Research ArticleEARTHQUAKES

Nucleation speed limit on remote fluid-induced earthquakes

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Science Advances  23 Aug 2017:
Vol. 3, no. 8, e1700660
DOI: 10.1126/sciadv.1700660

Figures

  • Fig. 1 Examples of known (6, 7, 28) remote dynamic triggering.

    (A) In the Basin and Range Province of the western United States. All M ≥ 2.0 western U.S. seismicity during the 24 hours after both the 1992 M = 7.4 Landers, CA (green dots) and the 2002 M = 7.9 Denali, AK (blue dots) earthquakes are shown. Both earthquakes have abundant local aftershocks and have produced significant triggered seismicity in the Basin and Range Province. Yellow stars show mainshock locations. The magnitude versus distance distribution from (B) the Landers earthquake shows continuous aftershocks trending into the Basin and Range Province out to ~1500 km away, whereas the (C) Denali earthquake has a clear spatial gap between local and remote aftershocks (~400 to 2300 km). Both earthquakes only have M ≥ 5 aftershocks within 300 km from mainshocks during the first 24 hours. The cumulative magnitude-frequency distribution of remotely triggered earthquakes in the Basin and Range Province during the first 24 hours after mainshocks is shown in (D). It is initially deficient in M ≥ 3.5 events (lighter blue points), whereas after 30 days [approximate duration of the rate increase (28)], it begins to fill in (darker blue dots) as evidenced by a significant decrease in the slope (b value) (Methods). Similarly, in (E), the temporal evolution of cumulative magnitude-frequency trends is shown for a global compilation (5) of remotely triggered earthquakes over 24 hours. The distributions are deficient at higher (M ≥ 4.5) magnitudes but gradually fill in over time, with b values decreasing.

  • Fig. 2

    Examples of the earliest occurrences of remotely triggered earthquakes across the magnitude spectrum worldwide in (A) Greece, (B) New Zealand, (C) China, (D) Chile, and (E) the Basin and Range Province of the United States. Remotely triggered earthquakes are defined as regional outbreaks occurring at rates significantly (>2σ) greater than day-to-day mean variation within local seismograph networks associated with surface wave arrivals from remote (r > 1000 km) global mainshocks (M ≥ 7.0). Blue spikes show ±24-hour variation in earthquake rates within regional networks, and red dashed lines show 1σ and 2σ variation on the mean variability (blue curves). Yellow histograms show daily seismicity rates for ±20 days before and after global mainshocks. Example maps show spatial distributions with red dots representing earthquakes in the 24 hours after surface wave arrivals and blue dots showing those in the 24 hours before. These examples demonstrate the origins of the earliest occurrences of 3.0 ≤ M ≤ 6.7 triggered earthquakes used to construct the relations shown in Fig. 3. Many more events are detected globally (5).

  • Fig. 3

    (A) Remotely triggered earthquakes (EQ) are part of a typical Omori law time decay that is similar to local aftershocks, which suggests that their timing should be advanced relative to a random process. The remotely triggered earthquake catalog was developed from 24-hour periods following 260 M ≥ 7.0 global mainshocks as recorded in 17 different seismograph networks around the world (white boxes on the map) (5). (B) Earthquakes from 38 randomly drawn 24-hour periods when no global or local mainshocks occurred demonstrate the anomalous nature of the rate increase associated with surface wave arrivals from global mainshocks (A). We use 38 24-hour periods because the observed data were triggered by 38 global mainshocks. (C) Locally (r ≤ 300 km) (blue dots) and remotely (r > 1000 km) (red dots) triggered earthquakes are plotted as a function of their magnitudes. Different shading of the locally triggered events represents five realizations of nonoverlapping sample sizes equal to the remotely triggered population. First-observed remotely triggered events in 0.1 magnitude-unit bins are plotted as bright red dots. There is an evident log-linear trend of delay time versus magnitude that is less evident in the locally triggered earthquakes (histogram in Fig. 1E shows the magnitude-frequency distribution of remotely triggered earthquakes). Lines fit to first-occurrence distributions from 100 sets of 38 random 24-hour periods within the 17 local networks are shown and are all earlier than, or overlap, the remotely triggered events.

  • Fig. 4 Comparison between locally (r ≤ 300 km; red) and remotely (r ≥ 1000 km; blue) triggered earthquakes versus time.

    Event rates are normalized and expressed as a percentage (per hour) of the total number in the ±10-day periods. The local mainshocks produce far more aftershocks than the remote mainshocks, which is evident when they are plotted on the same scale. Both sequences display Omori law rate increases that are followed by an inverse time decay.

  • Fig. 5 Nucleation dimension–dependent delay of remotely triggered earthquakes.

    First-observed earthquake times versus magnitude are also plotted against their magnitude-dependent, critical nucleation dimensions. These areas are the stuck patches (asperities) of earthquakes that must become unlocked for ruptures to occur (36, 37). Fault zone fluid pressurization rates are plotted for the range of observed (4145) fault zone diffusivity values, which illustrate consistency between delay times and asperity sizes. Generally, to obtain the earliest high-magnitude (largest rupture areas) induced earthquakes, fault zones must be highly diffusive.

  • Fig. 6 Calculated pore fluid pressure induced strength reduction as a function of earthquake magnitude and fault diffusivity.

    Points correspond to earliest-observed remotely triggered earthquakes in Figs. 2 and 3. Strength reduction is normalized by the initial shear strength on faults, which removes nucleation depth dependence. Low-magnitude (M < 3) earthquakes can show significant reduction (10 to 100% of initial resisting shear strength) regardless of diffusivity because the rupture areas are small. Higher-magnitude triggering appears to require high diffusivity, with D approaching 1.0 m2 s−1.

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