La Palma Seismicity 2021 - Plataforma de Riesgos

In September 2021, a significant eruption started on La Palma in the Canary Islands. This event was preceded by seismic unrest that had been ongoing since 2017, with particular increases in activity from September 2021. We use earthquake data from the Instituto Geográfico Nacional (IGN) to examine the earthquake swarms and their relationship to the eruption. We confirm that the seismic activity shows two distinct patterns — clusters consistent with the movement of magma at crustal depths (10-15 km), and deeper seismicity (20-35 km) from which the main eruption originated. The data shows that the deeper swarm began in the mid-2010s but became much more intense in 2021.

Introduction¶

La Palma is one of the most active volcanic islands in the Canary Islands archipelago. The island is formed by a large volcanic edifice rising from the ocean floor, with the youngest volcanic rocks being less than 2,000 years old Carracedo et al. (2001).

In September 2021, La Palma experienced one of the most significant volcanic eruptions in the recent history of the Canary Islands. This eruption was preceded by a period of increased seismic activity that began in 2017 but intensified dramatically in the weeks leading up to the eruption Torres-González et al. (2021).

Eruption History¶

Historical records indicate that La Palma has experienced seven eruptions in the past 500 years:

Date	Duration	Location
1470-1492	22 years	Montaña Quemada
1585	84 days	Tajuya near El Paso
1646	82 days	Todoque-Montaña Negra
1677	66 days	Fuencaliente
1712	56 days	El Charco
1949	38 days	Duraznero
1971	24 days	Fuencaliente

: Historical eruptions on La Palma {#tbl-eruptions}

The 2021 eruption began on September 19, 2021, at approximately 15:10 UTC and lasted for 85 days until December 13, 2021. This makes it the longest eruption on La Palma since the Montaña Quemada eruption that began in 1470.

Magma Reservoirs¶

Geological and geophysical studies have identified a complex magma plumbing system beneath La Palma Klügel et al. (2015). The system consists of multiple interconnected reservoirs at different depths:

Mantle reservoir (20-40 km depth): Primary magma storage
Crustal reservoir (8-15 km depth): Secondary storage and differentiation
Shallow conduits (0-8 km depth): Transport pathways to surface

The seismic data provides insights into the activation of these different levels of the magma system.

Dataset¶

The earthquake data comes from the IGN catalog, which records seismic events throughout Spain including the Canary Islands. For this analysis, we focus on earthquakes within a 20 km radius of the eruption site from January 2017 to December 2021.

Data Processing¶

The dataset contains the following parameters for each earthquake:

DateTime: UTC timestamp of the event
Latitude/Longitude: Geographic coordinates (WGS84)
Depth: Hypocentral depth in kilometers
Magnitude: Local magnitude (ML)
Quality: Location quality indicator

Events with magnitudes less than 1.0 or quality indicators below “C” were excluded from the analysis.

import pandas as pd
import numpy as np
import matplotlib.pyplot as plt

# Load the earthquake data
df = pd.read_csv('lapalma_earthquakes.csv')

# Filter the data
df = df[df['magnitude'] >= 1.0]
df = df[df['quality'].isin(['A', 'B', 'C'])]

# Convert datetime
df['datetime'] = pd.to_datetime(df['datetime'])

print(f"Total earthquakes analyzed: {len(df)}")

Results¶

Temporal Evolution¶

The temporal evolution of seismicity shows distinct phases:

Background activity (2017-2020): Low-level seismicity with occasional swarms
Pre-eruption intensification (July-September 2021): Dramatic increase in earthquake frequency
Syn-eruptive activity (September-December 2021): High-frequency, low-magnitude events

```{figure} temporal_evolution.png 31b8e172-b470-440e-83d8-e6b185028602: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:31b8e172-b470-440e-83d8-e6b185028602

Temporal evolution of seismicity on La Palma from 2017-2021. The plot shows both earthquake frequency (top) and cumulative seismic moment (bottom). Note the dramatic increase in activity starting in July 2021.


### Spatial Distribution

The spatial distribution of earthquakes reveals two main clusters:

**Cluster 1 - Shallow/Intermediate (8-15 km depth)**
- Located beneath the central part of the island
- Associated with crustal magma reservoir
- Earthquake magnitudes typically ML 2.0-3.5

**Cluster 2 - Deep (20-35 km depth)**  
- Located slightly offshore to the west
- Associated with mantle reservoir and deep magma ascent
- Earthquake magnitudes typically ML 1.5-4.5

```{figure} spatial_distribution.png
---
name: fig-spatial
width: 100%
---
Spatial distribution of earthquakes colored by depth. The shallow cluster (red) is located beneath the island, while the deep cluster (blue) extends offshore. The eruption site is marked with a star.

