【正文】 and earthquakes of magnitude in June 1988 and in August of this year in nearly the same region as the Loma Prieta Earthquake.The recent rupture is unusual for earthquakes on the San Andreas Fault. Past earthquakes have been the result of horizontal slip of the ground on the two sides of the fault, with the southwest side moving northwest relative to the northeast side. The recent rupture of the Loma Prieta Earthquake includes not only this horizontal ponent of slip, but also a significant thrusting of the southwest side up and over the northeast side. The fault has steep dip to the southwest in this area.Ground Motion RecordsStrongmotion records were obtained from hundreds of instruments, permitting a detailed understanding of the ground motion throughout the affected region. Preliminary results indicated that peak ground shaking as strong as (where g is the acceleration due to gravity) was recorded in both the horizontal and vertical directions in the epicentral area. Relatively low accelerations were recorded in locations between the epicenter and Oakland and San Francisco。 damage to gypsum board interior walls appeared to be significantly less severe. Chimney damage ranging from cracking to twisting and collapse was observed throughout the epicentral area. Such damage was observed in as many as 30% to 40% of the homes in the older areas of Watsonville. Chimney damage in Santa Cruz was less frequent and was primarily limited to older, unreinforced chimneys.Severe structural damage to homes observed could mainly be attributed to failure of unbraced cripple walls and lack of sill anchorage. These cripple walls are short wood stud framing present between the foundation and first floor of the building. Because they are not sheathed with plaster, as are walls in the living area, they are substantially weaker and a mon location for failures.Such damage was mainly observed in pre1940s homes with horizontal wood sheathing. These homes moved laterally from several inches to a foot, until the cripple walls became unstable and collapsed. Damage to utility lines due to this motion was frequent. In Watsonville, cripple wall failures were observed in approximately 10% to 20% of the pre1940s residences throughout the area, with many blocks suffering close to 100% failures. In Santa Cruz, damage appeared to occur mainly in the area of poorer soils. Nearly all of the homes on Pacific Avenue south of the Pacific Garden Mall had cripple wall failuresSevere damage was also apparent on Myrtle Street, near the wastewater treatment plant at Neary Lagoon. On one block, five of twentyfive homes had cripple wall failures. On this street, correlation of damage with quality of maintenance and general preearthquake condition was evident. Apparently, older homes that had been restored architecturally often also had structural upgrades. The table on the following page shows none of the homes with good preearthquake condition to be severely damaged, while only one poor condition home was in the light damage category. In one Watsonville block, only one house did not fall from its foundations. It had just been seismically upgraded and had a new foundation without cripples. Its interior also had no damage, not even sheetrock cracking.San Francisco Marina District The most severely damaged area in San Francisco was the Marina district. The soils in this area consist mainly of soft, saturated sands and silts, some of which are naturally occurring, but most of which were filledin for the PanamaPacific Exposition circa 1915.Most of the buildings in the Marina district are apartment buildings and singlefamily houses of woodframe construction, two to five stories high, built in the 1920s. Most of these buildings are constructed with parking garages at the first story. A large percentage of the buildings in the Marina district suffered damage. Although plaster and stucco cracking was evident in all buildings in the Marina district, the most severe damage was observed in midrise (four to fivestory) apartment buildings. Several of these buildings collapsed pletely and many more were damaged beyond repair. Failure of these buildings was due to two main effects. First, the mass and stiffness of the buildings may have produced fundamental periods of vibration close to the predominant period of vibration that was amplified by the soft soils in the area. Second, it appeared that nearly all of the collapsed and partially collapsed buildings had the same configuration: these buildings all had numerous garage door openings at the first level, resulting in a lack of story shear strength or a soft story. Many of the severely damaged buildings were located on corners, and hence had two of their four exterior walls perforated with garage door openings. Some of these corner buildings also exhibited twisting behavior, which further worsened their performance. Seismic settlement and liquefaction caused severe cracking and buckling of streets and sidewalks in the Marina district. Soil failures did not appear to be a direct cause of building collapses。s business interruption and some capital loss. Many businesses must relocate to new facilities until their buildings are repaired or replaced. In future stronger shocks or in earthquakes located closer to the major population centers, much more extensive damage and mercial loss are likelyResidential Structures Most residential buildings in the affected area are of wo