahl, E.R., Zoback, M.D., and Blackwell, D.D., eds., Neotectonics of North Yelin, T.S., and Patton, H.J., 1991, Seismotectonics of America: Boulder, Colorado, Geological Society of the Portland, Oregon, region: Bulletin of the America, Decade Map Volume 1, p. 77-98. Seismological Society of America, v.81, p. 109-130. STRONG-MOTION RECORDS 15 .
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CN a ^ m D CN i 1 i 1 s 1 m^ Ee 1 4J ^ 1 CN > ' CN 0) W gO 1 ^ O O O * H U O *-* , 1 k ^ 2 i »n u 1 r- rH Co nVO u O £ . 1 ^i Q) <«tf< <0 * ^ CN m -»J 1 O ^ ro W V£> rH rH -H 1 TJ ^ TJ ^ 2 1 CN (0 o ro »"j ' Q ^ cr» ro : 1 ^ ON 0» ^ ' « 0 H Q) ^ -^ . * e 20)..* C > 0 Station No. 2137 44.13N, 122.24W 076° Sens. =1.95 cm/g 0.03 g Freq. = 24.8 Hz Cougar Dam, Oregon - Center Crest Damp. = 0.6 crit SMA-1 No. 859 (ACOE) Up Sens. = 1.77 cm/g 0.02 Freq. = 26.1 Hz Earthquake of Damp. = 0.6 crit 25 March 1993, 1334:36.5 G.m.t. 346° Sens. = 1.75 cm/g 0.02 Freq. = 26.0 Hz Damp. = 0.6 crit Epicentral distance = 106 km Film speed = 1 cm/sec c/a 3 iI §D c/a Figure 4 (Continued). to OJ t UJ O CN SENSITIVITY IS 21 cm/g t UJ O Figure 5. Strong-motion record obtained from the earthquake of November 6,1962,03:36 UTC, at Portland, Oregon, in the basement of the Oregon State Office Building. Station coordinates are 45.52°N., 122.68°W. Epfcentral distance - 15 km. One cm - 1 sec. TELESEISMICDATA 25 P-wave Displacement 9.0E+02q COR A 0.7 -y^ufl^y^^ 0228.1 -9.0E+02 2.5E+02 LON A 1.8 0 17.7 -2.5E+02 2.0E+01 WDC A 4.4 0179.3 -2.0E+01 5.0E+00 CMB A 7.2 0165.8 -5.0E+00 4.0E+00 GSC A 10.7 0153.5 -4.0E+00 0 20 40 60 80 100 Time (sec) 2.0E+00 TUC A 15.7 0140.1 -2.0E+00 2.0E+00 ANMO A 15.9 : 0123.7 -2.0E+00 CCM A 24.4 0 95.6 -2.5E+00 1.4E+00 MIAR A 24.5 0105.2 i 7.0E-01q GOGA A 32.2 0 97.4 \ -7.0E-01 20 40 60 80 100 Time (sec) Figure 6 . P-wave displacements recorded at stations of the Federation of Digital Seismograph Networks. Stations are ordered by increasing distance (A, given in degrees). The azimuth of each station from the source is given by G. Amplitudes of displacements are given in microns. Stations are identified by standard abbreviations used by the National Earthquake Information Center. 26 SCOTTS MILLS, OREGON, EARTHQUAKE OF MARCH 25,1993 P-wave Displacement 7.0E+00 YSS A 61.7 0309.1 -7.0E+00 6.0E+00 TLY A 75.7 0332.4 -6.0E+00 WMQ A 87.5 0338.6 -5.0E+00 6.0E+00 XAN A 88.5 0319.4 e.OE+OOq ENH A 91.3 0316.9 -6.0E+00 20 40 60 80 too Time (sec) Figure 7. Observed P-wave displacements recorded at stations of the Federation of Digital Seismograph Networks, together with synthetic displacements (dashed lines) computed for the preferred focal mechanism. Otherwise, representation of the data is as in Figure 6. P SV Figure 8. Focal mechanism of the Scotts Mills earthquake, showing the distribution of data used to construct the focal mechanism. Lower hemispheres of the focal sphere are represented in equal-area projection, (a.) P-wave data: triangle - P-wave compression; diamond - pP wave that left the source as a compression; square - nodal P-wave or pP-wave; diagonal cross - P-wave dilatation; curved lines - projections of P-wave nodal-planes, one of which corresponds to the earthquake fault-plane, (b.) sP first motions: diagonal cross - dilatational sP first-motion; curved lines - nodal lines for SV radiation pattern. m 01 1 CM g CL e CO z io *j e a: CD < CM a) >: 01 CO a CM M 6 i X « c »H 4J . ro - h- ; J I C W CD ^ T -M i 0 0} a: ** C - ? i r : QJ a Z * m U 1 p