Seismic evidence for thermally-controlled dehydtration reaction in

subducting oceanic crust

火山センター D2 宮林佐和子

2010/4/15

Abstract簡単なマトメ1 Tomo DDで、東北から関東にかけて地下の地震波速度構造を推定した。2 沈み込むスラブの上面にLow-velocity zoneが存在することを確認した。3 Low-velocity zoneの分布は東北と関東で異なる。(関東のほうが深くまで存在する)4 Low-velocity zoneのおわりる場所で、地震が多発している。

Introduction島弧の地下構造の復習沈み込み帯では水が含水鉱物という形で深部に供給されている。

あるP,Tのとき、脱水、serpentite layerの形成

Iwamori(2007)

Introductionターゲットとする地域について日本のテクトニクス2つの海洋プレートが大陸プレートに沈み込んでいる。

PAC plate…8-10cm/yrPHS plate…3-5cm/yr

Iwamori(2007)

Introduction東北地方のテクトニクス太平洋プレートが 8-10cm/yr の速度で北米プレートに沈み込んでいる。【沈み込み帯のマグマ学より】一般的な島弧といわれる。沈み込むプレートの深度が約110km,160kmのところに火山列が見られる。(噴出量はあまり変わらないが、1列目のほうがFeO/MgO比が高い。)

Introduction関東地方のテクトニクスとっても複雑な場所。PHS plateがPAC plateにくっついている部分がある。mantle wedge からの熱供給が遮断され、東北よりも同じ深さにおいて温度が低いと予想することができる。

IntroductionLow-velocity zoneについて先行研究Tsuji et al., 2008PAC plate の上面にあつさ10km程度のLow velocity zone が存在し、70-90kmの深さでおわる。PAC上面から速度の遅いものがでてる。

IntroductionLow-velocity zoneについて含水鉱物が70-90kmで脱水し、Hydrous layerを形成(その後も水は残っている)Hydrous layerは120kmくらいまで沈み込む。Hydrous layerがはっきりと確認できる場所の上には火山がある。

purpose Tsuji et al., 2008の研究をデータなど改善し、さらに関東地方についても同じ解析を行う。地域によるLow velocity zoneの分布の違いを議論する。Low velocity zone の温度による影響を考える。

MethodTomo DD絶対走時と相対走時を両方使用することで速度構造、震源を同時に求める手法(相対走時は、波形が似ているぺアに関しては相互相関から求める。)

1876 H. Zhang and C. H. Thurber

be biased due to velocity heterogeneity (Got et al., 1994;Waldhauser and Ellsworth, 2000; Wolfe, 2002). In contrast,the latter approach uses the relative arrival times to deter-mine a much smaller number of adjusted arrival time picks,but these picks are absolute arrival times and so can be usedto determine absolute locations (in an existing velocitymodel, or using tomography).

We have developed a new method that combines theadvantages and avoids the disadvantages of the previous ap-proaches. It is based on the code hypoDD of Waldhauser(2001) and makes use of both absolute and relative arrivaltime data. The method determines a 3D velocity modeljointly with the absolute and relative event locations. Thisapproach has the advantage of including relative arrivaltimes with their quality values along with absolute arrivaltimes, thereby not discarding valuable information by onlyusing adjusted picks, and at the same time dispensing withsimplifying assumptions about ray path geometries or pathanomalies and producing absolute locations, not just rela-tive locations. The velocity model obtained with double-difference (DD) tomography should also be superior to thatfrom standard tomography. With standard tomography,event locations will be somewhat scattered due to imprecisepicks and correlated errors, but in DD tomography, the useof the differential arrival times (including both the WCC andcatalog time difference data) removes most of these errors,which will in turn remove some fuzziness from the velocitymodel. To demonstrate the effectiveness of the method, wehave applied it to a synthetic dataset based on the idealizedvelocity structure of the San Andreas fault in central Cali-fornia and to the Hayward fault dataset of Waldhauser andEllsworth (2002).

