Pelatihan : Techniques in Active Tectonic Study Juni 20-Juli 2, 2013 Instruktur: Prof. J Ramon Arrowsmith (JRA) Dari Arizona State University (ASU) - US Tempat Pelaksanaan: Ruang Pangea, Laboratorium Gempabumi (LabEarth) – Puslit Geoteknologi LIPI dan Kuliah lapangan akan dilakukan disekitar Sesar Lembang, Jawa Barat. * Lebih jelas baca TOR/KAK dan daftar acara

Earthquake Magnitude Outline of this lecture Crustal faults (and another example of rupture pulse model) Seismic moment and moment magnitude Examples: Haiti/Chile and Japan (along with seismic waves) Empirically defined moments Threshold for surface rupture

Three views of a crustal-scale strike-slip fault. Map view illustrates the fault as a zone of deformation. Cross section A-A' in the fault plane includes a contour map of the slip (u) which goes to zero at the fault tipline and is greatest near the hypocenter (star). Cross section B-B' perpendicular to the fault plane suggests that slip mechanisms are frictional resistance (FR) in the upper part of the crust and localized quasi-plastic flow (QP) in the lower part. The graph at the right indicates a linearly increasing resistance to shearing with depth to the brittle-ductile transition, and then a non-linear decreasing resistance to shearing with depth. Reprinted from (Sibson, 1989) Pollard and Fletcher, 2005

Map and cross sections of the Imperial Fault and the Brawley Fault for the October 15, 1979 earthquake in southern California (Archuleta, 1984). a) Map of the rupture trace. b) – e) vertical cross sections parallel to the fault trace with contours of the model rupture time, slip duration, strike-slip offset, and dip-slip offset. Pollard and Fletcher, 2005

Moment Magnitude Scale: Seismic moment (M o ) –Measures amount of strain energy released by movement along whole rupture surface; more accurate for big earthquakes –Calculated using rocks’ shear strength times rupture area of fault times displacement (slip) on the fault Moment magnitude scale uses seismic moment: –M w = 2/3 log 10 (M o ) – 6 Magnitude of Earthquakes

Seismic moment (M 0 ) Measures amount of strain energy released by movement along whole rupture surface; more accurate for big earthquakes Calculated using rocks’ shear strength times rupture area of fault times displacement (slip) on the fault M 0 = mu*A*u_bar --mu is shear modulus (how stiff is the rock?), typical value is 30 GPa (3x10 10 N/m 2 ) --A is surface area that slipped (m 2 ) [width x length] --u_bar is the average slip in meters Width Length Don’t forget to convert all to meters

Schematic diagram of four different methods for estimating the slip on a fault (Thatcher and Bonilla, 1989). The actual slip is contoured on the fault surface in a). Illustrations b) – d) show how geologists, geodesists, and seismologists gather data (left column), and graphical representations of these data are shown to the right. e) Interferometric synthetic aperture radar (InSAR) data provide the field of displacement at the surface near a fault which can be inverted to estimate the slip distribution. Pollard and Fletcher, 2005

How do we determine that the recent Chile earthquake was “500 times larger than” the recent Haiti earthquake? earthquake-chilean-earthquake-stronger- haitis/story?id=

First, one might say, well, if it is a log scale and it is M8.8 for Chile and M7.0 for Haiti, then that is a 1.8 magnitude difference, so what is ? Only 64 times larger. (someone at ABC is good with their exponents!) But that is not meaningful in terms of energy

Seismic moment (M 0 ) Measures amount of strain energy released by movement along whole rupture surface; more accurate for big earthquakes Calculated using rocks’ shear strength times rupture area of fault times displacement (slip) on the fault M 0 = mu*A*u_bar --mu is shear modulus (how stiff is the rock?), typical value is 30 GPa (3x10 10 N/m 2 ) --A is surface area that slipped (m 2 ) [width x length] --u_bar is the average slip in meters Width Length Don’t forget to convert all to meters

Although magnitude is still an important measure of the size of an earthquake, particularly for public consumption, seismic moment is a more physically meaningful measure of earthquake size. Seismic moment is proportional to the product of the slip on the fault and the area of the fault that slips. These “maps” of the slip on the fault surfaces of the January 12th M7.0 Haitian earthquake and the M8.8 Chilean earthquake show that, although the slip in Chile was only about 50% greater, the fault area was vastly larger. This accounts for the release of approximately 500 times more energy in the Chilean earthquake than in the Haiti earthquake. Chile Haiti Magnitude 8.8 OFFSHORE MAULE, CHILE Saturday, February 27, 2010 at 06:34:17 UTC Images courtesy of the U.S. Geological Survey

How do we determine that the recent Chile earthquake was “500 times larger than” the recent Haiti earthquake? Constant: mu = 3 x N/m 2 EarthquakeChileHaiti Length (m)600,00030,000 Width (m)150,00010,000 U_bar (m)53 M0M x Nm 2.7 x Nm MwMw M 0 = mu*Length*Width*U_bar M w = 2/3 log 10 (M 0 ) – 6 Energy difference: 1.35 x Nm / 2.7 x Nm = 500!

Tohoku, Japan Earthquake: Aftershock (and Foreshock) Sequence, 03/08/ /16/11

Tohoku, Japan Earthquake: Aftershock (and Foreshock) Sequence, M:Time History

Simple view of the seismic wave propagation Movie 3

Seismic wave propagation across US Movie 4

Evolving displacement field Movie 5

Tohoku, Japan Earthquake: Finite Fault Model USGS V1 - 7 hrs after OT Compact rupture, mostly bilateral about epicenter, peak slip up dip of hypocenter. Rupture was likely restricted to the shallow trench, and GPS vectors suggest slip did not reach the plate boundary beneath the coastline. Peak slips closer to 40+ m, inferred from updated modeling. Box is 580 km long and 190 km wide Zone of main slip is 300 km long and ~150 wide M 0 = mu*A*u_bar -mu = 30 GPa (3x10 10 N/m 2 ) -A is surface area that slipped (m 2 ) [width x length] -u_bar is the average slip in meters japan = mu.*500.*km.*200.*km.*11 = e+022 Nm japan_mw=(2./3).*log10(japan)-6 =

Empirically determined moments

Rupture map Slip distribution

Empirically determined moments

Three views of a crustal-scale strike-slip fault. Map view illustrates the fault as a zone of deformation. Cross section A-A' in the fault plane includes a contour map of the slip (u) which goes to zero at the fault tipline and is greatest near the hypocenter (star). Cross section B-B' perpendicular to the fault plane suggests that slip mechanisms are frictional resistance (FR) in the upper part of the crust and localized quasi-plastic flow (QP) in the lower part. The graph at the right indicates a linearly increasing resistance to shearing with depth to the brittle-ductile transition, and then a non-linear decreasing resistance to shearing with depth. Reprinted from (Sibson, 1989) Pollard and Fletcher, 2005

Circular (‘penny-shaped) crack m slip or opening 1 Mpa stress drop

Earthquake Magnitude Outline of this lecture Crustal faults (and another example of rupture pulse model) Seismic moment and moment magnitude Examples: Haiti/Chile and Japan (along with seismic waves) Empirically defined moments Threshold for surface rupture