Ground Motion Characteristics in Sendai during the 2011 off the Pacific Coast of Tohoku Earthquake-Comparison of 1978 Miyagi-ken Oki earthquake at the same observation pointMasato Motosaka (Professor, Disaster Control Research Center, Tohoku University)
Susumu Ohno(Assoc. Professor, DCRC, Tohoku University)
Kazuya Mitsuji (Assoc. Professor, Yamagata University)
It is impotant to discuss the ground motion characteristics at the same observation point for different scaleof earthquakes. As a quick report, we inform the characteristics of the observation records at B2F of Sumitomo
Building near Sendai Station, one of DCRC, Tohoku University observation points. At the observation site,
the vauable observation record was obtained during the 1978 Miyagi-ken Oki earthquake (M7.4) and also
during 2005 Miyagi-ken Oki earthquake (M7.2). It is noted that the Sumitomo Buiding is on the reratively stiff
soil (delivium terrace) and the record at B2F can be referred as engineering bedrock motion. The observation
record during the 1978 eathquake has been used in seismic design of building structures as Sendai 038.
Ground motion distribution characteristics in Sendai is necessary to be investigated by collecting andanalyzing observation data at many observation points. The records at Sumitomo building can be reference
point for discussing ground motion characteristics of alluvium sites like Orosimachi and hill zone like
Aobayama canpus of Tohoku University, where structural damage is recognized.
Fig. 1 shows the acceleration and velocity waveforms of 3 components and Fourier spectra and response
spectra (Tripatyte)
Table 1 shows PGA values of the 2011 off the Pacific Coast of Tohoku earthquake (M9.0), the 1978
Miyagi-ken Oki earthquake (M7.4) and the 2005 Miyagi-ken Oki earthquake (M7.2).
Fig. 2 shows comparison of waveforms of the three earthquakes in the NS direction (S25E) and theresponse spectra of the three earthquakes are comparatively shown in Fig.3 for the three components.
From these figures, the following findings are obtained.
Regarding ground motion characteristics of the 2011 earthquake, findings are;1) Duration of the records of the 2011 earthquake is very long about 180 seconds due to large magnitude(M9.0). From the envelope, two large phases are recognized and the other two smaller phases arefollowed.
2) Regarding spectral characteristics, dominant periods of about 1s and about 3s are recognized in the2011 earthquake.
3) Regarding the spectral values in the period range shorter than 1.5s, NS component is larger than EW
direction.
4) But in the period range around 3s, EW component is larger than NS component
Regarding comparison of the 1978 Miyagi-ken Oki earthquake and the 2005 earthquake, findings are;5) The envelope shape of the 1978 earthquake is almost same as the first phase of the 2011 earthquake.
6) Period contents shorter than 1.5s of the 2011 earthquake is larger than those of 1978 earthquake, about
20% larger at 1s, and twice as large at 0.5s
7) The period contents around 3s of the 2011 earthquake is twice as large compared to those of 1978earthquake.

Table
1 Comparisonofof PGA
PGA values
at theSendai
Table
1 Comparisonvalus atthe Same
Same Observation
ObservationPoint
Pointin inSendai(Sumitomo Building
B2F)Building
Unit: B2F)
cm/s/s Unit: cm/s/s(SumitomoEarthquakeNS direction

EWdirection

UDType ofdirectionSeismometeryr/m/dMagnitude

S25E

2011/3/11

9.0

317.7

234.1

160.3

SSA-1

1978/6/12

7.4

250.9

240.9

90.8

SMAC-Q

2005/8/16

7.2

120.8

78.0

56.4

SSA-1

S65W

Fig.1 Acceleration and Velocity Wwaveforms of 3 Components and Fourier Spectra andResponse Spectra

200gal2011 Off Pacific Coast of Tohoku(M9.0)
1978 Miyagi-ken Oki(M7.4)
2005 Miyagi-ken Oki(M7.2)

050

100150Time(sec)Fig.2 Comparison of Observed Acceleration Waveforms (NS Direction) at Sumitomo
Building near Sendai Station for 3 earthquakes

Fig.3 Comparison of Pseudo Velocity Spectra for 3 Earthquakes at B2F ofSumitomo Building