DD Tomography

The body-wave arrival time T from an earthquake i toa seismic station k is expressed using ray theory as a pathintegral,

ki iT ! s " u ds, (1)k !

i

where s i is the origin time of event i, u is the slowness field,and ds is an element of path length. The source coordinates(x1, x2, x3), origin times, ray paths, and the slowness fieldare the unknowns. The relationship between the arrival timeand the event location is highly nonlinear, so a truncatedTaylor series expansion is generally used to linearize equa-tion (1). This linearly relates the misfit between the observedand predicted arrival times to the desired perturbations toirk

the hypocenter and velocity structure parameters:

3 i k!Tki i ir ! Dx " Ds " du ds. (2)k " li !!x il!1 l

Subtracting a similar equation for event j observed at stationk from equation (2), we have

3 i k!Tki j i ir # r ! Dx " Ds " du dsk k " li !!x il!1 l (3)3 j k!Tk j j# Dx # Ds # du ds." lj !!x jl!1 l

Assuming that these two events are near each other so thatthe paths from the events to a common station are almostidentical and the velocity structure is known, then equation(3) can be simplified as

3 i!Tkij i j i idr ! r # r ! Dx " Dsk k k " li!xl!1 l (4)3 j!Tk j j# Dx # Ds ," lj!xl!1 l

where is the so called double-difference (Waldhauserijdrk

and Ellsworth, 2000). This term is the difference betweenobserved and calculated differential arrival times for the twoevents and can also be written as

ij i j i j obs i j caldr ! r # r ! (T # T ) # (T # T ) . (5)k k k k k k k

The observed differential arrival times can bei j obs(T # T )k k

calculated from both WCC techniques for similar waveformsand absolute catalog arrival times. Equation (4) is known asthe DD earthquake location algorithm (Waldhauser and Ells-worth, 2000).

In this approach, earthquake locations may be biasedwhen interevent distances exceed the scale length of velocityvariations. Waldhauser and Ellsworth (2000) applied a dis-tance-weighting factor to reduce or exclude data from eventpairs that are far apart. Although the arrival difference datafrom such events may be excluded, they can still be linkedin the inversion via a series of intermediate pairs (Got et al.,1994). For example, the pair can be linked if the twoi jT –Tk k

pairs and are included.i m m jT –T T –Tk k k k

To overcome this limitation, we use the differential ar-rival time data and equation (3) directly. It is known thatthere is a coupling effect between the event hypocenters andthe velocity structure (Thurber, 1992). Our purpose is todetermine not only the relative event locations, but also theirabsolute locations and the velocity structure. Also note thatthe ray paths from two nearby events will substantially over-lap, meaning that the model derivatives in equation (3) willessentially cancel outside the source region. For this reason,we include the absolute arrival times in the inversion to re-solve the velocity structure outside the source region. Bydoing this, we can jointly determine the velocity structureand the relative event locations as well as the absolute eventlocations accurately.

We developed a DD tomography code tomoDD basedon the DD location code hypoDD (Waldhauser, 2001). In the

1876 H. Zhang and C. H. Thurber

be biased due to velocity heterogeneity (Got et al., 1994;Waldhauser and Ellsworth, 2000; Wolfe, 2002). In contrast,the latter approach uses the relative arrival times to deter-mine a much smaller number of adjusted arrival time picks,but these picks are absolute arrival times and so can be usedto determine absolute locations (in an existing velocitymodel, or using tomography).

We have developed a new method that combines theadvantages and avoids the disadvantages of the previous ap-proaches. It is based on the code hypoDD of Waldhauser(2001) and makes use of both absolute and relative arrivaltime data. The method determines a 3D velocity modeljointly with the absolute and relative event locations. Thisapproach has the advantage of including relative arrivaltimes with their quality values along with absolute arrivaltimes, thereby not discarding valuable information by onlyusing adjusted picks, and at the same time dispensing withsimplifying assumptions about ray path geometries or pathanomalies and producing absolute locations, not just rela-tive locations. The velocity model obtained with double-difference (DD) tomography should also be superior to thatfrom standard tomography. With standard tomography,event locations will be somewhat scattered due to imprecisepicks and correlated errors, but in DD tomography, the useof the differential arrival times (including both the WCC andcatalog time difference data) removes most of these errors,which will in turn remove some fuzziness from the velocitymodel. To demonstrate the effectiveness of the method, wehave applied it to a synthetic dataset based on the idealizedvelocity structure of the San Andreas fault in central Cali-fornia and to the Hayward fault dataset of Waldhauser andEllsworth (2002).

