Report on“Evidence for tidal triggering of earthquakes as revealed from statistical analysis of global data”

by S. Tanaka and M. Ohtake and H. Sato

Carl TapeGe277 --- J.-P. Avouac

February 20, 2007

Outline

• Introduction to tides

• Overview of Tanaka papers

• Observational evidence of what triggers earthquakes:

• Tidal phase angle

• Regional tectonic stress

• Proximity in space and time to “event” in earthquake cycle

• Stress values to keep in mind:

• Earthquake-induced stress changes: 100,000 - 1,000,000 Pa

• Tides-induced stress changes: 1000 - 5000 Pa

Example of oceanic tidal data

3 years of high-tide-level data (1999 to 2001)at Kwajelein Island (southwestern Pacific Ocean)

Topography and bathymetry govern the propagation of

the tides

What are their data?

• Global earthquakes, 1977 - 2003, in the Harvard CMT catalog

• Mw 5.5 to Mw 8.0

• Focal mechanism, origin time, hypocenter (lat, lon, depth)

• Oceanic tidal loading model (need ocean tide model)

• Earth tide model (need PREM-like model to describe solid Earth)

Oceanic loading + solid-Earth term = stress tensor at a particular hypocenter

For this, we need a PREM-like model (density (r)),then assume hydrostatic equilibrium.

Examples of tidal shear stress on

fault planes

What is their modeling technique?

Schuster’s test

Schuster’s test for random distributions

For Schuster’s test, about 5% of the time you get a p-value <= 5% for RANDOM distributions with N = 9350.

For reverse fault type earthquakes, the correlation between the Earth tide and

earthquake occurrence is the most remarkable. For normal fault type, significant

correlations appear in some cases. On the other hand, we find no correlation for

strike-slip type. Surveying the cases of small p values ( p < 5%), we find a

remarkable regularity in the phase angle of the peak frequency. For the shear

stress, ranging between -58º and -22º appears near and in advance to the

phase angle 0º without exception. This means that many earthquakes tend to

occur a little before the shear stress in the slip direction attains the maximum. In

the case of volumetric stress (J1), the peak is found near the phase angle ±180º

(maximum compression) for reverse fault type, and near the phase angle 0º

(maximum tension) for normal fault type.

“Spatio-temporal variation of the tidal triggering effect on earthquake occurrence associated with the 1982 South Tonga earthquake of Mw 7.5” by Tanaka, Ohtake, and Sato, GRL, 2002

--> “…tide-earthquake correlation may emerge by focusing the analysis on the focal zone and pre-seismic period of large earthquakes.”

“Tidal triggering of earthquakes in Japan related to the regional stress field”

by Tanaka, Ohtake, and Sato, EPS, 2004

“…earthquakes are triggered by amplifying the tectonic stresses under which faults operate. A large stress change that would force a fault to slip in a direction other than the one in which it has evolved has little effect, whereas a tiny nudge in its natural slip direction can nucleate an earthquake.”

--(R. Stein, Nature, 2002)

“Tidal triggering of earthquakes in the subducting Philippine Sea plate beneath the locked zone of the plate interface in the Tokai region, Japan”

by Tanaka, Sato, Matsumura, and Ohtake, and Sato, Tectonophysics, 2006

--> “…a systematic monitoring of tidal effect may be feasible for detecting the final stage of tectonic stress prior to the occurrence of a main rupture…”

“[S]tudies on tidal triggering may provide a new insight into asperities of pending earthquakes.”

Questions for discussion

• How do the tidal triggering results shed light on the nucleation of earthquakes?

• How can the results from tidal triggering be used to estimate rate-and-state parameters?

• What factors appear to control tidal triggering of earthquakes?

• Tidal stress, tidal phase angle, earthquake magnitude, earthquake focal mechanism,

earthquake depth

• What is the relative importance of:

• the tidal stress (Tanaka 2002a)

• the regional tectonic stress (2004)

• the time within the characteristic earthquake cycle (Tanaka 2002b, 2006)

• What are the consequences of repeatedly subdividing the dataset (fault type, magnitude,

depth, etc)?

• Which cycle is the tidal phase angle tied to (frequency dependence)? Other cycles?

• Why do we observe the tidal triggering effect if it is so much weaker than Coulomb failure

criterion, which is not that well-observed?

• If tidal triggering might be used as a predictor for earthquakes, then how successful does

it appear to be (i.e., what is its failure rate)?