Depth Distribution¶

The depth distribution analysis confirms the two-reservoir model:

N(z) = A_1 \exp\left(-\frac{(z-z_1)^2}{2\sigma_1^2}\right) + A_2 \exp\left(-\frac{(z-z_2)^2}{2\sigma_2^2}\right)

(1)

Where: - $z_1 = 12 \pm 2$ km (crustal reservoir depth) - $z_2 = 28 \pm 4$ km (mantle reservoir depth) - $A_1, A_2$ are amplitude parameters - $\sigma_1, \sigma_2$ are depth uncertainties

Magnitude-Frequency Analysis¶

The Gutenberg-Richter relationship for La Palma seismicity follows:

\log_{10} N = a - bM

(2)

Where: - $N$ = cumulative number of earthquakes ≥ magnitude $M$ - $a = 4.2 \pm 0.1$ (activity parameter) - $b = 1.1 \pm 0.1$ (slope parameter)

The b-value of 1.1 is typical for volcanic environments and indicates a high proportion of smaller earthquakes relative to larger ones McNutt (2005).

```{figure} magnitude_frequency.png 31b8e172-b470-440e-83d8-e6b185028602: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:31b8e172-b470-440e-83d8-e6b185028602

Magnitude-frequency distribution for La Palma earthquakes. The plot shows the characteristic power-law relationship with a b-value of 1.1, typical for volcanic seismicity.


## Statistical Analysis

### Seismic Rate Changes

We applied change-point analysis to identify significant changes in seismic rate:

```python
import ruptures as rpt

# Prepare time series data
daily_counts = df.groupby(df['datetime'].dt.date).size()
signal = daily_counts.values

# Change-point detection
algo = rpt.Pelt(model="rbf").fit(signal)
result = algo.predict(pen=10)

print(f"Change points detected: {result}")

The analysis identified three major change points: 1. May 2020: Transition from background to elevated activity
2. July 2021: Onset of pre-eruptive intensification 3. September 19, 2021: Eruption onset

Correlation Analysis¶

Cross-correlation between shallow and deep seismicity reveals:

r(τ) = \frac{\sum_{t} [x_s(t) - \bar{x}_s][x_d(t+τ) - \bar{x}_d]}{\sqrt{\sum_t [x_s(t) - \bar{x}_s]^2 \sum_t [x_d(t+τ) - \bar{x}_d]^2}}

(3)

Where $x_s(t)$ and $x_d(t)$ are shallow and deep earthquake counts at time $t$ .

Results show maximum correlation at τ = 0 days with r = 0.72, indicating synchronized activation of both reservoir levels.

Discussion¶

Magma System Dynamics¶

The seismic data supports a model where:

Deep reservoir activation (2017-2020): Slow magma accumulation at mantle depths
Crustal reservoir charging (2020-2021): Magma ascent and storage at crustal levels\
Final ascent (September 2021): Rapid magma transport to surface

Hazard Implications¶

The analysis has important implications for volcanic hazard assessment:

Key Findings :class: important

Early warning: Deep seismicity provides months of advance warning
Eruption timing: Shallow swarms indicate imminent eruption (days to weeks)
Duration prediction: Correlation between pre-eruptive seismic energy and eruption duration :::

Comparison with Other Systems¶

La Palma’s seismic patterns are similar to those observed at other ocean island volcanoes:

Volcano	Deep Swarm Depth	Shallow Swarm Depth	Lead Time
Kilauea	30-60 km	3-8 km	Weeks-Months
Etna	20-30 km	5-15 km	Days-Weeks
La Palma	20-35 km	8-15 km	Months
Stromboli	10-20 km	2-6 km	Hours-Days

: Comparison of seismic patterns at ocean island volcanoes {#tbl-comparison}

Conclusions¶

The 2021 La Palma eruption was preceded by a complex pattern of seismic activity that provides insights into the magma plumbing system:

Two-reservoir system: Seismicity confirms mantle (20-35 km) and crustal (8-15 km) magma reservoirs
Long-term precursors: Deep seismicity began increasing in 2017, providing years of warning
Short-term precursors: Shallow seismicity intensified weeks before eruption
Synchronized activation: Strong correlation between deep and shallow earthquake rates

These findings have important implications for: - Volcanic monitoring: Multi-depth seismic networks essential for eruption forecasting - Hazard assessment: Different phases of unrest require different response protocols\

Scientific understanding: Confirms conceptual models of ocean island volcano plumbing systems

Future work should focus on: - Real-time implementation of change-point detection algorithms - Integration with other monitoring techniques (geodesy, gas emissions) - Development of probabilistic eruption forecasting models

Acknowledgments¶

The authors thank the Instituto Geográfico Nacional (IGN) for providing the earthquake catalog data. We also acknowledge the staff at the Observatorio Geofísico Central for their continuous monitoring efforts during the crisis.

Data Availability¶

The earthquake catalog data used in this study is available from the IGN at https://www.ign.es/web/ign/portal/sis-catalogo-terremotos. Processing scripts and analysis code are available at https://github.com/curvenote/la-palma-seismicity.

References¶

Carracedo, J. C., Badiola, E. A., Guillou, H., De La Nuez, J., Pérez Torrado, F. J., Javier, F., Klugel, A., & Schmincke, H. U. (2001). Geology and volcanology of La Palma and El Hierro, Western Canaries. Estudios Geológicos, 57(5–6), 175–273. 10.3989/egeol.01575-6134
Torres-González, P. A., Prieto, J. F., García-Cañada, L., & Fernández, J. (2021). The 2021 volcanic eruption of La Palma: Seismic and geodetic precursors. Journal of Volcanology and Geothermal Research, 421, 107437. 10.1016/j.jvolgeores.2021.107437
Klügel, A., Longpré, M.-A., García-Cañada, L., & Stix, J. (2015). Structure and evolution of the magma plumbing system beneath La Palma (Canary Islands): Constraints from thermobarometry of fluid inclusions. Contributions to Mineralogy and Petrology, 170(1), 1–22. 10.1007/s00410-015-1207-7
McNutt, S. R. (2005). Seismic monitoring and eruption forecasting of volcanoes: a review of the state-of-the-art and case histories. Monitoring and Mitigation of Volcano Hazards, 99–146. 10.1007/978-3-642-80087-0_3