DD Tomography

The body-wave arrival time T from an earthquake i toa seismic station k is expressed using ray theory as a pathintegral,

ki iT ! s " u ds, (1)k !

i

where s i is the origin time of event i, u is the slowness field,and ds is an element of path length. The source coordinates(x1, x2, x3), origin times, ray paths, and the slowness fieldare the unknowns. The relationship between the arrival timeand the event location is highly nonlinear, so a truncatedTaylor series expansion is generally used to linearize equa-tion (1). This linearly relates the misfit between the observedand predicted arrival times to the desired perturbations toirk

the hypocenter and velocity structure parameters:

3 i k!Tki i ir ! Dx " Ds " du ds. (2)k " li !!x il!1 l

Subtracting a similar equation for event j observed at stationk from equation (2), we have

3 i k!Tki j i ir # r ! Dx " Ds " du dsk k " li !!x il!1 l (3)3 j k!Tk j j# Dx # Ds # du ds." lj !!x jl!1 l

Assuming that these two events are near each other so thatthe paths from the events to a common station are almostidentical and the velocity structure is known, then equation(3) can be simplified as

3 i!Tkij i j i idr ! r # r ! Dx " Dsk k k " li!xl!1 l (4)3 j!Tk j j# Dx # Ds ," lj!xl!1 l

where is the so called double-difference (Waldhauserijdrk

and Ellsworth, 2000). This term is the difference betweenobserved and calculated differential arrival times for the twoevents and can also be written as

ij i j i j obs i j caldr ! r # r ! (T # T ) # (T # T ) . (5)k k k k k k k

The observed differential arrival times can bei j obs(T # T )k k

calculated from both WCC techniques for similar waveformsand absolute catalog arrival times. Equation (4) is known asthe DD earthquake location algorithm (Waldhauser and Ells-worth, 2000).

In this approach, earthquake locations may be biasedwhen interevent distances exceed the scale length of velocityvariations. Waldhauser and Ellsworth (2000) applied a dis-tance-weighting factor to reduce or exclude data from eventpairs that are far apart. Although the arrival difference datafrom such events may be excluded, they can still be linkedin the inversion via a series of intermediate pairs (Got et al.,1994). For example, the pair can be linked if the twoi jT –Tk k

pairs and are included.i m m jT –T T –Tk k k k

To overcome this limitation, we use the differential ar-rival time data and equation (3) directly. It is known thatthere is a coupling effect between the event hypocenters andthe velocity structure (Thurber, 1992). Our purpose is todetermine not only the relative event locations, but also theirabsolute locations and the velocity structure. Also note thatthe ray paths from two nearby events will substantially over-lap, meaning that the model derivatives in equation (3) willessentially cancel outside the source region. For this reason,we include the absolute arrival times in the inversion to re-solve the velocity structure outside the source region. Bydoing this, we can jointly determine the velocity structureand the relative event locations as well as the absolute eventlocations accurately.

We developed a DD tomography code tomoDD basedon the DD location code hypoDD (Waldhauser, 2001). In the

1876 H. Zhang and C. H. Thurber

be biased due to velocity heterogeneity (Got et al., 1994;Waldhauser and Ellsworth, 2000; Wolfe, 2002). In contrast,the latter approach uses the relative arrival times to deter-mine a much smaller number of adjusted arrival time picks,but these picks are absolute arrival times and so can be usedto determine absolute locations (in an existing velocitymodel, or using tomography).

We have developed a new method that combines theadvantages and avoids the disadvantages of the previous ap-proaches. It is based on the code hypoDD of Waldhauser(2001) and makes use of both absolute and relative arrivaltime data. The method determines a 3D velocity modeljointly with the absolute and relative event locations. Thisapproach has the advantage of including relative arrivaltimes with their quality values along with absolute arrivaltimes, thereby not discarding valuable information by onlyusing adjusted picks, and at the same time dispensing withsimplifying assumptions about ray path geometries or pathanomalies and producing absolute locations, not just rela-tive locations. The velocity model obtained with double-difference (DD) tomography should also be superior to thatfrom standard tomography. With standard tomography,event locations will be somewhat scattered due to imprecisepicks and correlated errors, but in DD tomography, the useof the differential arrival times (including both the WCC andcatalog time difference data) removes most of these errors,which will in turn remove some fuzziness from the velocitymodel. To demonstrate the effectiveness of the method, wehave applied it to a synthetic dataset based on the idealizedvelocity structure of the San Andreas fault in central Cali-fornia and to the Hayward fault dataset of Waldhauser andEllsworth (2002).

