The　U.S.　Geological　Survey　Parkfield　Dense　Seismograph　Array　(UPSAR)　recorded　successfully　strong　ground　motions　during　the　2004　Parkfield　earthquake　(M　6.0)　and　its　aftershock　series　after　waiting　for　15　years　for　an　anticipated　event　like　this.　The　array　also　recorded　the　2003　San　Simeon　earthquake　(M　6.5).　Because　the　array　covers　a　very　small　area　(0.45　km(2)),　these　data　offer　some　interesting　fresh　insights　into　intrasite　variations　of　seismic　ground　motions.　In　this　article,　we　study　strong-motion　data　recorded　at　the　UPSAR　from　the　San　Simeon　event,　the　Parkfield　event,　and　its　seven　aftershocks.　We　find　that　the　variations　of　high-frequency　ground　motions　(e.g.,　＞　3　Hz)　are　very　considerable.　The　largest　horizontal　peak　ground　acceleration　(PGA)　(P11,　408　cm/sec(2))　from　the　Parkfield　event　is　close　to　three　times　of　the　smallest　one　(P01,　157　cm/sec(2));　the　largest　peak　response　spectrum　is　even　over　three　times　of　the　smallest　one.　The　shortest　station-to-station　distance　(between　P06　and　P07)　in　the　array　is　only　25　m,　but　three-component　PGAs　of　the　two　stations　differ　from　a　factor　of　1.5　for　the　Parkfield　event.　The　coefficient　of　variation　(C(v)　=　sigma/mean)　of　Fourier　acceleration　is　about　50%　at　frequencies　higher　than　about　3　Hz.　We　find　that　C,　depends　strongly　on　frequency,　while　it　is　nearly　stable　for　different　earthquakes.　The　significant　variation　of　the　high-frequency　ground　motions　seems　to　be　brought　about　mostly　by　the　local　and　neighboring　topographic　effects,　which　have　a　larger　effect　in　horizontal　than　vertical　directions.　We　also　calculate　the　ratio　of　vertical　to　horizontal　response　spectrum.　Our　plotting　shows　that　the　ratio　is　not　sensitive　to　earthquake　magnitude.　We　compare　the　observed　motions　(PGA,　5%-damped　pseudoacceleration　response　spectrum　[PSA]　from　0.02　to　5　sec)　with　estimations　from　four　commonly　used　prediction　equations　(Abrahamson　and　Silva,　1997;　Boore　et　al.,　1997;　Campbell　and　Bozorgnia,　2003;　and　Sadigh　et　al.,　1997).　The　comparisons　indicate　that　the　significant　station-to-station　variation　reduces　largely　the　accuracy　with　which　a　site-specific　estimation　can　be　predicted.　However,　the　mean　of　the　observations　at　the　UPSAR　compares　reasonably　well　with　these　estimations.