報告書番号

U98007

タイトル（和文）

不整形地盤における入射波動場と地盤増幅特性－山梨県東部地震（１９９６）による足柄平野久野地点に関する検討－

タイトル（英文）

SITE AMPLIFICATION AND INCIDENT WAVE FIELD IN IRREGULARLY LAYERED MEDIA-A CASE STUDY FOR KUNO SITE IN ASHIGARA VALLEY DURING EAST YAMANASHI EARTHQUAKE IN 1996-

概要 （図表や脚注は「報告書全文」に掲載しております）

１９９６年３月６日と１０月２５日に発生したマグニチュード４～６の山梨県東部地震において，礫質地盤が不整形に堆積した足柄平野久野地点と岩盤上強震観測点で良好な記録を得た。震源の応力降下量は最大で４３Ｍｐａを示し，今まで一般に知られている平均値（３～１０ＭＰａ）と比べて明らかにに高く，観測された最大加速度値は，既往の距離減衰式の予測値を上回る。また，入射波動場は久野地点近傍の深部地下構造の影響により東西方向にＳ波の震動主軸方向が偏り，久野地点では，岩盤上に比べて後続波群の震動が長く続いており，不整形な地盤構造が地震波の伝播に影響を及ぼしている。これらの結果にもとづき，不整形な形状を示す東西断面での地盤構造を二次元でモデル化して解析した場合，震源からの波の放射と地下構造で卓越する震動主軸方向のＳＶ波に対する応答解析の増幅特性に対する結果が観測結果と良く対応し，強震動予測法として同手法が有用であることが示された。

概要 （英文）

DURING THE EAST YAMANASHI EARTHQUAKE SCALED FROM 4 TO 6 OF MAGNITUDE OCCURRED IN MARCH AND OCTOBER IN 1996, LARGE ACCELERATION AMPLITUDE RECORDS WERE OBSERVED ON FUE IRREGULARLY LAYERED MEDIA AT KUNO IN ASHIGARA VALLEY AND A SEISMOGRAPH OBSERVATION SITE ON AN OUTCROP OF ROCK INSTALLED BY CRIEPI AROUND KUNO. IN THIS EARTHQUAKE,THE STRESS DROP OF THE MAIN SHOCK INVERTED AS A SOURCE PARAMETER BY USING THE OBSERVED RECORDS WAS 43MPA. IT WAS HIGHER THAN THE AVERAGED VALUE FROM 3 TO 10MPA OF PAST EARTHQUAKES IN GENERAL. THE OBSERVED MAXIMUMACCELERATION VALUES WERE LARGER THAN THOSE OF THE ATTENUATION EQUATION. THE PRINCIPAL AXIS OF THE S-WAVE MOTION IN THE BASEMENT ROCK WAS INCLINED RELATIVELY TO EAST-WEST DIRECTION BY THE DEEP UNDERGROUND STRUCTURE AROUND THE KUNO SITE. THE S-WAVE MOTION AFTER THE PRINCIPAL SHOCK IS STILL REMAINING AT THE BASEMENT ROCK INSIDE THE SITE WHEN COMPARED WITHTHAT ON THE OUTCROP OF ROCK. THUS,THE SEISMIC WAVE PATH IS AFFECTED BYTHE LATERAL IRREGULARITY OF THE DEEP UNDERGROUND STRUCTURE. WHEN AN IRREGULARLY LAYERED MODEL WAS MADE IN TWO-DIMENSIONS (2-D) ON THE BASIS OF THE LATERAL IRREGULARITY OF THE LAYERED MEDIA ALONG THE DIRECTION FROM EAST TO WEST. THE SITE RESPONSE SIMULATED BY 2-D ANALYSIS DUE TO INCIDENT SV-WAVE AGREED WELL WITH OBSERVED SPECTRAL RATIOS, WHEN THE S WAVEWAS PREDOMINANT IN THE EW COMPONENT ACCORDING TO BOTH THE FOCAL RADIATION PATTERN AND PATH EFFECT OF THE UNDERGROUND STRUCTURE. THEREFORE,IT WAS CONFIRMED THAT THE ANALYTICAL METHOD WAS USEFUL TO EVALUATE STRONG MOTION UNDER SUCH CONDITIONS.

報告者

キーワード