DD Tomography

The body-wave arrival time T from an earthquake i toa seismic station k is expressed using ray theory as a pathintegral,

ki iT ! s " u ds, (1)k !

i

where s i is the origin time of event i, u is the slowness field,and ds is an element of path length. The source coordinates(x1, x2, x3), origin times, ray paths, and the slowness fieldare the unknowns. The relationship between the arrival timeand the event location is highly nonlinear, so a truncatedTaylor series expansion is generally used to linearize equa-tion (1). This linearly relates the misfit between the observedand predicted arrival times to the desired perturbations toirk

the hypocenter and velocity structure parameters:

3 i k!Tki i ir ! Dx " Ds " du ds. (2)k " li !!x il!1 l

Subtracting a similar equation for event j observed at stationk from equation (2), we have

3 i k!Tki j i ir # r ! Dx " Ds " du dsk k " li !!x il!1 l (3)3 j k!Tk j j# Dx # Ds # du ds." lj !!x jl!1 l

Assuming that these two events are near each other so thatthe paths from the events to a common station are almostidentical and the velocity structure is known, then equation(3) can be simplified as

3 i!Tkij i j i idr ! r # r ! Dx " Dsk k k " li!xl!1 l (4)3 j!Tk j j# Dx # Ds ," lj!xl!1 l

where is the so called double-difference (Waldhauserijdrk

and Ellsworth, 2000). This term is the difference betweenobserved and calculated differential arrival times for the twoevents and can also be written as

ij i j i j obs i j caldr ! r # r ! (T # T ) # (T # T ) . (5)k k k k k k k

The observed differential arrival times can bei j obs(T # T )k k

calculated from both WCC techniques for similar waveformsand absolute catalog arrival times. Equation (4) is known asthe DD earthquake location algorithm (Waldhauser and Ells-worth, 2000).

In this approach, earthquake locations may be biasedwhen interevent distances exceed the scale length of velocityvariations. Waldhauser and Ellsworth (2000) applied a dis-tance-weighting factor to reduce or exclude data from eventpairs that are far apart. Although the arrival difference datafrom such events may be excluded, they can still be linkedin the inversion via a series of intermediate pairs (Got et al.,1994). For example, the pair can be linked if the twoi jT –Tk k

pairs and are included.i m m jT –T T –Tk k k k

To overcome this limitation, we use the differential ar-rival time data and equation (3) directly. It is known thatthere is a coupling effect between the event hypocenters andthe velocity structure (Thurber, 1992). Our purpose is todetermine not only the relative event locations, but also theirabsolute locations and the velocity structure. Also note thatthe ray paths from two nearby events will substantially over-lap, meaning that the model derivatives in equation (3) willessentially cancel outside the source region. For this reason,we include the absolute arrival times in the inversion to re-solve the velocity structure outside the source region. Bydoing this, we can jointly determine the velocity structureand the relative event locations as well as the absolute eventlocations accurately.

We developed a DD tomography code tomoDD basedon the DD location code hypoDD (Waldhauser, 2001). In the

1876 H. Zhang and C. H. Thurber

be biased due to velocity heterogeneity (Got et al., 1994;Waldhauser and Ellsworth, 2000; Wolfe, 2002). In contrast,the latter approach uses the relative arrival times to deter-mine a much smaller number of adjusted arrival time picks,but these picks are absolute arrival times and so can be usedto determine absolute locations (in an existing velocitymodel, or using tomography).

We have developed a new method that combines theadvantages and avoids the disadvantages of the previous ap-proaches. It is based on the code hypoDD of Waldhauser(2001) and makes use of both absolute and relative arrivaltime data. The method determines a 3D velocity modeljointly with the absolute and relative event locations. Thisapproach has the advantage of including relative arrivaltimes with their quality values along with absolute arrivaltimes, thereby not discarding valuable information by onlyusing adjusted picks, and at the same time dispensing withsimplifying assumptions about ray path geometries or pathanomalies and producing absolute locations, not just rela-tive locations. The velocity model obtained with double-difference (DD) tomography should also be superior to thatfrom standard tomography. With standard tomography,event locations will be somewhat scattered due to imprecisepicks and correlated errors, but in DD tomography, the useof the differential arrival times (including both the WCC andcatalog time difference data) removes most of these errors,which will in turn remove some fuzziness from the velocitymodel. To demonstrate the effectiveness of the method, wehave applied it to a synthetic dataset based on the idealizedvelocity structure of the San Andreas fault in central Cali-fornia and to the Hayward fault dataset of Waldhauser andEllsworth (2002).

