|Earthquake nucleation and triggering on an
optimally oriented fault (pdf,
Earth and Planetary Science Letters, v. 363, p. 231-241, 2013
Carl Tape, Michael West, Vipul Silwal, Natasha Ruppert
Seismic surface waves from large, distant earthquakes commonly trigger smaller earthquakes. However, delay times of hours to days between the surface waves and the triggered earthquakes weaken the causal connection. Furthermore, when there is no delay, the triggered earthquakes are typically too small or too obscured to obtain reliable source mechanisms. We present observations of instantaneous triggering of a strike-slip earthquake in central Alaska. Shear strain from the optimally-aligned teleseismic Love wave induced a 24 s exponential foreshock signal leading to the triggered earthquake. This nucleation phase, and the alignment of the triggered earthquake source mechanism with the teleseismic stress field, reveal the behavior of an existing fault under well-calibrated strain conditions. The Alaska earthquake provides the first observation of combined nucleation and triggering, and it suggests that transient stresses during nucleation may influence the subsequent earthquake rupture. Laboratory and theoretical studies of nucleation and triggering may help discriminate between different interpretations for the Alaska earthquake.
This material is based upon work supported by the National Science Foundation under Grant Number (NSF EAR 1215959).
|Love waves in Alaska from
the April 11, 2012 Mw 8.6 Sumatra earthquake. (a)
Propagation path from Sumatra to central Alaska, at an
azimuthal angle of 22.3 deg. The beachball symbol
depicts the predominantly strike-slip rupture of the
Sumatra earthquake. (b) Horizontal displacement field in
Alaska at the origin time of the Mw 3.9 Nenana earthquake
in central Alaska. The epicenter of this event is marked
by the white dot at the center of the thick gray lines.
The large gray arrow denotes the wave propagation
direction from Sumatra. (c) Expanded view of (b) in the
region of the triggered event. Major active faults are
labeled as Denali, Tintina, and MFSZ, the Minto Flats
seismic zone. (d) Shear strain computed from the
displacement field in (b).
|The occurrence of the Mw
3.9 earthquake near Nenana, Alaska, coincident in time and
space with 4 cm horizontal displacement Love wave from the
(a) Representative three-component ground motion of the Sumatra earthquake in Alaska. This recording at MDM is the closest station (32 km) to the triggered Nenana earthquake. The Love wave is the dominant waveform throughout Alaska. (b) High-frequency filtered version of the same seismogram in (a) showing the Mw 3.9 Nenana earthquake. (c)-(d) Transverse displacement (c) and corresponding shear strain (d) at the Nenana epicenter, computed by stacking waveforms from the 13 closest stations. The dashed black lines denote the positive shear strain pulses A and C. The dashed red line denotes the emergence of the foreshock signal from the noise at t = -24 s.
|Source inversion for the
Nenana earthquake, showing a subset of observed
three-component seismograms (black) compared with
synthetic seismograms (red), both filtered between 0.3 Hz
and 0.6 Hz.
|Earthquake source mechanism
for the 2012-04-11 Mw 3.9 Nenana earthquake in comparison
with the orientation of Sumatra Love waves. The comparison
is made at the Nenana earthquake epicenter at the origin
time of the Nenana earthquake. The Sumatra Love waves
propagate along the great-circle path from Sumatra (solid
cyan line) and impart horizontal ground motion in the
perpendicular direction (dashed cyan line). The principal
axes of the associated strain tensor are shown as cyan
arrows. The principal axes for the Nenana source mechanism
are plotted as red arrows and white arrows.
Inward-pointing arrowheads for P-axis directions are
hidden beneath the beachball. The 1995-10-06 Mw 6.0
earthquake is plotted for comparison. Both earthquakes are
consistent with left-lateral strike slip fault motion on
the western of the two faults comprising the Minto Flats
|Record section of the 24 s
foreshock signal prior to the Nenana earthquake. The
seismograms are the vertical component of velocity,
filtered 2-8 Hz, ordered by distance from the Nenana
epicenter, and aligned on the P onset of the mainshock.
The red line at -24 s is the time at which the foreshock
signal rises above the pre-Sumatra background noise level.
The alignment of this pulse, in addition to the decrease
in amplitude with distance, indicate that the foreshock
signal is in the same region as the mainshock.
|Envelope of velocity seismograms for two
representative stations. The amplitudes are plotted on a
log scale so that the noise level, foreshock signal, and
mainshock are all visible. The linear slope of the
foreshock signal represents an exponential growth in
amplitude. MLY.AK is 86 km northwest of the Nenana
epicenter; COLA.IU is 51 km east of the Nenana epicenter.
||Comparison between the 2012-04-11 Mw 3.9
and 2004-11-17 Mw 3.6earthquakes, recorded at COLA.IU. The
2004 event has similar hypocenter, magnitude, and source
mechanism to the 2012 event. The striking difference
between these two records is the 2012 foreshock signal,
interpreted as a nucleation phase.
|Guide to interpretation
scenarios for the Nenana earthquake. (bottom) The
example time series is the MLY record (see above). The
vertical line at t = -72.9 s is the peak positive shear
stress perturbation from the Sumatra Love wave. (top)
Shear stress perturbation due to the Sumatra Love waves,
|Key times and durations
in this study. Station MDM is the closest station (32
km) to the triggered Nenana earthquake. By "at Nenana,"
we mean at the epicenter of the Nenana earthquake.
|Figure S4 from Tape et al. (2013). Figure
S4: Inferring two faults from seismicity in the Minto
Flats seismic zone. (a) Seismicity from the AEIC
catalog, 1990–2010, M > 0. (b) Seismicity
density from (a), plotted on a logarithmic color scale,
which reveals the dominant two fault strands that we
digitized. Other faults plotted are from (Plafker et
al., 1994). The Minto Flats seismic zone (MFSZ) has
previously been described as a single, diffuse seismic
zone that extends approximately 200 km from SW to NE
(Pulpan, 1986; Biswas and Tytgat , 1988; Page et al.,
1995; Ratchkovski and Hansen, 2002). Despite the
prominence of the two seismic lineaments, neither
lineament appears as an active or inactive fault in the
fault map of Plafker et al. (1994). The eastern
lineament coincides with the Minto fault mapped by Pewe
et al. (1966), but later interpreted as geomorphic
feature (Page et al., 1991; Plafker et al., 1994). Here
we suggest that the lineaments be identified as two
active faults. The two faults of the MFSZ appear to
bound the gravity-low signature of the Nenana
sedimentary basin (Barnes, 1961; Van Kooten et al.,
Text files containing the longitude and latitude coordinates for the two MFSZ fault strands: delimited (west only, east only).