1

PICTURES and CEVICHE: controlled source seismic experiments to probe the relationship between subduction zone slip and geologic structure in Chile Anne Tréhu (Oregon State University) and Nathan Bangs (Un. Texas Institute for Geophysics) In late 2016 and early 2017, the R/V Marcus Langseth will be on the continental margin of Chile for two NSF-funded projects to explore crustal velocity, seismic reflectivity and margin structure for comparison with geodetically-determined interseismic creep and with the distribution of slip during large plate boundary earthquakes (Figure 1). These projects represent examples of the use of controlled source seismic data to complement continuous observatory geodetic and seismic observations in order to study subduction zone earthquakes. Controlled source seismic experiments provide critical data for understanding subduction (see the white paper by Shillington et al.), and any SZO should include a controlled source component for high-resolution characterization of the subsurface geologic structure. The PICTURES and CEVICHE projects will provide an excellent basis for potential complementary SZO projects along the South American margin and data to test innovative new analysis approaches. Lessons learned will be applicable to future SZO experiments. PICTURES (Pisagua/Iquique Crustal Tomography to Understand the Region of the Earthquake Source) is a collaborative effort between the Oregon State University, GEOMAR and the Un. Of Chile to image crustal structure in the region of the 2014 Pisagua/Iquique earthquake. The Pisagua/Iquique earthquake was exceptional in having a long and well-recorded foreshock sequence and a remarkable correlation between seismicity and the gravity field, suggesting strong structural control on megathrust slip. Moreover, it only ruptured half of a seismic gap that last ruptured in a very large event in 1877, heightening concerns about a future event in this region. PICTURES is the first of two planned cruises to image 3D crustal structure and focuses on the region that ruptured in 2014; phase 2, on the R/V Sonne, will focus on the unruptured gap. Seismic sources from the R/V Marcus Langseth will be recorded on 69 ocean-bottom seismometers (50 from the US OBSIP and 19 from GEOMAR) and on onshore seismometers of the Chilean National Seismic and IPOC seismic networks as well as on the R/V Langseth MCS streamer. In addition to obtaining data to develop a 3D velocity model of the source region, two longr profiles along and across the subduction zone will be acquired to characterize the incoming plate structure and an apparent change in structure between the remaining gap and the region that ruptured in 2014. CEVICHE (Crustal Experiment from Valdivia to Illapel to Characterize Huge Earthquakes) is an MCS experiment that spans the source region of the 2015 Illapel and 2010 Maule earthquakes, and the northern half of the great 1960 Valdivia earthquake. The objective of this project is to image changes in plate boundary reflectivity for comparison with the slip distribution during these recent large earthquakes and with variations in interseismic locking and post-seismic afterslip behavior as determined geodetically. MCS data will be acquired with the 15-km long streamer on the R/V Marcus Langseth along the continental slope and shelf and just seaward of the deformation front. A number of dip lines across the forearc will also be acquired. Although the placement of the dip lines is shown schematically on Figure 1C, locations will be fine-tuned during the cruise in response to results from the strike profile along the shelf.

!(

!(

!(

!(

!(

!(

!(

!(!(

!( !(!( !(

!(!(

!(!(!( !(!(!(

!(!(

!(!(

!(!(

!(!(

!(!(

!(!(

!(!(

!(

!(!(

!(!(

!(!( !(!(

!(

!(

!(

!(!(

!(!(

!(!(

!. !.

!.!.

!.

!.

!.

!.

!.

!.

!.

!.!.

!.

!(

100 m

1000 m

2000 m3000 m

4000 m5000 m6000 m

7000 m

6000 m

5000 m

4000 m

4000 m

PA C I F I CO C E A N

PERU

CHILE

Punta Coles

Iquique

70°W

70°W

71°W

71°W

72°W

72°W

73°W

18°S 18

°S

19°S 19

°S

20°S 20

°S

21°S 21

°S

22°S 22

°S

L E G E N D

!( Scripps OBS Location

!. GEOMAR OBS Location

Seismic Transect

Exclusive Economic Zone

Territorial Waters (12 nmi)

Ocean Depth Contour

Marine Protected Area

!!!

!!!! !!!!!!

!

!

!(

!(

PA C I F I CO C E A N

CH

ILE

BOLIVIA

ARGENTINA

Caldera

Isla ChañaralIsla Grande de Atacama

La Rinconada

Islas Choros - Damas

Iquique

Valparaíso

52°W

52°W

56°W

56°W

60°W

60°W

64°W

64°W

68°W

68°W

72°W

72°W

76°W

76°W

80°W

80°W

84°W

84°W

88°W

88°W

92°W

92°W

96°W 48°W

18°S 18

°S

22°S 22

°S

26°S 26

°S

30°S 30

°S

34°S 34

°S

38°S

38°S

42°S

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!(

PA C I F I CO C E A N

CHILE

100 m

1000 m2000 m

3000 m

4000 m

1000 m2000 m3000 m

4000 m5000 m

4000 m

6000 m5000 m

ARGENTINA

Tic-Toc

Península de Hualpén

Chiloé Island

Gulf of Ancud

Gulf of CorcovadoGuafo Island

Rocas de Constitución

Islotes Lobería y Lobería Iglesiade Piedra de Cobquecura

Las Cruces

Bosque de Calabacillode Navidad

ChiloéPutemún

Pitipalena-Añihué

PullinqueIslotes de Punihuil

Bahía TicToc-Golfode Corcovado

Bosque Fósil de Punta Pelluco

Lafken Mapu-Lahual

Fiordo Comau-San Ignacio de Huinay

Valparaíso

69°W

69°W

70°W

70°W

71°W

71°W

72°W

72°W

73°W

73°W

74°W

74°W

75°W

75°W

76°W

76°W

77°W

77°W

78°W79°W

30°S 30

°S

31°S

31°S

32°S

32°S

33°S

33°S

34°S

34°S

35°S

35°S

36°S

36°S

37°S

37°S

38°S

38°S

39°S

39°S

40°S

40°S

41°S

41°S

42°S

42°S

43°S

43°S

44°S

44°S

40 0 4020 Kilometers 100 0 10050 Kilometers

NorthernSurvey

CentralSurvey

SouthernSurvey

PICTURES (Pisagua/Iquique Crustal Tomography toUnderstand the Region of the Earthquake Source)

CEVICHE (Crustal Experiment from Valdivia to Ilapel to Characterize Huge Earthquakes)

Figure 1. Maps showning the general location (A) and planned experimental geometries (B,C).

A

B C