DD Tomography

The body-wave arrival time T from an earthquake i toa seismic station k is expressed using ray theory as a pathintegral,

ki iT ! s " u ds, (1)k !

i

where s i is the origin time of event i, u is the slowness field,and ds is an element of path length. The source coordinates(x1, x2, x3), origin times, ray paths, and the slowness fieldare the unknowns. The relationship between the arrival timeand the event location is highly nonlinear, so a truncatedTaylor series expansion is generally used to linearize equa-tion (1). This linearly relates the misfit between the observedand predicted arrival times to the desired perturbations toirk

the hypocenter and velocity structure parameters:

3 i k!Tki i ir ! Dx " Ds " du ds. (2)k " li !!x il!1 l

Subtracting a similar equation for event j observed at stationk from equation (2), we have

3 i k!Tki j i ir # r ! Dx " Ds " du dsk k " li !!x il!1 l (3)3 j k!Tk j j# Dx # Ds # du ds." lj !!x jl!1 l

Assuming that these two events are near each other so thatthe paths from the events to a common station are almostidentical and the velocity structure is known, then equation(3) can be simplified as

3 i!Tkij i j i idr ! r # r ! Dx " Dsk k k " li!xl!1 l (4)3 j!Tk j j# Dx # Ds ," lj!xl!1 l

where is the so called double-difference (Waldhauserijdrk

and Ellsworth, 2000). This term is the difference betweenobserved and calculated differential arrival times for the twoevents and can also be written as

ij i j i j obs i j caldr ! r # r ! (T # T ) # (T # T ) . (5)k k k k k k k

The observed differential arrival times can bei j obs(T # T )k k

calculated from both WCC techniques for similar waveformsand absolute catalog arrival times. Equation (4) is known asthe DD earthquake location algorithm (Waldhauser and Ells-worth, 2000).

In this approach, earthquake locations may be biasedwhen interevent distances exceed the scale length of velocityvariations. Waldhauser and Ellsworth (2000) applied a dis-tance-weighting factor to reduce or exclude data from eventpairs that are far apart. Although the arrival difference datafrom such events may be excluded, they can still be linkedin the inversion via a series of intermediate pairs (Got et al.,1994). For example, the pair can be linked if the twoi jT –Tk k

pairs and are included.i m m jT –T T –Tk k k k

To overcome this limitation, we use the differential ar-rival time data and equation (3) directly. It is known thatthere is a coupling effect between the event hypocenters andthe velocity structure (Thurber, 1992). Our purpose is todetermine not only the relative event locations, but also theirabsolute locations and the velocity structure. Also note thatthe ray paths from two nearby events will substantially over-lap, meaning that the model derivatives in equation (3) willessentially cancel outside the source region. For this reason,we include the absolute arrival times in the inversion to re-solve the velocity structure outside the source region. Bydoing this, we can jointly determine the velocity structureand the relative event locations as well as the absolute eventlocations accurately.

We developed a DD tomography code tomoDD basedon the DD location code hypoDD (Waldhauser, 2001). In the

MethodTomo DD初期構造は右図(ただし、PAC plateの形状は与えていて、そこは5%速度がはやい)

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Fig. S1

Data領域：240×200×200(grid25*10*5)地震M>2.5(0-40km)M>1.0(40km-)震源深さ>震央距離?2001-2007 year+1997-1999depth>100km for region1,2

checker board testTomo DD1 Tomo DDで、東北から関東にかけて地下の地震波速度構造を推定した。2 沈み込むスラブの上面にLow-velocity zoneが存在することを確認した。3 Low-velocity zoneの分布は東北と関東で異なる。4 PAC plateにPHS plateが重なることで、温度分布に差がでて、関東では深くまでLow-velocity zoneが分布するのではないか。

checker board test解像度のテスト速度のパータベーションを碁盤のように与えて、走時を計算し、そこにノイズを与えて、速度構造を逆算する。パータベーションがどの程度戻るかで、解像度の目安になる。

ResultPAC plateの上部に10km厚さのVs-4.1km/sの層がある。東北…80km程度まで関東…120km程度まで(コンタクトしてると150km)

DiscussionPACの表面から5km下の速度分布☆ 関東のほうが遅い地殻が深くまで沈んでいる。(特にコンタクトゾーン！)↑温度によって脱水が支配されている

PACとPHSのコンタクトゾーン

Discussion地震活動との比較☆上面地震帯二重深発面の上面中で地震の集中してる場所

上面地震帯とLow-V地殻のおわりがほぼ一致している。

二重地震面の上面の震源分布

Conclusion 1 Tomo DDで、東北から関東にかけて地下の地震波速度構造を推定した。2 沈み込むスラブの上面にLow-velocity zoneが存在することを確認した。3 Low-velocity zoneの分布は東北と関東で異なる。(関東のほうが深くまで存在する)4 Low-velocity zoneのおわりる場所で、地震が多発している。

The end